J. Phys. Chem. C 2008, 112, 6209-6210
6209
COMMENTS Comment on “Mechanisms for Photooxidation Reactions of Water and Organic Compounds on Carbon-Doped Titanium Dioxide, as Studied by Photocurrent Measurements”
The oxidation rate of D via an indirect hole transfer may be described by while that for a direct hole transfer can be written i
Vox (x) ) kiox[D]Nf
(3)
Vdox(x) ) kdox[D]h+ s
(4)
as
L. H. Zhu* College of Life Sciences, China Jiliang UniVersity, Hangzhou 310018, China ReceiVed: December 10, 2007 The title paper by Nakato et al.1 appearing in a recent issue of this journal studied the mechanisms of photooxidation reactions of water and organic compounds on carbon-doped TiO2 mainly by a method of photocurrent measurements. The paper contains useful experimental data and discussions and is an important contribution to the literature. Unfortunately, however, because the authors overlook many important affecting factors of the photocurrent as it was used for mechanism assessment, I believe that most of their conclusions must be met with some skepticism. Let us consider the steady-state photocurrent for a semiconductor film electrode. The continuity equation for the conduction band electron density (the photocurrent is the same as for holes) in presence of current doubling gives
∂Jn(x) ) RΦ(x) + Vinj(x) - Rn(x) ∂x
(1)
where Jn(x) is net electron particle flux in the direction of the substrate (decreasing x), R is the light absorption coefficient, RΦ(x) is the photogeneration rate due to the photon flux Φ(x), Vinj(x) is the rate of electron injection from the organic radical into the semiconductor, which equals the production rate of the radical, Vox(x), if it is only used for electron injection, and R(x) stands for the loss rate of photogenerated electrons. This loss at least includes those for transferring to the oxidation species in the electrolyte and recombining with holes; thus, it has
Rn(x) ) knr Nfr n(x) + kred[A]n(x)
(2)
where knr is the probability for electron trapping, fr is the electron occupancy factor of the recombination centers with number of N, n(x) is the free electron density within the film at a distance x, kred is the interfacial electron-transfer rate constant, and [A] is the concentration of electron acceptor A in the electrolyte. For photogenerated holes arriving at the surface, they can either transfer directly to electron donors D in solution with the rate constant kdox or be captured by the midgap levels with the rate constant ktrap. Holes captured at the surface may also transfer to solutions with the rate constant kiox or recombine with electrons. * To whom correspondence should be addressed. E-mail: zhulh@ cjlu.edu.cn.
Substituting eqs 2 and 3 into eq 1, the photocurrent expression for the former is
∂Jin(x) ) RΦ(x) + kiox[D]Nf - knr Nfr n(x) - kred[A]n(x) ∂x
(5)
Inserting eqs 2 and 4 into eq 1, we get the photocurrent expression for the latter (note that at a stationary point the recombination rate of electrons via midgap levels equals that of holes)
∂Jdn(x) n ) RΦ(x) + kdox[D]h+ s - kr Nfr n(x) - kred[A]n(x) ∂x
(6)
The terms of Nf and h+ s under steady-state can in principle be calculated if their production and consumption processes have been established. These must be a complex function of n(x), [D], knr , kiox, ktrap, light intensity, and so forth.2 Thus, the photocurrent is also the case (for instance see their cited refs 30 and 31). If the transport of electrons in the film is limited by diffusion, then the photocurrent also depends on diffusion coefficient for electrons, which is in most cases dependent on the intensity of light used.3 The authors found that the photocurrent was largely enhanced by the addition of methanol under the simulated solar irradiation, whereas under the visible light illumination the photocurrent was hardly increased. To account for this, they used the same mechanism4 as that for the N-doped TiO2 in which methanol oxidation occurred via a direct reaction of holes in the valence band under the simulated solar illumination while it proceeded by an indirect reaction of hole in the midgap level under the visible light irradiation. They explanation is that (i) the large increase in the photocurrent by the addition of methanol in the direct path was attributed to a significant decrease in the densities of surface reaction intermediates of the water photooxidation reaction on the electrode acting as efficient carrier recombination centers; (ii) the direct oxidation path, if it proceeded via the current doubling mechanism, produced no surface radical intermediates; (iii) alternatively, the indirect mechanism still produced high densities of surface radical intermediates of the water photooxidation reaction thus resulting in a large carrier recombination loss. This explanation is understandable. However, I point out that the above-mentioned difference in the photocurrent cannot be used as evidence of the proposed mechanisms. First, as stated above, the magnitude of the photocurrent depends on many factors, so the observed small photocurrent in the oxidation of water does not necessarily mean that it produces high densities of carrier recombination
10.1021/jp7116104 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/21/2008
6210 J. Phys. Chem. C, Vol. 112, No. 15, 2008 centers. Others possible reasons like the reaction of the photogenerated oxygen with electrons, reversible reaction, and/ or small kiox of water oxidation can also produce a small photocurrent. Second, the large increase in the photocurrent by the addition of methanol under the simulated light does not necessarily require that its oxidation must be via the direct path; conversely, if ktrap and kiox are quite large then it can be found from eq 5 that indirect hole transfer can also produce a large photocurrent. Similarly, the observed small photocurrent by the addition of methanol under the visible light does not necessarily mean that methanol oxidation must be via the indirect hole transfer because in the case of direct hole transfer if kdox is small it can also produce less photocurrent. To get support for the proposed mechanism, they investigated the effect of oxygen on the photocurrent. They argued that the increase in the photocurrent under visible light with methanol can be attributed to the fact that the reaction of dissolved O2 with •CH2OH reduced the carrier recombination centers such as Ti-O• and •CH2OH, which is in harmony with their proposed mechanisms. If it is the case, then why does the photocurrent decrease on the contrary under the simulated light where it also produced •CH2OH as shown in eq 1 of the commenting paper. In fact, from eqs 5 and 6 it can be found that whether the photocurrent increases or not by addition of O2 at least depends on the relative rate size between its reaction with conduction electrons (decreasing photocurrent) and that with •CH2OH (increasing the photocurrent as argued by the authors). Additionally, oxygen may also influence the rate of photocurrent doubling if it reacts with •CH2OH. Thus, the large increase in the photocurrent by dissolved oxygen only under the visible light cannot be entirely attributed to the indirect hole transfer path.
Comments In addition, the experimental results for formic acid are also unable to support their proposed mechanisms based on the similar grounds as discussed above. In short, because the photocurrent depends on many factors, it does not necessarily directly reflect the photooxidation rate of electron donors; thus, it cannot distinguish oxidation mechanisms simply from its variation. For instance, under open circuit and low band bending conditions where the photocurrent is null and low, respectively, many substances can still be oxidized. Furthermore, the photocurrent for a nanostructure film electrode is even limited by electron diffusion3 and even independent of the oxidation mechanism of substances sometimes. So if the photocurrent is employed for the mechanism assessment of water and organics, these data should be taken with care. In fact, in another paper4 the authors also used a baseless claim that “photocurrent measurement can in general distinguish the above two oxidation processes because the photocurrent largely increases if a direct reaction with photogenerated holes occurs, whereas it hardly increases if an indirect reaction via the intermediates of water photooxidation occurs”. References and Notes (1) Liu, H. M.; Imanishi, A.; Nakato, Y. J. Phys. Chem. C 2007, 111, 8603. (2) Villarreal, T. L.; Gomez, R.; Neumann-Spallart, M.; Alonso-Vante, N.; Salvador, P. J. Phys. Chem. B 2004, 108, 15172. (3) Fisher, A. C.; Peter, L. M.; Ponomarev, E. A.; Walker, A. B.; Wijayantha, K. G. U. J. Phys. Chem. B 2000, 104, 949. (4) Nakamura, R.; Tanaka, T.; Nakato, Y. J. Phys. Chem. B 2004, 108, 10617.
