Surface Modification of Small Particle TiO₂ Colloids with Cysteine for Enhanced Photochemical Reduction:  An EPR Study

Abstract

Surface complexation of colloidal titanium dioxide nanoparticles (40−60 Å) with cysteine was investigated by electron paramagnetic resonance (EPR) and infrared (diffuse reflectance infrared Fourier transform−DRIFT) spectroscopies. Cysteine was found to bind strongly to the TiO2 surface, resulting in formation of new trapping sites where photogenerated electrons and holes are localized. Illumination of cysteine-modified TiO2 at 77 K resulted in formation of cysteine radicals with the unpaired electron localized on the carboxyl group. Upon warming to 150 K, these radicals are transformed into sulfur-centered radicals as observed by EPR spectroscopy. We have demonstrated the existence of two surface Ti(III) centers on cysteine-modified TiO2 particles having different extents of tetragonal distortion of the octahedral crystal field. Upon addition of lead ions, a new complex of cysteine that bridges surface titanium atoms and lead ions was detected by IR spectroscopy. Illumination of lead/cysteine-modified TiO2 did not result in the formation of sulfur-centered radicals. Instead, a symmetrical, lattice defect type EPR signal for trapped holes was observed. Addition of methanol to this system resulted in the formation of a ·CH2OH radical at 8.2 K. After the temperature was raised to 120 K, doubling of the signal associated with electrons trapped at the particle surface (Ti(III)surf) was observed. On further increase of the temperature to 200 K, the EPR signal for trapped electrons disappeared due to the reduction of Pb2+ ions, and metallic lead precipitated at room temperature. Conversion of photogenerated holes in the presence of methanol into trapped electrons can lead to the doubled quantum efficiency of metallic lead precipitation.

Kinetic Study of The Photoelectrochemical Gold Recovery from Diluted Chloride Solutions

“…Photogenerated charge carriers may be also trapped at the surface and then the redox reactions may be realized via the surface states which are usually located in the middle of the band gap. It prevents from the recombination, nonetheless the reduction or oxidation potentials of electron and hole are reduced in comparison with the charge carriers in the conduction and valence band respectively [9].…”

Molecular Structure Determination for Photogenerated Intermediates in Photoinduced Electron Transfer Reactions Using Steady-State and Transient XAFS

“…In addition, we have modified the TiO, surface by amino acids and other organic thiol compounds. An EPR study [9] shows that the charge separation time on TiO, colloid surfaces is increased and the reaction efficiency is greatly enhanced upon such modifications. Some heavy metal ions which could not be easily reduced or reduced at all were photoreduced by modified TiO,.…”

Enhanced photocatalytic reduction of Hg(II) in aqueous medium by 2-aminothiazole-modified TiO2 particles

“…This modification also leads to the accumulation of trapped electrons and holes, permitting the injection of multielectrons into the sorbed metal, which is advantageous since multielectron transfers usually require lower reduction potentials than single electron transfers. [10][11][12] Another important factor is the enhanced sorption of mercury on the TiO 2 surface, which facilitates charge transfers from the TiO 2 to the sorbed mercury. 10 The present paper describes the preparation of titania chemically modified with 2-aminothiazole groups, its characterization, sorption studies and photocatalytic reduction of mercury (II) in aqueous medium.…”

“…9 Surface modification of TiO 2 with carboxylic acids and arginine has shown enhanced photoreduction of mercury (II). 10,11 This is attributed to the replacement of surface OH groups and the coordination of surface titanium atoms with a carboxyl group, generating new trapping sites that localize photogenerated holes and electrons outside the semiconductor. The modifier increases the distance between trapped electrons and holes, preventing the occurrence of recombinations.…”

“…[10][11][12] Another important factor is the enhanced sorption of mercury on the TiO 2 surface, which facilitates charge transfers from the TiO 2 to the sorbed mercury. 10 The present paper describes the preparation of titania chemically modified with 2-aminothiazole groups, its characterization, sorption studies and photocatalytic reduction of mercury (II) in aqueous medium. The principal objective was to demonstrate the direct correlation between the enhanced photocatalytic properties of modified TiO 2 and the effectiveness of mercury (II) sorption onto the modified TiO 2 surface and its photoreduction.…”