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Does carbon doping of TiO2 allow water splitting in visible light? Comments on “Nanotube enhanced photoresponse of carbon modified (CM)-n-TiO2 for efficient water splitting”
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 92 (2008) 363–367 www.elsevier.com/locate/solmat
Letter
Does carbon doping of TiO2 allow water splitting in visible light? Comments on ‘‘Nanotube enhanced photoresponse of carbon modified (CM)-n-TiO2 for efficient water splitting’’ A.B. Murphy CSIRO Industrial Physics, P.O. Box 218, Lindfield, NSW 2070, Australia Received 24 July 2007; received in revised form 25 October 2007; accepted 25 October 2007 Available online 4 December 2007
Abstract Reports of water splitting by carbon-doped titanium dioxide (TiO2) photoelectrodes under visible illumination are critically examined. Xu et al. [Sol. Energy Mater. Sol. Cells 91 (2007) 938] recently reported significant incident photon conversion efficiencies (IPCEs) at visible wavelengths for carbon-doped TiO2 in thin film and nanotube form. Evidence is given here that these results were due to an artefact in the measurements. Further, it is pointed out that the mechanism proposed for water splitting under visible illumination is unphysical, and the photocurrents presented are shown to be grossly inconsistent with the IPCE data. Other workers have also measured non-zero IPCEs at visible wavelengths for carbon-doped TiO2, but have not presented this as evidence of water splitting. In other cases, carbon doping was performed in a reducing atmosphere, and measured visible activity is most likely a result of oxygen vacancies. It is concluded that there is no convincing evidence in the literature of water splitting under visible light in carbon-doped TiO2. Crown Copyright r 2007 Published by Elsevier B.V. All rights reserved. Keywords: Water splitting; Photoelectrochemical; Anion doping; TiO2
The question of whether carbon doping of titanium dioxide (TiO2) can lead to photoelectrochemical water splitting under visible illumination remains controversial. TiO2 has many advantageous properties as a photoanode for water splitting: it is cheap and easy to produce, it is a good catalyst, it absorbs light strongly at wavelengths shorter than its band-gap wavelength, and it does not degrade in strong bases or acids. However, its band gap is large (3.0 eV for rutile and 3.2 eV for anatase), which means that it absorbs only the UV component of the solar spectrum. This limits water-splitting efficiency under AM1.5 global solar illumination to 2.2% for rutile and 1.3% for anatase [1], well under the 10% required for economic hydrogen production. These considerations have led to many efforts to reduce the band gap of TiO2 in order to allow utilization of visible light. Recently, much attention has been focussed on anion doping of TiO2, in particular by carbon, nitrogen and sulphur. Tel.: +61 2 94137150; fax: +61 2 94137200.
E-mail address: [email protected]
In a highly publicized paper, Khan et al. [2] reported that carbon-doped rutile TiO2, produced by oxidizing titanium sheet in a natural gas flame, absorbed visible light at wavelengths up to 535 nm (corresponding to a band gap of 2.32 eV). In comparison, rutile TiO2, formed by oxidizing titanium sheet in an oven, had the usual band gap of 3.0 eV (corresponding to 414 nm). Khan et al. obtained a watersplitting efficiency Z of 8.35% for a carbon-doped TiO2 photoanode using the standard expression Z ¼ j p ðV ws V B Þ=E s ,
(1)
where jp is the measured photocurrent density, Vws ¼ 1.229 eV is the potential corresponding to the Gibbs free energy change per photon in the water-splitting reaction, VB is the cell bias voltage and Es is the irradiance. Khan et al.’s results have been criticized since, even assuming 100% incident photon conversion efficiency (IPCE, sometimes called quantum efficiency; an IPCE of 100% at a given wavelength indicates that one water molecule is split per two incident photons of that wavelength), the maximum efficiency obtainable for a
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A.B. Murphy / Solar Energy Materials & Solar Cells 92 (2008) 363–367
band gap of 2.32 eV under AM1.5 global solar irradiation is 8.0% for the stated bias voltage of 0.3 eV [1,3]. Investigation of the spectral absorbance data given by Khan et al. has shown that the absorption coefficient at visible wavelengths is in fact very low, of order 104 m1 or less, compared with 108–106 m1 at UV wavelengths. Using these values to determine the depth profile of photon absorption, and assuming a diffusion length of charge carriers of 200 nm in the carbon-doped TiO2, gave a maximum possible efficiency of about 3% [4]. Explanations postulated for the anomalously high value of Z of 8.35% have included differences in the spectrum of xenon arc lamp with water filter used as a source by Khan et al. and the AM1.5 global solar spectrum [1,3,5], an underestimation of the bias voltage [1,5,6], and a possible underestimation of the irradiance [1]. The latter is difficult to confirm since while Khan et al. gave the type of radiometer they used, they did not specify the photon detector. There have been many subsequent attempts to produce carbon-doped TiO2 photoanodes that split water under visible light. Noworyta and Augustynski [7] and Barnes et al. [8] used similar hydrocarbon-flame-oxidation methods to Khan et al. [2], and Enache et al. [9] used spray pyrolysis of an organometallic precursor in a CO2/O2 atmosphere, without obtaining any measurable activity for visible light illumination. Neumann et al. [10] tested photoanodes prepared by hydrolysis of titanium tetrachloride with carbon-containing bases. They found no significant water-splitting activity for visible light illumination, and a reduction in water-splitting activity for UV illumination compared with undoped TiO2 photoanodes, and concluded that carbon doping led to the formation of defect states located in the band gap, at which holes recombined before they could oxidize water. Other recent studies have investigated water-splitting efficiencies and related quantities for carbon-doped TiO2 nanotube arrays. Nanostructured photoelectrodes have the advantage that the absorption depth of light (which is typically large, of the order of 1 mm or more at wavelengths near the band-gap wavelength for TiO2) in the electrode material is decoupled from the distance charge carriers can diffuse before recombination (which is typically small, of the order 200 nm for TiO2). Shankar et al. [11] reported a small (2%) IPCE across the whole visible spectrum (approximately 400–800 nm) for TiO2 nanotubes doped with carbon by annealing in a propane flame. Park et al. [12] doped TiO2 nanotubes with carbon by annealing in a carbon monoxide (CO) atmosphere, and found, using a filter that blocked UV light, that about 30–40% of the photocurrent under illumination by a xenon arc lamp with water filter was due to visible light at wavelengths longer than 420 nm. No IPCE data was given. In both cases, the photocurrents corresponded to a water-splitting efficiency below 1%, consistent with that expected for TiO2 under solar illumination. Raja et al. [13] and Mohapatra et al. [14] prepared carbon-doped TiO2 nanotubes by different means (respectively, by annealing TiO2 nanotubes in a mixture of
acetylene, hydrogen and carbon, and by producing nanotubes from ethylene glycol using a sonochemical method). They reported visible absorption, and large photocurrents, up to 3 mA cm2 for large bias voltages under illumination by a solar simulator. No IPCE data were presented. However, Raja et al. did compare photocurrents with and without a filter with zero transmission below 400 nm, finding that about 20% of the photocurrent was due to visible light. These studies will be considered in more detail below. Xu et al. [15] have recently investigated the effect of carbon doping of both thin films and nanotubes of TiO2. In both cases, doping performed by oxidation of the TiO2 in a natural gas flame was found to increase the photocurrent density under illumination by a xenon arc lamp. Also, both doped and undoped nanotube photoanodes were found to have higher photocurrent densities than the respective thin film photoanodes. Xu et al. also presented IPCE measurements that showed a small peak in the visible range at around 620 nm for all photoanodes, as well as a large peak in the UV range at around 310 nm. They used a Tauc plot of the IPCE data for the second peak to derive a second transition at 1.30 eV for the carbon-doped nanotube photoanode. Xu and Khan [16] presented similar results in an earlier letter for thin film carbon-doped TiO2 electrodes produced by spray pyrolysis of glucose-containing TiCl4 solution; in this case the second transition was reported to occur at 1.45 eV. Xu et al. [15] hypothesized that carbon doping led to the formation of an intragap band, at an energy 1.30 eV above the valence band, pointing to a study in which deep-level optical spectroscopy was used to reveal levels located at 1.30 and 2.34 eV below the conduction band due to carbon doping [17], and also to theoretical calculations predicting intragap levels [18,19]. There are, however, several problems and misconceptions apparent in Xu et al.’s results and interpretation, particularly those concerning water splitting by visible light. These apply equally to the earlier letter [16], but the focus here is on the full paper. Xu et al.’s Fig. 7 shows IPCE versus wavelength of the incident light. The method used for these measurements was not given, but presumably a monochromator was used to select the wavelength of the incident light, as is standard practice. As noted above, the IPCE was largest at about 310 nm, falling to zero at 400 nm; a second, much smaller peak was observed at about 620 nm. It seems very likely that this smaller peak is in fact an artefact. A monochromator set to pass light of wavelength l will also pass a small fraction of any incident light of wavelength l/2; this is referred to as second-order transmission. Fig. 1 presents strong evidence that light of wavelengths around 310 nm is responsible for the small IPCE peaks at 620 nm. The figure reproduces the data of Xu et al., but shows the smaller visible peaks superimposed on the large UV peaks. This was done by halving the wavelength of the smaller peaks and multiplying their amplitude by a constant factor. It can be seen that both sets of peaks coincide after this
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Fig. 1. IPCE curves for four different TiO2 electrodes, from Fig. 7 of Xu et al. [15]. The smaller visible peaks are compared with the large UV peaks by halving the wavelength of the visible measurements and multiplying their amplitude by 16.4.
transformation, exactly as expected if the second-order transmission was responsible for the smaller peaks. It is standard practice to use a long-pass filter to avoid such second-order transmission when a monochromator is used with a broadband source. In the case discussed here, a filter that blocks wavelengths below 400 nm should be placed between the lamp and the electrochemical cell when measuring IPCEs at wavelengths longer than about 450 nm. It is important to note that the transition at 1.3 eV that is derived from the IPCE peaks at 620 nm is at too low an energy to allow charge carriers that are effective in splitting water to be produced. The energy per electron required to split water is 1.23 eV. However, there are unavoidable losses of at least 0.4 eV due to the entropy of mixing associated with the formation of excited states by photon absorption (as required to raise electrons to higher energy levels) [1,20] and at least 0.4 eV due to overpotentials at both electrodes [21]. Hence, absorption of photons of at least 2.0 eV is required to split water. This also indicates that the photocurrents measured by Shankar et al. for carbon-doped TiO2 nanotube electrodes [11] under visible illumination are not related to the water-splitting reaction. As noted above, they reported a small (2%) IPCE for carbon-doped TiO2 nanotubes across the whole visible spectrum to 800 nm, with a local maxima around 700 nm. However, water splitting cannot occur for wavelengths longer than 620 nm (corresponding to 2.0 eV), so if the photocurrents were related to water splitting, the IPCE should fall to zero above about 620 nm. Shankar et al. did find that the IPCE for flame-annealed Ti foil showed a decrease at wavelengths longer than 600 nm; however, this was not correlated with any decrease in the absorption of light by the electrode, as would be expected if a band gap corresponding to 600 nm had been formed. These considerations are in agreement with the arguments presented
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by Shankar et al., who were sceptical that their results indicated any band-gap modification by carbon doping, and put forward alternative hypotheses to explain the changes in the photoresponse of the TiO2 nanotubes that they observed. In particular, they noted that the absence of a clear threshold in both the IPCE and the optical absorption spectrum suggested the creation of impurity levels by carbon doping deep into the gap, rather than a modification of the band gap itself. Xu et al. [15] also suggested that the increase in the photoresponse under UV illumination for the carbondoped TiO2, compared with the undoped TiO2, was due to photogenerated electrons and holes having extra energy with respect to the conduction band edge and valence band edge, respectively, meaning the holes had more chance of reaching the interface with water before recombining. This explanation is flawed, since the maximum energy a photon can transfer to a charge carrier is equal to the bandgap energy. When a photon is absorbed, the excited state that is formed reaches thermal equilibrium with the surroundings very rapidly, so the system relaxes to the band-gap energy within about 10 ps, which is faster than electron transfer [20]. Finally, the IPCE data presented by Xu et al. are grossly inconsistent with the photocurrents given in their Figs. 1–3. The efficiency under a given source of illumination can be determined from the IPCE using [1] Z 1 Z ¼ ½eðV ws V B Þ=E s IPCEðlÞI l ðlÞ dl, (2) 0
where Il(l) is the incident spectral photon flux, e is the electronic charge and l is the wavelength. Results of calculations performed using Eq. (2) are given in Table 1 for the AM1.5 global solar spectrum. Results are given for UV wavelengths only (thus excluding the visible IPCE peak) and for all wavelengths. The bias voltage for the IPCE measurements was not given by Xu et al., so efficiencies are given in the table for a range of bias voltages. Efficiencies calculated using Eq. (1) are also given in Table 1. Again results are given for a range of bias voltages. To determine the photocurrent at a given bias voltage from Xu et al.’s Fig. 1, it was assumed that VB ¼ VmeasVoc, where Vmeas is the measured potential with respect to a standard calomel reference electrode and that Voc ¼ 1.1 V, as given by Xu and Khan [16]. It should be noted that this is likely to underestimate VB, which should in fact be equated to the voltage between the working and counter electrodes [1,5], by a few tenths of a volt. However, for the purposes of this comparison, the expression is adequate. Table 1 shows that the maximum efficiency obtained using Eq. (2) for the carbon-doped nanotube photoanode, even taking into account the visible peak in the IPCE, is 0.14%. However, using Eq. (1) from the photocurrent densities given in Xu et al.’s Fig. 1 gives an efficiency of over 5% for the same photoanode. Similarly, large discrepancies are evident for all photoanodes.
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Table 1 Comparison of efficiencies calculated from IPCE measurements (Eq. (2)) and photocurrent densities (Eq. (1)) for photoelectrodes used by Xu et al. [15] Calculation method
VB (V)
Efficiency (%) Undoped thin film TiO2
Undoped nanotube TiO2
C-doped thin film TiO2
C-doped nanotube TiO2
Eq. (2), lp400 nm only
0.2 0.4 0.6
0.007 0.006 0.004
0.021 0.017 0.013
0.050 0.041 0.030
0.087 0.070 0.053
Eq. (2), all l
0.2 0.4 0.6
0.012 0.010 0.007
0.041 0.033 0.023
0.088 0.071 0.053
0.141 0.114 0.086
Eq. (1)
0.2 0.4 0.6
0.48 0.90 0.78
0.82 1.56 1.55
2.09 3.34 3.17
5.30 6.79 5.77
Efficiencies calculated using Eq. (2) should be more reliable than those obtained using Eq. (1), since the results should be independent of the lamp spectrum. This strongly suggests the efficiencies calculated using Eq. (1) are greatly overestimated. Possible reasons are that the spectrum of the xenon lamp used by Xu et al. has a much stronger UV component than the AM1.5 global solar spectrum, which leads to higher photocurrents than under solar illumination, or that the detector used to measure the lamp irradiance is not linear across the full spectrum of the lamp (in particular, poor detection of IR radiation would lead to a major underestimate of Es, but would not affect the IPCE measurement). Since Khan et al. [2] used Eq. (1) to calculate a water-splitting efficiency of 8.35%, and since they presumably used similar apparatus and methods to those reported by Xu et al., it seems likely that this efficiency was also greatly overestimated. Recent work investigating nitrogen-doped TiO2 has underlined the difficulty in definitively assigning photocatalytic activity under visible light to the influence of anion dopants. Measurements using angle-resolved X-ray photoelectron spectroscopy showed that, in some cases, the anion dopant was present in the bulk of the photocatalyst but not at the surface, and was hence unlikely to contribute towards photocatalytic activity [22]. Further, Lin et al. [23] showed that the formation of oxygen vacancies by reduction of TiO2 can lead to visible light absorption and photocatalytic activity. This is relevant to carbon-doped TiO2 photocatalysts, since exposure of TiO2 to reducing atmospheres is likely to lead to the formation of oxygen vacancies. In addition, the calculations of Di Valentin et al. [19] indicate that carbon doping of rutile favours the formation of oxygen vacancies, which may be responsible for photocatalytic activity under visible illumination. These considerations are particularly relevant to the work of Park et al. [12] and Raja et al. [13], whose results, as mentioned above, suggested some water-splitting activity under visible illumination. In both cases, carbon doping of TiO2 nanotubes was carried out under reducing atmospheres (CO, and a mixture of acetylene, argon and hydrogen, respectively). Raja et al. [13] noted the
possibility that this led to the reduction of Ti4+ to Ti3+, and to the formation of oxygen vacancy states. They presented evidence that this occurred for the case TiO2 nanotubes annealed in a nitrogen–hydrogen atmosphere: XPS measurements showed the presence of Ti3+ ions, and a Mott–Schottky plot indicated an increase in charge carrier density after annealing, as expected if oxygen vacancies were formed. The authors stated that similar results were obtained for the TiO2 nanotubes annealed in the acetylene–argon–hydrogen atmosphere. Park et al. [12] postulated that the reaction of their TiO2 nanotubes with CO led to the formation of titanium oxycarbides, but provided no evidence for this. They noted that their XPS measurements did not show any evidence of Ti–C bonds, despite the large carbon concentration (between 8% and 42%), but did not discuss whether any peaks associated with oxycarbides were present. The likelihood of formation of oxycarbides can be assessed by examining the literature on carbothermal synthesis of TiC from TiO2. The first step in the reaction of TiO2 with CO is the formation of titanium suboxides of the form TinO2n1, such as Ti3O5 or Ti2O3; only in subsequent steps are titanium oxycarbides, and finally TiC, formed [24]. However, chemical equilibrium calculations [25] indicate that TiO2 does not form such suboxides in a CO atmosphere at temperatures below 1000 1C, well above the 500–800 1C used by Park et al. This indicates that oxycarbides will not form at these temperatures, which suggests that the water-splitting activity under visible illumination observed by Park et al. is unlikely to be related to carbon doping. However, it is possible for oxygen vacancies to form at lower temperatures [24], and these could account for the observations. In conclusion, most of the reports, including that of Xu et al. [15], of water-splitting activity under visible light illumination using carbon-doped TiO2 electrodes appear to be based on flawed measurements and interpretations. Only the results of Park et al. [12] and Raja et al. [13] provide plausible evidence of some water-splitting activity under visible light illumination, but it is likely that this is a result of oxygen vacancies in the TiO2, rather than carbon
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doping. Hence, there do not appear to be any published data that convincingly demonstrate water splitting under visible illumination in carbon-doped TiO2. Such a demonstration would require IPCE measurements, which, properly done, provide more information and are more reliable than photocurrent measurements. Further, careful measurements would be needed to confirm that the dopant is indeed present at the surface of the photoanode and is responsible for the visible activity. References [1] A.B. Murphy, P.R.F. Barnes, L.K. Randeniya, I.C. Plumb, I.E. Grey, M.D. Horne, J.A. Glasscock, Int. J. Hydrogen Energy 31 (2006) 1999. [2] S.U.M. Khan, M. Al-Shahry, W.B. Ingler Jr., Science 297 (2002) 2243. [3] C. Ha¨gglund, M. Gra¨tzel, B. Kasemo, Science 301 (2003) 1673b. [4] A.B. Murphy, Sol. Energy Mater. Sol. Cells 91 (2007) 1326. [5] V.M. Aroutiounian, V.M. Arakelyan, G.E. Shahnazaryan, Sol. Energy 78 (2005) 581. [6] K.S. Lackner, Science 301 (2003) 1673c. [7] K. Noworyta, J. Augustynski, Electrochem. Solid State Lett. 7 (2004) E31. [8] P.R.F. Barnes, L.K. Randeniya, A.B. Murphy, P.B. Gwan, I.C. Plumb, J.A. Glasscock, I.E. Grey, C. Li, Dev. Chem. Eng. Miner. Process. 14 (2006) 51. [9] C.S. Enache, J. Schoonman, R. van de Krol, J. Electroceram. 13 (2004) 177.
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[10] B. Neumann, P. Bogdanoff, H. Tributsch, S. Sakthivel, H. Kisch, J. Phys. Chem. B 109 (2005) 16,579. [11] K. Shankar, M. Paulose, G.K. Mor, O.K. Varghese, C.A. Grimes, J. Phys. D: Appl. Phys. 38 (2005) 3543. [12] J.H. Park, S. Kim, A.J. Bard, Nano Lett. 6 (2006) 24. [13] K.S. Raja, M. Misra, V.K. Mahajan, T. Gandhi, P. Pillai, S.K. Mohapatra, J. Power Sources 161 (2006) 1450. [14] S.K. Mohapatra, M. Misra, V.K. Mahajan, K.S. Raja, J. Catal. 246 (2007) 362. [15] C. Xu, Y.A. Shaban, W.B. Ingler Jr., S.U.M. Khan, Sol. Energy Mater. Sol. Cells 91 (2007) 938. [16] C. Xu, S.U.M. Khan, Electrochem. Solid State Lett. 10 (2007) B56. [17] Y. Nakano, T. Morikawa, T. Ohwaki, Y. Taga, Appl. Phys. Lett. 87 (2005) 052111. [18] H. Wang, J.P. Lewis, J. Phys.: Condens. Matter 17 (2005) L209. [19] C. Di Valentin, G. Pacchioni, A. Selloni, Chem. Mater. 17 (2005) 6656. [20] J.R. Bolton, A.F. Haught, R.T. Ross, Photochemical energy storage: an analysis of limits, in: J.S. Connolly (Ed.), Photochemical Conversion and Storage of Solar Energy, Proceedings of the Third International Conference, 1980, Boulder, CO, Academic Press, New York, 1981, pp. 297–339. [21] M.F. Weber, M.J. Dignam, J. Electrochem. Soc. 131 (1984) 1258. [22] A. Orlov, M.S. Tikhov, R.M. Lambert, C.R. Chimie 9 (2006) 794. [23] Z. Lin, A. Orlov, R.M. Lambert, M.C. Payne, J. Phys. Chem. B 109 (2005) 20,948. [24] L.-M. Berger, W. Gruner, E. Langholf, S. Stolle, Int. J. Refract. Metals Hard Mater. 17 (1999) 235. [25] P. Tristant, J. Aubreton, P. Lefort, High Temp. Chem. Process 3 (1994) 375.
Solar Energy Materials & Solar Cells 92 (2008) 363–367 www.elsevier.com/locate/solmat
Letter
Does carbon doping of TiO2 allow water splitting in visible light? Comments on ‘‘Nanotube enhanced photoresponse of carbon modified (CM)-n-TiO2 for efficient water splitting’’ A.B. Murphy CSIRO Industrial Physics, P.O. Box 218, Lindfield, NSW 2070, Australia Received 24 July 2007; received in revised form 25 October 2007; accepted 25 October 2007 Available online 4 December 2007
Abstract Reports of water splitting by carbon-doped titanium dioxide (TiO2) photoelectrodes under visible illumination are critically examined. Xu et al. [Sol. Energy Mater. Sol. Cells 91 (2007) 938] recently reported significant incident photon conversion efficiencies (IPCEs) at visible wavelengths for carbon-doped TiO2 in thin film and nanotube form. Evidence is given here that these results were due to an artefact in the measurements. Further, it is pointed out that the mechanism proposed for water splitting under visible illumination is unphysical, and the photocurrents presented are shown to be grossly inconsistent with the IPCE data. Other workers have also measured non-zero IPCEs at visible wavelengths for carbon-doped TiO2, but have not presented this as evidence of water splitting. In other cases, carbon doping was performed in a reducing atmosphere, and measured visible activity is most likely a result of oxygen vacancies. It is concluded that there is no convincing evidence in the literature of water splitting under visible light in carbon-doped TiO2. Crown Copyright r 2007 Published by Elsevier B.V. All rights reserved. Keywords: Water splitting; Photoelectrochemical; Anion doping; TiO2
The question of whether carbon doping of titanium dioxide (TiO2) can lead to photoelectrochemical water splitting under visible illumination remains controversial. TiO2 has many advantageous properties as a photoanode for water splitting: it is cheap and easy to produce, it is a good catalyst, it absorbs light strongly at wavelengths shorter than its band-gap wavelength, and it does not degrade in strong bases or acids. However, its band gap is large (3.0 eV for rutile and 3.2 eV for anatase), which means that it absorbs only the UV component of the solar spectrum. This limits water-splitting efficiency under AM1.5 global solar illumination to 2.2% for rutile and 1.3% for anatase [1], well under the 10% required for economic hydrogen production. These considerations have led to many efforts to reduce the band gap of TiO2 in order to allow utilization of visible light. Recently, much attention has been focussed on anion doping of TiO2, in particular by carbon, nitrogen and sulphur. Tel.: +61 2 94137150; fax: +61 2 94137200.
E-mail address: [email protected]
In a highly publicized paper, Khan et al. [2] reported that carbon-doped rutile TiO2, produced by oxidizing titanium sheet in a natural gas flame, absorbed visible light at wavelengths up to 535 nm (corresponding to a band gap of 2.32 eV). In comparison, rutile TiO2, formed by oxidizing titanium sheet in an oven, had the usual band gap of 3.0 eV (corresponding to 414 nm). Khan et al. obtained a watersplitting efficiency Z of 8.35% for a carbon-doped TiO2 photoanode using the standard expression Z ¼ j p ðV ws V B Þ=E s ,
(1)
where jp is the measured photocurrent density, Vws ¼ 1.229 eV is the potential corresponding to the Gibbs free energy change per photon in the water-splitting reaction, VB is the cell bias voltage and Es is the irradiance. Khan et al.’s results have been criticized since, even assuming 100% incident photon conversion efficiency (IPCE, sometimes called quantum efficiency; an IPCE of 100% at a given wavelength indicates that one water molecule is split per two incident photons of that wavelength), the maximum efficiency obtainable for a
0927-0248/$ - see front matter Crown Copyright r 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.10.007
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band gap of 2.32 eV under AM1.5 global solar irradiation is 8.0% for the stated bias voltage of 0.3 eV [1,3]. Investigation of the spectral absorbance data given by Khan et al. has shown that the absorption coefficient at visible wavelengths is in fact very low, of order 104 m1 or less, compared with 108–106 m1 at UV wavelengths. Using these values to determine the depth profile of photon absorption, and assuming a diffusion length of charge carriers of 200 nm in the carbon-doped TiO2, gave a maximum possible efficiency of about 3% [4]. Explanations postulated for the anomalously high value of Z of 8.35% have included differences in the spectrum of xenon arc lamp with water filter used as a source by Khan et al. and the AM1.5 global solar spectrum [1,3,5], an underestimation of the bias voltage [1,5,6], and a possible underestimation of the irradiance [1]. The latter is difficult to confirm since while Khan et al. gave the type of radiometer they used, they did not specify the photon detector. There have been many subsequent attempts to produce carbon-doped TiO2 photoanodes that split water under visible light. Noworyta and Augustynski [7] and Barnes et al. [8] used similar hydrocarbon-flame-oxidation methods to Khan et al. [2], and Enache et al. [9] used spray pyrolysis of an organometallic precursor in a CO2/O2 atmosphere, without obtaining any measurable activity for visible light illumination. Neumann et al. [10] tested photoanodes prepared by hydrolysis of titanium tetrachloride with carbon-containing bases. They found no significant water-splitting activity for visible light illumination, and a reduction in water-splitting activity for UV illumination compared with undoped TiO2 photoanodes, and concluded that carbon doping led to the formation of defect states located in the band gap, at which holes recombined before they could oxidize water. Other recent studies have investigated water-splitting efficiencies and related quantities for carbon-doped TiO2 nanotube arrays. Nanostructured photoelectrodes have the advantage that the absorption depth of light (which is typically large, of the order of 1 mm or more at wavelengths near the band-gap wavelength for TiO2) in the electrode material is decoupled from the distance charge carriers can diffuse before recombination (which is typically small, of the order 200 nm for TiO2). Shankar et al. [11] reported a small (2%) IPCE across the whole visible spectrum (approximately 400–800 nm) for TiO2 nanotubes doped with carbon by annealing in a propane flame. Park et al. [12] doped TiO2 nanotubes with carbon by annealing in a carbon monoxide (CO) atmosphere, and found, using a filter that blocked UV light, that about 30–40% of the photocurrent under illumination by a xenon arc lamp with water filter was due to visible light at wavelengths longer than 420 nm. No IPCE data was given. In both cases, the photocurrents corresponded to a water-splitting efficiency below 1%, consistent with that expected for TiO2 under solar illumination. Raja et al. [13] and Mohapatra et al. [14] prepared carbon-doped TiO2 nanotubes by different means (respectively, by annealing TiO2 nanotubes in a mixture of
acetylene, hydrogen and carbon, and by producing nanotubes from ethylene glycol using a sonochemical method). They reported visible absorption, and large photocurrents, up to 3 mA cm2 for large bias voltages under illumination by a solar simulator. No IPCE data were presented. However, Raja et al. did compare photocurrents with and without a filter with zero transmission below 400 nm, finding that about 20% of the photocurrent was due to visible light. These studies will be considered in more detail below. Xu et al. [15] have recently investigated the effect of carbon doping of both thin films and nanotubes of TiO2. In both cases, doping performed by oxidation of the TiO2 in a natural gas flame was found to increase the photocurrent density under illumination by a xenon arc lamp. Also, both doped and undoped nanotube photoanodes were found to have higher photocurrent densities than the respective thin film photoanodes. Xu et al. also presented IPCE measurements that showed a small peak in the visible range at around 620 nm for all photoanodes, as well as a large peak in the UV range at around 310 nm. They used a Tauc plot of the IPCE data for the second peak to derive a second transition at 1.30 eV for the carbon-doped nanotube photoanode. Xu and Khan [16] presented similar results in an earlier letter for thin film carbon-doped TiO2 electrodes produced by spray pyrolysis of glucose-containing TiCl4 solution; in this case the second transition was reported to occur at 1.45 eV. Xu et al. [15] hypothesized that carbon doping led to the formation of an intragap band, at an energy 1.30 eV above the valence band, pointing to a study in which deep-level optical spectroscopy was used to reveal levels located at 1.30 and 2.34 eV below the conduction band due to carbon doping [17], and also to theoretical calculations predicting intragap levels [18,19]. There are, however, several problems and misconceptions apparent in Xu et al.’s results and interpretation, particularly those concerning water splitting by visible light. These apply equally to the earlier letter [16], but the focus here is on the full paper. Xu et al.’s Fig. 7 shows IPCE versus wavelength of the incident light. The method used for these measurements was not given, but presumably a monochromator was used to select the wavelength of the incident light, as is standard practice. As noted above, the IPCE was largest at about 310 nm, falling to zero at 400 nm; a second, much smaller peak was observed at about 620 nm. It seems very likely that this smaller peak is in fact an artefact. A monochromator set to pass light of wavelength l will also pass a small fraction of any incident light of wavelength l/2; this is referred to as second-order transmission. Fig. 1 presents strong evidence that light of wavelengths around 310 nm is responsible for the small IPCE peaks at 620 nm. The figure reproduces the data of Xu et al., but shows the smaller visible peaks superimposed on the large UV peaks. This was done by halving the wavelength of the smaller peaks and multiplying their amplitude by a constant factor. It can be seen that both sets of peaks coincide after this
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Fig. 1. IPCE curves for four different TiO2 electrodes, from Fig. 7 of Xu et al. [15]. The smaller visible peaks are compared with the large UV peaks by halving the wavelength of the visible measurements and multiplying their amplitude by 16.4.
transformation, exactly as expected if the second-order transmission was responsible for the smaller peaks. It is standard practice to use a long-pass filter to avoid such second-order transmission when a monochromator is used with a broadband source. In the case discussed here, a filter that blocks wavelengths below 400 nm should be placed between the lamp and the electrochemical cell when measuring IPCEs at wavelengths longer than about 450 nm. It is important to note that the transition at 1.3 eV that is derived from the IPCE peaks at 620 nm is at too low an energy to allow charge carriers that are effective in splitting water to be produced. The energy per electron required to split water is 1.23 eV. However, there are unavoidable losses of at least 0.4 eV due to the entropy of mixing associated with the formation of excited states by photon absorption (as required to raise electrons to higher energy levels) [1,20] and at least 0.4 eV due to overpotentials at both electrodes [21]. Hence, absorption of photons of at least 2.0 eV is required to split water. This also indicates that the photocurrents measured by Shankar et al. for carbon-doped TiO2 nanotube electrodes [11] under visible illumination are not related to the water-splitting reaction. As noted above, they reported a small (2%) IPCE for carbon-doped TiO2 nanotubes across the whole visible spectrum to 800 nm, with a local maxima around 700 nm. However, water splitting cannot occur for wavelengths longer than 620 nm (corresponding to 2.0 eV), so if the photocurrents were related to water splitting, the IPCE should fall to zero above about 620 nm. Shankar et al. did find that the IPCE for flame-annealed Ti foil showed a decrease at wavelengths longer than 600 nm; however, this was not correlated with any decrease in the absorption of light by the electrode, as would be expected if a band gap corresponding to 600 nm had been formed. These considerations are in agreement with the arguments presented
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by Shankar et al., who were sceptical that their results indicated any band-gap modification by carbon doping, and put forward alternative hypotheses to explain the changes in the photoresponse of the TiO2 nanotubes that they observed. In particular, they noted that the absence of a clear threshold in both the IPCE and the optical absorption spectrum suggested the creation of impurity levels by carbon doping deep into the gap, rather than a modification of the band gap itself. Xu et al. [15] also suggested that the increase in the photoresponse under UV illumination for the carbondoped TiO2, compared with the undoped TiO2, was due to photogenerated electrons and holes having extra energy with respect to the conduction band edge and valence band edge, respectively, meaning the holes had more chance of reaching the interface with water before recombining. This explanation is flawed, since the maximum energy a photon can transfer to a charge carrier is equal to the bandgap energy. When a photon is absorbed, the excited state that is formed reaches thermal equilibrium with the surroundings very rapidly, so the system relaxes to the band-gap energy within about 10 ps, which is faster than electron transfer [20]. Finally, the IPCE data presented by Xu et al. are grossly inconsistent with the photocurrents given in their Figs. 1–3. The efficiency under a given source of illumination can be determined from the IPCE using [1] Z 1 Z ¼ ½eðV ws V B Þ=E s IPCEðlÞI l ðlÞ dl, (2) 0
where Il(l) is the incident spectral photon flux, e is the electronic charge and l is the wavelength. Results of calculations performed using Eq. (2) are given in Table 1 for the AM1.5 global solar spectrum. Results are given for UV wavelengths only (thus excluding the visible IPCE peak) and for all wavelengths. The bias voltage for the IPCE measurements was not given by Xu et al., so efficiencies are given in the table for a range of bias voltages. Efficiencies calculated using Eq. (1) are also given in Table 1. Again results are given for a range of bias voltages. To determine the photocurrent at a given bias voltage from Xu et al.’s Fig. 1, it was assumed that VB ¼ VmeasVoc, where Vmeas is the measured potential with respect to a standard calomel reference electrode and that Voc ¼ 1.1 V, as given by Xu and Khan [16]. It should be noted that this is likely to underestimate VB, which should in fact be equated to the voltage between the working and counter electrodes [1,5], by a few tenths of a volt. However, for the purposes of this comparison, the expression is adequate. Table 1 shows that the maximum efficiency obtained using Eq. (2) for the carbon-doped nanotube photoanode, even taking into account the visible peak in the IPCE, is 0.14%. However, using Eq. (1) from the photocurrent densities given in Xu et al.’s Fig. 1 gives an efficiency of over 5% for the same photoanode. Similarly, large discrepancies are evident for all photoanodes.
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Table 1 Comparison of efficiencies calculated from IPCE measurements (Eq. (2)) and photocurrent densities (Eq. (1)) for photoelectrodes used by Xu et al. [15] Calculation method
VB (V)
Efficiency (%) Undoped thin film TiO2
Undoped nanotube TiO2
C-doped thin film TiO2
C-doped nanotube TiO2
Eq. (2), lp400 nm only
0.2 0.4 0.6
0.007 0.006 0.004
0.021 0.017 0.013
0.050 0.041 0.030
0.087 0.070 0.053
Eq. (2), all l
0.2 0.4 0.6
0.012 0.010 0.007
0.041 0.033 0.023
0.088 0.071 0.053
0.141 0.114 0.086
Eq. (1)
0.2 0.4 0.6
0.48 0.90 0.78
0.82 1.56 1.55
2.09 3.34 3.17
5.30 6.79 5.77
Efficiencies calculated using Eq. (2) should be more reliable than those obtained using Eq. (1), since the results should be independent of the lamp spectrum. This strongly suggests the efficiencies calculated using Eq. (1) are greatly overestimated. Possible reasons are that the spectrum of the xenon lamp used by Xu et al. has a much stronger UV component than the AM1.5 global solar spectrum, which leads to higher photocurrents than under solar illumination, or that the detector used to measure the lamp irradiance is not linear across the full spectrum of the lamp (in particular, poor detection of IR radiation would lead to a major underestimate of Es, but would not affect the IPCE measurement). Since Khan et al. [2] used Eq. (1) to calculate a water-splitting efficiency of 8.35%, and since they presumably used similar apparatus and methods to those reported by Xu et al., it seems likely that this efficiency was also greatly overestimated. Recent work investigating nitrogen-doped TiO2 has underlined the difficulty in definitively assigning photocatalytic activity under visible light to the influence of anion dopants. Measurements using angle-resolved X-ray photoelectron spectroscopy showed that, in some cases, the anion dopant was present in the bulk of the photocatalyst but not at the surface, and was hence unlikely to contribute towards photocatalytic activity [22]. Further, Lin et al. [23] showed that the formation of oxygen vacancies by reduction of TiO2 can lead to visible light absorption and photocatalytic activity. This is relevant to carbon-doped TiO2 photocatalysts, since exposure of TiO2 to reducing atmospheres is likely to lead to the formation of oxygen vacancies. In addition, the calculations of Di Valentin et al. [19] indicate that carbon doping of rutile favours the formation of oxygen vacancies, which may be responsible for photocatalytic activity under visible illumination. These considerations are particularly relevant to the work of Park et al. [12] and Raja et al. [13], whose results, as mentioned above, suggested some water-splitting activity under visible illumination. In both cases, carbon doping of TiO2 nanotubes was carried out under reducing atmospheres (CO, and a mixture of acetylene, argon and hydrogen, respectively). Raja et al. [13] noted the
possibility that this led to the reduction of Ti4+ to Ti3+, and to the formation of oxygen vacancy states. They presented evidence that this occurred for the case TiO2 nanotubes annealed in a nitrogen–hydrogen atmosphere: XPS measurements showed the presence of Ti3+ ions, and a Mott–Schottky plot indicated an increase in charge carrier density after annealing, as expected if oxygen vacancies were formed. The authors stated that similar results were obtained for the TiO2 nanotubes annealed in the acetylene–argon–hydrogen atmosphere. Park et al. [12] postulated that the reaction of their TiO2 nanotubes with CO led to the formation of titanium oxycarbides, but provided no evidence for this. They noted that their XPS measurements did not show any evidence of Ti–C bonds, despite the large carbon concentration (between 8% and 42%), but did not discuss whether any peaks associated with oxycarbides were present. The likelihood of formation of oxycarbides can be assessed by examining the literature on carbothermal synthesis of TiC from TiO2. The first step in the reaction of TiO2 with CO is the formation of titanium suboxides of the form TinO2n1, such as Ti3O5 or Ti2O3; only in subsequent steps are titanium oxycarbides, and finally TiC, formed [24]. However, chemical equilibrium calculations [25] indicate that TiO2 does not form such suboxides in a CO atmosphere at temperatures below 1000 1C, well above the 500–800 1C used by Park et al. This indicates that oxycarbides will not form at these temperatures, which suggests that the water-splitting activity under visible illumination observed by Park et al. is unlikely to be related to carbon doping. However, it is possible for oxygen vacancies to form at lower temperatures [24], and these could account for the observations. In conclusion, most of the reports, including that of Xu et al. [15], of water-splitting activity under visible light illumination using carbon-doped TiO2 electrodes appear to be based on flawed measurements and interpretations. Only the results of Park et al. [12] and Raja et al. [13] provide plausible evidence of some water-splitting activity under visible light illumination, but it is likely that this is a result of oxygen vacancies in the TiO2, rather than carbon
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doping. Hence, there do not appear to be any published data that convincingly demonstrate water splitting under visible illumination in carbon-doped TiO2. Such a demonstration would require IPCE measurements, which, properly done, provide more information and are more reliable than photocurrent measurements. Further, careful measurements would be needed to confirm that the dopant is indeed present at the surface of the photoanode and is responsible for the visible activity. References [1] A.B. Murphy, P.R.F. Barnes, L.K. Randeniya, I.C. Plumb, I.E. Grey, M.D. Horne, J.A. Glasscock, Int. J. Hydrogen Energy 31 (2006) 1999. [2] S.U.M. Khan, M. Al-Shahry, W.B. Ingler Jr., Science 297 (2002) 2243. [3] C. Ha¨gglund, M. Gra¨tzel, B. Kasemo, Science 301 (2003) 1673b. [4] A.B. Murphy, Sol. Energy Mater. Sol. Cells 91 (2007) 1326. [5] V.M. Aroutiounian, V.M. Arakelyan, G.E. Shahnazaryan, Sol. Energy 78 (2005) 581. [6] K.S. Lackner, Science 301 (2003) 1673c. [7] K. Noworyta, J. Augustynski, Electrochem. Solid State Lett. 7 (2004) E31. [8] P.R.F. Barnes, L.K. Randeniya, A.B. Murphy, P.B. Gwan, I.C. Plumb, J.A. Glasscock, I.E. Grey, C. Li, Dev. Chem. Eng. Miner. Process. 14 (2006) 51. [9] C.S. Enache, J. Schoonman, R. van de Krol, J. Electroceram. 13 (2004) 177.
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