Nanoscale View Article Online PAPER View Journal | View Issue Tailoring of the photocatalytic activity of CeO2 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Cite this: Nanoscale, 2022, 14, 12048 nanoparticles by the presence of plasmonic Ag nanoparticles† Open Access Article. Published on 27 July 2022. Downloaded on 11/2/2022 7:53:33 AM. Shuang Zhao,a Marc Riedel,b Javier Patarroyo, c Neus G. Bastús, c Victor Puntes, c,d,e Zhao Yue, *f Fred Lisdat*b and Wolfgang J. Parak *a The present study investigates basic features of a photoelectrochemical system based on CeO2 nano- particles fixed on gold electrodes. Since photocurrent generation is limited to the absorption range of the CeO2 in the UV range, the combination with metal nanoparticles has been studied. It can be shown that the combination of silver nanoparticles with the CeO2 can shift the excitation range into the visible light wavelength range. Here a close contact between both components has been found to be essential and thus, hybrid CeO2@Ag nanoparticles have been prepared and analyzed. We have collected arguments that electron transfer occurs between both compositional elements of the hybrid nanoparticles.The photo- current generation can be rationalized on the basis of an energy diagram underlying the necessity of surface plasmon excitation in the metal nanoparticles, which is also supported by wavelength-dependent photocurrent measurements. However, electrochemical reactions seem to occur at the CeO2 surface and consequently, the catalytic properties of this material can be exploited as exemplified with the photoelec- Received 7th March 2022, trochemical reduction of hydrogen peroxide. It can be further demonstrated that the layer-by layer tech- Accepted 27th July 2022 nique can be exploited to create a multilayer system on top of a gold electrode which allows the adjust- DOI: 10.1039/d2nr01318e ment of the sensitivity of the photoelectrochemical system. Thus, with a 5-layer electrode with hybrid rsc.li/nanoscale CeO2@Ag nanoparticles submicromolar hydrogen peroxide concentrations can be detected. Introduction underlying electrode and on the other hand reactions of accep- tor or donor molecules with the excited nanostructures. In this Semiconductor nanostructures are valuable building blocks in way, the detected photocurrent becomes a measure of the con- electrochemical systems since they enlarge the possibilities in centration of the respective species.1,2 Conceptually the photo- sensing and energetic applications. They introduce light as current can be thus, used as analytical tool with a good signal additional tool in order to adjust electrochemical reactions. As to noise ratio. Since only the illuminated area will be analyzed, a basic feature, charge carriers are generated inside these the application of using a focused light source for multiplexed nanostructures, which cannot only show recombination detection on a small electrode surface has been recently leading to luminescence properties, but also allow the involve- demonstrated experimentally.3,4 Another interesting results ment of these light-triggered charge carriers in electrochemical from the fact that the generation of electron–hole pairs also reactions. This can be on the one hand electron transfer to the means, that the energetic situation of the charge carriers is changed upon illumination. This will allow the collection of electrons from oxidation processes at very low potential and a Fachbereich Physik, CHyN, Universität Hamburg, 22761 Hamburg, Germany. the transfer of electrons to acceptor molecules at rather high E-mail: wolfgang.parak@uni-hamburg.de potentials. b Biosystems Technology, Institute of Life Sciences and Biomedical Technologies, The attractiveness of photoelectrochemical detection Technical University of Applied Sciences Wildau, 15745 Wildau, Germany. schemes has resulted in numerous developments in the field. E-mail: flisdat@th-wildau.de c Institut Català de Nanociència i Nanotecnologia (ICN2), CSIC and BIST, Campus One cannot only directly detect donor or acceptor molecules, UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain but also combinations with specific enzymatic reactions are d Vall d’Hebron Institut de Recerca (VHIR), 08035 Barcelona, Catalonia, Spain feasible. Here different strategies from amperometric bio- e ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain sensors have been transferred to photobioelectrochemical f Department of Microelectronics, Nankai University, 30071 Tianjin, China. detection schemes. In the focus so far have been mainly coup- E-mail: yuezhao@nankai.edu.cn † Electronic supplementary information (ESI) available. See DOI: https://doi.org/ ling strategies in which the detection of enzymatic products 10.1039/d2nr01318e (or co-substrates) has been used. Examples are oxygen,5–7 12048 | Nanoscale, 2022, 14, 12048–12059 This journal is © The Royal Society of Chemistry 2022 View Article Online Nanoscale Paper Table 1 Summary of reports studying the photocatalytic properties of Ag/CeO2 nanocomposites Nanomaterial Application Enhancement mechanism Ref. Ag NPs decorated with Realizing enhanced surface enhanced Raman Charge transfer, more SERS hot spots present in the 29 CeO2 NPs scattering (SERS) properties and improved sensing Ag on the CeO2 surfaces performance of 4-aminothiophenol (4-ATP). Ag/CeO2 Ag/CeO2 antibacterial activity towards Gram positive Soot oxidation of CeO2 30 and Gram negative strains of bacteria Ag/CeO2 Better photocatalytic efficacy and degradation towards Lower rate recombination of charge carriers 31 nanocomposites organic pollutants This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Ag/Au doped CeO2 NPs Degradation of organic pollutants Morphology, particle size, electron–hole 32 recombination, and oxygen vacancies. Ag NPs decorated on Superior photocatalytic activity for the dye auramine O Localized surface plasmon resonance (LSPR) 33 Open Access Article. Published on 27 July 2022. Downloaded on 11/2/2022 7:53:33 AM. CeO2 supports Ag@CeO2 core–shell NPs O2 evolution and methylene blue (MB) dye degradation Localized surface plasmon resonance (LSPR) 34 3D flower-like Ag–CeO2– Enhanced performance for photocatalytic CO2 Prolonged absorption in the visible light region 35 ZnO nanocomposites reduction to CO and CH4 induced by the surface plasmon resonance (SPR) effect; efficient separation of photogenerated charges, and the Z-scheme configuration Ag@CeO2–Au nanorods Improved conversion efficiency of benzyl alcohol to Plasmonic scattering-enhanced absorption 36 benzaldehyde CeO2/Ag Enhanced photocatalytic activity and electrochemical Electron transfer 37 nanocomposites behavior towards Hg(II) NADH,8–10 thiocholine,11,12 or phenolic substances.13 Only few order to address the following questions: is the mechanism to systems have been reported so far on photoelectrochemical couple plasmonic NPs as light harvesters and charge transfer systems exploiting direct electron transfer with proteins,14–17 mediators to photocatalytic NPs of general nature? (i.e. can the although the coupling of enzymatic reactions to semi- approach be extended to plasmonic NPs others than Au?) Can conductor nanoparticles (NPs) has gained increasing interest. the multi-interface between redox couples in solution, the As an alternative, mediators can also be used for the com- immobilized hybrids of plasmonic and photocatalytic NPs, munication of enzymes with semiconductor nanostructures – and the underlying gold electrode be described by a band first examples have been reported here.18 In addition to structure diagram? CeO2 and Ag have already been combined enzymes, also biospecific binding reactions have been coupled in several studies, but with very different background. In to photoactive electrodes, enlarging the applicability of such Table 1 a short summary about previous work on using Ag/ systems even further.19–22 CeO2 nanocomposites for photocatalysis is given. There is a variety of semiconductor nanostructures which have been used for the construction of such photoelectro- chemical schemes. Examples are quantum dots,6,22,23 Results and discussion nanowires,8,24,25 nanoclusters,26–28 and others. The light inter- action of such structures can only be exploited when the wave- Characterization of the nanoparticles length used for excitation matches with the absorption pro- CeO2 NPs, Ag NPs of different size, and hybrid Ag/CeO2 NPs perties of the material. Here one can see a limitation since have been synthesized adopting previously published many NPs can only be excited by UV light. For the application protocols.38,39 In Fig. 1, representative transmission electron however, one should avoid the simultaneous excitation of the microscopy (TEM) images of the different NPs used in the transducer and the analyte molecules since this may give raise study are shown. Sodium citrate-coated (and thus, negatively to unwanted reactions or disturbances of the defined concen- charged) Ag NPs are hereby synthesized that their inorganic tration analysis as has been demonstrated with InGaN core diameter dc corresponds to the diameter of the Ag core in nanowires.8 the Ag/CeO2 NPs. From the size distribution (cf. Fig. 1d) Different strategies can be seen to overcome this issue. One obtained from the TEM images (Fig. 1a), a mean value of dc = is based on the application of hybrid NPs. Recently we have 5.8 ± 0.4 nm can be found for the Ag NPs. The CeO2 NPs (see shown how the photocatalytic properties of CeO2 NPs can be Fig. 1b for a TEM image) with a mean core diameter of dc = enhanced by the presence of plasmonic Au NPs.3 Presence of 15.4 ± 1.6 nm (cf. Fig. 1e for their size distribution) appear to the Au NPs allowed for amplifying the photocurrent response have a lower compactness, and may be porous. The TEM in the visible light regions where the Au NPs, but not the CeO2 images suggest that these NPs are not crystalline over their NPs absorb light. The best effect was observed when the Au entire volume. TEM images of the Ag/CeO2 hybrid NPs (cf. NPs were directly connected to the CeO2 NPs in the form of Fig. 1c) reveal the formation of structures composed of a hybrid NPs, without interfacial organic separation layer. central Ag core surrounded by cerium oxide domains. The In the present work we extend this strategy to the case of CeO2 domains covering the Ag cores have also a low compact- plasmonic Ag NPs (i.e. Ag/CeO2 hybrid NPs are investigated) in ness and very similar to the pure CeO2 NPs. The CeO2 domains This journal is © The Royal Society of Chemistry 2022 Nanoscale, 2022, 14, 12048–12059 | 12049 View Article Online Paper Nanoscale The UV/vis absorption spectra of the three different types of NPs as dispersed in aqueous solution can be seen in Fig. 2. The absorption of pure citrate-stabilized Ag NPs exhibits a surface plasmon resonance peak at λ = 392 nm, which agrees with literature for NPs of the obtained size.40 The CeO2 NPs show a characteristic absorption in the near-ultraviolet region (λ ≈ 270 nm), which arises from the electronic transitions within the sample. The absorption spectrum of the hybrid Ag/ CeO2 NPs is given in Fig. 2c. As can be seen, CeO2 coating This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. translates into: (i) the increase of the well-defined absorption of CeO2 in the near-ultraviolet region, (ii) the red-shift of the Open Access Article. Published on 27 July 2022. Downloaded on 11/2/2022 7:53:33 AM. surface plasmon resonance (SPR) Ag peak position (from 392 to 421 nm), and (iii) the progressive broadening of the plasmon band. These results can be explained by the increased refractive index (n ≈ 2.2) of the dielectric environment sur- rounding the Ag cores upon CeO2 coating, resulting in a red- shift whose extension depends on the thickness and degree of coating on the Ag cores. To further study and prove the successful formation of Ag/ CeO2 hybrid NPs X-ray photoelectron spectroscopy (XPS) measurements of Ag/CeO2 hybrid NPs and plain Ag NPs were carried out in a previous study.41 In the case of plain Ag NPs, only a doublet has been observed corresponding to the well- known spin–orbit splitting. The determined binding energy (BE) at 368.2 and 374.2 eV, assigned to 3d5/2 and 3d3/2, respectively, corresponds well to metallic Ag. Essentially, the same spectrum is obtained in the case of Ag/CeO2 hybrid NPs. The crystal structure of the Ag/CeO2 NPs has been further investigated by X-ray diffraction (XRD). As shown in Fig. S2,† Fig. 1 Transmission electron microscopy (TEM) images of the NPs two series of peaks are present in the diffraction patterns, including (a) Ag NPs, (b) CeO2 NPs, and (c) Ag/CeO2 hybrid NPs, together with the size distribution N(dc) of their inorganic diameters dc, which can be assigned to the fluorite (cubic) CeO2 phase as shown in (d)–(g): (d) Ag NPs, (e) CeO2 NPs, (f ) the Ag core size of the (JCPDS 34-0394) and the Ag cubic phase (JCPDS 04-0783). The Ag/CeO2 hybrid NPs, and (g) the whole size of the Ag/CeO2 hybrid NPs. diffraction peaks of the CeO2 domain are broader and weaker, The insets are an artistic guide to the eye in which a sphere symbolizes according to the smaller size of the CeO2 crystal domains. the Ag cores. Additional TEM images are shown in Fig. S1 in the ESI.† Representative images of high-angle annular dark field scanning TEM (HAADF STEM) of the Ag/CeO2 hybrid NPs demonstrate the systematic formation of Ag/CeO2 NPs consist- ing of an Ag core surrounded by a CeO2 shell, see Fig. S3.† Of do not fully isolate the Ag cores, therefore the Ag cores are in notice, the Ag cores present the brightest contrast in the HAAD direct contact with the aqueous solution in which they are dis- STEM images, due to their Z-contrast. Unlike other common persed. The diameter of the hybrid Ag/CeO2 NPs as deter- shell components with a continuous phase – such as SiO2, mined by TEM is dc = 15.8 ± 2.3 nm (cf. Fig. 1g), while the TiO2, Cu2O, and ZnO – the CeO2 layer is not compact nor con- inner Ag core has a diameter of dc = 7.8 ± 2.1 nm (cf. Fig. 1f ). tinuous, indicating that the growth of CeO2 follows Volmer– We want to point out that although the Ag parts of the Ag/ Weber growth modes, as expected from the large mismatch of CeO2 hybrid NPs and the Ag NPs have almost the same average lattice parameters between CeO2 (0.5412 nm) and Ag sizes, the size distributions of the TEM images given in Fig. 1 (0.4046 nm). An energy-dispersive X-ray spectroscopy show that the size distribution of the Ag NPs is more uniform (STEM-EDS) line scan obtained on the hybrid reveals that the than that of the Ag/CeO2 NPs. This is due to the fact that the hybrid NPs are composed of a Ag and Ce domain, see Fig. S3.† Ag cores have been produced following two different synthetic Additionally, the continuous Ce profile in the junction procedures, and consequently the level of size control and uni- between the 2 domains constitutes a further indication of the formity of the product is different. While the Ag/CeO2 hybrid tight interaction between Ag and CeO2. NPs have been produced following a one-pot method,39 The accessibility of the inner metal core in the hybrid struc- whereas the plain Ag NPs of similar size are produced follow- tures was proven by studying the catalytic degradation of 4-nitro- ing a seeded-growth procedure.38 Despite the versatility of the phenol (4-NP) by borohydride ions39 in the case of Au/CeO2 former, it does not allow for controlling the size of the Ag hybrid NPs. Although a decrease of the reduction rate of the dye cores with the same accuracy. has been observed for the hybrid NPs in comparison to bare Au 12050 | Nanoscale, 2022, 14, 12048–12059 This journal is © The Royal Society of Chemistry 2022 View Article Online Nanoscale Paper This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 27 July 2022. Downloaded on 11/2/2022 7:53:33 AM. Fig. 2 UV-vis absorption spectra A(λ) of (a) pure citric acid stabilized Ag NPs (–), pure CeO2 NPs, and (c) hybrid Ag/CeO2 NPs in the water. Note that the “–” means negative charged, in contrast to the positively charged Ag NPs (+) reported later in this work. Additional spectra are shown in Fig. S1.† NPs of similar core sizes, the presence of the CeO2 shell can not charge carrier generation in the CeO2 NPs. In a first approach prevent the 4-NP from reaching the inner metal core. plasmonic Ag NPs with dc = 5.8 ± 0.4 nm are mixed with the Altogether, these results confirm the growth of the CeO2 to CeO2 NPs and immobilized on the Au electrode. For this, the the Ag core, and the successful formation of the Ag/CeO2 aqueous solvent of Ag NP solutions is removed by ultrafiltra- hybrid NPs. tion,42 followed by redispersion of the Ag NPs in an aqueous solution of CeO2 NPs. The modified Au electrode is then incu- Photocurrent measurements of single layers of CeO2 NPs in bated with the NP mixture. Again electrostatic interactions presence of different types and amounts of Ag NPs with the positively charged cysteamine layer on the Au surface Since pure CeO2 NPs have a negative surface charge, direct have been exploited. The surface coverage of the electrode with assembly of these NPs to cysteamine modified gold electrode Ag NPs and CeO2 NPs can be determined by dissolving the has been used for immobilization (for details we refer to the NPs with aqua regia and measuring elemental Ag and Ce con- ESI†). Here a surface density of approximately n(CeO2 NP) = centrations with inductively coupled plasma mass spec- 5.5 × 1010 cm−2 (cf. Fig. S6†) has been achieved. Note that this trometry (ICP-MS). Based on the surface area of the coated Au low surface coverage is in agreement with previous results,3 electrode and the amount of Ag and Ce atoms per Ag NP and which however can be enhanced by the growth of multilayers CeO2 NP, respectively, from the elemental concentrations, the as reported in the next section (cf. Fig. S10†). For the calcu- number Ag NPs and CeO2 NPs per surface area of the electrode lations we refer to the ESI.† A bias voltage U has been applied can be determined. The coatings have been done in a way that to the Au electrode in the buffer solution in which the CeO2 the surface coverage of CeO2 NPs is maintained at about NP coated Au electrode surface is immersed and controlled n(CeO2 NP) = 5.5 × 1010 cm−2, while the amount of immobi- versus an Ag/AgCl reference electrode. Upon pulsed white light lized Ag NPs n(Ag NP) is varied up to a maximum of n(Ag NP) = illumination, a modulated photocurrent can flow, driven by 6 × 1010 cm−2. As shown in Fig. 3b, the presence of Ag NPs light-generated electron–hole pairs. For details of the set-up we increases the photocurrent in a concentration-dependent refer to the ESI.† In Fig. 3a the amplitude of the photocurrent manner for positive and negative bias potential (i.e. for catho- I is plotted versus the bias potential U. As the CeO2 NPs absorb dic and anodic photocurrents) under white light illumination. light mainly in the UV region (see Fig. 2b), upon white light In a second approach, mixtures of CeO2 NPs and Ag NPs of illumination, only a small fraction of the incident photons lie different core diameter (CeO2: one diameter dc = 15.4 ± 1.6 nm in the appropriate spectral range (i.e. the energy of most Ag: three different diameters: dc = 5.8 ± 0.4 nm, 20 ± 1.2 nm, photons is smaller than the band gap of the CeO2 NPs) and and 40 ± 3.8 nm) have been applied to the surface. For these thus, the amplitude of the photocurrent is low. experiments the number of NPs has been fixed in the range: In order to introduce visible light sensitivity, we have intro- n(CeO2 NP) = 5.5 × 1010 cm−2 and n(Ag NP) = 6 × 1010 cm−2, see duced plasmonic Ag NPs as a light harvester, which shall allow Fig. 3c. Data show that the photocurrent upon white light illu- This journal is © The Royal Society of Chemistry 2022 Nanoscale, 2022, 14, 12048–12059 | 12051 View Article Online Paper Nanoscale This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 27 July 2022. Downloaded on 11/2/2022 7:53:33 AM. Fig. 3 (a) The photocurrent I of pure CeO2 NPs after their immobilization on the cysteamine-modified gold electrode in phosphate-buffered saline (PBS, 0.1 M, pH = 7.4) versus the applied bias voltage U under pulsed white light illumination and without illumination. n(CeO2 NP) = 5.5 × 1010 cm−2. The bias voltage U is the potential difference between the Au electrode and an Ag/AgCl electrode in the buffer. (b) Photocurrent I at different bias potentials U under white light illumination as recorded with electrodes coated with a mixture of CeO2 NPs and Ag (−) NPs with dc = 5.8 ± 0.4 nm. The surface coverage of CeO2 NPs has been maintained at n(CeO2 NP) = 5.5 × 1010 cm−2, whereas the surface coverage of the Ag (−) NPs n(Ag NP) was varied. (c) Photocurrent I in dependence of the bias potential U under white light illumination for mixtures of CeO2 NPs and Ag (−) NPs of different size for the surface coverage of n(CeO2 NP) = 5.5 × 1010 cm−2 (see Fig. S6 and S7†) n(Ag NP) = 6 × 1010 cm−2. The “(−)” refers to the negative charge of the Ag NPs by the citrate ligands. mination is more enhanced for the smaller Ag NPs (at the fact, the recorded photocurrents given in Fig. 4b demonstrate same number of NPs per surface area). This might be attribu- higher photocurrent for the mixture of Ag (+) NPs and CeO2 ted to the fact that for the smaller Ag NPs a larger fraction of NPs than for the mixture of Ag (−) NPs and CeO2 NPs. their surface may be in contact with the CeO2 NPs, see the In a third approach we have applied hybrid NPs since an sketch in Fig. 3c. For this reason, all further measurements in even closer contact can be obtained when the CeO2 NPs are this work have been done with Ag NPs of dc = 5.8 ± 0.4 nm. directly grown on top of the plasmonic Ag NPs. Indeed, data From these experiments we can conclude that light-excited from Fig. 4b demonstrate that the photocurrent is highest for charge carriers in the plasmonic Ag NPs can be used for photo- hybrid Ag/CeO2 NPs. In addition, the stability of the photo- current generation with the electrode-fixed CeO2 NPs. This current in the hybrid Ag/CeO2 NPs is higher than for the mix- finding is also following the observation in a previous study by tures of Ag NPs and CeO2 NPs (see Fig. S10†). This is most mixing Au NPs and CeO2 NPs.3 One can assume electron trans- likely due to a more defined interface in the hybrid Ag/CeO2 fer as an essential mechanism. Such charge carrier transfer NPs which may also decrease photocorrosion. consequently should be distance-dependent, i.e. more effective the closer the connection between the CeO2 NPs and the plas- Photocurrent measurements of multiple layers of CeO2 NPs in monic NPs is. For the case of Ag NPs we thus, have tried to presence of different types of Ag NPs reduce the distance between the Ag NPs and the negatively One advantageous property of applying NPs on top of electro- charged CeO2 NPs by using positively-charged Ag NPs. In des is the potential for multilayer formation. Since inter-par- Fig. 4 a comparison of negatively and positively charged Ag ticle electron transfer is feasible, this can result in significant NPs is shown. Positive charge has been obtained by replacing photocurrent amplification. Consequently, multiple layers of the citrate ligand by a polyallylamine hydrochloride – PAH – at Ag NPs and CeO2 NPs have been arranged on top of the gold the Ag NPs. As given in the sketch of Fig. 4a we anticipate that working electrode using the layer-by-layer assembly and poly the distance between Ag (+) NPs and CeO2 NPs should be (diallyldimethylammonium chloride) (PDDA) as polyelectro- closer than the distance between Ag (−) NPs and CeO2 NPs. In lyte. For the case of mixtures of Ag NPs and CeO2 NPs here 12052 | Nanoscale, 2022, 14, 12048–12059 This journal is © The Royal Society of Chemistry 2022 View Article Online Nanoscale Paper This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 27 July 2022. Downloaded on 11/2/2022 7:53:33 AM. Fig. 4 (a) Sketch showing the closer distance between Ag (+) NPs and CeO2 NPs due to electrostatic attraction as compared mixtures of Ag (−) NPs and CeO2 NPs. A closer distance will facilitate charge transfer between both types of NPs. (b) Comparison of photocurrents are recorded with single layers of pure CeO2 NPs, mixtures of CeO2 NPs and Ag (−) NPs, mixtures of CeO2 NPs and Ag (+) NPs, and hybrid Ag/CeO2 NPs. All data were obtained under pulsed white light excitation in PBS (0.1 M, pH = 7.4). The “(−)” and “(+)” refer to the charge of the Ag NPs introduced by different ligands used, citrate and PAH respectively. The NP densities are similar to those in Fig. 3c (see Fig. S8 and S9†). and in the following only the negatively charged Ag (−) NPs are photocurrent only in the UV spectral range, i.e. there is no sig- used to assemble them in the same layer as the negatively nificant photocurrent for wavelengths 405 nm and above. This charged CeO2 NPs. The Ag (+) NPs with better PEC response is valid in the negative and positive potential range. The photo- have not been used to form these mixed layers because the current maximum cannot be reached, as the absorption positively charged PDDA could not be applied to immobilize maximum of CeO2 NPs is at 270 nm (cf. Fig. 2b) and thus the likewise charged Ag (+) NPs. Data in Fig. 5 exemplify that below the lowest used wavelength of 340 nm. For the CeO2 NP for all three cases: CeO2 NPs only, mixtures of Ag NPs and and Ag NP mixtures and for hybrid Ag/CeO2 NPs there is CeO2 NPs, and hybrid Ag/CeO2 NPs, the photocurrent clearly a photocurrent peak at the illumination between increases with the number of NP layers (both for the anodic measurements using the filters of 405 nm and 470 nm, which and cathodic current direction). Based on the comparison fits to the surface plasmon peak of the Ag NPs at about between Fig. 5b and c, it can be seen that the multilayers of 421 nm (cf. Fig. 2a and c). the Ag (−) NP and CeO2 NP mixture have always a larger photo- While the shape of the wavelength-dependence of the current than the multilayers formed by CeO2 NPs only. As the photocurrents is the same for both types of NP arrangements, CeO2 NP numbers in the just CeO2 NP layers and in the CeO2 the photocurrent is higher for the hybrid Ag/CeO2 NPs than for NP/Ag (−) NP mix layers are controlled to be rather similar the CeO2 NP and Ag NP mixtures. This confirms again that when (Fig. S10a†), it can be verified that the Ag (−) NPs can enhance surface plasmon excitation is applied for photocurrent enhance- the photo-electrochemical (PEC) response of the CeO2 NPs. ment this effect is larger for the closest connection between the Likewise, the hybrid Ag/CeO2 NPs have better PEC perform- two parts, i.e. for the case of hybrid Ag/CeO2 NPs. Note that also ance than the Ag (−) NP and CeO2 NP mixtures. Also here, in for the data recorded in Fig. 6 the NPs surface coating densities each layer, the number of CeO2 NPs (Fig. S10a†) and Ag NPs have been determined to be rather similar (ESI, Fig. S10†), thus (Fig. S10b†) has been controlled to be similar for the Ag (−) NP the enhancement effect is not due to different coating densities. and CeO2 NP mixture and the hybrid Ag/CeO2 NPs. The high For low wavelengths, the photocurrents of the CeO2 NP and Ag photocurrent is likely due to the fact that in the hybrid Ag/ NP mixtures and the hybrid Ag/CeO2 NPs are similar to the CeO2 NPs there is intimate contact between CeO2 and Ag, but photocurrent for the CeO2 NPs. At these low wavelengths the Ag in the mixture of Ag (−) NPs and CeO2 NPs the distance NPs do barely absorb light, and thus, charge transfer between between CeO2 and Ag is larger. In addition, photocurrents are the Ag NPs and the CeO2 NPs does not contribute to the photo- also most stable for the hybrid Ag/CeO2 NPs configuration (cf. current generation. As another control it should be also men- Fig. S13 and S14†). tioned here that Ag NPs alone – fixed on Au/cysteamine – do not result in photocurrent generation. Experimental and theoretical consideration about the Since a significant photocurrent can be obtained for the wavelength-dependence of the photocurrent combination of the two types of NPs at wavelengths where the While the data shown in Fig. 3–5 have been obtained under Ag and the CeO2 alone cannot generate a photocurrent, this white light illumination, in Fig. 6 the photocurrent for the points strongly to a charge transfer between the two types of different types of NP configurations on the modified gold elec- NPs. Furthermore, this strong photocurrent seems to be trodes is provided for illumination with different distinct wave- coupled to the plasmon excitation inside the Ag NPs. Thus, lengths, which are obtained using optical filters (340 ± 26 nm, the experiments clearly support our hypothesis that the exci- 405 ± 10 nm, 470 ± 40 nm, 540 ± 25 nm, 620 ± 20 nm, 725 ± tation of surface plasmon states in the metal NPs can be 50 nm, and 870 ± 50 nm). In the case of CeO2 NPs there is a advantageously applied for photocurrent generation. This journal is © The Royal Society of Chemistry 2022 Nanoscale, 2022, 14, 12048–12059 | 12053 View Article Online Paper Nanoscale This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 27 July 2022. Downloaded on 11/2/2022 7:53:33 AM. Fig. 5 (a) Sketch showing the layer-by-layer (LbL) assembly used to produce multiple layers of NPs by means of a polyelectrolyte (10% PDDA, layer number: N, cf. Fig. S10†). Photocurrents at different bias U are shown in dependence of the layer number N for (b) CeO2 NPs, (c) CeO2 NP and Ag (−) NP mixtures, and (d) hybrid Ag/CeO2 NPs. Measurements are carried out in 0.1 M PBS ( pH = 7.4) under white light illumination. In principle it should be possible to explain the wavelength to zero potential, and thus the applied bias U at the working dependence of the photocurrent with an energy band diagram. electrode is set against the reference electrode. A negative bias This is however complicated by the fact that some values U < 0 leads to a positive electronic energy level E = −e·U, with needed for this are not exactly known (such as the precise −e being the charge of one electron. In Fig. 7a first the situ- band gap of the CeO2 NPs). As the NPs are no tight core/shell ation without the plasmonic part, i.e. CeO2 NPs alone are systems, but the CeO2 shells around the Ag cores are not shown under bias U = −0.5 V. At the Au electrode – CeO2 NP compact, the electrolyte can be in contact with both, the CeO2 junction electrons cannot be transferred from the working NPs and with the Ag NPs. In addition, also the working elec- electrode into the conductance band (CB), but only into the trode can be connected with both NP species. Last, but not valence band (VB) of the CeO2 NPs, as electrons can only flow least also the redox couples in the electrolyte are not unequivo- into lower electronic levels. From the valence band of CeO2 no cally known. Thus, a model for an energy band diagram of the electron transfer to solution is possible. However, in case of here used NP geometry needs to be based on certain assump- light excitation (h·ν) with energy greater than the band gap of tions. At any rate it needs to demonstrate that at light exci- the CeO2 NPs electrons can be excited from the valence to the tation at the surface plasmon absorption peak of Ag, there are conductance band, where they are consumed for the reduction charge carriers in the CeO2 NPs. In general, we assume that of a suitable electron acceptor (e.g. hydrogen peroxide). Thus, the photocatalytic reaction takes place at the CeO2 NPs. This the current can only flow in case of UV light excitation. assumption is based on the rather similar potential behavior The situation changes if Ag NPs are introduced, cf. Fig. 7b. of the hydrogen peroxide reaction which will be used in the Now electrons can be injected from the working electrode into next chapter as electron acceptor to demonstrate sensing the Fermi level of the plasmonic Ag NPs. Upon excitation in applications. the purple blue range corresponding to the surface plasmon In Fig. 7 a semi-quantitative diagram of the energy levels in resonance energy of the Ag NPs (i.e. at lower energies com- case of negative bias voltage is given which could explain the pared to the CeO2), a current can flow. If electrons in the Ag observed behavior. Here the Ag/AgCl reference electrode is set NPs are excited at the plasmon resonance frequency, they have 12054 | Nanoscale, 2022, 14, 12048–12059 This journal is © The Royal Society of Chemistry 2022 View Article Online Nanoscale Paper This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 27 July 2022. Downloaded on 11/2/2022 7:53:33 AM. Fig. 6 Wavelength dependence of the photocurrent for bias U of (a) −500 mV, (b) +500 mV, (c) −300 mV, and (d) +300 mV versus an Ag/AgCl refer- ence electrode in 0.1 M PBS ( pH = 7.4). The cysteamine modified gold working electrode is coated with 5 layers of either pure CeO2 NPs, a mixture of CeO2 NPs and Ag (−) NPs, Ag/CeO2 hybrid NPs, and pure Ag NPs. For the illumination discrete wavelengths are used as obtained by optical filters in front of a white light source. The data points are connected by polynomial fits, and thus the “negative” photocurrent in (b) for CeO2 NPs around 420 nm is a fit artifact. Fig. 7 Plausible band diagram for negative applied bias involving (a) CeO2 NPs and (b) Au/CeO2 NPs under applied bias U = −0.5 V. The energy of the electronic states −e·U is plotted in reference to the Ag/AgCl electrode which is set to zero potential. As electron acceptor hydrogen peroxide is chosen since with this redox molecule application for sensing will be demonstrated. Additional information on the band gap of the CeO2 NPs and potential values used can be found at the end of the ESI.† This journal is © The Royal Society of Chemistry 2022 Nanoscale, 2022, 14, 12048–12059 | 12055 View Article Online Paper Nanoscale sufficient energy to be transferred to the conduction band of prised the background current without illumination plus the the CeO2 NPs, where again electrons can be consumed for the chopped unrectified photocurrent due to the modulated illu- reduction of an electron acceptor. Thus, introduction of the Ag mination. The current I′ is thus, given for the results of NPs allows to reduce the required light excitation from the UV chopped light voltammetry (light pulses of 50 s length; scan range to the purple blue range for obtaining a photocurrent. rate 5 mV s−1), the displayed photocurrent I in contrast, is the lock-in rectified current difference arising from the change of Photoelectrochemical detection of H2O2 and glucose dark to illuminated conditions (i.e. photocurrent only). CeO2 has pronounced catalytic properties and this can be used From the voltammetric current response on the left side of e.g. for the conversion of hydrogen peroxide. This feature can Fig. 8(a, c and e) one can clearly see that the different electro- This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. also be applied in a photoelectrochemical setup as recently des have a rather similar and moderate electrochemical activity shown by us.3 In order to demonstrate the general applicability for oxygen reduction (background current behavior in the Open Access Article. Published on 27 July 2022. Downloaded on 11/2/2022 7:53:33 AM. of the concept of enhancing the photocatalytic properties of a periods without illumination). The addition of hydrogen per- semiconductor nanomaterial by combining it with metal NPs oxide in contrast, results in a more pronounced electro- showing plasmon excitation, we have analyzed the electro- chemical reduction of this acceptor molecule. The comparable chemical and photoelectrochemical properties of the different electrochemical activity of all three electrodes also underlines NP combinations on a gold electrode. In Fig. 8 the current the rather similar amount of CeO2 NPs on the three different response I′ versus the bias voltage U is given for working elec- electrode systems, since this material is responsible for the trodes with 5 layers of CeO2, mixtures of CeO2 NPs and Ag observed activity. Due to the use of a white light source for the NPs, and hybrid Ag/CeO2 NPs in PBS, saturated with air or intermittent illumination nearly no photocurrent can be gener- with 5 mM H2O2 concentration. Note that the current I′ com- ated at the CeO2/Au electrode. In contrast, a cathodic photo- Fig. 8 Chopped light voltammetry recorded on working electrodes coated with 5 layers of (a and b) CeO2 NPs, (c and d) CeO2 NP Ag (−) NP mixture, and (e and f ) hybrid Ag/CeO2 NPs. On the left side the current I’ as recorded versus the applied bias U in PBS (which has been in equilibrium with the O2 in the air) and PBS with 5 mM H2O2 under white light illumination is shown. On the right side the corresponding photocurrents I at different bias U = −100 mV, −300 mV, and −500 mV versus an Ag/AgCl reference electrode are shown in dependence of the H2O2 concentration. 12056 | Nanoscale, 2022, 14, 12048–12059 This journal is © The Royal Society of Chemistry 2022 View Article Online Nanoscale Paper This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 27 July 2022. Downloaded on 11/2/2022 7:53:33 AM. Fig. 9 (a) Glucose detection using the electrodes coated with 5 layers of hybrid Ag/CeO2 NPs at U = −500 mV in PBS with 10% fetal bovine serum from Gibco with dissolved GOx (160 kDa, 1 mM, activity = 10 kU g−1) under white light illumination. (b) Selectivity performance of the PEC glucose measurements with different common interfering substances that have been added to the 10% fetal bovine serum at U = −500 mV. The concen- tration of the glucose is 1 mM and the concentration of the other analytes is 10 mM. current can be generated at Au electrodes modified with a As an outlook of this study we have also been testing mixture of CeO2 and Ag NPs and with the hybrid Ag/CeO2 NPs whether this NP electrode can be coupled to an enzymatic (its magnitude appears small, but this is only because of the reaction. For this proof-of-principle measurement glucose significant background current). This photocurrent is signifi- oxidase (GOx) has been applied in solution and different cantly enhanced when hydrogen peroxide is added to the solu- glucose concentrations have been added. Data given in Fig. 9 tion. This photocurrent response is highest for the case of the for a 5-layer electrode applying the hybrid NPs show that con- hybrid Ag/CeO2 NPs (Fig. 8e, blue graph). centration dependent signals can be obtained. The experiment By fixing the potential at different cathodic values and ana- also exemplifies that the photoelectrochemical response of the lyzing the photocurrent behavior in the presence of different Ag/CeO2/Au electrode can be maintained in protein-containing hydrogen peroxide concentrations a more sensitive detection solutions (since fetal bovine serum (FBS) has been added into of the reaction capabilities can be obtained. The results have the solution) and in the presence of millimolar sugar concen- been summarized on the left side of Fig. 8(b, d and f ) for the trations. This may provide the basis for further sensor develop- three electrode systems studied. Here it becomes clear, that all ments. These first experiments also demonstrate that many three electrodes can show a concentration- and bias-dependent potentially interfering substances do not result in photo- photocurrent originated by photoelectrochemical reduction of current changes (see Fig. 9b). This is mainly connected to the hydrogen peroxide. So even for the inefficiently excited CeO2 fact that here hydrogen peroxide reduction is exploited and NPs there is a response. However, when comparing the photo- thus, rather low electrode potentials can be used. current magnitude of the systems it is obvious, that the appli- cation of the hybrid NPs tremendously amplifies the photo- response (about 1.5 orders of magnitude). The results also Conclusions demonstrate that the hydrogen peroxide reduction can be further enhanced by decreasing the electrode potential. This is We have systematically investigated the possibilities for enhan- valid for all three NP-modified electrodes and thus, points to a cing the charge carrier generation and thus, photocurrent similar reaction mechanism of hydrogen peroxide conversion generation by combing the CeO2 NPs with Ag NPs. In this way at the CeO2 surface. However, for the hybrid NPs we have the catalytic properties of CeO2 NPs have been combined with already at a rather moderate electrode potential and a very pro- the surface plasmon excitation capability of Ag NPs in a photo- nounced response allowing a defined concentration analysis. electrochemical detection system based on NP-modified gold Consequently, high overpotentials can be avoided which may electrodes. We exploit the photocurrent generation with the cause unwanted interfering reactions when such electrodes electrode-fixed CeO2 NPs and use the property of hydrogen per- will be applied for sensing. For the 5 layers CeO2 NPs, the sen- oxide conversion at this material. Such NP-electrode structures sitivity and limit of detection (LOD) at −500 mV are 0.1 nA are limited with respect to the excitation wavelength. In a first μM−1 and 12 μM, and the linear range is from 20 μM to approach we have mixed both types of NPs and studied the 400 μM. For the 5 layers mixture NPs, the sensitivity and LOD influence of the size of the metal NPs. In a second approach at −500 mV are 0.23 nA μM−1 and 2.6 µM, and the linear range we succeed to show that a closer contact of the semiconductor is 20–1000 μM. For 5 layers hybrid NPs, the sensitivity and with the metal NPs is essential for signal enhancement and in LOD at −500 mV are 2.5 nA μM−1 and 200 nM, and the linear a third approach we have used hybrid NPs with the CeO2 range is 0.2–1000 μM. directly grown on the Ag core. By fixing these hybrid NPs on a This journal is © The Royal Society of Chemistry 2022 Nanoscale, 2022, 14, 12048–12059 | 12057 View Article Online Paper Nanoscale Table 2 Comparison of the H2O2 sensing performance of difference assays. CDs = carbon dots; PDI = N,N’-dicarboxy methyl perylene diimide; NWs = nanowire; LOD = limit of detection Read-out Transducer Nanomaterial LOD Linear range Ref. Electrical Photoelectrochemical CeO2/Ag NPs 200 nM 0.2–1000 μM This work CeO2/Co3O4 NPs 86 µM — 43 CeO2/Au NPs 3 μM 4 μM–2000 μM 3 CeO2/Co NPs 3.7 μM 5–10 μM 44 CeO2/MnO2 NPs 0.07 μM 45 PbS NPs 18 μM 46 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. CdS QDs 0.06 μM 19–356 μM 47 ZnO NPs 0.27 μM 0.1–100 μM 48 Au NPs 2 μM 30 μM–5 mM 27 Open Access Article. Published on 27 July 2022. Downloaded on 11/2/2022 7:53:33 AM. Electrochemical CeO2/graphene NPs 1 μM 2.8–160 μM 49 CeO2 Pt/C NPs 2 μM 0.01–30 mM 50 CeO2/Pt NPs 0.47 μM 3.3–17.5 mM 51 CeO2/Au NPs 7 μM 0.05–2.5 mM 52 CeO2/polyaniline NPs 50 mM 53 Optical Fluorescence spectroscopy CeO2/CDs NPs 0.35 µM 1.7 µM–2 mM 54 Colorimetric assay CeO2/PDI NPs 2.23 μM 55 Absorption spectroscopy CeO2/C NWs 0.69 µM 1–100 μM 56 CeO2/CePO4 NPs 2.9 µM 5–150 μM 57 CeO2/Cu2(OH)3Cl NPs 50 µM 0.02–0.05 mM 58 CeO2/TiO2 NPs 3.2 µM 5–100 μM 31 cysteamine modified gold electrode we have obtained the 390715994. SZ acknowledges funding by the Chinese highest photocurrents with a visible light source. Additionally, Scholarship Council (CSC). NGB and VP acknowledge financial it can be shown that the application of multilayers of such support from the Spanish Ministerio de Ciencia, Innovación y hybrid NPs can be applied for adjusting the sensitivity in a Universidades (MCIU) (RTI2018-099965-B-I00, AEI/FEDER, defined way. UE). ICN2 is supported by the Severo Ochoa program from In order to demonstrate the involvement of surface Spanish MINECO (SEV-2017-0706) and is funded by the plasmon state excitation inside the metal NPs in the photo- CERCA Programme/Generalitat de Catalunya. ZY acknowledges current generation we have analyzed the photocurrent support from the National Natural Science Foundation of response wavelength dependent. Clearly the excitation of the China (Grant No. 61871240). metal part influences the photocurrent thus, implicating charge carrier transfer from/to the CeO2 NPs. The observed effects can be rationalized with the help of a plausible energy References diagram for the reductive reaction pathway. The experiments also verify previous observations in combining a semi- 1 B. Gao, X. Zhao, Z. Liang, Z. Wu, W. Wang, D. Han and conductor with Au NPs and may open a new route for L. Niu, Anal. Chem., 2020, 93, 820–827. improved sensing strategies applying hybrid nanostructures on 2 N. Fu, L. 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