Quantitative analysis of defective sites in titanium(IV) oxide photocatalyst powders

Abstract

______________________________________________________________________The molar amounts of defective sites (M d ) in several titanium(IV) oxide (TiO 2 ) powders were determined using photoinduced reaction of electron accumulation in deaerated aqueous solutions containing sacrificial hole scavengers and subsequent reduction of methylviologen to its cation radical. The measurements of pH dependence of typical anatase and rutile TiO 2 powders showed that these defective sites were of electronic energy just below the conduction band edge of TiO 2 in ranges of 0-0.35 V for anatase and 0-0.25 V for rutile. A linear relation of M d with the rate constant of electron-hole recombination determined by femtosecond pump-probe diffuse reflection spectroscopy revealed that M d could be a quantitative parameter of recombination between a 2 photoexcited electron and a positive hole. The tendency for M d to increase, but not proportionally, with increase in the specific surface area suggested that these defective sites are mainly located near the external surfaces of TiO 2 particles. A reciprocal correlation between photocatalytic activity for water oxidation in aqueous silver sulfate solution and M d revealed that the rate of recombination is one of the predominant physical properties governing the activities of TiO 2 powders in this reaction system. ______________________________________________________________________ IntroductionPolycrystalline titanium(IV) oxide (TiO 2 ) has been widely used as semiconductor photocatalysts in liquid-solid as well as in gas-liquid regimes 1-9 because of its sufficient photostability in various circumstances, nontoxicity for environments, and potent ability to induce various types of redox reactions. 10,11 It is well known that photocatalytic activity strongly depends on bulk and surface physicochemical properties of the TiO 2 powder, such as the kind of crystal structure, 12-15 surface area and particle size, [16][17][18][19][20] Experimental SectionMaterials. Several kinds of titanium oxide (TiO 2 ) powders, those from commercial sources (Degussa P-25, Merck, Ishihara CR-EL, and Hombikat UV-100), reference catalysts supplied from the Catalysis Society of Japan (JRC-TIO-1, -2, -3, and -5), and powders synthesized by the HyCOM method 6,28,29 followed by a calcination at 1073 K under O 2 (designated as HyCOM (1073)), were used. In a separate experiment to quantify the amount of accumulated electrons by another chemical reaction, a deaerated aqueous solution containing colloidal platinum particles was added to the photoirradiated suspension under Ar instead of the addition of MV 2+ . After shaking the mixture for a few minutes, the gas phase was analyzed using a gas chromatograph (GC, Shimadzu, GC-8A, equipped with an MS-5A column (GL Sciences) and a TCD detector). Measurement of the amount of accumulated electrons in photoirradiated TiOPP-DR spectroscopy. Details of PP-DR spectroscopy were reported previously 40,41 and are briefly summarized as follows. The light source consisted of a mode-lo...

Photoacoustic spectroscopic analysis of photoinduced change in absorption of titanium(IV) oxide photocatalyst powders: A novel feasible technique for measurement of defect density

“…The amount of Ti 3+ was measured by photoinduced electron accumulation in TiO 2 suspended in deaerated aqueous solutions containing sacrificial hole scavengers and subsequent reduction of methylviologen to its cation radical [3].…”

“…(2)). (2) Figure 4 shows the correlation of I 520 measured under nitrogen atmosphere with the density of Ti 3+ measured photochemically [3]. The reported density of Ti 3+ for Merck, CR-EL, Hombikat UV-100, Degussa P-25 and JRC-TIO-1-5 was used [3].…”

“…We have developed a photochemical method to measure the molar amount of trivalent titanium species (Ti 3+ ), a trapped electron on surface and bulk titanium atoms, as an empirical measure of the density of crystalline defects in TiO 2 samples and its reciprocal relation with photocatalytic activity. We have proposed a linearity of the Ti 3+ density with the defect density but with no certified claim that these two species are identical [3]. Electron paramagnetic resonance (EPR) has also been used to measure such species [4][5][6][7][8], though active oxygen species liberated simultaneously may interfere.…”

Revisiting the fundamental physical chemistry in heterogeneous photocatalysis: its thermodynamics and kinetics

“…As an example of missing properties of photocatalysts, structural characterization of the recombination center in particles, which is believed to be a crystalline lattice defect, 38 is needed. An example of missing primary steps in photocatalysis is reduction of O 2 molecules adsorbed on the particle surfaces, the fate after reduction and the role of intermediate species in the mechanism.…”

Development of a novel photocatalytic reaction system for oxidative decomposition of volatile organic compounds in water with enhanced aeration

“…The production of small amount of acetone under Ar atmosphere is attributable to the oxidation of 2-propanol by positive holes (h + ) along with the formation of Ti 3+ species by trapping of electrons (e -) at defective sites in TiO 2 [20][21][22]. As discussed in previous report [22], such accumulation of e -was also observed by photoirradiation to a deaerated aqueous TiO 2 suspension containing methanol and triethanolamine in the absence of reduction catalyst such as platinum. The most striking feature in these plots is that just switching the atmosphere in the gas space changes the rates of acetone productions; rates of acetone production in the presence of O 2 in the gas space were about three times higher than those in the presence of Ar.…”