Sputter deposited titanium oxide films for photoelectrochemical water purification

Abstract

Non-stoichiometric polycrystalline Ti oxide thin films were prepared by reactive DC magnetron sputtering of Ti in Q2+Ar. Sputtered Ti oxide samples with four values of the film thickness were investigated by transmission electron microscopy (TEM) Cross-sections of (he specimens shows thai iheir most salienl feature is a parallel penniform structure extending over che fall cross-sections of the films. A very large intemal surface is manifest Rutile and anatase phase films were obtained by heating the substraia during the deposition. Incident photon-to-currem efficiency (IPCE) was studied as a function of several spuuenng parameters such as temperature of the substrate, film thickness, and 02/Ar gas flow ratio. The IPCE increased as the crystallinity of Ti oxide was systematically varied from amorphous to a mixture of analise and rutile, and it was also enhanced in films depositad at !ow O2/Ar ratios. The photogenerated current was found to be highly dependen on the composition, thickness, and structure of Ti oxide films. Optical constans of the films were determined from spectrophotometric measuremems of reflectante and transmittance. The amorphous films had a band gap of -3.4 cV and a wide absorption tail extending 10 lower energics. The crystalime films displayed a band gap of 3,3-3.35 eV and a more narrow absorption tail. The optical constants were dependem on film ihickness The crystalline films showed evidence of structural inhomogeneilics Icading to diffuse scatiering in the thicker films as well as to a grading of the refraciive Index. The complex dielectric permittivity £ was determined in the 10'3 to 107 H2 range from measurements using a capacitor configuration. The real and imaginary parts of s displayed power-law dependencies from - 10 Hz up to a value between 1 and 10 kHz, Our data could be rationalized in terms of a model for screened hopping of vacancy-inouced charge carriers, but details remain poorly undcrstood. The internal surface area was enhanced with increasing film ihickness, and that property could be correlated with an enlarged photoelectric conversión eficiency. Ti oxide films with thicknesses in the 1,9-8.0-mm range were sputter deposited orno etectrically conductive transparenl glass substrates. X-ray diftraction indicated a dominating rutile structure. The films were used to investigate photo-electrocatalytic degradation of 4-chlorophenol in water solution, employing a specially designed reactor. ITV light was used for (he photocatalysis, and a bias potential was applied for avoiding electron-hole recombination. The photocurrent and the kineiics of the catalytic reaaion were tested with potentiosiatic and spectrophotometric measurements, The reaction producis were investigated using different arrangements of a Ti oxide working electrode and a Pt counter electrode, IA irradiation through the gfass substrate yielded an enhancement of the reaction kinetics for increased film thicknesses and applied bias potential. X-ray diffractomeiry showed that nitile-like and anatase-Hke films were produced depending on the oxygen conteni in the sputter plasma. Rutherford backscatiering spectrometry documented some oxygen overstoichiometry. Spectral optical measurements were used to analyze the absorption around ihe fundamentari band gap and lo give evidence of some hydration and hydroxylation in ihe films. The various Ti oxide films were broughi in contact wiih 4--chlorophenol (4-CP), whose photo-electrocatlytically induced degradation under ukraviolet irradiation was investigated in a reacior allowing optical probing of 4-CP as well as of intermediate reaction products such as benzoquinone. A miile-üke structure was conductive to the degradation of 4-CP, which can be reconciled with the band gap being suitable for producing photoinduced heles capable 10 effecting oxidation of the pollutant.