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Enhanced photoelectrochemical performance of nanoparticle ZnO photoanodes for water-splitting application
Accepted Manuscript Title: Enhanced photoelectrochemical performance of nanoparticle ZnO photoanodes for water-splitting application Author: M.S. Islam M.F. Hossain PII: DOI: Reference:
S1010-6030(16)30002-8 http://dx.doi.org/doi:10.1016/j.jphotochem.2016.04.002 JPC 10191
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
4-1-2016 29-3-2016 5-4-2016
Please cite this article as: M.S.Islam, M.F.Hossain, Enhanced photoelectrochemical performance of nanoparticle ZnO photoanodes for water-splitting application, Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2016.04.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced Photoelectrochemical Performance of Nanoparticle ZnO Photoanodes for Water-Splitting Application
M. S. Islam, M. F. Hossain* Department of Electrical & Electronic Engineering, Rajshahi University of Engineering and Technology, Rajshahi-6204, Bangladesh.
*
Corresponding Author: Tel./Fax: +88-0721-750356 Email: [email protected] [email protected]
or
HIGHLIGHTS The modified thermal evaporation setup has the opportunity to vary source-substrate distance vertically by moving ups and downs the substrate holder. The proposed system has the ability to use different size of substrates. The deposition time has great influenced on the morphology, crystallinity and Photoconversion efficiency of ZnO photoanodes. The ZnO photoanode deposited on 20 min deposition time has better Photoconversion efficiency than the ZnO photoanode deposited on 10 or 30 min. Abstract: In this paper, fabrication of ZnO nanoparticle photoanodes for water splitting application is presented. At first, metal zinc films have been deposited on bare fluorine-doped tin oxide glass substrates by thermal evaporation method with chamber pressure 0.05 mbar, source temperature 7000C and source-substrate distance 3cm for fabrication of the photoanodes. This deposited zinc films have been annealed at 5000C for 2 hrs in atmosphere for obtaining ZnO films. The prepared ZnO films with deposition times (10, 20 and 30 min) are characterized and investigated to find out the photoelectrochemical properties and their suitability for splitting of water. It has been observed that all the ZnO films have high crystallinity wurtzite structure and uniformly distributed nanoparticles with the size of 32-58 nm. ZnO film with 20 min sample has smallest nanoparticles (32 nm) than the others. All the ZnO photoanodes have exhibited higher solar to hydrogen conversion efficiency (0.29 to 0.42%) than the other researchers data. Among them, ZnO with 20 min sample has been showed maximum efficiency of 0.64% for UV illumination and 0.42% for visible light illumination than the ZnO photoanodes prepared with other deposition times (10 and 30 min). Keywords: Zinc oxide; thermal evaporation; thin film; water splitting, nanoparticle. 1. Introduction Energy is one of the greatest challenges facing humanity in the coming years, and therefore, the development of sustainable and renewable energy sources is a key concern [1]. Hydrogen is an efficient energy carrier that has highest energy density and is also environmental friendly [2]. Currently, the main production of hydrogen comes from steam reforming of
natural gas, which involves energy losses and the release of greenhouse gases [3]. Electrochemical solar driven water splitting is an attractive, environment friendly and nonconventional energy resource [4]. Light active semiconductors or metal oxides have shown great prospects for this method of hydrogen generation [5]. In 1972, Fujishima and Honda first used titanium dioxide (TiO2) photoanodes for hydrogen evolution [6]. TiO2 is the most promising photoelectrode because of its high efficiency, chemical and optical stability, and environmental and biological compatibility [1]. However, the use of TiO2 as photoanode for water splitting is limited by its redox potential with reference to the normal hydrogen electrode [7]. The ZnO is thought to be a potential substitute of TiO2 [8] because of its similar band gap (3.37 eV), large excitation binding energy (60 meV), high electrochemical stability, electron affinity and 10-100 times higher electron mobility [9]. ZnO also can be tailored to different nanostructures as compared to TiO2 films [10]. Nanostructures ZnO have high electronic properties to improve the light absorption and the photocurrent of ZnO-based water splitting applications because of their high surface area [11, 12] and excellent light trapping properties.
Many fabrication methods have been developed to control the morphology of ZnO nanostructures such as sol-gel process [13], spray pyrolysis [14], chemical bath deposition [15], metal organic chemical vapor deposition [16], pulsed laser deposition [17], RF sputtering [18] and thermal evaporation [19]. Among these, thermal evaporation is widely used in nanofabrication processes due to its low cost, simplicity [20], high yield, easy implementation [21], high-chemical purity and low contamination of deposited films, easy to control the film thickness and deposition rate, and thermal stability [22]. Recently, thermally evaporated ZnO nano-tetrapod photocatalyst has exhibited the maximum photocurrent of ~0.4 mA/cm2 at applied potential of 0.8 VAg/AgCl [23]. In this paper, authors have used
horizontal tube furnace system, where smallest size of wafer can be used. Moreover, two quartz tubes have been used in their system. The advantages of vertical systems are already evident for wafer size. However, searching of nanostructured ZnO photoanodes deposited by thermal evaporation technique with better solar-to-hydrogen conversion efficiency is more desirable.
So, the aim of this work is to fabricate ZnO films on bare fluorine-doped tin oxide (FTO) glass substrate by thermal evaporation method [24] with modified source-substrate orientation. The photoelectrochemical cell performances are evaluated for nanostructured ZnO films with deposition times of 10, 20 and 30 min. The effects of deposition time on structural, optical and surface morphological properties of ZnO films with their water splitting applications have been investigated and discussed.
2. Experimental details 2.1. Preparation of the photoanodes The experiment was conducted in a thermal evaporation method with modified sourcesubstrate orientation, made of cast iron work-chamber of about 300 mm in diameter and 250 mm in height. The deposition was carried out in a vertically set heating furnace which consists of a crucible and a heating element, as shown in Fig. 1. First, zinc sheet (15 mg) with purity higher than 99.99% was placed in a small alumina crucible, then the bare FTO glass substrate was placed on the sample holder and the distance between the source and substrate was adjusted (3cm in our case) by moving the substrate holder. After that, the chamber was evacuated using a rotary vacuum pump (0.05 mbar in our case) up to its base pressure. The crucible was then heated from room temperature to about 7000C in 2 or 3 minutes and maintained at this temperature for 10, 20 and 30 min. After end of the deposition time, the
heater was switched off and the crucible was cooled to room temperature, a thick layer of black-brown products was obtained on the FTO glass substrate. The deposited zinc films were annealed at 5000C for 2 hours in atmosphere. During the annealing process the color of the films changes from black-brown to transparent white.
2.2. Characterization of the photoanodes The crystallinity and crystal orientations of the deposited ZnO films were determined by Xray diffractometer (XRD) analysis (Model: Bruker AXS D8 Advance) pattern measured with Cu-K radiations (λ = 1.54178 Å) in the 2θ range of 20–70.The optical properties of the films were measured with UV-spectrometer (Model: Dynamica HALLO SB-10) at room temperature. The surface morphologies were examined by using field emission scanning electron microscope (FESEM; Model: JEOL JSM 7600F). F-4600 FL Spectrophotometer was used for the measurement of Photoluminance (PL) spectra in the excitation wavelength 380 nm. The PEC properties were investigated in a three electrode system. KOH solution (0.1 M) was used as an electrolyte. As-prepared nanoparticles ZnO films were used as the working electrode (active/uniform illumination area of 1.56 cm2). Pt foil and standard Ag/AgCl electrode were used as counter and reference electrode, respectively. The samples were illuminated by an artificial sunlight simulator, consisting of a SOLAX lamp (model: SET140F, SERIC Ltd.) and an AM 1.5 filter (100 mW/cm2). 3. Results and discussions Figure 2 shows the typical XRD patterns of the deposited ZnO nanoparticles grown on bare FTO glass substrate via thermal evaporation system with modified source-substrate orientation. All samples shows sharp, highly intense peaks that match very well with bulk ZnO of hexagonal structure (JCPDS No. 800075). Three pronounced ZnO diffraction peaks that related to the (100), (002) and (101) plans are observed. Some other weak peaks (102),
(211), (110), (103) and (201) are also found. In addition, XRD pattern has some Zn peaks (110), (104) and (203). For all prepared samples, the most intense peak of ZnO is related to the (101) plane. However, the intensity of the peaks, and therefore the crystallinity of the deposited nanoparticles are dependent on the deposition time. The average crystallite size (D) of the films was calculated from the (101) reflection peak using the Debye-Scherrer’s relation [25]: D
0.94 Cos
(1)
Where ‘β’ is the broadening of diffraction line measured at the half of its maximum intensity of the peak in radians, ‘’ is the X-ray wavelength (1.5406A˚ for Cu Ka1) and ‘θ’ is the Bragg’s angle. The measured full width at half maximum (FWHM) of the (101) peak intensity are 0.37, 0.33, and 0.29 for the deposition time of 10, 20 and 30 min, and the corresponding crystallite sizes are 23.61, 26.47 and 30.12 nm respectively. The value of FWHM decreases with the increase of deposition time, which may be due to increase of the crystalline-peak intensity. The crystallite size of the ZnO films increases from 23.61 to 30.12 nm with an increase in deposition time from 10 to 30 min. A similar behavior was observed by V. Kumar et al. in ZnO thin films synthesized by sol-gel [26]. Figure 3 shows FESEM images of ZnO films prepared with various deposition times. The left-side and right-side of Fig. 3 shows the low and high magnification FESEM images, respectively. It is cleared that the surface morphology of ZnO films are strongly depends on deposition time. The surface of ZnO film prepared with 10 min deposition time exhibits the compact, a reduced amount of open surface with less porous structure and more connectivity with particles. The nanoparticles (grains) are clearly visible in the larger cluster which have the average size is around 58 nm. From the Fig. 3(b), it is clearly observed that the surface of the ZnO film for 20 min shows the distinct, uniformly distributed grain-clusters with more open surface and porous. The average nanoparticles size is around 32 nm and average grain-
clusters size are about 300-420 nm. Further increase in deposition time of 30 min [as shown Fig. 3(c)], the grain-clusters become larger with less porous and open surface than the 20 min sample. The individual nanoparticles (grains) are little bit large (~40 nm) and almost invisible in the cluster. The morphological property of ZnO films were also investigated by AFM microscopy. Fig. 4 (a) and (b) illustrates the rms roughness and average roughness calculated from (AFM images, not shown) of amplitude & phase, respectively of ZnO films at different deposition times. It is confirmed that the 20 min sample shows the highest rms and average roughness. The chemical (wt%) compositional analysis for zinc (Zn) and oxygen (O) has estimated from EDS spectra. Fig. 5 shows the EDS spectrum of ZnO films prepared at 20 min deposition time. Except Zn and O, no other peak for any other element has been found in the spectrum. It is observed from the inset table of Fig. 5 that, the percentage of oxygen decreases with the increase of deposition time. The amount of zinc level increases with the increase of deposition time. The same amounts of zinc are oxidized due to constant annealing time and temperature. But for the sample of longer deposition time, some zinc is remains un-oxidized. Due to this, the amount of zinc increases and oxygen decreases with the increase of deposition time. The composition of 20 min sample, calculated from EDS and quantitative analysis data is very close to that of stoichiometric ZnO (80-20 ratio for Zn and O). Figure 6(a) and (b) shows the optical transmittance and absorbance spectra respectively in the wavelength range of 350- 900 nm for the prepared ZnO films at different deposition time. It is observed that the transmittance edge shows a little red shift for 20 min sample, may be by reason of the difference of surface morphologies. The ZnO film prepared with 20 min shows maximum absorbance shown in Fig. 6(b), may be due to better rms and average roughness, red shift of transmittance edge and more open surface.
For direct transition, the optical band gap (Eg) of ZnO film is determined using the equation [27]:
(h ) A(h E g )
1
2
(2)
where α is the absorption coefficient, A is the edge width parameter and hν is the photon energy. The optical band gap of the ZnO films increases from 3.12 to 3.22 eV with the increase of the deposition time from 10 to 30 min. The photoluminescence (PL) spectra of the ZnO films for different deposition time are shown in Fig. 6(d). All the samples show two peaks; near band edge (NBE) emission in the ultra violate (UV) region and a broad deep level emission (DLE) in the green emission region. The NBE peak is believed to be generated by the recombination of the excitons through an exciton–exciton collision process [28]. The broad DLE from the ZnO films is related to subband transition, which seems to be intrinsic in nature. As shown in Fig. 6(d), the UV emission intensities increase relatively to their green emission intensities as the growth time of ZnO nanostructures increase. It is seen from Fig. 6(d) that the peak position for the NBE UV emission (at 378 nm) is almost the same for all tested samples. The narrow peak width of the NBE emission as well as the decrease in the peak intensity of the green emission could be related to the improvement of crystal quality of the deposited nanostructures [29]. The sample prepared with 20 min exhibits much smaller visible emission PL peak which means it shows less oxygen related defects. The UV emission falling edge or falling delay time for 20 min sample is faster than the others, which is more suitable for PEC applications. This finding indicates that the ZnO film prepared with 20 min is better than the others. Figure 7(a) shows the rise and fall of the photocurrent at fixed bias voltage (500 mV) during on–off cycles of UV/Visible (300-900 nm) light illumination (100mW/cm2) and Fig. 7(b) exhibits the one cycle of Fig. 7(a). The ZnO film prepared with 20 min shows minimum current (0.002 mA/cm2) under dark condition while 0.21 mA/cm2 and 0.24 mA/cm2 are
measured for the sample with 10 and 30 min, respectively. The 20 min sample shows lower dark current than 10 and 30 min sample which may be due to sharp UV falling edge in PL spectra [as shown in Fig. 6(d)]. When the light is ON state, the value of photocurrent reach instantly to 1.04, 1.14 and 2.83 mA/cm2 for deposition time of 10, 20 and 30 min respectively, then decreases to a saturated value 0.44, 0.60 and 0.79 mA/cm2. Fig. 7(c) shows the measured current density as a function of the applied potential V (versus Ag/AgCl) for different deposition time under both dark and illumination conditions. It may be noticed that no current was detected when applying a linear sweep within a range from -1.0 to 1.0 V under dark conditions. The saturation photocurrent density (~0.98 mA/cm2) for the sample of 20 min is significantly larger than that of the film deposited on 10 min (~0.53 mA/cm2) and 30 min (~0.92 mA/cm2). Although 30 min sample shows low band gap as compared to other samples, but the sample prepared with 20 min exhibits much smaller visible emission PL peak which means that it shows less oxygen defect-states. The UV emission falling edge for 20 min sample is faster than the others, which is more suitable for PEC water splitting applications. On the other hand, rms and average roughness of 20 min sample has larger values than other samples. This finding indicates that the ZnO film prepared with 20 min is better photocurrents than the others. To better evaluate the solar-to-hydrogen efficiency of PEC water splitting of the ZnO films, the photoconversion efficiency (η) is calculated based on the equation [30]:
J (1.23 Vbias ) / Plight (3) where J is the photocurrent density, Vbias is the bias potential vs. RHE and Plight is the power density of incident light.
Figure 7(d) shows the solar-to-hydrogen conversion efficiencies of different deposition time (10, 20 and 30 min) with respect to applied potential -0.5 to 1.0 V (Ag/AgCl). The maximum efficiency of the film of deposition time 20 min is calculated to be 0.64% at 0.27 V (Ag/AgCl) while it is found to be 0.39% in case of 10 min (at 0.27 V Ag/AgCl) and 0.49% at 0.17 V (Ag/AgCl) for 30 min deposition time, respectively shown in Table 1.
Similarly, Figure 8(a) shows the rise and fall of the photocurrent at bias voltage (0~500 mV) on–off cycles of visible light illumination (100mW/cm2) at different deposition time and Fig. 8(b) exhibits the one cycle of Fig. 8(a). It is observed from Fig. 7(a)-(b) and Fig. 8 (a)-(b) that all the ZnO photoanode shows spike in photocurrent when light is turned ON, these are mostly related with surface electron–hole recombination [31]. When the light is ON state, the value of photocurrent reach to 0.83, 1.42 and 0.44 mA/cm2 for deposition time of 10, 20 and 30 min respectively, then decreases to a saturated value 0.33, 0.34 and 0.47 mA/cm2 shown in Fig. 8(b). Upon illumination, the ZnO photoanode prepared with 10, 20 and 30 min show photocurrent density of 0.43, 0.55 and 0.42 mA/cm2 at 1.0 VAg/AgCl, respectively. The photocurrent density of 20 min sample is significantly larger than the other samples, shown in Fig. 8(c). Fig. 8(d) shows the solar-to-hydrogen conversion efficiencies at different deposition time. The 20 min sample shows maximum efficiency of
0.42% at 0.21 V
(Ag/AgCl) while it is found to be 0.29% in case of 10 min (at 0.27 V Ag/AgCl) and 0.30% (at 0.20 V Ag/AgCl) for 30 min, respectively shown in Table 1. A comparative study of the water splitting efficiencies of various ZnO nanostructured photoanodes in relation with the present work is shown in Table 2.
4. Conclusions Nanostructure ZnO photoanodes were successfully deposited on bare FTO glass substrate via thermal evaporation method. The morphology and crystallinity of the deposited ZnO photoanodes were found to depend on the deposition time. While deposition for 10 min resulted in the formation of nanoporous structure of ZnO photoanodes, extending the deposition time to 20 min led to the growth of ZnO nanoparticles. Further increase in the deposition time to 30 min resulted in the formation of randomly oriented nanoporous structure. The ZnO nanoparticles deposited after 20 min showed the best crystallinity. The band gap of the deposited ZnO photoanodes increased from 3.12 to 3.22 eV with the increased of deposition time 10 to 30 min. The ZnO photoanode with 20 min deposition time showed the maximum photoconversion efficiencies of 0. 64% and 0.42% with UV and visible light illumination, respectively, which suggested that the uniformly covered ZnO nanoparticles were a much better and suitable catalyst for water splitting application.
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[10] M. F. Hossain, T. Takahashi, IEEE trans. on Nanotechnol,13, 755 (2014). [11] J.Qiu, M. Guo, Y. Feng and X. Wang, Electrochim. Acta , 56. 5776 (2011). [12] O. Lupan, V. M Guérin, I. M Tiginyanu, V. V. Ursaki, L. Chow, H. Heinrich and T. Pauporté, J. Photochem. Photobiol. A, 11, 65 (2010). [13] G. Srinivasan, J. Kumar, Crys. Res. and Technol, 41, 893 (2006). [14] R. Ayouchi, D. Leinen, F. Martin, M. Gabas, E. Dalchiele, J. R. Ramos-Barrado, Thin Solid Films, 426, 68 (2003). [15] B. Cao, W. Cai, J. Phys. Chem. C, 112, 680 (2008). [16] B. H. Kong, D. C. Kim, S. K. Mohanta, H. K. Cho, Thin Solid Films, 518, 2975 (2010). [17] L. Zhao, J. Lian, Y. Liu, Q. Jiang, Appl. Surf. Sci. 252, 8451( 2006). [18] S. J. Lim, S. Kwon, H. Kim, Thin Solid Films, 516, 1523 (2008). [19] N.M. Shaalan, T. Yamazaki, T. Kikuta, Sens. Actuators, 153, 11 (2011). [20] A. Mohanta, R. K. Thareja, J. Appl. Phys. 104, 044906 (2008). [21] E. Joanni, R. Savu, L. Valadares, M. Cilense, M. A. Zaghete, Rev. of sci. Instr. 82, 065101 (2011). [22] X. S. Peng, L. D. Zhang, G. W. Meng, Y. T. Tian, Y. Lin, B. Y. Geng, S. H. Sun, J. Appl. Phys. 93, 1760 (2003). [23] N.K. Hassan, M.R. Hashim, Nageh K. Allam, Chem. Phys. Lett. 549, 62 (2012). [24] M. S. Islam, M. F. Hossain, N.M. Shaalan and M. M. Ali, J. of Modern Sci. and Tech. 1, 120 (2013). [25] B. D Cullity, S. R. Stock, Elements of X-Ray Diffraction, 3rd Ed., Prentice Hall, New York, (2001). [26] V. Kumar, N. Singh, R.M. Mehra, A. Kapoor, L.P. Purohit, H.C. Swart, Thin Solid Films, 539, 161 (2013).
[27] J. I. Pankove, Optical Processes in Semiconductors (Dover, New York, 1970), Chap.3, p.34. [28] Z.-G. Chen, A. Ni, F. Li, H. Cong, H.-M. Cheng, G.Q. Lu, Chem. Phys. Lett. 434, 301 (2007). [29] A. Wolcott, W.A. Smith, T.R. Kuykendall, Y. Zhao, J.Z. Zhang, Adv. Functional Mater. 19 1849 (2009). [30] N. S Lewis, Science,315, 7982007 (2007). [31] Amol U. Pawar, Chang Woo Kim, Myung Jong Kang,Young Soo Kang, Nano Energy, 20, 156 (2016). [32] Y. Hu, X. Yan, X. Chen, Z. Bai, Z. Kang, F.Long, Y. Zhang, Appl.Surf. Sci. 02, 074 (2015). [33] D.T. Nguyen, E. C. Shin, D. C. Cho, K. W. Chae, J.S. Lee, Int. J. of hydrogen energy 39, 20764 (2014). [34] Z. Wang, P. Xiao, L.Qiao, X. Meng, Y. Zhang, X. Li, F. Yang , Physica B 419, 51(2013). [35] Q. Li, X. Sun, K. Lozano, and Y. Mao, J. Phys. Chem. C, 118, 13467 (2014).
Figure caption: Fig.1. Schematic diagram of the thermal evaporation system with modified source-substrate orientation.
Fig.2. XRD pattern of (a) bare FTO substrate; ZnO films prepared with: (b) 10 min, (c) 20 min and (d) 30 min deposition time.
Fig.3. FESEM images of ZnO films prepared with (a) 10 min, (b) 20 min and (c) 30 min deposition time. In all images left side and right side indicate the low and high magnification, respectively.
Fig. 4. The rms and average roughness of (a) amplitude & (b) phase of ZnO films at different deposition time.
Fig.5. An EDS analysis of ZnO film deposited at 20 min, Inset of Fig. 5 (Table x: Chemical composition in wt% of ZnO films at different deposition time).
Fig.6. Optical spectra of (a) transmittance, (b) absorbance; and (c) Plot of (αhν)2 versus hν and (d) PL spectra of ZnO films for different deposition time.
Fig.7. Electrochemical photoresponse (a) and (b) under UV/Visible light illumination, (c) Linear sweep voltammetric plot under dark and illumination of 100 mW/cm2 white light and (d) Solar-to-hydrogen conversion efficiency plot with respect to applied voltage.
Fig.8. Electrochemical photoresponse (a) and (b) under visible light illumination, (c) Linear sweep voltammetric plot under illumination of 100 mW/cm2 white light and (d) Solar-tohydrogen conversion efficiency plot with respect to applied voltage.
Table 1: Photoelectrochemical water splitting properties of ZnO photoanodes for the deposition time of 10, 20 and 30 min:
SL. No 1 2 3
Deposition time
Current (mA/cm2) UV/Visible light
density
10 min
0.53
0.43
0.39
0.29
20 min
0.98
0.55
0.64
0.42
30 min
0.92
0.42
0.49
0.30
Visible light
Photoconversion efficiency (%) UV/Visible Visible light light
Table 2: Photoelectrochemical water splitting efficiencies of various ZnO nanostructured photoanodes: S.N. Photoanode structure
Growth process
η%
Ref.
1
ZnO nanorod arrays
Hydrothermal
0.18
[32]
2
ZnO thin film
Solution method
0.42
[33]
3
ZnO nanorod array
Aqueous solution
0.39
[34]
4
“Caterpillar-like” ZnO Nanostructures Hydrothermal
0.16
[35]
5
ZnO nano-tetrapod
Thermal evaporation
0.25
[23]
6
ZnO nanoporous structure
Thermal Evaporation (UV/Vis 100 mW/cm2) 0.64
Present
(Visible 100 mW/cm2)
Work
0.42
S1010-6030(16)30002-8 http://dx.doi.org/doi:10.1016/j.jphotochem.2016.04.002 JPC 10191
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
4-1-2016 29-3-2016 5-4-2016
Please cite this article as: M.S.Islam, M.F.Hossain, Enhanced photoelectrochemical performance of nanoparticle ZnO photoanodes for water-splitting application, Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2016.04.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced Photoelectrochemical Performance of Nanoparticle ZnO Photoanodes for Water-Splitting Application
M. S. Islam, M. F. Hossain* Department of Electrical & Electronic Engineering, Rajshahi University of Engineering and Technology, Rajshahi-6204, Bangladesh.
*
Corresponding Author: Tel./Fax: +88-0721-750356 Email: [email protected] [email protected]
or
HIGHLIGHTS The modified thermal evaporation setup has the opportunity to vary source-substrate distance vertically by moving ups and downs the substrate holder. The proposed system has the ability to use different size of substrates. The deposition time has great influenced on the morphology, crystallinity and Photoconversion efficiency of ZnO photoanodes. The ZnO photoanode deposited on 20 min deposition time has better Photoconversion efficiency than the ZnO photoanode deposited on 10 or 30 min. Abstract: In this paper, fabrication of ZnO nanoparticle photoanodes for water splitting application is presented. At first, metal zinc films have been deposited on bare fluorine-doped tin oxide glass substrates by thermal evaporation method with chamber pressure 0.05 mbar, source temperature 7000C and source-substrate distance 3cm for fabrication of the photoanodes. This deposited zinc films have been annealed at 5000C for 2 hrs in atmosphere for obtaining ZnO films. The prepared ZnO films with deposition times (10, 20 and 30 min) are characterized and investigated to find out the photoelectrochemical properties and their suitability for splitting of water. It has been observed that all the ZnO films have high crystallinity wurtzite structure and uniformly distributed nanoparticles with the size of 32-58 nm. ZnO film with 20 min sample has smallest nanoparticles (32 nm) than the others. All the ZnO photoanodes have exhibited higher solar to hydrogen conversion efficiency (0.29 to 0.42%) than the other researchers data. Among them, ZnO with 20 min sample has been showed maximum efficiency of 0.64% for UV illumination and 0.42% for visible light illumination than the ZnO photoanodes prepared with other deposition times (10 and 30 min). Keywords: Zinc oxide; thermal evaporation; thin film; water splitting, nanoparticle. 1. Introduction Energy is one of the greatest challenges facing humanity in the coming years, and therefore, the development of sustainable and renewable energy sources is a key concern [1]. Hydrogen is an efficient energy carrier that has highest energy density and is also environmental friendly [2]. Currently, the main production of hydrogen comes from steam reforming of
natural gas, which involves energy losses and the release of greenhouse gases [3]. Electrochemical solar driven water splitting is an attractive, environment friendly and nonconventional energy resource [4]. Light active semiconductors or metal oxides have shown great prospects for this method of hydrogen generation [5]. In 1972, Fujishima and Honda first used titanium dioxide (TiO2) photoanodes for hydrogen evolution [6]. TiO2 is the most promising photoelectrode because of its high efficiency, chemical and optical stability, and environmental and biological compatibility [1]. However, the use of TiO2 as photoanode for water splitting is limited by its redox potential with reference to the normal hydrogen electrode [7]. The ZnO is thought to be a potential substitute of TiO2 [8] because of its similar band gap (3.37 eV), large excitation binding energy (60 meV), high electrochemical stability, electron affinity and 10-100 times higher electron mobility [9]. ZnO also can be tailored to different nanostructures as compared to TiO2 films [10]. Nanostructures ZnO have high electronic properties to improve the light absorption and the photocurrent of ZnO-based water splitting applications because of their high surface area [11, 12] and excellent light trapping properties.
Many fabrication methods have been developed to control the morphology of ZnO nanostructures such as sol-gel process [13], spray pyrolysis [14], chemical bath deposition [15], metal organic chemical vapor deposition [16], pulsed laser deposition [17], RF sputtering [18] and thermal evaporation [19]. Among these, thermal evaporation is widely used in nanofabrication processes due to its low cost, simplicity [20], high yield, easy implementation [21], high-chemical purity and low contamination of deposited films, easy to control the film thickness and deposition rate, and thermal stability [22]. Recently, thermally evaporated ZnO nano-tetrapod photocatalyst has exhibited the maximum photocurrent of ~0.4 mA/cm2 at applied potential of 0.8 VAg/AgCl [23]. In this paper, authors have used
horizontal tube furnace system, where smallest size of wafer can be used. Moreover, two quartz tubes have been used in their system. The advantages of vertical systems are already evident for wafer size. However, searching of nanostructured ZnO photoanodes deposited by thermal evaporation technique with better solar-to-hydrogen conversion efficiency is more desirable.
So, the aim of this work is to fabricate ZnO films on bare fluorine-doped tin oxide (FTO) glass substrate by thermal evaporation method [24] with modified source-substrate orientation. The photoelectrochemical cell performances are evaluated for nanostructured ZnO films with deposition times of 10, 20 and 30 min. The effects of deposition time on structural, optical and surface morphological properties of ZnO films with their water splitting applications have been investigated and discussed.
2. Experimental details 2.1. Preparation of the photoanodes The experiment was conducted in a thermal evaporation method with modified sourcesubstrate orientation, made of cast iron work-chamber of about 300 mm in diameter and 250 mm in height. The deposition was carried out in a vertically set heating furnace which consists of a crucible and a heating element, as shown in Fig. 1. First, zinc sheet (15 mg) with purity higher than 99.99% was placed in a small alumina crucible, then the bare FTO glass substrate was placed on the sample holder and the distance between the source and substrate was adjusted (3cm in our case) by moving the substrate holder. After that, the chamber was evacuated using a rotary vacuum pump (0.05 mbar in our case) up to its base pressure. The crucible was then heated from room temperature to about 7000C in 2 or 3 minutes and maintained at this temperature for 10, 20 and 30 min. After end of the deposition time, the
heater was switched off and the crucible was cooled to room temperature, a thick layer of black-brown products was obtained on the FTO glass substrate. The deposited zinc films were annealed at 5000C for 2 hours in atmosphere. During the annealing process the color of the films changes from black-brown to transparent white.
2.2. Characterization of the photoanodes The crystallinity and crystal orientations of the deposited ZnO films were determined by Xray diffractometer (XRD) analysis (Model: Bruker AXS D8 Advance) pattern measured with Cu-K radiations (λ = 1.54178 Å) in the 2θ range of 20–70.The optical properties of the films were measured with UV-spectrometer (Model: Dynamica HALLO SB-10) at room temperature. The surface morphologies were examined by using field emission scanning electron microscope (FESEM; Model: JEOL JSM 7600F). F-4600 FL Spectrophotometer was used for the measurement of Photoluminance (PL) spectra in the excitation wavelength 380 nm. The PEC properties were investigated in a three electrode system. KOH solution (0.1 M) was used as an electrolyte. As-prepared nanoparticles ZnO films were used as the working electrode (active/uniform illumination area of 1.56 cm2). Pt foil and standard Ag/AgCl electrode were used as counter and reference electrode, respectively. The samples were illuminated by an artificial sunlight simulator, consisting of a SOLAX lamp (model: SET140F, SERIC Ltd.) and an AM 1.5 filter (100 mW/cm2). 3. Results and discussions Figure 2 shows the typical XRD patterns of the deposited ZnO nanoparticles grown on bare FTO glass substrate via thermal evaporation system with modified source-substrate orientation. All samples shows sharp, highly intense peaks that match very well with bulk ZnO of hexagonal structure (JCPDS No. 800075). Three pronounced ZnO diffraction peaks that related to the (100), (002) and (101) plans are observed. Some other weak peaks (102),
(211), (110), (103) and (201) are also found. In addition, XRD pattern has some Zn peaks (110), (104) and (203). For all prepared samples, the most intense peak of ZnO is related to the (101) plane. However, the intensity of the peaks, and therefore the crystallinity of the deposited nanoparticles are dependent on the deposition time. The average crystallite size (D) of the films was calculated from the (101) reflection peak using the Debye-Scherrer’s relation [25]: D
0.94 Cos
(1)
Where ‘β’ is the broadening of diffraction line measured at the half of its maximum intensity of the peak in radians, ‘’ is the X-ray wavelength (1.5406A˚ for Cu Ka1) and ‘θ’ is the Bragg’s angle. The measured full width at half maximum (FWHM) of the (101) peak intensity are 0.37, 0.33, and 0.29 for the deposition time of 10, 20 and 30 min, and the corresponding crystallite sizes are 23.61, 26.47 and 30.12 nm respectively. The value of FWHM decreases with the increase of deposition time, which may be due to increase of the crystalline-peak intensity. The crystallite size of the ZnO films increases from 23.61 to 30.12 nm with an increase in deposition time from 10 to 30 min. A similar behavior was observed by V. Kumar et al. in ZnO thin films synthesized by sol-gel [26]. Figure 3 shows FESEM images of ZnO films prepared with various deposition times. The left-side and right-side of Fig. 3 shows the low and high magnification FESEM images, respectively. It is cleared that the surface morphology of ZnO films are strongly depends on deposition time. The surface of ZnO film prepared with 10 min deposition time exhibits the compact, a reduced amount of open surface with less porous structure and more connectivity with particles. The nanoparticles (grains) are clearly visible in the larger cluster which have the average size is around 58 nm. From the Fig. 3(b), it is clearly observed that the surface of the ZnO film for 20 min shows the distinct, uniformly distributed grain-clusters with more open surface and porous. The average nanoparticles size is around 32 nm and average grain-
clusters size are about 300-420 nm. Further increase in deposition time of 30 min [as shown Fig. 3(c)], the grain-clusters become larger with less porous and open surface than the 20 min sample. The individual nanoparticles (grains) are little bit large (~40 nm) and almost invisible in the cluster. The morphological property of ZnO films were also investigated by AFM microscopy. Fig. 4 (a) and (b) illustrates the rms roughness and average roughness calculated from (AFM images, not shown) of amplitude & phase, respectively of ZnO films at different deposition times. It is confirmed that the 20 min sample shows the highest rms and average roughness. The chemical (wt%) compositional analysis for zinc (Zn) and oxygen (O) has estimated from EDS spectra. Fig. 5 shows the EDS spectrum of ZnO films prepared at 20 min deposition time. Except Zn and O, no other peak for any other element has been found in the spectrum. It is observed from the inset table of Fig. 5 that, the percentage of oxygen decreases with the increase of deposition time. The amount of zinc level increases with the increase of deposition time. The same amounts of zinc are oxidized due to constant annealing time and temperature. But for the sample of longer deposition time, some zinc is remains un-oxidized. Due to this, the amount of zinc increases and oxygen decreases with the increase of deposition time. The composition of 20 min sample, calculated from EDS and quantitative analysis data is very close to that of stoichiometric ZnO (80-20 ratio for Zn and O). Figure 6(a) and (b) shows the optical transmittance and absorbance spectra respectively in the wavelength range of 350- 900 nm for the prepared ZnO films at different deposition time. It is observed that the transmittance edge shows a little red shift for 20 min sample, may be by reason of the difference of surface morphologies. The ZnO film prepared with 20 min shows maximum absorbance shown in Fig. 6(b), may be due to better rms and average roughness, red shift of transmittance edge and more open surface.
For direct transition, the optical band gap (Eg) of ZnO film is determined using the equation [27]:
(h ) A(h E g )
1
2
(2)
where α is the absorption coefficient, A is the edge width parameter and hν is the photon energy. The optical band gap of the ZnO films increases from 3.12 to 3.22 eV with the increase of the deposition time from 10 to 30 min. The photoluminescence (PL) spectra of the ZnO films for different deposition time are shown in Fig. 6(d). All the samples show two peaks; near band edge (NBE) emission in the ultra violate (UV) region and a broad deep level emission (DLE) in the green emission region. The NBE peak is believed to be generated by the recombination of the excitons through an exciton–exciton collision process [28]. The broad DLE from the ZnO films is related to subband transition, which seems to be intrinsic in nature. As shown in Fig. 6(d), the UV emission intensities increase relatively to their green emission intensities as the growth time of ZnO nanostructures increase. It is seen from Fig. 6(d) that the peak position for the NBE UV emission (at 378 nm) is almost the same for all tested samples. The narrow peak width of the NBE emission as well as the decrease in the peak intensity of the green emission could be related to the improvement of crystal quality of the deposited nanostructures [29]. The sample prepared with 20 min exhibits much smaller visible emission PL peak which means it shows less oxygen related defects. The UV emission falling edge or falling delay time for 20 min sample is faster than the others, which is more suitable for PEC applications. This finding indicates that the ZnO film prepared with 20 min is better than the others. Figure 7(a) shows the rise and fall of the photocurrent at fixed bias voltage (500 mV) during on–off cycles of UV/Visible (300-900 nm) light illumination (100mW/cm2) and Fig. 7(b) exhibits the one cycle of Fig. 7(a). The ZnO film prepared with 20 min shows minimum current (0.002 mA/cm2) under dark condition while 0.21 mA/cm2 and 0.24 mA/cm2 are
measured for the sample with 10 and 30 min, respectively. The 20 min sample shows lower dark current than 10 and 30 min sample which may be due to sharp UV falling edge in PL spectra [as shown in Fig. 6(d)]. When the light is ON state, the value of photocurrent reach instantly to 1.04, 1.14 and 2.83 mA/cm2 for deposition time of 10, 20 and 30 min respectively, then decreases to a saturated value 0.44, 0.60 and 0.79 mA/cm2. Fig. 7(c) shows the measured current density as a function of the applied potential V (versus Ag/AgCl) for different deposition time under both dark and illumination conditions. It may be noticed that no current was detected when applying a linear sweep within a range from -1.0 to 1.0 V under dark conditions. The saturation photocurrent density (~0.98 mA/cm2) for the sample of 20 min is significantly larger than that of the film deposited on 10 min (~0.53 mA/cm2) and 30 min (~0.92 mA/cm2). Although 30 min sample shows low band gap as compared to other samples, but the sample prepared with 20 min exhibits much smaller visible emission PL peak which means that it shows less oxygen defect-states. The UV emission falling edge for 20 min sample is faster than the others, which is more suitable for PEC water splitting applications. On the other hand, rms and average roughness of 20 min sample has larger values than other samples. This finding indicates that the ZnO film prepared with 20 min is better photocurrents than the others. To better evaluate the solar-to-hydrogen efficiency of PEC water splitting of the ZnO films, the photoconversion efficiency (η) is calculated based on the equation [30]:
J (1.23 Vbias ) / Plight (3) where J is the photocurrent density, Vbias is the bias potential vs. RHE and Plight is the power density of incident light.
Figure 7(d) shows the solar-to-hydrogen conversion efficiencies of different deposition time (10, 20 and 30 min) with respect to applied potential -0.5 to 1.0 V (Ag/AgCl). The maximum efficiency of the film of deposition time 20 min is calculated to be 0.64% at 0.27 V (Ag/AgCl) while it is found to be 0.39% in case of 10 min (at 0.27 V Ag/AgCl) and 0.49% at 0.17 V (Ag/AgCl) for 30 min deposition time, respectively shown in Table 1.
Similarly, Figure 8(a) shows the rise and fall of the photocurrent at bias voltage (0~500 mV) on–off cycles of visible light illumination (100mW/cm2) at different deposition time and Fig. 8(b) exhibits the one cycle of Fig. 8(a). It is observed from Fig. 7(a)-(b) and Fig. 8 (a)-(b) that all the ZnO photoanode shows spike in photocurrent when light is turned ON, these are mostly related with surface electron–hole recombination [31]. When the light is ON state, the value of photocurrent reach to 0.83, 1.42 and 0.44 mA/cm2 for deposition time of 10, 20 and 30 min respectively, then decreases to a saturated value 0.33, 0.34 and 0.47 mA/cm2 shown in Fig. 8(b). Upon illumination, the ZnO photoanode prepared with 10, 20 and 30 min show photocurrent density of 0.43, 0.55 and 0.42 mA/cm2 at 1.0 VAg/AgCl, respectively. The photocurrent density of 20 min sample is significantly larger than the other samples, shown in Fig. 8(c). Fig. 8(d) shows the solar-to-hydrogen conversion efficiencies at different deposition time. The 20 min sample shows maximum efficiency of
0.42% at 0.21 V
(Ag/AgCl) while it is found to be 0.29% in case of 10 min (at 0.27 V Ag/AgCl) and 0.30% (at 0.20 V Ag/AgCl) for 30 min, respectively shown in Table 1. A comparative study of the water splitting efficiencies of various ZnO nanostructured photoanodes in relation with the present work is shown in Table 2.
4. Conclusions Nanostructure ZnO photoanodes were successfully deposited on bare FTO glass substrate via thermal evaporation method. The morphology and crystallinity of the deposited ZnO photoanodes were found to depend on the deposition time. While deposition for 10 min resulted in the formation of nanoporous structure of ZnO photoanodes, extending the deposition time to 20 min led to the growth of ZnO nanoparticles. Further increase in the deposition time to 30 min resulted in the formation of randomly oriented nanoporous structure. The ZnO nanoparticles deposited after 20 min showed the best crystallinity. The band gap of the deposited ZnO photoanodes increased from 3.12 to 3.22 eV with the increased of deposition time 10 to 30 min. The ZnO photoanode with 20 min deposition time showed the maximum photoconversion efficiencies of 0. 64% and 0.42% with UV and visible light illumination, respectively, which suggested that the uniformly covered ZnO nanoparticles were a much better and suitable catalyst for water splitting application.
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Figure caption: Fig.1. Schematic diagram of the thermal evaporation system with modified source-substrate orientation.
Fig.2. XRD pattern of (a) bare FTO substrate; ZnO films prepared with: (b) 10 min, (c) 20 min and (d) 30 min deposition time.
Fig.3. FESEM images of ZnO films prepared with (a) 10 min, (b) 20 min and (c) 30 min deposition time. In all images left side and right side indicate the low and high magnification, respectively.
Fig. 4. The rms and average roughness of (a) amplitude & (b) phase of ZnO films at different deposition time.
Fig.5. An EDS analysis of ZnO film deposited at 20 min, Inset of Fig. 5 (Table x: Chemical composition in wt% of ZnO films at different deposition time).
Fig.6. Optical spectra of (a) transmittance, (b) absorbance; and (c) Plot of (αhν)2 versus hν and (d) PL spectra of ZnO films for different deposition time.
Fig.7. Electrochemical photoresponse (a) and (b) under UV/Visible light illumination, (c) Linear sweep voltammetric plot under dark and illumination of 100 mW/cm2 white light and (d) Solar-to-hydrogen conversion efficiency plot with respect to applied voltage.
Fig.8. Electrochemical photoresponse (a) and (b) under visible light illumination, (c) Linear sweep voltammetric plot under illumination of 100 mW/cm2 white light and (d) Solar-tohydrogen conversion efficiency plot with respect to applied voltage.
Table 1: Photoelectrochemical water splitting properties of ZnO photoanodes for the deposition time of 10, 20 and 30 min:
SL. No 1 2 3
Deposition time
Current (mA/cm2) UV/Visible light
density
10 min
0.53
0.43
0.39
0.29
20 min
0.98
0.55
0.64
0.42
30 min
0.92
0.42
0.49
0.30
Visible light
Photoconversion efficiency (%) UV/Visible Visible light light
Table 2: Photoelectrochemical water splitting efficiencies of various ZnO nanostructured photoanodes: S.N. Photoanode structure
Growth process
η%
Ref.
1
ZnO nanorod arrays
Hydrothermal
0.18
[32]
2
ZnO thin film
Solution method
0.42
[33]
3
ZnO nanorod array
Aqueous solution
0.39
[34]
4
“Caterpillar-like” ZnO Nanostructures Hydrothermal
0.16
[35]
5
ZnO nano-tetrapod
Thermal evaporation
0.25
[23]
6
ZnO nanoporous structure
Thermal Evaporation (UV/Vis 100 mW/cm2) 0.64
Present
(Visible 100 mW/cm2)
Work
0.42