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. 2020 Sep 24;5(39):25125–25134. doi: 10.1021/acsomega.0c02754

Solution-Processed Synthesis of Copper Oxide (CuxO) Thin Films for Efficient Photocatalytic Solar Water Splitting

Asma Aktar 1, Shamim Ahmmed 1, Jaker Hossain 1,*, Abu Bakar Md Ismail 1,*
PMCID: PMC7542592  PMID: 33043191

Abstract

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This article reports a solution-processed synthesis of copper oxide (CuxO) to be used as a potential photocathode for solar hydrogen production in the solar water-splitting system. CuxO thin films were synthesized through the reduction of copper iodide (CuI) thin films by sodium hydroxide (NaOH), which were deposited by the spin coating method from CuI solution in a polar aprotic solvent (acetonitrile). The phase and crystalline quality of the synthesized CuxO thin films prepared at various annealing temperatures were investigated using various techniques. The X-ray diffraction and energy dispersive X-ray spectroscopy studies confirm the presence of Cu2O, CuO/Cu2O mixed phase, and pure CuO phase at annealing temperatures of 250, 300, and 350 °C, respectively. It is revealed from the experimental findings that the synthesized CuxO thin films with an annealing temperature of 350 °C possess the highest crystallinity, smooth surface morphology, and higher carrier density. The highest photocurrent density of −19.12 mA/cm2 at −1 V versus RHE was achieved in the photoelectrochemical solar hydrogen production system with the use of the CuxO photocathode annealed at a temperature of 350 °C. Therefore, it can be concluded that CuxO synthesized by the spin coating method through the acetonitrile solvent route can be used as an efficient photocathode in the solar water-splitting system.

1. Introduction

Energy consumption is increasing rapidly because of the increase in population and industrialization in the world. Production of hydrogen (H2) as chemical energy using the abundant solar power can be an excellent resolution to this growing energy crisis.1 A simple technique to produce H2 is water splitting in a photoelectrochemical (PEC) cell.1,2 In the PEC system, the photoelectrode is irradiated by sunlight, light is absorbed in the photoelectrode, and electron–hole pairs are generated, which are driven into the solution by the electric field at the photoelectrode/water junction to produce H2 and oxygen (O2) in the PEC water-splitting system.3 Efficient photoelectrode materials are selected based on some fundamental properties such as a high absorption coefficient, excellent charge transport capability, photocorrosion stability in the electrolyte solution, and low kinetic overpotentials.4,5 However, there is no single material that meets all the criteria for an efficient photoelectrode. Therefore, finding suitable materials for the absorption of a full portion of incident photons, developing active catalysts, and realizing the appropriate interface design between the catalyst, photoabsorber, and electrolyte to minimize losses are the main challenges for an efficient PEC system.5

A number of photocatalysts including titanium oxide (TiO2),6−10 zinc oxide (ZnO),11−13 tungsten oxide (WO3),14−16 reduced graphene oxide-polypyrrole (rGO@PPy),17 iron oxide (Fe2O3),2,18 bismuth vanadium tetra-oxide (BiVO4),19 cuprous oxide (Cu2O),1,20−23 and the more active catalyst cupric oxide (CuO)24−28 have been investigated for perceiving a high-efficiency PEC water-splitting system. Among these catalysts, copper oxides (CuO and Cu2O) have recently attracted special attention because they are abundant in the crust of the earth, low-cost and nontoxic.29 Moreover, copper oxide photocathodes can be prepared using electrodeposition,21,23 spin coating,30−32 RF-magnetron sputtering,28 and spray pyrolysis techniques.33 Depending on the techniques and criteria of preparation, the direct band gap of the p-type Cu2O and CuO has been observed to be nearly 2.0–2.5 and 1.3–1.7 eV, respectively.34 The band alignment of the Cu2O and CuO is favorable for the generation of the H2 fuel in the PEC system.33,35,36 The theoretical solar-to-hydrogen conversion efficiency of both Cu2O and CuO is >18%.33,35 However, the practical CuxO photoelectrodes suffer from the poor-stability problem in the aqueous solution because its redox potential lies within the band gap.1,37−40 Surface passivation of the photocathodes through the use of conformal coating could be one possible way to enhance the stability of the CuxO photoelectrodes.1,21,23,37,38 Low efficiency of the CuxO photocathodes is another critical issue. The unfavorable minority carrier diffusion length and slow carrier transfer rate at the photoelectrode surface are the main issues for the practical copper oxide (Cu2O, CuO) photocathodes that are responsible for the low efficiency of the devices.29,33

However, the performance of the copper oxide photocathode can be enhanced by improving the crystal quality and surface morphology of the material. To address this problem, researchers have applied different techniques such as doping41 and high-temperature treatment42 to copper oxide for the fabrication of an efficient PEC system. So far, the electrodeposited Cu2S-coated Cu2O NW photoelectrode has been reported to produce a photocurrent of −1.0 mA/cm2 at −0.6 V versus Ag/AgCl.21 A photocurrent of 0.49 mA/cm2 at −0.25 V versus Ag/AgCl has been reported for the electrodeposited Cu2O electrode with NiFe-layered double hydroxides.23 Son et al. also reported a photocurrent density of −5.2 mA/cm2 at 0 V versus RHE for the electrodeposited Cu2O photocathode using CuO/NiO as a back protected layer.37 Lim et al. reported a photocurrent of −0.28 and −0.35 mA/cm2 at 0.05 V versus RHE for the sol–gel deposited Cu2O and CuO photoelectrode, respectively.24 The RF-magnetron-sputtered CuO photocathode yielded a photocurrent of −3.1 mA/cm2 at 0 V (RHE).28 A solution-processed CuO nanoparticle films provided a photocurrent of −1.2 mA/cm2 at −0.7 V versus Ag/AgCl.30 Baran et al. reported a photocurrent of −1.3 mA/cm2 at 0.2 V versus RHE for the solution-processed CuxO photocathode.31 A photocurrent of −24 mA/cm2 at 0.25 V versus RHE has also been reported for the spray-pyrolyzed tenorite CuO thin films.33

In addition, the solution-processed synthesis of CuxO thin films by the spin coating method could produce potential candidates as efficient photoelectrodes for application in solar hydrogen production as it reduces cost and fabrication complexity. Recently, a simple one-step synthesis of p-type CuxO thin films via in situ reaction of a spin-coated CuI thin film in aqueous NaOH solution has been reported for the fabrication of efficient thin-film transistors.32 The CuxO thin films synthesized by the same route have been used as a hole transport layer for the perovskite solar cells.43 Moreover, the deposition of CuI thin films by the spin coating method from a CuI solution dissolved in an acetonitrile solvent has already been reported.44 However, there are few reports on the application of CuO photocathodes in PEC solar water splitting synthesized from the reduction of spin-coated CuI thin films.

In this article, we demonstrate the facile one-step synthesis of CuxO thin films through the reduction of CuI thin films prepared from a CuI solution dissolved in acetonitrile by the spin coating method for the efficient application in the solar water-splitting system. The synthesized CuxO thin films as a photocathode exhibited excellent performance in the PEC solar water-splitting system with better reproducibility. It is revealed from the findings that the acetonitrile-assisted synthesized CuxO thin film could be a deserving candidate as a photocathode in the PEC system for solar H2 production by water splitting.

2. Results and Discussion

2.1. X-ray Diffraction and TGA–DTA Studies of Copper Oxide Thin Films

The photocatalytic performance of a photocathode is dependent on the phase purity and crystallinity of the materials used as an electrode. The phase and crystallinity of the synthesized CuxO thin films were determined by X-ray diffraction (XRD) analysis. The XRD pattern of the CuxO thin films annealed at different temperatures is depicted in Figure 1a. For the film annealed at 250 °C, the peaks at 36.52 and 42.44° are corresponding to (111) and (200) diffraction planes of the face centred cubic (fcc) Cu2O crystalline structure (JCPDS Card no. 65-3288). This result indicates that a well-crystallized Cu2O film with no CuO phase formed at an annealing temperature of 250 °C. The phase change of the film from Cu2O to CuO was revealed at an annealing temperature of 300 °C. The XRD peaks at 2θ of 35.7, 38.9, and 49° for the film annealed at 300 °C corresponding to (002), (111), and (−202) planes, respectively, are attributed to the monoclinic crystal structure of CuO (JCPDS Card no. 48-1548) with improved crystallinity. The presence of the Cu2O phase was also confirmed by the peak positioned at 61.53° with orientation to the (220) plane in the film annealed at 300 °C. On the other hand, only the CuO phase was confirmed for the film annealed at 350 °C, as observed in Figure 1a. The sharp peaks signify the improved crystallinity of the CuO film with no other phase of copper oxide after annealing at 350 °C. These results on the thermal oxidation behavior of CuxO thin films are in good agreement with the reported work.45

Figure 1.

Figure 1

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(a) XRD patterns of the CuxO thin film annealed at different temperatures and (b) TGA–DTA of the copper oxide sample (dried at 120 °C).

The crystallite size (D) of the synthesized CuxO thin films was calculated using the Debye–Scherrer formula and is shown in Table 1. It is found that the crystallite size increases with annealing temperature and the CuxO thin film annealed at 350 °C showed the highest crystallinity.

Table 1. Different Crystal Parameters of the Synthesized CuxO Thin Films.

annealing temperature (°C) phase (hkl) 2θ (deg) fwhm (deg) crystallite size (nm)
250 Cu2O (111) 36.52 0.89 11.5
  Cu2O (200) 42.44 0.71 14.2
300 CuO (002) 35.70 0.54 18.2
  CuO (111) 38.90 0.74 13.9
  CuO (−202) 49.00 0.71 16.7
  Cu2O (220) 61.53 0.91 18.4
350 CuO (002) 35.70 0.48 20.4
  CuO (111) 38.90 0.54 18.6
  CuO (−202) 49.00 0.53 22.8
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Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) are the popular methods to investigate the thermal stability of the semiconducting materials.46−49 Therefore, TGA and DTA of the CuxO thin film (preannealed at 120 °C) were carried out to determine the proper annealing temperature. The TGA–DTA spectra were taken by scanning the temperature from 30 to 500 °C at 20 °C/min under a continuous flow of nitrogen gas at 20 mL/min. Figure 1b delineates the TGA–DTA spectra of the copper oxide sample. About 22% weight loss appeared from temperature 273–400 °C with no weight loss found except for this range. Such a huge weight loss reveals the transformation of the Cu2O phase to the CuO phase and the evaporation of other volatile elements. Further weight loss was not observed above 400 °C, indicating the thermal stability of CuxO from 400 to 500 °C. Therefore, the appropriate annealing temperature is >400 °C under a nitrogen atmosphere. Simultaneous thermal decomposition and improved crystallinity of the prepared sample appear from the DTA spectra. It also predicts that an optimum crystallinity of the sample could be obtained at a temperature of >400 °C.

2.2. Surface Morphological Study of CuxO Thin Films

Figure 2a–c illustrate the surface morphology of the synthesized CuxO thin films synthesized from the reduction of spin-coated CuI thin films in aqueous NaOH and annealed at different temperatures. The surface of the CuxO thin film annealed at 250 °C appears to be covered almost uniformly with grains of ∼0.9 μm. Several void spaces were also observed in the film. The surface morphology of the film significantly changed for the films annealing at 300 °C. The grains of the 300 °C-annealed copper oxide were found to be randomly distributed. The increase in void spaces, grain boundaries, and the surface roughness of the films might be due to the phase transition of the copper oxide from Cu2O to CuO and evaporation of the volatile elements.50 The nonuniformity of grains and a large number of grain boundaries in the photocathode increase the probability of charge scattering and carrier recombination rate, which leads to the performance degradation of the photocathode in the PEC system.

Figure 2.

Figure 2

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SEM images of the CuxO thin films annealed at (a) 250, (b) 300, and (c) 350 °C. EDX spectra of the CuxO thin films annealed at (d) 250, (e) 300, and (f) 350 °C.

The nucleation rate was found maximum at 350 °C and ∼0.34 μm grains were observed with a few amounts of grain boundaries. As shown in Figure 2c, the surface of the CuxO film annealed at 350 °C is very compact and smoother with reduced void spaces than compared to that of the films annealed at other temperatures. Improved surface morphology of the photocathode is favorable for the PEC performance,51 which resists the recombination of the charge carriers and benefits in the enhancement of the charge transport property.52,53

Figure 2d–f depict the energy dispersive X-ray spectroscopy (EDX) spectra used to evaluate the chemical composition of the CuxO thin films annealed at a temperature from 250 to 350 °C. The atomic ratio of “Cu” and “O” in the synthesized CuxO thin films is listed in Table 2, which indicates that CuxO annealed at 250 °C was entirely in the Cu2O phase whereas the film annealed at 300 °C was CuO with a small amount of the Cu2O phase and the film annealed at 350 °C was purely CuO phase. The EDX results are consistent with the XRD analysis.

Table 2. Partial Element Concentrations of the CuxO Films Annealed at 250, 300, and 350 °C Obtained from EDX Spectra.

  Cu (%)
 O (%)
  
annealing temperature (°C) atomic % weight % error (%) atomic % weight % error (%) [Cu]/[O]
250 71.43 90.78 5.3 28.57 9.22 8.3 2.50
300 55.56 83.11 5.5 44.44 16.89 8.2 1.25
350 46.51 77.39 5.4 53.49 22.61 8.0 0.87
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2.3. Optical Properties of CuxO Thin Films

The optical properties of the synthesized CuxO thin films annealed at different temperatures are shown in Figure 3. The absorption spectra of the CuxO thin films are depicted in Figure 3a. The absorbance increases with increasing annealing temperature. The CuxO thin film annealed at 350 °C showed better absorbance than other films that may arise because of large grain size, less void spaces, a smooth surface, and compactness of the film. The band gap of CuxO thin films was calculated using the Tauc plot,44 as shown in Figure 3b. It is reported that both Cu2O and CuO are p-type semiconductors having direct and indirect optical band gap, respectively.54−57 It was revealed from the XRD and EDX studies that the films annealed at 250, 300, and 350 °C were the pure Cu2O, Cu2O/CuO mixed, and pure CuO phase, respectively. Therefore, the direct band gap of Cu2O and Cu2O/CuO thin films was calculated using (αhν)2 versus hν plot, whereas (αhν)0.5 versus hν plot was used for the calculation of the indirect band gap of CuO thin films, as depicted in Figure 3b. The direct band gaps of Cu2O and Cu2O/CuO thin films were found to be 2.1 and 1.9 eV, respectively. On the other hand, the indirect band gap of CuO thin films was found to be 1.3 eV. This observed band gap of CuxO thin films is consistent with the previous studies.54−57 It is also theoretically predicted that around 35 mA/cm2 photocurrent could be produced by the CuO having a band gap of 1.3 eV.58

Figure 3.

Figure 3

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(a) Optical absorbance spectra and (b) Tauc plots of the synthesized CuxO thin films annealed at a temperature from 250 to 350 °C.

2.4. Carrier Transfer Property of CuxO Thin Films

In the previously reported work, CuxO photocathodes exhibited very low solar-to-hydrogen conversion efficiency because of the poor carrier transfer property of the surface.29,33,50 Therefore, enhancement of the carrier transfer property of the CuxO photocathodes is of high demand to the researchers. The carrier transfer property and semiconducting nature of the synthesized CuxO thin films were determined from the Mott–Schottky (MS) plot, illustrated in Figure 4a. The MS plot of the CuxO thin films annealed at different temperatures was constructed using the capacitance–voltage data observed in the impedance analyzer. The carrier density (ND) was calculated using the following equation35

2.4. 1

where C is the differential capacitance, ε is the dielectric constant of the semiconductor, ε0 is the permittivity of free space, ND is the density of dopants, V is the applied potential, Vfb is the flat band potential, k is the Boltzmann constant, and T is the absolute temperature. The negative slope of the MS plot confirms the p-type conductivity of the CuxO thin films. The value of the flat band potential (Vfb) was calculated by the slope of the extrapolating linear part of the MS plot. The value of Vfb and the carrier concentration were found to vary from 1.04 to 1.16 V and 3.4 × 1021 to 3.95 × 1021 cm–3 at the variation of annealing temperature from 250 to 350 °C, respectively. The films annealed at 350 °C show high carrier concentration, which might be the result of the better quality of the film, as revealed by the XRD and scanning transmission microscopy (SEM) studies. The enhancement of carrier concentration with annealing temperature signifies a high density of copper vacancies. The increase in charge carrier density promotes the degree of band bending at the CuxO, which promotes the charge separation at the CuxO/electrolyte interface.28

Figure 4.

Figure 4

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(a) MS plots of the CuxO thin films and (b) energy band diagram of the Cu2O and CuO photocathodes at pH = 1.6.

Figure 4b shows the energy band diagram of the Cu2O and CuO photocathodes, considering the band gap of 2.1 eV for Cu2O and 1.3 eV for CuO. The reduction of proton (H+) is in favor because the H+/H2 potential is more positive than the conduction band (CB) potential of both Cu2O and CuO photocathodes.59 The schematic diagram of the two-electrode PEC water-splitting system using the synthesized CuxO films as the working electrode (WE) and Pt as the counter electrode (CE) is also shown in the inset of Figure 4b. When the light is incident on the CuxO photocathode, the electron–hole pairs will be generated. The protons will be reduced at the CuxO/electrolyte interface by the CB electrons. Besides, oxidation of oxygen will occur at the CE because of the transfer of photogenerated holes from the CuxO photocathode to the CE.

2.5. Photocatalytic Performance of CuxO Thin Films

The current–voltage characteristics of the CuxO photocathodes in the three-electrode PEC water-splitting system are depicted in Figure 5a. The plot shows the photocurrent density of the best performing CuxO photocathodes among a number of samples with better reproducibility. First, the experimental potential was measured with respect to the Ag/AgCl reference electrode (RE). Then, the potential versus Ag/AgCl was converted into the potential versus RHE (reversible hydrogen electrode) scale with the help of the following formula

2.5. 2

where, ERHE is the potential converted into the RHE scale, EAg/AgCl is the experimentally measured potential with respect to the Ag/AgCl RE, pH is the pH of the electrolyte, and EAg/AgCl0 = 0.1976 V at 25 °C.

Figure 5.

Figure 5

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(a) J–V characteristics of the synthesized CuxO photocathodes in a three-electrode system (CuxO WE, Pt CE, and Ag/AgCl RE) and (b) J–V characteristics and ABPE (%) of the synthesized CuxO photocathodes in a two-electrode system (CuxO WE and Pt CE).

At an applied potential of −0.5 V versus RHE (onset potential), H2 production was initiated from the photocathode annealed at 250 °C (production of bubbles), and the production of bubbles increased with increasing applied bias potential; the onset potential of −0.47 V versus RHE and −0.4 V versus RHE was observed for the photocathodes annealed at 300 and 350 °C, respectively. The onset potential followed the variation of the flat band potential in a reversible way; a low onset potential is desirable for an efficient photocathode. The synthesized photocathode annealed at 250, 300, and 350 °C produced a photocurrent density of −12.50, −2.13, and −19.12 mA/cm2 , respectively, at −1 V versus RHE. As it was confirmed from the XRD and EDX results that CuxO thin films annealed at 250, 300, and 35 °C were Cu2O, CuO/Cu2O, and CuO, respectively; therefore, photocurrent densities of −12.50, −2.13, and −19.12 mA/cm2 were produced by the Cu2O, CuO/Cu2O, and CuO photocathodes, respectively, at −1 V versus RHE. It is observed from Figure 5a that the photocathode annealed at 300 °C shows low photocurrent density than the other photocathodes. The presence of mixed copper oxide phases (CuO/Cu2O) in the film annealed at 300 °C might be the reason for the low photocurrent density because the mixed (nonstoichiometric) copper oxide phases induce more defects (trapping centers)60 and reduced the carrier mobility. The CuO photocathode produced a higher photocurrent than the Cu2O photocathode. The higher crystallite size, uniform surface morphology, and higher carrier density of the CuO photocathode helped to produce higher photocurrent than the Cu2O photocathode,33,61 which is consistent with the XRD, SEM, and MS plot analysis discussed in the previous sections. The dark current was found significantly very low compared with the light current, which indicates that the corrosion rate of our synthesized CuxO photoelectrode in aqueous solution is low.

The PEC performance of the synthesized CuxO photoelectrodes was investigated by calculating the applied bias photon-to-current efficiency (ABPE) in a two-electrode system using the following formula62

2.5. 3

where Vb is the applied potential between the WE and CE in a two-electrode configuration and its corresponding photocurrent density is Jph and Ptotal is the total light (100 mW/cm2) incident on the photocathode. As illustrated in Figure 5b, the optimum ABPE for the Cu2O, CuO/Cu2O, and CuO photocathodes is 4.69% (at 0.9 V vs Pt), 0.79% (at 0.9 V vs Pt), and 7.06% (at 0.85 V vs Pt), respectively. The observed PEC performance of the synthesized photocathodes is much higher than the previously reported performance.1,19−22,24,27,28,30,31,33,63−65Table 3 shows a comparison of CuxO-based photocathode performance with that of our synthesized CuxO photocathode.

Table 3. Comparison of CuxO-Based Photocathode Performance Synthesized using Different Techniques.

photocathode thickness (nm) fabrication technique photocurrent refs
NiO–CuO thin film 850 sol–gel spin coating –1.07 mA/cm2 at –0.55 V (Ag/AgCl) (24)
CuO thin film ∼500 sputtering –0.65 mA/cm2 at –0.45 V (Ag/AgCl) (66)
Li-doped CuO nanoparticles 1700 flame spray pyrolysis –1.7 mA/cm2 at –0.55 V (Ag/AgCl) (27)
CuO thin film 500 RF-magnetron sputtering –3.1 mA/cm2 at 0 V (RHE) (67)
CuO nanoparticles films 1340 spin coating –1.2 mA/cm2 at –0.55 V (Ag/AgCl) (68)
CuO thin film with TiO2 protecting layer and Au–Pd co-catalyst nanostructures (CuO–TiO2–AuPd) ∼500 sol–gel method and RF sputtering –1.9 mA/cm2 at 0 V vs RHE (69)
tenorite CuO thin films ∼500 spray pyrolysis –24 mA/cm2 at 0.25 V vs RHE (33)
Cu2O/CuO/C ∼2000 electrodeposition –7.5 mA/cm2 at –0.1 V vs RHE (70)
CuO ∼700 spin coating –19.12 mA/cm2 at –1 V vs RHE present work
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The objective of the present work was to synthesize an efficient CuxO photocathode for the PEC system. Because the stability of the CuxO photocathodes is an important issue, the stability of the photocurrent of the synthesized photocathodes was observed under 100 mW/cm2 sun light conditions at −1 V versus Pt using the two-electrode system and is shown in Figure 6. Till now, the photostability of the CuxO photocathodes is far from that of the typical photoelectrodes (WO3 and α-Fe2O3); the fast photocorrosion deactivated the CuxO photocathodes.63,64,71 It has been demonstrated that the copper oxides spontaneously decompose to metal Cu under illumination; Cu accumulated on the surface and deactivated the inner copper oxide after a certain period.63,71 It is observed from the Figure 6 that the photocathodes which were annealed at 250 (Cu2O), 300 (CuO/Cu2O), and 350 °C (CuO) retained about ∼85% of their initial photocurrent for ∼30, ∼34, and ∼39 min, respectively. After this period, the photocurrent was drastically suppressed because of photocorrosion. The CuxO photocathode annealed at 350 °C showed better stability against photocorrosion among the all synthesized CuxO photoelectrodes. However, many researchers have already tried to overcome the stability issue of the CuxO photocathodes through a successful way of using a surface protective layer on the CuxO photocathodes.1,21,23,37,38,70 Following the same strategy, future research could be conducted to improve the stability of CuxO photocathodes synthesized by the reduction of spin-coated CuI thin films.

Figure 6.

Figure 6

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Photocurrent stability of the CuxO photocathodes annealed at (a) 250, (b) 300, and (c) 350 °C measured using the two-electrode system.

3. Conclusions

The CuxO thin films were successfully synthesized by the reduction of copper iodide (CuI) thin films deposited by the spin coating method from CuI solution dissolved in an acetonitrile solvent. The experimental findings reveal that the CuxO thin film annealed at 350 °C has a larger grain size with a smooth surface and high carrier density. This synthesized CuxO exhibited a photocurrent density of −19.12 mA/cm2 at −1 V versus RHE and ABPE of 7.06% with better reproducibility when applied as a photocathode in the solar water-splitting system. Therefore, this work reveals that CuxO thin films synthesized by spin coating and reduction of CuI thin films using the acetonitrile solvent route have huge potential as photocathodes for solar hydrogen production by water splitting.

4. Experimental Details

4.1. Materials

Copper(I) iodide powder (CuI, 99.5%), acetonitrile (CH3CN, 99.8%), sodium hydroxide pellets (NaOH, 97%), and ITO-coated glass slides (surface resistivity of 8–12 Ω/sq) were purchased from Sigma-Aldrich. All the chemicals were used as purchased without further purification.

4.2. Preparation of the CuxO Electrode

Figure 7 shows the preparation steps of the copper oxide photocathode. First, the CuI thin films were prepared on commercialized indium-doped tin oxide (ITO)-coated glass substrates from a copper iodide (CuI) solution of 30 mg/mL in acetonitrile following a recipe reported in previous work.44 The ITO-coated glass substrates were cleaned using an ultrasonic vibrator where acetone, isopropanol, and distilled water were used in sequence for cleaning before the deposition of the films. CuI thin films were deposited on the ITO glass substrates by spin coating at a speed of 2000 rpm for 30 s following a preannealing process at 150 °C for 10 min. The process was repeated five times to deposit thicker films. Then, the CuxO films were produced through dipping of the CuI thin films in aqueous sodium hydroxide (NaOH) solution (0.25 mol L–1). The CuxO thin films were then rinsed in methanol and dried in air to eliminate the undesired surplus. Because thermal treatment is an essential factor in the one-step solution process for the phase transformation of copper oxide, the synthesized photocathodes were finally annealed at temperatures of 250, 300, and 350 °C for 1 h. The thicknesses of the CuxO thin films were in the range of 650–700 nm.

Figure 7.

Figure 7

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Preparation steps of CuxO photocathodes.

4.3. Characterization

Structural properties of the copper oxide thin films were investigated using XRD with Cu Kα radiation having wavelength 1.540598 Å using the EMMA X-ray diffractometer (GBC Corporation, Australia). The surface morphology of the films was observed using a scanning electron microscope from EVO 18, Carl Zeiss, UK. The elemental composition of the films was measured by EDX using a Team EDS system (EDAX, AMETEK, USA; beam voltage: 15 kV). The Bruker Dektak XTL thickness profiler was used to measure the thickness of the films. The optical properties of the films were investigated using UV–visible spectrophotometry using a T60 UV–visible spectrophotometer (PG Instruments) with 1 nm increment in wavelength. Capacitance–voltage (C–V) measurement was done using an impedance analyzer (model: Agilent 4192 A). All the films were characterized at room temperature (∼27 °C) and a relative humidity of ∼50%.

4.4. PEC Measurement

The PEC performance of the synthesized CuxO photocathodes was explored under 1.5 AM (100 mW/cm2) simulated sun illumination using a Solar Light 16S-Series 150–300 W UVB solar simulator. The measurement was carried out in a well-recognized three-electrode PEC system, where platinum (Pt) as the CE, saturated Ag/AgCl as the RE and CuxO photocathode as the WE having an active area of 1.5 cm2 were immersed in a 0.025 M H2SO4 aqueous electrolyte solution. For all samples, current versus potential measurements were recorded at a scanning rate of 10 mV s–1 using a Squidstat plus potentiostat from Admiral Instruments, USA. The CuxO photocathode was installed perpendicularly to the direction of light illumination and illuminated from the front side because such alignment is convenient to utilize the performance of the photoelectrode properly.2 The two-electrode system (CuxO as the WE and Pt as the CE) was used to evaluate the PEC performance of the CuxO photocathodes in terms of ABPE (%).

Acknowledgments

This study was partially supported by a grant #A-1205/5/52/RU/F.ENG.-06/2019–2020 from the Faculty of Engineering, University of Rajshahi, Bangladesh. The authors also gratefully acknowledge the financial support from ICT Division, Ministry of Posts, Telecommunications and IT, Government of the Peoples Republic of Bangladesh. The authors highly appreciate Abdul Al Murtaza, atomic energy commission, Dhaka, Bangladesh for his help during thickness measurement.

The authors declare no competing financial interest.

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