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Article

Thin-Film Nanocrystalline Zinc Oxide Photoanode Modified with CdO in Photoelectrocatalytic Degradation of Alcohols

by
Vitali A. Grinberg
,
Victor V. Emets
,
Alexander D. Modestov
*,
Aleksey A. Averin
and
Andrei A. Shiryaev
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky Prospekt 31, Building 4, 119071 Moscow, Russia
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(6), 1080; https://doi.org/10.3390/coatings13061080
Submission received: 9 May 2023 / Revised: 1 June 2023 / Accepted: 8 June 2023 / Published: 11 June 2023
(This article belongs to the Collection Feature Papers of Coatings for Energy Applications)
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Abstract

:
Thin-film nanocrystalline zinc oxide electrodes were fabricated by electrochemical deposition of ZnO on FTO-coated glass slides. ZnO electrodes were promoted by CdO coating on top of ZnO in amounts corresponding to 0.8, 0.1, and 0.05 C cm−2 in electric units. Modification of ZnO by a small amount of CdO, corresponding to 0.05 C cm−2, shifts the photoactivity of the composite photoanode into the visible part of the solar spectrum. It is shown that the ZnO/(0.05C)CdO/FTO electrode demonstrates high efficiency in photoelectrochemical degradation of methanol, ethylene glycol, and glycerol when irradiated by a simulated sunlight. According to intensity-modulated photocurrent spectroscopy (IMPS), the effect is due to suppression of the electron–hole pairs recombination and increase in the rate of photo-induced charge transfer. Therefore, thin-film photoanodes based on zinc oxide modified by CdO can be used for photoelectrochemical degradation of byproducts of biofuel production glycerol, and of other alcohols.

1. Introduction

Environmental pollution is the crucial problem that is to be solved for sustainable development of society. Degradation of aqueous organic pollutants by photocatalytic and photoelectrochemical methods are very attractive since solar irradiation energy is employed in the energy-intense processes [1,2,3,4]. Since the degradation of organic pollutants involves electrooxidation, gaseous hydrogen evolves. Some useful products can be produced by photoelectrochemical oxidation of organic pollutants; e.g., in photoelectrochemical oxidation of glycerin, a byproduct of biodiesel production, a number of useful products can be formed, including organic acids, dihydroxyacetone, etc. [5,6,7,8,9,10,11]. Semiconductors based on metal oxides such as titanium dioxide and zinc oxide are the most attractive materials of photoanodes because of their availability at low price, high chemical, optical, and thermal stabilities, high electron mobility, and low toxicity [3,12]. TiO2 and ZnO are wide-band-gap semiconductors which are able to absorb only a small fraction of solar spectrum; for instance, a ZnO photoanode with a band gap of 3.37 eV absorbs only 4%. Furthermore, due to incomplete separation of photogenerated electron–hole pairs in ZnO, the efficiency of this semiconductor electrode is rather low. The application of heterostructure electrodes allows for better separation of photogenerated electron–hole pairs and significantly improves the properties of ZnO-based photoelectrochemical devices [13,14,15,16,17,18,19,20,21,22].
CdO is a semiconductor with a direct band gap of 2.5 eV and an indirect band gap of 1.98 eV. Presumably, it can serve as an effective photocatalyst due to marked absorption in the visible range of solar spectrum and high mobility of charge carriers [23]. Only few reports on photoelectrocatalysis by ZnO/CdO heterojunction photoanodes exist. In [24], a ZnO/CdO heterojunction was deposited on a reduced graphene oxide ZnO/CdO/(rGO). Upon irradiation with visible light (λ = 428 nm), current density reached 0.62 mA cm−2 at ZnO/CdO and 1.15 mA cm−2 at ZnO/CdO/(rGO) photoanodes at E = 1.23 V vs. RHE. It was also mentioned that on a Cd-doped zinc oxide photoanode the photocurrent density was 5.42 μA cm−2. It is worth mentioning that photoelectrocatalytic activity of thin-film photoanodes CdO/ZnO was not tested in degrading pollutants. For this reason, research on photocatalytic degradation of rather chemically stable alcohols by thin-film photoanodes based on ZnO promoted by CdO is of particular interest, especially from the point of view of photodegradation of organic pollutants. Photodegradation of polyalcohols, such as glycerin, deserves special attention since oxidation products of these alcohols can be valuable compounds. It should be noted that the use of light within the solar spectrum in the research paves the way for development of the practical process.
In the present work, we investigated the photoelectrocatalytic activity of ZnO films prepared by electrochemical deposition followed by modification with different amounts of CdO in the reaction of photoelectrocatalytic degradation of methanol, ethylene glycol, and glycerol under visible light illumination. The influence of the organic species present in the electrolyte on recombination of photogenerated charge carriers in CdO-modified ZnO films is elucidated.
Stability of the synthesized photoanodes was demonstrated in this work. This point is of particular interest since cadmium compounds are toxic [25].
End products of the photoelectrochemical oxidation of alcohols were not addressed.

2. Materials and Methods

Chemically pure zinc chloride ZnCl2, potassium chloride KCl, and Cd(NO3)2·4H2O (Aldrich, St. Louis, MO, USA) were used without further purification. To manufacture the photoanodes, glass substrates with electrically conductive coating of fluorine-stabilized tin dioxide (FTO) were employed (Aldrich; specific resistance ≈ 7 Ω cm−2). The electrolyte was purged with high-purity oxygen (99.999%).

2.1. Synthesis of the ZnO Photoanode

Zinc oxide was deposited on an FTO-coated glass substrate. The FTO substrate was cleaned in an ultrasonic bath using the following solvents sequentially: acetone, isopropyl alcohol, and distilled water. The treatment time in each solvent was 15 min. The cleaned substrate was fixed in a Teflon frame (Figure S1) exposing 1 cm2 area. A three-electrode system was assembled on a Teflon® cover of 100 mL electrochemical cell equipped with a water jacket. A Pt90Ir10 alloy plate with a geometric area of 8 cm2 was used as a counter electrode; a silver chloride electrode (Ag/AgCl + KClstd) served as a reference electrode. All potentials are given relative to this reference unless otherwise stated. Potentials relative to a reversible hydrogen electrode can be determined from the equation ERHE = EAg/AgCl + 0.059 × pH + EoAg/AgCl, where EoAg/AgCl = 0.197. The distance between the cathode and the counter electrode was 3 cm. The aqueous 5 mM ZnCl2 + 0.1 M KCl electrolyte was stirred and saturated with oxygen. Zinc oxide was deposited on an FTO slide at a constant potential E = −1.0 V. The electric charge passed in ZnO electrodeposition was 2.0–2.2 C cm−2. The resulting film thickness was about 500–600 nm.
Washed and dried samples were calcined in air at 400 °C for 1 h. After cooling in the oven for 12 h, the obtained samples coated with the uniform ZnO film were used for further investigation.

2.2. Synthesis of ZnO/CdO Photoanode

Cadmium oxide was deposited on the preformed ZnO film at a constant potential E = −0.75 V (vs. Ag/AgCl) from a 5 mM Cd(NO3)2·4H2O + 0.1 M KCl solution while stirring the electrolyte. The same electrochemical cell with a Pt–Ir alloy anode was used. Depending on the deposition time, the electric charge that passed varied between 50 and 800 mC. CdO formation via generation of intermediate Cd(OH)2 was proposed in [26]:
NO3− + H2O + 2e− → NO2− + 2OH−
Cd2+ + 2OH− → Cd(OH)2
After rinsing and drying, the sample was calcined in an air oven at 350 °C for 3.0 h and then cooled as described above. Cd(OH)2 is converted to CdO at temperatures above 280 °C by the following reaction [27]:
Cd(OH)2 → CdO + H2O
The prepared photoanodes with zinc oxide film modified with CdO were used in all experiments.

2.3. Characterization of Zinc Oxide and Modified Zinc Oxide Films

2.3.1. Study of Structure, Phase, and Chemical Composition

X-ray Diffraction

The phase composition of the deposited film coatings was studied by X-ray diffraction (XRD) analysis on an Empyrean X-ray diffractometer (Panalytical BV, Almelo, The Netherlands). Ni-filtered Cu–Kα (λ = 1.542 Å) radiation was used; the samples were studied in the Bragg–Brentano geometry. Experimental diffraction patterns were processed using the Highscore program; the phase composition was identified using the ICDD PDF-2 diffraction database. The average size of crystallites of the identified phase was determined from the broadening of the observed diffraction peaks using Scherrer method.

X-ray Fluorescence

Spatial distribution of Cd, Zn, and of other elements on the glass substrate was studied using energy-dispersive X-ray fluorescence (XRF). An XGT-7200V (Horiba, Kyoto, Japan) device operating with a Rh tube was used. In order to maximize contribution of thin film, a minimal accessible excitation energy of 15 keV was used. The sample was scanned with a 1.2 mm beam with a strong overlap of the analyzed points to improve the resolution; Cd L and Zn K lines were used to map distribution of these elements.

Light Absorption Spectra

Absorption spectra of the obtained films were studied in a range of 300–700 nm at room temperature using a Lambda35 Perkin Elmer spectrometer.

Raman Spectra

Raman spectra were recorded using an inVia “Reflex” Raman spectrometer (Renishaw, New Mills Wotton-under-Edge, UK) with a 50× objective. The 405 nm line of a diode laser was used for excitation, and light beam power on the sample was less than 0.2 mW.

Film Thickness Measurement

The thickness of the electrodeposited films was determined by a thin-films measurement system MProbe 20 (Semiconsoft Inc., Southborough, MA, USA). The measurement range was from 1 nm to 1 mm.

Photoelectrooxidation

Photoelectrochemical measurements were performed using a (dedicated) setup comprising a photoelectrochemical three-electrode cell PECC-2 (Zahner Elektrik, Kronach, Germany), a 150 W Solar spectrum simulator 96,000 (Newport, RI, USA) with an AM1.5G filter, and an IPC-Pro MF potentiostat (IPChE RAS, Moscow, Russia). The working electrode in the cell was a 1 cm2 photoanode with a film coating made of zinc oxide or zinc oxide doped with cadmium oxide. A Pt wire mesh with a surface area of ≈3 cm2 was used as an auxiliary electrode. An Ag/AgCl reference electrode was employed. Illumination hit the oxide film from the rear side of the photoanode. The illumination power density was determined using a Nova instrument (OPHIR-SPIRICON Inc., Jerusalem, Israel). Visible light illumination of 1 sun at a power density of 100 mW cm−2 was applied to the prepared photoanodes for photoelectrochemical oxidation of the selected alcohols.
Intensity-modulated photocurrent spectroscopy (IMPS) data were obtained using a Zahner CIMPS-QE/IPCE computerized photoelectrochemical workstation (Zahner-Elektrik Gmbh & Co. KG, Kronach, Germany). The workstation included TLS03 monochromatic light source with a set of LEDs covering light spectrum 320 to 1020 nm and the software package. Incident photon-to-current conversion efficiency (IPCE) data were collected in the wavelength range 350−800 nm with 10 nm spectral resolution. The IMPS spectra were measured using monochromatic light of 407 nm wavelength. Light intensity on the photoanode surface was 14 mW cm−2. A sinusoidal disturbance (~10% of stationary illumination) in the frequency range from 0.02 to 2·103 Hz was superimposed on a constant base light intensity. Normalized IMPS curves were obtained by dividing the real and imaginary components of the experimental IMPS curve (Re(Iph) and Im(Iph)) by the maximum value of Re(Iph) denoted as I2.

3. Results

Several photoanode samples were fabricated for research. Samples with electrodeposited films of pure zinc oxide are designated as ZnO/FTO. Photoanodes with films of modified zinc oxide are denoted as ZnO/(0.05)CdO/FTO, ZnO/(0.1)CdO/FTO, and ZnO/(0.8)CdO/FTO, where the number in parentheses denotes the electric charge spent on the electrodeposition of CdO in Coulombs per cm2 of the geometric surface area of the photoanode.
X-ray diffraction patterns of the FTO glass slide and the films deposited on the FTO glass are shown in Figure 1.
Phase composition of ZnO–CdO compounds deposited on FTO glass substrate was analyzed using X-ray diffraction in Bragg–Brentano geometry (Figure 1). The employed out-of-plane diffraction scheme is sensitive to crystallographic planes oriented parallel to the substrate. The only crystalline phase observable in all films is the hexagonal ZnO (zincite, ICDD card 01-079-0205). Despite wide variations in applied amount of electricity during CdO deposition and clear effect of the Cd cations on electrophysical properties of the films, no individual Cd phase was observed. Most likely, concentration of incorporated Cd cations does not exceed solubility limits, and a CdO–ZnO solid solution is formed [28]. At high amounts of the CdO dopant (e.g., 0.1 C), the size of crystallites is difficult to determine using the Scherrer method due to narrow peaks (thus, the sizes are in the range of several hundred nm); at (0.05) the size of crystallites is approximately 50–60 nm. At the same time, at high amounts of CdO, the microstrain is much more significant. With reservation due to limitations of the analysis based on individual peaks, it appears that the strain is the largest in the (100) plane of ZnO (2θ is 31.91°); other directions are less strained. The absence of a separate Cd-containing phase in a ZnO–CdO thin film with low concentration of Cd admixture (5.5%) was also noted in [29].

3.1. Distribution of Cd in the Films

Presence of Cd in the deposited films was confirmed using XRF, Figure 2A. Maps of spatial distribution of various chemical elements (Figure 2B,C) show very close correspondence of Cd and Zn distributions, confirming their successful deposition in employed process. The mapped area is 24 mm × 24 mm.
Figure 3 shows the Raman spectra of the deposited films. The observed characteristic peaks at 438 cm−1 and 575 cm−1 belong to hexagonal zinc oxide. Small shifts of these peaks from the reference positions may arise from substrate-induced strain [30]. Ideal stoichiometric cubic CdO does not possess Raman-active vibrations; however, deviations from stoichiometry (e.g., oxygen vacancies) may lead to the appearance of Raman peaks at 233 cm−1, 264 cm−1, and 364 cm−1 [31]. Even if present, these peaks are very weak and may be very difficult to discern on strong background from ZnO and glass substrate. Senthil et al. also did not find a Raman peak for CdO of composite ZnO–CdO [32].
Absorption spectra of the original ZnO and CdO-modified ZnO films are shown in Figure 4a. To facilitate comparison of films with different thicknesses, the curves were normalized to [0, 1] (Figure S2). Comparison of curves in Figure 4a shows that modification of ZnO by CdO increases the light absorption in the whole spectral range. The band gap of the electrodeposited films was estimated in Tauc coordinates [33,34] (Figure 4b). The direct band gap of a semiconductor (Eg) can be obtained by extrapolation of the linear part of the function (αhν)2 to the X axis (photon energy hν). As seen from Figure 4b, the modification of ZnO with cadmium oxide lowers the Eg to 3.06–3.09 eV, compared with 3.14 eV for the unmodified ZnO/FTO sample. It should be noted that for all samples, an absorption feature characteristic for pure ZnO [35,36] is observed between 360 nm and 380 nm.

3.2. Photoelectrocatalytic Oxidation of Water, Methanol, Ethylene Glycol, and Glycerol on the Modified ZnO Photoanodes

Combining CdO and ZnO at a nanoscale by formation of a solid solution enables the formation of a material with enhanced, compared to pure ZnO, photoelectrochemical properties in a water photoelectrooxidation reaction.
According to Figure 5, modification of a thin-film ZnO photoanode by electrodeposition of cadmium oxide in small amounts results in an increase in the photocurrent density of water oxidation by a factor of 1.2 at E = 0.6 V, with the curves 6 and 3 corresponding to ZnO/(0.05)CdO/FTO and ZnO/FTO, respectively. Presumably, the effect occurs due to the suppression of the surface recombination of photogenerated electron–hole pairs. This result can be explained in terms of an increase in the density of surface states (SS), as was previously observed in [37].
However, excessive modification of ZnO by CdO leads to a decrease in the photocurrent density of water oxidation (Figure 5, curves 4 and 5), probably due to a decrease in the SS density. Possible precipitation of poorly ordered CdO particles lacking clear Raman and/or XRD features may also partially block the photoanode surface, since the photoelectrocatalytic activity of CdO is much lower than that of ZnO (Figure 5, curve 2).
It was also of interest to study the photoelectrooxidation of organic depolarizers, namely, alcohols, on a CdO-promoted ZnO photoanode. Figure 6 shows the polarization curves of photoelectrooxidation of CH3OH, C2H4(OH)2, and C3H5(OH)3 in an aqueous 0.5 M Na2SO4 on ZnO/(0.05)CdO/FTO film photoanode.
According to Figure 6, the voltammetry curves of photoelectrochemical oxidation of all these alcohols are shifted to more negative potentials compared to electrooxidation of the alcohols in dark conditions. At the photoanode potential 0.6 V (vs. Ag/AgCl electrode), the photocurrent in electrolytes containing 20% methanol, 20% ethylene glycol, or 20% glycerol is increased by a factor of 2.1–2.3, compared to photocurrent measured in a 0.5 M Na2SO4 solution. It indicates acceleration of the electrooxidation reaction by irradiation in the series: H2O < MeOH < C2H2(OH)2 < C3H5(OH)3. The results obtained can be explained by the influence of the chemical nature of the depolarizer both on the rate constant of photoelectrooxidation and on recombination processes on the surface states of the CdO-promoted zinc oxide photoanode. The sequence of voltammetric curves obtained during the photoelectrooxidation of alcohols implies that the recombination losses on CdO-modified zinc oxide photoanodes are somewhat higher during the oxidation of methanol and ethylene glycol compared to glycerol. It can be assumed that the decrease in recombination losses during the photoelectrooxidation of glycerol is due to its stronger adsorption on the surface of the composite photoanode compared to water, methanol, and ethylene glycol. A similar observation for the same series of alcohols was made when studying the photoelectrocatalytic properties of a titanium-promoted nanocrystalline hematite photoanode [38].
Voltammograms of the composite photoanode with higher amount of electrodeposited cadmium oxide (0.8) in reactions of photoelectrocatalytic oxidation of methanol, ethylene glycol, and glycerol are shown in Figure S3. Comparison of Figure 6 and Figure S3 shows that at high cadmium oxide loading the photoelectrooxidation currents of alcohols are significantly reduced, keeping the shapes of curves and their sequence unchanged. It can be assumed that since the photocurrent densities of alcohol oxidation are determined by the concentration of photogenerated holes, an increase in the concentration of the modifying component results in partial blocking of the photoanode surface and a decrease in the concentration of photogenerated holes. Apparently, an increase in adsorption sites on the surface of the photoanode of polyatomic alcohols leads to an increase in the consumption of holes, which is more typical for trihydric glycerol. Voltammograms of photoelectrooxidation of glycerol for low and high loadings of the promoter (CdO) are shown in Figure S4. According to it, optimization of CdO loading makes it possible to increase the efficiency of ZnO photoanodes in the processes of photoelectrooxidation of organic substrates. The stability of photoanode ZnO/(0.05)CdO/FTO is made evident by measurements of current vs. time curves showing independence of photocurrent on time at potentials 0.4 V and 0.62 V vs. Ag/AgCl (Figure S5).
Comparing the results of this study with published data, it can be noted that partial photocurrent of glycerol oxidation in 0.5 M Na2SO4 at ZnO/(0.05)/CdO/FTO photoanode is 0.86 mA cm−2 (E = 1.23 V vs. RHE), while photoelectrochemical oxidation on a hematite photoanode did not exceed 0.25 mA cm−2 [39], and on a cobalt-promoted zinc oxide photoanode, photocurrent was 0.4 mA cm−2 [40]. However, under similar conditions, glycerol oxidation current on a BiVO4 photoanode in a 0.5 M Na2SO4 solution at pH 5 and pH 7 reached 1.0 mA cm−2 [1].

3.3. Evaluation of Recombination Losses in the Photoelectrooxidation of Alcohols

Figure 7 shows dependence of the quantum efficiency (IPCE%) of photocurrent generation of ZnO/(0.05C)CdO/FTO film photoanode on the wavelength of incident light measured in an aqueous 0.5 M Na2SO4 solution. It can be seen that the modification of ZnO with a small amount of CdO extends the photoactivity of the photoanode up to 420 nm, which is consistent with the absorption spectrum of this sample (Figure 4, curve 3).
IMPS measurements were used to quantify recombination losses and charge transfer rate constants for photoelectrooxidation of water and alcohols at photoanodes [41,42,43,44]. In the IMPS measurements (see Figure 8), monochromatic irradiation with a wavelength of 407 nm (blue region of the visible spectrum) was used.
Results of IMPS measurements with ZnO/(0.05)CdO/FTO in 0.5 M Na2SO4 are given in Figure S6. Similar IMPS curves were measured with ZnO/(0.05)CdO/FTO photoanode in the presence of different alcohols in 0.5 M Na2SO4.
The curves were normalized to determine the rate constants of hole recombination and hole transfer rate to the alcohol molecules of adsorbed at the photoanode surface. The normalized IMPS curves for a ZnO/(0.05)CdO/FTO film photoanode in solutions with various depolarizers are shown in Figure 8. The high-frequency intercept of these curves with the X-axis gives the value of total photogenerated current I2. It includes nonstationary recombination current and the electrooxidation photocurrent (I1) of the species present in the electrolyte. The low-frequency intercepts give the values related solely to oxidation of the species present in the electrolyte I1. The IMPS curves were normalized by I2 values. The ratio (I2-I1)/I2 gives the fraction of recombination losses. As follows from Figure 8 curve 1, during the photoelectrooxidation of water a significant surface recombination of photogenerated charge carriers occurs. At a potential of 0.6 V, the electrooxidation photocurrent of species in the electrolyte I1 was ~55% of the total photogenerated current I2. Hence, the recombination losses during the photooxidation of water amounted to 45% of the photogenerated current.
The introduction of alcohols (20%) into an aqueous solution of 0.5 M Na2SO4 reduces the recombination losses at the photoanode at E = 0.6 V from ~45% for water to ~22% in the case of CH3OH, to ~14% for C2H4(OH)2, and to 8% for C3H5(OH)3 (Figure 8, curves 2, 3, and 4). The result obtained shows that the studied alcohols are more efficiently oxidized by the photogenerated holes than water molecules.
From the normalized IMPS data, the recombination rate constants Krec and charge transfer rate constants Kct were calculated. The low-frequency intercept on the X-axis of photocurrent with X-IMPS (I1/I2 in Figure 8) is related to these constants as I1/I2 = Kct/(Krec + Kct). The light modulation frequency that corresponds to maximum value of Im(Iph/I1), Figure 8, enables us to find the sum (Krec + Kct) from the relation 2πfmax = (Krec + Kct). The values of Krec and Kct calculated from these equations for the photoelectrooxidation of water, methanol, ethylene glycol, and glycerol are summarized in Table 1. The high-frequency part of the IMPS response is attenuated by the RC time constant of the electrochemical cell (Figure 8). In the case of the oxide films studied here, the RC time constant arises from the series resistance of the FTO glass used as substrate and the high space charge capacitance of the oxide film. The fmin/fmax ratio for the ZnO/(0.05)CdO/FTO photoanode in the studied solutions is ≥50, which ensures sufficient accuracy of the calculated values of Krec and Kct [44]. Results shown in Figure 8 imply that the chemical nature of the depolarizer affects both constants. An increase in Kct is observed in the series H2O < MeOH < C2H4(OH)2 < C3H5(OH)3. The accelerated consumption of holes entering the surface states (SSs) of the photoanode in the reaction of photoelectrooxidation of the studied alcohols, in turn, causes a decrease in Krec in the same sequence. The same pattern was observed for this series of alcohols on a nanocrystalline hematite photoanode promoted with titanium [38].

4. Conclusions

Thin-film ZnO/CdO/FTO composite photoanodes were prepared by electrochemical deposition of oxides on an FTO glass substrate. Modification of a zinc oxide film by a small amount of cadmium oxide (0.05) shifts light absorption and photoactivity of the composite photoanode into the visible part of the solar spectrum, up to 420 nm, and improves the electrocatalytic properties of the composite photoanode in photoelectrooxidation reaction of water. Namely, at E = 1.23 V (vs. RHE) photoelectrooxidation, current density at thin-film ZnO/CdO/FTO composite photoanodes increases by 20% compared to a pure zinc oxide photoanode. The results can be explained by an increase in the density of surface states on the composite photoelectrode. It is shown that zinc oxide electrodes promoted by CdO demonstrate high efficiency in the photoelectrochemical destruction of methanol, ethylene glycol, and glycerol, when irradiated by a sunlight simulator. According to IMPS measurements, the improvement is due to the suppression of recombination of electron–hole pairs and an increase in the rate of photoinduced charge transfer. Both factors increase in the sequence H2O < MeOH < C2H4(OH)2 < C3H5(OH)3. Thus, thin-film photoanodes based on zinc oxide modified with cadmium oxide can be used for photoelectrochemical degradation of a byproduct of biofuel production—glycerol and other alcohols.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings13061080/s1, Figure S1: Three-electrode electrochemical cell used for deposition of ZnO and CdO on FTO-coated glass slide. Part of the FTO slide, which is not intended for oxide deposition, is isolated by a Teflon frame; Figure S2: Normalized to [0, 1] absorption spectra for film photoanodes: (1) ZnO/FTO; (2) ZnO/(0.8C)CdO/FTO; (3) ZnO/(0.05C)CdO/FTO; (4) ZnO/(0.1C)CdO/FTO; Figure S3: Voltammograms of a ZnO/(0.8C)CdO /FTO film photoanode obtained: (1) in dark conditions, and (2–5) under visible light illumination with a power density of 100 mW cm−2 in aqueous solutions of (2) 0.5 M Na2SO4; (3) 0.5 M Na2SO4 + 20% CH3OH; (4) 0.5 M Na2SO4 + 20% C2H4(OH)2; and (5) 0.5 M Na2SO4 + 20% C3H5(OH)3. Potential scan rate is 10 mV s−1. The dark curves for all solutions practically coincide; Figure S4: Voltammograms obtained for film photoanodes in: (1) dark conditions, (2) ZnO/(0.8)CdO/FTO (3) ZnO/(0.05)CdO/FTO in aqueous solutions of 0.5 M Na2SO4 + 20% C3H5(OH)3 (2-3) under visible light illumination with a power density of 100 mW cm−2. Potential scan rate is 10 mV s−1. The dark curves for two films practically coincide; Figure S5: I-t curves of film photoanode ZnO/(0.05)CdO/FTO at various bias potentials (1) 0.4 V and (2) 0.62 V vs. Ag/AgCl in 0.5 M Na2SO4 + 20% C3H5(OH)3 under visible light illumination with a power density of 100 mW cm−2; Figure S6: IMPS dependence for a ZnO/(0.05C)CdO/FTO film photoanode illuminated with monochromatic light 407 nm wavelength in 0.5 M Na2SO4; illumination power density 14 mW cm−2; photoanode potential 0.57 V.

Author Contributions

Conceptualization and samples preparation, V.A.G.; conceptualization and IMPS investigation, V.V.E.; photoelectrochemical measurements, A.D.M.; UV and Raman spectroscopic measurements, A.A.A.; XRD and X-ray fluorescence studies, A.A.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the Ministry of Science and Higher Education of the Russian Federation.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This research was performed using the equipment of the Center for Collective Use of Physical Investigation Methods of the IPCE RAS.

Conflicts of Interest

The authors declare no conflict of interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. X-ray diffraction patterns (Cu–Kα radiation) of the samples: (1)—FTO glass substrate, (2)—ZnO/FTO; (3)—ZnO/(0.1)CdO/FTO; (4)—ZnO/(0.05)CdO/FTO.
Figure 1. X-ray diffraction patterns (Cu–Kα radiation) of the samples: (1)—FTO glass substrate, (2)—ZnO/FTO; (3)—ZnO/(0.1)CdO/FTO; (4)—ZnO/(0.05)CdO/FTO.
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Figure 2. Results of X-ray fluorescence analysis. (A)—Part of the EDX spectrum highlighting region of Cd L-lines; the curves are displaced vertically for clarity. The inset zooms into the Cd Lα1 region. (B,C)—Intensity maps of Zn Kα1 and Cd Lα1 lines, respectively.
Figure 2. Results of X-ray fluorescence analysis. (A)—Part of the EDX spectrum highlighting region of Cd L-lines; the curves are displaced vertically for clarity. The inset zooms into the Cd Lα1 region. (B,C)—Intensity maps of Zn Kα1 and Cd Lα1 lines, respectively.
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Figure 3. Raman spectra of the film samples: (1) ZnO/FTO, (2) ZnO/(0.8)CdO/FTO.
Figure 3. Raman spectra of the film samples: (1) ZnO/FTO, (2) ZnO/(0.8)CdO/FTO.
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Figure 4. Light absorption spectra—(a) and absorption spectra in Tauc coordinates (αhν)2 versus hν—(b) for the film photoanodes: (1) ZnO/FTO; (2) ZnO/(0.8)CdO/FTO; (3) ZnO/(0.05)CdO/FTO; (4) ZnO/(0.1)CdO/FTO.
Figure 4. Light absorption spectra—(a) and absorption spectra in Tauc coordinates (αhν)2 versus hν—(b) for the film photoanodes: (1) ZnO/FTO; (2) ZnO/(0.8)CdO/FTO; (3) ZnO/(0.05)CdO/FTO; (4) ZnO/(0.1)CdO/FTO.
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Figure 5. Voltammograms measured at photoanodes in aqueous 0.5 M Na2SO4 under dark conditions—(1), and (2–6) under illumination with visible light of 100 mW cm−2: (2) CdO/FTO, (3) ZnO/FTO, (4) ZnO/(0.8)CdO/FTO, and (5) ZnO/(0.1)CdO/FTO; (6) ZnO/(0.05)CdO/FTO. Potential scan rate is 10 mV s−1. The dark curves for all films practically coincide.
Figure 5. Voltammograms measured at photoanodes in aqueous 0.5 M Na2SO4 under dark conditions—(1), and (2–6) under illumination with visible light of 100 mW cm−2: (2) CdO/FTO, (3) ZnO/FTO, (4) ZnO/(0.8)CdO/FTO, and (5) ZnO/(0.1)CdO/FTO; (6) ZnO/(0.05)CdO/FTO. Potential scan rate is 10 mV s−1. The dark curves for all films practically coincide.
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Figure 6. Voltammograms of a ZnO/(0.05)CdO/FTO film photoanode obtained under dark conditions—(1), and (2–5) under illumination with visible light 100 mW cm−2: in aqueous solutions of (2) 0.5 M Na2SO4; (3) 0.5 M Na2SO4 + 20% CH3OH; (4) 0.5 M Na2SO4 + 20% C2H4(OH)2; and (5) 0.5 M Na2SO4 + 20% C3H5(OH)3. Potential scan rate is 10 mV s−1. Dark voltammetry curves for all solutions practically coincide.
Figure 6. Voltammograms of a ZnO/(0.05)CdO/FTO film photoanode obtained under dark conditions—(1), and (2–5) under illumination with visible light 100 mW cm−2: in aqueous solutions of (2) 0.5 M Na2SO4; (3) 0.5 M Na2SO4 + 20% CH3OH; (4) 0.5 M Na2SO4 + 20% C2H4(OH)2; and (5) 0.5 M Na2SO4 + 20% C3H5(OH)3. Potential scan rate is 10 mV s−1. Dark voltammetry curves for all solutions practically coincide.
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Figure 7. IPCE% spectrum of the ZnO/(0.05)CdO/FTO photoanode in aqueous 0.5 M Na2SO4.
Figure 7. IPCE% spectrum of the ZnO/(0.05)CdO/FTO photoanode in aqueous 0.5 M Na2SO4.
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Figure 8. Normalized IMPS dependences for a ZnO/(0.05C)CdO/FTO film photoanode illuminated with monochromatic light of 407 nm at the illumination power density of 14 mW cm−2, obtained at a potential of 0.57 V in aqueous solutions of (1) 0.5 M Na2SO4, (2) 0.5 M Na2SO4 + 20% CH3OH, (3) 0.5 M Na2SO4 + 20% C2H4(OH)2, and (4) 0.5 M Na2SO4 + 20% C3H5(OH)3.
Figure 8. Normalized IMPS dependences for a ZnO/(0.05C)CdO/FTO film photoanode illuminated with monochromatic light of 407 nm at the illumination power density of 14 mW cm−2, obtained at a potential of 0.57 V in aqueous solutions of (1) 0.5 M Na2SO4, (2) 0.5 M Na2SO4 + 20% CH3OH, (3) 0.5 M Na2SO4 + 20% C2H4(OH)2, and (4) 0.5 M Na2SO4 + 20% C3H5(OH)3.
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Table
Table 1. Charge transfer (Kct) and recombination (Krec) rate constants for the ZnO/(0.05 CdO/FTO film photoanode at E = 0.6 V (vs. Ag/AgCl) under monochromatic light (407 nm) illumination at a 14 mW cm−2 power density in aqueous solutions of 0.5 M Na2SO4, 0.5 M Na2SO4 + 20%CH3OH, 0.5 M Na2SO4 + 20% C2H4(OH)2, and 0.5 M Na2SO4 + 20% C3H5(OH)3.
Table 1. Charge transfer (Kct) and recombination (Krec) rate constants for the ZnO/(0.05 CdO/FTO film photoanode at E = 0.6 V (vs. Ag/AgCl) under monochromatic light (407 nm) illumination at a 14 mW cm−2 power density in aqueous solutions of 0.5 M Na2SO4, 0.5 M Na2SO4 + 20%CH3OH, 0.5 M Na2SO4 + 20% C2H4(OH)2, and 0.5 M Na2SO4 + 20% C3H5(OH)3.
DepolarizerKct, s−1Krec, s−1
H2O28.5 24.2
CH3OH30.5 8.1
C2H4(OH)233 5.6
C3H5(OH)335.13.5
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Grinberg, V.A.; Emets, V.V.; Modestov, A.D.; Averin, A.A.; Shiryaev, A.A. Thin-Film Nanocrystalline Zinc Oxide Photoanode Modified with CdO in Photoelectrocatalytic Degradation of Alcohols. Coatings 2023, 13, 1080. https://doi.org/10.3390/coatings13061080

AMA Style

Grinberg VA, Emets VV, Modestov AD, Averin AA, Shiryaev AA. Thin-Film Nanocrystalline Zinc Oxide Photoanode Modified with CdO in Photoelectrocatalytic Degradation of Alcohols. Coatings. 2023; 13(6):1080. https://doi.org/10.3390/coatings13061080

Chicago/Turabian Style

Grinberg, Vitali A., Victor V. Emets, Alexander D. Modestov, Aleksey A. Averin, and Andrei A. Shiryaev. 2023. "Thin-Film Nanocrystalline Zinc Oxide Photoanode Modified with CdO in Photoelectrocatalytic Degradation of Alcohols" Coatings 13, no. 6: 1080. https://doi.org/10.3390/coatings13061080

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