Cu 2 ZnSnS 4 -Ag 2 S Nanoscale P-N Heterostructures as Sensitizers for Photoelectrochemical Water Splitting

A cation exchange-based route was used to produce Cu 2 ZnSnS 4 (CZTS)-Ag 2 S nanoparticles with controlled composition. We report a detailed study of the formation of such CZTS-Ag 2 S nano-heterostructures and of their photocatalytic properties. When compared to pure CZTS, the use of nanoscale p-n heterostructures as light absorbers for photocatalytic water splitting provides superior photocurrents. We associate this experimental fact to a higher separation efficiency of the photogenerated electron–hole pairs. We believe this and other type-II nano-heterostructures will open the door to the use of CZTS, with excellent light absorption properties and made of abundant and environmental friendly elements, to the field of photocatalysis.


Introduction
Colloidal nanocrystals are highly suitable building blocks for the fabrication of energy conversion and storage devices.ACS Nano, 2015, 9, 1012-1057] However, current functional nanomaterials must meet too many very demanding properties not realizable with a unique compound.Thus the design of multicomponent nanostructures with improved, multiple and/or new physical-chemical properties, such as enhanced photoluminescence, [4][5] modified magnetic behavior, [6] improved (photo)catalytic performances, [7][8][9] and superior thermoelectric efficiencies, [10] is essential.In particular, p-n junction heterostructured nanoparticles provide new ways to manipulate electron and hole wave functions. [11]4][15] Several synthesis strategies have been developed to produce type-II nano-heterostructures.
Among them, ion-exchange methods based on solubility constant difference have been proven as particularly suitable. [16][22] Within this group of materials, a particularly environmental friendly direct band gap semiconductor is Cu2ZnSnS4 (CZTS).In the last decade, CZTS has attracted much attention in the field of thin film photovoltaics as alternative to CdTe and CuIn1-xGaxSe2.CZTS uniquely combines both, outstanding optoelectronic properties, and a composition based on elements that abundant in the Earth's crust.Particularly, its direct band gap at 1.5 eV matches well with the energy requirement for solar water splitting, what makes CZTS a strong candidate for water splitting. [23]However, in spite of its outstanding properties, and except for few examples on the photocatalytic generation of hydrogen and other value-added chemicals, [24][25][26][27][28] this material has been mostly ignored in the photocatalysis field.This is in part due to its high surface charge carrier recombination, which becomes critical in high surface area materials as those required for photocatalysis.
One solution to minimize recombination of photogenerated charge carriers is the rapid spatial separation of electrons and holes in heterostructures.[31][32][33][34][35] However, due to the difficulties in tuning the composition, phase, size and shape of such complex materials, the preparation of CZTS-based nanoscale p-n junction heterostructures still remains challenging.
In this work, we demonstrate that partial cation exchange is an excellent strategy to produce CZTS-based nanoheterostructures due to the high mobility of cations in the CZTS lattice.In particular, we describe the preparation of CZTS-based type-II nanoheterostructures via cation exchange of CZTS with Ag + ions to form CZTS-Ag2S nanoparticles.The progressive exchange in the initial CZTS nanoparticles by Ag + ions allows controlling the relative ratio between the two compounds.We further demonstrate here that the use of CZTS-Ag2S nano-heterostructures as sensitizers provides significantly enhanced photocurrent response for photoelectrochemical water splitting under visible-light illumination.

Synthesis of CZTS-Ag 2 S heterostructures
Quasi-spherical CZTS nanoparticles with wurtzite structure were prepared using the method we described before 25 by the reaction of copper, tin and zinc salts with a mixture of tertdodecylmercaptan and dodecanethiol in the presence of OLA.In a typical synthesis, 5.4 mmol of CuCl2•2H2O, 4.8 mmol of ZnO, and 1.8 mmol of SnCl4•5H2O were dissolved in THF.Then, 24 mmol of distilled OLA and 20 g of ODE were added to the reaction mixture.The solution was heated to 175 °C under argon flow and maintained at this temperature for 1 h.After purging, the mixture was cooled to 100 °C, and 50 mmol of tert-dodecylmercaptan and 5 mmol of dodecanethiol were added.The solution was then heated to 250 °C and kept for 1 h.The obtained CZTS nanocrystals were thoroughly purified by multiple precipitation and re-dispersion steps using 2-propanol and chloroform.
To produce CZTS-Ag2S heterostructures, first, a stock precursor solution of AgNO3 in OLA was made by dissolving 0.34 g AgNO3 (2.0 mmol) in 10.0 mL OLA at 60 °C.Dissolution took ~15 min under magnetic stirring and N2 atmosphere.When cooled down to room temperature, the colorless solution gradually turned into a white waxy paste.Then, 1 mL THF dispersion of the CZTS nanoparticles (~40 mg) was mixed with 10 mL ODE and 0.5 mL OLA under N2 atmosphere.The solution was maintained under N2 flow at 120 °C for 30 min to remove THF.
Depending on the relative CZTS/Ag2S ratio targeted, between 2 and 6 mL of AgNO3 stock solution was quickly injected into the CZTS nanoparticle dispersion at 120 °C.The reaction was maintained at this temperature for 30 seconds before it was rapidly cooled down to room temperature using a water bath.Nanoparticles were finally washed by multiple precipitation and re-dispersion steps using toluene and ethanol.

Characterization techniques
The morphological, chemical and structural characterization of the nanoparticles was carried out by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM).Carboncoated TEM grids from Ted-Pella were used as substrates.HRTEM images were obtained using a JEOL 2010F field-emission gun microscope with a 0. Photoelectrochemical response was measured using a CHI852C electrochemical workstation with conventional three-electrode setup under visible-light illumination.Fluorine doped tin oxide (FTO) glass substrates were cleaned with acetone and ethanol, and dried with pure nitrogen.To prepare the electrode, aqueous TiO2 paste (25% w/w TiO2, 10% w/w polyethylene glycol (MW 20 000)) was doctor bladed onto 1×1 cm 2 FTO substrates.The films were then calcined at 450 • C for 2 h.Then, the as-prepared CZTS-Ag2S were first dispersed in 1 mL toluene with concentration of 1 mg/mL followed by sonication.After that, the solution was deposited on the working electrode substrate by spin-coating (rotation speed: 3,000 rpm).A platinum wire and Ag/AgCl were used as the counter and reference electrodes respectively.0.1 M Eu(NO3)3 aqueous solution were used as electrolyte.The illumination source was a 300 W xenon lamp with a cut-off filter (λ>420 nm) to provide visible light illumination.

Results and Discussion
Figure 1a shows a representative TEM micrograph of the CZTS nanocrystals used to produce CZTS-based nanoheterostructures. CZTS nanocrystals were highly monodisperse, with an average size of 12±1 nm, and presented quasi-spherical geometries with faceted surfaces.CZTS-Ag2S heterostructures were obtained by adding Ag + ions to a solution containing CZTS nanoparticles at 120 °C.CZTS-Ag2S heterodimers were formed by the preferential cation exchange at the vertexes of the hexagonal structures, which are the most reactive sites (Figure 1b-d).JACS 2009, 131, 5285] When increasing the amount of Ag + precursor added, the interface between the two compounds, CZTS and Ag2S, moved inward the CZTS nanoparticle.The diameter of the Ag2S nanocrystal increased from ~2 nm (Figure 1b, sample H1), to ~4 nm (Figure 1c, sample H2) and ~7 nm (Figure 1d, sample H3) when the amount of Ag + precursor used increased from 0.4 mmol, 0.8 mmol and 1.2 mmol, respectively.In accordance with the rapid kinetics characterizing cation exchange reactions, [36] the reaction was completed in less than 5 seconds and a further prolongation of the reaction time did not translate in any morphology change.XRD patterns of the CZTS nanocrystals before and after the addition of increasing amounts of Ag + cations are shown in Figure 2. Initial CZTS nanoparticles presented a wurtzite crystallographic structure. [37]As increasing amounts of Ag + were added, monoclinic Ag2S (JCPDS card no.14-0072) was formed, as it can be observed by the higher intensities of the peaks associated to Ag2S crystallographic phase.Energy-dispersive x-ray spectroscopy (EDX) analysis (Table S1) verified the increasing ratio of [Ag]/[Cu+Zn+Sn] when increasing the amount of Ag + ions introduced.Further confirmation of the element oxidation states was obtained by XPS.Besides the existence of Cu, Zn, Sn, S, the XPS spectrum for both sample H1 and H3 (Figure S1) also showed the components at 368.0 and 374.0 eV, which were attributed to Ag 3d5/2 and Ag 3d3/2 of Ag + ions in the Ag2S phase.Based on these results, we believe Cu, Zn and Sn atoms in CZTS behave virtually like a "fluid", which diffuses towards the solid-liquid interface through the anion sub-lattice and is replaced by Ag + , with a slight preferential exchange of Zn 2+ ions by Ag + .The exchange is in part driven by the extremely small solubility constant of Ag + , Ksp=1.0×10 -49 at 18 °C. [38]The nanoparticle shape is conserved because of the lower activation barriers for the diffusion of smaller cations compared to anions, which sub-lattice integrity is conserved.HRTEM analysis confirmed the crystallographic phase of both compounds within the CZTS-Ag2S nano-heterostructure and allowed determining the structural relationship at the interface.Pradhan group. [39]s a proof-of-concept application, CZTS-Ag2S nanoparticles were used as sensitizers in a photoelectrochemical cell (PEC) for water splitting.Within the PEC cell, electron-hole pairs are generated under visible light illumination.Photogenerated holes are used to oxidize H2O and produce O2 and the photogenerated electrons are driven towards the Pt electrode to produce hydrogen.As shown in Figure 4a, the photocurrent of TiO2/CZTS-Ag2S nanoheterostructures (H1) at 0.5 V is 0.15 mA/cm 2 , what is much higher than that of the electrode made from pure TiO2 (0.03 mA/cm 2 ) or that of the one incorporating CZTS nanocrystals (TiO2/CZTS, 0.06 mA/cm 2 ).These values indicate that the CZTS-Ag2S heterostructure promotes photocatalytic activity under visible light illumination.This result was further supported by the corresponding amperometric I-t cycles.Figure 4b shows the rapid and consistent photocurrent responses for each switch-on and -off event in multiple 10 second on-off cycles under visible-light illumination.The sample H2 showed the highest photocurrent density (ca.0.58 mA/cm 2 ), which was about 10 times higher than that of the pure CZTS electrode (ca.0.06 mA/cm 2 ).However, the photocurrent decreased when further increasing the amount of Ag2S (sample H3).This indicates that an optimal compositional ratio of the two phases exist that provides the highest photocatalytic activity as will be discussed below.The corresponding amperometric I-t curves under switching light ON-OFF at the applied potential of 0.5 V.
Main key parameters determining the PEC activity are charge carriers photogeneration, separation and transport efficiency. [40]To be able to account for the performance enhancement, light absorption and charge separation and transfer properties of the CZTS-Ag2S (H2) heterostructures were studied.As shown in Figure S2, just a slight extension of the absorbance edge toward the infrared was observed after coupling CZTS (1.5 eV) with Ag2S.This observation agrees well with the fact that 4 nm Ag2S nanocrystals do not exhibit quantum confinement effects and thus possess a bulk-like band gap (∼ 1.0 eV). [41]To better understand the electronic properties of CZTS-Ag2S heterostructures, Mott-Sckottky (M-S) measurements were performed in dark using impedance spectroscopy.Figure S3 shows M-S plots of TiO2 and CZTS-Ag2S (H2) sensitized TiO2.The flat band potential for TiO2, calculated from the x intercept of the linear region, was found to be -0.8V vs. Ag/AgCl.Just a slight positive shift of potential was found for the TiO2-CZTS-Ag2S sample.This sample was also characterized by a lower slope of the linear region, which suggested a much higher donor density.The energy band structures of TiO2, Ag2S and CZTS are plotted in Figure 5a.The Fermi level in n-type TiO2 and Ag2S are close to their conduction band minimum (CBM), while that of p-type CZTS are close to its valence band maximum (VBM).As the CBM and VBM of p-type CZTS are situated at higher energy levels than Ag2S, [42] under visible-light irradiation, photogenerated electrons in the CB of CZTS will transfer to the CB of Ag2S, while holes travel in the opposite direction within the VB, as shown in Figure 5a.This type-II band alignment in the CZTS-Ag2S nanoscale p-n heterostructure increases the driving force for photogenerated charge carrier separation, thus reducing the chance for their recombination, increasing their lifetime and enhancing the overall material photocatalytic performance. [43]Besides, the relatively low CBM of Ag2S still facilitates the electron transfer to the CB of TiO2.In a third step, as shown in Figure 5b, the efficiently separated electrons are collected on the surface of electrodes and injected into the external circuit, which are subsequently consumed in the reduction of water into hydrogen, [44] while holes participate in the oxidation reaction taking place at the CZTS surface.While the combination of CZTS and Ag2S clearly results in an enhancement of the photocatalytic properties, it is also clear that an optimum composition exist.One possible explanation for the composition-dependent PEC activities measured might be ascribed to the structural change of CZTS in the presence of different concentration of Ag + .Along with the increase of Ag + concentration, the molar ratio of Zn/(Cu+Sn) decreases from 0.32 to 0.17.As previously reported, the content of Zn can affect the CBM of the particles. For CZTS-Ag2S nano-heterostructures, a narrowing of the CZTS band gap from 1.5 to 1.37 eV was observed when increasing the Ag+ concentration (Figure S4), what may difficult the transfer of electrons from CZTS to Ag2S.A second explanation for the PEC activity decrease with the increase of Ag + is related to the loss of effective donors for the oxidation reaction.The decreased amount of CZTS with good hole transport ability results in a lower donor density.Furthermore, excessive Ag2S in the heterostructures surface might reduce the CZTS reactive sites for the oxidation reaction, thus deteriorating the hole extraction from CZTS.7d]

Conclusions
In summary, a series of CZTS-Ag2S nanoscale p-n heterostructures with different CZTS/Ag2S ratios was obtained by partial cation exchange.From HRTEM characterization, we conclude that the heterojunction has coincident site epitaxy.Such CZTS-Ag2S nano-heterostructures exhibited significantly enhanced photoelectrochemical current response under visible-light illumination compared to pure CZTS.The ability to synthesize such structures with controlled composition and suitable band alignment promoting charge separation provides a better insight into fundamental synthetic chemistry as well as opening avenues for diverse optoelectronic applications.

Figure 5 .
Figure 5. (a) Schematic of the energy band diagram of the heterostructures and the possible