pubs.acs.org/cm Article Surfactant-Free Colloidal Syntheses of Gold-Based Nanomaterials in Alkaline Water and Mono-alcohol Mixtures Jonathan Quinson,* Olivia Aalling-Frederiksen, Waynah L. Dacayan, Joachim D. Bjerregaard, Kim D. Jensen, Mads R. V. Jørgensen, Innokenty Kantor, Daniel R. Sørensen, Luise Theil Kuhn, Matthew S. Johnson, María Escudero-Escribano, Søren B. Simonsen, and Kirsten M. Ø. Jensen* Cite This: Chem. Mater. 2023, 35, 2173−2190 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Gold nanoparticles (Au NPs) and gold-based nano- materials combine unique properties relevant for medicine, imaging, optics, sensing, catalysis, and energy conversion. While the Turkevich−Frens and Brust−Schiffrin methods remain the state-of- the-art colloidal syntheses of Au NPs, there is a need for more sustainable and tractable synthetic strategies leading to new model systems. In particular, stabilizers are almost systematically used in colloidal syntheses, but they can be detrimental for fundamental and applied studies. Here, a surfactant-free synthesis of size-controlled colloidal Au NPs stable for months is achieved by the simple reduction of HAuCl4 at room temperature in alkaline solutions of low-viscosity mono-alcohols such as ethanol or methanol and water, without the need for any other additives. Palladium (Pd) and bimetallic AuxPdy NPs, nanocomposites and multimetallic samples, are also obtained and are readily active (electro)catalysts. The multiple benefits over the state-of-the-art syntheses that this simple synthesis bears for fundamental and applied research are highlighted. ■ INTRODUCTION Gold (Au)-based nanomaterials (NMs) are used in applica- appealing for large scale production as they minimize hazards and cost.6,17 A range of RT syntheses in alcohols (ROHs) such tions ranging from biomedical diagnostics and treatments to as polyols are typically performed under alkaline conditions to facilitate the reduction of the metal precursor by the formation imaging and catalysis.1−3 A crucial achievement for a successful of alkoxides.18 RT syntheses using plant extracts and/or Au-based nanotechnology is the controlled synthesis of (micro)organisms are possible in aqueous media.13,14 How- nanoparticles (NPs) at sizes below 100 nm. Numerous ever, impurities in these bioderived compounds can be syntheses have been reported, and interested readers will challenging to control, and different feedstocks and/or find numerous reviews of the synthesis of Au NPs4 with an purification processes can lead to different Au NP properties.19 increasing focus on developing more sustainable syntheses.5,6 Furthermore, due to the rich chemistry of biomolecules, it is In particular, wet-chemical colloidal syntheses in which a gold challenging to identify the functional group(s) responsible for molecular precursor is converted into Au NPs by reduction in the reduction of the gold precursor, which might impair further the liquid phase are widely used, typically in the presence of optimization of the synthesis.19 stabilizers such as surfactants and at relatively low concen- More reagents also mean more potential sources of trations.4 Examples of surfactant-based colloidal syntheses are impurities, leading to irreproducibility and unwarranted summarized in Table 1. The state-of-the-art colloidal syntheses complexity for the synthesis.20 In addition, while the use of of Au NPs remain the Turkevich−Frens method,7,8 performed additives and surfactants is claimed to be necessary to achieve in close-to-boiling water using trisodium citrate (Na3Ct) as a colloidal stability, size, and/or shape control, surfactants are reducing and protecting agent, and the Brust−Schiffrin often not desirable for most applications. Surfactant removal is method,9 performed at room temperature (RT) in organic typically required, in catalysis to optimize the availability of media using NaBH4 as a reducing agent and thiol-based compounds as stabilizers. Overall, Au NP syntheses often require organic solvents Received: January 13, 2023 and/or strong and hazardous reducing agents.12 There is thus Revised: February 10, 2023 an interest in developing cheaper and more sustainable Published: February 24, 2023 methods. Water-based strategies facilitate scaling by minimiz- ing organic and hazardous waste. RT syntheses are possible due to the high reduction potentials of Au precursors and are © 2023 The Authors. Published by American Chemical Society https://doi.org/10.1021/acs.chemmater.3c00090 2173 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article Table 1. Overview of Colloidal Au NP Synthetic Strategies green solvent? green reducing agent? surfactant- low-Tbp solvent type ref solvent reducing agent (see ref 10) (see refs 10 and 11) freea (≤100 °C) RT Turkevich−Frens 7, 8 water Na3Ct yes no no yes no Brust−Schiffrin 9 toluene, water NaBH4 no no no no yes DMFb 12 DMF DMF no no yes no no biogenic 13 water biomolecules yes yes no yes yes (e.g., from plants) microorganisms 14 water microorganisms yes yes no yes yes polyols 15 polyols with water polyols yes yes yes no yes (e.g., glycerol) mono-alcohols this work mono-alcohol(s) mono-alcohols yes yes yes yes yes (with water) a Or free of additives other than solvents or reducing agents or “is the reducing agent a simple molecule with a molar mass of <100 g mol−1?” bN,N- Dimethylformamide. active surface sites21 and in medicine to best functionalize NPs free syntheses of Au NPs to date accounts for the overlooked with desired molecules.22 Surfactant removal involves chemical potentials of the promising and surprisingly simple synthesis or physical treatments, e.g., a high temperature,21 that are approach with the combined benefits (i−iv) reported here. typically only moderately successful and challenging to scale. We first provide a detailed account of overlooked parameters While often deemed impossible, surfactant-free syntheses of in the literature regarding surfactant-free and RT synthesis of colloidal precious metal (PM) NPs can be achieved.23 To the Au NPs. We then show the crucial importance of several best of our knowledge, there is no consensus on the definition experimental parameters that severely impact the properties of of a surfactant-free synthesis.23−25 We here considered that a the resulting NPs. We finally give the first examples of the synthesis is surfactant-free if no species other than the metal promising range of applications of this new synthesis to precursor has a molar mass of >100 g mol−1.23 Polyols in address the remaining challenges in studies and applications of alkaline solutions,26 e.g., in glycerol for Au NPs,15 are examples Au-based nanomaterials. of suitable reductants for surfactant-free PM NP syntheses. It is believed that the high viscosity of the polyols accounts for the stabilization of the NPs. Unfortunately, the high viscosity of ■ EXPERIMENTAL SECTION Chemicals. All chemicals were used as received: HAuCl4·3H2O polyols prevents a simple use of the as-prepared NPs.27 It has (Sigma Aldrich), PdCl2 (anhydrous, 60% Pd basis, Aldrich), H2PtCl6· recently been show that mono-alcohols could lead to stable 6H2O (99.9%, Alfa Aesar), OsCl3 (Premion, 99.99% metals basis, Alfa surfactant-free colloidal NPs under alkaline conditions, such as Aesar), H2IrCl6 (99.9%, Alfa Aesar), RuCl3·xH2O (99.9%, Alfa Pt,28 Ir,28,29 Ru,28 or Os30,31 NPs even at high precursor Aesar), LiOH (98%, Alfa Aesar), NaOH (Puriss., Sigma-Aldrich), KOH (ACS reagent, Sigma-Aldrich), CsOH (99.95%, Aldrich), water concentrations around 50 mM and at temperatures close to the [Milli-Q, Millipore, resistivity of >18.2 MΩ cm, total organic carbon boiling point of methanol (∼65 °C) or ethanol (∼78 °C). (TOC) < 5 ppb], methanol (MeOH, 99.8%, VWR), ethanol (EtOH, However this approach does not easily lead to stable absolute, VWR), trisodium citrate dehydrate (Na3Ct, 99%, Alfa colloidal Pd NPs that are best prepared at RT in the presence Aesar), polyvinylpyrrolidone (PVP, Alfa Aesar, MW of 58 000), of a support.32 Equally, this approach does not lead to Au NPs. H2SO4 (96%, Merck, Suprapur), HCl (37%, 37%, EMSURE ACS, Due to the high reduction potentials of Pd(NO3)2 and ISO, Reag. Ph Eur), and HNO3 (67%, Normatom VWR). HAuCl4, using alkaline methanol or ethanol as a reducing Syntheses. Au NPs. Unless otherwise specified, the experiments agent leads, at RT, to large and/or agglomerated materials in were performed at RT (∼22 °C), ambient pressure, and ambient light, with stirring, in air, in closed 15 or 50 mL centrifuge tubes (VWR) the micrometer to millimeter range but not to colloidal made of polypropylene (PP) used as a reactor. HAuCl4 was added surfactant-free NPs. Obtaining stable surfactant-free Au NPs from a 20 mM aqueous stock solution. The base (LiOH, NaOH, with size control using mono-alcohols as the reducing agent KOH, or CsOH) was added from a 57 mM aqueous stock solution. therefore remained a daunting challenge. Nevertheless, Freshly prepared base solutions were preferred. Unless otherwise inspired by the possibility of developing surfactant-free yet specified, the base, water, and mono-alcohol (ROH) were mixed in stable colloidal NPs of PMs using mono-alcohols as reducing the PP tube, and then the tube was closed with the dedicated cover. agents, the hypothesis driving the work presented here is that The solution was shaken manually or left to stir for few seconds on a carefully controlling the reaction conditions for Au NP magnetic plate with a magnetic stirrer at ambient light and pressure. The required amount of the HAuCl4 solution was then added last synthesis, e.g., lower concentration of the precursor and fine under stirring, and the container closed. Although the reaction can be balance between the amount of base and the amount of completed in a few hours, the samples were left to react while being reducing agent, could lead to stable surfactant-free colloidal Au stirred for 24 h at ambient light, ambient pressure, and RT before NPs. stirring was stopped. Samples for TEM or high-resolution TEM An ultimate, scalable and green synthesis of Au NPs would (HRTEM) characterization were prepared after 24 h. The colloidal be (i) surfactant-free and carried out (ii) at RT, (iii) in dispersions were further kept in a fridge at ∼4−5 °C for other aqueous media, and (iv) using low-viscosity mono-alcohols as characterization, e.g., by ultraviolet−visible (UV−vis), X-ray dif- reducing agents, and yet lead to stable colloids.6 We here fraction (XRD), X-ray photoelectron spectroscopy (XPS), pair distribution function (PDF), and electrochemistry (see below). The demonstrate that using LiOH and NaOH as a base and ethanol stirrers were cleaned with aqua regia [3:1 (v:v) HCl:HNO3 mixtures, (EtOH) as a reducing agent in aqueous media very simply to be handled with care following the related safety procedures in leads at RT to stable and size-controlled surfactant-free place in the laboratory].4 As much as possible, a control experiment colloidal Au and Au-based NPs that are stable for months. was performed the same day as a set of experiments investigating the We believe that the lack of a detailed study of RT surfactant- influence of a given parameter. With the approach proposed, it is 2174 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article relatively easy to perform up to 30 syntheses a day, so the experiments for the largest NPs (>50 nm) and more typically 100−200 NPs (for are performed under the same RT and ambient light. smaller NPs) up to 500 NPs in the case of the synthesis performed Modifications of the general procedure described above are using mixtures of ROH, for AuxPdy NPs and for the [xAu + yPd] highlighted in the dedicated sections in the Supporting Information. sample where the latter is a mixture of presynthesized Au and Pd NPs. When N2 was used, the gas was humidified first by bubbling N2 in a HRTEM and Scanning TEM-EDS (STEM-EDS). HRTEM and solution of 30 vol % ROH in water. The humidified stream of gas was STEM-EDS analysis was performed by using a JEOL 3000F then bubbled in the synthesis mixture (plastic connections and tubing microscope equipped with a scan unit and an EDS detector from were used so that no metal was in contact with the solution). To Oxford Instruments. For STEM-EDS, a nominal probe size of 1 nm create dark conditions, the samples were covered in aluminum foil. and a camera length of 12 cm were used, and the sample holder was For RT syntheses, the samples were simply left on a stirring plate. For tilted 20° toward the EDS detector. M- and L-α peaks were used for samples subjected to a heat treatment, the same conditions were used both EDS maps and standardless quantification for Au and Pd, but the samples were left to equilibrate at the desired temperature in a respectively. To reduce noise in the HRTEM images of the Pd water bath for 15 min before the addition of HAuCl4. The samples nanoparticle, an ABSF filter was applied by using a D. R. G. Mitchell were left at the desired temperature as indicated for 1 or 0.5 h and plugin for Digital Micrograph. It was found to be unnecessary to filter then left for 23 or 23.5 h at RT and ambient light, respectively, after the HRTEM images of Au NPs. The EDS spectra were acquired on a the addition of HAuCl4. For the scale-up experiments to 1 L of FEI Talos F200X instrument equipped with a ChemiSTEM system solution, a glass container cleaned with ∼10 mL of aqua regia and operated at 200 kV using the Velox Software. HAuCl4 was added from a 200 mM aqueous solution. UV−vis. UV−vis spectra were acquired with a Lambda 1050 UV/ The final concentrations of each chemical and the volume obtained vis/NIR absorption spectrometer (PerkinElmer). A solvent mixture for different syntheses conditions are listed in the tables in the with the same water:ROH ratio as the sample was used as a baseline. Supporting Information. Note that the volume percentage expressed The as-prepared solutions (or diluted to a maximum Au is the volume percentage before volume contraction upon mixing of concentration of 0.5 mM) were placed in dedicated quartz UV−vis the stock solutions, water, and ROH. The concentrations are also cuvettes for absorption measurements with a 1 cm path length, and expressed before taking into account the volume contraction. spectra recorded from 200 to 800 nm at ∼1 nm s−1. Au NMs show a As described below, in situ X-ray total scattering (XRTS) was signal in UV−vis characterization that corresponds to a size-, shape-, performed to follow the synthesis. For such experiments, a relatively and solvent-dependent localized surface plasmon resonance. This high Au concentration, e.g., 50 mM HAuCl4, is required to obtain a spectrum is rich in information but still complex to fully and suitable signal.29,33 Unfortunately, upon addition of HAuCl4 last as it accurately interpret.34 In the case of spherical NPs, the position λspr is preferred at a lower HAuCl4 concentration, the reaction proceeds (wavelength at the surface plasmon resonance) of the maximum too fast for any measurement to be performed. For these XRTS absorption peak intensity (Aspr) provides information about the size of experiments, 150 mM base and 50 mM HAuCl4 were used, and ROH the NPs. As the NMs grow and/or aggregate, larger features appear at was added last, unlike the general synthesis experiments, in which higher wavelengths and the plasmon peak position will shift to higher HAuCl4 was added last. The stock solution of the base was at 428 wavelengths. An absolute comparison of sizes is challenging based on mM, and the stock solution of HAuCl4 at 143 mM, both in water. UV−vis characterization only, and displaying all of the spectra ROH was added to obtain 30 vol % ROH in the final mixture. To reported for this study will be tedious. To be comprehensive, several comply with the requirement of the measurement, the synthesis was metrics used in the literature that capture different features of the performed in 3 mm diameter NMR (Wildmad, 3 mm outer diameter, UV−vis data are reported in the tables below and in the Supporting 0.27 mm wall thickness, Type 1 Class A) tubes at RT.30 The time of Information. synthesis is expressed as the time after mixing all of the chemicals, and λspr gives an indication of the size of spherical NPs. In a first no data could be acquired for a few minutes, which corresponds to the approximation, if 525 nm < λspr < 579 nm, a lower λspr corresponds to time needed to start the measurement. smaller NPs, although very small NPs, ≲3 nm, do not show a AuxPdy and Pd NPs. The synthesis of the AuxPdy NMs with pronounced surface plasmon resonance.35 Δλ/λspr at 90% of Aspr, the different Au and Pd ratios was performed by mixing different amounts relative width at 90% of Aspr, gives an evaluation of the broadness of of a stock solution of HAuCl4 at 20 mM in water and a stock solution the surface plasmon resonance peak, so indirectly it gives an of PdCl2 at 20 mM in EtOH or water. The two precursors were mixed indication on the broadness of the size distribution.36 Aspr/A450 first in the desired ratios and then added to a solution containing the gives an indication of the size of the NMs with a lower value base, water, and ROH so that the final total PM concentration was 0.5 corresponding to smaller NPs, although the related quantitative mM. The rest of the synthesis was performed as detailed for Au NPs. models were developed in 100% water and are not directly applicable Au x Pd y Pt z Os u Ir v Ru w Samples. The synthesis of the here in a mixture of water and ROH.35 The relative intensity AuxPdyPtzOsuIrvRuw NM samples was performed by mixing different measured at 400 nm was suggested to indicate the relative amount of amounts of a 20 mM stock solution of HAuCl4, PdCl2, H2PtCl6, Au0 in the sample and so provides an estimation of the yield of the OsCl3, RuCl3, and H2IrCl6 in MeOH or EtOH. The six precursors synthesis.34 Previous quantitative analyses were performed in 100% were first mixed together and then added to a solution containing the water, and the related absolute conversion coefficients to Au NP yield base, water, and 30 vol % ROH. The rest of the synthesis was retrieved34 are not directly applicable here in a mixture of water and performed as detailed for the Au NPs so that each precursor was at ROH. A650/Aspr is reported to indicate the extent of aggregation of the 0.5 mM with a LiOH:Au molar ratio of 4 or 10. Although no stable NPs.37,38 The higher this ratio, the more aggregated the NPs; it must colloids were obtained, NPs were formed. The materials were washed be noted that this is technically relevant only if the NPs are with water and redispersed in a 30 vol % EtOH aqueous solution so characterized by a well-defined plasmon resonance peak. A380/A800 is that the final PM concentration based on the nominal amount used reported to indicate the stability of the colloid and was here used to from the precursors is 1 mgPM mL−1. compare different samples, where the most stable colloids display a Characterization. Transmission Electron Microscopy (TEM) and higher ratio.39 Energy Dispersive X-ray Spectroscopy (EDS). The as-prepared or X-ray Diffraction (XRD). X-ray powder diffraction was measured on supported Au or Au x Pd y , alternatively the redispersed a Bruker D8 diffractometer with a Cu anode equipped with a Ni filter AuxPdyPtzOsuIrvRuw samples or the latter supported on a support in Bragg−Brentano geometry. The samples were prepared by drop- material, were dropped onto carbon films of nickel or copper TEM casting a dispersion of the NPs onto a microscope slide and washed grids and left to dry at RT before being imaged on a JEOL 2100 with water. Each sample was measured from 5° to 80° 2θ for 4 h, and instrument operated at 200 kV. The samples were characterized by the background measured under the same conditions subtracted from recording micrographs in at least three randomly selected areas of the the data. grid and at least three different magnifications. The size of the NPs ζ Potential Measurements. A Litesizer 500 instrument (Anton was evaluated using the ImageJ software by measuring at least 30 NPs Paar) was employed using the analysis software Kalliope (Anton 2175 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article Paar). Courtesy of Ms. J. Schröder from the Prof. M. Arenz group and ERHE = ESCE + 1.07 in 1 M KOH. The as-prepared dispersions of (University of Bern, Bern, Switzerland). Au or AuxPdy NPs were dropped directly on the glassy carbon X-ray Photoelectron Spectroscopy (XPS). XPS measurements on electrode, polished to a mirror finish using alumina paste as previously the AuxPdy NPs prepared at RT were performed with a Thermo reported,46 and left to dry in 6 × 5 μL for a total volume of 30 μL. For Scientific Theta Probe employing a monochromatic Al X-ray source the AuxPdyPtzOsuIrvRuw NM samples, 10 μL of the solution deposited (EAl Kα = 1486.6 eV) operating at a base pressure of <5 × 10−9 mbar in 2 × 5 μL was used because the solution of NPs was concentrated to using an X-ray spot size of 400 μm and pass energies of 50 eV with ∼1 mgPM mL−1. dwell times of 10 ms and 0.1 eV detection steps. Avantage and The electrochemically active surface area (ECSA) of the Au NPs CasaXPS softwares were used for quantitative analysis (see details in was evaluated by the charge passed under the reduction peak of gold the Supporting Information). in H2SO4 converted to an ECSA using a conversion factor of 386 μC X-ray Total Scattering (XRTS) and Pair Distribution Function cm−2.47 The ECSA of Pd NPs was evaluated using the charge passed (PDF) Analysis. TS data for Au and AuxPdy samples were collected at under the reduction peak of palladium in KOH using a conversion the DanMAX beamline at the MAX IV Laboratory, Sweden. Data for factor of 424 μC cm−2.32,48 In the case of samples comprising both Au the Au NPs were also collected using an Empyrean Series 3 and Pd, the total ECSA was obtained by the sum of the ECSA instrument with a Ag source. In situ measurements of Au NP evaluated for Au and Pd. The mass activity (MA) was evaluated on formation were performed at beamline P21.1 at the Deutsches the basis of the nominal content of Au or PM in the colloidal Electron Synchrotron, DESY. Data for the AuxPdyPtzOsuIrvRuw dispersion. The MA was corrected by a factor of −3% to take into samples were measured at beamline P02.1, DESY. account the fact that for the EtOH:water ratio used [3:7 (v:v)] the Measurements at Beamline DanMAX, MAX IV Laboratory. The volume contraction is ∼3%. The specific activity (SA) was evaluated colloidal dispersions of AuxPdy NPs prepared using a RT synthesis as SA = MA/ECSA. were drop-casted on a Kapton polyimide thin film. XRTS data were The samples were then tested following different protocols. collected in transmission geometry using a DECTRIS PILATIUS3 × Protocol A: (1) three scans at 50 mV s−1 in 0.5 M H2SO4 between 2M CdTe detector with a pixel size of 172 μm × 172 μm. A 0.00 and 1.50 VSCE (0.26−1.76 VRHE), (2) three scans at 50 mV s−1 in wavelength of 0.3542 Å was used, and the sample−detector distance 1 M KOH between −0.80 and 0.50 VSCE (0.27−1.57 VRHE), and (3) was determined to be 91.667 mm from calibration of a NIST LaB6 50 scans at 50 mV s−1 in 1 M KOH and 1 M EtOH between −0.80 standard using Fit2D.40 The two-dimensional (2D) data were and 0.50 VSCE (0.27−1.57 VRHE). This protocol includes steps at high furthermore integrated using Dioptas.41 The exposure time used for potential to best evaluate the ECSA related to Au that is our reference each sample was 15 min. The XRTS data were Fourier transformed material in this study.49 Alternatively, and as indicated, a direct using the PDFgetX342 software, and a Qmax of 19.9 Å−1 was used to characterization using only one step was used. Protocol B: 50 scans at obtain the PDF. TS data for an empty Kapton film were measured for 50 mV s−1 in 1 M KOH and 1 M EtOH between −0.80 and 0.50 VSCE 45 min and used in the background subtraction. Instrumental (0.27−1.57 VRHE). For chromoamperometry experiments, a different resolution parameters (Qdamp and Qbroad) were determined from a protocol was used. Protocol C: (1) 10 scans at 50 mV s−1 in 1 M LaB6 standard. KOH and 1 M EtOH between −0.80 and 0.20 VSCE (0.27−1.27 In-House TS Measurements. The data for the Au NPs were collected at a Panalytical Empyrean Series 3 instrument with a Ag VRHE), (2) chromoamperometry in 1 M KOH and 1 M EtOH at source. The measurements were conducted in the Bragg−Brentano −0.20 VSCE (0.87 VRHE), and (3) 10 scans at 50 mV s−1 in 1 M KOH geometry using a flat plate sample holder with a Kapton polyimide and 1 M EtOH between −0.80 and 0.20 VSCE (0.27−1.27 VRHE). For thin film. Here a wavelength of 0.56 Å and a GaliPIX detector were the comparison of the surfactant-free NPs obtained from different used. The exposure time was 7.5 h in the 2θ range of 3−56° and 22.5 ROH species, the electrochemical protocol was used. Protocol D: (1) h in the 2θ range of 54−110°. The XRD data were furthermore three scans at 50 mV s−1 in 0.5 M H2SO4 between 0.50 and 1.50 VSCE analyzed with the Le Bail method using FULLPROF.43,44 (0.76−1.76 VRHE), (2) 25 scans at 50 mV s−1 in 1 M KOH and 1 M Measurements at Beamline P21.1, DESY. For in situ experiments, EtOH between −0.50 and 0.50 VSCE (0.57−1.57 VRHE), and (3) 10 the reaction mixture was injected into a NMR tube and the tube scans at 20 mV s−1 in 1 M KOH and 1 M ethylene glycol (EG) placed in a customized setup with an aluminum block suitable for between −0.50 and 0.50 VSCE (0.57−1.57 VRHE). For this comparative NMR tubes.30 The samples were measured in the transmission study of the effect of different ROH species on the electrocatalytic geometry using a Perkin-Elmer flat panel detector with a pixel size of performances, the chromoamperometry experiments reported were 200 μm × 200 μm, a wavelength of 0.122 Å, and a sample−detector performed at 0.20 VSCE (1.27 VRHE). distance of 396 mm. The exposure time was 30 s. A Qmax of 18 Å−1 was used to obtain the PDF. Measurements at Beamline P02.1, DESY. AuxPdyPtzOsuIrvRuw samples were loaded on a Kapton polyimide thin film. Data were ■ RESULTS AND DISCUSSION Overlooked Synthesis Parameters. An overview of the measured in the transmission geometry with an exposure time of 300 literature in which polyols and mono-alcohols were used as s. A wavelength of 0.20734 Å, a Varex XRD 4343CT detector with a reducing agents to perform surfactant-free RT synthesis of Au pixel size of 150 μm × 150 μm, and a sample−detector distance of NPs is provided in Table S1. For subsequent discussions, it is 253.7 mm were used. A Qmax of 18 Å−1 was furthermore used to important to stress and keep in mind a few important points. obtain the PDF. (A) The syntheses are preferentially performed using Modeling. All PDF modeling was performed using PDFgui.45 The Au and Pd fcc structures in space group Fm3̅m were used as the glycerol as the reducing agent. The syntheses are also structural starting model. The refinement was conducted over the preferentially performed in the presence of PVP.18,50,51 It is time range of 60−120 min, where the fcc structure was dominating here shown that none are required. and an only minimal contribution from the precursor structure was (B) In one study using glycerol as the reducing agent, the observed. amount of glycerol is found to control the size and to some Electrochemical Measurements (see also sections SP and SV of degree the shape of the NPs, with larger NPs obtained at the Supporting Information). The electrochemical testing was higher glycerol contents.52 In another study, while the glycerol performed in a three-electrode setup using a glassy carbon tip (5 mm diameter) as the working electrode, a saturated calomel electrode content was roughly in the same range, there was little effect (SCE) as the reference electrode, and a carbon rod as the counter on the resulting Au NP size.15 The differences observed might electrode in Teflon cells. The voltage expressed versus the SCE (ESCE) be related to the very different concentrations of chemicals and was converted to the voltage versus the relative hydrogen electrode so different NaOH/HAuCl4 molar ratio used, varying from 1− (ERHE) using the relationships ERHE = ESCE + 0.26 in 0.5 M H2SO4 1052 to 400.15 2176 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article Figure 1. Synthesis and characterization of Au NPs obtained at RT. (a) Schematic representation of the surfactant-free synthesis of Au NPs at RT under ambient light. The bottom image is a collection of pictures of solutions in a UV−vis quartz cuvette with a 1 cm width, for aliquots from a 39 mL solution taken at different times of the synthesis, as indicated, without stirring. (b) Time-resolved UV−vis spectra during Au NP synthesis (min, minutes; h, hours; D, days; M, months). (c) UV−vis spectra of the colloidal Au NPs obtained using NaOH for a total volume of (black) 6.5 mL or (red) 1 L of solution. (d and e) TEM micrographs of Au NPs obtained using ethanol and methanol, respectively. (f) Size retrieved by TEM analysis using different amounts of methanol and ethanol mixed with water. In the experiments depicted in panels a−f, unless specified otherwise, the total volume fraction of alcohol was 30 vol % in water for a total volume of 13 mL, with 2 mM LiOH and HAuCl4 added last for a final concentration of 0.5 mM. The synthesis was performed at RT for at least 24 h with stirring. (g) Time-resolved PDF obtained during the formation of Au NPs using 50 mM HAuCl4 and 150 mM LiOH in 30 vol % ethanol and for the alcohol added last. (h) Time-resolved peak intensity related to the Au−Cl interatomic distance in HAuCl4 and the Au−Au interatomic distance in fcc Au NPs as a function of reaction time retrieved from PDF analysis. (C) High glycerol contents are preferred.15 This has led to here because it omits the value of 30 vol % that is here shown the conclusion that such surfactant-free NPs are mainly to be important for achieving a successful synthesis. stabilized due to the high viscosity of glycerol. This statement (E) NaOH is the preferred base for the reaction, if not the is challenged here. only one investigated so far. (D) The actual need for polyols was challenged in previous (F) The order of addition of the chemicals is not necessarily work in which EtOH was used as the reducing agent,16 but no detailed. Also, it is often mentioned that the reactions proceed satisfying control over the synthesis was achieved. In particular, in few minutes15,53 or “immediately”50 upon mixing the it is not possible to retrieve the concentration of NaOH used reactants, but without more details about how long the in that work. Furthermore, only 10, 50, and 100 vol % EtOH experiments were performed. For instance, it is stated in ref 16 were investigated. This is not the optimal range we identify that NaOH was added last. We show here that the complete 2177 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article control of the NP structures would actually require specifying Size and a Certain Degree of Shape Control. A detailed various experimental parameters and in particular the time of investigation of the parameters allowing size control is given in addition of the base after HAuCl4 and the ROH have been section SN. EtOH leads to smaller NPs that are ∼10 nm in mixed. diameter, as concluded from TEM micrographs in Figure 1d, (G) There is generally less work on Au-based multimetallic as opposed to ∼20 nm NPs with MeOH, see Figure 1e. In each NPs obtained by the surfactant-free approach than on case, the Au NPs exhibit the fcc structure as seen from XRD monometallic Au NPs. diffractograms (Figure S4). The differences in size obtained by In light of the gaps and discrepancies in the literature using EtOH or MeOH are to be related to the different highlighted above, we investigated in detail the use of mono- kinetics of formation observed by following the color change of alcohols in the RT synthesis of surfactant-free colloidal Au the dispersion over time. To further follow the reaction, we use NPs. In particular, we focused on achieving colloidal stability in situ XRTS with PDF analysis. Refinement of the data can be and size control. To comply with the principles of Green found in Figures S10 and S11. Time-resolved PDFs are shown Chemistry,54 we optimized the synthesis to minimize the in Figure 1g. The PDF represents a histogram of interatomic amount of chemicals required to achieve this goal. A specific distances present in the sample and thus allows following focus was placed on the use of different alcohols (EtOH and structural changes occurring during the reaction. Following the MeOH), different volume percents of alcohols (10−70 vol %), peak intensity of the Au−Cl interatomic distance characteristic and different bases (LiOH, NaOH, KOH, and CsOH). of the precursor HAuCl4 and the Au−Au interatomic distance Controlled Surfactant-Free Synthesis of Au NPs. To characteristic of fcc Au NPs in Figure 1h, the PDFs confirm establish if size-controlled Au-based NPs could be obtained by that the formation of crystalline NPs characterized by a a surfactant-free synthesis in low-boiling point solvents, we significant increase in the Au−Au peak intensity is slower in investigated a large experimental parameter space, and the MeOH. In simple terms, and assuming that the classical influence of each parameter is discussed in the dedicated nucleation theory applies here,55 EtOH is a reducing agent that section of the Supporting Information. The study includes leads to fast and multiple nucleation events with subsequently more than 350 experiments to establish the influence of the moderate growth. MeOH is a less favorable reducing agent, nature of the alcohol used as the reducing agent (sections SC− allowing few nucleation events but a slow yet significant SJ), alcohol content (SC and SH), light (SC, SF, and SJ), growth over time. Conveniently, EtOH is a safe and green10 temperature (SD, SG, SI, and SJ), base concentration (SD and reducing agent for further scaling (section SM). SH), HAuCl4 concentration (SD and SJ), gas atmosphere (SI), Having established that the reducing agent controls NP size, the order in which the chemicals are added (SJ and SM), and we also show that the size can be tuned using the the nature of the cation in the base (SC, SD, and SG−SJ). MeOH:EtOH ratio, as illustrated in Figure 1f. Size control is Other variables that are found to have a weaker influence of a general challenge in Au NP synthesis. Typically, concen- the synthesis are the volume of the solution (SD and SM), trations (precursor, surfactants, etc.), time of synthesis, pH, stirring (SD), or the type of containers used (SD). and temperature should all be carefully tuned to control Au NP Optimized Synthesis. Mono-alcohols, e.g., EtOH and/or MeOH in alkaline water, are actually suitable reducing agents size.8 Considering that no surfactants are used here, we achieve for obtaining at RT colloidal Au NPs that are stable for a relatively fine size control with relatively uniform size months. Figure 1a shows the process taking place during NP distributions in the range of 10−20 nm using a single formation. Upon addition of HAuCl4 to a mixture of a mono- parameter, the MeOH:EtOH ratio. Larger NPs are obtained alcohol, water, and a base, the solutions turn from gray to red, with more MeOH. Because the surfactant-free synthesis indicative of the formation of small ∼10 nm Au NPs.8 From proceeds in different alcohols (section SE), this approach the parametric study performed, we found that ∼10 nm Au opens a range of opportunities to develop further new NPs are easily and reproducibly synthesized from 0.5 mM synthetic strategies. HAuCl4 and 2 mM LiOH or NaOH in 30 vol % EtOH in Formation Pathway and Stabilization. A detailed water, under ambient conditions, with or without stirring, in understanding of the mechanism of formation of Au NPs is polypropylene centrifuge tubes or glassware. There is no need ultimately the key to developing complex Au NPs with for any extra chemicals such as surfactants. desirable properties. Despite intense research over the past The particles form in ∼2 h at RT and within a few minutes several decades, the mechanism of formation of Au NPs is still at 50 °C using EtOH. Figure 1b shows the progressive subject to debate, and it is expected that new model systems appearance of a localized surface plasmon resonance (LSPR) with unique combinations of features will contribute to solving during the synthesis, marked by a maximum absorbance the remaining challenges.4,8,56 While a complete understanding intensity around 520 nm in UV−vis measurements, character- of the mechanism at play in NP formation can be the subject of istic of small Au NPs. Only minor changes are observed in the several years of research,57 e.g., the mechanism of the UV−vis spectra after ∼2 h at RT, indicative of a relatively fast Turkevich−Frens method reported in 1951 is still debated synthesis. The LSPR and the related peak intensity remain over and investigated8,58,59 and the synthesis continues to be refined time for UV−vis measurements conducted after 1, 2, and 3 over the years,60−63 we rationalize the observations reported months, indicative of colloids stable for months in a here by assuming, for now, that the classical nucleation theory refrigerator, but also when left on a bench at RT under applies.55 To obtain small NPs, it is therefore expected that ambient light. The synthesis is reproducible as discussed in nucleation events and/or seed formation of Au clusters must section SL. Figure 1c shows that no large differences are be fast relative to the growth of the NPs from these seeds and/ observed between the UV−vis spectra of Au NP dispersions or clusters. Accordingly, many seeds and/or nuclei will obtained using 6.5 mL or 1 L of solvent. The synthesis thus ultimately grow at the same rate to lead to relatively conveniently scales to 1 L of solution where ∼100 mg of Au monodisperse NMs. In contrast, if nucleation events occur NPs can be produced as documented in section SM. over a long period of time, NPs with polydisperse size 2178 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article Table 2. Influence of the EtOH Content and Nature of the Basea ROH H2O (vol %) ROH (vol %) base λspr (Δλ/λspr) nm Aspr/A450 relative yieldb A650/Aspr A380/A800 diameter (nm) PdI EtOH 90 10 LiOH 523 (6.9%) 1.61 1.00 0.09 32.0 7.5 ± 3.3 0.19 NaOH 530 (7.0%) 1.75 0.98 0.20 8.20 9.1 ± 6.4 0.49 KOH 536 (6.9%) 1.82 0.99 0.18 18.4 13.8 ± 5.7 0.17 CsOH 523 (7.5%) 1.54 1.00 0.16 21.5 10.2 ± 3.8 + network 0.14 70 30 LiOH 515 (6.6%) 1.53 0.97 0.10 26.3 8.6 ± 2.0 0.05 NaOH 518 (6.9%) 1.52 0.83 0.10 34.0 8.9 ± 2.2 0.06 KOH 526 (6.5%) 1.68 0.97 0.16 19.3 13.1 ± 2.9 0.05 CsOH 534 (12.0%) 1.46 1.00 0.60 2.5 7.6 ± 1.5 0.04 50 50 LiOH 592 (18.9%) 1.62 1.00 0.92 1.2 >50 X NaOH X X X X X X X KOH 554 (15.0%) 1.58 0.3 0.77 1.2 >50 X CsOH X X X X X X X 30 70 LiOH X X X X X X X NaOH X X X X X X X 0 100 LiOH X X X X X X X a Physical characteristics of the Au NPs obtained using different ROH contents and cations when EtOH is used as the reducing agent. The final concentration of HAuCl4 is 0.5 mM, and the base concentration is 2 mM (base:Au molar ratio of 4) for a volume of 13 mL; the experiments were performed at RT and left to react for 24 h. An X indicates that the measurements could not be performed typically due to the formation of nonstable colloids. See Figure S1 for the definition of a network in this work. bEvaluated as the ratio of A400 for the sample and the maximum values of A400 for the data set at a given volume percent of ROH. distributions will be obtained. The influence of the most no stable colloids, characterized by larger A650/Aspr values, are important parameters identified is discussed below. obtained. This corresponds to a case in which there is too Influence of the ROH. Certainly, the redox properties of the much reducing agent. This value hints toward the need for ROH are an important factor,11 because the ROH or derived reaching a balance between having enough reducing agent to alkoxides play the role of reducing agents.18 As detailed above, allow for the reaction to proceed and not having too much the nature of the ROH used strongly influences the kinetics of reducing agent to avoid overgrowth of the NPs. the reaction. Larger NPs are obtained using MeOH compared Following the Green Chemistry principles for the develop- to using EtOH (see Figure 1). However, MeOH and EtOH are ment of NPs,54 we observed that 30 vol % is also a suitable not expected to show strong differences in redox potentials.11 amount of polyols for the development of surfactant-free Because the maximum absorbance of a solution of HAuCl4 is syntheses. Furthermore, 35 vol % EtOH was found to be the shifted in different solvents (see Figure S27), one can expect optimal content in previous studies to favor fast kinetics of the HAuCl4 complex to undergo some interaction with the reduction, e.g., using sonication.65 In another study using solvent and possibly ligand exchange with methoxy and ethoxy plasma synthesis,66 faster kinetics of reduction were observed groups.64 This could explain the Au−O distances probed by for a mole fraction of ROH around ∼0.09. Note that 30 vol % PDF (Figure S28). This ultimately accounts for the different ROH corresponds to a mole fraction of ∼0.11. At these values, resulting materials. In addition, mono-alcohols with higher the interaction of water with ROH changes at the microscopic molecular weights and larger alkyl chains, such as isopropanol level. In light of the discussion in Influence of ROH, this and tert-butanol, do not lead to small NPs (Figure S6). physical phenomenon may also influence the reduction Another piece of evidence pointing in the direction of the mechanism. interaction of Au complexes with ROH/RO− is that using Influence of Light. We also observed that light had an MeOH and adding it last can lead to a large network of Au influence on the properties of the resulting NPs (see sections NMs whereas EtOH leads to NPs (see Figure S21). The SC, SF, and SJ). When the synthesis is performed in the dark, properties of the ROH/RO− species, rationalized as the ability the NPs tend to form slower, and this leads to larger NPs. The to be oxidized and to interact differently with the Au complex, influence of light and the need for ROH for the synthesis to therefore matter. proceed suggest that the formation of radicals plays a role in Furthermore, when the reaction was performed with the reduction process.67 glycerol, we observed that a number of rods were obtained Influence of the Temperature. To partially alleviate the (Figure S6), which suggests that despite the challenges in influence of light, we investigated the effect of temperature on developing surfactant-free syntheses, a certain degree of shape the resulting Au NPs (see sections SD, SG, SI, and SJ). As the control is possible. temperature increases, the time needed to observe the Influence of the ROH Content. We here identified 30 vol % formation of red colloidal Au NPs decreases. However, we as an optimal value for obtaining small Au NPs. This key value did not observe a significant improvement over the resulting is missing in a previous screening and attempt to use EtOH as size of the NPs. Above 50 °C, no stable colloids were obtained the reducing agent.16 Table 2 lists the properties Au NPs using EtOH. This suggests that the stabilization of the Au NPs obtained with different volume percents of EtOH (see also becomes too weak and/or the reducing power of the ROH section SC for complementary data and results with MeOH). becomes too strong as the temperature increases. At <30 vol %, no Au NPs form. This corresponds to a case in Influence of the Base Concentration. We now turn to the which there is not enough reducing agent. At higher values, effect of pH, a parameter typically tuned in the synthesis of Au larger NPs, characterized by larger λspr values, are obtained or NPs8,16 and other PMs,68 although it was shown to have little 2179 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article Table 3. Influence of the Base:Au Molar Ratioa H2O ROH LiOH:Au molar λspr (Δλ/λspr) relative ROHb (vol %)b (vol %)b ratio nm Aspr/A450 yieldb A650/Aspr A380/A800 diameter (nm) PdI EtOH 70 30 2 542 (6.6%) 2.02 0.52 0.19 20.9 >100 X 3 539 (8.0%) 1.84 1.00 0.17 18.6 >40 X 3.5 523 (6.4%) 1.69 0.90 0.09 35.4 11.7 ± 3.5 0.09 4 523 (6.5%) 1.63 0.85 0.17 29.4 9.3 ± 2.8 0.09 517 (6.6%) 1.55 0.88 0.17 12.0 9.2 ± 2.6 0.08 522 (6.7%) 1.65 0.76 0.17 16.6 8.5 ± 2.1 0.06 4.5 539 (12.2%) 1.48 0.50 0.52 4.6 7.6 ± 2.6 (not spherical) 0.12 5 543 (12.2%) 1.48 0.44 0.48 4.9 7.6 ± 3.9 (not spherical) 0.26 6 549 (15.6%) 1.39 0.49 0.68 1.9 8.5 ± 2.3 + >100 0.07 (not all spherical) 8 537 (11.4%) 1.41 0.60 0.50 2.5 7.9 ± 2.7 + >100 0.12 (not spherical) 10 557 (17.2%) 1.44 0.69 0.77 1.8 7.0 ± 2.6 + >100 0.14 a Physical characteristics of the Au NPs obtained using different water contents and cations when EtOH is used as the reducing agent. The final concentration of HAuCl4 is 0.5 mM for a volume of 13 mL, and the experiments were performed for 1 h at 50 °C and left to react further for 23 h. An X indicates that the measurements could not be performed typically due to the formation of nonstable colloids. bEvaluated as the ratio of A400 for the sample and the maximum values of A400 for the data set. influence in the surfactant-free synthesis of Pt NPs in alkaline importance of this ratio of 4 between the molar quantities of mono-alcohols.28,69 Because we here have alcohol/water the base and Au is best understood in light of the equation for mixtures in which the pH is not a well-defined variable, we the formation of Au NPs (Au0), further detailed in section SK: prefer to quote the expected experimental concentration of the base and therefore the base:Au molar ratio (see Figure S14 and 2HAuCl4 + 3H3CCH 2OH + 8OH 3H3CCOH Table S8). The detailed results of the parametric study, including different volume percents, different cations, and + 8H 2O + 2Au 0 + 8Cl different ROHs, are given in section SH. As an illustrative Interestingly, this ratio of 4 is also suitable for obtaining Au example, we focus here on NPs obtained using EtOH and NPs using EG or glycerol (see section SE), which shows that LiOH, which leads to small ∼10 nm NPs. The influence of the lowe amounts of chemicals (here the base can be used base concentration was evaluated using a 1 h step at 50 °C, to compared to what was reported previously (see Table S1). limit the possible effects of light that may arise from different This result paves the way for further optimization of the kinetics of reaction observed at RT when using different base polyol-based syntheses. Tuning the base:Au molar ratio concentrations. Table 3 lists the results obtained with 30 vol % therefore gives a relative yet promising degree of size and to EtOH. The trends observed are consistent with observations some extent shape control. for different volume percents of ROH and different bases. Influence of the Gas Atmosphere. Although we here aim to The optimal dispersion characterized by low λspr, low Δλ/λ, develop a simple synthetic method, we investigated the effect and low PdI values, indicative of small NPs in the absence of of the gas atmosphere using a N2 purge (see the Experimental different NP structures (e.g., absence of chunky materials), was Section and the Supporting Information). Smaller NPs (∼7 obtained around a LiOH:Au molar ratio of 4 leading to NPs nm) can be obtained using N2. Although we do not have a that were ∼10 nm in diameter. For lower LiOH:Au molar clear understanding of the effect of N2 at this stage, this could ratios, larger NPs were obtained. A LiOH:Au molar ratio of 3.5 be because acetaldehyde (Tbp = 20 °C), a possible oxidation led to ∼11 nm NPs. At overly low LiOH:Au molar ratios, the product of EtOH, is removed from the solution by the gas structures were large and not well defined (LiOH:Au molar stream as it forms. It could also be that O2 plays the role of an ratio of 2−3) and showed poor colloidal stability. At higher oxidizing agent and/or favors the formation of radicals from LiOH:Au molar ratios, i.e., 4.5−5/6, while some NPs were still the ROH. Therefore, the N2 purge will ultimately lead to a small, they did not show a spherical shape, which can explain different set of reducing agents in solution. While using N2 the larger λspr recorded, and as the ratio increased, larger leads to a relative size control toward smaller NP sizes, using structures could be observed on the TEM grids (see Figure gases unnecessarily complicates the syntheses at the practical S15), in agreement with the shift of the λspr to a higher level and increases the cost of the synthesis. Therefore, using wavelength. N2 was not considered further. An optimal value of ∼4 for the base:HAuCl4 molar ratio was Influence of the Order of Addition of the Chemicals. As probably overlooked in previous studies (see Table S1 and stressed in Overlooked Synthesis Parameters, a crucially Overlooked Synthesis Paramaters). This partially accounts for missing or not detailed parameter in the synthesis of Au NPs the fact that only relatively large NPs were obtained,16 and the using ROH as reducing agents is the effect of the order of approach was disregarded for the development of a very simple addition of the chemicals. We observed across various synthesis of surfactant-free colloidal NPs, as achieved herein. A experiments from RT to 50 °C that the speed at which the base ratio of 4−5 was shown to be a key threshold in the reaction proceeds (marked by the appearance of a gray color) formation of Au NPs in previous work using MeOH as the is faster, and the resulting NPs are smaller, as the last added solvent but in a synthesis using PVP.70 UV−vis data in that chemical is HAuCl4 < base ≪ ROH (see section SJ for detailed study showed an increase in the plasmon resonance at 525 nm examples, including the use of LiOH, NaOH, EtOH, or for this ratio, related to an increased number of Au NPs. The MeOH). Table 4 illustrates further these results where a UV− 2180 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article Table 4. Influence of the Order of Addition of the Chemicalsa ROH H2O (vol %) ROH (vol %) added last λspr (Δλ/λspr) nm Aspr/A450 relative yieldb A650/Aspr A380/A800 diameter (nm) PdI EtOH 70 30 HAuCl4 523 (6.4%) 1.69 1.00 0.10 32.3 10.0 ± 3.9 0.16 LiOH 525 (6.7%) 1.72 0.91 0.11 29.5 15.4 ± 4.3 0.08 ROH 547 (10.7%) 1.62 0.91 0.61 1.6 >50 X a Physical characteristics of the Au NPs obtained using EtOH as the reducing agent for different chemicals added last. The final concentration of HAuCl4 is 0.5 mM, and the LiOH concentration is 2 mM (LiOH:Au molar ratio of 4) for a volume of 6.5 mL. The experiments were performed 1 h at 50 °C and left to react further for 23 h. An X indicates that the measurements could not be performed. bEvaluated as the ratio of A400 for the sample and the maximum values of A400 for the data set. vis spectrum with a well-defined plasmon resonance and a tail to rather large NPs. The approach of adding the base last as with a lower absorbance at higher wavelengths is obtained preferred in previous works16 would then require controlling when HAuCl4 is added last. A slight shift toward higher λspr the time between mixing ROH and HAuCl4 before adding the values indicative of larger NPs is obtained when the base is base. added last, and a spectrum with larger features at higher Nanoparticle Growth. Once the NPs form, their growth is wavelengths is obtained when ROH is added last. Because we key to control. TEM characterization of the colloidal could not find an investigation of this phenomenon using dispersion over time confirms that the time needed to obtain polyols, we made sure this phenomenon is also observed at RT NPs increases when MeOH is used compared to EtOH (see using EtOH and glycerol (see details in section SJ). section SF). We observed, in agreement with previous reports, The identity of the intermediate chemical species formed the formation of networks.74 However, these networks are during Au complex reduction is not yet definitively known.71,72 probably artifacts from the TEM sample preparation, the It is expected that the initial HAuIIICl4 complex, [AuIIICl4]− in possible residual amount of unreacted HAuCl4, the vacuum, solution, is in equilibrium with various hydroxyl-substituted and the high-energy beam. Our UV−vis data, however, point species with a general formula of [AuClx(OH)y]n− (x + y = 4). toward a growth of the NPs from smaller seeds, in agreement For alkaline conditions, the equilibria are shifted toward the with recent work.56 In addition here, in situ PDF data suggest a formation of more hydroxyl-substituted complexes (y > 1). growth of the crystalline domains over time (Figures S9−S11), However, the higher the value of y, the less likely the which also rules out the formation of large crystals that would complexes are to be reduced in solution.73 Nevertheless, it is subsequently shrink in size. expected that these hydroxy-substituted complexes can be The Turkevich−Frens synthesis has been a preferred model reduced on preexisting NPs, thus initiating a growth phase.8 system to date because it “includes only three starting Under the slightly alkaline conditions used here, it is materials”:75 water, Na3Ct, and HAuCl4. Nevertheless, the expected that the gold complex will therefore either not rich chemistry of Au and Na3Ct leaves many questions undergo reduction in solution or do so only poorly. Such is the unanswered.8 If the Turkevich−Frens method leads to small case if the reducing agent is added last; i.e., ROH is added to a NPs [typically ∼10 nm (see section SP)] and relatively solution with an expected large amount of hydroxyl-substituted monodisperse NPs due to the use of surfactants, it is also species and therefore with no or few features related to commonly reported that the method or the use of surfactants HAuCl4 at low wavelengths in UV−vis characterization (see leads to other NP shapes such as triangles and rods in small Figure S27). A direct consequence is that if more base is used, percentages.62,67 The surfactant-free synthesis presented here, the equilibria are also more strongly shifted toward the while leading to slightly more polydisperse NPs, did not lead to formation of hydroxyl-substituted species, which are expected rods or triangles, as assessed by TEM characterization. to form more quickly upon addition of HAuCl4, therefore Therefore, despite being surfactant-free, the approach leading to the formation of fewer seeds that slowly (over)grow, presented here leads to a relatively homogeneous product. as it is observed at relatively high base:Au molar ratios as Influence of Cations. A last aspect of the formation of the discussed in Influence of the Base Concentration. NPs is their stabilization. Previous results obtained with The key to a successful synthesis is then to allow enough glycerol as the reducing agent suggest that viscosity might be a AuIIICl4− to reduce upon mixing of the different species. This is reason for the stabilization of surfactant-free Au NPs.15 The here achieved by adding HAuCl4 last8 to a moderately basic achievement here of stable colloidal dispersions without a solution. The fast reaction that occurs when HAuCl4 is added surfactant in low-viscosity solvents shows that viscosity is not last can be interpreted as arising from a favorable situation in the only and not necessarily the main stabilization mechanism, which the local pH around the HAuCl4 that has just been in agreement with recent results for Pt and Ir NPs obtained in added does not become too high during the time scale in mono-alcohols.29,76 which the complex starts to be reduced while the ROH is Identifying in detail the stabilization mechanism(s) of oxidized, so that no or few [AuClx(OH)y]n− species with y > 1 surfactant-free NPs certainly remains a challenge.23,77 In ζ are formed, but rather many seeds and/or nuclei. As a further potential measurements, the NPs prepared using LiOH show a support of this interpretation, the concentration of the stock value of −14.5 or −78 mV when the analysis is performed by solution of HAuCl4 also plays a role and smaller NPs are best inputting water or EtOH, respectively, as the solvent in the obtained using concentrated solutions of HAuCl4 added last data treatment (analysis software, Kalliope, Anton Paar). (see Figure S23). These results suggest a strong stability of the colloids, in The case in which the base is added last is more complex agreement with the long-term stability observed over months because the ROH alone (without base) can start reducing the and/or after centrifugation. Certainly, electrostatic interactions HAuCl4 (see Figure S27). Upon addition of a base, the will play a role in the stabilization of surfactant-free NPs,78 and reaction proceeds faster probably on preexisting seeds, leading this points toward the role of the cations. 2181 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article Figure 2. Synthesis and characterization of AuxPdy NPs. (a) Schematic representation of the surfactant-free synthesis of AuxPdy NPs using alkaline mixtures of ethanol and HAuCl4 and PdCl2 as precursors at RT or 50 °C. (b) Illustrative HRTEM micrographs of Au and Pd NPs obtained at RT. (c) Diameter of the AuxPdy NPs evaluated by TEM as a function of their composition evaluated by EDS. (d) XRTS data and PDF refinements using the fcc structural model of AuxPdy NPs obtained at RT. The NPs were obtained in 30 vol % ethanol in water using 2 mM LiOH and 0.5 mM {HAuCl4 + PdCl2} for a total volume of 13 mL and RT (blue) for a synthesis time of at least 24 h or after low-temperature synthesis (50 °C, red) for 1 h, with stirring. A complex interplay is likely at play among ROH type, of the base will change, as suggested by the equation presented volume percent of ROH, and cations, not to mention the used above, further shifting the many chemical equilibria consid- of additives illustrated in section SO. Overall, LiOH seems to ered. stabilize the NPs better than NaOH or KOH (see Table 2 and Interestingly, a certain degree of shape control is achieved section SC), as we also observed for Pt NPs.76 For a large scale from NPs to networks by changing the order of the addition of (section SM), NaOH is a cheaper and more promising option the chemicals (section SJ) or the base concentration (section than LiOH. The stabilization can be explained by stronger SH). The synthesis detailed here therefore offers a range of interaction of the smallest cation Li+ with metal surfaces. The experimental knobs anticipated to provide new insights into role of Cl− also cannot be ruled out, nor that of small the mechanism(s) of formation of NMs, e.g. regarding the molecules and solvent interactions.23,77 The interlinked roles of effect of surfactants and/or cations as pointed out in section the ROH, ROH amount, cation, and cation/OH− concen- SO. tration are difficult to disentangle at this stage. We here introduce a new model system with “only” four Conclusions. Not only the nature of the ROH but also light chemicals: a base, water, HAuCl4, and ROH. However, the rich and the order of addition of the chemicals strongly influence chemistry already demonstrated suggests that this synthesis can the outcome of the Au NP synthesis. This can be understood be a suitable model system for further studying Au NP considering multiple equilibria between different gold com- synthesis. Arguably, ROH/RO− species are ultimately simpler plexes formed, with different redox potentials, and an interplay species than Na3Ct,58,79 and the fact that the synthesis with the type of ROH used as the reducing agent. It must be proceeds here at RT in only a couple of hours is certainly a kept in mind that as the reaction proceeds, the concentrations positive feature for future mechanistic studies. 2182 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article Figure 3. Toward multimetallic NM samples. (a) Schematic representation of the surfactant-free synthesis of AuxPdyPtzOsuIrvRuw NP samples using alkaline mixtures of 30 vol % ethanol and 0.5 mM HAuCl4, 0.5 mM H2PtCl6, 0.5 mM PdCl2, 0.5 mM OsCl3, 0.5 mM H2IrCl6, and 0.5 mM RuCl3 at RT or 50 °C. (b) TEM micrograph and (c) PDF refinement using the fcc structural model for multimetallic samples obtained at RT using LiOH and 30 vol % ethanol where ethanol was added last. The data are colored blue. The fit is colored red. The difference between the data and fit is colored green. Composition Control. Au NPs are important materials activity (SA, activity per electrochemically active surface area), due to their unique properties. Alloying Au with other and the stability of the catalyst must be improved. elements opens an even wider range of opportunities.80,81 Representative cyclic voltammograms of the EOR catalyzed The presented synthesis strategy is suitable for obtaining by Au NPs are reported in Figure 4a (see also Figure 5). A AuxPdy NPs, which have a wide range of applications, as high current intensity related to alcohol oxidation in the illustrated below for (electro)catalysis. Here, ∼3−4 nm Pd forward (F) scan around 1.1−1.2 VRHE is desirable, and a high NPs are easily obtained (see Figure 2a,b). The Au:Pd ratio ratio of this current to the current in the backward (B) scan between 0 and 1 is controlled by the Au:Pd precursor ratio. around 1.0−1.1 VRHE, related to the oxidation of poisoning The composition was assessed by TEM-EDS (Figure 2c) and species, is indicative of the desirable poisoning resistance to XPS (section SR), and the compositions retrieved from the detrimental intermediates formed during alcohol oxida- two techniques agree well. The NP size decreases as the tion.49,85 amount of Pd increases (Figure 2c), and PDF characterization Benefits of Surfactant-Free NPs. First, we investigate the confirms that fcc structured AuxPdy NPs are obtained (Figure electrochemical properties of the Au NPs and the benefits of 2d). surfactant-free NPs (see details in section SP). For catalysis, The synthesis presented is also suitable for RT synthesis of the NPs are typically supported on a material. This supporting other NPs. Recently, high-entropy alloys (HEAs) made of at step can be challenging with NPs protected by PVP or Na3Ct, least five different elements have received more attention due due to unfavorable interactions between protective agents and to their promising catalytic properties.82,83 The RT synthesis the support surface. In contrast, the surfactant-free Au NPs are carried out with six different metal precursors (illustrated in easily supported on various materials by direct adsorption (e.g., Figure 3a) leads to NMs in the size range of 1.5−2.5 nm (see on Al2O3) or solvent removal (e.g., on carbon) (see details in the TEM image in Figure 3b) with the bulk composition section SQ). To allow a fair comparison and avoid any effects Au33Pd19Pt18Os20Ir1Ru9 estimated by EDS. The presence of of the supporting steps, Au NPs prepared with or without HAuCl4 seems to be key for the reaction to proceed at RT. surfactants are here readily deposited on conductive electrodes Structural and compositional characterization of individual (section SP). HEA NPs is a general challenge,84 and the particles synthesized The use of PVP leads to NPs (7.1 ± 1.5 nm in diameter) here are too small for most STEM-EDS equipment for smaller than the surfactant-free Au NPs characterized by an elemental mapping of individual NPs (see the discussion in average diameter of 8.6 ± 2.0 nm, whereas the NPs obtained section SU). PDF analysis confirms small crystalline NPs with using the adapted Turkevich−Frens synthesis are 12.3 ± 1.2 a fcc structure (Figure 3c). These results show the suitability of nm in diameter (see Table S19). However, using PVP the synthesis method proposed to develop Au, Pd, and a range completely passivates the surface of the NPs and no of Au-based NMs. electrochemical measurements can be performed. This Electrocatalytic Oxidation of Alcohols. Surfactant-free illustrates the benefits of developing surfactant-free approaches. NPs are relevant for multiple applications and in particular for The surfactant-free NPs present an ECSA of 8.2 ± 1.1 m2 g−1, catalysis in which clean NP surfaces are required.23,28 As an higher than that for NPs prepared by the Turkevich−Frens illustrative example, we use alcohol oxidations, e.g., ethanol method characterized by an ECSA of 7.1 ± 0.4 m2 g−1. This (EOR) and glycerol (EGOR) electro-oxidation reactions, that observation agrees with the smaller size of the surfactant-free are key reactions at the anode of direct alcohol fuel cells NPs and the expectation that ECSA increases when the NP envisioned to enable more sustainable energy conversion.83,85 size decreases. The SA for the EOR in alkaline media is also To develop optimal electrodes, the mass activity of the catalyst higher for surfactant-free NPs than for NPs prepared using (MA, current retrieved from the oxidation process normalized water and Na3Ct. This is explained by the surfactant-free by the mass of PMs), the electrochemically active surface area synthesis allowing a better access to the NP surface. The MA (ECSA, measure of the catalytically active surface), the specific of the NPs, e.g., EGOR in alkaline media, is also higher, and/or 2183 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article is a rewarding strategy for balancing activity, stability, and tolerance to poisoning.85 We then characterize the AuxPdy NPs (section SR), as illustrated in Figure 5, following two different protocols detailed in the experimental section and summarized in Figure 5b. The ECSA of the NPs increases as the size decreases or as the Pd content increases (Figure 5c). This increase in ECSA correlates with an increase in MA (see, for instance, MAs in Figure 5d, where the NPs were subjected to a relatively harsh protocol A that includes scanning at relatively high potentials). The AuxPdy nanocatalysts are more stable (higher MA after harsh treatments) than Pd NPs. Attention is paid to the NMs obtained by the simpler RT synthesis. An optimal NP composition for achieving the highest MA was obtained for ∼3.5 nm Au65Pd35 NPs (Figure 5e and section ST), in agreement with previous work.85 These results illustrate the opportunity to tune the properties of surfactant-free Au-based NPs by alloying to easily screen for optimal compositions. Beyond alloying, and as a further example of the benefits of the simple syntheses reported here, the surfactant-free NMs allow the exploration of two emerging concepts in nanocatalyst design. First, mixtures of different NPs were suggested to enhance catalytic performances compared to those of alloys.86 These [xAu + yPd] nanocomposites lead to MAs comparable to those obtained with AuxPdy NPs when the harsher protocol A is used, as summarized in section ST and illustrated in Figure 6a for the most active [40Au + 60Pd]. For protocol B (see Figure 6b), no significant improvement is observed, e.g., compared to Pd NPs. The reason for this enhanced activity in mixtures of NPs is not yet clear; it could originate from the alloying of the two metals during electrochemical testing.86 Nevertheless, starting with well-defined (surfactant-free) Au and Pd NP batches that are simpler to characterize and mix in the desired ratios is arguably easier, especially at a large scale, than preparing and characterizing the corresponding bimetallic NPs. Figure 4. (a) Cyclic voltammograms of Au NPs prepared by the Another approach to tuning the NP properties comes with Turkevich−Frens method (Na 3 Ct) or without a surfactant the sample Au33Pd19Pt18Os20Ir1Ru9 exhibiting different electro- (Surfactant-free) for the EOR in alkaline media in 1 M EtOH and chemical properties compared to those of individual AuxPdy or 1 M KOH recorded at RT at a scan rate of 50 mV s−1. The 10th scan is displayed. (b) Chronoamperometry of the EGOR performed at [xAu + yPd] samples (see further details in section SU). The 1.27 VRHE for 1 h in 1 M EG and 1 M KOH. NPs prepared with PVP MA for the EOR remains relatively low due to the large did not show any measurable electrocatalytic activity. amount of PMs (Figure 6a,b), but an interesting feature of this sample is its resistance to poisoning. Of all of the surfactant- free Pd, AuxPdy, [xAu + yPd], and Au33Pd19Pt18Os20Ir1Ru9 NPs characterized here, only the latter showed an improved more stable NPs are prepared by a surfactant-free approach. tolerance to poisoning (see the intensity ratio of forward-to- The surfactant-free NPs obtained here also lead to activities backward peak intensity of >1.5 in Figure 6d). Although a full that are higher than those of Au NPs obtained using EG or understanding of the reasons for this observation is beyond the glycerol as the reducing agent (section SP). scope of this first report, the results overall illustrate the The surfactant-free synthesis allows a better access to the NP promising features of Au-based NMs obtained by the RT surface for the reactants, and these results emphasize the synthesis introduced for catalytic studies toward improved benefits of surfactant-free NPs as readily active catalysts NMs. without the need for (pre)treatment or washing, avoiding the Comparing NPs obtained by different methods remains a complications related to extra NP treatments when surfactants general challenge because the NPs are often characterized by or viscous solvents are used.17 different protocols using, for instance, different electrolyte Influence of Different Composition Control and Different concentrations, different EtOH concentrations for the EOR, Testing Protocols. The results obtained with AuxPdy NPs are different upper and lower limit potentials, different scan rates, summarized in Figure 5 with the illustrative examples of Au, different numbers of scans, different pretreatments in general, Au83Pd17, and Pd NPs for the EOR. Pd NPs show higher and different ink formulations in the case of supported catalytic activity at lower overpotentials compared to that of electrocatalysts, not to mention that different normalizations Au NPs for the EOR and EGOR. However, Au NPs show are used to report the results, etc.27,49,87 Nevertheless, the improved stability and better tolerance to poisoning. activity achieved here is relatively high for AuxPdy catalysts88,89 Developing Au- and Pd-based materials, e.g., AuxPdy alloys, or comparable to that reported elsewhere but using more 2184 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article Figure 5. Electrochemical properties of AuxPdy NPs as a function of NP composition. Case study of the EOR. (a) Cyclic voltammogram (50th scan) of Au, Au83Pd17, and Pd NPs in 1 M KOH and 1 M ethanol recorded at 50 mV s−1, as indicated. The scales used are different to best identify important features. The arrows indicate the forward (F) and backward (B) scans, and the peaks of maximum intensity corresponding to Au (subscript A) and Pd (subscript B) sites are marked by vertical dashed lines. The NPs were characterized following protocol A detailed in panel b. (b) Different characterization protocols used. (c) ECSA estimated following protocol A. (d and e) Mass activity for the EOR estimated following protocols A and B, respectively, detailed in panel b. The NPs were obtained using 2 mM LiOH, 30 vol % ethanol, and different amount of HAuCl4 and PdCl2 for a total concentration of 0.5 mM. complex synthetic approaches.85 In this respect, the readily and their performances are better than those of NPs prepared active NPs obtained here by a simple method can be using surfactants or polyols or obtained by the Turkevich− envisioned as a promising reference material for future studies Frens synthesis. across different laboratories for improved comparative studies. We limit our discussion to a putative formation pathway that we hope will be investigated further and (dis)proved by the ■ CONCLUSION Stable surfactant-free Au NPs are readily obtained by the community. We document and highlight parameters that we anticipate will be the key to further understanding. (1) One reduction of HAuCl4 in moderately alkaline mixtures of water main finding is that the ROH used influences the kinetics of and EtOH or MeOH at room or relatively low temperatures the reduction. (2) A second finding is the critical importance (≤50 °C). This approach is a surprisingly simple, straightfor- of the order of additions of the chemicals. (3) A third is the ward, ultimately cost-effective, and leads to ideal new model influence of light. The NP size distribution and shape remain systems for the development of surfactant-free yet stable relatively non-uniform compared to those of most syntheses colloidal Au NPs and Au-based multimetallic NMs. The using surfactants. It is expected that a deeper understanding of multiple benefits of this simple approach have probably not the mechanism of formation will help to control even more been exploited to date due to the overlooked influence of finely the surfactant-free Au NP properties. several parameters on the synthesis. A reproducible synthetic The use of low-boiling point solvents makes the NPs simple protocol is here reported after screening of a large experimental to further process, e.g., for supporting steps for heterogeneous parameter space across more than 350 experiments. The strong catalyst design. The RT synthesis offers opportunities for high influence on Au NP synthesis of the mono-alcohol nature and throughout, automation, and safe scale-up. The surfactant-free content, base nature, and content as well as the order of NPs are anticipated to be ideal building blocks for other addition of the reactants is established. Surfactant-free Au NPs materials. After 70 years of study and primary focus on the around 10 nm in diameter are easily obtained within 2 h at Turkevich−Frens synthesis as the model system for Au NP room temperature by adding HAuCl4 from a concentrated synthesis, the surfactant-free approach reported here offers new aqueous stock solution to alkaline (NaOH or LiOH) mixtures opportunities for education purposes and for fundamental and containing ∼30 vol % EtOH for a final base:HAuCl4 molar applied studies of Au-based NMs. It is anticipated that this ratio of preferably ∼4. The surfactant-free NMs are readily surfactant-free colloidal synthesis will provide valuable insights relevant for catalytic applications and studies, e.g., for the EOR, into more rational nanomaterial design and syntheses. 2185 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article Figure 6. Comparison of different surfactant-free NPs and strategies for electrocatalyst design for the EOR. Au NPs, Pd NPs, the most active AuxPdy NPs, i.e., Au65Pd35, the most active nanocomposite, i.e., [40Au + 60Pd], and Au33Pd19Pt18Os20Ir1Ru9 are compared, as indicated. All materials were obtained at RT using LiOH as the base and 30 vol % ethanol. (a and b) Mass activity for the EOR evaluated following protocols A and B, respectively, detailed in Figure 5b. The related materials are schematized in panel c with (i) Au NPs, (ii) Pd NPs, (iii) AuxPdy NPs, (iv) nanocomposite [xAu + yPd], and (v) multimetallic samples. (d) Ratio of the forward and backward scan intensity for the highest peak observed in the characterization of the different materials. I(FB)/I(BB) was used for all materials expect for Au NPs, for which I(FA)/I(BA) was used. The related peaks are illustrated in Figure 5a. ■ * ASSOCIATED CONTENT sı Supporting Information Authors The Supporting Information is available free of charge at Olivia Aalling-Frederiksen − Department of Chemistry, https://pubs.acs.org/doi/10.1021/acs.chemmater.3c00090. Copenhagen University, DK-2100 Copenhagen, Denmark; orcid.org/0000-0003-1462-7173 Materials and Methods, detailed influence of various Waynah L. Dacayan − Department of Energy Conversion and experimental parameters, and additional characterization Storage, Technical University of Denmark, DK-2800 Kgs. of the nanomaterials (PDF) Lyngby, Denmark Joachim D. Bjerregaard − Department of Chemistry, ■ AUTHOR INFORMATION Corresponding Authors Copenhagen University, DK-2100 Copenhagen, Denmark Kim D. Jensen − Department of Chemistry, Copenhagen University, DK-2100 Copenhagen, Denmark; orcid.org/ Jonathan Quinson − Department of Chemistry, Copenhagen 0000-0001-7466-8458 University, DK-2100 Copenhagen, Denmark; Biochemical Mads R. V. Jørgensen − Department of Chemistry and and Chemical Engineering Department, Aarhus University, iNANO, Aarhus University, DK-8000 Aarhus, Denmark; DK-8200 Aarhus, Denmark; orcid.org/0000-0002-9374- MAX IV Laboratory, Lund University, SE-224 84 Lund, 9330; Email: jquinson@bce.au.dk Sweden; orcid.org/0000-0001-5507-9615 Kirsten M. Ø. Jensen − Department of Chemistry, Innokenty Kantor − MAX IV Laboratory, Lund University, Copenhagen University, DK-2100 Copenhagen, Denmark; SE-224 84 Lund, Sweden; Department of Physics, The orcid.org/0000-0003-0291-217X; Email: kirsten@ Technical University of Denmark, DK-2800 Kgs. Lyngby, chem.ku.dk Denmark 2186 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article Daniel R. Sørensen − Department of Chemistry and iNANO, under Contract 2018-07152, the Swedish Governmental Aarhus University, DK-8000 Aarhus, Denmark; MAX IV Agency for Innovation Systems under Contract 2018-04969, Laboratory, Lund University, SE-224 84 Lund, Sweden and Formas under Contract 2019-02496. DanMAX is funded Luise Theil Kuhn − Department of Energy Conversion and by NUFI Grant 4059-00009B. Storage, Technical University of Denmark, DK-2800 Kgs. Notes Lyngby, Denmark The authors declare the following competing financial Matthew S. Johnson − Department of Chemistry, Copenhagen interest(s): The presented nanotechnology is subject to a University, DK-2100 Copenhagen, Denmark; orcid.org/ patent application. Applicant: University of Copenhagen, 0000-0002-3645-3955 Denmark; Inventors: J.Q., K.M.Ø.J.; Application number: María Escudero-Escribano − Department of Chemistry, EP21193770; Status: Patent filed. The general principle of the Copenhagen University, DK-2100 Copenhagen, Denmark; surfactant-free synthesis is covered by the patent application. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, 08193 Barcelona, Spain; ICREA, 08010 Barcelona, Spain; orcid.org/0000-0002-6432- 3015 ■ ACKNOWLEDGMENTS Prof. Ib Chorkendorff, Danish Technical University (DTU), is Søren B. Simonsen − Department of Energy Conversion and thanked for access to XPS equipment. The authors acknowl- Storage, Technical University of Denmark, DK-2800 Kgs. edge Espen D. Bøjesen (AU), Denmark, for training and Lyngby, Denmark; orcid.org/0000-0001-7172-1225 facilitating access to the Talos F200X equipment. The authors Complete contact information is available at: are grateful to the staff at DESY beamlines P21.1 and P02.1 for https://pubs.acs.org/10.1021/acs.chemmater.3c00090 support during the experiments. The staff at MAX IV beamline DanMAX are furthermore thanked for the support during Author Contributions remote beamtime. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J.Q. designed the work, established the proof ■ REFERENCES (1) Ishida, T.; Murayama, T.; Taketoshi, A.; Haruta, M. Importance of concept, proposed the research direction, and supervised the of Size and Contact Structure of Gold Nanoparticles for the Genesis overall project. J.Q. performed the syntheses and sample of Unique Catalytic Processes. Chem. Rev. 2020, 120 (2), 464−525. preparation for TEM-EDS, HRTEM, XRD, XPS, PDF, ζ (2) Kumari, Y.; Kaur, G.; Kumar, R.; Singh, S. K.; Gulati, M.; potential, UV−vis, and electrochemical characterization. J.Q. Khursheed, R.; Clarisse, A.; Gowthamarajan, K.; Karri, V.; performed TEM-EDS, XRD, UV−vis, and electrochemical Mahalingam, R.; et al. Gold nanoparticles: New routes across old characterization, data analysis, and interpretation. O.A.-F. boundaries. Adv. Colloid Interface Sci. 2019, 274, 102037. helped in the sample preparation for PDF and XPS. O.A.-F. (3) Li, C. J.; Chai, O. J. H.; Yao, Q. F.; Liu, Z. H.; Wang, L.; Wang, H. J.; Xie, J. P. Electrocatalysis of gold-based nanoparticles and supervised and performed PDF measurements. O.A.-F. nanoclusters. Mater. Horiz. 2021, 8 (6), 1657−1682. performed PDF data analysis and interpretation. O.A.-F. (4) Daruich De Souza, C.; Ribeiro Nogueira, B.; Rostelato, M. contributed to the draft of the manuscript, in particular the Review of the methodologies used in the synthesis gold nanoparticles sections related to PDF. W.L.D. and S.B.S. performed the by chemical reduction. J. Alloys Compd. 2019, 798, 714−740. HRTEM data acquisition, analysis, and interpretation and (5) Nasaruddin, R. R.; Chen, T. K.; Yao, Q. F.; Zang, S. Q.; Xie, J. P. wrote the related sections. J.D.B. performed syntheses. K.D.J. Toward greener synthesis of gold nanomaterials: From biological to performed XPS data acquisition, analysis, and interpretation biomimetic synthesis. Coord. Chem. Rev. 2021, 426, 213540. and wrote the related sections. M.R.V.J., I.K., and D.R.S. (6) Gilbertson, L. M.; Zimmerman, J. B.; Plata, D. L.; Hutchison, J. collected XRTS data at DanMAX. L.T.K. helped with TEM E.; Anastas, P. T. Designing nanomaterials to maximize performance and minimize undesirable implications guided by the Principles of data acquisition. M.S.J. supervised J.D.B. M.E.-E. supervised Green Chemistry. Chem. Soc. Rev. 2015, 44 (16), 5758−5777. J.Q. and K.D.J. S.B.S. supervised W.L.D. K.M.Ø.J. supervised (7) Turkevich, J.; Stevenson, P. C.; Hillier, J. A study of the J.Q. and O.A.-F. J.Q., M.E.-E., and K.M.Ø.J. finalized the nucleation and growth processe in the synthesis of colloidal gold. paper. All of the authors discussed the overall results and Discuss. Faraday Soc. 1951, 11 (11), 55. participated in writing the final version of the manuscript. (8) Wuithschick, M.; Birnbaum, A.; Witte, S.; Sztucki, M.; Vainio, Funding U.; Pinna, N.; Rademann, K.; Emmerling, F.; Kraehnert, R.; Polte, J. Turkevich in New Robes: Key Questions Answered for the Most J.Q. acknowledges the European Union’s Horizon 2020 Common Gold Nanoparticle Synthesis. ACS Nano 2015, 9 (7), research and innovation program under Marie Skłodowska- 7052−7071. Curie Grant Agreement 840523 (CoSolCat). M.E.-E. and (9) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. K.D.J. thank the Independent Research Fund Denmark for the Synthesis of thio-derivatized gold nanoparticles in a 2-phase liquid- award of a DFF-Research Project 1 grant (9041-00224B). The liquid system. J. Chem. Soc., Chem. Commun. 1994, 7, 801−802. authors are grateful to the Villum Foundation for financial (10) Prat, D.; Wells, A.; Hayler, J.; Sneddon, H.; McElroy, C. R.; support through a Villum Young Investigator grant Abou-Shehada, S.; Dunn, P. J. CHEM21 selection guide of classical- (VKR00015416). Funding from the Danish Ministry of Higher and less classical-solvents. Green Chem. 2016, 18 (1), 288−296. Education and Science through the SMART Lighthouse is (11) Rodrigues, T. S.; Zhao, M.; Yang, T. H.; Gilroy, K. D.; da Silva, A. G. M.; Camargo, P. H. C.; Xia, Y. N. Synthesis of Colloidal Metal gratefully acknowledged. The Danish Research Council Nanocrystals: A Comprehensive Review on the Reductants. Chem. (DanScatt) is acknowledged for covering travel expenses for Eur. J. 2018, 24 (64), 16944−16963. the synchrotron experiments. The authors acknowledge MAX (12) Yamamoto, H.; Yano, H.; Kouchi, H.; Obora, Y.; Arakawa, R.; IV Laboratory for time on DanMAX under Proposal Kawasaki, H. N,N-Dimethylformamide-stabilized gold nanoclusters as 20200731. Research conducted at MAX IV, a Swedish national a catalyst for the reduction of 4-nitrophenol. Nanoscale 2012, 4 (14), user facility, is supported by the Swedish Research council 4148−4154. 2187 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article (13) Khan, T.; Ullah, N.; Khan, M. A.; Mashwani, Z. U. R.; Escudero-Escribano.; et al. Surfactant-free syntheses and pair Nadhman, A. Plant-based gold nanoparticles; a comprehensive review distribution function analysis of osmium nanoparticles. Beilstein J. of the decade-long research on synthesis, mechanistic aspects and Nanotechnol. 2022, 13, 230−235. diverse applications. Adv. Colloid Interface Sci. 2019, 272, 102017. (32) Quinson, J.; Simonsen, S. B.; Kuhn, L. T.; Kunz, S.; Arenz, M. (14) Ahmed, S.; Annu; Ikram, S.; Yudha, S. S. Biosynthesis of gold Size effect studies in catalysis: a simple surfactant-free synthesis of sub nanoparticles: A green approach. J. Photochem. Photobiol., B 2016, 3 nm Pd nanocatalysts supported on carbon. RSC Adv. 2018, 8 (59), 161, 141−153. 33794−33797. (15) Parveen, R.; Ullah, S.; Sgarbi, R.; Tremiliosi-Filho, G. One-pot (33) Mathiesen, J. K.; Quinson, J.; Dworzak, A.; Vosch, T.; Juelsholt, ligand-free synthesis of gold nanoparticles: The role of glycerol as M.; Kjaer, E. T. S.; Schroder, J.; Kirkensgaard, J. J. K.; Oezaslan, M.; reducing-cum-stabilizing agent. Colloids Surf., A 2019, 565, 162−171. Arenz, M.; et al. Insights from In Situ Studies on the Early Stages of (16) Junqi, T.; Shiqing, M. Green Synthesis of Colloidal Gold by Platinum Nanoparticle Formation. J. Phys. Chem. Lett. 2021, 12 (12), Ethyl Alcohol and NaOH at Normal Temperature. Xiyou Jinshu 3224−3231. Cailiao Yu Gongcheng 2013, 42 (11), 2232−2236. (34) Hendel, T.; Wuithschick, M.; Kettemann, F.; Birnbaum, A.; (17) Hutchison, J. E. The Road to Sustainable Nanotechnology: Rademann, K.; Polte, J. In Situ Determination of Colloidal Gold Challenges, Progress and Opportunities. ACS Sustain. Chem. Eng. Concentrations with UV-Vis Spectroscopy: Limitations and Perspec- 2016, 4 (11), 5907−5914. tives. Anal. Chem. 2014, 86 (22), 11115−11124. (18) Gomes, J. F.; Garcia, A. C.; Ferreira, E. B.; Pires, C.; Oliveira, V. (35) Haiss, W.; Thanh, N. T. K.; Aveyard, J.; Fernig, D. G. L.; Tremiliosi-Filho, G.; Gasparotto, L. H. S. New insights into the Determination of size and concentration of gold nanoparticles from formation mechanism of Ag, Au and AgAu nanoparticles in aqueous UV-Vis spectra. Anal. Chem. 2007, 79 (11), 4215−4221. alkaline media: alkoxides from alcohols, aldehydes and ketones as (36) Larm, N. E.; Essner, J. B.; Thon, J. A.; Bhawawet, N.; Adhikari, universal reducing agents. Phys. Chem. Chem. Phys. 2015, 17 (33), L.; St. Angelo, S. K.; Baker, G. A. Single Laboratory Experiment 21683−21693. Integrating the Synthesis, Optical Characterization, and Nanocatalytic (19) Dauthal, P.; Mukhopadhyay, M. Noble Metal Nanoparticles: Assessment of Gold Nanoparticles. J. Chem. Educ. 2020, 97 (5), Plant-Mediated Synthesis, Mechanistic Aspects of Synthesis, and 1454−1459. Applications. Ind. Eng. Chem. Res. 2016, 55 (36), 9557−9577. (37) Ye, Y. J.; Lv, M. X.; Zhang, X. Y.; Zhang, Y. X. Colorimetric (20) Liz-Marzan, L. M.; Kagan, C. R.; Millstone, J. E. determination of copper(II) ions using gold nanoparticles as a probe. Reproducibility in Nanocrystal Synthesis? Watch Out for Impurities! RSC Adv. 2015, 5 (124), 102311−102317. ACS Nano 2020, 14 (6), 6359−6361. (38) Agarwal, S.; Mishra, P.; Shivange, G.; Kodipelli, N.; Moros, M.; (21) Cargnello, M.; Chen, C.; Diroll, B. T.; Doan-Nguyen, V. V. T.; de la Fuente, J. M.; Anindya, R. Citrate-capped gold nanoparticles for Gorte, R. J.; Murray, C. B. Efficient Removal of Organic Ligands from the label-free detection of ubiquitin C-terminal hydrolase-1. Analyst Supported Nanocrystals by Fast Thermal Annealing Enables Catalytic 2015, 140 (4), 1166−1173. Studies on Well-Defined Active Phases. J. Am. Chem. Soc. 2015, 137 (39) Merk, V.; Rehbock, C.; Becker, F.; Hagemann, U.; Nienhaus, (21), 6906−6911. H.; Barcikowski, S. In Situ Non-DLVO Stabilization of Surfactant- (22) Nicol, J. R.; Dixon, D.; Coulter, J. A. Gold nanoparticle surface Free, Plasmonic Gold Nanoparticles: Effect of Hofmeister’s Anions. functionalization: a necessary requirement in the development of Langmuir 2014, 30 (15), 4213−4222. novel nanotherapeutics. Nanomedicine 2015, 10 (8), 1315−1326. (40) Hammersley, A. P. FIT2D: a multi-purpose data reduction, (23) Quinson, J. Surfactant-free precious metal colloidal nano- analysis and visualization program. J. Appl. Crystallogr. 2016, 49, 646− particles for catalysis. Front. Nanotechnol. 2021, 3, 770281. 652. (24) Schulz-Dobrick, M.; Sarathy, K. V.; Jansen, M. Surfactant-free (41) Prescher, C.; Prakapenka, V. B. DIOPTAS: a program for synthesis and functionalization of gold nanoparticles. J. Am. Chem. reduction of two-dimensional X-ray diffraction data and data Soc. 2005, 127 (37), 12816−12817. exploration. High Press. Res. 2015, 35 (3), 223−230. (25) Kawasaki, H. Surfactant-free solution-based synthesis of (42) Juhas, P.; Davis, T.; Farrow, C. L.; Billinge, S. J. L. PDFgetX3: a metallic nanoparticles toward efficient use of the nanoparticles’ rapid and highly automatable program for processing powder surfaces and their application in catalysis and chemo-/biosensing. diffraction data into total scattering pair distribution functions. J. Nanotechnol. Rev. 2013, 2 (1), 5−25. Appl. Crystallogr. 2013, 46, 560−566. (26) Wang, Y.; Ren, J. W.; Deng, K.; Gui, L. L.; Tang, Y. Q. (43) Rodriguez-Carvajal, J. Recent advances in magnetic-structure Preparation of tractable platinum, rhodium, and ruthenium nano- determination by neutron powder diffraction. Phys. B 1993, 192 (1− clusters with small particle size in organic media. Chem. Mater. 2000, 2), 55−69. 12 (6), 1622−1627. (44) Le Bail, A.; Duroy, H.; Fourquet, J. L. Ab-initio structure (27) Quinson, J.; Kunz, S.; Arenz, M. Beyond Active Site Design: A determination of LiSbWO6 by X-ray powder diffraction. Mater. Res. Surfactant-Free Toolbox Approach for Optimized Supported Nano- Bull. 1988, 23 (3), 447−452. particle Catalysts. ChemCatChem 2021, 13 (7), 1692−1705. (45) Farrow, C. L.; Juhas, P.; Liu, J. W.; Bryndin, D.; Bozin, E. S.; (28) Quinson, J.; Neumann, S.; Wannmacher, T.; Kacenauskaite, L.; Bloch, J.; Proffen, T.; Billinge, S. J. L. PDFfit2 and PDFgui: computer Inaba, M.; Bucher, J.; Bizzotto, F.; Simonsen, S. B.; Theil Kuhn, L.; programs for studying nanostructure in crystals. J. Phys.: Condens. Bujak, D.; et al. Colloids for Catalysts: A Concept for the Preparation Matter 2007, 19 (33), 335219. of Superior Catalysts of Industrial Relevance. Angew. Chem., Int. Ed. (46) Inaba, M.; Quinson, J.; Arenz, M. pH matters: The influence of 2018, 57 (38), 12338−12341. the catalyst ink on the oxygen reduction activity determined in thin (29) Mathiesen, J. K.; Quinson, J.; Blaseio, S.; Kjær, E. T. S.; film rotating disk electrode measurements. J. Power Sources 2017, 353, Dworzak, A.; Cooper, S.; Pedersen, J. K.; Wang, B.; Bizzotto, F.; 19−27. Johanna, S.; et al. Chemical insights on the formation of colloidal (47) Padayachee, D.; Golovko, V.; Ingham, B.; Marshall, A. T. iridium nanoparticles from in situ X-ray total scattering: Influence of Influence of particle size on the electrocatalytic oxidation of glycerol precursors and cations on the reaction pathway. J. Am. Chem. Soc. over carbon-supported gold nanoparticles. Electrochim. Acta 2014, 2023, 145 (3), 1769−1782. 120, 398−407. (30) Mathiesen, J. K.; Cooper, S. R.; Anker, A. S.; Kinnibrugh, T. L.; (48) Lukaszewski, M.; Soszko, M.; Czerwinski, A. Electrochemical Jensen, K. M. Ø.; Quinson, J. Simple Setup Miniaturization with Methods of Real Surface Area Determination of Noble Metal Multiple Benefits for Green Chemistry in Nanoparticle Synthesis. Electrodes - an Overview. Int. J. Electrochem. Sci. 2016, 11 (6), ACS Omega 2022, 7, 4714−4721. 4442−4469. (31) Juelsholt, M.; Quinson, J.; Kjær, E. T. S.; Wang, B.; Pittkowski, (49) Silva, L. S. R.; Almeida, C. V. S.; Meneses, C. T.; Batista, E. A.; R.; Cooper, S. R.; Kinnibrugh, T. L.; Simonsen, S. B.; Theil Kuhn, L.; Santos, S. F.; Eguiluz, K. I. B.; Salazar-Banda, G. R. AuPd/C core-shell 2188 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article and alloy nanoparticles with enhanced catalytic activity toward the (68) Quinson, J.; Inaba, M.; Neumann, S.; Swane, A. A.; Bucher, J.; electro-oxidation of ethanol in alkaline media. Appl.Catal. B- Simonsen, S. B.; Theil Kuhn, L.; Kirkensgaard, J. J. K.; Jensen, K. M. Environmental. 2019, 251, 313−325. Ø.; Oezaslan, M.; et al. Investigating Particle Size Effects in Catalysis (50) Gasparotto, L. H. S.; Garcia, A. C.; Gomes, J. F.; Tremiliosi- by Applying a Size-Controlled and Surfactant-Free Synthesis of Filho, G. Electrocatalytic performance of environmentally friendly Colloidal Nanoparticles in Alkaline Ethylene Glycol: Case Study of synthesized gold nanoparticles towards the borohydride electro- the Oxygen Reduction Reaction on Pt. ACS Catal. 2018, 8 (7), oxidation reaction. J. Power Sources 2012, 218, 73−78. 6627−6635. (51) Harada, M.; Einaga, H. In situ XAFS studies of au particle (69) Quinson, J.; Kacenauskaite, L.; Bucher, J.; Simonsen, S. B.; formation by photoreduction in polymer solutions. Langmuir 2007, Theil Kuhn, L.; Oezaslan, M.; Kunz, S.; Arenz, M. Controlled 23 (12), 6536−6543. Synthesis of Surfactant-Free Water-Dispersible Colloidal Platinum (52) Nalawade, P.; Mukherjee, T.; Kapoor, S. Green Synthesis of Nanoparticles by the Co4Cat Process. ChemSusChem 2019, 12 (6), Gold Nanoparticles Using Glycerol as a Reducing Agent. Adv. 1229−1239. Nanopart. 2013, 2 (2), 78−86. (70) Siiman, O.; Hsu, W. P. Surface-enhanced Raman scaterring (53) Parveen, R.; Tremiliosi-Filho, G. A step ahead towards the (SERS) enhancements and excitation profiles for 3,5-pyridinedicar- green synthesis of monodisperse gold nanoparticles: the use of crude boxylate and dabsyl aspartate on colloidal gold. J. Chem. Soc. Faraday glycerol as a greener and low-cost reducing agent. RSC Adv. 2016, 6 Trans. I 1986, 82, 851−867. (97), 95210−95219. (71) Yao, T.; Sun, Z. H.; Li, Y. Y.; Pan, Z. Y.; Wei, H.; Xie, Y.; (54) Duan, H. H.; Wang, D. S.; Li, Y. D. Green chemistry for Nomura, M.; Niwa, Y.; Yan, W. S.; Wu, Z. Y.; et al. Insights into Initial nanoparticle synthesis. Chem. Soc. Rev. 2015, 44 (16), 5778−5792. Kinetic Nucleation of Gold Nanocrystals. J. Am. Chem. Soc. 2010, 132 (55) Thanh, N. T. K.; Maclean, N.; Mahiddine, S. Mechanisms of (22), 7696−7701. Nucleation and Growth of Nanoparticles in Solution. Chem. Rev. (72) Gao, Y. H.; Torrente-Murciano, L. Mechanistic insights of the 2014, 114 (15), 7610−7630. reduction of gold salts in the Turkevich protocol. Nanoscale 2020, 12 (56) Panariello, L.; Radhakrishnan, A. N. P.; Papakonstantinou, I.; (4), 2740−2751. Parkin, I. P.; Gavriilidis, A. Particle Size Evolution during the (73) Machesky, M. L.; Andrade, W. O.; Rose, A. W. Interactions of Synthesis of Gold Nanoparticles Using In Situ Time-Resolved UV-Vis gold(III) chloride and elemental gold with peat-derived humic Spectroscopy: An Experimental and Theoretical Study Unravelling substances. Chem. Geol. 1992, 102 (1−4), 53−71. the Effect of Adsorbed Gold Precursor Species. J. Phys. Chem. C 2020, (74) Pong, B. K.; Elim, H. I.; Chong, J. X.; Ji, W.; Trout, B. L.; Lee, J. 124 (50), 27662−27672. Y. New insights on the nanoparticle growth mechanism in the citrate (57) Quinson, J.; Jensen, K. M. Ø. From platinum atoms in reduction of Gold(III) salt: Formation of the au nanowire molecules to colloidal nanoparticles: A review on reduction, intermediate and its nonlinear optical properties. J. Phys. Chem. C nucleation and growth mechanisms. Adv. Colloid Interface Sci. 2020, 2007, 111 (17), 6281−6287. 286, 102300. (75) Ji, X.; Song, X.; Li, J.; Bai, Y.; Yang, W.; Peng, X. Size control of (58) Al-Johani, H.; Abou-Hamad, E.; Jedidi, A.; Widdifield, C. M.; gold nanocrystals in citrate reduction: The third role of citrate. J. Am. Viger-Gravel, J.; Sangaru, S. S.; Gajan, D.; Anjum, D. H.; Ould-Chikh, Chem. Soc. 2007, 129 (45), 13939−13948. S.; Hedhili, M. N.; et al. The structure and binding mode of citrate in (76) Quinson, J.; Bucher, J.; Simonsen, S. B.; Kuhn, L. T.; Kunz, S.; the stabilization of gold nanoparticles. Nat. Chem. 2017, 9 (9), 890− Arenz, M. Monovalent Alkali Cations: Simple and Eco-Friendly 895. (59) Mendez, E.; Fagundez, P.; Sosa, P.; Gutierrez, M. V.; Botasini, Stabilizers for Surfactant-Free Precious Metal Nanoparticle Colloids. S. Experimental evidences support the existence of an aggregation/ ACS Sustain. Chem. Eng. 2019, 7 (16), 13680−13686. disaggregation step in the Turkevich synthesis of gold nanoparticles. (77) Schrader, I.; Warneke, J.; Neumann, S.; Grotheer, S.; Swane, A. Nanotechnology 2021, 32 (4), 045603. A.; Kirkensgaard, J. J. K.; Arenz, M.; Kunz, S. Surface Chemistry of (60) Kimling, J.; Maier, M.; Okenve, B.; Kotaidis, V.; Ballot, H.; ″Unprotected″ Nanoparticles: A Spectroscopic Investigation on Plech, A. Turkevich method for gold nanoparticle synthesis revisited. Colloidal Particles. J. Phys. Chem. C 2015, 119 (31), 17655−17661. J. Phys. Chem. B 2006, 110 (32), 15700−15707. (78) Reichenberger, S.; Marzun, G.; Muhler, M.; Barcikowski, S. (61) Ojea-Jimenez, I.; Bastus, N. G.; Puntes, V. Influence of the Perspective of Surfactant-free Colloidal Nanoparticles in Heteroge- Sequence of the Reagents Addition in the Citrate-Mediated Synthesis neous Catalysis. ChemCatChem 2019, 11 (18), 4489−4518. of Gold Nanoparticles. J. Phys. Chem. C 2011, 115 (32), 15752− (79) Franco-Ulloa, S.; Tatulli, G.; Bore, S. L.; Moglianetti, M.; 15757. Pompa, P. P.; Cascella, M.; De Vivo, M. Dispersion state phase (62) Schulz, F.; Homolka, T.; Bastus, N. G.; Puntes, V.; Weller, H.; diagram of citrate-coated metallic nanoparticles in saline solutions. Vossmeyer, T. Little Adjustments Significantly Improve the Turkevich Nat. Commun. 2020, 11 (1), 5422. Synthesis of Gold Nanoparticles. Langmuir 2014, 30 (35), 10779− (80) Lu, P. Y.; Zhou, J. W.; Hu, Y. K.; Yin, J. W.; Wang, Y. H.; Yu, J. 10784. L.; Ma, Y. B.; Zhu, Z. L.; Zeng, Z. Y.; Fan, Z. X. Gold-based (63) Huang, H.; du Toit, H.; Besenhard, M. O.; Ben-Jaber, S.; nanoalloys: synthetic methods and catalytic applications. J. Mater. Dobson, P.; Parkin, I.; Gavriilidis, A. Continuous flow synthesis of Chem. A 2021, 9 (35), 19025−19053. ultrasmall gold nanoparticles in a microreactor using trisodium citrate (81) Zhu, X. J.; Guo, Q. S.; Sun, Y. F.; Chen, S. J.; Wang, J. Q.; Wu, and their SERS performance. Chem. Eng. Sci. 2018, 189, 422−430. M. M.; Fu, W. Z.; Tang, Y. Q.; Duan, X. Z.; Chen, D.; et al. (64) Quinn, M.; Mills, G. Surface-mediated formation of gold Optimising surface d charge of AuPd nanoalloy catalysts for enhanced particles in basic methanol. J. Phys. Chem. 1994, 98 (39), 9840−9844. catalytic activity. Nat. Commun. 2019, 10, 1428. (65) Mihata, A.; Usman, R.; Kurawaki, J. Selective Synthesis of Gold (82) Batchelor, T. A. A.; Pedersen, J. K.; Winther, S. H.; Castelli, I. Nanoparticles in Water/Alcohol Binary Solution Systems by Ultra- E.; Jacobsen, K. W.; Rossmeisl, J. High-Entropy Alloys as a Discovery sonic Irradiation. E-J. Surf. Sci. Nanotechnol. 2015, 13, 427−430. Platform for Electrocatalysis. Joule 2019, 3, 834−845. (66) Sudare, T.; Ueno, T.; Watthanaphanit, A.; Saito, N. Accelerated (83) Wu, D. S.; Kusada, K.; Yamamoto, T.; Toriyama, T.; nanoparticles synthesis in alcohol-water-mixture-based solution Matsumura, S.; Kawaguchi, S.; Kubota, Y.; Kitagawa, H. Platinum- plasma. Phys. Chem. Chem. Phys. 2015, 17 (45), 30255−30259. Group-Metal High-Entropy-Alloy Nanoparticles. J. Am. Chem. Soc. (67) Harada, M.; Kizaki, S. Formation Mechanism of Gold 2020, 142 (32), 13833−13838. Nanoparticles Synthesized by Photoreduction in Aqueous Ethanol (84) Yao, Y. G.; Huang, Z. N.; Li, T. Y.; Wang, H.; Liu, Y. F.; Stein, Solutions of Polymers Using In Situ Quick Scanning X-ray Absorption H. S.; Mao, Y. M.; Gao, J. L.; Jiao, M. L.; Dong, Q.; et al. High- Fine Structure and Small-Angle X-ray Scattering. Cryst. Growth Des. throughput, combinatorial synthesis of multimetallic nanoclusters. 2016, 16 (3), 1200−1212. Proc. Natl. Acad. Sci. U. S. A. 2020, 117 (12), 6316−6322. 2189 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190 Chemistry of Materials pubs.acs.org/cm Article (85) Lin, H. H.; Muzzio, M.; Wei, K. C.; Zhang, P.; Li, J. R.; Li, N.; Yin, Z. Y.; Su, D.; Sun, S. H. PdAu Alloy Nanoparticles for Ethanol Oxidation in Alkaline Conditions: Enhanced Activity and C1 Pathway Selectivity. ACS Appl. Energy Mater. 2019, 2 (12), 8701−8706. (86) Du, J.; Quinson, J.; Zhang, D. M.; Wang, B. Y.; Wiberg, G. K. H.; Pittkowski, R. K.; Schroeder, J.; Simonsen, S. B.; Kirkensgaard, J. J. K.; Li, Y.; et al. Nanocomposite Concept for Electrochemical In Situ Preparation of Pt-Au Alloy Nanoparticles for Formic Acid Oxidation. JACS Au 2022, 2 (7), 1757−1768. (87) Schreyer, O. A. H.øj; Quinson, J.; Escudero-Escribano, M. Toward Overcoming the Challenges in the Comparison of Different Pd Nanocatalysts: Case Study of the Ethanol Oxidation Reaction. Inorganics 2020, 8 (11), 59. (88) Zhu, L. D.; Zhao, T. S.; Xu, J. B.; Liang, Z. X. Preparation and characterization of carbon-supported sub-monolayer palladium decorated gold nanoparticles for the electro-oxidation of ethanol in alkaline media. J. Power Sources 2009, 187 (1), 80−84. (89) Liang, Z. X.; Song, L.; Deng, S. Q.; Zhu, Y. M.; Stavitski, E.; Adzic, R. R.; Chen, J. Y.; Wang, J. X. Direct 12-Electron Oxidation of Ethanol on a Ternary Au(core)-PtIr(Shell) Electrocatalyst. J. Am. Chem. Soc. 2019, 141 (24), 9629−9636. 2190 https://doi.org/10.1021/acs.chemmater.3c00090 Chem. Mater. 2023, 35, 2173−2190