Study on Method and Mechanism of Noble Metals Photoelectric Deposition by Directly Utilizing Solar Energy

The recovery of noble metals is crucial in terms of resource utilization and ecological environment. Herein, a green strategy for the recovery of three noble metals, Au, Ag, and Pt, by photoelectric deposition at the cathode without external voltage is described. The realizability and mechanism of noble metals deposition are investigated and analyzed in terms of the band structure and the reduction potential of metal ions using the special heterostructure ZnO/ZnS composites derived from metal–organic frameworks, as well as ZnO and Fe2O3 semiconductors. It is found that ZnO/ZnS has preferable photoelectrochemical performance than pure ZnO due to their effective electron–hole separation and appropriate band matching structure. To deposit metals Au, Ag, and Pt, the potential of electrons in the conduction band of ZnO/ZnS should be more negative than the reduction potential of these metals, allowing for the deposition of these metals while simultaneously undergoing an oxygen evolution reaction, mediated by the photogenerated holes on the surface of the photoanode, and the collection of conduction band electrons by the back electrode.


DOI: 10.1002/aesr.202300061
The recovery of noble metals is crucial in terms of resource utilization and ecological environment.Herein, a green strategy for the recovery of three noble metals, Au, Ag, and Pt, by photoelectric deposition at the cathode without external voltage is described.The realizability and mechanism of noble metals deposition are investigated and analyzed in terms of the band structure and the reduction potential of metal ions using the special heterostructure ZnO/ZnS composites derived from metal-organic frameworks, as well as ZnO and Fe 2 O 3 semiconductors.It is found that ZnO/ZnS has preferable photoelectrochemical performance than pure ZnO due to their effective electron-hole separation and appropriate band matching structure.To deposit metals Au, Ag, and Pt, the potential of electrons in the conduction band of ZnO/ZnS should be more negative than the reduction potential of these metals, allowing for the deposition of these metals while simultaneously undergoing an oxygen evolution reaction, mediated by the photogenerated holes on the surface of the photoanode, and the collection of conduction band electrons by the back electrode.
In this study, we conducted a photoelectric deposition experiment utilizing semiconductors with varying conduction band levels (ZnO, Fe 2 O 3 , and ZnO/ZnS heterostructures) which can generate electrons with different electrical potentials.The deposition process is linked to the energy band of the semiconductor and the reduction/oxidation potential of the metal (oxide).We prepared ZnO/ZnS following the method of Zhou et al. which involves in situ growth of zeolite imidazole framework (ZIF-8) on semiconductor ZnO nanorods followed by vulcanization.The results indicate that the MOF-derived composites with heterojunctions exhibit enhanced photoelectrochemical properties compared to pure ZnO due to their efficient electron-hole separation and suitable band matching structure.Additionally, owing to the conduction band position of ZnO being more negative than the reduction potential of Au 3þ /Au, Ag þ /Ag, and Pt 4þ /Pt and the valence band (VB) energy level (E v ) being more positive than the oxidation potential of water helping facilitate the oxygen evolution reaction (OER), NMs can be deposited from the appropriate solution (HAuCl 4 , AgNO 3 , H 2 PtCl 6 ).Besides, Fe 2 O 3 can only deposit Au due to its more positive conduction band energy levels than the reduction potential of Ag þ /Ag and Pt 4þ /Pt.

Results and Discussion
Figure 2m demonstrates working mechanism of the photoelectric position of Au, Ag, and Pt (M).The Na 2 SO 4 solution and photoanode were poured into the anode chamber, the precursor solution and fluorine-doped tin oxide (FTO) as a blank substrate were placed in the cathode chamber, and the FTO and photoanode were connected with wires.Under the excitation of light energy, the semiconductor photoelectrode generates photogenerated electrons, which enter the cathode chamber through the wire and undergo reduction reaction with the corresponding precursor solution (HAuCl 4 , AgNO 3 , H 2 PtCl 6 ) to deposit Au, Ag, and Pt, while the photogenerated holes undergo OER reaction at the anode.The reactions occurring at cathode and anode are as follows Cathode∶ Figure 1 illustrates the synthesis process of the ZnO, ZnO/ZnS, and Fe 2 O 3 photoanodes.The morphologies of ZnO, ZnO/ZIF-8, ZnO/ZnS, and Fe 2 O 3 electrodes were characterized by field emission scanning electron microscopy (FESEM) (Figure 2a-h).Figure 2a shows that ZnO film is formed by nanorods perpendicular to the FTO substrate, with regular hexagonal edge structure on its surface, and a diameter measuring 320 nm and a length of approximately 3 μm (as shown in Figure 2e).It has been reported that nanostructured semiconductor materials grown perpendicular to the substrate can simplify the electron transfer path. [26]Therefore, the probability of recombination between photogenerated electrons and holes is reduced.The growth of ZIF-8 on the surface of the ZnO array is clearly visible in Figure 2b, indicating an in situ formation process, the growth thickness is about 2.8 μm (Figure 2f ), and the top profile is still clearly visible.Figure 2c shows the image after further vulcanization.The ZnO/ZnS array presents a uniform honeycomb structure with a thickness of 3 μm (Figure 2g).The 1D nanorod structure Fe 2 O 3 grows uniformly perpendicular to the FTO substrate with a thickness of about 600 nm (Figure 3h).
The X-ray diffraction (XRD) patterns of ZnO, ZnO/ZIF-8, ZnO/ZnS, and Fe 2 O 3 films are depicted in Figure 2i-l.Based on the findings in Figure 2i, it can be concluded that the identified peaks of ZnO are consistent with the hexagonal standard ZnO (JCPDS No. 36-1451).Furthermore, the significant peak observed at 2θ = 36.25°suggeststhat the nanorods have a preferred orientation at the (101) crystal plane.The peaks at 7.38°, 10.42°, 12.77°, and 18.04°are shown in Figure 2j, which are attributed to (011), (002), (112), and (222) reflections of ZIF-8. [27]The XRD analysis showed two minor peaks at 33.68°and 78.30°, which correspond to the (200) and (331) orientations of ZnS (JCPDS No. 79-0043), respectively.The weak characteristic peak intensity of ZnS may be caused by the in situ growth of ZIF-8 on ZnO surface and its further transformation into ZnS.It is evident that the discernible diffraction peaks of Fe 2 O 3 are in line with the standard card Fe 2 O 3 (JPCDS No. 33-0664) (Figure 2I).
The insets in Figure 3a-c are the UVÀvis diffuse reflectance spectra of ZnO, ZnO/ZnS, and Fe 2 O 3 photoelectrodes with absorption edges at 420, 410, and 600 nm, respectively.As previously reported, ZnS has a bandgap of approximately 3.67 eV and an absorption edge of about 340 nm. [28]As ZnO exhibits a narrower bandgap in ZnO/ZnS nanocomposites, the absorption peak of ZnO/ZnS is basically consistent with ZnO, and furthermore, the peak of ZnO/ZnS nanocomposites at 350 nm may correspond to the absorption of ZnS. Figure 3a-c shows the transformed Kubelka-Munk functions versus photoenergy, and the bandgap energy (E g ) can be estimated from the tangent intercept of the (αhν) 2 versus photoenergy.According to the formula αhν = A (hν À E g ) n/2 , the bandgap energies (E g ) of ZnO, ZnO/ZnS and Fe 2 O 3 are obtained to be 3.16, 3.14, and 2.09 eV, respectively, where ZnO, ZnS, and Fe 2 O 3 are all direct bandgap semiconductors and therefore n is taken as 1.Because ZnS has a larger bandgap, the bandgap width of ZnO is evident in the overall bandgap of ZnO/ZnS.
Figure 3d displays the photoluminescence (PL) spectra of both ZnO and ZnO/ZnS.It is evident that the PL signal of ZnO/ZnS composite structure is weaker than that of pure ZnO, implying that the introduction of ZnS helps to suppress the recombination of photogenerated electron and hole, and that the formation of a ZnO/ZnS heterostructure enhances the interfacial charge separation, resulting in improved charge transport performance.Figure 3e shows the linear sweep voltammetry (LSV) curves for the OER on ZnO, ZnO/ZnS, and Fe 2 O 3 electrodes under dark and light.ZnO, ZnO/ZnS, and Fe 2 O 3 photoelectrodes show high onset potential above %1.75V (vs RHE) for water oxidation in the dark.Under simulated sunlight, the onset potential of ZnO and ZnO/ZnS are reduced significantly to 0.38 V (vs RHE) and 0.13 V (vs RHE), and that of Fe 2 O 3 decreased slightly.The results show that all three materials have light-induced OER activity.In addition, the photocurrent density of ZnO/ZnS is always greater than that of ZnO with the increase of voltage.The photocurrent density of ZnO/ZnS (7.76 mA cm À2 ) is 1.6 times that of ZnO (4.83 mA cm À2 ) at 1.23 V (vs RHE), which indicates that ZnO/ZnS accelerates the oxidation kinetics of water under illumination and improves the photocatalytic performance compared with ZnO.To gain a deeper understanding of the charge transport behavior at the photoelectrode interface, we utilized electrochemical impedance spectroscopy (EIS) in our analysis, and its Nyquist plot is shown in Figure 3f.The size of the semicircle in an EIS plot is indicative of the charge transfer resistance. [29]Specifically, a larger semicircle corresponds to a higher charge transfer resistance.The EIS curves with semicircular shape and decreased in diameter for all three electrode spectra compared with that in the dark illustrate that light can reduce the load transfer resistance and thus reduce the electrode impedance.The spectral arch of the ZnO/ZnS electrode is smaller than that of the ZnO electrode, which represent that the addition of ZnS can reduce the resistance of charge carrier movement at the photoanode/electrolyte interface, resulting in faster charge transport and inhibiting the electron-hole pair recombination.Figure 3g,h reflects the transient photocurrent of ZnO and ZnO/ZnS, Fe 2 O 3 photoelectrodes.They exhibit a steady and prompt photocurrent response throughout the entire cycle of illumination.The photocurrent intensity of Fe 2 O 3 is about 1.4 μA cm À2 without external bias.The photocurrent intensity of ZnO/ZnS electrode is 0.2 mA cm À2 larger than that of ZnO and 1.7 times of ZnO photocurrent density, which indicates that both ZnO and ZnO/ZnS exhibit a powerful photocurrent, and the ZnO/ZnS heterostructure demonstrates a notably enhanced transfer rate of photogenerated electrons.The sharp peak observed in the electrochemical response curve indicates the build-up of a significant amount of photogenerated charge on the electrode surface following illumination.However, due to the presence of surface reaction resistance, only a portion of this charge can actively participate in the OER, while the remaining charge is prone to recombine, leading to the transient sharp peak.This phenomenon highlights the importance of optimizing the surface reaction kinetics to enhance the overall efficiency of the OER process. [30]In Figure 3i all present n-type semiconductor properties, and ZnS and ZnO form n-n homotypic heterostructure.For n-type semiconductors, there will be an upward band bend at the solution interface, which facilitates photogenic hole migration to the surface and thus promotes OER. [31]Figure S1, Supporting Information, shows that the FB potentials of ZnO, ZnO/ZnS, and Fe 2 O 3 are 0.42, 0.13, and 1.05 V (vs RHE), respectively.Compared with pure ZnO, the FB potential becomes more negative after the introduction of ZnS, which further demonstrates that the conduction band level positions of ZnS are more negative than that of ZnO, both of which form type II heterojunction, consistent with the literature. [32,33]igure 4a-c shows the photocurrent-time curves for the deposition of the NM ions Au, Ag, and Pt for 30 min.Upon deposition of gold, the photocurrent can reach 0.15 mA cm À2 for ZnO and 0.27 mA cm À2 for ZnO/ZnS.When Ag is deposited, the photocurrent is 0.60 mA cm À2 for ZnO; the photocurrent of ZnO/ZnS gradually decreases from 2 to 0.81 mA cm À2 .The photocurrent of ZnO/ZnS is 28 μA larger than that of pure ZnO for the deposition of Pt.Fe 2 O 3 is used as a photoanode for the deposition of gold, silver, and platinum; the photocurrent can only reach the level of microcurrent.The above test results make known that ZnO/ZnS electrode produces more current and has better performance than ZnO electrode.The reduction potentials of Au 3þ to Au, Ag þ to Ag, and Pt 4þ to Pt are measured in chloroauric acid, silver nitrate, chloroplatinic acid solutions with mass fractions of 0.02% utilizing a three electrodes system.The reduction peak at 1.24 V (vs RHE) indicates that Au 3þ begins to be reduced to Au, and the reduction potential of Au 3þ /Au is 1.24 V (Figure 4d).Similarly, the reduction potentials of Ag þ /Ag and Pt 4þ /Pt are 0.86 and 0.84 V (vs RHE) (Figure 4e,f ), respectively.The XRD patterns of Au, Ag, and Pt deposited by illuminating photoanode are shown in Figure 5a-c.The diffraction peaks centered at 38.2°and 44.4°can be attributed to the characteristic peak of Au (JCPDS No. 65-8601) (Figure 5a), with the (111) and (200) crystal planes being the dominant orientations.Similarly, in Figure 5b, two discernible peaks at 38.1°and 44.3°are assigned to the (111) and (200) orientations of Ag, based on the characteristic peaks of the standard card (JCPDS No. 65-2871).The strong (111) diffraction peak at 38.1°indicates its high exposure.Due to the small amount of Pt deposition (the concentration of the precursor used was 0.02 wt% of Pt), the characteristic peak of Pt in Pt/SnO 2 is very weak (Figure 5c), which is consistent with previous literature reports. [34]The two characteristic peaks appearing at 40.0°and 46.5°correspond to the (110) and ( 200) crystal planes of Pt (JCPDS No. 01-1194), respectively.The morphology of the deposited Au, Ag, and Pt was investigated by FESEM (Figure 5d-i), and the insets in Figure 5d-f are the Au, Ag, and Pt films deposited on FTO in the experiment.As shown in Figure 5d, the Au NPs present irregular spherical structure with a uniform size and a thickness of about 400 nm (Figure 5g).The two structures of Ag are rod-shaped and block-shaped, and the size of the block-shaped structure is significantly larger than that of the rod-shaped structure and the thickness of the Ag film was about 18 μm (Figure 5e,h).Interestingly, the surface of Ag is covered by a layer of fluff-like structure (Figure 5j-l), which has not been studied in the currently known reports.Figure 5i shows that that Pt deposited by light is tightly attached to the surface of FTO with a thickness of 2.4 μm.In general, the deposition layer of silver is the thickest, followed by gold and platinum and the metallic layers of Au, Ag, and Pt appear as thin films with some differences in apparent compactness.
It is known from the experimental phenomenon that Au, Ag, and Pt can all be deposited from the corresponding precursor solution under irradiation using ZnO, ZnO/ZnS electrodes, while under the same conditions, only Au can be deposited, but not Ag, Pt, when Fe 2 O 3 is used.According to reports, for photodeposition to occur, the metal (oxides) being deposited must have a reduction/oxidation potential that is positioned favorably with respect to the energy band positions of the semiconductor. [35]To investigate the charge transfer mechanism, we analyzed the band structure of ZnO, ZnO/ZnS, and Fe 2 O 3 electrodes, as well as discussed OER and the reduction potential of Au 3þ /Au, Ag þ /Ag, and Pt 4þ /Pt, which help to study the possibility of electrodeposition from thermodynamic perspective.The conduction band positions of ZnO, ZnS, and Fe 2 O 3 are À0.31,À1.04, [36] and 0.46 V [37] (vs NHE), respectively, with bandgap energies of 3.16, 3.6, [36] and 2.09 eV, and the positions of VBs are 2.85, 2.56, and 2.55 V (vs NHE), obtained by the formula It is known above that the conduction band energy level of ZnO (0.095 V vs RHE) is more negative than the reduction potentials of gold silver platinum (1.24, 0.86, and 0.84 V vs RHE), and thus Au, Ag, and Pt can be deposited.While the conduction band energy level of Fe 2 O 3 (0.87 V vs RHE) is more positive than the reduction potentials of Ag and Pt and more negative than Au, as a result only Au is deposited.The experimental results are consistent with the literature.
After illumination, semiconductors can excite photogenerated electrons and holes.If the potentials of the photogenerated electrons in the conduction band are higher than the deposition potential of the metal, the entire deposition process is spontaneous.It can be seen that the conduction band energy levels of ZnO and ZnS are higher (more negative) than the NM deposition potential, and the conduction level of Fe 2 O 3 is higher than that of gold (Figure 6).For ZnO/ZnS heterostructures, ZnS has higher bandgap level and shows a slightly negative potential when incorporated in composite materials.The type II structure is highly beneficial in promoting the separation of photogenerated charges, leading to the production of a greater number of photogenerated electrons that can be utilized for reduction.In the VB test, the VB energy levels of the three semiconductor materials are lower (more positive) than the water oxidation potential, and thus OER can occur.

Conclusions
In summary, the photoelectric deposition of Au, Ag, and Pt at the cathode by ZnO, ZIF-8-derived ZnO/ZnS, and Fe 2 O 3 photoanodes has been investigated from the perspective of energy band structure and metal reduction potential.The results show that ZnO can deposit Au, Ag, and Pt from the precursor fluid, and so can ZnO/ZnS, which is due to its proper band matching structure, more effective hole-electron separation, and faster charge transfer, which improves its photocatalytic performance.By contrast, Fe 2 O 3 can only deposit Au, but not Ag and Pt, showing the importance of selecting a semiconductor with a conduction band potential more negative than the reduction potential of the metals and a VB position more positive than the oxidation-reduction potential of water.This method of retrieving these metals by photoelectric deposition is conducted without the need for external voltage or sacrifice agent, making it a safe, environmentally friendly, energy-saving, and cost-effective process.
Preparation of the ZnO Photoelectrode: A small amount of ethanol solution containing 0.005 M Zn(NO 3 ) 2 •6H 2 O was added dropwise onto the conductive side of the FTO substrate, dried in a vacuum oven at 60 °C, and the process was repeated 3 times.Subsequently, the FTO glass was annealed in a muffle furnace at 450 °C for 15 min, and ZnO seed crystals were formed on the FTO substrate.The FTO with ZnO seed layer, cooled to room temperature, was placed into a Teflon-lined stainless steel autoclave containing a mixed solution of 0.1 M Zn(NO 3 ) 2 •6H 2 O and 0.2 M HMT and heated at 95 °C for 6 h.After cooling down, the ZnO electrode was obtained by rinsing it 3 times with deionized water and drying it naturally.
Preparation of the ZnO/ZnS Photoelectrode: The ZnO/ZnS photoelectrode was prepared via in situ hydrothermal and sulfurization methods. [25]irst, 2-methylimidazole (0.1 mg) was added to a mixed solution of DMF (8 mL) and H 2 O (8 mL) and dissolved by stirring.ZnO/ZIF-8 arrays were grown on the ZnO arrays in stainless steel autoclave using this mixed solvent, which was then heated at 70 °C for 2 h.The ZnO/ZIF-8 arrays were rinsed with DMF and ethanol for 3 times and then dried at 60 °C for 12 h in an oven.Next, thioacetamide (0.05 g) was dissolved in ethanol (20 mL) and the ZnO/ZIF-8 array was immersed in this mixed solvent for 2 h at 80 °C.Finally, the ZnO/ZnS photoelectrode was obtained by washing with deionized water and ethanol for 3 times after vulcanization, and drying at 60 °C for 4 h.
Preparation of the Fe 2 O 3 Photoelectrode: Preparation of Fe 2 O 3 nanorods arrays on the FTO glass has been reported in our previous research. [38]It was achieved by employing a combination of a one-step hydrothermal process and a one-step annealing procedure.Specifically, FTO glass was submerged in a mixed solution of 0.3 M FeCl 3 and 2 M NaNO 3 inside a stainless steel autoclave.The hydrothermal reaction was then carried out at a temperature of 100 °C for a duration of 12 h.Once cooled to room temperature, the resulting products were rinsed with deionized water and subjected to calcination at 550 °C for a period of 2 h, ultimately yielding the Fe 2 O 3 photoelectrode.
Characterization: The FESEM (Shimadzu JSM-6700F) was used to observe the morphology of the samples.The XRD patterns were acquired on a Bruker (Germany) D8 Advance diffractometer with Cu Kα radiation in the range of 5°-80°(2θ).UVÀvis diffuse reflectance spectroscopy (DRS) was used to measure the semiconductor films using a Shimadzu UV-2550 spectrometer.In addition, the reflection was converted to absorbance through the Kubelka-Munk function.Electrochemical tests were performed using an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co., Ltd.).A 500 W Xenon lamp (CHFXQ500W, Beijing Trusttech Co., Ltd.) was used as the light source to simulate sunlight.The light radiation intensity was adjusted to 100 mW cm À2 , and evaluated by a radiometer (Photoelectronic Instrument Co. Attached to Beijing Normal University, China).Subsequently, the contact angle was calculated using a DSA 100 Kruss contact angle meter.
Photoelectrochemical Measurements: The OER performance of ZnO, ZnO/ZnS, and Fe 2 O 3 photoelectrodes in 0.5 M Na 2 SO 4 solution was measured using LSV.The EIS of ZnO, ZnO/ZnS, and Fe 2 O 3 photoelectrodes was measured in 0.5 M Na 2 SO 4 at a frequency range of 10 6-1 Hz.To determine the FB potentials of ZnO, ZnO/ZnS, and Fe 2 O 3 electrodes in 0.5 M Na 2 SO 4 solution, the Mott-Schottky (MS) plots were obtained by using MS method and run at a frequency of 1 kHz.The transient photocurrent of ZnO, ZnO/ZnS, and Fe 2 O 3 photoelectrodes was measured in 0.5 M KOH solution under AM1.5 illumination with a power density of 100 mW cm À2 .The above tests were performed in the three-electrode system, where ZnO, ZnO/ZnS, and Fe 2 O 3 photoelectrode were used as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as the reference electrode.Cyclic voltammetry (CV) curves were measured in the three-electrode system in 0.02 wt% chlorauric acid, silver nitrate, and chlorplatinic acid solutions with FTO as working electrodes.The I-t curves were tested in the two-electrode system, wherein the ZnO, ZnO/ZnS, and Fe 2 O 3 photoelectrode served as the working electrode and a Pt wire as the counter electrode in 0.02% chloroplatinic acid, 0.02% chloroauric acid, or 0.02% silver nitrate, respectively.The potentials were calibrated by the Nernst equation: E(RHE) = E(NHE) þ 0.0591 pH, E(NHE) = E(Ag/ AgCl) þ 0.197.
Experimental Design: The experiments were performed in a setup with a separate cathode chamber and an anode chamber with a quartz window, and the cathode and anode solutions were separated by a proton exchange membrane (PEM).0.5 M Na 2 SO 4 solution was poured into the anode side with the quartz window, and the corresponding precursor solution (0.02% chloroplatinic acid, 0.02% chloroauric acid, or 0.02% silver nitrate) was poured into the cathode side.The photoanode (ZnO, ZnO/ZnS, Fe 2 O 3 ) was immersed into the anode solution, the blank FTO was immersed into the precursor solution as substrate, and the FTO and photoanode were connected by wire.Finally, the photoanodes were illuminated under a xenon lamp.

Figure 3 .
Figure 3.The plots of (αhν) 2 against photo energy of a) ZnO, b) ZnO/ZnS, and c) Fe 2 O 3 films (the inset is the UV-vis absorption spectra).d) The PL spectra of ZnO and ZnO/ZnS.e) OER polarization curves of ZnO, ZnO/ZnS, and Fe 2 O 3 with and without illumination in 0.5 M Na 2 SO 4 solution.f ) EIS of ZnO, ZnO/ZnS, and Fe 2 O 3 photoelectrodes with and without illumination.I-t characteristic curves under intermittent illumination without applied bias voltages on g) ZnO, ZnO/ZnS, and Fe 2 O 3 , h) Fe 2 O 3 in 0.5 M KOH aqueous solution with a power density of 100 mW cm À2 .i) Contact angle of ZnO, ZnO/ZnS and Fe 2 O 3 .
, the contact angles of ZnO, ZnO/ZnS, and Fe 2 O 3 films are presented.The pure ZnO and ZnO/ZnS films have contact angles of 45.4°and 39°, respectively.The introduction of ZnS improves the hydrophilicity compared with pure ZnO.Fe 2 O 3 films exhibit superhydrophilicity due to their small contact angle of 8.4°.Mott-Schottky analysis was performed to evaluate the flatband (FB) potential of ZnO, ZnO/ZnS, and Fe 2 O 3 .The positive slope of the curve indicates that ZnO, ZnS, and Fe 2 O 3 electrodes

Figure 4 .
Figure 4. I-t curves of a) Au, b) Ag, and c) Pt deposited by Fe 2 O 3 , ZnO and ZnO/ZnS under illumination for 30 min, respectively.Cyclic voltammetric curves of FTO in d) 0.02 wt% chloroauric acid, e) silver nitrate, and f ) chloroplatinic acid solutions, respectively.

Figure 5 .
Figure 5. XRD pattern of a) Au film, b) Ag film, and c) Pt film on FTO.FESEM image of d) Au film, e) Ag film, and f ) Pt film.Cross-sectional view of g) Au film, h) Ag film, and i) Pt film.j-l) FESEM image of Ag film with different magnification.

Figure 6 .
Figure 6.Working mechanisms of photoelectric deposition of NMs.