TY - JOUR

T1 - Mechanistic Investigations of Photoelectrochemical Water and Methanol Oxidation on Well-Defined TiO2 Anatase (101) and Rutile (110) Surfaces

AU - Ahmed, Amira Y.

AU - Kandiel, Tarek A.

AU - Oekermann, Torsten

AU - Günnemann, Carsten

AU - Bahnemann, Detlef

N1 - Funding information: T.A.K. acknowledges financial support provided by the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum & Minerals (KFUPM) through the Project DF181025 and by the King Abdullah City for Atomic and Renewable Energy (K.A.CARE) through the Project KACARE182-RFP-12. D.B. acknowledges financial support from Saint Petersburg State University (Research Grant 39054581).

PY - 2019/7/22

Y1 - 2019/7/22

N2 - The mechanisms of photoelectrochemical water and methanol oxidation on TiO2 anatase (101) and rutile (110) surfaces have been studied using the intensity modulated photocurrent spectroscopy (IMPS) technique. The phenomenological rate constants for the photogenerated charge carriers transfer and recombination have been determined at different band bending values in the presence and absence of methanol as well as at different methanol concentrations. The obtained results have been analyzed based on the IMPS theory for a bulk semiconductor surface together with a model to define the nature of the surface-bound intermediates. The results of the analysis indicated that (i) water oxidation proceeds via coupling of two >OH• br radicals on both surfaces and the coupling of >OH• br radicals is about 1 order of magnitude faster on the anatase (101) surface than that on the rutile (110) surface; (ii) the rate constant of surface recombination at the flatband potential on anatase (101) surface is much lower than that on rutile (110) surface; and (iii) in the presence of methanol, the coupling of the >OH• br radicals on the anatase (101) surface is still the dominating reaction whereas on the rutile (110) surface the photogenerated >OH• br reacts rapidly with methanol, faster by a factor of 18 times than on anatase (101) surface so that the >OH• br coupling is less dominant. On the basis of those findings, it is concluded that the coupling of the >OH• br radicals is an essential step to trap the photogenerated holes and reduce the surface recombination on anatase (101) surface in the absence and presence of methanol. The faster reaction of methanol with the >OH• br on rutile (110) compared with that on anatase (101), while the former usually exhibits lower photocatalytic activity, indicates that the charge separation (not the charge transfer) is the rate controlling step in the photocatalytic process.

AB - The mechanisms of photoelectrochemical water and methanol oxidation on TiO2 anatase (101) and rutile (110) surfaces have been studied using the intensity modulated photocurrent spectroscopy (IMPS) technique. The phenomenological rate constants for the photogenerated charge carriers transfer and recombination have been determined at different band bending values in the presence and absence of methanol as well as at different methanol concentrations. The obtained results have been analyzed based on the IMPS theory for a bulk semiconductor surface together with a model to define the nature of the surface-bound intermediates. The results of the analysis indicated that (i) water oxidation proceeds via coupling of two >OH• br radicals on both surfaces and the coupling of >OH• br radicals is about 1 order of magnitude faster on the anatase (101) surface than that on the rutile (110) surface; (ii) the rate constant of surface recombination at the flatband potential on anatase (101) surface is much lower than that on rutile (110) surface; and (iii) in the presence of methanol, the coupling of the >OH• br radicals on the anatase (101) surface is still the dominating reaction whereas on the rutile (110) surface the photogenerated >OH• br reacts rapidly with methanol, faster by a factor of 18 times than on anatase (101) surface so that the >OH• br coupling is less dominant. On the basis of those findings, it is concluded that the coupling of the >OH• br radicals is an essential step to trap the photogenerated holes and reduce the surface recombination on anatase (101) surface in the absence and presence of methanol. The faster reaction of methanol with the >OH• br on rutile (110) compared with that on anatase (101), while the former usually exhibits lower photocatalytic activity, indicates that the charge separation (not the charge transfer) is the rate controlling step in the photocatalytic process.

KW - anatase (101)

KW - IMPS

KW - photocatalysis

KW - rutile (110)

KW - single crystal

KW - water oxidation

UR - http://www.scopus.com/inward/record.url?scp=85070548529&partnerID=8YFLogxK

U2 - 10.1021/acsaem.9b01163

DO - 10.1021/acsaem.9b01163

M3 - Article

AN - SCOPUS:85070548529

VL - 2

SP - 5308

EP - 5318

JO - ACS Applied Energy Materials

JF - ACS Applied Energy Materials

IS - 7

ER -