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Effect of hydrogen treatment on the photoelectrochemical properties of quantum dots sensitized ZnO nanorod array
Journal of Power Sources 272 (2014) 647e653
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Effect of hydrogen treatment on the photoelectrochemical properties of quantum dots sensitized ZnO nanorod array Yuyu Bu, Zhuoyuan Chen* Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
CdS sensitized hydrogenated ZnO nanorod array (NRA) was prepared in this work. Oxygen vacancies significantly enhance the charge carrier density of HeZnO NRA. Oxygen vacancies significantly enhance the electron migration ability of HeZnO NRA. H treatment significantly improve the photoelectrochemical performance of ZnO NRA.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2014 Received in revised form 25 August 2014 Accepted 28 August 2014 Available online 10 September 2014
The hydrogenated ZnO nanorod array (NRA) was, for the first time, applied to the field of quantum dots (QDs) sensitized photoanodes. Hydrogen treatment at high temperature can significantly change the surface states of ZnO NRA. Partial oxygen atoms on the surface of ZnO are taken away by hydrogen, resulting in the formation of oxygen vacancies. The formation of oxygen vacancies significantly enhances the charge carrier density and the electron migration ability of ZnO NRA. Meanwhile, the energy barrier of the injection of the photoinduced electrons generated by CdS into ZnO NRA decreases due to the formation of doping energy levels under the conducting band of ZnO. Therefore, the photoelectrochemical performance of CdS QDs sensitized hydrogenated ZnO NRA is significantly improved. This work has potential significance in enhancing the performance of QDs sensitized solar cell as well as dye sensitized solar cell. © 2014 Elsevier B.V. All rights reserved.
Keywords: Hydrogen treatment ZnO nanorod array Quantum dots Photoelectrochemical property Oxygen vacancy
1. Introduction Since Fujishima [1] pioneered the use of TiO2 photoanode in realizing the splitting of water under UV illumination, the technique of the photoelectrochemical hydrogen production from water splitting has caused widespread concern in the research community [2e6]. How to extend the light absorption region and how to improve the quantum yield of the semiconductor materials
* Corresponding author. Tel.: þ86 532 82898731; fax: þ86 532 82880498. E-mail address: [email protected] (Z. Chen). http://dx.doi.org/10.1016/j.jpowsour.2014.08.127 0378-7753/© 2014 Elsevier B.V. All rights reserved.
have become the focus of research [7,8]. Recently, Chen et al. [9] reported that high-temperature treating TiO2 under hydrogen atmosphere results in the formation of ‘black TiO2’. Hydrogen treatment allows the surface of TiO2 to lose oxygen atoms and therefore to become disordered. Oxygen vacancies were formed on the positions where oxygen was taken away by hydrogen. The existence of these oxygen vacancies results in the formation of new doping energy levels under the conduction band of TiO2 [10]. Therefore, the band gap of TiO2 was narrowed, leading to a red shift of the absorption band-edge of TiO2. Wang et al. [11] treated TiO2 nanowire array photoanode with hydrogen and they found that TiO2 nanowire array photoanode treated under hydrogen atmosphere at
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350 C possesses the best photoelectrochemical performance, which is mainly caused by the enhancing of the photoelectric conversion efficiency of TiO2 in the UV region due to the hydrogen treatment. Research on the hydrogen treatment of WO3 nanoflakes photoanode conducted by Wang et al. [12] shows that the photoelectrochemical performance and stability of WO3 nanoflakes photoanode are significantly enhanced by hydrogen treatment. Xia et al. [13] studied the effect of hydrogen treatment on the nanocrystal structure of TiO2. They found that hydrogen treatment induces the contraction of TiO2 lattice and changes the pressure of the crystal surface, therefore affecting the photocatalytic performance of hydrogenated TiO2. As an excellent semiconductor material with wide band gap, ZnO possesses the characteristics of high electron mobility, high stability and low price, and it has been extensively studied and applied in the fields of photoelectrochemical hydrogen production from water splitting [14,15], photocatalytic degradation of organic pollutants [16,17] and organic dye/inorganic quantum dots (QDs) sensitized solar cell [18e24]. And, hydrogen treatment can also enhance the photoelectrochemical and photocatalytic performance of ZnO. Research carried out by Cooper et al. [25] shows that hydrogen treatment can promote the photoelectrochemical performance of ZnO nanorod array (NRA), and the performance promotion is mainly ascribed to the enhancing of migration efficiency of charger carriers. Lu et al. [26] found that hydrogenated ZnO NRA possesses a very strong capability of photoelectrochemical hydrogen production from water splitting in the presence of the hole scavenger, S2þ. QDs sensitized solar cell has attracted widespread attention of researchers because of its excellent characteristics, such as special quantum effect, low price and simple preparation process. In this kind of solar cell, the inorganic QDs materials with narrow band gap are mainly acted as the light absorption materials, while the semiconductor materials with wide band gap, such as TiO2 or ZnO, are mainly acted as the electron migration materials [27e29]. Because high-temperature treating of the inert semiconductor materials under hydrogen atmosphere will cause some of the oxygen atoms on the surface be seized by hydrogen, which results in a significant change of the surface states and thus leads to the changes of the electron mobility and the energy band structure. To our best known, there is no report concerning about using hydrogenated inert semiconductor materials as the electron migration layer in the QDs sensitized solar cell. In this paper, the authors treated the ZnO NRA under hydrogen atmosphere at high temperature, and subsequently in situ deposited CdS quantum dots on the surface of hydrogenated ZnO NRA by the chemical bath deposition method. Furthermore, the significant promotion mechanism of the photoelectrochemical performance of the QDs sensitized ZnO NRA photoanode caused by treating the ZnO NRA under hydrogen atmosphere at high temperature was studied and proposed in this work.
nanoparticle seed layer was formed after heating the sol film at 500 C for 10 min. A total volume of 100 mL mixed solution containing 2.5 mmol zinc acetate, 2.5 mmol hexamethylenetetramine, and 0.6 mmol polyethyleneimine was then prepared and stirred for 20 min in an ice bath. Subsequently, 80 mL of the mixed solution was transferred to a polytetrafluoroethylene tube with a total volume of 100 mL. The FTO glass prepared with a ZnO nanoparticle seed layer was immersed into the solution and faced down, maintained at a certain angle versus the tube wall, and hydrothermally reacted for 4 h at 95 C. After that, the reaction solution in the polytetrafluoroethylene tube was updated, and the hydrothermal reaction was repeated one more time to increase the length of the ZnO nanorods under the same conditions. The prepared sample was repeatedly rinsed with deionized water and anhydrous alcohol, after which it was annealed at 500 C for 1 h. Series hydrogenated ZnO NRA samples were obtained by directly heating the prepared ZnO NRA at a given temperature (250, 300, 350, 400 and 450 C) for 30 min in a conventional tube furnace under high pure (99.999%) hydrogen atmosphere. All reagents used in this work were analytical ones from Aladin Industrial Corporation, China. 2.2. Preparation of CdS-sensitized ZnO NRA and CdS-sensitized hydrogenated ZnO NRA photoanodes The CdS quantum dots were deposited on the surface of the prepared ZnO NRA and hydrogenated ZnO NRA by chemical bath deposition method. ZnO NRA and hydrogenated ZnO NRA were sequentially immersed in 0.5 M Cd(NO3)2, deionized water, 0.5 M Na2S, and deionized water for 30 s, and this cyclic operation was repeated for 10 times. Finally, the samples were dried at 100 C for 2 h in vacuum and CdS quantum dots sensitized ZnO NRA or hydrogenated ZnO NRA samples were obtained. 2.3. Characterizations of CdS-sensitized ZnO NRA and CdSsensitized hydrogenated ZnO NRA The morphology of the prepared ZnO NRA was analyzed using a scanning electron microscope (SEM) (JSM-6700F; JEOL, Tokyo, Japan). The surface information, elemental compositions and bonding information of the synthetic products were analyzed using a field emission high-resolution transmission electron microscope (FE-HRTEM, Tecnai G2 F20, FEI Company, USA), an energy dispersive spectrometer (EDS, FEI Tecnai G20, FEI Company, USA) and Xray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd., England). The optical absorption properties were investigated using a UV/Vis diffuse reflectance spectrophotometer (U-41000; HITACHI, Tokyo, Japan). The photoluminescence intensity of the prepared samples was characterized using a fluorescence spectrometer (PL, Fluoro Max-4, HORIBA Jobin Yvon, France). 2.4. Photoelectrochemical measurements
2. Experimental 2.1. Preparation of ZnO NRA and hydrogenated ZnO NRA The ZnO nanorod array (NRA) was fabricated on fluorine-doped tin oxide (FTO) glass and the preparation of ZnO NRA composite photoelectrode was primarily based on the method used by Law et al. [30] with minor modifications. First, a ZnO nanoparticle seed layer was deposited onto the FTO glass (>99.6% purity; 5 cm 1 cm) by applying the following steps: 0.01 mol zinc acetate and 0.012 mol diethanolamine were dissolved in 25 mL anhydrous alcohol. A homogeneous sol was formed after 30 min of stirring at 60 C. An even sol film was formed on the FTO glass by using a dipcoating method (1 cm min1 pulling rate). A uniform ZnO
A three-electrode system was employed to measure the photoinduced volt-ampere characteristic curve (ieV curve) and the variations of the photoinduced current density with time (iet curve) of the prepared photoelectrodes using the CHI660D Electrochemical Workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). The photoelectrodes (0.5 cm 0.5 cm), saturated calomel electrode (SCE), and platinum electrode act as the working, reference, and counter electrodes, respectively. The ieV curves were measured from 1.6 V to 1.0 V with a scan rate of 0.05 V s1. The gap between the switching on and turning off of the light was 1 s. The iet curves were measured at a 0 V bias potential. The white light source was a 150 W Xe arc lamp (PLS-SXE300, Beijing Changtuo Co. Ltd., Beijing, China) with an optical intensity of
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265 mW cm2. A 420 nm cutoff filter was used to remove light with wavelengths less than 420 nm, ultimately generating visible light with an optical intensity of 200 mW cm2. The photoelectric conversion efficiency of the photoelectrodes was studied based on their monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra, which were measured using a 500 W Xe lamp with a monochromator. The light-sensing surface area was 0.5 cm 0.5 cm. Electrochemical impedance spectroscopy (EIS) tests were performed at open circuit potential over the frequency range between 105 and 102 Hz, with an AC voltage magnitude of 5 mV, using 12 points/decade. MotteSchottky plots were measured at the potential range of 1.0 Ve1.0 V and the frequency of 1000 Hz with an AC voltage magnitude of 10 mV. 3. Results and discussion Fig. S1 shows the SEM images of the prepared ZnO NRA (Fig. S1a) and that after CdS sensitization (Fig. S1b). The ZnO rod body, with a length of approximately 5 mm and a diameter of approximately 100 nm, was uniformly grown and distributed on the surface of fluorine-doped tin oxide (FTO) glass. After CdS sensitization, a large number of nanoparticles with length <10 nm were observed on the surface of ZnO rod body. Field emission highresolution transmission electron microscope (FE-HRTEM) is further used to characterize the tiny changes at the surface of ZnO NRA after hydrogen treatment, and the relevant results are shown in Fig. 1. Fig. 1a and b shows the HRTEM microscopic morphologies of ZnO NRA under low (Fig. 1a) and high (Fig. 1b) magnification, respectively. ZnO (110) crystal plane can be observed in Fig. 1a, and the ZnO nanorod body is uniform without apparent difference
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between the surface and bulk phases. The atomic arrays at the surface of ZnO NRA can be clearly observed in Fig. 1b. Fig. 1c and d shows the HRTEM microscopic morphologies of ZnO NRA treated under hydrogen atmosphere at 400 C at low (Fig. 1c) and high (Fig. 1d) magnification, respectively. Compared with Fig. 1a, significant changes in the surface morphology of hydrogenated ZnO NRA are observed in Fig. 1c. Complete lattice arrangement on the surface crystal planes of hydrogenated ZnO NRA is not observed, and moreover, obvious difference between the surface and bulk phases of hydrogenated ZnO NRA occurs, as shown in Fig. 1c. By comparing with Fig. 1b, surface roughening of hydrogenated ZnO NRA can be clearly observed in Fig. 1d, and the atomic arrangement becomes disordered. The aforementioned results indicate that some oxygen atoms on the surface of ZnO NRA were taken away during the hydrogen treatment and oxygen vacancies were formed on the surface of hydrogenated ZnO NRA, which greatly change the surface tension of ZnO and thus leading to the formation of disordered atomic arrangement on the surface of hydrogenated ZnO NRA. X-ray photoelectron spectroscopy (XPS) was used to further verify whether the formation of disordered atomic arrangement on the surface of hydrogenated ZnO NRA is due to the oxygen vacancies formed on the surface or not. Fig. S2a show the total XPS survey spectra of ZnO NRA and hydrogenated ZnO NRA treated at 400 C. As the results shown in Fig. S2a, hydrogen treatment at high temperature does not influence the elemental compositions. Fig. S2b is the Zn2p core level XPS sprectra of ZnO NRA before and after hydrogen treatment at 400 C, and no obvious changes on the location and shape of the Zn2p binding energy peaks are observed before and after hydrogen treatment, indicating that the valance
Fig. 1. HRTEM morphologies of ZnO NRA (a, b) and hydrogenated ZnO NRA treated at 400 C (c, d).
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state of Zn is not influenced by the hydrogen treatment at 400 C for 30 min Fig. 2 is the O1s core level XPS spectra of ZnO NRA before and after hydrogen treatment at 400 C. The peak centred at the binding energy of 529.9 eV is a characteristic peak of ZneOeZn [31], while the peak centred at the binding energies of 531.5 (for ZnO NRA without hydrogen treatment) is assigned to ZneOeH and the hydroxyl residual bonds on the surface of ZnO without hydrogen treatment. After treated at 400 C for 30 min, the binding energy peak at 531.5 eV is positively shifted to 531.9 eV, which can be attributed to the doping energy levels of oxygen vacancies, Vo [26,32,33]. The density of the binding energy peak at 531.9 eV for hydrogenated ZnO NRA is higher than that of the binding energy peak at 531.5 eV for ZnO NRA without hydrogen treatment, which is due to the increased density of hydroxyl groups and oxygen vacancies doping energy levels on hydrogenated ZnO surface. As mentioned above, Wang et al. [11] systematically studied the photoelectrochemical performance of hydrogenated TiO2 nanowire array photoanode, and they found that hydrogen treatment can significantly enhance the photoelectric conversion efficiency of the photoanode in the UV region, while the corresponding performance in visible light region cannot be improved. In the present work, the photoelectrochemical performance and the light absorption region of hydrogenated ZnO NRA are studied, and the results are shown in Fig. S3. Fig. S3a shows the variations in the photo-induced current densities of ZnO NRA (Curve a) and hydrogenated ZnO NRA treated at 250 (Curve b), 300 (Curve c), 350 (Curve d), 400 (Curve e) and 450 C (Curve f) at a bias potential of 0 V (vs SCE). Positive excitation current densities, indicated anodic current, were obtained under white light illumination. As the results shown in Fig. S3a, hydrogen treatment can enhance the photoinduced current density of ZnO NRA. The photoinduced current reaches a maximum value, 2.1 mA cm2, when the ZnO NRA was treated under hydrogen atmosphere at 300 C, which is approximately two times greater than ZnO NRA without hydrogen treatment. The photoinduced current density decreased with the further increase of the hydrogen treatment temperature. By increasing the hydrogen treatment temperature to 450 C, the photoinduced current decreased significantly and became very unstable, which is attributed to the significant decrease of the conductivity of the FTO glass caused by the formation of Sn due to the reduction of SnO2 in FTO glass. Fig. S3b show the photoinduced
Fig. 2. The O1s XPS core level spectra of ZnO NRA (lower) and hydrogenated ZnO NRA treated at 400 C (upper).
volteampere characteristic curves (i-V curves) of ZnO NRA and hydrogenated ZnO NRA treated at 300 C. The photoinduced current density significantly increased from 0.2 V and increased slowly when the bias potential reached to 0.2 V. The photoinduced current density of ZnO NRA is 4.6 mA cm2 when the bias potential is 1 V. However, different changing tendency is observed for hydrogenated ZnO NRA treated at 300 C. The photoinduced current density continued to increase quickly when the bias potential was larger than 0.2 V. The photoinduced current density of hydrogenated ZnO NRA treated at 300 C is 12.5 mA cm2 when the bias potential is 1 V, which is approximately three times higher than that of ZnO NRA. With respect to photoanode, the applied positive bias can increase the separation efficiency of the photoinduced electronehole pairs and promote the interface reaction capability, thus increasing the photoinduced current density of the photoanode. As shown in Fig. S3b, the increasing tendency of the photoinduced current density is not obvious for ZnO NRA when the applied bias potential is larger than 0.2 V, illustrating that the main controlling factor for the photoinduced current has been gradually transformed from the separation efficiency of the photogenerated electronehole pairs and the interfacial reaction capability into the intrinsic electronic migration efficiency for ZnO NRA. The low intrinsic electron migration efficiency of ZnO NRA inhibits the further promotion of photoinduced current density. However, this experimental phenomenon cannot be observed for hydrogenated ZnO NRA treated at 300 C, which illustrates, to some extent, that hydrogen treatment can improve the electron migration efficiency of ZnO NRA. Fig. S3c displays the monochromatic incident photonto-electron conversion efficiency (IPCE) spectra of ZnO NRA and hydrogenated ZnO NRA treated at 300 C at a bias potential of 1 V (vs SCE). As shown in Fig. S3c, the IPCE value of ZnO NRA can maintain at approximately 40% under the incident light with the wavelength below 360 nm; however, the IPCE value is reduced to 5% under the incident light with the wavelength of 380 nm, which corresponds to the standard light absorption bandedge of ZnO with the band gap of 3.2 eV. However, the IPCE value of hydrogenated ZnO NRA treated at 300 C keeps at approximately 80% due to its strong photoelectric conversion ability under the incident light with the wavelength below 360 nm. Significant decrease of the IPCE value is observed only under the incident light with the wavelength >400 nm and the photoelectric conversion capability has been extended to the vicinity of 440 nm, as shown in Fig. S3c. Similar results were obtained by Wang et al. [11] Fig. S3d shows the UV/Vis diffuse reflectance spectra of ZnO NRA and hydrogenated ZnO NRA treated at 300 C. As shown in Fig. S3d, hydrogen treatment can enhance the adsorption capacity of ZnO NRA in the region of UV and can result in a slightly red shift in the light absorption region, which can be ascribed to the promotion of the migration efficiency of the electrons due to the existence of oxygen vacancies at the surface of hydrogenated ZnO NRA. Meanwhile, new doping energy levels, Vo, are formed under the conduction band of ZnO which shortens the band gap of ZnO to some extent and thus results in a slightly red shift in the light absorption region. To further explore the effects of hydrogenated ZnO NRA treated at high temperature on the photoelectrochemical performance of QDs sensitized ZnO NRA photoanode, CdS QDs were deposited on the hydrogenated ZnO NRA surface using chemical bath deposition method, and the relevant morphologies and elemental compositions are presented in Fig. S4. As shown in Fig. S4, a large number of CdS QDs particles with the diameter of approximately 5 nm were accumulated on the surface of ZnO NRA, and the elements of Zn, O, Cd and S are accorded with the elemental compositions of CdS/ZnO NRA photoanode. The Cu element is observed in Fig. S4 because the elemental composition test was performed using HRTEM with a copper mesh substrate.
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Fig. 3 shows the photoelectrochemical performances of ZnO NRA and series CdS QDs sensitized hydrogenated ZnO NRA treated at different temperature. Fig. 3A shows the variations in currents with time (iet) for series photoanodes with visible and white light switched on and off. The light used for the first three cycles of light switched on and off is white light while that used for the last three cycles is visible light. Curve a in Fig. 3A is the iet curve of CdS QDs sensitized ZnO NRA photoanode and the photoinduced current densities under white and visible light are 9.8 and 4.5 mA cm2, respectively. However, the photoinduced current densities of series CdS QDs sensitized hydrogenated ZnO NRA photoanodes are significantly enhanced under both the white and visible light. Under white light, the photoinduced current density of CdS QDs sensitized hydrogeneated ZnO NRA rise first and decline later with the increase of the hydrogen treatment temperature. When the hydrogen treatment temperature is 400 C, the maximum photoinduced current densities of CdS QDs sensitized hydrogeneated ZnO NRA photoanode under white and visible light are observed and they are 19.4 and 11.9 mA cm2, respectively. These values are much higher than that of CdS QDs sensitized ZnO NRA under the same conditions. Fig. 3B is the ieV curves of CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C. According to the ieV curves shown in Fig. 3B, a photoinduced current is generated at a bias of 1.5 V. And the photoinduced current density significantly increases with the increase of bias voltage. For CdS QDs sensitized ZnO NRA photoanode, this significant upward change trend of the photoinduced current stopped at the bias potential of 0.3 V and the photoinduced
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current turns to be stabilized and slightly increase after the bias potential > 0.3 V. When the bias potential is 1 V, the photoinduced current density under white light is 11.1 mA cm2. For CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C, the significant increase of the photoinduced current density with the bias voltage is continued to the bias potential of 0 V, and subsequently the photoinduced current turns to be stabilized and slightly increase after the bias potential >0 V. When the bias potential is 1 V, the photoinduced current density under white light is 20.3 mA cm2, which is much higher than that obtained for CdS QDs sensitized ZnO NRA photoanode at the same condition. Fig. 3C is the IPCE curves of CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C. As shown in Fig. 3C, the IPCE value of CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C is higher than that of CdS QDs sensitized ZnO NRA under both the regions of UV and visible light. Especially in the region of UV with the wavelength less than 400 nm, the IPCE value of CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C is significantly higher than that of CdS QDs sensitized ZnO NRA. The thresholds of the photoelectric conversion for both photoanodes are at approximately 560 nm, which corresponds to the standard absorption value of CdS. The aforementioned results indicate that CdS QDs sensitized hydrogenated ZnO NRA photoanodes have much better photoelectrochemical performance than CdS QDs sensitized ZnO NRA photoanode, and the best photoelectrochemical performance was observed on the CdS QDs sensitized hydrogenated ZnO NRA photoanode at a hydrogen treatment temperature of 400 C.
Fig. 3. (A) The variations in currents with time (iet) at a bias potential of 0 V (vs SCE) for CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at different temperature with visible and white light switched on and off; (B) The photoinduced volteampere characteristic curves of ZnO NRA and hydrogenated ZnO NRA treated at 400 C; (C) The monochromatic incident photon-to-electron conversion efficiency spectra of ZnO NRA and hydrogenated ZnO NRA treated at 400 C at a bias potential of 0 V (vs SCE). All of the measurements were in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte.
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Fig. 4. (a) MotteSchottky plots of ZnO NRA and hydrogenated ZnO NRA treated at 300 and 400 C obtained at a frequency of 1 kHz in 0.1 M Na2SO4 electrolyte and under dark condition. (b) The photoluminescence spectra of ZnO NRA and hydrogenated ZnO NRA treated at 400 C. (c) The Nyquist plots of the photoanodes prepared by CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte under dark condition. (d) The Nyquist plots of the photoanodes prepared by CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte under white light.
Hydrogen treatment can significantly enhance the photoelectrochemical performance of the QDs sensitized photoanodes. In this work, MotteSchottky plots, photoluminescence (PL) spectra, and electrochemical impedance spectroscopy (EIS) were employed to determine how hydrogen treatment affects the photoelectrochemical performance of the QDs sensitized photoanodes, and the relevant results are shown in Fig. 4. Fig. 4a shows the MotteSchottky plots of ZnO NRA and series hydrogenated ZnO NRA treated at different temperature. MotteSchottky method describes the relation between capacitance of the space charge region and the applied potential with the specific formula for an n-type semiconductor listed as follows:
. 1 C 2 ¼ 2ðeεε0 ND Þ1 $ E Efb kT=e
(1)
where C is the capacitance of the space charge region in the semiconductor; ND is the electron carrier density, e is the elemental charge, ε0 is the permittivity of free space, ε is the relative permittivity of the semiconductor, E is the applied potential, Efb is the flat band potential, T is the temperature, and k is the Boltzmann constant [34]. According to Equation (1), the carrier density, ND, which is bound up to the electron migration efficiency in semiconductor, is inversely proportional to the slope of the straight-line portion in the MotteSchottky plot. The higher the charge carrier density, the larger the electron migration efficiency [2]. As shown in Fig. 4a, with the increase of the hydrogen treatment temperature, the slope
of the straight-line portion in the MotteSchottky plot for the hydrogenated ZnO NRA decreases gradually, therefore, the carrier density increases gradually, and the electron migration efficiency also increases gradually. This could be caused by the formation of oxygen vacancies on the surface of hydrogenated ZnO NRA. Hydrogen treatment at high temperature will seize partial oxygen on the surface of ZnO by hydrogen and oxygen vacancies were formed on the surface of ZnO, which made the ZnO surface positively charged. New doping energy levels were formed under the conduction band of ZnO in the mean time [10,25]. Because of the small difference between the formed new doping energy levels and the conduction band of ZnO, electrons in the doping energy levels can easily transfer to the conduction band of ZnO and thus enhances the carrier density of ZnO NRA and the electron migration efficiency. The electron migration efficiency takes great role in the enhancing of the photoelectrochemical performance of the QDs sensitized photoanodes. Fig. 4b shows the PL spectra of ZnO NRA and hydrogenated ZnO NRA treated at 400 C. With the increase of the electron migration efficiency in semiconductor, the lifetime of the photogenerated electrons will increase and the fluorescence intensity in the PL spectrum will decrease. For both ZnO NRA and hydrogenated ZnO NRA treated at 400 C, a strong, luminous broad peak emerged at 400 nme600 nm, as shown in Fig. 4b. However, the luminous intensity of the peak at 400 nme600 nm was significantly weakened for hydrogenated ZnO NRA treated at 400 C by comparing with that for ZnO NRA, illustrating that hydrogenated ZnO NRA treated at 400 C possesses a stronger electron migration capability.
Fig. 5. Proposed mechanism for the promotion of the photoelectrochemical performance of CdS QDs sensitized ZnO NRA due to hydrogen treatment.
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EIS can be used to study the migration ability of the electrons and the interface reaction ability of the electrons in the photocatalytic materials, which are very closely related to the photoelectrochemical performance of semiconductor materials [35]. The smaller the impedance value is, the greater the electron migration ability is and the stronger the interface reaction ability is. In this work, the impedance values of the photoanodes, prepared by CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C, were measured in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte in the dark and under white light illumination. Fig. 4c shows the impedance of the photoanodes prepared by CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte under dark condition, as a function of frequency. The impedance of the photoanode prepared by CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C is much lower than that of the photoanode prepared by CdS QDs sensitized ZnO NRA at the same frequency, indicating that the electron migration ability in both the thin film of the hydrogenated ZnO NRA and the interface of hydrogenated ZnO and CdS was dramatically improved. Fig. 4d shows the EIS results of the photoanodes prepared by CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte under white light. As shown in Fig. 4d, the impedance values of CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C are dramatically lower than those of CdS QDs sensitized ZnO NRA, indicating that hydrogen treatment can significantly increase the separation efficiency of the photoinduced electrons generated by the photoanode, and increase the interface reaction ability and the electron migration ability in the thin film of the photoanode. Fig. 5 schematically shows the proposed mechanism for the promotion of the photoelectrochemical performance of CdS QDs sensitized ZnO NRA due to hydrogen treatment based on aforementioned analysis. After hydrogen treatment at high temperature, partial oxygen on the surface of ZnO were seized by hydrogen, leading to the formation of oxygen vacancies, the disorder of the surface atoms of ZnO and a positive charge of ZnO. The formation of oxygen vacancies result in the formation of discontinuous doping energy levels under the conduction band of ZnO, leading to a significant increase of the charge carrier density and therefore increasing the electron mobility of ZnO. Meanwhile, because the potential of the doping energy levels, Vo, are more positive than that of the conduction band of ZnO, the photoinduced electrons generated by CdS can inject into not only the conduction band of ZnO but also the doping energy levels, Vo. This makes the difference of the electron migration potential difference of the photogenerated electrons are larger, thus lowering down the electron migration energy barrier. Hydrogen treatment at high temperature brings a positive charge on ZnO surface and the electric field formed by these positive charges on ZnO surface is propitious to the injection of the photoinduced electrons generated by CdS. For all of these reasons, the photoelectrochemical performance is significantly improved due to the hydrogen treatment of ZnO NRA. 4. Conclusion In general, hydrogen treatment can significantly increase the photoelectrochemical performance of the QDs sensitized ZnO NRA photoanodes. When hydrogen treatment temperature is 400 C, the CdS QDs sensitized hydrogenated ZnO NRA photoanode possesses the best photoelectrochemical performance. According to the experimental results from MotteSchottky, EIS and PL, the main reason for causing the significant increase of the photoelectrochemical performance is due to the formation of oxygen vacancies on the surface of ZnO, leading to the increase of the
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electron mobility and the formation of new doping energy levels under the conduction band of ZnO, which can lower down the energy barrier of transferring the photoinduced electrons generated by CdS to ZnO. Based on the experimental results obtained in this work, hydrogen treatment at high temperature has potential significance and application value for promoting the performance of the QDs sensitized solar cell. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 41376126) and the Hundreds-Talent Program of the Chinese Academy of Sciences (Y02616101L). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2014.08.127. References [1] A. Fujishima, Nature 238 (1972) 37. [2] W. Luo, Z. Yang, Z. Li, J. Zhang, J. Liu, Z. Zhao, Z. Wang, S. Yan, T. Yu, Z. Zou, Energy Environ. Sci. 4 (2011) 4046. [3] H.M. Chen, C.K. Chen, R. Liu, L. Zhang, J. Zhang, D.P. Wilkinson, Chem. Soc. Rev. 41 (2012) 5654. [4] J. Kamimura, P. Bogdanoff, J. Lahnemann, C. Hauswald, L. Geelhaar, S. Fiechter, H. Riechert, J. Am. Chem. Soc. 135 (2013) 10242. [5] Y. Bu, Z. Chen, W. Li, Dalt. Trans. 42 (2013) 16272. [6] M. Wang, L. Sun, Z. Lin, J. Cai, K. Xie, Changjian Lin, Energy Environ. Sci. 6 (2013) 1211. [7] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253. [8] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Chem. Rev. 110 (2010) 6446. [9] X. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 746. [10] A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, F. Fabbri, S. Cappelli, C.L. Bianchi, R. Psaro, V.D. Santo, J. Am. Chem. Soc. 134 (2012) 7600. [11] G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R.C. Fitzmorris, C. Wang, J.Z. Zhang, Y. Li, Nano Lett. 11 (2011) 3026. [12] G. Wang, Y. Ling, H. Wang, X. Yang, C. Wang, J.Z. Zhang, Y. Li, Energy Environ. Sci. 5 (2012) 6180. [13] T. Xia, X. Chen, J. Mater. Chem. A 1 (2013) 2983. [14] X. Yang, A. Wolcott, G. Wang, A. Sobo, R.C. Fitzmorris, F. Qian, J.Z. Zhang, Yat Li, Nano Lett. 9 (2009) 2331. [15] K. Sun, Y. Jing, C. Li, X. Zhang, R. Aguinaldo, A. Kargar, K. Madsen, K. Banu, Y. Zhou, Y. Bando, Z. Liu, D. Wang, Nanoscale 4 (2012) 1515. [16] H. Zhang, R. Zong, Y. Zhu, J. Phys. Chem. C 113 (2009) 4605. [17] Y. Zheng, C. Chen, Y. Zhan, X. Lin, Q. Zheng, K. Wei, J. Zhu, J. Phys. Chem. C 12 (2008) 10773. [18] W. Chiu, C. Lee, H. Cheng, H. Lin, S. Liao, J. Wu, W. Hsieh, Energy Environ. Sci. 2 (2009) 694. [19] Y. Shi, C. Zhu, L. Wang, C. Zhao, W. Li, K.K. Fung, T. Ma, A. Hagfeldt, N. Wang, Chem. Mater. 25 (2013) 1000. [20] H. Ming Chen, C.K. Chen, Y. Chang, C. Tsai, R. Liu, S. Hu, W. Chang, K. Chen, Angew. Chem. Int. Ed. 49 (2010) 5966. [21] G. Wang, X. Yang, F. Qian, J.Z. Zhang, Y. Li, Nano Lett. 10 (2010) 1088. [22] J. Jean, S. Chang, P.R. Brown, J.J. Cheng, P.H. Rekemeyer, M.G. Bawendi, S. Gradecak, V. Bulovic, Adv. Mater. 25 (2013) 2790. [23] M. Seol, H. Kim, W. Kim, K. Yong, Electrochem. Comm. 12 (2010) 1416. [24] Y. Bu, Z. Chen, W. Li, J. Yu, ACS Appl. Mater. Interfaces 5 (2013) 509. [25] J.K. Cooper, Y. Ling, C. Longo, Y. Li, J.Z. Zhang, J. Phys. Chem. C 116 (2012) 17360. [26] X. Lu, G. Wang, S. Xie, J. Shi, W. Li, Y. Tong, Y. Li, Chem. Commun. 48 (2012) 7717. [27] M.C. Beard, J.M. Luther, O.E. Semonin, A.J. Nozik, Acc. Chem. Res. 46 (2013) 1252. [28] I. Mora-Sero, S. Gimenez, F. Fabregat-Santiago, R. Gomez, Q. Shen, T. Toyoda, J. Bisquert, Acc. Chem. Res. 42 (2009) 1848. [29] M. Shalom, S. Buhbut, S. Tirosh, A. Zaban, J. Phys. Chem. Lett. 3 (2012) 2436. [30] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nat. Mater. 4 (2005) 455. [31] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, Appl. Surf. Sci. 158 (2000) 134. [32] H.K. Yadav, K. Sreenivas, V. Gupta, S.P. Singh, R.S. Katiyar, J. Mater. Res. 22 (2007) 2404. [33] S.F.J. Cox, E.A. Davis, S.P. Cottrell, P.J.C. King, J.S. Lord, J.M. Gil, H.V. Alberto, R.C. Vilao, J.P. Duarte, N.A. de Campos, A. Weidinger, R.L. Lichti, S.J.C. Irvine, Phys. Rev. Lett. 86 (2001) 2601. [34] Y. Bu, Z. Chen, W. Li, App. Cat. B: Envir. 144 (2014) 622. [35] X. Yu, J. Liao, K. Qiu, D. Kuang, C. Su, ACS Nano 5 (2011) 9494.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Effect of hydrogen treatment on the photoelectrochemical properties of quantum dots sensitized ZnO nanorod array Yuyu Bu, Zhuoyuan Chen* Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
CdS sensitized hydrogenated ZnO nanorod array (NRA) was prepared in this work. Oxygen vacancies significantly enhance the charge carrier density of HeZnO NRA. Oxygen vacancies significantly enhance the electron migration ability of HeZnO NRA. H treatment significantly improve the photoelectrochemical performance of ZnO NRA.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2014 Received in revised form 25 August 2014 Accepted 28 August 2014 Available online 10 September 2014
The hydrogenated ZnO nanorod array (NRA) was, for the first time, applied to the field of quantum dots (QDs) sensitized photoanodes. Hydrogen treatment at high temperature can significantly change the surface states of ZnO NRA. Partial oxygen atoms on the surface of ZnO are taken away by hydrogen, resulting in the formation of oxygen vacancies. The formation of oxygen vacancies significantly enhances the charge carrier density and the electron migration ability of ZnO NRA. Meanwhile, the energy barrier of the injection of the photoinduced electrons generated by CdS into ZnO NRA decreases due to the formation of doping energy levels under the conducting band of ZnO. Therefore, the photoelectrochemical performance of CdS QDs sensitized hydrogenated ZnO NRA is significantly improved. This work has potential significance in enhancing the performance of QDs sensitized solar cell as well as dye sensitized solar cell. © 2014 Elsevier B.V. All rights reserved.
Keywords: Hydrogen treatment ZnO nanorod array Quantum dots Photoelectrochemical property Oxygen vacancy
1. Introduction Since Fujishima [1] pioneered the use of TiO2 photoanode in realizing the splitting of water under UV illumination, the technique of the photoelectrochemical hydrogen production from water splitting has caused widespread concern in the research community [2e6]. How to extend the light absorption region and how to improve the quantum yield of the semiconductor materials
* Corresponding author. Tel.: þ86 532 82898731; fax: þ86 532 82880498. E-mail address: [email protected] (Z. Chen). http://dx.doi.org/10.1016/j.jpowsour.2014.08.127 0378-7753/© 2014 Elsevier B.V. All rights reserved.
have become the focus of research [7,8]. Recently, Chen et al. [9] reported that high-temperature treating TiO2 under hydrogen atmosphere results in the formation of ‘black TiO2’. Hydrogen treatment allows the surface of TiO2 to lose oxygen atoms and therefore to become disordered. Oxygen vacancies were formed on the positions where oxygen was taken away by hydrogen. The existence of these oxygen vacancies results in the formation of new doping energy levels under the conduction band of TiO2 [10]. Therefore, the band gap of TiO2 was narrowed, leading to a red shift of the absorption band-edge of TiO2. Wang et al. [11] treated TiO2 nanowire array photoanode with hydrogen and they found that TiO2 nanowire array photoanode treated under hydrogen atmosphere at
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350 C possesses the best photoelectrochemical performance, which is mainly caused by the enhancing of the photoelectric conversion efficiency of TiO2 in the UV region due to the hydrogen treatment. Research on the hydrogen treatment of WO3 nanoflakes photoanode conducted by Wang et al. [12] shows that the photoelectrochemical performance and stability of WO3 nanoflakes photoanode are significantly enhanced by hydrogen treatment. Xia et al. [13] studied the effect of hydrogen treatment on the nanocrystal structure of TiO2. They found that hydrogen treatment induces the contraction of TiO2 lattice and changes the pressure of the crystal surface, therefore affecting the photocatalytic performance of hydrogenated TiO2. As an excellent semiconductor material with wide band gap, ZnO possesses the characteristics of high electron mobility, high stability and low price, and it has been extensively studied and applied in the fields of photoelectrochemical hydrogen production from water splitting [14,15], photocatalytic degradation of organic pollutants [16,17] and organic dye/inorganic quantum dots (QDs) sensitized solar cell [18e24]. And, hydrogen treatment can also enhance the photoelectrochemical and photocatalytic performance of ZnO. Research carried out by Cooper et al. [25] shows that hydrogen treatment can promote the photoelectrochemical performance of ZnO nanorod array (NRA), and the performance promotion is mainly ascribed to the enhancing of migration efficiency of charger carriers. Lu et al. [26] found that hydrogenated ZnO NRA possesses a very strong capability of photoelectrochemical hydrogen production from water splitting in the presence of the hole scavenger, S2þ. QDs sensitized solar cell has attracted widespread attention of researchers because of its excellent characteristics, such as special quantum effect, low price and simple preparation process. In this kind of solar cell, the inorganic QDs materials with narrow band gap are mainly acted as the light absorption materials, while the semiconductor materials with wide band gap, such as TiO2 or ZnO, are mainly acted as the electron migration materials [27e29]. Because high-temperature treating of the inert semiconductor materials under hydrogen atmosphere will cause some of the oxygen atoms on the surface be seized by hydrogen, which results in a significant change of the surface states and thus leads to the changes of the electron mobility and the energy band structure. To our best known, there is no report concerning about using hydrogenated inert semiconductor materials as the electron migration layer in the QDs sensitized solar cell. In this paper, the authors treated the ZnO NRA under hydrogen atmosphere at high temperature, and subsequently in situ deposited CdS quantum dots on the surface of hydrogenated ZnO NRA by the chemical bath deposition method. Furthermore, the significant promotion mechanism of the photoelectrochemical performance of the QDs sensitized ZnO NRA photoanode caused by treating the ZnO NRA under hydrogen atmosphere at high temperature was studied and proposed in this work.
nanoparticle seed layer was formed after heating the sol film at 500 C for 10 min. A total volume of 100 mL mixed solution containing 2.5 mmol zinc acetate, 2.5 mmol hexamethylenetetramine, and 0.6 mmol polyethyleneimine was then prepared and stirred for 20 min in an ice bath. Subsequently, 80 mL of the mixed solution was transferred to a polytetrafluoroethylene tube with a total volume of 100 mL. The FTO glass prepared with a ZnO nanoparticle seed layer was immersed into the solution and faced down, maintained at a certain angle versus the tube wall, and hydrothermally reacted for 4 h at 95 C. After that, the reaction solution in the polytetrafluoroethylene tube was updated, and the hydrothermal reaction was repeated one more time to increase the length of the ZnO nanorods under the same conditions. The prepared sample was repeatedly rinsed with deionized water and anhydrous alcohol, after which it was annealed at 500 C for 1 h. Series hydrogenated ZnO NRA samples were obtained by directly heating the prepared ZnO NRA at a given temperature (250, 300, 350, 400 and 450 C) for 30 min in a conventional tube furnace under high pure (99.999%) hydrogen atmosphere. All reagents used in this work were analytical ones from Aladin Industrial Corporation, China. 2.2. Preparation of CdS-sensitized ZnO NRA and CdS-sensitized hydrogenated ZnO NRA photoanodes The CdS quantum dots were deposited on the surface of the prepared ZnO NRA and hydrogenated ZnO NRA by chemical bath deposition method. ZnO NRA and hydrogenated ZnO NRA were sequentially immersed in 0.5 M Cd(NO3)2, deionized water, 0.5 M Na2S, and deionized water for 30 s, and this cyclic operation was repeated for 10 times. Finally, the samples were dried at 100 C for 2 h in vacuum and CdS quantum dots sensitized ZnO NRA or hydrogenated ZnO NRA samples were obtained. 2.3. Characterizations of CdS-sensitized ZnO NRA and CdSsensitized hydrogenated ZnO NRA The morphology of the prepared ZnO NRA was analyzed using a scanning electron microscope (SEM) (JSM-6700F; JEOL, Tokyo, Japan). The surface information, elemental compositions and bonding information of the synthetic products were analyzed using a field emission high-resolution transmission electron microscope (FE-HRTEM, Tecnai G2 F20, FEI Company, USA), an energy dispersive spectrometer (EDS, FEI Tecnai G20, FEI Company, USA) and Xray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd., England). The optical absorption properties were investigated using a UV/Vis diffuse reflectance spectrophotometer (U-41000; HITACHI, Tokyo, Japan). The photoluminescence intensity of the prepared samples was characterized using a fluorescence spectrometer (PL, Fluoro Max-4, HORIBA Jobin Yvon, France). 2.4. Photoelectrochemical measurements
2. Experimental 2.1. Preparation of ZnO NRA and hydrogenated ZnO NRA The ZnO nanorod array (NRA) was fabricated on fluorine-doped tin oxide (FTO) glass and the preparation of ZnO NRA composite photoelectrode was primarily based on the method used by Law et al. [30] with minor modifications. First, a ZnO nanoparticle seed layer was deposited onto the FTO glass (>99.6% purity; 5 cm 1 cm) by applying the following steps: 0.01 mol zinc acetate and 0.012 mol diethanolamine were dissolved in 25 mL anhydrous alcohol. A homogeneous sol was formed after 30 min of stirring at 60 C. An even sol film was formed on the FTO glass by using a dipcoating method (1 cm min1 pulling rate). A uniform ZnO
A three-electrode system was employed to measure the photoinduced volt-ampere characteristic curve (ieV curve) and the variations of the photoinduced current density with time (iet curve) of the prepared photoelectrodes using the CHI660D Electrochemical Workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). The photoelectrodes (0.5 cm 0.5 cm), saturated calomel electrode (SCE), and platinum electrode act as the working, reference, and counter electrodes, respectively. The ieV curves were measured from 1.6 V to 1.0 V with a scan rate of 0.05 V s1. The gap between the switching on and turning off of the light was 1 s. The iet curves were measured at a 0 V bias potential. The white light source was a 150 W Xe arc lamp (PLS-SXE300, Beijing Changtuo Co. Ltd., Beijing, China) with an optical intensity of
Y. Bu, Z. Chen / Journal of Power Sources 272 (2014) 647e653
265 mW cm2. A 420 nm cutoff filter was used to remove light with wavelengths less than 420 nm, ultimately generating visible light with an optical intensity of 200 mW cm2. The photoelectric conversion efficiency of the photoelectrodes was studied based on their monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra, which were measured using a 500 W Xe lamp with a monochromator. The light-sensing surface area was 0.5 cm 0.5 cm. Electrochemical impedance spectroscopy (EIS) tests were performed at open circuit potential over the frequency range between 105 and 102 Hz, with an AC voltage magnitude of 5 mV, using 12 points/decade. MotteSchottky plots were measured at the potential range of 1.0 Ve1.0 V and the frequency of 1000 Hz with an AC voltage magnitude of 10 mV. 3. Results and discussion Fig. S1 shows the SEM images of the prepared ZnO NRA (Fig. S1a) and that after CdS sensitization (Fig. S1b). The ZnO rod body, with a length of approximately 5 mm and a diameter of approximately 100 nm, was uniformly grown and distributed on the surface of fluorine-doped tin oxide (FTO) glass. After CdS sensitization, a large number of nanoparticles with length <10 nm were observed on the surface of ZnO rod body. Field emission highresolution transmission electron microscope (FE-HRTEM) is further used to characterize the tiny changes at the surface of ZnO NRA after hydrogen treatment, and the relevant results are shown in Fig. 1. Fig. 1a and b shows the HRTEM microscopic morphologies of ZnO NRA under low (Fig. 1a) and high (Fig. 1b) magnification, respectively. ZnO (110) crystal plane can be observed in Fig. 1a, and the ZnO nanorod body is uniform without apparent difference
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between the surface and bulk phases. The atomic arrays at the surface of ZnO NRA can be clearly observed in Fig. 1b. Fig. 1c and d shows the HRTEM microscopic morphologies of ZnO NRA treated under hydrogen atmosphere at 400 C at low (Fig. 1c) and high (Fig. 1d) magnification, respectively. Compared with Fig. 1a, significant changes in the surface morphology of hydrogenated ZnO NRA are observed in Fig. 1c. Complete lattice arrangement on the surface crystal planes of hydrogenated ZnO NRA is not observed, and moreover, obvious difference between the surface and bulk phases of hydrogenated ZnO NRA occurs, as shown in Fig. 1c. By comparing with Fig. 1b, surface roughening of hydrogenated ZnO NRA can be clearly observed in Fig. 1d, and the atomic arrangement becomes disordered. The aforementioned results indicate that some oxygen atoms on the surface of ZnO NRA were taken away during the hydrogen treatment and oxygen vacancies were formed on the surface of hydrogenated ZnO NRA, which greatly change the surface tension of ZnO and thus leading to the formation of disordered atomic arrangement on the surface of hydrogenated ZnO NRA. X-ray photoelectron spectroscopy (XPS) was used to further verify whether the formation of disordered atomic arrangement on the surface of hydrogenated ZnO NRA is due to the oxygen vacancies formed on the surface or not. Fig. S2a show the total XPS survey spectra of ZnO NRA and hydrogenated ZnO NRA treated at 400 C. As the results shown in Fig. S2a, hydrogen treatment at high temperature does not influence the elemental compositions. Fig. S2b is the Zn2p core level XPS sprectra of ZnO NRA before and after hydrogen treatment at 400 C, and no obvious changes on the location and shape of the Zn2p binding energy peaks are observed before and after hydrogen treatment, indicating that the valance
Fig. 1. HRTEM morphologies of ZnO NRA (a, b) and hydrogenated ZnO NRA treated at 400 C (c, d).
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state of Zn is not influenced by the hydrogen treatment at 400 C for 30 min Fig. 2 is the O1s core level XPS spectra of ZnO NRA before and after hydrogen treatment at 400 C. The peak centred at the binding energy of 529.9 eV is a characteristic peak of ZneOeZn [31], while the peak centred at the binding energies of 531.5 (for ZnO NRA without hydrogen treatment) is assigned to ZneOeH and the hydroxyl residual bonds on the surface of ZnO without hydrogen treatment. After treated at 400 C for 30 min, the binding energy peak at 531.5 eV is positively shifted to 531.9 eV, which can be attributed to the doping energy levels of oxygen vacancies, Vo [26,32,33]. The density of the binding energy peak at 531.9 eV for hydrogenated ZnO NRA is higher than that of the binding energy peak at 531.5 eV for ZnO NRA without hydrogen treatment, which is due to the increased density of hydroxyl groups and oxygen vacancies doping energy levels on hydrogenated ZnO surface. As mentioned above, Wang et al. [11] systematically studied the photoelectrochemical performance of hydrogenated TiO2 nanowire array photoanode, and they found that hydrogen treatment can significantly enhance the photoelectric conversion efficiency of the photoanode in the UV region, while the corresponding performance in visible light region cannot be improved. In the present work, the photoelectrochemical performance and the light absorption region of hydrogenated ZnO NRA are studied, and the results are shown in Fig. S3. Fig. S3a shows the variations in the photo-induced current densities of ZnO NRA (Curve a) and hydrogenated ZnO NRA treated at 250 (Curve b), 300 (Curve c), 350 (Curve d), 400 (Curve e) and 450 C (Curve f) at a bias potential of 0 V (vs SCE). Positive excitation current densities, indicated anodic current, were obtained under white light illumination. As the results shown in Fig. S3a, hydrogen treatment can enhance the photoinduced current density of ZnO NRA. The photoinduced current reaches a maximum value, 2.1 mA cm2, when the ZnO NRA was treated under hydrogen atmosphere at 300 C, which is approximately two times greater than ZnO NRA without hydrogen treatment. The photoinduced current density decreased with the further increase of the hydrogen treatment temperature. By increasing the hydrogen treatment temperature to 450 C, the photoinduced current decreased significantly and became very unstable, which is attributed to the significant decrease of the conductivity of the FTO glass caused by the formation of Sn due to the reduction of SnO2 in FTO glass. Fig. S3b show the photoinduced
Fig. 2. The O1s XPS core level spectra of ZnO NRA (lower) and hydrogenated ZnO NRA treated at 400 C (upper).
volteampere characteristic curves (i-V curves) of ZnO NRA and hydrogenated ZnO NRA treated at 300 C. The photoinduced current density significantly increased from 0.2 V and increased slowly when the bias potential reached to 0.2 V. The photoinduced current density of ZnO NRA is 4.6 mA cm2 when the bias potential is 1 V. However, different changing tendency is observed for hydrogenated ZnO NRA treated at 300 C. The photoinduced current density continued to increase quickly when the bias potential was larger than 0.2 V. The photoinduced current density of hydrogenated ZnO NRA treated at 300 C is 12.5 mA cm2 when the bias potential is 1 V, which is approximately three times higher than that of ZnO NRA. With respect to photoanode, the applied positive bias can increase the separation efficiency of the photoinduced electronehole pairs and promote the interface reaction capability, thus increasing the photoinduced current density of the photoanode. As shown in Fig. S3b, the increasing tendency of the photoinduced current density is not obvious for ZnO NRA when the applied bias potential is larger than 0.2 V, illustrating that the main controlling factor for the photoinduced current has been gradually transformed from the separation efficiency of the photogenerated electronehole pairs and the interfacial reaction capability into the intrinsic electronic migration efficiency for ZnO NRA. The low intrinsic electron migration efficiency of ZnO NRA inhibits the further promotion of photoinduced current density. However, this experimental phenomenon cannot be observed for hydrogenated ZnO NRA treated at 300 C, which illustrates, to some extent, that hydrogen treatment can improve the electron migration efficiency of ZnO NRA. Fig. S3c displays the monochromatic incident photonto-electron conversion efficiency (IPCE) spectra of ZnO NRA and hydrogenated ZnO NRA treated at 300 C at a bias potential of 1 V (vs SCE). As shown in Fig. S3c, the IPCE value of ZnO NRA can maintain at approximately 40% under the incident light with the wavelength below 360 nm; however, the IPCE value is reduced to 5% under the incident light with the wavelength of 380 nm, which corresponds to the standard light absorption bandedge of ZnO with the band gap of 3.2 eV. However, the IPCE value of hydrogenated ZnO NRA treated at 300 C keeps at approximately 80% due to its strong photoelectric conversion ability under the incident light with the wavelength below 360 nm. Significant decrease of the IPCE value is observed only under the incident light with the wavelength >400 nm and the photoelectric conversion capability has been extended to the vicinity of 440 nm, as shown in Fig. S3c. Similar results were obtained by Wang et al. [11] Fig. S3d shows the UV/Vis diffuse reflectance spectra of ZnO NRA and hydrogenated ZnO NRA treated at 300 C. As shown in Fig. S3d, hydrogen treatment can enhance the adsorption capacity of ZnO NRA in the region of UV and can result in a slightly red shift in the light absorption region, which can be ascribed to the promotion of the migration efficiency of the electrons due to the existence of oxygen vacancies at the surface of hydrogenated ZnO NRA. Meanwhile, new doping energy levels, Vo, are formed under the conduction band of ZnO which shortens the band gap of ZnO to some extent and thus results in a slightly red shift in the light absorption region. To further explore the effects of hydrogenated ZnO NRA treated at high temperature on the photoelectrochemical performance of QDs sensitized ZnO NRA photoanode, CdS QDs were deposited on the hydrogenated ZnO NRA surface using chemical bath deposition method, and the relevant morphologies and elemental compositions are presented in Fig. S4. As shown in Fig. S4, a large number of CdS QDs particles with the diameter of approximately 5 nm were accumulated on the surface of ZnO NRA, and the elements of Zn, O, Cd and S are accorded with the elemental compositions of CdS/ZnO NRA photoanode. The Cu element is observed in Fig. S4 because the elemental composition test was performed using HRTEM with a copper mesh substrate.
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Fig. 3 shows the photoelectrochemical performances of ZnO NRA and series CdS QDs sensitized hydrogenated ZnO NRA treated at different temperature. Fig. 3A shows the variations in currents with time (iet) for series photoanodes with visible and white light switched on and off. The light used for the first three cycles of light switched on and off is white light while that used for the last three cycles is visible light. Curve a in Fig. 3A is the iet curve of CdS QDs sensitized ZnO NRA photoanode and the photoinduced current densities under white and visible light are 9.8 and 4.5 mA cm2, respectively. However, the photoinduced current densities of series CdS QDs sensitized hydrogenated ZnO NRA photoanodes are significantly enhanced under both the white and visible light. Under white light, the photoinduced current density of CdS QDs sensitized hydrogeneated ZnO NRA rise first and decline later with the increase of the hydrogen treatment temperature. When the hydrogen treatment temperature is 400 C, the maximum photoinduced current densities of CdS QDs sensitized hydrogeneated ZnO NRA photoanode under white and visible light are observed and they are 19.4 and 11.9 mA cm2, respectively. These values are much higher than that of CdS QDs sensitized ZnO NRA under the same conditions. Fig. 3B is the ieV curves of CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C. According to the ieV curves shown in Fig. 3B, a photoinduced current is generated at a bias of 1.5 V. And the photoinduced current density significantly increases with the increase of bias voltage. For CdS QDs sensitized ZnO NRA photoanode, this significant upward change trend of the photoinduced current stopped at the bias potential of 0.3 V and the photoinduced
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current turns to be stabilized and slightly increase after the bias potential > 0.3 V. When the bias potential is 1 V, the photoinduced current density under white light is 11.1 mA cm2. For CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C, the significant increase of the photoinduced current density with the bias voltage is continued to the bias potential of 0 V, and subsequently the photoinduced current turns to be stabilized and slightly increase after the bias potential >0 V. When the bias potential is 1 V, the photoinduced current density under white light is 20.3 mA cm2, which is much higher than that obtained for CdS QDs sensitized ZnO NRA photoanode at the same condition. Fig. 3C is the IPCE curves of CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C. As shown in Fig. 3C, the IPCE value of CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C is higher than that of CdS QDs sensitized ZnO NRA under both the regions of UV and visible light. Especially in the region of UV with the wavelength less than 400 nm, the IPCE value of CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C is significantly higher than that of CdS QDs sensitized ZnO NRA. The thresholds of the photoelectric conversion for both photoanodes are at approximately 560 nm, which corresponds to the standard absorption value of CdS. The aforementioned results indicate that CdS QDs sensitized hydrogenated ZnO NRA photoanodes have much better photoelectrochemical performance than CdS QDs sensitized ZnO NRA photoanode, and the best photoelectrochemical performance was observed on the CdS QDs sensitized hydrogenated ZnO NRA photoanode at a hydrogen treatment temperature of 400 C.
Fig. 3. (A) The variations in currents with time (iet) at a bias potential of 0 V (vs SCE) for CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at different temperature with visible and white light switched on and off; (B) The photoinduced volteampere characteristic curves of ZnO NRA and hydrogenated ZnO NRA treated at 400 C; (C) The monochromatic incident photon-to-electron conversion efficiency spectra of ZnO NRA and hydrogenated ZnO NRA treated at 400 C at a bias potential of 0 V (vs SCE). All of the measurements were in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte.
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Fig. 4. (a) MotteSchottky plots of ZnO NRA and hydrogenated ZnO NRA treated at 300 and 400 C obtained at a frequency of 1 kHz in 0.1 M Na2SO4 electrolyte and under dark condition. (b) The photoluminescence spectra of ZnO NRA and hydrogenated ZnO NRA treated at 400 C. (c) The Nyquist plots of the photoanodes prepared by CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte under dark condition. (d) The Nyquist plots of the photoanodes prepared by CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte under white light.
Hydrogen treatment can significantly enhance the photoelectrochemical performance of the QDs sensitized photoanodes. In this work, MotteSchottky plots, photoluminescence (PL) spectra, and electrochemical impedance spectroscopy (EIS) were employed to determine how hydrogen treatment affects the photoelectrochemical performance of the QDs sensitized photoanodes, and the relevant results are shown in Fig. 4. Fig. 4a shows the MotteSchottky plots of ZnO NRA and series hydrogenated ZnO NRA treated at different temperature. MotteSchottky method describes the relation between capacitance of the space charge region and the applied potential with the specific formula for an n-type semiconductor listed as follows:
. 1 C 2 ¼ 2ðeεε0 ND Þ1 $ E Efb kT=e
(1)
where C is the capacitance of the space charge region in the semiconductor; ND is the electron carrier density, e is the elemental charge, ε0 is the permittivity of free space, ε is the relative permittivity of the semiconductor, E is the applied potential, Efb is the flat band potential, T is the temperature, and k is the Boltzmann constant [34]. According to Equation (1), the carrier density, ND, which is bound up to the electron migration efficiency in semiconductor, is inversely proportional to the slope of the straight-line portion in the MotteSchottky plot. The higher the charge carrier density, the larger the electron migration efficiency [2]. As shown in Fig. 4a, with the increase of the hydrogen treatment temperature, the slope
of the straight-line portion in the MotteSchottky plot for the hydrogenated ZnO NRA decreases gradually, therefore, the carrier density increases gradually, and the electron migration efficiency also increases gradually. This could be caused by the formation of oxygen vacancies on the surface of hydrogenated ZnO NRA. Hydrogen treatment at high temperature will seize partial oxygen on the surface of ZnO by hydrogen and oxygen vacancies were formed on the surface of ZnO, which made the ZnO surface positively charged. New doping energy levels were formed under the conduction band of ZnO in the mean time [10,25]. Because of the small difference between the formed new doping energy levels and the conduction band of ZnO, electrons in the doping energy levels can easily transfer to the conduction band of ZnO and thus enhances the carrier density of ZnO NRA and the electron migration efficiency. The electron migration efficiency takes great role in the enhancing of the photoelectrochemical performance of the QDs sensitized photoanodes. Fig. 4b shows the PL spectra of ZnO NRA and hydrogenated ZnO NRA treated at 400 C. With the increase of the electron migration efficiency in semiconductor, the lifetime of the photogenerated electrons will increase and the fluorescence intensity in the PL spectrum will decrease. For both ZnO NRA and hydrogenated ZnO NRA treated at 400 C, a strong, luminous broad peak emerged at 400 nme600 nm, as shown in Fig. 4b. However, the luminous intensity of the peak at 400 nme600 nm was significantly weakened for hydrogenated ZnO NRA treated at 400 C by comparing with that for ZnO NRA, illustrating that hydrogenated ZnO NRA treated at 400 C possesses a stronger electron migration capability.
Fig. 5. Proposed mechanism for the promotion of the photoelectrochemical performance of CdS QDs sensitized ZnO NRA due to hydrogen treatment.
Y. Bu, Z. Chen / Journal of Power Sources 272 (2014) 647e653
EIS can be used to study the migration ability of the electrons and the interface reaction ability of the electrons in the photocatalytic materials, which are very closely related to the photoelectrochemical performance of semiconductor materials [35]. The smaller the impedance value is, the greater the electron migration ability is and the stronger the interface reaction ability is. In this work, the impedance values of the photoanodes, prepared by CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C, were measured in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte in the dark and under white light illumination. Fig. 4c shows the impedance of the photoanodes prepared by CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte under dark condition, as a function of frequency. The impedance of the photoanode prepared by CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C is much lower than that of the photoanode prepared by CdS QDs sensitized ZnO NRA at the same frequency, indicating that the electron migration ability in both the thin film of the hydrogenated ZnO NRA and the interface of hydrogenated ZnO and CdS was dramatically improved. Fig. 4d shows the EIS results of the photoanodes prepared by CdS QDs sensitized ZnO NRA and CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C in 0.25 M Na2S þ 0.35 M Na2SO3 electrolyte under white light. As shown in Fig. 4d, the impedance values of CdS QDs sensitized hydrogenated ZnO NRA treated at 400 C are dramatically lower than those of CdS QDs sensitized ZnO NRA, indicating that hydrogen treatment can significantly increase the separation efficiency of the photoinduced electrons generated by the photoanode, and increase the interface reaction ability and the electron migration ability in the thin film of the photoanode. Fig. 5 schematically shows the proposed mechanism for the promotion of the photoelectrochemical performance of CdS QDs sensitized ZnO NRA due to hydrogen treatment based on aforementioned analysis. After hydrogen treatment at high temperature, partial oxygen on the surface of ZnO were seized by hydrogen, leading to the formation of oxygen vacancies, the disorder of the surface atoms of ZnO and a positive charge of ZnO. The formation of oxygen vacancies result in the formation of discontinuous doping energy levels under the conduction band of ZnO, leading to a significant increase of the charge carrier density and therefore increasing the electron mobility of ZnO. Meanwhile, because the potential of the doping energy levels, Vo, are more positive than that of the conduction band of ZnO, the photoinduced electrons generated by CdS can inject into not only the conduction band of ZnO but also the doping energy levels, Vo. This makes the difference of the electron migration potential difference of the photogenerated electrons are larger, thus lowering down the electron migration energy barrier. Hydrogen treatment at high temperature brings a positive charge on ZnO surface and the electric field formed by these positive charges on ZnO surface is propitious to the injection of the photoinduced electrons generated by CdS. For all of these reasons, the photoelectrochemical performance is significantly improved due to the hydrogen treatment of ZnO NRA. 4. Conclusion In general, hydrogen treatment can significantly increase the photoelectrochemical performance of the QDs sensitized ZnO NRA photoanodes. When hydrogen treatment temperature is 400 C, the CdS QDs sensitized hydrogenated ZnO NRA photoanode possesses the best photoelectrochemical performance. According to the experimental results from MotteSchottky, EIS and PL, the main reason for causing the significant increase of the photoelectrochemical performance is due to the formation of oxygen vacancies on the surface of ZnO, leading to the increase of the
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electron mobility and the formation of new doping energy levels under the conduction band of ZnO, which can lower down the energy barrier of transferring the photoinduced electrons generated by CdS to ZnO. Based on the experimental results obtained in this work, hydrogen treatment at high temperature has potential significance and application value for promoting the performance of the QDs sensitized solar cell. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 41376126) and the Hundreds-Talent Program of the Chinese Academy of Sciences (Y02616101L). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2014.08.127. References [1] A. Fujishima, Nature 238 (1972) 37. [2] W. Luo, Z. Yang, Z. Li, J. Zhang, J. Liu, Z. Zhao, Z. Wang, S. Yan, T. Yu, Z. Zou, Energy Environ. Sci. 4 (2011) 4046. [3] H.M. Chen, C.K. Chen, R. Liu, L. Zhang, J. Zhang, D.P. Wilkinson, Chem. Soc. Rev. 41 (2012) 5654. [4] J. Kamimura, P. Bogdanoff, J. Lahnemann, C. Hauswald, L. Geelhaar, S. Fiechter, H. Riechert, J. Am. Chem. Soc. 135 (2013) 10242. [5] Y. Bu, Z. Chen, W. Li, Dalt. Trans. 42 (2013) 16272. [6] M. Wang, L. Sun, Z. Lin, J. Cai, K. Xie, Changjian Lin, Energy Environ. Sci. 6 (2013) 1211. [7] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253. [8] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Chem. Rev. 110 (2010) 6446. [9] X. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 746. [10] A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, F. Fabbri, S. Cappelli, C.L. Bianchi, R. Psaro, V.D. Santo, J. Am. Chem. Soc. 134 (2012) 7600. [11] G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R.C. Fitzmorris, C. Wang, J.Z. Zhang, Y. Li, Nano Lett. 11 (2011) 3026. [12] G. Wang, Y. Ling, H. Wang, X. Yang, C. Wang, J.Z. Zhang, Y. Li, Energy Environ. Sci. 5 (2012) 6180. [13] T. Xia, X. Chen, J. Mater. Chem. A 1 (2013) 2983. [14] X. Yang, A. Wolcott, G. Wang, A. Sobo, R.C. Fitzmorris, F. Qian, J.Z. Zhang, Yat Li, Nano Lett. 9 (2009) 2331. [15] K. Sun, Y. Jing, C. Li, X. Zhang, R. Aguinaldo, A. Kargar, K. Madsen, K. Banu, Y. Zhou, Y. Bando, Z. Liu, D. Wang, Nanoscale 4 (2012) 1515. [16] H. Zhang, R. Zong, Y. Zhu, J. Phys. Chem. C 113 (2009) 4605. [17] Y. Zheng, C. Chen, Y. Zhan, X. Lin, Q. Zheng, K. Wei, J. Zhu, J. Phys. Chem. C 12 (2008) 10773. [18] W. Chiu, C. Lee, H. Cheng, H. Lin, S. Liao, J. Wu, W. Hsieh, Energy Environ. Sci. 2 (2009) 694. [19] Y. Shi, C. Zhu, L. Wang, C. Zhao, W. Li, K.K. Fung, T. Ma, A. Hagfeldt, N. Wang, Chem. Mater. 25 (2013) 1000. [20] H. Ming Chen, C.K. Chen, Y. Chang, C. Tsai, R. Liu, S. Hu, W. Chang, K. Chen, Angew. Chem. Int. Ed. 49 (2010) 5966. [21] G. Wang, X. Yang, F. Qian, J.Z. Zhang, Y. Li, Nano Lett. 10 (2010) 1088. [22] J. Jean, S. Chang, P.R. Brown, J.J. Cheng, P.H. Rekemeyer, M.G. Bawendi, S. Gradecak, V. Bulovic, Adv. Mater. 25 (2013) 2790. [23] M. Seol, H. Kim, W. Kim, K. Yong, Electrochem. Comm. 12 (2010) 1416. [24] Y. Bu, Z. Chen, W. Li, J. Yu, ACS Appl. Mater. Interfaces 5 (2013) 509. [25] J.K. Cooper, Y. Ling, C. Longo, Y. Li, J.Z. Zhang, J. Phys. Chem. C 116 (2012) 17360. [26] X. Lu, G. Wang, S. Xie, J. Shi, W. Li, Y. Tong, Y. Li, Chem. Commun. 48 (2012) 7717. [27] M.C. Beard, J.M. Luther, O.E. Semonin, A.J. Nozik, Acc. Chem. Res. 46 (2013) 1252. [28] I. Mora-Sero, S. Gimenez, F. Fabregat-Santiago, R. Gomez, Q. Shen, T. Toyoda, J. Bisquert, Acc. Chem. Res. 42 (2009) 1848. [29] M. Shalom, S. Buhbut, S. Tirosh, A. Zaban, J. Phys. Chem. Lett. 3 (2012) 2436. [30] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nat. Mater. 4 (2005) 455. [31] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, Appl. Surf. Sci. 158 (2000) 134. [32] H.K. Yadav, K. Sreenivas, V. Gupta, S.P. Singh, R.S. Katiyar, J. Mater. Res. 22 (2007) 2404. [33] S.F.J. Cox, E.A. Davis, S.P. Cottrell, P.J.C. King, J.S. Lord, J.M. Gil, H.V. Alberto, R.C. Vilao, J.P. Duarte, N.A. de Campos, A. Weidinger, R.L. Lichti, S.J.C. Irvine, Phys. Rev. Lett. 86 (2001) 2601. [34] Y. Bu, Z. Chen, W. Li, App. Cat. B: Envir. 144 (2014) 622. [35] X. Yu, J. Liao, K. Qiu, D. Kuang, C. Su, ACS Nano 5 (2011) 9494.