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A facile way to synthesize Er2O3@ZnO core-shell nanorods for photoelectrochemical water splitting
Inorganic Chemistry Communications 45 (2014) 116–119
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A facile way to synthesize Er2O3@ZnO core-shell nanorods for photoelectrochemical water splitting Kai-Hang Ye, Ji-Yu Wang, Nan Li ⁎, Zhao-Qing Liu, Shi-Heng Guo, Yun-Ping Guo, Yu-Zhi Su School of Chemistry and Chemical Engineering/Guangzhou, Key Laboratory for Environmentally Functional Materials and Technology, Guangzhou University, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 18 February 2014 Received in revised form 2 April 2014 Accepted 10 April 2014 Available online 24 April 2014
a b s t r a c t In this study, we demonstrated large-scale Er2O3@ZnO core-shell nanorods with great photoelectrochemical water splitting ability, which were successfully fabricated by a simple and facile electrodeposition. These Er2O3@ZnO core-shell nanorods exhibited improved photocurrent under the light irradiation, which can be attributed to the enhanced donor density by the covered Er2O3. © 2014 Elsevier B.V. All rights reserved.
Keywords: ZnO Er2O3 Core-shell Photoelectrochemical water splitting
Hydrogen, as a clean energy, which will not bring any greenhouse gases after combustion, has drawn continuous attentions. One of the most promising and environmental friendly methods is the water splitting by the photoelectrochemical cells over the semiconductors [1–3]. Among these semiconductors, metal oxide, such as ZnO, TiO2 and Fe2O3, has been widely studied as a photoanode in the photoelectrochemical (PEC) cell and some achievements have been made [4–11]. ZnO with a band gap of 3.2 eV has drawn continuous attention due to its favorable band-edge positions that can straddle the redox potential of water photoelectrolysis, high photocatalytic activity, natural abundance, economical and low toxicity to the environment. However, the efficiency of the ZnO is not satisfying as it possesses large band gap and suffers rapidly recombination of photoexcited electron-hole pairs [12]. Up to now, an enormous amount of research has been made to enhance the absorption of ZnO in the visible light region. Generally, there are many approaches to improve the conversion efficiency by sensitization ZnO with small band gap semiconductors or elemental doping, and many achievements have been made [13,14]. On the other hand, it has been proved that the morphology of ZnO can affect the electronic structure and then enhance the separation and transportation of the photoexcited charge carriers. For instance, Yang et al. [12] reported the N doped ZnO nanowires to improve light-harvest ability, which show an enhanced photocurrent and stability in photoelectrochemical hydrogen volution. Rare earth elements, with outstanding catalytic capacity, have shown noteworthy activity owing to its potential applications in various fields, such as, photodegradation, catalysts, high-quality phosphors and upconversion [15–17]. Yousefi et al. [18] have demonstrated that the ⁎ Corresponding author. Tel./fax: +86 20 39366908. E-mail address: [email protected] (N. Li).
http://dx.doi.org/10.1016/j.inoche.2014.04.018 1387-7003/© 2014 Elsevier B.V. All rights reserved.
addition of Ce to ZnO thin film increased photocurrent density to about double amount at applied potential of 0.6 V vs. Ag/AgCl compared to undoped ZnO film in photoelectrochemical water splitting reaction. Anandan and his co-works [19] observed that the doping of La in ZnO helps to achieve complete mineralization of monocrotophos within a short irradiation time. However, no attention have been paid to the erbium (Er) doped ZnO it is highly desirable to investigate the influence of the Er2O3 coating. Herein, we demonstrated the design and synthesis of Er2O3@ZnO core-shell structure on F doped SnO2 (FTO) glass substrates by a facile electrochemical deposition process, following by studying their PEC performance for water splitting. The electrodeposition of ZnO was carried out in solution of 0.02 M Zn(NO3)2, 0.01 M NH4Ac and 0.01 M hexamethylenetetramine with current density of − 2.0 mA cm−2 at 90 °C for 50 min, and the typical SEM images are presented in Fig. 1a–c. It can be seen that hexagonal ZnO nanorods were successfully synthesized on FTO substrates. The average diameter of the nanorods is about 200–300 nm. Then the asprepared ZnO film served as the electrode and template to take electrodeposition of Er2O3, and the SEM image of a typical Er2O3@ZnO composite material was illustrated in Fig. 1d–f. As shown in Fig. 1f, the surface of the ZnO nanorods became rather coarse after electrodeposition of Er2O3, which confirmed that the Er2O3 was successfully deposited onto the ZnO nanorods. It is noteworthy that the average diameter was similar to that of ZnO, suggesting that a very thin layer of Er2O3 was constructed. The crystal structure of Er2O3@ZnO core-shell nanorod was investigated by TEM. A low-magnification TEM image of part of an individual nanorod with rod-like structure is illustrated in Fig. 2a, which clearly shows the partial surface of the ZnO nanorod is covered by Er2O3 layer. The enlarged HRTEM image (Fig. 2b) of the selected area marked
K.-H. Ye et al. / Inorganic Chemistry Communications 45 (2014) 116–119
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Fig. 1. SEM images of (a–c) pristine ZnO and (d–f) Er2O3@ZnO shell-core nanorod arrays.
by blue cycle in Fig. 2a reveals the lattice fringe spacing of 0.26 nm, which paralleled to its growth axis are attributed to the (0001) plane of wurtzite ZnO, indicating the ZnO nanorod grow along the direction of [0001]. In Addition, the surface of the ZnO nanorod is uniformly coated with a shell of amorphous Er2O3 layer. Energy-dispersive X-ray spectroscopy (EDS) analysis was also conducted for the Er2O3@ZnO composites. As shown in Fig. 2c, besides the Cu, C and Sn signals coming from the TEM grid and FTO substrates, respectively, Er, Zn, and O were
detected from the Er2O3@ZnO composites, suggesting the composite is made up of Er2O3 and ZnO. In order to identify the crystalline phase of the products, X-ray powder diffraction (XRD) was conducted, and the results were presented in Fig. 3a. All of the reflections of the XRD pattern could be indexed to hexagonal ZnO with a lattice constant a = b = 3.253 Å, c = 5.215 Å, compatible with JCPDS card no. 80-0047. The diffraction peaks appeared at 2θ = 31.73°, 34.36°, 36.21°, 47.47° and 62.75° corresponding to (100),
Fig. 2. (a) TEM, (b) HRTEM and (c) the corresponding EDS pattern of Er2O3@ZnO shell-core nanorod.
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properties of pristine ZnO and Er2O3@ZnO electrodes were characterized by measuring the linear sweep voltammogram (LSW) curves under light irradiation. As demonstrated in Fig. 4a, the dark scans collected in the potential range between −0.6 and +1.0 V vs. Ag/AgCl revealed a negligible background current. Under the white light illumination, both pristine ZnO and Er2O3@ZnO showed pronounced photoresponse. Significantly, the Er2O3@ZnO films and annealed Er2O3@ZnO films achieved the highest photocurrent density of 3.5 mA cm−2 at 1.0 V vs. Ag/AgCl, exhibiting substantially larger photocurrent density than that of ZnO films with a maximum photocurrent density of 2.2 mA cm− 2 at the same potential. This result confirmed the positive role of Er2O3 shell in enhancing the photoactivity of pristine ZnO electrodes under illumination. To further elucidate the effect of Er2O3 shell on the electronic properties of ZnO, electrochemical impedance measurements for both electrodes were performed in the dark. Carrier densities of these samples were calculated from the slopes of Mott–Schottky plots using the equation: h i‐1 2 Nd ¼ ð2=e0 εε0 Þ d 1=c =dV where e0 is the electron charge, ε is the dielectric constant of ZnO (ε = 8) [24], ε0 is the permittivity of vacuum and Nd is the dopant density and V the electrode applied potential.
Fig. 3. (a) X-ray diffraction patterns of prinstine ZnO, Er2O3@ZnO and annealed Er2O3@ ZnO shell-core nanorod arrays (550 °C); (b) UV-vis absorption spectra of pristine ZnO and Er2O3@ZnO shell-core nanorod arrays.
(002), (101), (102) and (103) planes were observed, and the strong intensity of the diffraction peaks indicates good crystallinity of the microwire. Besides SnO2 (coming from FTO substrate) and ZnO diffraction peaks, no other obvious peaks corresponding to Er2O3 were obtained. In addition, the diffraction peaks of annealed Er2O3@ZnO film are also consistent with the pristine ZnO and Er2O3@ZnO film. The results confirm that the crystal structure of Er2O3 shell is amorphous. Furthermore, the UV-vis absorption spectra of pristine ZnO and the Er2O3@ZnO film were measured to investigate the influence of Er2O3 shell layer. Similar to the shape absorption edge of the pristine of ZnO film, the edge of Er2O3@ZnO was also at around 400 nm. The band gap (Eg) of semiconductor material can be calculated from the equation of (Rhν)n = A (hν − Eg), where hν is the photon energy, R is the absorption coefficient, A is a constant for the material and n is 2 for a direct transition or 1/2 for an indirect transition [20,21]. Therefore, as a direct transition semiconductor, the estimated direct Eg values of the ZnO and Er2O3@ZnO were both about 3.2 eV. It is concluded that the thin layer has little effect on the band gap of ZnO. Well-aligned 1D nanostructures with narrow band gap are very acceptable and efficient for the conversion of solar energy because the 1D structures can provide a direct electron transport pathway and thus enhance the separation of the photo-excited electron–hole pairs [20,22,23]. Based on these hypothesizes, the as prepared are expected to be used as photoelectrode in a PEC cell. The photoelectrochemical
Fig. 4. (a) I − V curves recorded for pristine ZnO, Er2O3@ZnO and annealed Er2O3@ZnO shell-core nanorod arrays (550 °C) with a scan rate of 10 mV/s in 0.5 M Na2SO4. (b) Mott– Schottky plots of pristine ZnO and Er2O3@ZnO core-shell nanorod arrays obtained at each potential with 10 kHz frequency.
K.-H. Ye et al. / Inorganic Chemistry Communications 45 (2014) 116–119
Fig. 4b displays the Mott–Schottky plots collected for ZnO and Er2O3@ZnO. The plots were generated based on capacitances that were derived from the electrochemical impedance obtained at each potential with 10 kHz frequency. As shown in Fig. 4b, both two samples exhibit positive slopes in the Mott–Schottky plots, suggesting that both samples expected for n-type semiconductors. Moreover, the Er2O3@ ZnO sample exhibited the smaller slope compared to pristine ZnO, suggesting the Er2O3@ZnO sample possess the more donor densities (carrier densities) after deposition Er2O3 shell. Then the calculated carrier densities from the Mott–Schottky equation are 2.44 × 1018 cm−3 for Er2O3@ZnO and 1.24 × 1016 cm− 3 for pristine ZnO, respectively. It should be noted that the Mott–Schottky equation was derived based on a planar electrode model, while we used projected area instead of effective surface area of the nanorod structures for the calculation, which could cause errors in determining the carrier densities. However, a qualitative comparison of carrier densities between our samples is valid, as they have similar morphology and surface area because only a very thin Er2O3 shell layer was covered on the ZnO surface. As discussed above, the calculated carrier density of Er2O3@ZnO sample is about several orders of magnitude to that of ZnO. Therefore, the donor density can be large enhanced by deposition Er2O3 shell. In summary, we have successfully synthesized the Er2O3@ZnO coreshell nanostructures for the PEC water splitting. The SEM and XRD indicated that a thin layer covered the ZnO nanorods, while the UV-vis absorption spectra showed almost no difference between these two samples. Under the white light illumination, the Er2O3@ZnO films achieved the highest photocurrent density of 3.8 mA cm−2 at 1.0 V vs. Ag/AgCl, exhibiting substantially larger photocurrent density than that of ZnO films with a maximum photocurrent density of 2.8 mA cm− 2 at the same potential, suggesting that this core-shell structure can greatly enhance the photo-response of the ZnO. Moreover, it is concluded from the Mott–Schottky plots that this improvement may be due to the increased donor density. This achievement makes it possible to design composite photoelectrodes to more effectively utilize the solar spectrum. Characterization and measurements The Er2O3@ZnO core-shell nanorods here were grown on FTO substrates via two-step synthetic process. In general, ZnO nanorod arrays were firstly fabricated on FTO substrates by galvanostatic electrolysis in an aqueous solution of 0.02 M Zn(NO3)2, 0.01 M NH4Ac and 0.01 M hexamethylenetetramine with a current density of −2.0 mA cm−2 at 90 °C for 50 min. Then the ZnO films were washed with deionized water, ethanol and dried at 80 °C for 1 h in air. The preparation of the Er2O3@ZnO core-shell structure nanorod array films was similar to that of the ZnO, while the galvanostatic electrolysis was proceed at + 0.5 mA cm−2 at 90 °C for 30 min in an aqueous solution of 0.01 M ErCl3 containing 10% (V/V) DMSO. Then the Er2O3@ZnO core-shell films were washed for two times with DI water and ethanol, following by drying at 80 °C for 3 h. At last, the as-prepared Er2O3@ZnO core-shell films were annealed in air at 550 °C for 3 h. The phase of the Er2O3@ZnO core-shell nanorod array film was determined by X-ray powder diffraction (PIGAKU, D/MAX2200VPC), and the surface morphology and structure of the as-deposited films were observed by thermal field emission environment scanning electron microscope (FE-SEM, JSM-6330 F) and transmission electron microscopy (TEM, JEM2010-HR, 200 KV). The optical properties were measured on a UV-vis-NIR spectrophotometer (UV, Shimadzu UV-2450). PEC measurements were carried out in a three-electrode cell with a flat quartz window to facilitate illumination of the photoelectrode surface. The electrolyte is 0.5 M Na2SO4 solution, and the working electrode is the Er2O3@ZnO film, while a Pt wire was used as the counter electrode and a saturated Ag/AgCl electrode used as reference electrode. The illumination source was a 150 W Xe arc lamp (Newport, 6255, 150 W) directed at the quartz photoelectrochemical cell. The photocurrent
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densities were recorded with a CHI 760d electrochemical workstation (CHI, Shanghai). Acknowledgments This work was supported by the Natural Science Foundations of China (grant no. 21306030), the Natural Science Foundations of Guangdong Province (grant no. s2012010009719 and s2013040015229), the Innovative Talents Cultivation Project of Guangdong Province (grant no. LYM11096), the Science and Technology Project of Guangzhou (grant no. 12C52011621) and the Scientific Research Project of Guangzhou Municipal Colleges and Universities (grant no. 2012A064). References [1] A. Fujishima, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Efficient photochemical water splitting by a chemically modified n-TiO2, Science 297 (2002) 2243–2245. [3] X. Chen, S. Shen, L. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570. [4] W. Smith, A. Wolcott, R.C. Fitzmorris, J.Z. Zhang, Y. Zhao, Quasi-core-shell TiO2/WO3 and WO3/TiO2 nanorod arrays fabricated by glancing angle deposition for solar water splitting, J. Mater. Chem. 21 (2011) 10792–10800. [5] G. Wang, Y. Ling, H. Wang, X. Yang, C. Wang, J.Z. Zhang, Y. Li, Hydrogen-treated WO3 nanoflakes show enhanced photostability, Energy Environ. Sci. 5 (2012) 6180–6187. [6] G. Wang, Y. Ling, D.A. Wheeler, K.E.N. George, K. Horsley, C. Heske, J.Z. Zhang, Y. Li, Facile synthesis of highly photoactive α-Fe2O3-based films for water oxidation, Nano Lett. 11 (2011) 3503–3509. [7] G.M. Wang, H.Y. Wang, Y.C. Ling, Y.C. Tang, X.Y. Yang, R.C. Fitzmorris, C.C. Wang, J.Z. Zhang, Y. Li, Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting, Nano Lett. 11 (2011) 3026–3033. [8] Y.-Z. Su, K. Xiao, Z.-J. Liao, Y.-H. Zhong, N. Li, Y.-B. Chen, Z.-Q. Liu, Directed electrochemical synthesis of ZnO/PDMcT core/shell nanorod arrays with enhanced photoelectrochemical properties, Int. J. Hydrog. Energy 38 (2013) 15019–15026. [9] R. Solarska, A. Królikowska, J. Augustyński, Silver nanoparticle induced photocurrent enhancement at WO3 photoanodes, Angew. Chem. Int. Ed. 49 (2010) 7980–7983. [10] S. Xie, X. Lu, T. Zhai, J. Gan, W. Li, M. Xu, M. Yu, Y.-M. Zhang, Y. Tong, Controllable synthesis of ZnxCd1-xS@ZnO core-shell nanorods with enhanced photocatalytic activity, Langmuir 28 (2012) 10558–10564. [11] S. Xie, T. Zhai, W. Li, M. Yu, C. Liang, J. Gan, X. Lu, Y. Tong, Hydrogen production from solar driven glucose oxidation over Ni(OH)2 functionalized electroreduced-TiO2 nanowire arrays, Green Chem. 15 (2013) 2434–2440. [12] X. Yang, A. Wolcott, G. Wang, A. Sobo, R.C. Fitzmorris, F. Qian, J.Z. Zhang, Y. Li, Nitrogen-doped ZnO nanowire arrays for photoelectrochemical water splitting, Nano Lett. 9 (2009) 2331–2336. [13] J. Qiu, M. Guo, Y. Feng, X. Wang, Electrochemical deposition of branched hierarchical ZnO nanowire arrays and its photoelectrochemical properties, Electrochim. Acta 56 (2011) 5776–5782. [14] Z.-Q. Liu, X.-H. Xie, Q.-Z. Xu, S.-H. Guo, N. Li, Y.-B. Chen, Y.-Z. Su, Electrochemical synthesis of ZnO/CdTe core-shell nanotube arrays for enhanced photoelectrochemical properties, Electrochim. Acta 98 (2013) 268–273. [15] S. Obregón, A. Kubacka, M. Fernández-García, G. Colón, Kinetic study of 1-butene isomerization on alumina at 510-550 ° C, J. Catal. 299 (2013) 298–306. [16] S. Liu, Y. Cai, X. Cai, H. Li, F. Zhang, Q. Mu, Y. Liu, Y. Wang, Catalytic photodegradation of Congo red in aqueous solution by Ln(OH)3 (Ln = Nd, Sm, Eu, Gd, Tb, and Dy) nanorods, Appl. Catal. A Gen. 453 (2013) 45–53. [17] B. Dong, H. Song, H. Yu, H. Zhang, R. Qin, X. Bai, G. Pan, S. Lu, F. Wang, L. Fan, Upconversion properties of Ln3+ doped NaYF4/polymer composite fibers prepared by electrospinning, J. Phys. Chem. C 112 (2008) 1435–1440. [18] M. Yousefi, M. Amiri, R. Azimirad, A.Z. Moshfegh, Enhanced photoelectrochemical activity of Ce doped ZnO nanocomposite thin films under visible light, J. Electroanal. Chem. 661 (2011) 106–112. [19] S. Anandan, A. Vinu, K.L.P. Sheeja Lovely, N. Gokulakrishnan, P. Srinivasu, T. Mori, V. Murugesan, V. Sivamurugan, K. Ariga, Photocatalytic activity of La-doped ZnO for the degradation of monocrotophos in aqueous suspension, J. Mol. Catal. A Chem. 266 (2007) 149–157. [20] S. Xie, X. Lu, T. Zhai, W. Li, M. Yu, C. Liang, Y. Tong, Enhanced photoactivity and stability of carbon and nitrogen co-treated ZnO nanorod arrays for photoelectrochemical water splitting, J. Mater. Chem. 22 (2012) 14272–14275. [21] D. Pradhan, K.T. Leung, Controlled growth of two-dimensional and one-dimensional ZnO nanostructures on indium tin oxide coated glass by direct electrodeposition, Langmuir 24 (2008) 9707–9716. [22] Y. Tak, S.J. Hong, J.S. Lee, K. Yong, Fabrication of ZnO/CdS core/shell nanowire arrays for efficient solar energy conversion, J. Mater. 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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A facile way to synthesize Er2O3@ZnO core-shell nanorods for photoelectrochemical water splitting Kai-Hang Ye, Ji-Yu Wang, Nan Li ⁎, Zhao-Qing Liu, Shi-Heng Guo, Yun-Ping Guo, Yu-Zhi Su School of Chemistry and Chemical Engineering/Guangzhou, Key Laboratory for Environmentally Functional Materials and Technology, Guangzhou University, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 18 February 2014 Received in revised form 2 April 2014 Accepted 10 April 2014 Available online 24 April 2014
a b s t r a c t In this study, we demonstrated large-scale Er2O3@ZnO core-shell nanorods with great photoelectrochemical water splitting ability, which were successfully fabricated by a simple and facile electrodeposition. These Er2O3@ZnO core-shell nanorods exhibited improved photocurrent under the light irradiation, which can be attributed to the enhanced donor density by the covered Er2O3. © 2014 Elsevier B.V. All rights reserved.
Keywords: ZnO Er2O3 Core-shell Photoelectrochemical water splitting
Hydrogen, as a clean energy, which will not bring any greenhouse gases after combustion, has drawn continuous attentions. One of the most promising and environmental friendly methods is the water splitting by the photoelectrochemical cells over the semiconductors [1–3]. Among these semiconductors, metal oxide, such as ZnO, TiO2 and Fe2O3, has been widely studied as a photoanode in the photoelectrochemical (PEC) cell and some achievements have been made [4–11]. ZnO with a band gap of 3.2 eV has drawn continuous attention due to its favorable band-edge positions that can straddle the redox potential of water photoelectrolysis, high photocatalytic activity, natural abundance, economical and low toxicity to the environment. However, the efficiency of the ZnO is not satisfying as it possesses large band gap and suffers rapidly recombination of photoexcited electron-hole pairs [12]. Up to now, an enormous amount of research has been made to enhance the absorption of ZnO in the visible light region. Generally, there are many approaches to improve the conversion efficiency by sensitization ZnO with small band gap semiconductors or elemental doping, and many achievements have been made [13,14]. On the other hand, it has been proved that the morphology of ZnO can affect the electronic structure and then enhance the separation and transportation of the photoexcited charge carriers. For instance, Yang et al. [12] reported the N doped ZnO nanowires to improve light-harvest ability, which show an enhanced photocurrent and stability in photoelectrochemical hydrogen volution. Rare earth elements, with outstanding catalytic capacity, have shown noteworthy activity owing to its potential applications in various fields, such as, photodegradation, catalysts, high-quality phosphors and upconversion [15–17]. Yousefi et al. [18] have demonstrated that the ⁎ Corresponding author. Tel./fax: +86 20 39366908. E-mail address: [email protected] (N. Li).
http://dx.doi.org/10.1016/j.inoche.2014.04.018 1387-7003/© 2014 Elsevier B.V. All rights reserved.
addition of Ce to ZnO thin film increased photocurrent density to about double amount at applied potential of 0.6 V vs. Ag/AgCl compared to undoped ZnO film in photoelectrochemical water splitting reaction. Anandan and his co-works [19] observed that the doping of La in ZnO helps to achieve complete mineralization of monocrotophos within a short irradiation time. However, no attention have been paid to the erbium (Er) doped ZnO it is highly desirable to investigate the influence of the Er2O3 coating. Herein, we demonstrated the design and synthesis of Er2O3@ZnO core-shell structure on F doped SnO2 (FTO) glass substrates by a facile electrochemical deposition process, following by studying their PEC performance for water splitting. The electrodeposition of ZnO was carried out in solution of 0.02 M Zn(NO3)2, 0.01 M NH4Ac and 0.01 M hexamethylenetetramine with current density of − 2.0 mA cm−2 at 90 °C for 50 min, and the typical SEM images are presented in Fig. 1a–c. It can be seen that hexagonal ZnO nanorods were successfully synthesized on FTO substrates. The average diameter of the nanorods is about 200–300 nm. Then the asprepared ZnO film served as the electrode and template to take electrodeposition of Er2O3, and the SEM image of a typical Er2O3@ZnO composite material was illustrated in Fig. 1d–f. As shown in Fig. 1f, the surface of the ZnO nanorods became rather coarse after electrodeposition of Er2O3, which confirmed that the Er2O3 was successfully deposited onto the ZnO nanorods. It is noteworthy that the average diameter was similar to that of ZnO, suggesting that a very thin layer of Er2O3 was constructed. The crystal structure of Er2O3@ZnO core-shell nanorod was investigated by TEM. A low-magnification TEM image of part of an individual nanorod with rod-like structure is illustrated in Fig. 2a, which clearly shows the partial surface of the ZnO nanorod is covered by Er2O3 layer. The enlarged HRTEM image (Fig. 2b) of the selected area marked
K.-H. Ye et al. / Inorganic Chemistry Communications 45 (2014) 116–119
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Fig. 1. SEM images of (a–c) pristine ZnO and (d–f) Er2O3@ZnO shell-core nanorod arrays.
by blue cycle in Fig. 2a reveals the lattice fringe spacing of 0.26 nm, which paralleled to its growth axis are attributed to the (0001) plane of wurtzite ZnO, indicating the ZnO nanorod grow along the direction of [0001]. In Addition, the surface of the ZnO nanorod is uniformly coated with a shell of amorphous Er2O3 layer. Energy-dispersive X-ray spectroscopy (EDS) analysis was also conducted for the Er2O3@ZnO composites. As shown in Fig. 2c, besides the Cu, C and Sn signals coming from the TEM grid and FTO substrates, respectively, Er, Zn, and O were
detected from the Er2O3@ZnO composites, suggesting the composite is made up of Er2O3 and ZnO. In order to identify the crystalline phase of the products, X-ray powder diffraction (XRD) was conducted, and the results were presented in Fig. 3a. All of the reflections of the XRD pattern could be indexed to hexagonal ZnO with a lattice constant a = b = 3.253 Å, c = 5.215 Å, compatible with JCPDS card no. 80-0047. The diffraction peaks appeared at 2θ = 31.73°, 34.36°, 36.21°, 47.47° and 62.75° corresponding to (100),
Fig. 2. (a) TEM, (b) HRTEM and (c) the corresponding EDS pattern of Er2O3@ZnO shell-core nanorod.
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properties of pristine ZnO and Er2O3@ZnO electrodes were characterized by measuring the linear sweep voltammogram (LSW) curves under light irradiation. As demonstrated in Fig. 4a, the dark scans collected in the potential range between −0.6 and +1.0 V vs. Ag/AgCl revealed a negligible background current. Under the white light illumination, both pristine ZnO and Er2O3@ZnO showed pronounced photoresponse. Significantly, the Er2O3@ZnO films and annealed Er2O3@ZnO films achieved the highest photocurrent density of 3.5 mA cm−2 at 1.0 V vs. Ag/AgCl, exhibiting substantially larger photocurrent density than that of ZnO films with a maximum photocurrent density of 2.2 mA cm− 2 at the same potential. This result confirmed the positive role of Er2O3 shell in enhancing the photoactivity of pristine ZnO electrodes under illumination. To further elucidate the effect of Er2O3 shell on the electronic properties of ZnO, electrochemical impedance measurements for both electrodes were performed in the dark. Carrier densities of these samples were calculated from the slopes of Mott–Schottky plots using the equation: h i‐1 2 Nd ¼ ð2=e0 εε0 Þ d 1=c =dV where e0 is the electron charge, ε is the dielectric constant of ZnO (ε = 8) [24], ε0 is the permittivity of vacuum and Nd is the dopant density and V the electrode applied potential.
Fig. 3. (a) X-ray diffraction patterns of prinstine ZnO, Er2O3@ZnO and annealed Er2O3@ ZnO shell-core nanorod arrays (550 °C); (b) UV-vis absorption spectra of pristine ZnO and Er2O3@ZnO shell-core nanorod arrays.
(002), (101), (102) and (103) planes were observed, and the strong intensity of the diffraction peaks indicates good crystallinity of the microwire. Besides SnO2 (coming from FTO substrate) and ZnO diffraction peaks, no other obvious peaks corresponding to Er2O3 were obtained. In addition, the diffraction peaks of annealed Er2O3@ZnO film are also consistent with the pristine ZnO and Er2O3@ZnO film. The results confirm that the crystal structure of Er2O3 shell is amorphous. Furthermore, the UV-vis absorption spectra of pristine ZnO and the Er2O3@ZnO film were measured to investigate the influence of Er2O3 shell layer. Similar to the shape absorption edge of the pristine of ZnO film, the edge of Er2O3@ZnO was also at around 400 nm. The band gap (Eg) of semiconductor material can be calculated from the equation of (Rhν)n = A (hν − Eg), where hν is the photon energy, R is the absorption coefficient, A is a constant for the material and n is 2 for a direct transition or 1/2 for an indirect transition [20,21]. Therefore, as a direct transition semiconductor, the estimated direct Eg values of the ZnO and Er2O3@ZnO were both about 3.2 eV. It is concluded that the thin layer has little effect on the band gap of ZnO. Well-aligned 1D nanostructures with narrow band gap are very acceptable and efficient for the conversion of solar energy because the 1D structures can provide a direct electron transport pathway and thus enhance the separation of the photo-excited electron–hole pairs [20,22,23]. Based on these hypothesizes, the as prepared are expected to be used as photoelectrode in a PEC cell. The photoelectrochemical
Fig. 4. (a) I − V curves recorded for pristine ZnO, Er2O3@ZnO and annealed Er2O3@ZnO shell-core nanorod arrays (550 °C) with a scan rate of 10 mV/s in 0.5 M Na2SO4. (b) Mott– Schottky plots of pristine ZnO and Er2O3@ZnO core-shell nanorod arrays obtained at each potential with 10 kHz frequency.
K.-H. Ye et al. / Inorganic Chemistry Communications 45 (2014) 116–119
Fig. 4b displays the Mott–Schottky plots collected for ZnO and Er2O3@ZnO. The plots were generated based on capacitances that were derived from the electrochemical impedance obtained at each potential with 10 kHz frequency. As shown in Fig. 4b, both two samples exhibit positive slopes in the Mott–Schottky plots, suggesting that both samples expected for n-type semiconductors. Moreover, the Er2O3@ ZnO sample exhibited the smaller slope compared to pristine ZnO, suggesting the Er2O3@ZnO sample possess the more donor densities (carrier densities) after deposition Er2O3 shell. Then the calculated carrier densities from the Mott–Schottky equation are 2.44 × 1018 cm−3 for Er2O3@ZnO and 1.24 × 1016 cm− 3 for pristine ZnO, respectively. It should be noted that the Mott–Schottky equation was derived based on a planar electrode model, while we used projected area instead of effective surface area of the nanorod structures for the calculation, which could cause errors in determining the carrier densities. However, a qualitative comparison of carrier densities between our samples is valid, as they have similar morphology and surface area because only a very thin Er2O3 shell layer was covered on the ZnO surface. As discussed above, the calculated carrier density of Er2O3@ZnO sample is about several orders of magnitude to that of ZnO. Therefore, the donor density can be large enhanced by deposition Er2O3 shell. In summary, we have successfully synthesized the Er2O3@ZnO coreshell nanostructures for the PEC water splitting. The SEM and XRD indicated that a thin layer covered the ZnO nanorods, while the UV-vis absorption spectra showed almost no difference between these two samples. Under the white light illumination, the Er2O3@ZnO films achieved the highest photocurrent density of 3.8 mA cm−2 at 1.0 V vs. Ag/AgCl, exhibiting substantially larger photocurrent density than that of ZnO films with a maximum photocurrent density of 2.8 mA cm− 2 at the same potential, suggesting that this core-shell structure can greatly enhance the photo-response of the ZnO. Moreover, it is concluded from the Mott–Schottky plots that this improvement may be due to the increased donor density. This achievement makes it possible to design composite photoelectrodes to more effectively utilize the solar spectrum. Characterization and measurements The Er2O3@ZnO core-shell nanorods here were grown on FTO substrates via two-step synthetic process. In general, ZnO nanorod arrays were firstly fabricated on FTO substrates by galvanostatic electrolysis in an aqueous solution of 0.02 M Zn(NO3)2, 0.01 M NH4Ac and 0.01 M hexamethylenetetramine with a current density of −2.0 mA cm−2 at 90 °C for 50 min. Then the ZnO films were washed with deionized water, ethanol and dried at 80 °C for 1 h in air. The preparation of the Er2O3@ZnO core-shell structure nanorod array films was similar to that of the ZnO, while the galvanostatic electrolysis was proceed at + 0.5 mA cm−2 at 90 °C for 30 min in an aqueous solution of 0.01 M ErCl3 containing 10% (V/V) DMSO. Then the Er2O3@ZnO core-shell films were washed for two times with DI water and ethanol, following by drying at 80 °C for 3 h. At last, the as-prepared Er2O3@ZnO core-shell films were annealed in air at 550 °C for 3 h. The phase of the Er2O3@ZnO core-shell nanorod array film was determined by X-ray powder diffraction (PIGAKU, D/MAX2200VPC), and the surface morphology and structure of the as-deposited films were observed by thermal field emission environment scanning electron microscope (FE-SEM, JSM-6330 F) and transmission electron microscopy (TEM, JEM2010-HR, 200 KV). The optical properties were measured on a UV-vis-NIR spectrophotometer (UV, Shimadzu UV-2450). PEC measurements were carried out in a three-electrode cell with a flat quartz window to facilitate illumination of the photoelectrode surface. The electrolyte is 0.5 M Na2SO4 solution, and the working electrode is the Er2O3@ZnO film, while a Pt wire was used as the counter electrode and a saturated Ag/AgCl electrode used as reference electrode. The illumination source was a 150 W Xe arc lamp (Newport, 6255, 150 W) directed at the quartz photoelectrochemical cell. The photocurrent
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densities were recorded with a CHI 760d electrochemical workstation (CHI, Shanghai). Acknowledgments This work was supported by the Natural Science Foundations of China (grant no. 21306030), the Natural Science Foundations of Guangdong Province (grant no. s2012010009719 and s2013040015229), the Innovative Talents Cultivation Project of Guangdong Province (grant no. LYM11096), the Science and Technology Project of Guangzhou (grant no. 12C52011621) and the Scientific Research Project of Guangzhou Municipal Colleges and Universities (grant no. 2012A064). References [1] A. Fujishima, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Efficient photochemical water splitting by a chemically modified n-TiO2, Science 297 (2002) 2243–2245. [3] X. Chen, S. Shen, L. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570. [4] W. Smith, A. Wolcott, R.C. 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