Straightforward electrochemical synthesis of a Co3O4 nanopetal/ZnO nanoplate p–n junction for photoelectrochemical water splitting

Hydrogen production through photoelectrochemical (PEC) reactions is an innovative and promising approach to producing clean energy. The PEC working electrode of a Co3O4/ZnO-based p–n heterojunction was prepared by a straightforward electrochemical deposition with different deposition times onto an FTO (Fluorine-doped Tin Oxide) glass substrate. The successful synthesis of the materials was confirmed through analysis using XRD, FTIR, SEM-EDX, DRS, and PL techniques. Mott–Schottky plots and some characterization studies also checked the determination of the formation of the p–n junction. Co3O4/ZnO/FTO with a Co3O4 deposition time of 2 minutes exhibited the lowest onset potential of 0.82 V and the lowest overpotential of 470 mV at a current density of 10 mA cm −2. Furthermore, the photo-conversion efficiency of the Co3O4/ZnO/FTO sample showed 1.4 times higher current density than the ZnO/FTO sample. A mechanism is also proposed to enhance the Co3O4/ZnO/FTO electrode photo-electrocatalytic activity involved in the water-splitting reaction. The Co3O4/ZnO/FTO electrode shows significant potential as a promising PEC electrode to produce hydrogen.


Introduction
Solar-driven water splitting using semiconductors, including photochemical and photoelectrochemical (PEC) approaches, is receiving signicant attention in energy research as a promising solution for clean and renewable energy generation. 1,2The research of particulate semiconductor materials for photocatalytic water splitting has emerged as a straightforward and environmentally friendly method for the efficient production of hydrogen. 3Efficient photochemical water splitting requires photocatalysts with a band gap higher than the thermodynamic energy requirement of 1.23 eV.5][6] Also, the efficiency of photocatalytic water splitting, which converts solar energy into hydrogen, has remained relatively low.For instance, Zhou et al. 7 conducted a study using InGaN/GaN nanowire semiconductors, which achieved one of the highest solar-to-hydrogen efficiencies of 9.2% when using pure water under a xenon lamp with an AM1.5G lter.Integrating electrochemical processes with solar energy in PEC water splitting holds great potential as a highly effective approach for enhancing hydrogen production efficiency. 80][11] However, photoelectrodes in PEC water splitting based on metal oxide semiconductors, i.e., TiO 2 , ZnO, are challenging due to their narrow visible light absorption, unfavorable band positions, low charge mobilities, and less photostability, making it challenging to achieve efficient overall performance. 124][15] In type-II heterojunctions, the movement of holes and electrons occurs in opposite directions, enabling effective separation and transfer of charge carriers at the interface. 16inc oxide (ZnO ∼3.37 eV) is a semiconductor that exhibits excellent efficiency for PEC due to its CB position (E CB ) at −0.22 V vs. NHE, which indicates an opposing alignment relative to the favorable water reduction potential for the hydrogen evolution half-reaction (HER). 17,18However, ZnO is responded to UV light excitation, which constitutes only 5% of the total solar energy spectrum. 19Besides, the high photo-corrosion of ZnO under UV irradiation in aqueous environments also limits its widespread application. 20Meanwhile, cobalt oxide (Co 3 O 4 ) possesses two distinct optical band gaps that correspond to the energy state transitions of O 2− / Co 3+ (∼1.4 eV) and O 2− / Co 2+ (∼2.1 eV), leading to the enhancement of electron charge transfer. 21,22The nanostructured Co 3 O 4 lms remained stable during chronopotentiometry tests in acidic and alkaline environments. 23,24By constructing a Co 3 O 4 layer onto the ZnO layer, the exposure of ZnO to the aqueous solution can be prevented, thereby mitigating its photo-corrosion.The rapid recombination rate of photoexcited electron-hole pairs is a notable drawback observed in many individual semiconductor materials.As a result, recent studies proposed that the integration of both broad-band gap and narrow-band gap materials or metalsemiconductors could lead to efficient heterojunctions. 25he fabrication of ZnO and Co 3 O 4 p-n junctions has gained attention due to the unique properties of these materials.ZnO, excited by UV light, is employed as the n-type material, while Co 3 O 4 , which can absorb visible light, is the p-type material. 26hese junctions have shown promising applications in photocatalytic CO 2 reduction, 27 degradation of dyes, 28 and photoelectrochemical water oxidation. 29To fabricate a Co 3 O 4 /ZnO pn heterojunction, ZnO nanomaterials and Co 3 O 4 nanoparticles were synthesized by many methods, for instance, a combination of the vapor-liquid-solid method at 800 °C and the hydrothermal method for 10 hours at 200 °C. 30In another approach, the Co 3 O 4 -ZnO core-shell structure was synthesized by hydrothermally fabricating pristine Co 3 O 4 nanowires on an FTO glass substrate at 110 °C for 5 hours and the Co 3 O 4 -ZnO lm was obtained by annealing in air at 500 °C for 15 minutes using a muffle furnace. 31In another study, Markhabayeva et al. 32 employed a combination of spin coating and chemical bath deposition, with two annealing stages at each step to synthesize the ZnO/Co 3 O 4 on ITO.In contrast to alternative synthesis methods that involve expensive equipment, high temperatures, and long synthesis time, the Co 3 O 4 /ZnO p-n heterojunction was successfully fabricated using the electrodeposition method.This approach not only saves time but also requires a simple equipment setup.The ZnO layer was electrodeposited for 8000 seconds, and the Co(OH) 2 layer was electrodeposited for 360 seconds.The Co 3 O 4 /ZnO junction was annealed in an ambient environment at 300 °C for 3 hours. 33n this study we synthesized a photoelectrode Co 3 O 4 /ZnO/ FTO specically for HER applications.The inuence of deposition time on the electrode properties was investigated.The fabricated electrode's characteristics were evaluated using XRD (X-ray diffraction), FTIR (Fourier-transform infrared spectroscopy), SEM-EDX (scanning electron microscopy with energydispersive X-ray spectroscopy), DRS (Diffuse Reectance Spectroscopy), and PL (photoluminescence)-various techniques were used to assess electrochemical properties, including LSV (Linear Sweep Voltammetry) and Mott-Schottky plot.The photoelectrochemical efficiency was also determined by measuring the photocurrent density under simulated solar light conditions.Finally, the mechanism of the hydrogen evolution reaction using the Co 3 O 4 /ZnO/FTO photocathode under simulated solar light illumination is proposed.

Synthesis of Co 3 O 4 /ZnO/FTO
A two-step fabrication process was used to synthesize Co 3 O 4 / ZnO/FTO materials.In the rst step ZnO was synthesized by electrochemical deposition in a three-electrode system from a 0.1 M zinc nitrate electrolyte solution.FTO glass, a pure zinc sheet (99.99%), and an Ag/AgCl electrode (in a saturated KCl solution) were used as the working electrode (WE), counter electrode (CE), and reference electrode (RE), respectively.FTO glass with a surface resistivity of about 13-15 U sq −1 was cut into samples of 1.0 cm × 2.0 cm, cleaned with acetone and DI water sequentially in an ultrasonic bath for 20 minutes, then dried.The electrolyte solution was maintained at 60 °C throughout the experiment.The deposition time was 5 minutes with a constant voltage of −1.0 V (vs.Ag/AgCl).Aer deposition, the ZnO/FTO lm was washed with DI water and dried in an oven.
In the second step, the previously prepared ZnO-coated FTO glass was used to continue the deposition of Co(OH) 2 at room temperature.The electrochemical three-electrode system at this time consisted of ZnO-coated FTO glass, a platinum (Pt) wire, and an Ag/AgCl electrode (in a saturated KCl solution) used as the working electrode, counter electrode, and reference electrode, respectively.The electrolyte solution contained 0.01 mol Co(NO 3 ) 2 $6H 2 O, 60 ml DI water, and 40 ml ethylene glycol.The deposition process was carried out at a constant voltage of −1.0 V (vs.Ag/AgCl) with the respective survey times of 2, 4, 6, and 8 minutes.Then, Co(OH) 2 /ZnO/FTO was dried and annealed at 300 °C for 2 hours with a constant heating rate of 3 °C min −

Characterization of materials
The crystal structures of the ZnO/FTO and Co 3 O 4 /ZnO/FTO samples were characterized by XRD using a Bruker D8 Advanced instrument with Cu Ka radiation (l = 1.5406Å) as the X-ray source, an electron acceleration voltage of 45 kV and a current of 45 mA.The instrument was set to scan from 10°to 80°with a scan rate of 0.02°s −1 .For FTIR measurements, a JASCO FT/IR-4700 was used to identify the vibrational features of the material's surface functional groups.The sample and KBr mixture were compressed into a round pellet with a diameter of 1 cm and a mass ratio of sample/KBr of 1/300.The compressed Nanoscale Advances Paper pellets were then placed in the instrument for analysis; the IR signal was scanned from wavenumber 4000 cm −1 to 400 cm −1 with a wavenumber resolution of 1 cm −1 .The surface morphology of the material was investigated by SEM using a JEOL JSM-IT500.Before observing the SEM images, the electrodes were placed in the measurement chamber with an accelerating voltage of 20 kV.JSM-IT500 was also used for EDX mapping to obtain the elements present in the sample and the weight percentage of each element.The optical properties of the material were determined from the DRS spectrum and the Tauc plot.The DRS method was performed on a JASCO V-770 spectrophotometer in the wavelength range from 300 to 800 nm.The PL spectrum was measured using a Cary Eclipse Fluorescence Spectrophotometer with an excitation wavelength of 300 nm.Photoelectrochemical measurements, including LSV, photocurrent density, Mott-Schottky, and EIS of the synthesized samples, were performed using a Biologic SP-200 potentiostat and were carried out in a three-electrode system.The working electrode was the investigated material, the counter electrode was a Pt wire, and the Ag/AgCl electrode (in saturated KCl solution) was the reference electrode.All electrochemical experiments were performed at room temperature in 0.1 M Na 2 SO 4 solution (pH ∼7).LSV measurements were performed at a scan rate of 10 mV s −1 .The potential was linearly scanned with time while the current was measured and recorded through a reversible hydrogen electrode (V vs. RHE) according to eqn (1): In this equation, E RHE is the potential concerning the reversible hydrogen electrode, E Ag/AgCl is the experimentally measured potential, and E 0 Ag/AgCl = 0.197 V at 25 °C.The photocurrent density was determined using a solar simulator device from Abet Technologies.During the measurement, the applied voltage was kept constant at 0.80 V (vs.RHE), the light window was turned on and off every 60 seconds per cycle, and the measurement was repeated seven times.Furthermore, Mott-Schottky measurements were performed at a xed frequency of 100 kHz to obtain the material's conduction band.Electrochemical impedance spectroscopy (EIS) measurements were carried out with a frequency range from 100 kHz to 0.1 Hz and an amplitude of 10 mV.

Structural and morphological properties
The synthesized samples' structural and crystalline characteristics were analyzed by XRD and FTIR (Fig. 1).The XRD patterns of ZnO/FTO and Co 3 O 4 /ZnO/FTO samples are shown in Fig. 1a.Accordingly, the characteristic diffraction peaks of ZnO from the standard pattern (JCPDS #36-1451) are observed at diffraction angles (2q) of 31.8°,34.3°, 36.5°,47.6°, 57.2°, 63.2°, 67.9°, 69.0°, and 72.7°, corresponding to the (100), (002), ( 101), ( 102), (110), ( 103), ( 112), (201), and (004) planes, respectively.The XRD results of the synthesized ZnO sample are consistent with the standard pattern, indicating that ZnO has been successfully deposited on the FTO substrate with a hexagonal wurtzite crystal structure. 33The prominent diffraction peaks of the ZnO sample corresponding to the (100), (002), and (101) planes have strong intensities, indicating that the formed material has a relatively good crystallinity.In addition, the diffraction peaks in the XRD patterns of Co 3 O 4 /ZnO/FTO samples synthesized with different electrolysis times all match well with the standard diffraction pattern of ZnO (JCPDS #36-1451), indicating that ZnO has been successfully deposited on the FTO substrate in all samples.It was observed that the intensity of the ZnO (101) peak gradually increases, and the intensity of the (002) peak gradually decreases with increasing Co 3 O 4 deposition time.This was explained by the diffraction peak corresponding to the (311) plane of Co 3 O 4 in the standard pattern (JCPDS #42-1467), which is the strongest.However, the position of the diffraction peak (Fig. 1b).In detail, the appearance of the bands at a wavelength of approximately 3440 cm −1 and 1600 cm −1 can be attributed to the stretching and bending vibrations of the O-H bond, which may be caused by adsorbed water on the material's surface or by atmospheric humidity. 34The signal at a wavelength of approximately 455 cm −1 is characteristic of the stretching vibration of the Zn-O bond. 35At a wavelength of 572 cm −1 and 664 cm  2c  and d.In detail, the Co 3 O 4 layer on top has a nanopetal structure that is intertwined and developed uniformly on the ZnO substrate, similar to previous reports on electrochemically deposited Co 3 O 4 . 37Thus, based on the SEM images of the samples, it can be concluded that the Co 3 O 4 /ZnO bilayer material with a nanopetal/nanoplate structure on the FTO substrate was successfully synthesized by the electrochemical deposition method with high coverage and uniformity.
In addition, EDX was employed to investigate the elemental composition and potential presence of impurities on the surface of both the ZnO/FTO and Co 3 O 4 /ZnO/FTO samples.The EDX mapping of the ZnO/FTO sample is shown in Fig. 2e-i.The appearance of Zn and O element peaks (Fig. 2e) has proved the successful synthesis of ZnO by electrochemical deposition.In addition, there are also peaks of other elements, such as Sn and F, which are thought to be from the FTO substrate.The mapping images (Fig. 2f-i) show that the elements Zn (red) and O (blue) appear mainly in the ZnO deposition area; at the same time, the uniform distribution of the mapping once again conrms that ZnO was deposited successfully and uniformly on the FTO substrate.Based on EDX mapping of the Co 3 O 4 /ZnO/ FTO sample (Fig. 2j-n), the existence of the elements Co, Zn, and O is shown, which implies the successful synthesis of Co 3 O 4 nanopetals on ZnO/FTO by the electrochemical deposition/ annealing method.In addition, no other elements were iden-tied, suggesting that the purity of the synthesized sample is high.

Optical properties
To evaluate the optical properties of the materials aer fabrication, the DRS spectra of the synthesized Co

Nanoscale Advances Paper
photocatalytic effects of the material.Moreover, the emission peak at 550 nm of ZnO is typically assigned to the oxygen vacancy defect states of ZnO.Meanwhile, the typical emission peak of Co 3 O 4 located at 563 nm could be assigned to the bandto-band emission of Co 3 O 4 , which is in agreement with the DRS result analysis in Fig. 3c.

Photoelectrochemical water splitting activity
The electrochemical water splitting activity of the material is shown by the LSV results (Fig. 5a and b).compared to the other samples.All the material samples used as catalytic electrodes showed a high current density at more negative potentials, and the LSV curves were also linear and straight, indicating that this material is quite stable in the electrochemical water splitting reaction to generate hydrogen.
Similarly, LSV measurements were also performed to evaluate the material's electrochemical water-splitting activity via the oxygen evolution reaction (OER).Fig. 5b is  Non-illuminated and illuminated LSV measurements were performed using a solar simulator for ZnO/FTO and Co 3 O 4 -2/ ZnO/FTO samples to investigate the photoelectrochemical activity of water splitting.LSV curves were also recorded at a scan rate of 10 mV s −1 in 0.1 M Na 2 SO 4 electrolyte solution.
The LSV results for the hydrogen evolution reaction of the materials are shown in Fig. 5c.The results show that both the ZnO and Co 3 O 4 -2/ZnO samples have lower onset and overpotentials when illuminated than when not illuminated.The photocurrent density of the illuminated samples is also higher From the illuminated LSV results, it can be concluded that light plays an important role in increasing the water splitting efficiency of the material.When light is irradiated onto the material, a higher water splitting current density is observed than in the non-illuminated state (under dark conditions).This suggests that light has excited the electrons in the material, giving them sufficient energy to participate in the watersplitting reaction, thereby increasing the number of electrons participating in the reaction and making the process more efficient.
The light-chopped LSV and photocurrent density measurements were performed to determine the material's lightresponsive ability, and the results are shown in Fig. 6.
The optimized Co 3 O 4 -2/ZnO and ZnO electrodes were utilized in the chopped LSV experiment to investigate the photoresponse during on-off cycling in the voltage range of 0.5 to 1.5 V vs. RHE.The results, shown in Fig. 6a, demonstrate that the chopped LSV curves were consistently similar to the continuous dark and light LSV curves.Additionally, all electrodes exhibited rapid and reproducible photoresponse throughout each on-off cycle.To assess the enhancement of the Co 3 O 4 /ZnO heterojunction compared to ZnO, the steady-state photocurrent was measured at 0.8 V vs. RHE.The results in Fig. 6b show that the photocurrent density of the Co 3 O 4 -2/ZnO sample is 1.4 times higher than that of the ZnO sample, from which it can be concluded that the ZnO material has a lower photo-response ability under the action of solar light than the Co 3 O 4 /ZnO material.This may be because ZnO has a large band gap, so it can only absorb energy from the UV region.In contrast, the combined sample may have a smaller band gap, resulting in more light energy being obtained (under the same illumination conditions).At the same time, the light energy conversion efficiency of the ZnO sample also decreases faster than the Co 3 O 4 /ZnO sample aer seven cycles, which shows that the combined material has higher stability and photoelectric efficiency than the ZnO material.
Table 1 shows the comparison of different PEC electrodes on the FTO substrate.The electrode fabrication process, involving the electrodeposition of ZnO for 5 minutes and Co(OH) 2 for 2 minutes, followed by a 3 hours annealing step at 300 °C, offers several advantages.It is a time-saving method that requires simple equipment and operates at a lower temperature than previous studies.Furthermore, the current density at 1.23 V under the same illumination conditions was 5.9 mA cm −2 .][43][44][45] The Mott-Schottky method evaluates synthesized material samples' carrier density and electronic conductivity type (p-type or n-type).The type of semiconductor is determined by the slope of the Mott-Schottky plot, with a positive slope indicating an ntype semiconductor and a negative slope indicating a p-type semiconductor.Additionally, Mott-Schottky measurements have been used to determine the materials' conduction band energy level (E CB ) before combination.A Mott-Schottky plot is a graph of the inverse square of the capacitance and the resistance components of the impedance versus the applied potential (C −2 vs. E) according to eqn (2).
where C sc is the capacitance of the space charge region, 3 the relative dielectric constant of the semiconductor, 3 0 the vacuum permittivity of the free space, N is the donor or the acceptor density, E the electrode potential, E  the at band potential, k is the Boltzmann constant, and T the absolute temperature.The plot of 1/C 2 against the applied voltage will show a tangent that

Paper
Nanoscale Advances cuts the horizontal axis, the point of intersection will indicate the value of E  , also known as the E CB of the material.The Mott-Schottky plot of the ZnO material presented in Fig. 7a shows a positive slope, which indicates that ZnO is an ntype semiconductor.The results show that the conduction band energy level of the ZnO sample is −0.22 V (vs.NHE).This result is consistent with the previously reported values of ZnO. 46Upon application of external reverse bias to a p-type semiconductor, the depletion region experiences an expansion, consequently leading to an increase in the space-charge capacitance and a subsequent decrease in the value of 1/C 2 . 47As a result, the negative slope in the Mott-Schottky plot of the Co 3 O 4 sample (Fig. 7b) provides conclusive evidence for its p-type conductivity, consistent with the presence of majority holes as charge carriers.In addition, the Mott-Schottky results also show that the conduction band energy level of the Co 3 O 4 material is 0.74 V (vs.NHE).
To study the charge-transport behavior existing between the electrode and the electrolyte junction, the data obtained from EIS measurements were analyzed using a simple Randles equivalent circuit (Fig. 7c and d).In this circuit, R 1 is the solution resistance, R 2 (R CT ) is the charge transfer resistance, Q 2 is the double layer capacitance, and W is the Warburg impedance.The diameter of the semicircle in the Nyquist plot of EIS is related to the charge transfer resistance and the electron-hole separation efficiency.Therefore, a smaller semicircle diameter indicates a lower charge transfer resistance, i.e., a higher charge transfer efficiency.Based on the EIS results shown in Fig. 7c, the semicircle diameter in the Nyquist plot of the ZnO sample is smaller than that of the Co 3 O 4 sample.This implies that the electrical resistivity for charge transport for the n-type semiconductor ZnO is lower than that for the p-type semiconductor Co 3 O 4 , and thus promotes the kinetics of surface reactions on ZnO.
From the conduction band energy level E CB , combined with the above band gap values, the energy diagram characteristic of the photoelectrochemical water splitting ability of the material is shown in These electrons directly participate in the water splitting reaction.Therefore, the increasing concentration of electrons in the CB of ZnO increases the photocatalytic activity of the material.
In addition, the internal electric eld generated at the interface of the two layers of material also plays an important role in slowing down the recombination of the electron-hole pair. 33his also signicantly enhances the photocatalytic activity of the material.
Paper Nanoscale Advances corresponding to the (311) plane of Co 3 O 4 was overlapped with that of the ZnO (101) peak.The overlap of the two planes explains the change in the symmetry of the peak at this position, and when the electrolysis time increases, the amount of Co 3 O 4 on ZnO will increase, leading to increased diffraction intensity.This result partly shows the successful deposition of Co 3 O 4 on ZnO.However, the XRD patterns of all the samples did not show any clear diffraction peaks of Co 3 O 4 .Therefore, the FTIR method was performed to verify the presence of Co 3 O 4 −1 , two characteristic peaks appear that correspond to the stretching vibrations of the two bonds: Co(III)-O when the Co 3+ ions are in an octahedral coordination state and Co(II)-O when the Co 2+ ions are in a tetrahedral coordination state. 36These characteristic peaks are present in all Co 3 O 4 /ZnO/FTO samples, demonstrating the successful synthesis of Co 3 O 4 on the ZnO structure.At the same time, these are also two characteristics of the spinel structure of Co 3 O 4 .This result again conrms the successful synthesis of Co 3 O 4 on the ZnO/FTO substrate.In addition, the sharp peak at around 1398 cm −1 is considered the vibrational signal of the NO 3 − radical.This is likely since both the syntheses of ZnO and Co 3 O 4 use metal nitrate salts as the electrolyte solution, leading to residual NO 3 − on the material's surface.It is also noted that because the ZnO synthesis process only involves drying without requiring high-temperature annealing, the amount of NO 3 − on the ZnO sample is the highest, leading to the strongest absorption intensity.The surface morphology of ZnO synthesized by electrochemical deposition on the FTO substrate was observed by SEM.Fig. 2a is a low magnication SEM image (2000×) showing that the surface image of the ZnO lm developed uniformly on the FTO substrate.Fig. 2b is a high magnication SEM image (20 000×), indicating that the ZnO obtained has a thin nanoplate structure arranged closely together.SEM also provided the surface morphology of the Co 3 O 4 /ZnO/FTO sample, as in Fig.
3 O 4 /ZnO samples and the Tauc plots are calculated from the DRS spectra of ZnO, Co 3 O 4 -2, and Co 3 O 4 -2/ZnO samples, which are shown in Fig. 3.The Co 3 O 4 /ZnO samples were synthesized with different deposition times (Fig. 3a).For samples with Co 3 O 4 deposition times
Co 3 O 4 /ZnO samples with different Co 3 O 4 deposition times were subjected to LSV (HER) measurements to evaluate the HER activity.The LSV curves were recorded at a scan rate of 10 mV s −1 in 0.1 M Na 2 SO 4 electrolyte solution.In addition, FTO glass and ZnO/FTO (same size) samples were also prepared for LSV measurements for comparison.Fig. 5a shows that the Co 3 O 4 -2/ZnO, Co 3 O 4 -6/ZnO, and Co 3 O 4 -8/ZnO samples have the lowest onset potential, which is close to each other (−0.7 V).However, the Co 3 O 4 -2/ZnO sample has the highest current density (127.7 mA cm −2 ) a characteristic LSV curve for the OER reaction performed in 0.1 M Na 2 SO 4 electrolyte recorded at a scan rate of 10 mV s −1 .The results show that the Co 3 O 4 -2/ZnO sample has the lowest onset potential (0.82 V) and the smallest overpotential of 470 mV at a current density of 10 mA cm −2 .Based on the two LSV (HER and OER) results, it can be concluded that the Co 3 O 4 -2/ZnO material exhibits the best electrochemical water-splitting activity.Therefore, the optimal deposition time for Co 3 O 4 on ZnO is 2 minutes.It was also found that the deposition of Co 3 O 4 on the surface of ZnO can increase the efficiency of the electrochemical water-splitting reaction.

Fig. 5
Fig. 5 HER (a) and OER (b) curves of FTO, ZnO and Co 3 O 4 /ZnO samples; HER (c) and OER (d) curves of ZnO and Co 3 O 4 -2/ZnO samples under the illuminated and non-illuminated conditions.