Properties of spray deposited titanium dioxide thin

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Solar Energy Materials & Solar Cells 92 (2008) 283–290 www.elsevier.com/locate/solmat

Properties of spray deposited titanium dioxide thin films and their application in photoelectrocatalysis P.S. Shindea,b, S.B. Sadalec, P.S. Patilb, P.N. Bhosalea, A. Bru¨gerd, M. Neumann-Spallartc, C.H. Bhosalea, a

Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, Maharashtra, India b Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, Maharashtra, India c Groupe d’E´tude de la Matie`re Condenseé, C.N.R.S., 1, Place Aristide Briand, 92195 Meudon Cedex, France d Institute of Materials Chemistry, Vienna University of Technology, Veterina¨rplatz 1, A-1210 Vienna, Austria Received 7 April 2007; received in revised form 3 September 2007; accepted 4 September 2007 Available online 18 October 2007

Abstract Thin films of titanium dioxide were deposited onto optically transparent, electrically conducting substrates (fluorine doped tin oxide on glass). The two oxide layers, SnO2 and TiO2, were deposited sequentially by spray pyrolysis. TiO2 films of up to 800 nm thickness were prepared by varying the quantity of sprayed solution (titanyl acetylacetonate in methanol), at a growth rate of 0.15 nm/s. The effect of film thickness on the structural, optical and photoelectrochemical properties of TiO2 films was studied. Scanning electron microscopy showed that the polycrystalline anatase films were compact. The grain size increased up to 1100 nm with increase in film thickness, whereas the crystallite size remained constant (40 nm) as shown by X-ray diffraction. The films had a transmittance of more than 70% in the visible region. Junctions of the semiconducting films with aqueous electrolytes were rectifying and photoactive. Films of 330 or 600 nm were thick enough to exhibit maximum photoelectrochemical response for light of a wavelength of 313 or 365 nm, respectively. Under depletion conditions, an IPCE (incident photon to current conversion efficiency) of 0.8 for a 330 nm thick film at 313 nm was obtained. Oxalic acid degradation under UVA light and under sunlight, applying electrical bias, was demonstrated using these electrodes. r 2007 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Semiconductors; Thin films; Electrochemical properties; Photoelectrocatalysis

1. Introduction Titanium dioxide (TiO2) has been known for quite some time to afford oxidation of organic substances under illumination. This is due to the high oxidation potential of its valence band holes. Therefore, TiO2 has become an interesting candidate as a catalyst for photochemical water (and air) purification [1,2]. Other applications are in electronic and optical devices due to its electronic [3] and optical properties [4]. For the preparation of thin films of TiO2, numerous methods such as thermal or anodic oxidation of Ti, dip or spin coating and CVD were used, as well as spray pyrolysis [5–8] and aerosol pyrolysis (AP) Corresponding author.

E-mail address: [email protected] (C.H. Bhosale). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.09.001

[9–12], employing different metalorganic precursors such as Ti-ethoxide, Ti-isopropoxide, bis-(2,4-pentanedionato) titanium oxide, Ti-oxy acetylacetonate and diisopropoxytitanium bis-acetylacetonate. Of the crystalline phases, brookite is unstable and can only be obtained by a complex preparation but never as a single phase, whereas rutile is the thermodynamically most stable polymorph which is formed on layers at synthesis temperatures above 700 1C. The anatase modification has been shown in several investigations to be the most active for catalytic (oxidative decomposition) reactions [5,13–16]. For example, Kato et al. [15,16] have reported that the morphology and crystal structure of TiO2 coatings affected the photocatalytic activity for the decomposition of aqueous acetic acid. The use of TiO2 in photoelectrocatalytic processes (water purification) is particularly

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interesting [17]. As this application requires rather large surface areas, spray pyrolysis may be used, as it is a simple method for fast coating of large areas with thick films in a single step [18]. However, there are only few reports on photoelectrochemical properties of spray pyrolysis or AP deposited TiO2 films [7,11,19]. The efficiency of charge separation, IPCE (incident photon to current conversion efficiency) in junctions of dense, thin TiO2 (anatase) films with aqueous electrolytes was reported in Refs. [11,20,21], and the highest reported value at 365 nm was 0.6 [20] for 15–20 mm thick films. In this study, we report the spray pyrolysis synthesis of TiO2 films on a transparent conductor (TCO) with special attention to their photoelectrochemical properties, especially the efficiency of charge separation as a function of wavelength and electrical bias, which is the basis of their application in the photoelectrochemical oxidation of organic solutes in water (water detoxification), using a flow-through reactor with UV or sunlight irradiation, and electrical bias.

counter-electrode and a saturated calomel electrode (SCE) as a reference. All potentials are quoted vs. SCE. The electrolytes were aqueous solutions of either 0.1 M NaOH or 0.01 M Na2SO4, with and without added oxalic acid. A 150 W medium pressure mercury lamp in combination with band-pass filters (313 nm, 365 nm), and LEDs (376.9 nm, 403.11 nm) were used for illumination. IPCE was calculated using a Hamamatsu S1337-1010BQ calibrated photodiode. Large (10 cm 10 cm) electrodes were prepared in the same way as described above and tested in a thin (1 mm) flow-through photoelectrochemical reactor employing a stainless steel counter-electrode and broadband UVA (defined as the UV light range between 320 and 400 nm) irradiation using a ‘‘blacklight’’ lamp (Conrad) or solar light. With the thin film reactor, backside illumination (through the transparent conducting substrate) was used.

2. Experimental Glass substrates were cleaned ultrasonically in trichloroethylene, acetone, ethanol and double distilled water. First, fluorine doped tin oxide (FTO) conducting coatings on glass were prepared by spray pyrolysis using stannic chloride pentahydrate (SnCl4 5H2O) (purity 99.9%) and ammonium fluoride (NH4F) (98%) as precursor salts. The FTO had a transmittance of 90–95% and a sheet resistance of 10–15 O. This FTO-coated glass was used as substrate for the spray pyrolytic deposition of TiO2 films. The precursor solution consisted of titanyl acetylacetonate (TiAcAc, C10H14O5Ti, AR grade, 99.9% pure, Merck) in methanol. It was sprayed through a pneumatic glass nozzle using compressed air as a carrier gas onto either glass (soda lime) or FTO substrates maintained at a fixed temperature. The spraying rate was 4.5 ml/min and the quantity of the spraying solution was varied between 30 and 210 ml in order to obtain films of different thickness. Structural properties were studied using a Philips PW 1730 X-ray diffractometer operated at 30 kV, 20 mA. The surface morphology of thin films was examined using a scanning electron microscope JEOL JSM-6360. Optical transmission spectra were recorded in the wavelength range of 200–1000 nm using a double beam UV—vis–NIR spectrophotometer, Hitachi U-2800. Thickness of the FTO and TiO2 films on glass was calculated from the interference patterns of the transmission spectra [22,23] and the thickness of the FTO/TiO2 samples was derived from the reflection spectra using the ‘‘Nanocalc’’ (Mikropack) software. Photoelectrochemical characterization was performed using a potentiostatic set-up. The photoelectrochemical cell comprised TiO2/FTO/glass as working electrode (with an active surface area of 0.5 or 1 cm2), a Pt

Fig. 1. X-ray diffraction patterns of as-deposited (0.1 M TiAcAc spraying solution, 470 1C substrate temperature) anatase TiO2 films on soda lime glass, with different thicknesses ranging from 25 to 785 nm. Vertical lines—JCPDS 21-1272.

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Total organic carbon content (TOC) of solutions was measured using a colorimetric procedure (Machery– Nagel).

3. Results and discussion 3.1. TiO2 film formation The preparative parameters such as solution concentration, solution flow rate, nozzle-to-substrate distance, and substrate temperature were optimized to obtain uniform, homogenous and adherent thin films. Films of increasing thickness were prepared at 470 1C by spraying 30–210 ml of 0.1 M TiAcAc solution. Film formation occurred through thermal decomposition of the precursor under oxygen (air): C10 H14 O5 Ti þ 12O2 ! TiO2 þ 10CO2 þ 7H2 O:

(1)

This leads to clear, colorless films, whereas films obtained in the absence of oxygen contain carbon, as shown for AP of a similar metal organic precursor [11]. Deposits on glass were prepared for optical characterization and deposits on FTO/glass for photoelectrochemical characterization. Thickness values used in Fig. 2 were estimated from the reflectance and transmittance spectra using standard expressions [22] and published values for n and k [23]. From these values, an initial growth rate of 0.15 nm/s, somewhat increasing during the course of the deposition, can be deduced.

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3.2. X-ray diffraction (XRD) Fig. 1 shows the XRD patterns of as-deposited TiO2 thin films on soda lime glass for various film thicknesses. The formation of the crystalline, single-phase anatase is revealed at a thickness X123 nm (60 ml of sprayed solution). The diffraction peaks correspond to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 1 1) crystal planes, respectively, of the anatase phase [24], and additional peaks corresponding to the (2 0 4) and (1 1 6) planes can be observed for thicker films. The crystallite size was determined by the broadening of the (1 0 1) diffraction peak at 25.411, using the Scherrer formula and FWHM values corrected for the line broadening of the instrument. Crystallite sizes of 40.572.5 nm were found for all samples of a thickness of X123 nm. 3.3. Scanning electron microscopy (SEM) Fig. 2a–g shows typical scanning electron microscopy (SEM) images of as-deposited TiO2 thin films with different thicknesses. The surface morphology is essentially changing from granular to platelet rich with increasing film thickness. Densely packed round grains of 200 nm were seen for the films of 25 and 123 nm thickness. The formation of platelets was seen for 148 and 220 nm thick films. Individual platelets, perpendicular to the grains in the specimen plane were clearly seen for the 332 nm thick film, the surface morphology of which is similar to that

Fig. 2. Scanning electron microscopy (SEM) images of as-deposited sprayed (0.1 M TiAcAc solution and 470 1C substrate temperature) TiO2 films on FTO with different thicknesses, viz. (a) 25 nm, (b) 123 nm, (c) 148 nm, (d) 220 nm, (e) 332 nm, (f) 595 nm, and (g) 785 nm; (h) grain size as a function of thickness. A 15-nm-thick platinum layer was sputtered on the films before recording the images. The micrographs are recorded with 10,000 magnification and 30 kV accelerating voltage.



Fig. 2. (Continued)

reported in Ref. [6]. For thicker films, these platelets were clustered forming large grains of up to 1100 nm diameter. The size of the grains is plotted as a function of film thickness in Fig. 2h. As the grain size is always thicker than the film, up to the thicknesses studied here, the grains must be flattened spheres. It may further follow that such grains reach from the top surface of the electrode to the backplane. This would lead to unhindered charge transfer across the film, resulting in the good electronic behavior observed (see below). 3.4. Optical transmission Fig. 3 shows the transmittance spectra for as-deposited TiO2 films on glass in the spectral range of 200–1000 nm. The thickness of the TiO2 films varied from 25 to 785 nm for a variation of the quantity of sprayed solution from 30 to 210 ml. The transmittance of the films decreased with increasing film thickness due to scattering. Maximum transmission exceeds 90% for the thinnest films.

The transmittance curves for all samples show a strong decrease in the transmittance at wavelengths below 400 nm, superimposed by absorption of the substrate, soda lime glass. For a thick film deposited on quartz, a bandgap energy around 3.3 eV was estimated. 3.5. Photoelectrochemical studies Photoelectrochemical measurements of films deposited on FTO were carried out at 313, 365, 376.9, and 403.11 nm. Especially the wavelength of 365 nm was selected in view of later application of the TiO2 layers in water purification, as mercury lamps with a prominent line at, or continuum around, 365 nm are readily available [25] making photochemical processes convenient to be carried out. In Fig. 4, a typical current–potential curve in the dark and under illumination in 0.1 N NaOH is shown for a 330 nm thick film on FTO. The plateau photocurrents reached a few hundred millivolts above the photocurrent onset potential, showing that the semiconducting properties of the n-TiO2


Fig. 3. Plot of transmittance vs. wavelength for as-deposited sprayed (0.1 M TiAcAc solution and 470 1C substrate temperature) TiO2 thin films with different thicknesses ranging from 25 to 785 nm.

Fig. 4. Photoelectrochemical response of a 332 nm thick TiO2 film on FTO/glass substrate in a junction with 0.1 N NaOH under slow rectangular chopped light; scan rate of 20 mV/s. As-deposited TiO2 film (150 ml of 0.1 M TiAcAc solution sprayed, 470 1C substrate temperature).

layer are quite satisfactory. Films of increasing thickness show increasing photocurrent. No change in photocurrents was observed after post-annealing of the films at 500 1C in air for 1 h. IPCEs were calculated from the photocurrents at 0.8 V, a potential leading to plateau currents (depletion under reverse bias), as shown in Fig. 4. In Fig. 5, the IPCE is plotted as a function of thickness. A thickness of 330 nm is sufficient to reach a plateau value for IPCE at 313 nm, due to complete light absorption at this wavelength. At even higher thickness (595 nm) the IPCE at 365 nm reaches the value obtained for 313 nm, as photons of both wavelengths are completely absorbed within the film. A similar IPCE dependence on thickness was reported by Belaidi et al. [11]

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Fig. 5. Variation of IPCE on TiO2 film thickness at 365 nm (open circles) and 313 nm (filled squares) in 0.1 N NaOH at 0.8 V vs. SCE.

Fig. 6. IPCE as a function of wavelength in 0.1 M NaOH at 0.8 V vs. SCE for TiO2 samples of 25 nm (open triangles), 220 nm (filled triangles) and 332 nm (down-pointing triangles) thickness. Inset: IPCE as a function of wavelength for a 220 nm thick sample, for EE and SE illumination. Open squares, EE illumination; filled circles, SE illumination; open circles, SE illumination corrected for light absorption by the substrate (FTO/glass).

for TiO2 thin films deposited by AP, but the IPCE (365 nm) was limited to 0.2. Fig. 6 shows IPCE vs. l for TiO2 electrodes of three different thicknesses at an applied bias voltage of 0.8 V, at which plateau photocurrents are obtained. For short wavelengths (313 nm), where light absorption within the layer is complete, the IPCE is the highest, and the quantum yields of charge separation, considering light reflections at the different interfaces of the electrochemical cell and the electrolyte/electrode surface, is 0.9, thus approaching the theoretical limit of 1. For longer wavelengths, incomplete absorption of photons leads to a decreased IPCE. For thin enough TiO2/FTO/glass films, when going from higher to lower wavelengths, no difference between



backside (substrate–electrode, ‘‘SE’’) and front side (electrolyte–electrode, ‘‘EE’’) illumination was observed down to 365 nm (inset Fig. 6). At lower wavelengths, the two curves separate even for the SE-curve corrected for light absorption by the substrate. This could be due to the fact that light entering from the backside leads to the generation of charge carriers far from the semiconductor/ electrolyte junction. Such charge carriers cannot be separated efficiently if Lp+wod, where Lp is the minority carrier diffusion length, d the layer thickness, and w the width of the space charge layer (see also Ref. [26] for the case of thin WO3 films, and Ref. [27] for a theoretical description of the phenomenon). For thicker films, the IPCE for backside illumination drops below the one for front side illumination, already at higher wavelengths (results not shown), as expected [26]. The difficulty of extending the space charge layer all the way to the backside of a highly doped thick electrode using moderate bias may be responsible for the effect, but the fact that 313 nm SE radiation is absorbed within 10 nm from the interface with the conducting substrate leads certainly to a high probability of interface recombination and in turn, decrease of charge collection efficiency. In any case, it follows that for use in a photoelectrochemical reactor with backside illumination [25] (as intended with the present films), film thickness has to be optimized for the spectral characteristics of the light source. As typical for anatase electrodes, current multiplication is observed when the reaction of an oxidizable solute with valence band holes leads to a transient species (a radical), the redox potential of which is sufficiently low to inject electrons into the conduction band of the semiconductor [28]. This is also observed for the electrodes presented here, and it is an indication for direct hole transfer due to adsorption of electroactive species at the surface of the semiconductor (Fig. 7).

Fig. 7. IPCE as a function of oxalic acid concentration in 10 mM Na2SO4 for a TiO2/FTO/glass electrode at 0.8 V vs. SCE, front side illumination at 365 nm.

Experiments with large (10 cm 10 cm) electrodes were carried out under UVA (center wavelength 365 nm) or solar irradiation. Fig. 8 shows an i–E curve under UVA illumination in a two electrode configuration. In this case, the onset of plateau photocurrents occurs at comparatively high bias is due to iR drop, as there is no potentiostatic control. Fig. 9 shows that substantial photocurrents can be drawn even under sunlight where the UV part of the spectrum is relatively small. This is expected on the basis of measured IPCE for backside illumination (Fig. 7) and the AM1.5 solar spectrum (Fig. 10). The photocurrent as a function of wavelength (curve b in Fig. 10) is the product of

Fig. 8. i–E curve for a TiO2/FTO/glass electrode. Active area 64 cm2; electrolyte, 10 mM Na2SO4 at a flow rate of 10 ml/min; open circles, in the dark, and filled circles, under broadband UVA illumination.

Fig. 9. Photocurrent as a function of time for a TiO2/FTO/glass electrode. Active area 64 cm2; electrolyte, 1 mM oxalic acid at a flow rate of 0.044 ml/ min; bias 1.324 V vs. steel; solar illumination at location 161400 3700 N 741150 1800 E, at 13:24 (IST) (start of experiment), azimuth tracking.


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Fig. 10. Spectral dependence of a TiO2/FTO/glass electrode and of solar irradiation (AM1.5). Backside illumination (SE) under plateau photocurrent conditions in 0.1 M NaOH. The total photocurrent density is the area under curve b (7.594 105 A/cm2).

part of the solute was decomposed, showing the viability of the here proposed concept, using titanium dioxide under illumination and electrical bias. (Without bias or without illumination no decomposition occurs.) 4. Conclusions

Fig. 11. Oxidative degradation of oxalic acid as a function of flow rate. TiO2/FTO/glass electrode; active area 64 cm2; electrolyte, 2.5 mM oxalic acid in 10 mM Na2SO4; broadband UVA illumination; bias 2 V vs. steel.

IPCE and solar irradiance expressed as current. The area under curve b gives the total photocurrent density to be expected. It is in good agreement with the measured photocurrent density under solar illumination (0.125 mA/cm2, Fig. 9). Under either UVA or solar illumination and bias between 1.3 and 2 V, degradation of oxalic acid was shown to occur. In Fig. 11 concentrations are expressed in TOC/TOC0, where TOC0 and TOC are the total organic carbon content of the solution at the inlet and the outlet of the reactor, respectively. Upon single pass flow-through, the extent of oxidation of oxidizable solute varies with flow rate [25]. At the lowest flow rate, corresponding to the highest dwell time in the reactor and therefore highest number of photons absorbed, the concentration at the exit is the lowest (Fig. 11). Under low flow rate, a substantial

Spray deposited TiO2 films of different thicknesses in the range of 25–785 nm were prepared at a growth rate above 0.15 nm/s on FTO/glass which was spray deposited in a preceding step in the device fabrication. The grain size increased with increase in thickness. As-deposited n-type TiO2 thin films exhibited photoactivity under UV illumination in junctions with 0.1 N NaOH. Under depletion conditions, IPCEs of 0.8 were obtained for 330 nm thick films at a wavelength of 313 nm, and of 0.7 for 600 nm thick films at 365 nm illumination. These are exceptionally high values compared to values reported previously in the literature. Oxalic acid degradation in a thin film reactor under UVA light and under sunlight was demonstrated using such electrodes under electrical bias. Acknowledgment The authors wish to acknowledge the Department of Science and Technology (DST), Government of India, New Delhi, for the financial support through project No. SR/S2/ CMP-34/2003. References [1] [2] [3] [4] [5]

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