CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Comment on “Photocatalytic Oxidation Mechanism of As(III) on TiO2: Unique Role of As(III) as a Charge Recombinant Species”
C
hoi et al. claimed that they provided undisputed evidence to support that As(III) acted as a charge recombination center mainly by carrying out time-resolved diffuse reflectance (TDR) spectroscopy measurements.1 However, unfortunately, the authors neglected important affecting factors of TDR signal. We believe that most of their conclusions must be met with serious skepticism. The continuity equation for excess electron density, n, in TiO2 is given by Dn=Dt ¼ G - R total - Rtransfer
ð1Þ
where G is the photogeneration rate of electron, Rtotal is the total recombination rate of electron both inside (Rin) and on the surface (Rsurf), and Rtranfer is the net electron transfer rate to the acceptors in the solution. The authors concluded that As(III) served as recombination centers, based on an assumption that if As(III)/As(IV) works as recombination centers the presence of As(III) should accelerate the charge recombination. This conclusion is invalid because of the following: (i) According to eq 1, their assumption is valid only if both G and Rtranfer is unchanged. However, they did not rationalize this assumption. Absolute TDR intensity is initial light intensity-dependent. No evidence supports that this intensity was the same. Rtranfer can be neglected, if either it is much slower than the As(III)-mediated recombination, or it does not change. However, no substantial evidence supported that the two time domains (>20 μs vs submicroseconds to nanoseconds) could be assigned to the above two processes, respectively. Additionally, we found a higher Rtranfer with As(III) (higher dark cathodic steady-state current, not shown). Even if both G and Rtranfer are the same, no evidence indicates that other types of recombination, like Rin, were unchanged. (ii) More importantly, they claimed that the accelerated recombination was fully consistent with their observation that the photocurrent decreased upon spiking with As(III). Interestingly, we observed the opposite behavior (Figure 1). So, according to their logic, if As(III) totally acts a recombination center, how do they explain such an increase in photocurrent? In fact, that the decreased absorption of trapped hole and their speculation on the hole fate, is in turn consistent with the increase in the photocurrent we observed. The concomitant decrease in the absorption of electron is most probably due to acceleratory transferring to other acceptors rather than As(IV), which agrees with the higher Rtranfer .We have disputed against2 the validity of the photocurrent measurement.3 Their claim that the additive behaviour of any electron donor should increase the photocurrent4 is conditional because the photocurrent depends on both electron donor oxidation (charge separation) efficiency and electron collection efficiency.5 Whether the photocurrent varies with As(III) should depend on the change in the product of the two efficiencies. Considering this, we believe there will have no the so-called paradox in ref 3 at all. (iii) The authors speculated that a part of electrons had a long lifetime (>20 μs claimed), while the nonrecombined holes transferred to water not r 2011 American Chemical Society
Figure 1. Change of photocurrent upon spiking As(III) in the airequilibrated electrolyte. The Degussa P25 TiO2 was similar to that in ref 1, and the anatase TiO2 was similar to ref 2. The potential was 0.1 V (vs saturated calomel electrode). Other conditions were the same as that in ref 3.
As(III). This requires that this part of holes should transfer to the solution faster than 20 μs, otherwise it will rapidly recombine with the remaining electrons via As(III) (submicroseconds to nanoseconds). However, we found that the hole oxidizing water or hydroxyl is in the time scale of millseconds.6 So far we call most attention to the use of TDR spectroscopy measurements and reliable interpretations deduced thereof. Although superoxides were involved in the As(III) PCO process as suggested by the authors in several papers,3,4,7 none of them provided undisputed evidence to support As(III) acting as a recombination center and superoxides being the main oxidant. We argue that as long as any one opposite fact and other plausible explanations like the ones presented above are provided their conclusion is invalid. W. H. Leng,* X. Li, H. Fei, J. Q. Zhang, and C. N. Cao Department of Chemistry, Yuquan Campus, Zhejiang University, Hangzhou, 310027, China
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Project 50971116 supported by National Natural Science Foundation of China. Published: February 09, 2011 2028
dx.doi.org/10.1021/es1040046 | Environ. Sci. Technol. 2011, 45, 2028–2029
Environmental Science & Technology
CORRESPONDENCE/REBUTTAL
’ REFERENCES (1) Choi, W.; Yeo, J.; Ryu, J.; Tachikawa, T.; Majima, T. Photocatalytic oxidation mechanism of As(III) on TiO2: Unique role of As(III) as a charge recombinant species. Environ. Sci. Technol. 2010, 44, 9099–9104. (2) Leng, W. H.; Cheng, X. F.; Zhang, J. Q.; Cao, C. N. Comment on “Photocatalytic oxidation of arsenite on TiO2: Understanding the controversial oxidation mechanism involving superoxides and the effect of alternative electron acceptors. Environ. Sci. Technol. 2007, 41, 6311– 6312. (3) Ryu, J.; Choi, W. Y. Photocatalytic oxidation of arsenite on TiO2: Understanding the controversial oxidation mechanism involving superoxides and the effect of alternative electron acceptors. Environ. Sci. Technol. 2006, 40, 7034–7039. (4) Ryu, J.; Choi, W. Response to comment on “Photocatalytic oxidation of arsenite on TiO2: Understanding the controversial oxidation mechanism involving superoxides and the effect of alternative electron acceptors. Environ. Sci. Technol. 2007, 41, 6313–6314. (5) Leng, W. H.; Barnes, P. R. F.; Juozapavicius, M.; O’Regan, B. C.; Durrant, J. R. Electron diffusion length in mesoporous nanocrystalline TiO2 photoelectrodes during water oxidation. J. Phys. Chem. Lett. 2010, 1, 967–972. (6) Cowan, A. J.; Tang, J. W.; Leng, W. H.; Durrant, J. R.; Klug, D. R. Water splitting by nanocrystalline TiO2 in a complete photoelectrochemical cell exhibits efficiencies limited by charge recombination. J. Phys. Chem. C 2010, 114, 4208–4214. (7) Lee, H.; Choi, W. Photocatalytic oxidation of arsenite in TiO2 suspension: Kinetics and mechanisms. Environ. Sci. Technol. 2002, 36, 3872–3878.
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dx.doi.org/10.1021/es1040046 |Environ. Sci. Technol. 2011, 45, 2028–2029
