J. Phys. Chem. C 2009, 113, 10829
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COMMENTS Comment on “Rapid Photoelectrochemical Method for in Situ Determination of Effective Diffusion Coefficient of Organic Compounds” L. H. Zhu* College of Life Sciences, China Jiliang UniVersity, Hangzhou 310018, China ReceiVed: December 29, 2008; ReVised Manuscript ReceiVed: March 28, 2009 The title paper by Wen et al.1 appearing in a recent issue of this journal reported an in situ photoelectrochemical method for determination of effective diffusion coefficients of organic compounds. A great deal of thought and effort has been made in the commented paper. However, unfortunately, because the authors used a model that neglected an important affecting factor of photocurrent, that of light intensity, used for identifying the rate-limiting step and deriving effective diffusion coefficients of organic compounds, I believe their measurement principle and most important conclusion must be met with some skepticism. As the authors stated, only when the overall reaction is under diffusion-controlled conditions is their eq 5 valid for determination of the diffusion coefficient. This requires that the ratedetermining step in a photoelectrocatalysis system is the diffusion of organic compounds toward the surface of the electrode, and the limiting photocurrent is totally determined by the diffusion flux of solutes. The authors argued that at a given light intensity of 6.0 mW/cm2 (corresponding to incidence photons 1.1 × 1016 s-1 cm-2 or a maximum photocurrent of 1766 µA cm-2) their system can attain diffusion-controlled conditions if the applied potential bias is more positive than +0.30 V (versus Ag/AgCl) and the concentration of the substrate is below 0.8 mM for benzoic acid. If this light intensity is high enough to ensure such conditions, then just by further increasing the light intensity the saturated photocurrent should not increase because it is totally determined by the diffusion rate of the organic compound. Unfortunately, the authors did not provide light intensity-dependent photocurrent data. In fact, many studies have found that the saturated photocurrent was still increased when the light intensity was larger than this value even under such a similar concentration of the organic compound.2-5 Augustynski et al.6 reported that the saturated photocurrent linearly increased with light intensity even at levels to 700 mW cm-2. This leads us to conclude that, under a light intensity of 6.0 mW/cm2, the limiting photocurrent was not due to the photocatalytic oxidation of organic compounds under diffusion* To whom correspondence should be addressed. E-mail: zhulh@ cjlu.edu.cn.
controlled steady-state mass transfer conditions. Furthermore, according to the reported parameters for the organic species investigated, for example 0.8 mM oxalic acid and 0.8 mM benzoic acid, the limiting diffusion current is 32.12 and 338.91 µA, respectively. This means that at any other light intensity the limiting photocurrent cannot exceed these values at a concentration of 0.8 mM solute because these diffusion fluxes determine the saturated photocurrent. However, the saturated photocurrent reported in the literature can be larger than these values at a concentration of about 0.8 mM solute.2-5 Therefore, eq 5 cannot be used for calculation of the diffusion coefficient under the experimental conditions. Generally speaking, for most aqueous photoelectrocatalysis systems at any given potential, the photocurrents do not depend very much on mass transport or the presence of solutes in minor quantities but only on the flux of absorbed light.4,7,8 Even if the surface concentration of the solutes on the electrode is exhausted by photoholes, illuminated TiO2 under bias potential is still able to oxidize water (present at a concentration of 55 M, a process difficult to compete with) to produce a photocurrent. In summary, because the photocurrent depends on many factors, it does not necessarily directly reflect the photooxidation rate of electron donors.7-9 For instance, under open circuit and low band bending conditions where the photocurrent is null and very low, respectively, many substances can still be efficiently oxidized. Furthermore, under particular conditions, the photocurrent for a nanostructure film (see ref 2) may even be limited by electron diffusion6-8,10 and even be independent of the oxidation rate. So, if the saturated photocurrent is employed for the determination of diffusion coefficients of organics by eq 5 in their paper, these data should be taken with care, particularly the photogeneration rate of the hole (light intensitydependent) should be at least optimized to ensure that it is not the rate-determining step. References and Notes (1) Wen, W.; Zhao, H.; Zhang, S.; Pires, V. J. Phys. Chem. C 2008, 112, 3875. (2) Jiang, D.; Zhao, H.; Zhang, S.; John, R. J. Catal. 2004, 223, 212. (3) Byrne, J. A.; Eggins, B. R. J. Electroanal. Chem. 1998, 457, 61. (4) Waldner, G.; Pourmodjib, M.; Bauer, R.; Neumann-Spallart, M. Chemosphere 2003, 50, 989. (5) Waldner, G.; Gomez, R.; Neumann-Spallart, M. Electrochim. Acta 2007, 52, 2634. (6) Solarska, R.; Rutkowska1, I.; Morand, R.; Augustynski, J. Electrochim. Acta 2006, 51, 2230. (7) Peter, L. M. Chem. ReV. 1990, 90, 753. (8) Hagfeldtt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49. (9) Villarreal, T. L.; Gomez, R.; Neumann-Spallart, M.; Alonso-Vante, N.; Salvador, P. J. Phys. Chem. B 2004, 108, 15172. (10) Fisher, A. C.; Peter, L. M.; Ponomarev, E. A.; Walker, A. B.; Wijayantha, K. G. U. J. Phys. Chem. B 2000, 104, 949.
JP811466N
10.1021/jp811466n CCC: $40.75 2009 American Chemical Society Published on Web 05/22/2009
