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CuO photoelectrode with n-p heterogeneous structure for photoelectrocatalytic oxidation of formaldehyde
Accepted Manuscript Full Length Article ZnO/CuO photoelectrode with n-p heterogeneous structure for photoelectrocatalytic oxidation of formaldehyde Xi-Ming Song, Chunxue Yuan, Yanming Wang, Baoxin Wang, Hui Mao, Shuyao Wu, Yu Zhang PII: DOI: Reference:
S0169-4332(18)31524-1 https://doi.org/10.1016/j.apsusc.2018.05.196 APSUSC 39472
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
8 March 2018 28 April 2018 25 May 2018
Please cite this article as: X-M. Song, C. Yuan, Y. Wang, B. Wang, H. Mao, S. Wu, Y. Zhang, ZnO/CuO photoelectrode with n-p heterogeneous structure for photoelectrocatalytic oxidation of formaldehyde, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.05.196
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ZnO/CuO photoelectrode with n-p heterogeneous structure for photoelectrocatalytic oxidation of formaldehyde
Xi-Ming Song, Chunxue Yuan, Yanming Wang, Baoxin Wang, Hui Mao, Shuyao Wu, Yu Zhang* Liaoning Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang 110036, China *Corresponding author, E-mail: [email protected]
1
Abstract: We developed a novel ZnO/CuO heterogeneous structure for photoelectrochemical (PEC) cells, which was fabricated with CuO attached onto ZnO nanorod (NR) arrays by an electrochemical deposition method. CuO as a p-type and narrow band-gap sensitizer can make the n-type ZnO respond to visible light and promote the separation of photogenerated charge carriers by building a p-n heterogeneous structure. The photoelectric conversion and photocatalytic formaldehyde oxidation of ZnO/CuO were investigated in detail under simulated sunlight (AM 1.5) in comparison with single ZnO NRs and CuO film. In addition, the energy level diagram of the p-n heterogeneous interface was revealed according to a theoretical analysis on the basis of the flat band potential results. Keywords:
Zinc
Oxide;
Cupric
oxide;
photoelectrocatalytic
oxidation;
Heterogeneous structure; Band structure
1.
Introduction With energy and environment issues seriously increasing, photoelectrochemical
(PEC) cells have attracted much attention due to their capacity to convert light energy into electrical power or chemical energy [1-5]. In most of PEC cells, photoanodes are consist of metal oxides (ZnO, TiO2, etc.) modified with a layer of light-absorbing semiconductor with a high absorption coefficient and an ideal band gap value [6-8]. A variety of light-absorbing materials, such as CdS, CdTe, PbS, Ag2S and CuS have been developed as sensitizers for ZnO or TiO2, forming a typical type-II heterostructure [9-15]. However, their poor stability and high costs greatly limit their further application. One method to solve these problems is directly using stable and p-type material, which can build a p-n heterogeneous interface with ZnO or TiO2 to promote the separation of photogenerated charge carriers. CuO, a narrow band gap and p-type semiconductor, has wide application prospects in solar cells and photocatalytic cells [16-17]. Bhaumik et al. demonstrated that copper oxide nanopowders onto the thin film increased the power conversion efficiency by 80% compared to micropowder thin film solar devices. The phase 2
mixture of copper oxides onto the thin film demonstrated a higher solar cell efficiency (2.88%) for solar cells [18]. Xia et al. prepared CuO NL/c-Si solar cells, showed a great increase of the optical absorption and a reduction of the reflectance in the 250–1250 nm wavelength range. The CuO NL/c-Si structure reduced the optical loss, improved the carrier collection, and distinctly enhanced the c-Si solar cell efficiency [19]. Recently, Dong et al. reported a three-dimensional ZnO nanoparticle-loaded CuO heterostructure and their photoelectrochemical (PEC) water splitting under simulated solar light illumination. They concluded that the enhanced photoanode current was attributed to the efficient separation of the photogenerated electrons and holes driven by the intimate p-n junction between p-type CuO and n-type ZnO interface [20]. Therefore, these results indicating that the energy band structures of p-type CuO and some n-type semiconductors such as TiO2, c-Si would be matched along with the equilibrium of Fermi level. The ability to fabricate p-n junction thin film based on CuO should also find broad interest in PEC device applications. In this work, ZnO nanorod (NR) array was used to form a n-p heterostructure with CuO. The advantages of ZnO nanorod include proper band alignment with CuO, facile synthesis and high electron mobility [21-23]. Electrodeposition technology was carried out for CuO growing on the surface of ZnO NR arrays, which can ensure that CuO was fully filled in the interspace of ZnO nanorod to construct compact p-n hetero-interface. Mott-schottky equation was conducted to reveal the energy band structure of ZnO/CuO heterostructure and the PEC catalysis of formaldehyde were designed to study the photoelectric performance.
2. Experimental section 2.1 Preparation of ZnO NR arrays The ZnO nanorod arrays were prepared on the fluorine-doped tin oxide glass (FTO, Nippon Sheet Glass) via a hydrothermal method [24]. 0.055 g of zinc acetate dihydrate as a starting material was dissolved in 50 mL of anhydrous ethanol. The FTO glass was firstly cleaned by detergent, and then in deionized water and ethanol by using an ultrasonic cleaner. The anhydrous ethanol solution of zinc acetate 3
dihydrate was dropped into a clean FTO glass substrate and blew dry in the air, and the process was repeated three times. Then the FTO glass was calcined at 350 Ԩ for 15 min in a furnace. When the temperature dropped to room temperature, the FTO
with ZnO seed was transfered to an autoclave containing 0.1 M zinc acetate and 0.1 M hexamethylene tetramine in aqueous solution. Then the autoclave was put in an oven and heated to 95 Ԩ for 4 h, obtaining ZnO NR arrays on FTO. 2.2 Preparation of ZnO/CuO heterostructure The Cu2O was firstly prepared on ZnO NR arrays by an electrodeposition process in a three electrode system immersed in a water bath at the temperature of 60 Ԩ. The ZnO nanorod arrays were used as the working electrode, and the exposed area of 1×1
cm2. The reference electrode was Ag/AgCl electrode and copper sheet (2×2 cm2) as counter electrode. 0.02 M copper acetate solution was used for electrodeposition [25]. The applied potential was -0.4 V vs. Ag/AgCl and the electrodeposition time was 10 min to obtain ZnO/Cu2O heterostructure film. The ZnO/CuO heterostructure was fabricated by thermal treatment of the electrodeposited ZnO/Cu2O in air atmosphere at 400 Ԩ for 2 h. For comparison,
single CuO film based on FTO glass was also prepared by the electrodeposition method and calcination process.
2.3 Characterization The morphology of the samples were observed by field-emission scanning electron microscopy (FESEM JSM-6700F microscope). The XRD patterns were collected by using a Bruker (Germany) D8 Advance diffractometer with Cu-Ka radiation in the range of 20°–80°. The UV-vis transmission spectra were characterized on an UV-vis spectrometer (Shimadzu UV-2550). The photocurrent measurement was carried out in a two-electrode PEC cell, with the prepared thin-film photoanode and Pt-coated FTO counter electrode. The light source was a 500W xenon lamp (CHFXQ500W, Beijing Trusttech Co. Ltd.) with chopped simulated AM 1.5 illumination (100 mW/cm2). The photocurrent-time (I-t) curves were recorded under chopped light illumination by 4
electrochemical workstation system (CHI660E, Shanghai). The electrolyte was composed of I2, BMII, NBII, CuSCN, and 3-methoxypropionitrile (liquid electrolyte DHS-E36, Dalian HeptaChroma SolarTech Co. Ltd.). Mott-Schottky plot was measured by electrochemical workgroup (CHI660E, Shanghai), the three electrode configuration was immersed in 0.5 M Na2SO4 solution. The as-prepared film coated on FTO served as a working electrode. Ag/AgCl electrode and platinum wire were used as reference and counter electrodes, respectively. The PEC performance of the samples was investigated by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using a CHI600D Electrochemical Station (Shanghai CHENHUA Instrument Co. Ltd.) in light on/off. The three electrode system was used in PEC formaldehyde oxidation, with prepared samples on FTO as working electrode, Ag/AgCl reference electrode and Pt counter electrode. The measurements were performed in 0.05 M KOH (pH=12.18). The xenon lamp (CHFXQ500W, Beijing Trusttech Co. Ltd.) serve as light source under AM 1.5 optical filter (Beijing Trust tech Co.Ltd.), and the current was recorded by electrochemical workstation system (CHI660E, Shanghai).
3. Results and discussion Fig. 1 shows XRD spectra of the ZnO NR arrays, CuO film and ZnO/CuO heterostructure. The diffraction peaks of CuO were in agreement with the reference pattern (JCPDS No.48-1548). The crystal phase of ZnO NR arrays was hexagonal wurtzite structure which can be indexed on the basis of JCPDS file No. 36-1451. The enhanced diffraction peak (001) prove the vertical growth of ZnO NR [26-27]. In the ZnO/CuO composite, the peak intensities of ZnO are relatively weak, which may be caused by the surface coating of the CuO. All the diffraction peaks of ZnO/CuO heterostructure indicate no diffraction peaks of impurities could be found according to the XRD pattern. Therefore, CuO film was combined successfully on ZnO NR arrays. The FESEM images of ZnO NR arrays, CuO film, ZnO/CuO composite are shown in Fig. 2a, 2b and 2c, which were synthesized by the procedures as Scheme 1. The insets in Fig. 2a, 2b and 2c are the cross-sectional view of the samples. Fig. 2a, it can 5
be seen that the ZnO NR arrays have a well defined hexagonal flat tops with an average diameter of 400–450 nm. The cross-sectional image shows that ZnO nanorods grow vertically on FTO glass with a length of 2.2 mm. Fig. 2b indicates the CuO film has no special structure with thickness of 1.47 mm. As shown in Fig. 2c, CuO is covered completely on the ZnO NR. It can be seen that the CuO growing on the ZnO surface is more smooth and uniform. And the thickness of the ZnO/CuO heterogeneous structure is about 2.8 mm. Fig. 2d presents the UV-vis reflectance spectra of ZnO NR arrays, CuO film, ZnO/CuO heterostructure based on FTO substrates. It is well known that the absorption spectrum of a semiconductor should cross a wide range with a threshold based on its band gap. The absorption threshold of ZnO NR arrays are about 370 nm, corresponding to a band gap of 3.37 eV using the formula (E = 1240/λonset) [28]. According the reference [16], CuO has a narrow band gap of 1.2 eV, corresponding a threshold of 1000 nm. So, the absorption of CuO film exist in both visible and UV region. ZnO/CuO heterostructure has absorption spectrum range from 350 nm to 800 nm, indicating that CuO improve the visible light utilization of ZnO NR effectively. According to the photos of the samples shown in the Fig. 2d inset, the CuO has been deposited on ZnO nanorods uniformly, which can be attributed to the electrodeposition method. The Mott-Schottky equation was used to obtain the flat band potential of the ZnO NR arrays and CuO film. According to the Mott-Schottky equation, there will be a linear relationship of 1/C2 versus applied potential, and the negative (positive) slope means a sample has p-type (n-type) conductivity. The results in Fig. 3a and 3b show that the ZnO is an n-type semiconductor and CuO is p-type semiconductor [21]. Thus, the ZnO/CuO nanocomposite can be seen as an n-p heterogeneous structure, which would promote the separation of photogenerated charges. According to previous report [29], the flat band potential represents the apparent Fermi level of a semiconductor in equilibrium with redox couple. In Fig. 3a and 3b, the flat-band potentials of ZnO NR, CuO film are 0.42 V, 1.1 V, respectively. Therefore, we can propose the energy band structure of the two semiconductors. The positions of 6
conduction band (CB) of ZnO and CuO can be found in reference [30], and the positions of valence band (VB) were derived according to the CB levels and band gaps of the two semiconductors, which are 2.9 V and 1.46 V (vs Ag/AgCl). So, the relative energy band diagram was obtained and shown in Fig. 3c. According to semiconductor theory, when two semiconductors were brought in contact, the Fermi level should be at equilibrium conditions, a p-n heterogeneous structure can be formed after the contacting of the n-type ZnO and p-type CuO (see Fig. 3d) [31-32]. Under illumination, the electrons in the VB of CuO would be excited into the CB. Then, the photogenerated electrons were transferred from CB of CuO to the CB of ZnO according to thermodynamic principle, leaving the photogenerated holes in the VB of CuO. Thus, this n-p structure can allow the separation of photogenerated electrons and holes. To investigate the photoelectric activity of the n-p heterostructure, PEC experiments were performed under simulated AM1.5 sunlight as shown in Fig. 4. As ZnO only absorbs UV light, the photoelectric conversion ability in sunlight is poor and the photocurrent density is only -0.04 mA/cm2. Single CuO electrode shows a even weaker photocurrent. Although CuO has narrow band gap, the low carrier mobility, short carrier lifetime and poor photostability greatly limited its performance [21,31]. The ZnO/CuO electrode has a photocurrent density about -0.26 mA/cm2, indicating the efficient generation and separation of photogenerated electrons and holes. This result further prove the n-p heterostructure depicted in Fig. 3c and 3d exist in the nanocomposite. At last, PEC formaldehyde oxidation was designed to explore its actual application prospect. Fig. 5a present the C-V curves of ZnO/CuO and ZnO electrodes in 0.05 M potassium hydroxide solution, with 33.3 mM formaldehyde, as well as without formaldehyde at a scanning rate of 50 mVs-1. As is seen, there are two oxidation peaks for ZnO/CuO electrode in formaldehyde solution, which are around -0.15 V and 0.15 V. As a comparison, the peak around -0.15 V disappeared when no formaldehyde was added, indicating that the peak can be attributed to formaldehyde degradation. In addition, the oxidation peak at 0.15 V and reduction peak at -0.5 V can be confirmed 7
as the valence state changes of Cu when comparing with single ZnO electrode. According to previous report [33], the reduction peak should be the transition of Cu (II)/Cu (I), and the oxidation peak can be attributed to the transition of Cu (I)/Cu (II). Fig. 5b shows the C-V curves of ZnO/CuO in formaldehyde solution in dark and in light. It can be seen that the current of the oxidation peaks have an obvious increase, indicating photogenerated charge carriers were involved in the oxidation process. And it is interesting that the oxidation potential of the two peaks became more negative. This phenomenon can be explained by the non-equilibrium condition of illumination, and the concept of quasi-Fermi-levels can be used. One has to consider separately the quasi-Fermi levels for electrons EFn and for holes EFp which depart from the equilibrium Fermi-level in the area of essential light absorption and diffusion length as shown in the inset of Fig. 5b. Because the photogenerated holes would also participate in the oxidation process, the quasi-Fermi levels of holes (EFp) would determine the actual oxidation potential of the electrode, which is more positive than the scanning potential and the oxidation peak would appear earlier. Fig. 5c and 5d shows the differential pulse voltammograms (DPV) curves of ZnO/CuO photoanode in formaldehyde solution. Along with the increase of [formaldehyde], the current for formaldehyde oxidation gradually increased. Under illumination, enhanced DPV responses can be observed, indicating a changed dynamic mechanism exist in the heterogeneous photoanode. It is known that the current difference of each pulse voltammetry is plotted as a function of potential in DPV measurement. So, the enhanced DPV current may be due to a faster electron transfer from the electrolyte to the photoanode, which should be induced by the holes in the valence band of the semiconductor. The plot depicting the DPV current of formaldehyde versus [formaldehyde] is presented in Fig. 5e, which indicates that the current at -0.15 V increases linearly with [formaldehyde] from 33.3 mM to 199.8 mM (R2 = 0.9918). The DPV curves of single CuO, ZnO and blank FTO in both the dark and light are listed in Fig. S1 in the supporting information. The two oxidation processes can be also observed for the sample of CuO, but the light-induced current of CuO became weaker, which may be caused by its p-type conductivity. Because holes have good 8
mobility in p-type semiconductor, photogenerated holes in single CuO electrode could transfer to FTO substrate quickly. This phenomenon would not appear in ZnO/CuO heterostructure because the transfer of photogenerated holes would be hindered by ZnO layer. The single ZnO photoanode has an obvious light-induced current, but the peak of formaldehyde oxidation was not observed.
4. Conclusions In this study, we synthesized n-ZnO/p-CuO semiconductor heterostructure structure by hydrothermal and electrochemical deposition method. The Mott-Schottky curves reveal the energy band positions of the semiconductors. Efficient separation of photogenerated charge carriers has been found in photocurrent experiments. Light-induced electrooxidation of formaldehyde have also been carried out to explore its actual application. The enhanced CV and DPV peak currents confirm the ZnO/CuO heterostructure has good PEC formaldehyde oxidation properties. In addition more negative CV peak of formaldehyde oxidation appeared. So if we design a PEC cell with double-electrode structure for practical application, the smaller external bias can be applied, that is to say, the energy obtained from solar light will greatly reduce the energy consumption in electrolytic process.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 21203082 and 51773085).
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Figure captions: Fig. 1. XRD spectra of the ZnO NR arrays, CuO film and ZnO/CuO heterostructure based on FTO glass. Fig. 2. FESEM images of top view and cross-sectional view of (a) ZnO NR arrays, (b) single CuO film and (c) ZnO/CuO heterostructure, and the scale bar in the insets is 1mm. (d) UV–vis diffuse reflectance spectra of ZnO NR arrays, CuO film, ZnO/CuO heterostructure . The inset shows the photos of the samples. Fig. 3. Variation of capacitance (C) with the applied potential in 0.5 mol/L Na2SO4 presented in the Mott−Schottky relationship for (a) ZnO NR arrays and (b) CuO film . The capacitance was determined by electrochemical impedance spectroscopy. (c) Schematic energy-band diagram for isolated ZnO and CuO; (d) energy-band diagrams for ZnO/CuO heterostructure, and the separation of charge carriers under irradiation. Fig. 4. Photocurrent of ZnO NR arrays, CuO film, ZnO/CuO photoanodes under simulated AM 1.5 sunlight on/off cycles. Fig. 5. (a) Cyclic voltammograms of the ZnO/CuO heterostructure with formaldehyde (33.3 mM), ZnO/CuO heterostructure with out formaldehyde and single ZnO without formaldehyde in 0.05 M KOH solution, at a scanning rate at 50 mVs-1. (b) Cyclic voltammograms of the ZnO/CuO heterostructure in dark and under illumination of simulated AM 1.5 sunlight in 0.05 M KOH and 33.3 mM formaldehyde solution. Inset: The transfer mechanism of photogenerated electrons and holes and the Fermi level splitting under illumination. (c) differential pulse voltammograms of the ZnO/CuO heterostructure with different [formaldehyde]. (d) differential pulse voltammograms of the ZnO/CuO heterostructure with different [formaldehyde] under illumination of simulated AM 1.5 sunlight. (e) plot of current vs. [formaldehyde]. Scheme 1. Proposed growth routes of ZnO/CuO heterostructure.
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Graphical abstract
15
Highlights
ZnO/CuO n-p heterogeneous structure was prepared by an electrochemical deposition method. The Mott-Schottky plots reveal the energy band positions of the two semiconductors Light-induced electrocatalysis of formaldehyde has been designed and studied.
16
S0169-4332(18)31524-1 https://doi.org/10.1016/j.apsusc.2018.05.196 APSUSC 39472
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
8 March 2018 28 April 2018 25 May 2018
Please cite this article as: X-M. Song, C. Yuan, Y. Wang, B. Wang, H. Mao, S. Wu, Y. Zhang, ZnO/CuO photoelectrode with n-p heterogeneous structure for photoelectrocatalytic oxidation of formaldehyde, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.05.196
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ZnO/CuO photoelectrode with n-p heterogeneous structure for photoelectrocatalytic oxidation of formaldehyde
Xi-Ming Song, Chunxue Yuan, Yanming Wang, Baoxin Wang, Hui Mao, Shuyao Wu, Yu Zhang* Liaoning Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang 110036, China *Corresponding author, E-mail: [email protected]
1
Abstract: We developed a novel ZnO/CuO heterogeneous structure for photoelectrochemical (PEC) cells, which was fabricated with CuO attached onto ZnO nanorod (NR) arrays by an electrochemical deposition method. CuO as a p-type and narrow band-gap sensitizer can make the n-type ZnO respond to visible light and promote the separation of photogenerated charge carriers by building a p-n heterogeneous structure. The photoelectric conversion and photocatalytic formaldehyde oxidation of ZnO/CuO were investigated in detail under simulated sunlight (AM 1.5) in comparison with single ZnO NRs and CuO film. In addition, the energy level diagram of the p-n heterogeneous interface was revealed according to a theoretical analysis on the basis of the flat band potential results. Keywords:
Zinc
Oxide;
Cupric
oxide;
photoelectrocatalytic
oxidation;
Heterogeneous structure; Band structure
1.
Introduction With energy and environment issues seriously increasing, photoelectrochemical
(PEC) cells have attracted much attention due to their capacity to convert light energy into electrical power or chemical energy [1-5]. In most of PEC cells, photoanodes are consist of metal oxides (ZnO, TiO2, etc.) modified with a layer of light-absorbing semiconductor with a high absorption coefficient and an ideal band gap value [6-8]. A variety of light-absorbing materials, such as CdS, CdTe, PbS, Ag2S and CuS have been developed as sensitizers for ZnO or TiO2, forming a typical type-II heterostructure [9-15]. However, their poor stability and high costs greatly limit their further application. One method to solve these problems is directly using stable and p-type material, which can build a p-n heterogeneous interface with ZnO or TiO2 to promote the separation of photogenerated charge carriers. CuO, a narrow band gap and p-type semiconductor, has wide application prospects in solar cells and photocatalytic cells [16-17]. Bhaumik et al. demonstrated that copper oxide nanopowders onto the thin film increased the power conversion efficiency by 80% compared to micropowder thin film solar devices. The phase 2
mixture of copper oxides onto the thin film demonstrated a higher solar cell efficiency (2.88%) for solar cells [18]. Xia et al. prepared CuO NL/c-Si solar cells, showed a great increase of the optical absorption and a reduction of the reflectance in the 250–1250 nm wavelength range. The CuO NL/c-Si structure reduced the optical loss, improved the carrier collection, and distinctly enhanced the c-Si solar cell efficiency [19]. Recently, Dong et al. reported a three-dimensional ZnO nanoparticle-loaded CuO heterostructure and their photoelectrochemical (PEC) water splitting under simulated solar light illumination. They concluded that the enhanced photoanode current was attributed to the efficient separation of the photogenerated electrons and holes driven by the intimate p-n junction between p-type CuO and n-type ZnO interface [20]. Therefore, these results indicating that the energy band structures of p-type CuO and some n-type semiconductors such as TiO2, c-Si would be matched along with the equilibrium of Fermi level. The ability to fabricate p-n junction thin film based on CuO should also find broad interest in PEC device applications. In this work, ZnO nanorod (NR) array was used to form a n-p heterostructure with CuO. The advantages of ZnO nanorod include proper band alignment with CuO, facile synthesis and high electron mobility [21-23]. Electrodeposition technology was carried out for CuO growing on the surface of ZnO NR arrays, which can ensure that CuO was fully filled in the interspace of ZnO nanorod to construct compact p-n hetero-interface. Mott-schottky equation was conducted to reveal the energy band structure of ZnO/CuO heterostructure and the PEC catalysis of formaldehyde were designed to study the photoelectric performance.
2. Experimental section 2.1 Preparation of ZnO NR arrays The ZnO nanorod arrays were prepared on the fluorine-doped tin oxide glass (FTO, Nippon Sheet Glass) via a hydrothermal method [24]. 0.055 g of zinc acetate dihydrate as a starting material was dissolved in 50 mL of anhydrous ethanol. The FTO glass was firstly cleaned by detergent, and then in deionized water and ethanol by using an ultrasonic cleaner. The anhydrous ethanol solution of zinc acetate 3
dihydrate was dropped into a clean FTO glass substrate and blew dry in the air, and the process was repeated three times. Then the FTO glass was calcined at 350 Ԩ for 15 min in a furnace. When the temperature dropped to room temperature, the FTO
with ZnO seed was transfered to an autoclave containing 0.1 M zinc acetate and 0.1 M hexamethylene tetramine in aqueous solution. Then the autoclave was put in an oven and heated to 95 Ԩ for 4 h, obtaining ZnO NR arrays on FTO. 2.2 Preparation of ZnO/CuO heterostructure The Cu2O was firstly prepared on ZnO NR arrays by an electrodeposition process in a three electrode system immersed in a water bath at the temperature of 60 Ԩ. The ZnO nanorod arrays were used as the working electrode, and the exposed area of 1×1
cm2. The reference electrode was Ag/AgCl electrode and copper sheet (2×2 cm2) as counter electrode. 0.02 M copper acetate solution was used for electrodeposition [25]. The applied potential was -0.4 V vs. Ag/AgCl and the electrodeposition time was 10 min to obtain ZnO/Cu2O heterostructure film. The ZnO/CuO heterostructure was fabricated by thermal treatment of the electrodeposited ZnO/Cu2O in air atmosphere at 400 Ԩ for 2 h. For comparison,
single CuO film based on FTO glass was also prepared by the electrodeposition method and calcination process.
2.3 Characterization The morphology of the samples were observed by field-emission scanning electron microscopy (FESEM JSM-6700F microscope). The XRD patterns were collected by using a Bruker (Germany) D8 Advance diffractometer with Cu-Ka radiation in the range of 20°–80°. The UV-vis transmission spectra were characterized on an UV-vis spectrometer (Shimadzu UV-2550). The photocurrent measurement was carried out in a two-electrode PEC cell, with the prepared thin-film photoanode and Pt-coated FTO counter electrode. The light source was a 500W xenon lamp (CHFXQ500W, Beijing Trusttech Co. Ltd.) with chopped simulated AM 1.5 illumination (100 mW/cm2). The photocurrent-time (I-t) curves were recorded under chopped light illumination by 4
electrochemical workstation system (CHI660E, Shanghai). The electrolyte was composed of I2, BMII, NBII, CuSCN, and 3-methoxypropionitrile (liquid electrolyte DHS-E36, Dalian HeptaChroma SolarTech Co. Ltd.). Mott-Schottky plot was measured by electrochemical workgroup (CHI660E, Shanghai), the three electrode configuration was immersed in 0.5 M Na2SO4 solution. The as-prepared film coated on FTO served as a working electrode. Ag/AgCl electrode and platinum wire were used as reference and counter electrodes, respectively. The PEC performance of the samples was investigated by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using a CHI600D Electrochemical Station (Shanghai CHENHUA Instrument Co. Ltd.) in light on/off. The three electrode system was used in PEC formaldehyde oxidation, with prepared samples on FTO as working electrode, Ag/AgCl reference electrode and Pt counter electrode. The measurements were performed in 0.05 M KOH (pH=12.18). The xenon lamp (CHFXQ500W, Beijing Trusttech Co. Ltd.) serve as light source under AM 1.5 optical filter (Beijing Trust tech Co.Ltd.), and the current was recorded by electrochemical workstation system (CHI660E, Shanghai).
3. Results and discussion Fig. 1 shows XRD spectra of the ZnO NR arrays, CuO film and ZnO/CuO heterostructure. The diffraction peaks of CuO were in agreement with the reference pattern (JCPDS No.48-1548). The crystal phase of ZnO NR arrays was hexagonal wurtzite structure which can be indexed on the basis of JCPDS file No. 36-1451. The enhanced diffraction peak (001) prove the vertical growth of ZnO NR [26-27]. In the ZnO/CuO composite, the peak intensities of ZnO are relatively weak, which may be caused by the surface coating of the CuO. All the diffraction peaks of ZnO/CuO heterostructure indicate no diffraction peaks of impurities could be found according to the XRD pattern. Therefore, CuO film was combined successfully on ZnO NR arrays. The FESEM images of ZnO NR arrays, CuO film, ZnO/CuO composite are shown in Fig. 2a, 2b and 2c, which were synthesized by the procedures as Scheme 1. The insets in Fig. 2a, 2b and 2c are the cross-sectional view of the samples. Fig. 2a, it can 5
be seen that the ZnO NR arrays have a well defined hexagonal flat tops with an average diameter of 400–450 nm. The cross-sectional image shows that ZnO nanorods grow vertically on FTO glass with a length of 2.2 mm. Fig. 2b indicates the CuO film has no special structure with thickness of 1.47 mm. As shown in Fig. 2c, CuO is covered completely on the ZnO NR. It can be seen that the CuO growing on the ZnO surface is more smooth and uniform. And the thickness of the ZnO/CuO heterogeneous structure is about 2.8 mm. Fig. 2d presents the UV-vis reflectance spectra of ZnO NR arrays, CuO film, ZnO/CuO heterostructure based on FTO substrates. It is well known that the absorption spectrum of a semiconductor should cross a wide range with a threshold based on its band gap. The absorption threshold of ZnO NR arrays are about 370 nm, corresponding to a band gap of 3.37 eV using the formula (E = 1240/λonset) [28]. According the reference [16], CuO has a narrow band gap of 1.2 eV, corresponding a threshold of 1000 nm. So, the absorption of CuO film exist in both visible and UV region. ZnO/CuO heterostructure has absorption spectrum range from 350 nm to 800 nm, indicating that CuO improve the visible light utilization of ZnO NR effectively. According to the photos of the samples shown in the Fig. 2d inset, the CuO has been deposited on ZnO nanorods uniformly, which can be attributed to the electrodeposition method. The Mott-Schottky equation was used to obtain the flat band potential of the ZnO NR arrays and CuO film. According to the Mott-Schottky equation, there will be a linear relationship of 1/C2 versus applied potential, and the negative (positive) slope means a sample has p-type (n-type) conductivity. The results in Fig. 3a and 3b show that the ZnO is an n-type semiconductor and CuO is p-type semiconductor [21]. Thus, the ZnO/CuO nanocomposite can be seen as an n-p heterogeneous structure, which would promote the separation of photogenerated charges. According to previous report [29], the flat band potential represents the apparent Fermi level of a semiconductor in equilibrium with redox couple. In Fig. 3a and 3b, the flat-band potentials of ZnO NR, CuO film are 0.42 V, 1.1 V, respectively. Therefore, we can propose the energy band structure of the two semiconductors. The positions of 6
conduction band (CB) of ZnO and CuO can be found in reference [30], and the positions of valence band (VB) were derived according to the CB levels and band gaps of the two semiconductors, which are 2.9 V and 1.46 V (vs Ag/AgCl). So, the relative energy band diagram was obtained and shown in Fig. 3c. According to semiconductor theory, when two semiconductors were brought in contact, the Fermi level should be at equilibrium conditions, a p-n heterogeneous structure can be formed after the contacting of the n-type ZnO and p-type CuO (see Fig. 3d) [31-32]. Under illumination, the electrons in the VB of CuO would be excited into the CB. Then, the photogenerated electrons were transferred from CB of CuO to the CB of ZnO according to thermodynamic principle, leaving the photogenerated holes in the VB of CuO. Thus, this n-p structure can allow the separation of photogenerated electrons and holes. To investigate the photoelectric activity of the n-p heterostructure, PEC experiments were performed under simulated AM1.5 sunlight as shown in Fig. 4. As ZnO only absorbs UV light, the photoelectric conversion ability in sunlight is poor and the photocurrent density is only -0.04 mA/cm2. Single CuO electrode shows a even weaker photocurrent. Although CuO has narrow band gap, the low carrier mobility, short carrier lifetime and poor photostability greatly limited its performance [21,31]. The ZnO/CuO electrode has a photocurrent density about -0.26 mA/cm2, indicating the efficient generation and separation of photogenerated electrons and holes. This result further prove the n-p heterostructure depicted in Fig. 3c and 3d exist in the nanocomposite. At last, PEC formaldehyde oxidation was designed to explore its actual application prospect. Fig. 5a present the C-V curves of ZnO/CuO and ZnO electrodes in 0.05 M potassium hydroxide solution, with 33.3 mM formaldehyde, as well as without formaldehyde at a scanning rate of 50 mVs-1. As is seen, there are two oxidation peaks for ZnO/CuO electrode in formaldehyde solution, which are around -0.15 V and 0.15 V. As a comparison, the peak around -0.15 V disappeared when no formaldehyde was added, indicating that the peak can be attributed to formaldehyde degradation. In addition, the oxidation peak at 0.15 V and reduction peak at -0.5 V can be confirmed 7
as the valence state changes of Cu when comparing with single ZnO electrode. According to previous report [33], the reduction peak should be the transition of Cu (II)/Cu (I), and the oxidation peak can be attributed to the transition of Cu (I)/Cu (II). Fig. 5b shows the C-V curves of ZnO/CuO in formaldehyde solution in dark and in light. It can be seen that the current of the oxidation peaks have an obvious increase, indicating photogenerated charge carriers were involved in the oxidation process. And it is interesting that the oxidation potential of the two peaks became more negative. This phenomenon can be explained by the non-equilibrium condition of illumination, and the concept of quasi-Fermi-levels can be used. One has to consider separately the quasi-Fermi levels for electrons EFn and for holes EFp which depart from the equilibrium Fermi-level in the area of essential light absorption and diffusion length as shown in the inset of Fig. 5b. Because the photogenerated holes would also participate in the oxidation process, the quasi-Fermi levels of holes (EFp) would determine the actual oxidation potential of the electrode, which is more positive than the scanning potential and the oxidation peak would appear earlier. Fig. 5c and 5d shows the differential pulse voltammograms (DPV) curves of ZnO/CuO photoanode in formaldehyde solution. Along with the increase of [formaldehyde], the current for formaldehyde oxidation gradually increased. Under illumination, enhanced DPV responses can be observed, indicating a changed dynamic mechanism exist in the heterogeneous photoanode. It is known that the current difference of each pulse voltammetry is plotted as a function of potential in DPV measurement. So, the enhanced DPV current may be due to a faster electron transfer from the electrolyte to the photoanode, which should be induced by the holes in the valence band of the semiconductor. The plot depicting the DPV current of formaldehyde versus [formaldehyde] is presented in Fig. 5e, which indicates that the current at -0.15 V increases linearly with [formaldehyde] from 33.3 mM to 199.8 mM (R2 = 0.9918). The DPV curves of single CuO, ZnO and blank FTO in both the dark and light are listed in Fig. S1 in the supporting information. The two oxidation processes can be also observed for the sample of CuO, but the light-induced current of CuO became weaker, which may be caused by its p-type conductivity. Because holes have good 8
mobility in p-type semiconductor, photogenerated holes in single CuO electrode could transfer to FTO substrate quickly. This phenomenon would not appear in ZnO/CuO heterostructure because the transfer of photogenerated holes would be hindered by ZnO layer. The single ZnO photoanode has an obvious light-induced current, but the peak of formaldehyde oxidation was not observed.
4. Conclusions In this study, we synthesized n-ZnO/p-CuO semiconductor heterostructure structure by hydrothermal and electrochemical deposition method. The Mott-Schottky curves reveal the energy band positions of the semiconductors. Efficient separation of photogenerated charge carriers has been found in photocurrent experiments. Light-induced electrooxidation of formaldehyde have also been carried out to explore its actual application. The enhanced CV and DPV peak currents confirm the ZnO/CuO heterostructure has good PEC formaldehyde oxidation properties. In addition more negative CV peak of formaldehyde oxidation appeared. So if we design a PEC cell with double-electrode structure for practical application, the smaller external bias can be applied, that is to say, the energy obtained from solar light will greatly reduce the energy consumption in electrolytic process.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 21203082 and 51773085).
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Figure captions: Fig. 1. XRD spectra of the ZnO NR arrays, CuO film and ZnO/CuO heterostructure based on FTO glass. Fig. 2. FESEM images of top view and cross-sectional view of (a) ZnO NR arrays, (b) single CuO film and (c) ZnO/CuO heterostructure, and the scale bar in the insets is 1mm. (d) UV–vis diffuse reflectance spectra of ZnO NR arrays, CuO film, ZnO/CuO heterostructure . The inset shows the photos of the samples. Fig. 3. Variation of capacitance (C) with the applied potential in 0.5 mol/L Na2SO4 presented in the Mott−Schottky relationship for (a) ZnO NR arrays and (b) CuO film . The capacitance was determined by electrochemical impedance spectroscopy. (c) Schematic energy-band diagram for isolated ZnO and CuO; (d) energy-band diagrams for ZnO/CuO heterostructure, and the separation of charge carriers under irradiation. Fig. 4. Photocurrent of ZnO NR arrays, CuO film, ZnO/CuO photoanodes under simulated AM 1.5 sunlight on/off cycles. Fig. 5. (a) Cyclic voltammograms of the ZnO/CuO heterostructure with formaldehyde (33.3 mM), ZnO/CuO heterostructure with out formaldehyde and single ZnO without formaldehyde in 0.05 M KOH solution, at a scanning rate at 50 mVs-1. (b) Cyclic voltammograms of the ZnO/CuO heterostructure in dark and under illumination of simulated AM 1.5 sunlight in 0.05 M KOH and 33.3 mM formaldehyde solution. Inset: The transfer mechanism of photogenerated electrons and holes and the Fermi level splitting under illumination. (c) differential pulse voltammograms of the ZnO/CuO heterostructure with different [formaldehyde]. (d) differential pulse voltammograms of the ZnO/CuO heterostructure with different [formaldehyde] under illumination of simulated AM 1.5 sunlight. (e) plot of current vs. [formaldehyde]. Scheme 1. Proposed growth routes of ZnO/CuO heterostructure.
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Graphical abstract
15
Highlights
ZnO/CuO n-p heterogeneous structure was prepared by an electrochemical deposition method. The Mott-Schottky plots reveal the energy band positions of the two semiconductors Light-induced electrocatalysis of formaldehyde has been designed and studied.
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