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Light intensity effects on photocatalytic water splitting with a titania catalyst
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 ) 6 9 3 8 e6 9 4 7
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Light intensity effects on photocatalytic water splitting with a titania catalyst Stuart Bell 1, Geoffrey Will*, John Bell Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia
article info
abstract
Article history:
Photocatalytic water splitting is a process which could potentially lead to commercially
Received 8 October 2012
viable solar hydrogen production. In order to evaluate if solar concentration could be used
Received in revised form
to increase the feasibility of the process, the effect of light intensity on photocatalytic water
18 January 2013
splitting was examined.
Accepted 23 February 2013 Available online 24 April 2013
Degussa P25 TiO2 films were used to form a photocatalytic cell and illuminated with a Xenon arc lamp at intensities up to 52 suns. The reaction demonstrated a sub-linear relationship where photocurrent was proportional to intensity with an exponential value
Keywords: Photocatalysis
of 0.627. This is an important finding for photocatalytic water splitting. It could provide an
Water splitting
avenue for the development of a large scale photocatalytic water splitting system for
Solar hydrogen
hydrogen production and the further commercialisation of this technology.
Hydrogen generation
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Titanium dioxide
reserved.
Light intensity
1.
Introduction
Energy has been a major political, economic and environmental issue throughout the 20th century and will become the number one issue of the 21st century. The amount of energy that falls on the earth in sunlight is 3 1024 joules per year, or 10 000 times that of the world’s usage [1] making large scale conversion of solar energy to a usable form a potential solution to future energy needs. Photocatalytic water splitting is a technology that could produce renewable hydrogen using solar energy and water as inputs. Since the initial breakthrough demonstration in 1972 by Fujishima and Honda [2] much research has been devoted to developing materials for this process. Very little focus however, has been devoted to the systems aspect of photocatalytic water splitting. This is important as the optimal
conditions for water splitting may be very different to the conditions for which materials are currently being developed. Light intensity is an important parameter when considering using concentrated light in photocatalytic water splitting systems. The authors consider concentrated light systems to be promising due to the small amounts of hydrogen per unit area produced by one sun systems and the difficulties in capturing and containing the gas in such systems. To date there has been minimal research conducted into the effect of light intensity on photocatalytic water splitting reactions. In 1976 Carey & Oliver used an argon laser to illuminate their cell with UV light up to 400 mW/cm2 (approximately 65 suns) [3]. They found a non-linear response to light intensity over this range, but did not reach saturation or propose any type of relationship. Tabata, Ohnishi, Yagasaki, Ippommatsu, & Domen [4] conducted a study using a
* Corresponding author. Tel.: þ61 7 3138 2297. E-mail addresses: [email protected] (S. Bell), [email protected] (G. Will). 1 þ61 423345183. 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.02.147
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 ) 6 9 3 8 e6 9 4 7
List of nomenclature AM1.5 P25 QE TiO2 UV I j
air mass 1.5 grade of TiO2 quantum efficiency titanium dioxide ultraviolet light intensity, mW cm2 photocurrent density, A cm2
suspension of K4Nb6O17 as the photocatalyst, a Xe lamp for low intensity tests (<0.1 mW/cm2 UV) and Hg lamp for high intensity tests (1e100 mW/cm2 UV, or approximately 16 times the UV power in the AM1.5 spectrum). They found that the hydrogen evolution rate was proportional to I0.92 at low intensity, and proportional to I0.52 at high intensity. They proposed that a linear relationship existed at low intensities before recombination became dominant at high intensities and resulted in a half order relationship. A more recent study tested TiO2 nanotube array photocatalysts under two different light conditions; an AM1.5 solar simulator (1 sun) and a UV source with a power of 98 mW/cm2 (approximately 16 times the AM1.5 UV power) [5]. The photocurrent reported under the high intensity UV source is over 26 times greater than attained using the AM1.5 source. Whilst this investigation did not investigate the effect of light intensity further, its’ findings imply that photocatalytic water splitting can be undertaken under light intensity conditions significantly greater than one sun with little or no reduction in efficiency. In fact, this result is much higher than expected if a linear relationship existed and contrary to previous studies in the area. However, this does suggest that increasing light intensity may be a solution for large scale systems. More work has been undertaken into light intensity’s effect on photocatalytic degradation reactions. Nogueira & Jardim [6], Huang et al. [7], and Jiang, Zhao, Jia, Cao, & John [8], all reported linear responses to light intensity at irradiations levels up to 1 sun for water decontamination of various pollutants in different configurations. A study by Lim, Jeong, Kim, & Gyenis [9], into the decomposition of NO by TiO2 in flowing gas, described the relationship between reaction rate and light intensity in two regimes; first order at low intensities and half order at higher intensities. This finding agrees with that of Tabata et al. for water splitting reactions [4]. The linear regime at low light intensities in these findings was attributed to photogenerated electronehole pairs being consumed by chemical reactions faster than they can recombine. As the intensity is increased however, so too does the density of the photogenerated charges in the material impelling the recombination of electronehole pairs to become dominant and causing the half-order regime. Another explanation for this regime shift could be that the charge transfer rate from the photocatalyst to the electrolyte becomes limiting. This could be due to insufficient mass transfer through the electrolyte resulting in insufficient reactants at the electrolyte/photocatalyst interface [10]. This again would result in higher recombination and non-linearity with increasing light intensity.
h c hv n N l h
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Planck’s constant, J s speed of light in vacuum, m s1 photon energy, J number of tests number of photons wavelength, nm efficiency, %
In this study we investigate the effect of light intensity on the photocatalytic water splitting reaction using a Dugussa P25 TiO2 photocatalyst. The experimental apparatus and methods developed for this investigation are described, the details and results of our experiments are presented and their implications discussed.
2.
Apparatus
This investigation required the development of an experimental apparatus and testing protocol to obtain repeatable results under varying conditions. The key components of this apparatus were: - a light source and measurement apparatus - a reactor which allows light to illuminate the photocatalyst; and - a reliable, reasonably active and easily replicated photocatalyst with which to carry out the experiments. Measurements were carried out using the experimental setup described in Fig. 1. The solar simulator consisted of a 150 W Ozone free Xenon Lamp supplied by Oriel Instruments (model No. 6255) in a solar simulator housing with collimated output (Oriel model: 96000) followed by an Oriel 59060 Band pass filter (transmission range 300e800 nm). The spectrum of the light obtained from this device before and after the filter is depicted in Fig. 2. The light intensity was controlled by using the lens inbuilt in the housing to focus or defocus the light as required, or by moving the cell along the focal axis. The highly focussed light retained an image of the lamp, which limited the maximum intensity of the light. The light was focussed above a locating tray, into which could be loaded either a power meter (Newport 1918-C with Newport 818P-015-19 thermopile detector) or the test cell. This locating tray allowed movement in 3 dimensions and ensured that the power meter and the cell were situated in identical positions for accurate and consistent measurement of cell illumination (Fig. 1). A Keithley 236 sourcemeasure unit was used to control the voltage and measure the current produced by the cell. A thin sheet of metal was placed in front of the cell to shield it during dark measurements. A photocatalytic cell consisting of the photoanode and counter electrode separated by a rubber O-Ring and clamped together (depicted in Fig. 3) was the second key component required for this investigation. The cell was formed by placing a rubber O-ring between a fresh photoanode and a fresh counter electrode then clamping the system together with binder clips.
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Fig. 1 e Experimental apparatus under operation. Solar simulator illuminates either a thermopile intensity meter or the test cell under identical conditions. Measurements are recorded with a Keithley 236 Source-Measure unit and plotted as IV curves. Addition of heater and thermocouple for temperature based testing (further work).
Fig. 2 e Xenon Arc Lamp spectrum with (red) and without (blue) the filter, compared to the AM1.5 spectrum (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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 ) 6 9 3 8 e6 9 4 7
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where: IUV is the UV power of the solar simulator, IP is the intensity measured by the power meter, IAM1.5 is the UV power of the AM1.5 spectrum (6.15 mW/cm2), Isun equiv. is the power incident on the sample in suns equivalent to the AM1.5 spectrum. This calibration of the pyrometer, in conjunction with the locating tray, ensured accurate measurement of the light intensity illuminating the test cell. This allowed consistent and reliable testing of the effect of light intensity on photocatalytic water splitting. Fig. 3 e Layout of the O-Ring test system, disassembled and assembled. Assembly involves clamping photoanode and platinised cathode together with and O-Ring separator and injecting electrolyte through the O-Ring into the reaction chamber.
Electrolyte was injected through the o-ring into the reaction space with a needle and syringe. A second needle was used to allow trapped air to escape. These components were matched, ascribed a cell number and only tested together to ensure that all tests were comparable. The cell was back illuminated from the photoanode side through the glass, and masked with a 6.25 mm diameter aperture e resulting in an illuminated area of 0.307 cm2. Before assembly electrical contacts were soldered onto the conducting surface with an ultrasonic soldering iron and low temperature solder. With the use of ultrasonic soldered contacts this cell arrangement allowed for simple assembly and disassembly, and reliable electrical connection and performance at the conditions under which testing took place. The cell was situated approximately 15 cm from the housing light tube depending on the light intensity required. The UV power of the light was determined by using potassium ferrioxalate actinometry described by Murov, Carmichael, & Hug [11]. The light absorbance at 510 nm of 10 aliquots of irradiated solution over the range of achievable light intensity was compared to an unilluminated blank. As the absorbance properties of the potassium ferrioxalate solution and the spectrum of the light is known the flux of the light absorbed at each wavelength can be found. This information was used to calibrate the pyrometer which was more practical for power measurement during the experimentation process. This was achieved by plotting that actinometry data against the intensities measured by the power meter (Fig. 4). A non-linear relationship existed between the intensity measured by the power meter and that measured by actinometry. For ease of interpretation light intensity is expressed in suns. As the photocatalyst used only absorbs UV light, the power in the UV portion of the AM1.5 spectrum (6.15 mW/cm2) was used as a reference, from which the equivalent suns delivered by the system could be calculated. This allowed us to develop a method for calculating the UV equivalent suns of our incident light, using the power meter measurement: IUV ¼ 1:80 104 IP 2 þ 0:268IP
(1)
And: Isun
equiv:
¼
IUV IAM1:5
(2)
3.
Method
The TiO2 used in this investigation was P25 from Degussa. It was deposited on FTO conducting glass (obtained from Dyesol) by the following method. Raw P25 was ground in a mortar and pestle and added to methanol to form a suspension of 0.231 g per ml. This suspension was then sonicated for 30 min. Deposition was by doctor blading [12] onto cleaned (detergent, acetone, de-mineralised water then air dried) FTO glass. After the methanol had evaporated the films were placed in a furnace and calcined for 4 h at 450 C. Electrical contacts were attached to the electrode using ultrasonic soldering. The films obtained from this process had even coverage, a good adherence to the glass surface and a film thickness of approximately 10e12 microns. After multiple testing and rinsing there was no visible damage or material removed from the film. Field emission scanning electron microscopy (FE SEM) images of two films manufactured in different batches show minor differences between the films; mainly in the agglomeration of the P25 particles. This is shown by the difference between the films in Fig. 5a and b, where the particles are more densely packed. Fig. 5c and d also show this greater agglomeration in the second film. This agglomeration disparity is probably due to a slight difference in oven temperature, resulting in more melting of the particles together. This leads to lower surface area, but better electrical conductivity in the film. Whilst the film production process was kept as consistent as possible, slight differences between films are unavoidable and subsequent differences in film performance will occur. Counter electrodes were produced by submerging FTO glass in chloroplatinic acid (2.1 103 M concentration) with a silver chloride reference electrode and a platinum counter electrode. The voltage was scanned from 0 V to 0.8 V then held at 0.8 V for 30 s for electrodeposition. This process was repeated 5 times per electrode. Finally ultrasonic soldering was used to attach electrical contacts to the electrode. The cell was assembled using a 0.1 M Na2SO4 solution as electrolyte and placed in the locating tray. The power meter and lens inside the lamp housing were then used to adjust the light intensity to the desired level. The intensity was recorded and the power meter replaced with the cell assembly. The Keithley 236 source-measure unit was attached and an IV scan was performed between 0 and 1 V at a rate of 20 mV s1. After the scan, the cell was disassembled, rinsed with deionised water and re-assembled with new electrolyte for the next test.
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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 ) 6 9 3 8 e6 9 4 7
Fig. 4 e Comparison of <430 nm light power using the power meter and actinometry results at various light intensities. The fitted curve was used for calibration of power meter from the actinometry data.
The intensity of the light was initially zero (a dark curve) and increased with each subsequent test until the maximum light intensity was attained. A second dark curve was conducted to conclude the testing. The same TiO2 photoanode and
platinum counter electrode were used for each light intensity test and a 6.25 mm aperture was used for all experiments. This set of tests was undertaken 3 times, each time with a new photoanode and counter electrode.
Fig. 5 e FE SEM images of 2 TiO2 films produced using the same process; a) 250 0003 magnification of film 1, b) 250 0003 magnification of film 2, c) 80 0003 magnification of film 1, d) 80 0003 magnification of film 2 (JEOL 7100, operator e Eunice Grinan).
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 ) 6 9 3 8 e6 9 4 7
The stability of these TiO2 films was found experimentally. They were repeatedly tested under high light intensity conditions (approx. 28 suns UV Equivalent) at room temperature. The cells were disassembled, rinsed and reassembled between every test to ensure that changes in the electrolyte did not affect the measured performance. These tests were repeated 12e15 times under illumination, with dark current tests before and after the illuminated tests. The scans were undertaken from 0 to 1.0 V at a scan rate of 20 mV s1. The currents obtained at 0.5 V are presented in Fig. 6. Fig. 6 shows that each cell’s performance degraded consistently over the range of these tests and there is no indication that it will stabilise. The data was fitted with a linear function and the rate of degradation per test calculated from this. Cells 1, 2 and 3 lost 1.19%, 1.25% and 1.30% performance per test respectively, giving a mean degradation rate of approximately 1.25% per test. Equation (3) was used to account for this degradation in cell performance: ji ¼ jf ð1 þ rd Þn1
(3)
where ji and jf are the initial and final photocurrent densities, rd is the rate of degradation per test and n is the number of times the film has been tested (under light). This degradation rate allows results of the intensity tests to be corrected for the number of times the film has been tested. These variations are probably due to slight differences between films e from solution preparation, doctor blading and calcinations processes e leading to minor disparities in film thickness, morphology and electronic characteristics. For instance, the difference in particle agglomeration between the 2 films shown in Fig. 5 will lead to a lower surface area, but greater electronic conductivity in the more highly agglomerated film. Both of these factors are known to affect photocatalyst performance.
4.
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Results
This section presents the results of the light intensity experiments. The experimental results consist of IV curves conducted over a range of light power. The mean of the curves for all of the tested cells with standard deviation error is displayed in Fig. 7. Fig. 7 shows that as light intensity increases, the current also increases. It also shows that above about 0.5 V of applied voltage, the current saturates. However, the voltage at which this saturation occurs is higher as light intensity increases. There was a significant difference between the performances of each cell which are indicated by the error bars. These disparities are attributed to differences in the film preparation, deposition and calcination processes. The data presented in Fig. 8 has been converted into photocatalytic current density (A/cm2) by subtracting the dark current from the light current and dividing by the illuminated area (0.307 cm2). This graph shows the data at 0.5 V bias. The response at greater than 0.5 V bias is reasonably flat and plots taken at biases above 0.5 V produced similar results, therefore the 0.5 V biased system was considered to sufficiently represent the system for this investigation. There appears to be two different regimes with increasing light intensity. Below approximately 5 suns the slope of the data is higher than above 5 suns. Quantum efficiency was calculated by comparing the number of electrons in the photocurrent to the photon flux of the incident light; via Equation (4): QE ¼
jphoto $c N_ photo
(4)
where: Jphoto ¼ the photocurrent in A cm2, C is the number of electrons in 1 coulomb (6.24 1018), N_ photon is the photon flux
Fig. 6 e Reduction in Performance of 3 films from repeated testing (0.5 V Bias) and associated fitted curves. Used to predict the degradation in performance with repeated testing.
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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 ) 6 9 3 8 e6 9 4 7
Fig. 7 e Mean Light intensity test IV curves (0.307 cm2 aperture) with Standard Deviation error at each light intensity tested. s1 cm2 of the incident light with a wavelength below 430 nm. This was found by dividing the UV power of the incident light (IUV) by the average energy of an incident photon with a wavelength between 290 nm and 430 nm (hv): IUV N_ photon ¼ hv
(5)
where (hv) is obtained via the summation: hv ¼
290 nm X 430 nm
hc hvðlÞ $ l hv290 430
(6)
where: h ¼ Planck’s constant (6.63 1034 J s1), c is the speed of light in a vacuum (3.00 108 m s1), hv(l) is the number of photons at a specific wavelength of the incident light, hv290 430 is the total number of photons of the incident light between 290 nm and 430 nm. As the quantum efficiency is calculated for UV light only, it does not take into consideration visible and infrared radiation. The quantum efficiency results are presented in Fig. 9. This figure shows that as light intensity increases quantum efficiency drops, before stabilising at approximately 1%. This is about ¼ of the quantum efficiency value acquired under low light intensity.
Fig. 8 e Photocurrents vs. light intensity for the 3 films at 0.5 V applied bias. Mean and standard deviation shown.
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 ) 6 9 3 8 e6 9 4 7
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Fig. 9 e Quantum efficiency of intensity experiments at 0.5 V applied bias (at wavelengths below 430 nm). Mean and standard deviation shown.
The quantum efficiency data shows a steady decrease in the ratio of photons which produce a reaction as light intensity increases. However, above approximately 10e20 suns, the relationship approaches a steady value with intensity. This quantum efficiency data illustrates the non-linearity of the intensity relationship, but suggests as intensity increases the relationship approaches linearity.
5.
Discussion
In order to determine the relationship between light intensity and photocurrent Fig. 8 was modified into a logelog graph and
presented in Fig. 10. The theoretical model developed by Tabata et al. [4] suggested a half order relationship at high light intensity and a logelog plot allowed us to evaluate this relationship in our data. It was found that the results approximated linearity with an R2 value of 0.9687. A linear plot on a logelog graph produces an equation in the following form: x ¼ ðxm Þ 10b
(7)
where m is the slope and b is the intercept. Least squares fit of our data gave a slope of 0.627 and an intercept of 4.14; resulting in Equation (8): jphoto ¼ 7:24 105 $Isun equiv: 0:627
Fig. 10 e LogeLog plot of photocurrents at various intensities for 3 films at 0.5 V applied bias with fitted curve.
(8)
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Fig. 11 e Experimental results compared to those found by Carey and Oliver [3], with fitted curves shown.
The high accuracy of the fit of the equation with the experimental data indicates that photocurrent is proportional to I0.627 for these films (i.e. jA fI0:627 ). A plot of the fitted equation against linear intensity has been included in Fig. 11. This relationship is similar to that reported in the literature. Tabata et al. [4] found a relationship proportional to I0.52 at high light intensities for water splitting. Also, Lim et al. [9] reported an exponential value of 0.47 for NO decomposition. The exponential value of 0.627 obtained by our study is slightly higher than those reported values. The experimental data from this study is compared to that of Carey and Oliver [3], in Fig. 11. The data presented by Carey and Oliver [3] using a titania photocatalyst displays a similar relationship with intensity to our data. The Carey and Oliver data is collected at a bias of 0 V and compared to our data at an applied bias of 0.5 V in Fig. 11. This is due to our cell being a 2 electrode system (i.e. no reference electrode), whilst the other tests used an Ag/AgCl reference electrode resulting in a 0.5 V potential difference in switch on voltages of the films. The line fitted to the Carey & Oliver [3] data on a logelog graph has a slope of 0.395, an intercept of 3.442 and an R2 value of 0.9985, or: jphoto ¼ 3:63 104 $Isun equiv: 0:395
(9)
The exponential value of 0.395 is slightly below those found in more recent studies. Sub-linear relationships are generally attributed to either recombination of photogenerated charges, or reactant mass transfer limitations. Either of these explanations could apply to our system. It must be noted however; that as our reactant is water (with a concentration of 55.56 M) the mass transfer rate will be much higher than in degradation reactions where the reactant concentration is low. Thus, this sub-linearity is probably due to increased recombination of charges at high intensities.
The spread of the three exponential terms (0.395, 0.52 and 0.627 found by Carey and Oliver [3], Tabata et al. [4] and this work respectively) raises some questions. Is this exponential term in some way related to the materials used? Could it be affected by particle size? As thin film technology has progressed in its ability to produce films with particle sizes approaching the diffusion length of the material, the exponent appears to increase. This exponent may be directly related to charge recombination in the photocatalyst and independent of performance. If so then it could be an important factor in determining the photocatalyst’s ability to use generated charges effectively. If the particle size is reduced further, it may result in reduced recombination, as the distance which charges have to migrate to perform reactions approaches the diffusion length of the material. This could result in an increase in the exponent of the light intensity relationship, pushing the relationship towards linearity. Investigating this relationship should be a major focus of future work in this area.
6.
Conclusion
As the effect of light intensity on photocatalytic water splitting is not well understood, this study forms a valuable contribution to the area. Light intensity is an important parameter for system design and this study shows that light intensities up to 52 suns does not saturate the photocatalyst. This makes high intensity solar water splitting a plausible proposition. This is an important finding as it could inform direct development strategies towards active photocatalysts at high light intensities, a direction which will be essential for the commercialisation of this technology.
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 ) 6 9 3 8 e6 9 4 7
This presents a challenge for researchers, to better understand the mechanisms which affect the relationship between photocatalytic water splitting and light intensity, and apply that knowledge to improving photocatalyst performance under high light intensity.
Acknowledgements The authors wish to acknowledge Queensland University of Technology and the Queensland Government for their financial and equipment support of this work.
references
[1] Gra¨tzel M. Photoelectrochemical cells. Nature 2001;414:338e44. [2] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37e8. [3] Carey JH, Oliver BG. Intensity effects in the electrochemical photolysis of water at the titanium dioxide electrode. Nature 1976;259(5544):554e6. [4] Tabata S, Ohnishi H, Yagasaki E, Ippommatsu M, Domen K. Light-intensity dependence in photocatalytic decomposition of water over K4Nb6O17 catalyst. Catalysis Letters 1994;28(2):417e22.
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[5] Ruan C, Paulose M, Varghese OK, Grimes CA. Enhanced photoelectrochemical-response in highly ordered TiO2 nanotube-arrays anodized in boric acid containing electrolyte. Solar Energy Materials and Solar Cells 2006;90(9):1283e95. [6] Nogueira RFP, Jardim WF. TiO2-fixed-bed reactor for water decontamination using solar light. Solar Energy 1996;56(5):471e7. [7] Huang A, Cao L, Chen J, Spiess F-J, Suib SL, Obee TN, et al. Photocatalytic degradation of triethylamine on titanium oxide thin films. Journal of Catalysis 1999;188(1):40e7. [8] Jiang D, Zhao H, Jia Z, Cao J, John R. Photoelectrochemical behaviour of methanol oxidation at nanoporous TiO2 film electrodes. Journal of Photochemistry and Photobiology A: Chemistry 2001;144(2e3):197e204. [9] Lim TH, Jeong SM, Kim SD, Gyenis J. Photocatalytic decomposition of NO by TiO2 particles. Journal of Photochemistry and Photobiology A: Chemistry 2000;134(3):209e17. [10] Meng Y, Huang X, Wu Y, Wang X, Qian Y. Kinetic study and modeling on photocatalytic degradation of parachlorobenzoate at different light intensities. Environmental Pollution 2002;117(2):307e13. [11] Murov SL, Carmichael I, Hug GL. Handbook of photochemistry. 2nd ed. New York: M. Dekker; 1993. [12] Nazeeruddin MK, Kay A, Rodicio I, Humphry-Baker R, Mueller E, Liska P, et al. Conversion of light to electricity by cis-X2bis(2,20 -bipyridyl-4,40 -dicarboxylate)ruthenium(II) charge-transfer sensitizers (X ¼ Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes. Journal of the American Chemical Society 1993;115(14):6382e90.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Light intensity effects on photocatalytic water splitting with a titania catalyst Stuart Bell 1, Geoffrey Will*, John Bell Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia
article info
abstract
Article history:
Photocatalytic water splitting is a process which could potentially lead to commercially
Received 8 October 2012
viable solar hydrogen production. In order to evaluate if solar concentration could be used
Received in revised form
to increase the feasibility of the process, the effect of light intensity on photocatalytic water
18 January 2013
splitting was examined.
Accepted 23 February 2013 Available online 24 April 2013
Degussa P25 TiO2 films were used to form a photocatalytic cell and illuminated with a Xenon arc lamp at intensities up to 52 suns. The reaction demonstrated a sub-linear relationship where photocurrent was proportional to intensity with an exponential value
Keywords: Photocatalysis
of 0.627. This is an important finding for photocatalytic water splitting. It could provide an
Water splitting
avenue for the development of a large scale photocatalytic water splitting system for
Solar hydrogen
hydrogen production and the further commercialisation of this technology.
Hydrogen generation
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Titanium dioxide
reserved.
Light intensity
1.
Introduction
Energy has been a major political, economic and environmental issue throughout the 20th century and will become the number one issue of the 21st century. The amount of energy that falls on the earth in sunlight is 3 1024 joules per year, or 10 000 times that of the world’s usage [1] making large scale conversion of solar energy to a usable form a potential solution to future energy needs. Photocatalytic water splitting is a technology that could produce renewable hydrogen using solar energy and water as inputs. Since the initial breakthrough demonstration in 1972 by Fujishima and Honda [2] much research has been devoted to developing materials for this process. Very little focus however, has been devoted to the systems aspect of photocatalytic water splitting. This is important as the optimal
conditions for water splitting may be very different to the conditions for which materials are currently being developed. Light intensity is an important parameter when considering using concentrated light in photocatalytic water splitting systems. The authors consider concentrated light systems to be promising due to the small amounts of hydrogen per unit area produced by one sun systems and the difficulties in capturing and containing the gas in such systems. To date there has been minimal research conducted into the effect of light intensity on photocatalytic water splitting reactions. In 1976 Carey & Oliver used an argon laser to illuminate their cell with UV light up to 400 mW/cm2 (approximately 65 suns) [3]. They found a non-linear response to light intensity over this range, but did not reach saturation or propose any type of relationship. Tabata, Ohnishi, Yagasaki, Ippommatsu, & Domen [4] conducted a study using a
* Corresponding author. Tel.: þ61 7 3138 2297. E-mail addresses: [email protected] (S. Bell), [email protected] (G. Will). 1 þ61 423345183. 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.02.147
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 ) 6 9 3 8 e6 9 4 7
List of nomenclature AM1.5 P25 QE TiO2 UV I j
air mass 1.5 grade of TiO2 quantum efficiency titanium dioxide ultraviolet light intensity, mW cm2 photocurrent density, A cm2
suspension of K4Nb6O17 as the photocatalyst, a Xe lamp for low intensity tests (<0.1 mW/cm2 UV) and Hg lamp for high intensity tests (1e100 mW/cm2 UV, or approximately 16 times the UV power in the AM1.5 spectrum). They found that the hydrogen evolution rate was proportional to I0.92 at low intensity, and proportional to I0.52 at high intensity. They proposed that a linear relationship existed at low intensities before recombination became dominant at high intensities and resulted in a half order relationship. A more recent study tested TiO2 nanotube array photocatalysts under two different light conditions; an AM1.5 solar simulator (1 sun) and a UV source with a power of 98 mW/cm2 (approximately 16 times the AM1.5 UV power) [5]. The photocurrent reported under the high intensity UV source is over 26 times greater than attained using the AM1.5 source. Whilst this investigation did not investigate the effect of light intensity further, its’ findings imply that photocatalytic water splitting can be undertaken under light intensity conditions significantly greater than one sun with little or no reduction in efficiency. In fact, this result is much higher than expected if a linear relationship existed and contrary to previous studies in the area. However, this does suggest that increasing light intensity may be a solution for large scale systems. More work has been undertaken into light intensity’s effect on photocatalytic degradation reactions. Nogueira & Jardim [6], Huang et al. [7], and Jiang, Zhao, Jia, Cao, & John [8], all reported linear responses to light intensity at irradiations levels up to 1 sun for water decontamination of various pollutants in different configurations. A study by Lim, Jeong, Kim, & Gyenis [9], into the decomposition of NO by TiO2 in flowing gas, described the relationship between reaction rate and light intensity in two regimes; first order at low intensities and half order at higher intensities. This finding agrees with that of Tabata et al. for water splitting reactions [4]. The linear regime at low light intensities in these findings was attributed to photogenerated electronehole pairs being consumed by chemical reactions faster than they can recombine. As the intensity is increased however, so too does the density of the photogenerated charges in the material impelling the recombination of electronehole pairs to become dominant and causing the half-order regime. Another explanation for this regime shift could be that the charge transfer rate from the photocatalyst to the electrolyte becomes limiting. This could be due to insufficient mass transfer through the electrolyte resulting in insufficient reactants at the electrolyte/photocatalyst interface [10]. This again would result in higher recombination and non-linearity with increasing light intensity.
h c hv n N l h
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Planck’s constant, J s speed of light in vacuum, m s1 photon energy, J number of tests number of photons wavelength, nm efficiency, %
In this study we investigate the effect of light intensity on the photocatalytic water splitting reaction using a Dugussa P25 TiO2 photocatalyst. The experimental apparatus and methods developed for this investigation are described, the details and results of our experiments are presented and their implications discussed.
2.
Apparatus
This investigation required the development of an experimental apparatus and testing protocol to obtain repeatable results under varying conditions. The key components of this apparatus were: - a light source and measurement apparatus - a reactor which allows light to illuminate the photocatalyst; and - a reliable, reasonably active and easily replicated photocatalyst with which to carry out the experiments. Measurements were carried out using the experimental setup described in Fig. 1. The solar simulator consisted of a 150 W Ozone free Xenon Lamp supplied by Oriel Instruments (model No. 6255) in a solar simulator housing with collimated output (Oriel model: 96000) followed by an Oriel 59060 Band pass filter (transmission range 300e800 nm). The spectrum of the light obtained from this device before and after the filter is depicted in Fig. 2. The light intensity was controlled by using the lens inbuilt in the housing to focus or defocus the light as required, or by moving the cell along the focal axis. The highly focussed light retained an image of the lamp, which limited the maximum intensity of the light. The light was focussed above a locating tray, into which could be loaded either a power meter (Newport 1918-C with Newport 818P-015-19 thermopile detector) or the test cell. This locating tray allowed movement in 3 dimensions and ensured that the power meter and the cell were situated in identical positions for accurate and consistent measurement of cell illumination (Fig. 1). A Keithley 236 sourcemeasure unit was used to control the voltage and measure the current produced by the cell. A thin sheet of metal was placed in front of the cell to shield it during dark measurements. A photocatalytic cell consisting of the photoanode and counter electrode separated by a rubber O-Ring and clamped together (depicted in Fig. 3) was the second key component required for this investigation. The cell was formed by placing a rubber O-ring between a fresh photoanode and a fresh counter electrode then clamping the system together with binder clips.
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Fig. 1 e Experimental apparatus under operation. Solar simulator illuminates either a thermopile intensity meter or the test cell under identical conditions. Measurements are recorded with a Keithley 236 Source-Measure unit and plotted as IV curves. Addition of heater and thermocouple for temperature based testing (further work).
Fig. 2 e Xenon Arc Lamp spectrum with (red) and without (blue) the filter, compared to the AM1.5 spectrum (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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where: IUV is the UV power of the solar simulator, IP is the intensity measured by the power meter, IAM1.5 is the UV power of the AM1.5 spectrum (6.15 mW/cm2), Isun equiv. is the power incident on the sample in suns equivalent to the AM1.5 spectrum. This calibration of the pyrometer, in conjunction with the locating tray, ensured accurate measurement of the light intensity illuminating the test cell. This allowed consistent and reliable testing of the effect of light intensity on photocatalytic water splitting. Fig. 3 e Layout of the O-Ring test system, disassembled and assembled. Assembly involves clamping photoanode and platinised cathode together with and O-Ring separator and injecting electrolyte through the O-Ring into the reaction chamber.
Electrolyte was injected through the o-ring into the reaction space with a needle and syringe. A second needle was used to allow trapped air to escape. These components were matched, ascribed a cell number and only tested together to ensure that all tests were comparable. The cell was back illuminated from the photoanode side through the glass, and masked with a 6.25 mm diameter aperture e resulting in an illuminated area of 0.307 cm2. Before assembly electrical contacts were soldered onto the conducting surface with an ultrasonic soldering iron and low temperature solder. With the use of ultrasonic soldered contacts this cell arrangement allowed for simple assembly and disassembly, and reliable electrical connection and performance at the conditions under which testing took place. The cell was situated approximately 15 cm from the housing light tube depending on the light intensity required. The UV power of the light was determined by using potassium ferrioxalate actinometry described by Murov, Carmichael, & Hug [11]. The light absorbance at 510 nm of 10 aliquots of irradiated solution over the range of achievable light intensity was compared to an unilluminated blank. As the absorbance properties of the potassium ferrioxalate solution and the spectrum of the light is known the flux of the light absorbed at each wavelength can be found. This information was used to calibrate the pyrometer which was more practical for power measurement during the experimentation process. This was achieved by plotting that actinometry data against the intensities measured by the power meter (Fig. 4). A non-linear relationship existed between the intensity measured by the power meter and that measured by actinometry. For ease of interpretation light intensity is expressed in suns. As the photocatalyst used only absorbs UV light, the power in the UV portion of the AM1.5 spectrum (6.15 mW/cm2) was used as a reference, from which the equivalent suns delivered by the system could be calculated. This allowed us to develop a method for calculating the UV equivalent suns of our incident light, using the power meter measurement: IUV ¼ 1:80 104 IP 2 þ 0:268IP
(1)
And: Isun
equiv:
¼
IUV IAM1:5
(2)
3.
Method
The TiO2 used in this investigation was P25 from Degussa. It was deposited on FTO conducting glass (obtained from Dyesol) by the following method. Raw P25 was ground in a mortar and pestle and added to methanol to form a suspension of 0.231 g per ml. This suspension was then sonicated for 30 min. Deposition was by doctor blading [12] onto cleaned (detergent, acetone, de-mineralised water then air dried) FTO glass. After the methanol had evaporated the films were placed in a furnace and calcined for 4 h at 450 C. Electrical contacts were attached to the electrode using ultrasonic soldering. The films obtained from this process had even coverage, a good adherence to the glass surface and a film thickness of approximately 10e12 microns. After multiple testing and rinsing there was no visible damage or material removed from the film. Field emission scanning electron microscopy (FE SEM) images of two films manufactured in different batches show minor differences between the films; mainly in the agglomeration of the P25 particles. This is shown by the difference between the films in Fig. 5a and b, where the particles are more densely packed. Fig. 5c and d also show this greater agglomeration in the second film. This agglomeration disparity is probably due to a slight difference in oven temperature, resulting in more melting of the particles together. This leads to lower surface area, but better electrical conductivity in the film. Whilst the film production process was kept as consistent as possible, slight differences between films are unavoidable and subsequent differences in film performance will occur. Counter electrodes were produced by submerging FTO glass in chloroplatinic acid (2.1 103 M concentration) with a silver chloride reference electrode and a platinum counter electrode. The voltage was scanned from 0 V to 0.8 V then held at 0.8 V for 30 s for electrodeposition. This process was repeated 5 times per electrode. Finally ultrasonic soldering was used to attach electrical contacts to the electrode. The cell was assembled using a 0.1 M Na2SO4 solution as electrolyte and placed in the locating tray. The power meter and lens inside the lamp housing were then used to adjust the light intensity to the desired level. The intensity was recorded and the power meter replaced with the cell assembly. The Keithley 236 source-measure unit was attached and an IV scan was performed between 0 and 1 V at a rate of 20 mV s1. After the scan, the cell was disassembled, rinsed with deionised water and re-assembled with new electrolyte for the next test.
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Fig. 4 e Comparison of <430 nm light power using the power meter and actinometry results at various light intensities. The fitted curve was used for calibration of power meter from the actinometry data.
The intensity of the light was initially zero (a dark curve) and increased with each subsequent test until the maximum light intensity was attained. A second dark curve was conducted to conclude the testing. The same TiO2 photoanode and
platinum counter electrode were used for each light intensity test and a 6.25 mm aperture was used for all experiments. This set of tests was undertaken 3 times, each time with a new photoanode and counter electrode.
Fig. 5 e FE SEM images of 2 TiO2 films produced using the same process; a) 250 0003 magnification of film 1, b) 250 0003 magnification of film 2, c) 80 0003 magnification of film 1, d) 80 0003 magnification of film 2 (JEOL 7100, operator e Eunice Grinan).
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The stability of these TiO2 films was found experimentally. They were repeatedly tested under high light intensity conditions (approx. 28 suns UV Equivalent) at room temperature. The cells were disassembled, rinsed and reassembled between every test to ensure that changes in the electrolyte did not affect the measured performance. These tests were repeated 12e15 times under illumination, with dark current tests before and after the illuminated tests. The scans were undertaken from 0 to 1.0 V at a scan rate of 20 mV s1. The currents obtained at 0.5 V are presented in Fig. 6. Fig. 6 shows that each cell’s performance degraded consistently over the range of these tests and there is no indication that it will stabilise. The data was fitted with a linear function and the rate of degradation per test calculated from this. Cells 1, 2 and 3 lost 1.19%, 1.25% and 1.30% performance per test respectively, giving a mean degradation rate of approximately 1.25% per test. Equation (3) was used to account for this degradation in cell performance: ji ¼ jf ð1 þ rd Þn1
(3)
where ji and jf are the initial and final photocurrent densities, rd is the rate of degradation per test and n is the number of times the film has been tested (under light). This degradation rate allows results of the intensity tests to be corrected for the number of times the film has been tested. These variations are probably due to slight differences between films e from solution preparation, doctor blading and calcinations processes e leading to minor disparities in film thickness, morphology and electronic characteristics. For instance, the difference in particle agglomeration between the 2 films shown in Fig. 5 will lead to a lower surface area, but greater electronic conductivity in the more highly agglomerated film. Both of these factors are known to affect photocatalyst performance.
4.
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Results
This section presents the results of the light intensity experiments. The experimental results consist of IV curves conducted over a range of light power. The mean of the curves for all of the tested cells with standard deviation error is displayed in Fig. 7. Fig. 7 shows that as light intensity increases, the current also increases. It also shows that above about 0.5 V of applied voltage, the current saturates. However, the voltage at which this saturation occurs is higher as light intensity increases. There was a significant difference between the performances of each cell which are indicated by the error bars. These disparities are attributed to differences in the film preparation, deposition and calcination processes. The data presented in Fig. 8 has been converted into photocatalytic current density (A/cm2) by subtracting the dark current from the light current and dividing by the illuminated area (0.307 cm2). This graph shows the data at 0.5 V bias. The response at greater than 0.5 V bias is reasonably flat and plots taken at biases above 0.5 V produced similar results, therefore the 0.5 V biased system was considered to sufficiently represent the system for this investigation. There appears to be two different regimes with increasing light intensity. Below approximately 5 suns the slope of the data is higher than above 5 suns. Quantum efficiency was calculated by comparing the number of electrons in the photocurrent to the photon flux of the incident light; via Equation (4): QE ¼
jphoto $c N_ photo
(4)
where: Jphoto ¼ the photocurrent in A cm2, C is the number of electrons in 1 coulomb (6.24 1018), N_ photon is the photon flux
Fig. 6 e Reduction in Performance of 3 films from repeated testing (0.5 V Bias) and associated fitted curves. Used to predict the degradation in performance with repeated testing.
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Fig. 7 e Mean Light intensity test IV curves (0.307 cm2 aperture) with Standard Deviation error at each light intensity tested. s1 cm2 of the incident light with a wavelength below 430 nm. This was found by dividing the UV power of the incident light (IUV) by the average energy of an incident photon with a wavelength between 290 nm and 430 nm (hv): IUV N_ photon ¼ hv
(5)
where (hv) is obtained via the summation: hv ¼
290 nm X 430 nm
hc hvðlÞ $ l hv290 430
(6)
where: h ¼ Planck’s constant (6.63 1034 J s1), c is the speed of light in a vacuum (3.00 108 m s1), hv(l) is the number of photons at a specific wavelength of the incident light, hv290 430 is the total number of photons of the incident light between 290 nm and 430 nm. As the quantum efficiency is calculated for UV light only, it does not take into consideration visible and infrared radiation. The quantum efficiency results are presented in Fig. 9. This figure shows that as light intensity increases quantum efficiency drops, before stabilising at approximately 1%. This is about ¼ of the quantum efficiency value acquired under low light intensity.
Fig. 8 e Photocurrents vs. light intensity for the 3 films at 0.5 V applied bias. Mean and standard deviation shown.
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Fig. 9 e Quantum efficiency of intensity experiments at 0.5 V applied bias (at wavelengths below 430 nm). Mean and standard deviation shown.
The quantum efficiency data shows a steady decrease in the ratio of photons which produce a reaction as light intensity increases. However, above approximately 10e20 suns, the relationship approaches a steady value with intensity. This quantum efficiency data illustrates the non-linearity of the intensity relationship, but suggests as intensity increases the relationship approaches linearity.
5.
Discussion
In order to determine the relationship between light intensity and photocurrent Fig. 8 was modified into a logelog graph and
presented in Fig. 10. The theoretical model developed by Tabata et al. [4] suggested a half order relationship at high light intensity and a logelog plot allowed us to evaluate this relationship in our data. It was found that the results approximated linearity with an R2 value of 0.9687. A linear plot on a logelog graph produces an equation in the following form: x ¼ ðxm Þ 10b
(7)
where m is the slope and b is the intercept. Least squares fit of our data gave a slope of 0.627 and an intercept of 4.14; resulting in Equation (8): jphoto ¼ 7:24 105 $Isun equiv: 0:627
Fig. 10 e LogeLog plot of photocurrents at various intensities for 3 films at 0.5 V applied bias with fitted curve.
(8)
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Fig. 11 e Experimental results compared to those found by Carey and Oliver [3], with fitted curves shown.
The high accuracy of the fit of the equation with the experimental data indicates that photocurrent is proportional to I0.627 for these films (i.e. jA fI0:627 ). A plot of the fitted equation against linear intensity has been included in Fig. 11. This relationship is similar to that reported in the literature. Tabata et al. [4] found a relationship proportional to I0.52 at high light intensities for water splitting. Also, Lim et al. [9] reported an exponential value of 0.47 for NO decomposition. The exponential value of 0.627 obtained by our study is slightly higher than those reported values. The experimental data from this study is compared to that of Carey and Oliver [3], in Fig. 11. The data presented by Carey and Oliver [3] using a titania photocatalyst displays a similar relationship with intensity to our data. The Carey and Oliver data is collected at a bias of 0 V and compared to our data at an applied bias of 0.5 V in Fig. 11. This is due to our cell being a 2 electrode system (i.e. no reference electrode), whilst the other tests used an Ag/AgCl reference electrode resulting in a 0.5 V potential difference in switch on voltages of the films. The line fitted to the Carey & Oliver [3] data on a logelog graph has a slope of 0.395, an intercept of 3.442 and an R2 value of 0.9985, or: jphoto ¼ 3:63 104 $Isun equiv: 0:395
(9)
The exponential value of 0.395 is slightly below those found in more recent studies. Sub-linear relationships are generally attributed to either recombination of photogenerated charges, or reactant mass transfer limitations. Either of these explanations could apply to our system. It must be noted however; that as our reactant is water (with a concentration of 55.56 M) the mass transfer rate will be much higher than in degradation reactions where the reactant concentration is low. Thus, this sub-linearity is probably due to increased recombination of charges at high intensities.
The spread of the three exponential terms (0.395, 0.52 and 0.627 found by Carey and Oliver [3], Tabata et al. [4] and this work respectively) raises some questions. Is this exponential term in some way related to the materials used? Could it be affected by particle size? As thin film technology has progressed in its ability to produce films with particle sizes approaching the diffusion length of the material, the exponent appears to increase. This exponent may be directly related to charge recombination in the photocatalyst and independent of performance. If so then it could be an important factor in determining the photocatalyst’s ability to use generated charges effectively. If the particle size is reduced further, it may result in reduced recombination, as the distance which charges have to migrate to perform reactions approaches the diffusion length of the material. This could result in an increase in the exponent of the light intensity relationship, pushing the relationship towards linearity. Investigating this relationship should be a major focus of future work in this area.
6.
Conclusion
As the effect of light intensity on photocatalytic water splitting is not well understood, this study forms a valuable contribution to the area. Light intensity is an important parameter for system design and this study shows that light intensities up to 52 suns does not saturate the photocatalyst. This makes high intensity solar water splitting a plausible proposition. This is an important finding as it could inform direct development strategies towards active photocatalysts at high light intensities, a direction which will be essential for the commercialisation of this technology.
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This presents a challenge for researchers, to better understand the mechanisms which affect the relationship between photocatalytic water splitting and light intensity, and apply that knowledge to improving photocatalyst performance under high light intensity.
Acknowledgements The authors wish to acknowledge Queensland University of Technology and the Queensland Government for their financial and equipment support of this work.
references
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[5] Ruan C, Paulose M, Varghese OK, Grimes CA. Enhanced photoelectrochemical-response in highly ordered TiO2 nanotube-arrays anodized in boric acid containing electrolyte. Solar Energy Materials and Solar Cells 2006;90(9):1283e95. [6] Nogueira RFP, Jardim WF. TiO2-fixed-bed reactor for water decontamination using solar light. Solar Energy 1996;56(5):471e7. [7] Huang A, Cao L, Chen J, Spiess F-J, Suib SL, Obee TN, et al. Photocatalytic degradation of triethylamine on titanium oxide thin films. Journal of Catalysis 1999;188(1):40e7. [8] Jiang D, Zhao H, Jia Z, Cao J, John R. Photoelectrochemical behaviour of methanol oxidation at nanoporous TiO2 film electrodes. Journal of Photochemistry and Photobiology A: Chemistry 2001;144(2e3):197e204. [9] Lim TH, Jeong SM, Kim SD, Gyenis J. Photocatalytic decomposition of NO by TiO2 particles. Journal of Photochemistry and Photobiology A: Chemistry 2000;134(3):209e17. [10] Meng Y, Huang X, Wu Y, Wang X, Qian Y. Kinetic study and modeling on photocatalytic degradation of parachlorobenzoate at different light intensities. Environmental Pollution 2002;117(2):307e13. [11] Murov SL, Carmichael I, Hug GL. Handbook of photochemistry. 2nd ed. New York: M. Dekker; 1993. [12] Nazeeruddin MK, Kay A, Rodicio I, Humphry-Baker R, Mueller E, Liska P, et al. Conversion of light to electricity by cis-X2bis(2,20 -bipyridyl-4,40 -dicarboxylate)ruthenium(II) charge-transfer sensitizers (X ¼ Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes. Journal of the American Chemical Society 1993;115(14):6382e90.