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CuO thin film with enhanced visible light photoelectrochemical property
Materials Letters 154 (2015) 44–46
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hierarchically branched ZnO/CuO thin film with enhanced visible light photoelectrochemical property Juan Wang, Wei-De Zhang n, Wei-Xin Ouyang, Yu-Xiang Yu School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 November 2014 Accepted 8 April 2015 Available online 18 April 2015
Hierarchically branched ZnO/CuO film was fabricated by a two-step chemical process on Cu foil. CuO nanowires were firstly synthesized on Cu foil by a simple wet chemical method, and then ZnO nanowires were grown on the CuO nanowire scaffold through a hydrothermal process. Comparing with CuO film, the photocurrent response and incident photon to electron conversion efficiency yielded by the hierarchically branched ZnO/CuO film are significantly enhanced under visible light irradiation, which can be attributed to the large contact area with the electrolyte and direct pathways for charge carrier collection. The hierarchical ZnO/CuO thin film could be a promising material for solar energy conversion. & 2015 Elsevier B.V. All rights reserved.
Keywords: CuO ZnO Thin film Nanocomposite Photoelectrochemical property
1. Introduction One-dimensional (1D) nanostructured materials have drawn intensive attention due to their special physical properties and promising applications in fabrication of nanoscale devices. In the field of solar energy conversion, recent work has focused on 1D nanostructured photoelectrodes, such as nanotubes [1,2], nanowires [3] and nanorods [4]. The 1D nanostructure can provide a direct pathway for the electron transfer, thus charge separation can be accelerated. However, compared with nanoparticles, the photoelectrode composed of nanorods or nanotubes with smaller surface areas, would not favor to the charge transfer process, especially in the case of water oxidation since the kinetics are sluggish [5]. However, hierarchically branched 1D nanostructures with enhanced surface-to-volume ratios and a high packing density may solve this problem. For instance, Zheng and his colleagues reported that the photocurrent response of a hierarchically branched TiO2 nanorods photoelectrode is two times as that of TiO2 nanorods photoelectrode [5]. Our previous work also demonstrates the high photoelectrochemical properties of the hierarchically branched ZnO nanowiresmodified carbon nanotubes [6]. The hierarchically branched 1D nanostructures offer greatly enhanced surface area and direct conduction pathway for the rapid collection of photo-generated electrons. Because of their low cost, long term stability, easy availability, non-toxicity and favorable physical/chemical properties, TiO2 and ZnO are the semiconductive materials that have been investigated
n
Corresponding author. Tel./fax: þ 86 20 8711 4099. E-mail address: [email protected] (W.-D. Zhang).
http://dx.doi.org/10.1016/j.matlet.2015.04.048 0167-577X/& 2015 Elsevier B.V. All rights reserved.
most. Compared with TiO2, ZnO also has similar wide bandgap and band edge positions, but its electron mobility in the bulk singlecrystal phases is over two orders of magnitude higher than that in TiO2 [7], which can greatly enhance electron transport efficiency. However, the wide bandgap (3.37 eV) of ZnO limits its optical absorption in solar spectrum region and leads to low energy conversion efficiency. CuO is a typical p-type semiconductor with narrow direct bandgap which can be activated by visible light. Therefore, light absorption in the visible light region and solar conversion efficiency can be enhanced by combining cuprous oxide with ZnO as a photoelectrode. In the present work, hierarchically branched ZnO/CuO film was successfully fabricated via a simple wet chemical method combined with a hydrothermal process [8,9]. Photoelectrochemical properties of the ZnO/CuO photoelectrode were investigated under visible light irradiation.
2. Experiment CuO nanowires were synthesized by a simple wet chemical process [8]. The as-prepared CuO nanowires on Cu foil were immersed into an ethanol solution containing 0.0050 M Zn (CH3COO)2 for about 15 s and then dried in air. After several times' repetition, they were annealed at 350 1C for 30 min in atmosphere, which leads to the formation of ZnO seeds on the CuO nanowires. After that, the sample was put into a Teflon-sealed autoclave containing 60 mL 0.050 M Zn(NO3)2 6H2O and C6H12N4, and maintained at 95 1C for 10 h. Then, the autoclave was cooled down naturally to room temperature. After being washed and dried, the
J. Wang et al. / Materials Letters 154 (2015) 44–46
45
(IPCE) of the samples was calculated as follows: IPCE ¼ 1240I λJ light
hierarchically branched ZnO/CuO heterojunctions on Cu foil were prepared. The morphology of the samples was characterized by scanning electron microscopy (SEM, JSM-6380-LA, JEOL, Japan). Structure and phase characterization of the prepared ZnO/CuO were conducted using an X-ray diffractometer (XRD, Bruke-D8, with Cu Kα radiation). The photocurrent measurement was performed on an electrochemical working station (CHI-660C, China). 0.10 M Na2SO4 aqueous solution was used as a supporting electrolyte. AM 1.5G solar simulator (Oriel model 91192) was used as irradiation source with an output intensity of 100 mW/cm2. Bias potentials are given with reference to the Ag/ AgCl electrode. Wavelength-dependent photocurrent was also studied using a 500 W Xe lamp (CHF-XM-500, Changtuo Technology Co., Ltd.) and a monochromator (monochromator300, Changtuo Technology Co., Ltd.). The light intensity was measured with a radiometer (PM1 20V A, Thorlabs). The incident photon to electron conversion efficiency
where I is the measured photocurrent density, λ is the wavelength of the incident light, and Jlight is the measured irradiance at the measurement wavelength.
3. Results and discussion The morphology of the prepared samples was observed by SEM. Fig. 1a is a SEM image of the CuO nanowires on Cu foil, which reveals that the entire surface of the copper foil is covered by dense nanowires with uniform morphology, and some of the nanowires are intertwined. The nanowires have an average diameter of 300– 400 nm, and length of 3–5 μm. After the hydrothermal process in
Fig. 1. SEM images of (a) CuO nanowires, hierarchically branched ZnO/CuO at (b) low and (c) high magnifications and (d) XRD patterns of CuO and ZnO/CuO.
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Fig. 2. (a) Photocurrent density νs. time with the CuO and ZnO/CuO electrodes at bias potential of 0.30 V in 0.10 M Na2SO4 solution under AM 1.5G illumination. (b) Photocurrent density νs. time with the ZnO/CuO electrode at different bias potentials in 0.10 M Na2SO4 solution under AM 1.5G irradiation. (c) IPCEs of CuO and ZnO/CuO at 0.35 V νs. Ag/AgCl.
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J. Wang et al. / Materials Letters 154 (2015) 44–46
Zn(NO3)2 solution for 10 h, branches are found to appear from the surface of the CuO scaffold, as indicated in Fig. 1b. High magnification SEM image depicted in Fig. 1c reveals that the diameter of ZnO nanowires is about 50 nm, and the length ranges from 0.5 to 1 μm. The prepared samples were further characterized by XRD to reveal their phase composition and the XRD patterns of CuO and ZnO/CuO are illustrated in Fig. 1d. All the diffraction peaks are indexed to CuO (JCPDS Card no. 45-0937), Cu2O (JCPDS Card no. 65-3288), ZnO (JCPDS Card no. 65-3411) or Cu (JCPDS Card no. 659743). No diffraction peaks of impurities can be found according to the XRD pattern. For comparison, the XRD patterns of physical mixture of CuO and ZnO are also displayed in Fig. 1d. To evaluate the performance of ZnO/CuO as a photoelectrode, photoelectrochemical measurements were conducted in a threeelectrode electrochemical setup. Amperometric i–t curves collected at a bias of 0.30 V νs. Ag/AgCl reference electrode (RE) with light on/off cycles show that the photoelectrodes display reproducible photocurrent in response to AM 1.5G solar light (Fig. 2), and the responses are typical p-type and p–n junction semiconductor photocurrent responses under reversed bias potential [8,10,11]. The photocurrent density of the ZnO/CuO photoelectrode is approximately five times as that of the CuO photoelectrode under visible light irradiation (Fig. 2a). Furthermore, we measured the photocurrent density of the ZnO/CuO photoelectrode at different bias potentials. As shown in Fig. 2b, the more negative the bias potential is, the larger photocurrent is obtained. The current density is less than 0.10 mA cm 2 at 0.1 V, whereas it is approximately 0.5 mA cm 2 at 0.40 V. To further evaluate the performance of the hierarchical nanostructure, the IPCEs of CuO and ZnO/CuO are presented in Fig. 2c, and the highest IPCEs for ZnO/CuO and CuO are 10% and 5.5% separately at bias potential of 0.35 V νs. Ag/AgCl electrode. 4. Conclusions In summary, hierarchically branched ZnO/CuO film was fabricated through a simple wet chemical method combined with a
hydrothermal process. PEC property of hierarchically branched ZnO/CuO film was significantly enhanced comparing with that of CuO film under visible light irradiation. The adhesive growth of ZnO nanowires on the CuO scaffold greatly improves the exposed surface area, and the one-dimensional ZnO nanowires continuously provide direct pathways for the transfer of photo-induced electron. The hierarchically branched ZnO/CuO photoelectrode could be promising for potential applications in solar cells, waste water treatment and photoelectric devices.
Acknowledgment The authors thank the Guangdong Natural Science Foundation (2014A030311039) for financial support.
References [1] Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA. Nano Lett 2006;6:1165–71. [2] Martinson ABF, Elam JW, Hupp JT, Pellin MJ. Nano Lett 2007;7:2183–7. [3] Xu C, Wang XD, Wang ZL. J Am Chem Soc 2009;131:5866–72. [4] Zeng TW, Ho CC, Tu YC, Tu GY, Wang LY, Su WF. Langmuir 2011;27:15255–60. [5] Cho IS, Chen ZB, Forman AJ, Kim DR, Rao PM, Jaramillo TF, et al. Nano Lett 2011;11:4978–84. [6] Zhang WD, Jiang LC, Ye JS. J Phys Chem C 2009;113:16247–53. [7] Tiwana P, Docampo P, Johoston MB, Snaith HJ, Herz LM. ACS Nano 2011;5:5158–66. [8] Qian F, Wang GM, Li Y. Nano Lett 2010;10:4686–91. [9] Guo Z, Chen X, Li J, Liu JH, Huang XJ. Langmuir 2011;27:6193–200. [10] Hsueh HT, Chang SJ, Hung FY, Weng WY, Hsu CL, Hsueh TJ, et al. Superlattices Microstruct 2011;49:572–80. [11] Kargar A, Jing Y, Kim SJ, Riley CT, Pan XQ, Wang D. ACS Nano 2013;7:11112–20.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hierarchically branched ZnO/CuO thin film with enhanced visible light photoelectrochemical property Juan Wang, Wei-De Zhang n, Wei-Xin Ouyang, Yu-Xiang Yu School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 November 2014 Accepted 8 April 2015 Available online 18 April 2015
Hierarchically branched ZnO/CuO film was fabricated by a two-step chemical process on Cu foil. CuO nanowires were firstly synthesized on Cu foil by a simple wet chemical method, and then ZnO nanowires were grown on the CuO nanowire scaffold through a hydrothermal process. Comparing with CuO film, the photocurrent response and incident photon to electron conversion efficiency yielded by the hierarchically branched ZnO/CuO film are significantly enhanced under visible light irradiation, which can be attributed to the large contact area with the electrolyte and direct pathways for charge carrier collection. The hierarchical ZnO/CuO thin film could be a promising material for solar energy conversion. & 2015 Elsevier B.V. All rights reserved.
Keywords: CuO ZnO Thin film Nanocomposite Photoelectrochemical property
1. Introduction One-dimensional (1D) nanostructured materials have drawn intensive attention due to their special physical properties and promising applications in fabrication of nanoscale devices. In the field of solar energy conversion, recent work has focused on 1D nanostructured photoelectrodes, such as nanotubes [1,2], nanowires [3] and nanorods [4]. The 1D nanostructure can provide a direct pathway for the electron transfer, thus charge separation can be accelerated. However, compared with nanoparticles, the photoelectrode composed of nanorods or nanotubes with smaller surface areas, would not favor to the charge transfer process, especially in the case of water oxidation since the kinetics are sluggish [5]. However, hierarchically branched 1D nanostructures with enhanced surface-to-volume ratios and a high packing density may solve this problem. For instance, Zheng and his colleagues reported that the photocurrent response of a hierarchically branched TiO2 nanorods photoelectrode is two times as that of TiO2 nanorods photoelectrode [5]. Our previous work also demonstrates the high photoelectrochemical properties of the hierarchically branched ZnO nanowiresmodified carbon nanotubes [6]. The hierarchically branched 1D nanostructures offer greatly enhanced surface area and direct conduction pathway for the rapid collection of photo-generated electrons. Because of their low cost, long term stability, easy availability, non-toxicity and favorable physical/chemical properties, TiO2 and ZnO are the semiconductive materials that have been investigated
n
Corresponding author. Tel./fax: þ 86 20 8711 4099. E-mail address: [email protected] (W.-D. Zhang).
http://dx.doi.org/10.1016/j.matlet.2015.04.048 0167-577X/& 2015 Elsevier B.V. All rights reserved.
most. Compared with TiO2, ZnO also has similar wide bandgap and band edge positions, but its electron mobility in the bulk singlecrystal phases is over two orders of magnitude higher than that in TiO2 [7], which can greatly enhance electron transport efficiency. However, the wide bandgap (3.37 eV) of ZnO limits its optical absorption in solar spectrum region and leads to low energy conversion efficiency. CuO is a typical p-type semiconductor with narrow direct bandgap which can be activated by visible light. Therefore, light absorption in the visible light region and solar conversion efficiency can be enhanced by combining cuprous oxide with ZnO as a photoelectrode. In the present work, hierarchically branched ZnO/CuO film was successfully fabricated via a simple wet chemical method combined with a hydrothermal process [8,9]. Photoelectrochemical properties of the ZnO/CuO photoelectrode were investigated under visible light irradiation.
2. Experiment CuO nanowires were synthesized by a simple wet chemical process [8]. The as-prepared CuO nanowires on Cu foil were immersed into an ethanol solution containing 0.0050 M Zn (CH3COO)2 for about 15 s and then dried in air. After several times' repetition, they were annealed at 350 1C for 30 min in atmosphere, which leads to the formation of ZnO seeds on the CuO nanowires. After that, the sample was put into a Teflon-sealed autoclave containing 60 mL 0.050 M Zn(NO3)2 6H2O and C6H12N4, and maintained at 95 1C for 10 h. Then, the autoclave was cooled down naturally to room temperature. After being washed and dried, the
J. Wang et al. / Materials Letters 154 (2015) 44–46
45
(IPCE) of the samples was calculated as follows: IPCE ¼ 1240I λJ light
hierarchically branched ZnO/CuO heterojunctions on Cu foil were prepared. The morphology of the samples was characterized by scanning electron microscopy (SEM, JSM-6380-LA, JEOL, Japan). Structure and phase characterization of the prepared ZnO/CuO were conducted using an X-ray diffractometer (XRD, Bruke-D8, with Cu Kα radiation). The photocurrent measurement was performed on an electrochemical working station (CHI-660C, China). 0.10 M Na2SO4 aqueous solution was used as a supporting electrolyte. AM 1.5G solar simulator (Oriel model 91192) was used as irradiation source with an output intensity of 100 mW/cm2. Bias potentials are given with reference to the Ag/ AgCl electrode. Wavelength-dependent photocurrent was also studied using a 500 W Xe lamp (CHF-XM-500, Changtuo Technology Co., Ltd.) and a monochromator (monochromator300, Changtuo Technology Co., Ltd.). The light intensity was measured with a radiometer (PM1 20V A, Thorlabs). The incident photon to electron conversion efficiency
where I is the measured photocurrent density, λ is the wavelength of the incident light, and Jlight is the measured irradiance at the measurement wavelength.
3. Results and discussion The morphology of the prepared samples was observed by SEM. Fig. 1a is a SEM image of the CuO nanowires on Cu foil, which reveals that the entire surface of the copper foil is covered by dense nanowires with uniform morphology, and some of the nanowires are intertwined. The nanowires have an average diameter of 300– 400 nm, and length of 3–5 μm. After the hydrothermal process in
Fig. 1. SEM images of (a) CuO nanowires, hierarchically branched ZnO/CuO at (b) low and (c) high magnifications and (d) XRD patterns of CuO and ZnO/CuO.
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Fig. 2. (a) Photocurrent density νs. time with the CuO and ZnO/CuO electrodes at bias potential of 0.30 V in 0.10 M Na2SO4 solution under AM 1.5G illumination. (b) Photocurrent density νs. time with the ZnO/CuO electrode at different bias potentials in 0.10 M Na2SO4 solution under AM 1.5G irradiation. (c) IPCEs of CuO and ZnO/CuO at 0.35 V νs. Ag/AgCl.
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Zn(NO3)2 solution for 10 h, branches are found to appear from the surface of the CuO scaffold, as indicated in Fig. 1b. High magnification SEM image depicted in Fig. 1c reveals that the diameter of ZnO nanowires is about 50 nm, and the length ranges from 0.5 to 1 μm. The prepared samples were further characterized by XRD to reveal their phase composition and the XRD patterns of CuO and ZnO/CuO are illustrated in Fig. 1d. All the diffraction peaks are indexed to CuO (JCPDS Card no. 45-0937), Cu2O (JCPDS Card no. 65-3288), ZnO (JCPDS Card no. 65-3411) or Cu (JCPDS Card no. 659743). No diffraction peaks of impurities can be found according to the XRD pattern. For comparison, the XRD patterns of physical mixture of CuO and ZnO are also displayed in Fig. 1d. To evaluate the performance of ZnO/CuO as a photoelectrode, photoelectrochemical measurements were conducted in a threeelectrode electrochemical setup. Amperometric i–t curves collected at a bias of 0.30 V νs. Ag/AgCl reference electrode (RE) with light on/off cycles show that the photoelectrodes display reproducible photocurrent in response to AM 1.5G solar light (Fig. 2), and the responses are typical p-type and p–n junction semiconductor photocurrent responses under reversed bias potential [8,10,11]. The photocurrent density of the ZnO/CuO photoelectrode is approximately five times as that of the CuO photoelectrode under visible light irradiation (Fig. 2a). Furthermore, we measured the photocurrent density of the ZnO/CuO photoelectrode at different bias potentials. As shown in Fig. 2b, the more negative the bias potential is, the larger photocurrent is obtained. The current density is less than 0.10 mA cm 2 at 0.1 V, whereas it is approximately 0.5 mA cm 2 at 0.40 V. To further evaluate the performance of the hierarchical nanostructure, the IPCEs of CuO and ZnO/CuO are presented in Fig. 2c, and the highest IPCEs for ZnO/CuO and CuO are 10% and 5.5% separately at bias potential of 0.35 V νs. Ag/AgCl electrode. 4. Conclusions In summary, hierarchically branched ZnO/CuO film was fabricated through a simple wet chemical method combined with a
hydrothermal process. PEC property of hierarchically branched ZnO/CuO film was significantly enhanced comparing with that of CuO film under visible light irradiation. The adhesive growth of ZnO nanowires on the CuO scaffold greatly improves the exposed surface area, and the one-dimensional ZnO nanowires continuously provide direct pathways for the transfer of photo-induced electron. The hierarchically branched ZnO/CuO photoelectrode could be promising for potential applications in solar cells, waste water treatment and photoelectric devices.
Acknowledgment The authors thank the Guangdong Natural Science Foundation (2014A030311039) for financial support.
References [1] Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA. Nano Lett 2006;6:1165–71. [2] Martinson ABF, Elam JW, Hupp JT, Pellin MJ. Nano Lett 2007;7:2183–7. [3] Xu C, Wang XD, Wang ZL. J Am Chem Soc 2009;131:5866–72. [4] Zeng TW, Ho CC, Tu YC, Tu GY, Wang LY, Su WF. Langmuir 2011;27:15255–60. [5] Cho IS, Chen ZB, Forman AJ, Kim DR, Rao PM, Jaramillo TF, et al. Nano Lett 2011;11:4978–84. [6] Zhang WD, Jiang LC, Ye JS. J Phys Chem C 2009;113:16247–53. [7] Tiwana P, Docampo P, Johoston MB, Snaith HJ, Herz LM. ACS Nano 2011;5:5158–66. [8] Qian F, Wang GM, Li Y. Nano Lett 2010;10:4686–91. [9] Guo Z, Chen X, Li J, Liu JH, Huang XJ. Langmuir 2011;27:6193–200. [10] Hsueh HT, Chang SJ, Hung FY, Weng WY, Hsu CL, Hsueh TJ, et al. Superlattices Microstruct 2011;49:572–80. [11] Kargar A, Jing Y, Kim SJ, Riley CT, Pan XQ, Wang D. ACS Nano 2013;7:11112–20.