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Fabrication of large area nanorod like structured CdS photoanode for solar H2 generation using spray pyrolysis technique
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 3 6 e4 4
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Fabrication of large area nanorod like structured CdS photoanode for solar H2 generation using spray pyrolysis technique Alka Pareek, Rekha Dom, Pramod H. Borse* Solar H2 PEC Lab., International Advanced Research Centre for Powder Metallurgy and New Materials, Balapur PO, Hyderabad, AP 500 005, India
article info
abstract
Article history:
Large area nanorod like structured CdS films (9 9 cm2) were deposited on the FTO glass
Received 5 June 2012
substrate using simple and economic spray pyrolysis deposition technique for photo-
Received in revised form
electrochemical (PEC) hydrogen production. With an intention of electrode scaling-up, the
25 September 2012
deposition area of photoanode was varied to evaluate its effect on the PEC hydrogen
Accepted 19 October 2012
generation capability. High photocurrent of 5 mA has been achieved from the PEC active
Available online 22 November 2012
area of 37.5 cm2. Its unit area (1 cm2) counterpart yielded Solar-to-Hydrogen (STH) conversion efficiency of 0.20% at a bias of 0.2 V vs Ag/AgCl using sacrificial reagents under
Keywords:
solar simulator (AM1.5) with 80 mW/cm2 irradiance. The 500 nm thick film exhibiting
Nanostructure
uniformly distributed nano-rod features yielded 3-times more photocurrent, as well as
Photoelectrochemical hydrogen
hydrogen evolution than other films. It exhibited an enhanced photo-activity as indicated
Large area
by the higher IPCE values (5e9%) in the wavelength range of 450e550 nm. It exhibited
Solar-to-hydrogen
superior optical properties (Eg w2.4 eV), and formation of high crystallinity hexagonal CdS
Solar energy
phase with space group P63MC. The superior performance of the photoanode is attributed
Spray pyrolysis film deposition
to the nanostructured morphology acquired under optimized spray pyrolysis conditions. Large area photoanodes showed unaltered photo-activity indicating the homogeneity in the film properties even in scaled-up version. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The solar radiation induced photoelectrochemical hydrogen production is a green technology route to generate renewable energy from eco-friendly entities. The hydrogen thus generated from water and solar light over a photocatalytic system, is thus highly important for mankind today till an efficient and stable photocatalyst system is identified. There have been exploratory studies over semiconducting oxides, chalcogenides and ferrites systems, to identify an efficient photoanode [1e5]. The potential metal oxides of TiO2, SrTiO3 etc. though
are known as good photoanode candidates, but suffer by their large band gap (Eg 3.2 eV), which limits [6] their PEC activity to UV light. On the other hand ferrites, an n-type Fe2O3 semiconductor, do exhibit a low band gap (Eg w2.0e2.2 eV), but show a poor photo-oxidation efficiency due to its low absorption coefficient and high electron-hole (e-h) recombination rates [7]. Chalcogenides have been known as the most potent photocatalyst candidates working under visible light. They exhibit desirable low band gap, and well suited valence/ conduction band edges with respect to water redox potential levels. Thus till today, they are the most extensively sought
* Corresponding author. Tel.: þ91 4024452426; fax: þ91 4024442699. E-mail address: [email protected] (P.H. Borse). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.057
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semiconductors for the role of visible light absorbing photoanode. Cadmium sulphide, a well known low band gap, n-type semiconductor (Eg w2.4 eV), is the most promising candidate not only for PEC hydrogen production applications but also for various other applications; viz. in opto-electronic devices, such as laser material, transducers, photo-conducting cells, photo-sensor, optical waveguides and non-linear integrated optical devices [8]. Consequently it has been chosen in the present study. Additionally, advent of modern material development techniques needs to be exploited for the material nanostructuring. Such attempts not only pave path to achieve better material properties but also lead to demonstrate the industrial processability of the methodology. In past CdS has been deposited by several techniques viz. chemical vapor deposition (CVD) [9,10], chemical bath deposition (CBD) [11e14], vacuum evaporation [15], sputtering [16], electro deposition [17], pulsed laser deposition [18], spray pyrolysis deposition (SPD) [8,19e25], & SILAR [26] etc. SPD is one of the most simple, inexpensive and versatile film deposition techniques which displays high potential for its industrial applicability. It can yield desired metal-oxide, semiconductor film over a large substrate area. This technique involves a controlled droplet-spraying of the molecularly mixed liquid precursor over the thermally heated substrate. These droplets undergo thermal decomposition over the substrate by making use of available energy. It is controllably possible to modulate the nucleation-and-growth mediated film properties by tuning the deposition conditions e flow rate, deposition temperature, deposition time etc. Unlike CBD, it offers the advantage of yielding high film crystallinity, and film uniformity, without the loss of chemical precursor. The ability to deposit large area nanostructure thin film is an added asset of SPD [23]. Such capability of SPD is expected to be beneficial in the field of PEC and solar-cell applications. Thus we utilize this deposition method in the present study to deposit large area thin films. Study of large area photoanodes for H2 production is most desirable with respect to commercialization and scale-up efforts for the hydrogen fuel utilization. Though CdS is a very efficient PEC system, still the study of its large area PEC performance is not reported in past. Hence such study would be a great step toward the efforts to evaluate scale-up approach to generate the green energy. In present work we have focused on the large area (more than 40 cm2) film deposition and its photoanode performance for H2 generation. A rigorous optimization of the deposition parameters with respect to flow rate, deposition temperature, deposition time, precursor etc. has been done to achieve an efficient photoanode. The main work related to the photoanode application for hydrogen generation is further elaborated. It was expected that by tailoring the structural, optical and morphological film properties, one is able to achieve an improved nanostructured photoanode from such films. The study presents the work in two major sections one being unit area (1 1 cm2) photoanode and other as large area (9 9 cm2) photoanode. Accordingly, in the first section the deposition and PEC optimization have been described for unit area film. The final section demonstrates large area film deposition. As a step further, the study clearly demonstrates the feasibility of deposition of large area films and their utilization as an efficient solar-electrode, particularly for the PEC application.
2.
37
Experimental
2.1. Large area film deposition and photo-electrode fabrication Fig. 1 shows the schematic diagram of indigenously designed spray pyrolysis equipment that was used for large area (9 9 cm2) thin film deposition. The automated computer interface of this SPD system has ability to control the inert gas pressure, precursor flow rate, substrate temperature and the film thickness. In order to deposit the desired film, equal amounts of CdCl2 (0.1 M) & (NH2)2CS (0.1 M) chemicals were dissolved in double distilled water to make the film precursor solution. Unless otherwise stated, throughout the work, certain fixed substrate area was chosen to achieve PEC active area of 1.5 cm2. Fluorine doped tin oxide; FTO (Pilkington TCO15) with resistivity of 12e15 U/sq was used as a bottom electrode. Films were deposited at various temperatures of 300e500 C and characterized (Figs. SI.1 and 2). The film thickness was varied by increasing the number of deposition cycles in the range of 3e20 cycles. The nozzle-to-substrate distance was varied from 15 to 25 cm, while the spray-rate was maintained between 2 and 10 ml/min. Parameter optimization was done on the basis of PEC performance (Figs. SI.3 and 4). In order to deposit large area film on FTO, optimized parameters were used. Mainly the results of four different substrate areas 1.5 cm2, 12 cm2, 37.5 cm2 & 81 cm2 are described in the present work. For the fabrication of electrode, the proposed active area was located over the film. The electrical contact was made over the bottom electrode, to apply the desired voltage biasing from external circuit. Rest of the part was electrically insulated using an epoxy. Thus, photoelectrode active area of 1 cm2 was achieved. The large area electrodes were also fabricated in similar manner. The photo-electrode thus
Fig. 1 e Schematic of spray pyrolysis set up for large area film deposition.
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obtained was used for the photoelectrochemical hydrogen generation.
2.2.
Film characterization
X-ray diffraction pattern was recorded by using Bruker AX D8 XRD equipment having CuKa X-ray source. The scan was taken in the range 0e100 with an increase of 0.01 at duration of 2 s/step. The transmission spectra were recorded using UVeVis spectrometer (Perkin Elmer, lambda 650). The detailed analysis was carried out using Tauc relation [24]: a¼
K hn Eg hn
n2 (1)
where K is constant, Eg is optical band gap, and n ¼ 1 for direct band gap material such as CdS. The band gap Eg is determined from the E vs (ahn)2 graph. The intercept on the energy axis was obtained by extrapolating the tangent to the curve, which gives the value Eg. Surface morphology of CdS thin films was studied by field emission scanning electron microscopy, FESEM (Hitachi model S4300SE/N) operated at 20 kV. Energy dispersive spectroscopy (EDS) was carried out for all the films during SEM study. The film cross-sectional studies were carried out to estimate the film thickness. For this the films were cracked carefully to expose the cross-section and then were used for the further study. Thus deposited thin films were named on the basis of their deposition parameters as shown in Table 1.
2.3.
Photo-electro-chemical characterization
Photoelectrochemical measurements were carried out using two-electrode cell (or 3-electrode cell, wherever mentioned) with the photoanode as a working electrode and graphite (/platinum) as a counter electrode in a quartz PEC reactor. In case of 3-electrode cell Ag/AgCl was used as reference electrode. The electrolyte was made using 0.01 M Na2S & 0.02 M Na2SO3 so as to minimize the photo-corrosion. The solar simulator (Oriel Model 91160) equipped with AM0, AM1.0 and AM1.5 Global (Newport) filters was utilized for the experimentation. The irradiance of 80 mW/cm2 was used in present case. The PEC measurements were done using two different PEC cell reactors, one for unit area electrode and another being for large area. A schematic of typical set is shown in Fig. 2. The reactor cell is interfaced with gas chromatography (GC) equipment and with an electrochemical set up (PARSTAT 2273). As shown, the specially designed and fabricated PEC reactor consists of a quartz window on the top of the reactor.
Table 1 e Films deposited at 350 C were named on the basis of the deposition cycles and observed film thickness as characterized from cross-section measurements. Film name A B C
No. of deposition cycles
Thicknessa (nm)
5 10 15
200 500 1000
a FESEM cross-section studies.
Fig. 2 e Schematic of photoelectrochemical reactor set up used for hydrogen generation.
This enables one to vertically photo-illuminate the electrode using solar simulator. The air tight reactor was utilized to monitor the generated H2 evolution. The evolved gas was then extracted and characterized by GC. During the PEC measurements, the distance between working electrode and counter electrode was maintained around 1 cm. The electrode connections from the PEC reactor were directly connected to an electrochemical system for photocurrent-potential and chronoamperometric measurements. In order to carry out Incident-Photon-Current-Conversion Efficiency (IPCE) measurements, an Oriel monochromator (Oriel model 74125) fitted with a 300 W Xenon lamp, that was capable of generating wavelengths in the range of 200e900 nm was used. The quantitative estimation of photo-electrochemically generated H2 was done by GC spectrometer, Model GC-2010 Plus (Shimadzu) that was equipped with TCD (Thermal conductivity detector) detector.
3.
Results and discussions
3.1.
Physico-chemical characterization of thin films
Fig. 3 shows the XRD spectra for the films (A, B and C) deposited under optimized temperature (Figs. SI.1 and 2) of 350 C. All the films exhibited hexagonal crystal structure of CdS lattice belonging to P63MC (SG no. 186) space group. There was no trace of any impurity phase like CdO. The crystallinity was found to improve from A to C, with the increase in the net deposited CdS content over the substrate. The FTO substrate peak (200) was found in all cases. The inset of the figure displays the variation in the CdS (002) peak intensity with the film thickness from A to C, indicating the effect of film thickness on film crystallinity. On the contrary, the intensity of the FTO (200) peak was found to decrease due to increase in the film thickness. Further in order to estimate the crystallite size, Debye Scherer’s formula [27] was used:
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Fig. 4 e Transmission spectra of CdS films deposited for thickness of: A-200 nm, B-500 nm and C-1000 nm. Inset shows Tauc plot of CdS-B film that was used for band gap estimation. Fig. 3 e XRD patterns of CdS film deposited with different thicknesses viz., A (200 nm), B (500 nm) and C (1000 nm). Substrate (FTO) peaks are indicated by solid circle. Inset shows the change in (002) peak intensity with the increase in the film thickness from A to C.
1000 nm for films A, B and C respectively. In view of achieving rod like features in film B, it was further investigated with special focus on its PEC property.
3.2.
0:9l D¼ b cos q
(2)
where, D-particle size, l-wavelength, q-diffraction angle and b-FWHM. As displayed in Table 1, the crystallite size was found to increase from A, B to C as 22 nm, 28 nm and 28.9 nm respectively. The EDS spectrum of film-B confirmed that Cd and S exist in the stoichiometric ratio of 1:1 (Fig. SI.5). Fig. 4 shows the optical transmission spectra for the CdS films A to C, indicating the absorption edge for Film B at wavelength longer than 500 nm. Film B exhibited a very low transmission than Films A and C, confirming its high absorptivity toward visible light photons. Films A & C display a systematic variation in the transmission with respect to increase in the thickness from film A to C. The band gap (Eg) was estimated using Tauc plots analysis. Inset of Fig. 4 shows Tauc plot for the typical film B revealing its direct band gap as 2.4 eV, which is in accordance with the known reports [19,25]. In order to study the film surface morphology and to measure the film thickness, morphological and cross-sectional studies were carried out. Fig. 5 shows the surface morphology of the films A to C along with their respective cross-sectional view, exhibiting respective distinct features. Interestingly the surface of film B exhibited certain aligned rod like nanostructure throughout the surface. On the other hand the films with other thicknesses viz. film A (and C) display very smooth (and highly rough) surface respectively. The changes in the film thickness are clearly visible from the cross-sectional view, indicating the film thickness of 200 nm, 500 nm and
X-ray photoelectron spectroscopy (XPS)
Elemental characterization of the films was carried out using XPS. Typical XPS analysis is shown in Fig. 6 for film B. As clearly seen in Fig. 6(a), the survey scan indicates the presence of Cd 3d & S 2p lines along with the remnant carbon peak. The region wise scan of S 2p and Cd 3d states is shown in Fig. 6(b) and (c) respectively. A detail analysis using de-convolution of peaks indicated that S 2p can be fitted with two peaks, one at B.E. w160.9 eV, and other being at 162.2 eV. These peaks can be respectively attributed to the S2 of bulk S atoms [28] and to S2ions of surface S-atoms. Further, Fig. 6(c) displays the peaks of Cd 3d5/2 and Cd 3d3/2 centered at about 404.69 eV and 411.5 2 eV, validating the spin-orbit separation of 6.83 eV. The doublet-separation can be attributed to Cd2þ of CdeS bond [29]. The attempts to de-convolute, doublet peak did not yield any additional component, thus ruling out the existence of CdeO over the film surface. This result is in accordance with the XRD studies, which validate that no oxide phase is present in the film. It is also evident from EDS study that these films display Cd:S ratio of 1:1 (Fig. SI.5). The XPS study clearly demonstrates that, the film consists of a stoichiometric composition of CdS crystal structure.
3.3. Photoelectrochemical studies and hydrogen production Photoelectrochemical analysis is very important to identify and quantify the performance of a photoanode. Fig. 7(a) displays the high transient photocurrents from various photoanodes; while film-B being the highest among all. In all the
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Fig. 5 e FESEM images showing morphology and film cross-section for the film thickness of (a) 200 nm (Film-A), (b) 500 nm (Film-B), and (c) 1000 nm (Film-C).
cases, the photoanodic current showed a rapid (wms) increase when light illumination was made ON, however it decayed to a much lower value (steady state) in few tens of seconds. The exponential shape of the transient part possessing certain time constant is known to be function of e the applied bias, incident light intensity and the redox species present in the electrolyte. Thus the time constant can be correlated with the reaction kinetics. Such faster decay kinetics reveals the larger recombination rates in the photoanode, where once the charges acquire an equilibrium a steady state value is reached by the system. The steady state value was found to be highest (137 mA/cm2) in case of film-B indicating that film deposited at 350 C yields efficient photoanode due to its nanostructured morphology. It is noteworthy that film-B displays maximum absorption which contributes to its improved PEC performance. Fig. 7(b) shows the graph of variation of photocurrentdensity and solar-to-hydrogen (STH) efficiency of the photoanodes (A to C). Both the curves display a concurrent trend in
variation with the thickness, indicating maximum efficiency of film-B. Ideally, the hydrogen generation is directly proportional to the photocurrent (J ). The current is generated, when the photo-induced electrons are available for the H2 evolution reaction, where the reaction can be given by the following equation: 2Hþ þ 2e /H2 ðgasÞ
(3)
It indicates that during PEC reaction, the photo generated electrons react with Hþ-ions at the counter electrode to yield H2 gas. The high photocurrent (137 mA/cm2) of film-B is thus responsible for achieving a maximum solar-to-hydrogen (STH) efficiency (w0.20%) at an applied bias of 0.2 V vs Ag/ AgCl in it than other films. The solar-to-hydrogen conversion efficiency (h) was calculated by using following relation [30]: h% ¼
Jp 1:23 Vapp 100% I0
(4)
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Fig. 6 e XPS spectrum of 500 nm thick CdS film (Film-B) showing survey scan (a) and region wise scan for Cd 3d level (b) and S 1s level (c).
where Vapp is the absolute value of applied potential that can be expressed as:
Fig. 7 e (a) Plots showing variation of photocurrent and hydrogen generated in an electrolyte (Na2S(0.01 M) & Na2SO3 (0.02 M)) at 0.2 V over CdS photoanodes; and (b) respective chronoamperometric scans for the films A to C.
Vapp ¼ Vmeas Voc where Vmeas and Voc are applied potential and open circuit potential respectively and I0 is power density of source used in mW/cm2. This yields an overall efficiency. In order to understand the performance of a photoanode material system following section is discussed.
3.4.
3.5.
MotteSchottky studies
The behavior of a photoanode is not only dependent on the physical properties of film, but is also decided by the charge
IPCE measurements of CdS photoanodes
Photoanode performance can be analyzed by studying the Incident photon-to-current efficiency (IPCE) which is given by the relation [31]: IPCE ¼
1240 I 100% lP
[5]
IPCE is also known as wavelength dependent efficiency. Fig. 8 shows the IPCE variation with the film thickness. IPCE was highest for film-B which is 8.5% at around 500 nm. All the films showed considerate IPCE values in the wavelength range of 400e700 nm as expected in case of CdS system. Film-A showed a poor performance as compared to the other two films. Incidentally, the film-C shows higher IPCE values in the range of 600e700 nm, but film-B showed highest IPCE-values in the range of 450e600 nm. The behavior is also in accordance with the photo-absorption capability of the respective films as shown in optical characterization.
Fig. 8 e IPCE of CdS thin films deposited at different thicknesses.
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kinetics occurring at the semiconductor/electrolyte interface. The charge transport properties can be studied by the electrochemical characterization of films specifically by estimation of the donor density and by the flat band potential measurement. The value of the flat band potential (VFB) was calculated using MotteSchottky (MeS) relation [8] and the barrier capacitance were measured at 1 kHz in the range of 1.2 to 1.2 V (vs Ag/AgCl) in dark. Fig. 9 shows the MeS plots indicating the flat band potential values of 1.08 V, 0.502 V and 0.188 V for film C, A and B respectively. The donor density was found to increase with the increase in the thickness (A to C) from 1.0 1017 cm3 to 1.5 1017 cm3. The reason of large change in the value of VFB with thickness can be attributed to creation of new donor levels [32]. Film B showed high flat band potential, and comparatively a high donor density value, which probably contributes to improve its PEC performance.
3.6.
Large area film deposition and PEC performance
In order to evaluate the PEC behavior of large area films deposited in present work, PEC measurements were carried out over these films of different electrode area (1.5e37.5 cm2). Fig. 10(a) shows the results of chronoamperometric measurements over different photo-electrodes B1 (1.5 cm2), B2 (12 cm2) and B3 (37.5 cm2), which are summarized as graph of photocurrent vs the photoanode areas in Fig. 10(b). The photocurrent was generated when the light was turned ON validating the PEC behavior of the films. Importantly it can be seen that the variation in the photocurrent followed a linear function with respect to electrode area, indicating the superior quality of the film. Unlike this observation, in practice the large area films often suffer by the problem of film inhomogeneity, film non-uniformity which in turn expected to yield poor photocurrent-density in a large area film. The linear fit clearly demonstrates that the film retains an expected photocurrent density for all the films as discussed in more details in next section. Utilization of PEC concept for energy generation is next important salient feature of the present work. The open
Fig. 10 e (a) Chronoamperometry of CdS photo-electrodes deposited over different substrate areas viz. B1-1.5 cm2, B2-12 cm2 and B3-37.5 cm2, under solar simulator at 0.2 V bias; (b) graph shows the linear variation of photocurrent with film area.
circuit potentials and the short circuit currents were measured in direct sun for these films under unbiased conditions. Fig. 11 shows the generated open circuit potential and short circuit currents from these films of variable electrode areas. The photographs of actual films used for the electrode formation are shown in the inset of the Fig. 11. It is important to mention that achieving such efficient large area films entrusts the spray pyrolysis deposition technique as an economic and industrially viable technique, that displays high potential for generating solar conversion photoanodes.
4. Importance of large area film in energy application
Fig. 9 e MotteSchottky curves for CdS deposited at different thicknesses.
Deposition of an efficient large area thin film is a crucial factor with respect to its commercial applicability. The deposition of a large area photoelectrode can amplify the photovoltaic output, provided the physico-chemical properties of the
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achieve an adherent, defect free and pinhole free large-area film for the photoelectrode fabrication. Spray pyrolysis is the easiest method to deposit adherent large-area thin films with defects-free surface morphology. The improved photoanode performance of large area photoanode clearly implies that spray pyrolysis is an economic and reliable methods best suited for commercialization of large area photoanodes for energy generation.
5.
Fig. 11 e Variation of open circuit potential (Voc) and short circuit current (Isc) with photoanode area for B1-1.5 cm2, B2-12 cm2, B3-37.5 cm2 and B4-81 cm2 electrodes measured directly under sun, and photograph of the respective films in the inset. The measurements were done under natural sunlight between 11a.m.e2 p.m. at ARCI, Hyderabad (Andhra Pradesh, India; 78.47 E, 28.28 N), with sunlight irradiation density of 0.020 W cmL2.
physical property of the film are retained. In fact practically the drift in the photocurrent density with increase in the film deposition area is generally observed, where the deterioration in PEC property is attributed to the poor quality of the large film area. Ideally the photocurrent density of large area photoanode is expected to be constant and can be measure of film quality. Accordingly, in an n-type semiconducting material, the photocurrent produced is directly proportional to the area of photoelectrode which is given by following expression [33]: i ¼ n F A k0f nsc Co
[6]
where n is carrier density, F is Faraday constant, A is area, nsc is concentration of electrons at the interface, C0 is Capaci0 tance, kf is heterogeneous rate constant. The linear variation fit of photocurrent with area clearly validates the excellent film properties as shown in Fig. 10(b). Sustained hydrogen production needs the attention with respect to scale-up feature of any photoanode. In present report, the photocurrents generated were found to be directly proportional to the photoanode area as shown in Fig. 10(b). This is an important achievement with respect to scale-up the photoanodes for energy conversion. A step further, the large area photoanodes exhibited a correlateable high open circuit potential and short circuit current variation in the photoanode area (see Fig. 11), indicating that there is an immense scope of utilization of SPD methodology for making efficient photoanodes. There are quite a few reports on study of large area photoanodes [34,35], where it has been reported that with an increase in the PEC active area, practically yields lowered photocurrent density [36] thereby hampering the large area applicability. Such poor performance is owing to the increased electrical resistance of the conducting substrate as well as the defects formation in the film. Thus it is highly desirable to
Conclusions
Adherent and uniform large-area nanorod structured CdS films were deposited using spray pyrolysis deposition. The film deposition parameters viz. deposition temperature, rate of film deposition etc., were optimized, and later these optimized conditions were used for large area (37.5 cm2) film deposition over FTO substrate. These films were used to PEC hydrogen generation application. Film thickness of 500 nm was found to yield best PEC performance yielding solar-tohydrogen (STH) efficiency of 0.20%. Superior photoanode performance is attributed to the nanorod structured morphology of the film. PEC performance of large area CdS films were studied and high photocurrent of 5 mA was observed for active electrode area 37.5 cm2. It has been demonstrated that the formation of high quality large-area film yields enhanced PEC H2 production and electrical power generation, using a very simple economic film deposition method.
Acknowledgments The authors wish to thank Mr. K. Ramesh Reddy and L. Venkatesh, for their help during film characterization. Authors are also thankful to Dr. Neha Y. Hebalkar for her help in X-ray photoelectron spectroscopy. Permission granted by Director, ARCI to publish the work is also gratefully acknowledged.
Appendix A. Supporting information Supporting information related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2012.10.057.
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water splitting applications. Electrochem Solid-state Lett 2006;9:G248e50. Duret A, Gratzel M. Visible light-induced water oxidation on mesoscopic a-Fe2O3 films made by ultrasonic spray pyrolysis. J Phys Chem B 2005;109(36):17184e91. Kokorin AL, Bahnemann DW. Chemical physics of nanostructured semiconductors. Boston: VSP; 2003. More GK, Prakasam HE, Varghese OK, Shankar K, Grimes CA. Vertically oriented TieFeeO nanotube array films: toward a useful material architecture for solar water splitting. Nano Lett 2007;7(8):2356e64. Yadav AA, Barote MA, Masumdar EU. Studies on nanocrystalline cadmium sulphide (CdS) thin films deposited by spray pyrolysis. Solid State Sci 2010;12:1173e7. Shen XP, Yuan AH, Wang F, Hong JH, Zheng X. Fabrication of well-aligned CdS nanotubes by CVD-template method. Solid State Commun 2005;133:19e22. Hiroshi U, Yohezawa H, Oht subo Y, Kosaka M, Sonomura H. Thin CdS films prepared by metal organic chemical vapor deposition. Sol Energy Mater Sol Cells 2003;75:219e26. Hodes G. Semiconductor and ceramic nanoparticle films deposited by chemical bath deposition. Phys Chem Chem Phys 2007;9:2181e96. Ximello-Quiebras JN, Contreras-Puente G, AguilarHerna´ndez J, Santana-Rodriguez G, Arias-Carbajal Readigos A. Physical properties of chemical bath deposited CdS thin films. Sol Energy Mater Sol Cells 2004;82:263e8. Pawar SM, Pawar BS, Kim JH, Oh-Shim Joo, Lokhande CD. Recent status of chemical bath deposited metal chalcogenide and metal oxide thin films. Curr Appl Phys 2011;11:117e61. Habas Susan E, Platt Heather AS, van Hest Maikel FAM, Ginley David S. Low-cost inorganic solar cells: from ink to printed device. Chem Rev 2010;110:6571e94. Senthil K, Mangalaraj D, Narayandass Sa K. Structural and optical properties of CdS thin films. Appl Surf Sci 2001;476: 169e70. Podesta´ A, Armani N, Salviati G, Romeo N, Bosio A, Prato M. Influence of the fluorine doping on the optical properties of CdS thin films for photovoltaic applications. Thin Solid Films 2006;448:511e2. Yemei L, Maozhong A, Gouki L. Optical properties of cadmium sulphide nanocrystal film prepared by electrochemical synthesis at liquideliquid interface. Appl Surf Sci 2006;253:459e63. Ullrich B, Tomm JW, Dushkina NM, Tomm Y, Sakai H, Segawa Y. Photoelectric dichromism of oriented thin film CdS fabrication by pulsed-laser deposition. Solid State Commun 2000;116:33e5. Mooney JB, Radding SB. Spray pyrolysis processing. Annu Rev Mater Sci 1982;12:81e101. Perednis D, Gauckler LJ. Thin film deposition using spray pyrolysis. J Electroceram 2005;14:103e11. Mathew S, Mukerjee PS, Vijayakumar KP. Optical and surface properties of spray-pyrolysed CdS thin films. Thin Solid Films 1995;254:278e84.
[22] Yadav AA, Barote MA, Dongre PM, Masumdar EU. Studies on growth and characterization of CdS1xSex (0.0 < x < 1.0) alloy thin films by spray pyrolysis. J Alloys Compd 2010;493: 179e85. [23] Krishnakumar K, Subramanian V, Ramprakash Y, Lakshman AS. Thin film preparation by spray pyrolysis for solar cells. Mater Chem Phys 1987;15:385e95. [24] Bacaksiz E, Tomakin M, Altunbas M, Parlak M, Colakoglu T. Structural, optical and magnetic properties of Cd1xCoxS thin films prepared by spray pyrolysis. Phys B 2008;403: 3740e5. [25] Yadav AA, Masumdar EU. Photo electrochemical investigation of cadmium sulphide (CdS) thin film electrodes by spray pyrolysis. J Alloys Compd 2011;509:5394e9. [26] Lokhnande CD, Sankapal BR, Pathan HM, Muller M, Giersig M, Tributsch H. Some structural studies on successive ionic layer adsorption and reaction (SILAR) deposited CdS thin films. Appl Surf Sci 2001;181:277e82. [27] Culity BD. Elements of X-ray diffraction. 2nd ed. Reading, MA: Addison-Wesley Publishing Compay, Inc; 1978. [28] Kulkarni SK, Winkler U, Deshmukh N, Borse PH, Fink R, Umbach E. Investigations on chemically capped CdS, ZnS and ZnCdS nanoparticles. Appl Surf Sci 2001;169e170: 438e46. [29] Yan Xiaoxia, Liu Gang, Wang Lian Zhou, Wang yong, Zhu Xian fang, Zhou Jin, et al. Anti-photocorrosive photocatalyst containing CdS nanoparticles and exfoliated TiO2 nanosheets. J Mater Res 2010;25:182e8. [30] Kumari S, Tripathi C, Singh AP, Chauhan D, Shrivastav R, Dass S, et al. Characterization of Zn-doped hematite thin films for photoelectrochemical splitting of water. Curr Sci 2006;91:1062e4. [31] Chen Z, Jaramillo TF, Deutsch TG, Shwarsctein AK, Forman AJ, Gaillard N, et al. Accelerating materials and development for photoelectrochemical hydrogen production. Standards for methods, definitions and reporting protocols. J Mater Res 2010;25:3e16. [32] Mane RS, Sankapal BR, Lokhande CD. Photoelectrochemical cells based on chemically deposited nanocrystalline Bi2S3 thin films. Mater Chem Phys 1999;60:196e203. [33] Bard AJ, Faulkner LR. Electrochemical methods. 2nd ed. Austin: John Wiley & Sons; 1944. p. 752. [34] Hanafusa A, Aramoto T, Tsuji M, Yamamoto T, Nishio T, Veluchamy P, et al. Highly efficient large area (10.5%, 1376 cm2) thin film CdS/CdTe solar cell. Sol Energy Mater Sol Cells 2001;67:21e9. [35] Winkler T, Schmidt H, Flugge H, Nikolayzik F, Baumann I, Schmale S, et al. Efficient large area semitransparent organic solar cells based on highly transparent and conductive ZTO/ Ag/ZTO multilayer top electrodes. Org Electron 2011;12: 1612e8. [36] Lee WJ, Shinde PS, Go GH, Ramasamy E. Ag-grid induced photocurrent enhancement in WO3 photoanodes and their scale-up performance toward photoelectrochemical H2 generation. Int J Hydrogen Energy 2011;36:5262e70.
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Fabrication of large area nanorod like structured CdS photoanode for solar H2 generation using spray pyrolysis technique Alka Pareek, Rekha Dom, Pramod H. Borse* Solar H2 PEC Lab., International Advanced Research Centre for Powder Metallurgy and New Materials, Balapur PO, Hyderabad, AP 500 005, India
article info
abstract
Article history:
Large area nanorod like structured CdS films (9 9 cm2) were deposited on the FTO glass
Received 5 June 2012
substrate using simple and economic spray pyrolysis deposition technique for photo-
Received in revised form
electrochemical (PEC) hydrogen production. With an intention of electrode scaling-up, the
25 September 2012
deposition area of photoanode was varied to evaluate its effect on the PEC hydrogen
Accepted 19 October 2012
generation capability. High photocurrent of 5 mA has been achieved from the PEC active
Available online 22 November 2012
area of 37.5 cm2. Its unit area (1 cm2) counterpart yielded Solar-to-Hydrogen (STH) conversion efficiency of 0.20% at a bias of 0.2 V vs Ag/AgCl using sacrificial reagents under
Keywords:
solar simulator (AM1.5) with 80 mW/cm2 irradiance. The 500 nm thick film exhibiting
Nanostructure
uniformly distributed nano-rod features yielded 3-times more photocurrent, as well as
Photoelectrochemical hydrogen
hydrogen evolution than other films. It exhibited an enhanced photo-activity as indicated
Large area
by the higher IPCE values (5e9%) in the wavelength range of 450e550 nm. It exhibited
Solar-to-hydrogen
superior optical properties (Eg w2.4 eV), and formation of high crystallinity hexagonal CdS
Solar energy
phase with space group P63MC. The superior performance of the photoanode is attributed
Spray pyrolysis film deposition
to the nanostructured morphology acquired under optimized spray pyrolysis conditions. Large area photoanodes showed unaltered photo-activity indicating the homogeneity in the film properties even in scaled-up version. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The solar radiation induced photoelectrochemical hydrogen production is a green technology route to generate renewable energy from eco-friendly entities. The hydrogen thus generated from water and solar light over a photocatalytic system, is thus highly important for mankind today till an efficient and stable photocatalyst system is identified. There have been exploratory studies over semiconducting oxides, chalcogenides and ferrites systems, to identify an efficient photoanode [1e5]. The potential metal oxides of TiO2, SrTiO3 etc. though
are known as good photoanode candidates, but suffer by their large band gap (Eg 3.2 eV), which limits [6] their PEC activity to UV light. On the other hand ferrites, an n-type Fe2O3 semiconductor, do exhibit a low band gap (Eg w2.0e2.2 eV), but show a poor photo-oxidation efficiency due to its low absorption coefficient and high electron-hole (e-h) recombination rates [7]. Chalcogenides have been known as the most potent photocatalyst candidates working under visible light. They exhibit desirable low band gap, and well suited valence/ conduction band edges with respect to water redox potential levels. Thus till today, they are the most extensively sought
* Corresponding author. Tel.: þ91 4024452426; fax: þ91 4024442699. E-mail address: [email protected] (P.H. Borse). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.057
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semiconductors for the role of visible light absorbing photoanode. Cadmium sulphide, a well known low band gap, n-type semiconductor (Eg w2.4 eV), is the most promising candidate not only for PEC hydrogen production applications but also for various other applications; viz. in opto-electronic devices, such as laser material, transducers, photo-conducting cells, photo-sensor, optical waveguides and non-linear integrated optical devices [8]. Consequently it has been chosen in the present study. Additionally, advent of modern material development techniques needs to be exploited for the material nanostructuring. Such attempts not only pave path to achieve better material properties but also lead to demonstrate the industrial processability of the methodology. In past CdS has been deposited by several techniques viz. chemical vapor deposition (CVD) [9,10], chemical bath deposition (CBD) [11e14], vacuum evaporation [15], sputtering [16], electro deposition [17], pulsed laser deposition [18], spray pyrolysis deposition (SPD) [8,19e25], & SILAR [26] etc. SPD is one of the most simple, inexpensive and versatile film deposition techniques which displays high potential for its industrial applicability. It can yield desired metal-oxide, semiconductor film over a large substrate area. This technique involves a controlled droplet-spraying of the molecularly mixed liquid precursor over the thermally heated substrate. These droplets undergo thermal decomposition over the substrate by making use of available energy. It is controllably possible to modulate the nucleation-and-growth mediated film properties by tuning the deposition conditions e flow rate, deposition temperature, deposition time etc. Unlike CBD, it offers the advantage of yielding high film crystallinity, and film uniformity, without the loss of chemical precursor. The ability to deposit large area nanostructure thin film is an added asset of SPD [23]. Such capability of SPD is expected to be beneficial in the field of PEC and solar-cell applications. Thus we utilize this deposition method in the present study to deposit large area thin films. Study of large area photoanodes for H2 production is most desirable with respect to commercialization and scale-up efforts for the hydrogen fuel utilization. Though CdS is a very efficient PEC system, still the study of its large area PEC performance is not reported in past. Hence such study would be a great step toward the efforts to evaluate scale-up approach to generate the green energy. In present work we have focused on the large area (more than 40 cm2) film deposition and its photoanode performance for H2 generation. A rigorous optimization of the deposition parameters with respect to flow rate, deposition temperature, deposition time, precursor etc. has been done to achieve an efficient photoanode. The main work related to the photoanode application for hydrogen generation is further elaborated. It was expected that by tailoring the structural, optical and morphological film properties, one is able to achieve an improved nanostructured photoanode from such films. The study presents the work in two major sections one being unit area (1 1 cm2) photoanode and other as large area (9 9 cm2) photoanode. Accordingly, in the first section the deposition and PEC optimization have been described for unit area film. The final section demonstrates large area film deposition. As a step further, the study clearly demonstrates the feasibility of deposition of large area films and their utilization as an efficient solar-electrode, particularly for the PEC application.
2.
37
Experimental
2.1. Large area film deposition and photo-electrode fabrication Fig. 1 shows the schematic diagram of indigenously designed spray pyrolysis equipment that was used for large area (9 9 cm2) thin film deposition. The automated computer interface of this SPD system has ability to control the inert gas pressure, precursor flow rate, substrate temperature and the film thickness. In order to deposit the desired film, equal amounts of CdCl2 (0.1 M) & (NH2)2CS (0.1 M) chemicals were dissolved in double distilled water to make the film precursor solution. Unless otherwise stated, throughout the work, certain fixed substrate area was chosen to achieve PEC active area of 1.5 cm2. Fluorine doped tin oxide; FTO (Pilkington TCO15) with resistivity of 12e15 U/sq was used as a bottom electrode. Films were deposited at various temperatures of 300e500 C and characterized (Figs. SI.1 and 2). The film thickness was varied by increasing the number of deposition cycles in the range of 3e20 cycles. The nozzle-to-substrate distance was varied from 15 to 25 cm, while the spray-rate was maintained between 2 and 10 ml/min. Parameter optimization was done on the basis of PEC performance (Figs. SI.3 and 4). In order to deposit large area film on FTO, optimized parameters were used. Mainly the results of four different substrate areas 1.5 cm2, 12 cm2, 37.5 cm2 & 81 cm2 are described in the present work. For the fabrication of electrode, the proposed active area was located over the film. The electrical contact was made over the bottom electrode, to apply the desired voltage biasing from external circuit. Rest of the part was electrically insulated using an epoxy. Thus, photoelectrode active area of 1 cm2 was achieved. The large area electrodes were also fabricated in similar manner. The photo-electrode thus
Fig. 1 e Schematic of spray pyrolysis set up for large area film deposition.
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obtained was used for the photoelectrochemical hydrogen generation.
2.2.
Film characterization
X-ray diffraction pattern was recorded by using Bruker AX D8 XRD equipment having CuKa X-ray source. The scan was taken in the range 0e100 with an increase of 0.01 at duration of 2 s/step. The transmission spectra were recorded using UVeVis spectrometer (Perkin Elmer, lambda 650). The detailed analysis was carried out using Tauc relation [24]: a¼
K hn Eg hn
n2 (1)
where K is constant, Eg is optical band gap, and n ¼ 1 for direct band gap material such as CdS. The band gap Eg is determined from the E vs (ahn)2 graph. The intercept on the energy axis was obtained by extrapolating the tangent to the curve, which gives the value Eg. Surface morphology of CdS thin films was studied by field emission scanning electron microscopy, FESEM (Hitachi model S4300SE/N) operated at 20 kV. Energy dispersive spectroscopy (EDS) was carried out for all the films during SEM study. The film cross-sectional studies were carried out to estimate the film thickness. For this the films were cracked carefully to expose the cross-section and then were used for the further study. Thus deposited thin films were named on the basis of their deposition parameters as shown in Table 1.
2.3.
Photo-electro-chemical characterization
Photoelectrochemical measurements were carried out using two-electrode cell (or 3-electrode cell, wherever mentioned) with the photoanode as a working electrode and graphite (/platinum) as a counter electrode in a quartz PEC reactor. In case of 3-electrode cell Ag/AgCl was used as reference electrode. The electrolyte was made using 0.01 M Na2S & 0.02 M Na2SO3 so as to minimize the photo-corrosion. The solar simulator (Oriel Model 91160) equipped with AM0, AM1.0 and AM1.5 Global (Newport) filters was utilized for the experimentation. The irradiance of 80 mW/cm2 was used in present case. The PEC measurements were done using two different PEC cell reactors, one for unit area electrode and another being for large area. A schematic of typical set is shown in Fig. 2. The reactor cell is interfaced with gas chromatography (GC) equipment and with an electrochemical set up (PARSTAT 2273). As shown, the specially designed and fabricated PEC reactor consists of a quartz window on the top of the reactor.
Table 1 e Films deposited at 350 C were named on the basis of the deposition cycles and observed film thickness as characterized from cross-section measurements. Film name A B C
No. of deposition cycles
Thicknessa (nm)
5 10 15
200 500 1000
a FESEM cross-section studies.
Fig. 2 e Schematic of photoelectrochemical reactor set up used for hydrogen generation.
This enables one to vertically photo-illuminate the electrode using solar simulator. The air tight reactor was utilized to monitor the generated H2 evolution. The evolved gas was then extracted and characterized by GC. During the PEC measurements, the distance between working electrode and counter electrode was maintained around 1 cm. The electrode connections from the PEC reactor were directly connected to an electrochemical system for photocurrent-potential and chronoamperometric measurements. In order to carry out Incident-Photon-Current-Conversion Efficiency (IPCE) measurements, an Oriel monochromator (Oriel model 74125) fitted with a 300 W Xenon lamp, that was capable of generating wavelengths in the range of 200e900 nm was used. The quantitative estimation of photo-electrochemically generated H2 was done by GC spectrometer, Model GC-2010 Plus (Shimadzu) that was equipped with TCD (Thermal conductivity detector) detector.
3.
Results and discussions
3.1.
Physico-chemical characterization of thin films
Fig. 3 shows the XRD spectra for the films (A, B and C) deposited under optimized temperature (Figs. SI.1 and 2) of 350 C. All the films exhibited hexagonal crystal structure of CdS lattice belonging to P63MC (SG no. 186) space group. There was no trace of any impurity phase like CdO. The crystallinity was found to improve from A to C, with the increase in the net deposited CdS content over the substrate. The FTO substrate peak (200) was found in all cases. The inset of the figure displays the variation in the CdS (002) peak intensity with the film thickness from A to C, indicating the effect of film thickness on film crystallinity. On the contrary, the intensity of the FTO (200) peak was found to decrease due to increase in the film thickness. Further in order to estimate the crystallite size, Debye Scherer’s formula [27] was used:
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Fig. 4 e Transmission spectra of CdS films deposited for thickness of: A-200 nm, B-500 nm and C-1000 nm. Inset shows Tauc plot of CdS-B film that was used for band gap estimation. Fig. 3 e XRD patterns of CdS film deposited with different thicknesses viz., A (200 nm), B (500 nm) and C (1000 nm). Substrate (FTO) peaks are indicated by solid circle. Inset shows the change in (002) peak intensity with the increase in the film thickness from A to C.
1000 nm for films A, B and C respectively. In view of achieving rod like features in film B, it was further investigated with special focus on its PEC property.
3.2.
0:9l D¼ b cos q
(2)
where, D-particle size, l-wavelength, q-diffraction angle and b-FWHM. As displayed in Table 1, the crystallite size was found to increase from A, B to C as 22 nm, 28 nm and 28.9 nm respectively. The EDS spectrum of film-B confirmed that Cd and S exist in the stoichiometric ratio of 1:1 (Fig. SI.5). Fig. 4 shows the optical transmission spectra for the CdS films A to C, indicating the absorption edge for Film B at wavelength longer than 500 nm. Film B exhibited a very low transmission than Films A and C, confirming its high absorptivity toward visible light photons. Films A & C display a systematic variation in the transmission with respect to increase in the thickness from film A to C. The band gap (Eg) was estimated using Tauc plots analysis. Inset of Fig. 4 shows Tauc plot for the typical film B revealing its direct band gap as 2.4 eV, which is in accordance with the known reports [19,25]. In order to study the film surface morphology and to measure the film thickness, morphological and cross-sectional studies were carried out. Fig. 5 shows the surface morphology of the films A to C along with their respective cross-sectional view, exhibiting respective distinct features. Interestingly the surface of film B exhibited certain aligned rod like nanostructure throughout the surface. On the other hand the films with other thicknesses viz. film A (and C) display very smooth (and highly rough) surface respectively. The changes in the film thickness are clearly visible from the cross-sectional view, indicating the film thickness of 200 nm, 500 nm and
X-ray photoelectron spectroscopy (XPS)
Elemental characterization of the films was carried out using XPS. Typical XPS analysis is shown in Fig. 6 for film B. As clearly seen in Fig. 6(a), the survey scan indicates the presence of Cd 3d & S 2p lines along with the remnant carbon peak. The region wise scan of S 2p and Cd 3d states is shown in Fig. 6(b) and (c) respectively. A detail analysis using de-convolution of peaks indicated that S 2p can be fitted with two peaks, one at B.E. w160.9 eV, and other being at 162.2 eV. These peaks can be respectively attributed to the S2 of bulk S atoms [28] and to S2ions of surface S-atoms. Further, Fig. 6(c) displays the peaks of Cd 3d5/2 and Cd 3d3/2 centered at about 404.69 eV and 411.5 2 eV, validating the spin-orbit separation of 6.83 eV. The doublet-separation can be attributed to Cd2þ of CdeS bond [29]. The attempts to de-convolute, doublet peak did not yield any additional component, thus ruling out the existence of CdeO over the film surface. This result is in accordance with the XRD studies, which validate that no oxide phase is present in the film. It is also evident from EDS study that these films display Cd:S ratio of 1:1 (Fig. SI.5). The XPS study clearly demonstrates that, the film consists of a stoichiometric composition of CdS crystal structure.
3.3. Photoelectrochemical studies and hydrogen production Photoelectrochemical analysis is very important to identify and quantify the performance of a photoanode. Fig. 7(a) displays the high transient photocurrents from various photoanodes; while film-B being the highest among all. In all the
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Fig. 5 e FESEM images showing morphology and film cross-section for the film thickness of (a) 200 nm (Film-A), (b) 500 nm (Film-B), and (c) 1000 nm (Film-C).
cases, the photoanodic current showed a rapid (wms) increase when light illumination was made ON, however it decayed to a much lower value (steady state) in few tens of seconds. The exponential shape of the transient part possessing certain time constant is known to be function of e the applied bias, incident light intensity and the redox species present in the electrolyte. Thus the time constant can be correlated with the reaction kinetics. Such faster decay kinetics reveals the larger recombination rates in the photoanode, where once the charges acquire an equilibrium a steady state value is reached by the system. The steady state value was found to be highest (137 mA/cm2) in case of film-B indicating that film deposited at 350 C yields efficient photoanode due to its nanostructured morphology. It is noteworthy that film-B displays maximum absorption which contributes to its improved PEC performance. Fig. 7(b) shows the graph of variation of photocurrentdensity and solar-to-hydrogen (STH) efficiency of the photoanodes (A to C). Both the curves display a concurrent trend in
variation with the thickness, indicating maximum efficiency of film-B. Ideally, the hydrogen generation is directly proportional to the photocurrent (J ). The current is generated, when the photo-induced electrons are available for the H2 evolution reaction, where the reaction can be given by the following equation: 2Hþ þ 2e /H2 ðgasÞ
(3)
It indicates that during PEC reaction, the photo generated electrons react with Hþ-ions at the counter electrode to yield H2 gas. The high photocurrent (137 mA/cm2) of film-B is thus responsible for achieving a maximum solar-to-hydrogen (STH) efficiency (w0.20%) at an applied bias of 0.2 V vs Ag/ AgCl in it than other films. The solar-to-hydrogen conversion efficiency (h) was calculated by using following relation [30]: h% ¼
Jp 1:23 Vapp 100% I0
(4)
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41
Fig. 6 e XPS spectrum of 500 nm thick CdS film (Film-B) showing survey scan (a) and region wise scan for Cd 3d level (b) and S 1s level (c).
where Vapp is the absolute value of applied potential that can be expressed as:
Fig. 7 e (a) Plots showing variation of photocurrent and hydrogen generated in an electrolyte (Na2S(0.01 M) & Na2SO3 (0.02 M)) at 0.2 V over CdS photoanodes; and (b) respective chronoamperometric scans for the films A to C.
Vapp ¼ Vmeas Voc where Vmeas and Voc are applied potential and open circuit potential respectively and I0 is power density of source used in mW/cm2. This yields an overall efficiency. In order to understand the performance of a photoanode material system following section is discussed.
3.4.
3.5.
MotteSchottky studies
The behavior of a photoanode is not only dependent on the physical properties of film, but is also decided by the charge
IPCE measurements of CdS photoanodes
Photoanode performance can be analyzed by studying the Incident photon-to-current efficiency (IPCE) which is given by the relation [31]: IPCE ¼
1240 I 100% lP
[5]
IPCE is also known as wavelength dependent efficiency. Fig. 8 shows the IPCE variation with the film thickness. IPCE was highest for film-B which is 8.5% at around 500 nm. All the films showed considerate IPCE values in the wavelength range of 400e700 nm as expected in case of CdS system. Film-A showed a poor performance as compared to the other two films. Incidentally, the film-C shows higher IPCE values in the range of 600e700 nm, but film-B showed highest IPCE-values in the range of 450e600 nm. The behavior is also in accordance with the photo-absorption capability of the respective films as shown in optical characterization.
Fig. 8 e IPCE of CdS thin films deposited at different thicknesses.
42
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kinetics occurring at the semiconductor/electrolyte interface. The charge transport properties can be studied by the electrochemical characterization of films specifically by estimation of the donor density and by the flat band potential measurement. The value of the flat band potential (VFB) was calculated using MotteSchottky (MeS) relation [8] and the barrier capacitance were measured at 1 kHz in the range of 1.2 to 1.2 V (vs Ag/AgCl) in dark. Fig. 9 shows the MeS plots indicating the flat band potential values of 1.08 V, 0.502 V and 0.188 V for film C, A and B respectively. The donor density was found to increase with the increase in the thickness (A to C) from 1.0 1017 cm3 to 1.5 1017 cm3. The reason of large change in the value of VFB with thickness can be attributed to creation of new donor levels [32]. Film B showed high flat band potential, and comparatively a high donor density value, which probably contributes to improve its PEC performance.
3.6.
Large area film deposition and PEC performance
In order to evaluate the PEC behavior of large area films deposited in present work, PEC measurements were carried out over these films of different electrode area (1.5e37.5 cm2). Fig. 10(a) shows the results of chronoamperometric measurements over different photo-electrodes B1 (1.5 cm2), B2 (12 cm2) and B3 (37.5 cm2), which are summarized as graph of photocurrent vs the photoanode areas in Fig. 10(b). The photocurrent was generated when the light was turned ON validating the PEC behavior of the films. Importantly it can be seen that the variation in the photocurrent followed a linear function with respect to electrode area, indicating the superior quality of the film. Unlike this observation, in practice the large area films often suffer by the problem of film inhomogeneity, film non-uniformity which in turn expected to yield poor photocurrent-density in a large area film. The linear fit clearly demonstrates that the film retains an expected photocurrent density for all the films as discussed in more details in next section. Utilization of PEC concept for energy generation is next important salient feature of the present work. The open
Fig. 10 e (a) Chronoamperometry of CdS photo-electrodes deposited over different substrate areas viz. B1-1.5 cm2, B2-12 cm2 and B3-37.5 cm2, under solar simulator at 0.2 V bias; (b) graph shows the linear variation of photocurrent with film area.
circuit potentials and the short circuit currents were measured in direct sun for these films under unbiased conditions. Fig. 11 shows the generated open circuit potential and short circuit currents from these films of variable electrode areas. The photographs of actual films used for the electrode formation are shown in the inset of the Fig. 11. It is important to mention that achieving such efficient large area films entrusts the spray pyrolysis deposition technique as an economic and industrially viable technique, that displays high potential for generating solar conversion photoanodes.
4. Importance of large area film in energy application
Fig. 9 e MotteSchottky curves for CdS deposited at different thicknesses.
Deposition of an efficient large area thin film is a crucial factor with respect to its commercial applicability. The deposition of a large area photoelectrode can amplify the photovoltaic output, provided the physico-chemical properties of the
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achieve an adherent, defect free and pinhole free large-area film for the photoelectrode fabrication. Spray pyrolysis is the easiest method to deposit adherent large-area thin films with defects-free surface morphology. The improved photoanode performance of large area photoanode clearly implies that spray pyrolysis is an economic and reliable methods best suited for commercialization of large area photoanodes for energy generation.
5.
Fig. 11 e Variation of open circuit potential (Voc) and short circuit current (Isc) with photoanode area for B1-1.5 cm2, B2-12 cm2, B3-37.5 cm2 and B4-81 cm2 electrodes measured directly under sun, and photograph of the respective films in the inset. The measurements were done under natural sunlight between 11a.m.e2 p.m. at ARCI, Hyderabad (Andhra Pradesh, India; 78.47 E, 28.28 N), with sunlight irradiation density of 0.020 W cmL2.
physical property of the film are retained. In fact practically the drift in the photocurrent density with increase in the film deposition area is generally observed, where the deterioration in PEC property is attributed to the poor quality of the large film area. Ideally the photocurrent density of large area photoanode is expected to be constant and can be measure of film quality. Accordingly, in an n-type semiconducting material, the photocurrent produced is directly proportional to the area of photoelectrode which is given by following expression [33]: i ¼ n F A k0f nsc Co
[6]
where n is carrier density, F is Faraday constant, A is area, nsc is concentration of electrons at the interface, C0 is Capaci0 tance, kf is heterogeneous rate constant. The linear variation fit of photocurrent with area clearly validates the excellent film properties as shown in Fig. 10(b). Sustained hydrogen production needs the attention with respect to scale-up feature of any photoanode. In present report, the photocurrents generated were found to be directly proportional to the photoanode area as shown in Fig. 10(b). This is an important achievement with respect to scale-up the photoanodes for energy conversion. A step further, the large area photoanodes exhibited a correlateable high open circuit potential and short circuit current variation in the photoanode area (see Fig. 11), indicating that there is an immense scope of utilization of SPD methodology for making efficient photoanodes. There are quite a few reports on study of large area photoanodes [34,35], where it has been reported that with an increase in the PEC active area, practically yields lowered photocurrent density [36] thereby hampering the large area applicability. Such poor performance is owing to the increased electrical resistance of the conducting substrate as well as the defects formation in the film. Thus it is highly desirable to
Conclusions
Adherent and uniform large-area nanorod structured CdS films were deposited using spray pyrolysis deposition. The film deposition parameters viz. deposition temperature, rate of film deposition etc., were optimized, and later these optimized conditions were used for large area (37.5 cm2) film deposition over FTO substrate. These films were used to PEC hydrogen generation application. Film thickness of 500 nm was found to yield best PEC performance yielding solar-tohydrogen (STH) efficiency of 0.20%. Superior photoanode performance is attributed to the nanorod structured morphology of the film. PEC performance of large area CdS films were studied and high photocurrent of 5 mA was observed for active electrode area 37.5 cm2. It has been demonstrated that the formation of high quality large-area film yields enhanced PEC H2 production and electrical power generation, using a very simple economic film deposition method.
Acknowledgments The authors wish to thank Mr. K. Ramesh Reddy and L. Venkatesh, for their help during film characterization. Authors are also thankful to Dr. Neha Y. Hebalkar for her help in X-ray photoelectron spectroscopy. Permission granted by Director, ARCI to publish the work is also gratefully acknowledged.
Appendix A. Supporting information Supporting information related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2012.10.057.
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