Preparation Characterization and Photocatalytic Activity of Polycrystalline ZnO/TiO₂ Systems. 1. Surface and Bulk Characterization

Abstract

Polycrystalline ZnO/TiO2 solids have been prepared with four different methods using home prepared TiO2 (anatase) or TiO2 (rutile) as supports and Zn(NO3)2·6H2O or Zn(CH3COO)2·2H2O as precursors for ZnO. The bulk and surface properties of the samples were investigated by means of TG-DTA, XRD, TEM, SEM-EDAX, XPS, BET surface area determination, and porosity measurements. XRD and TEM results indicate that no significant defect structures exist in any of the samples. The ZnO crystallinity and its enrichment on the surface of TiO2 particles were dependent on the preparation method. The surface areas generally decrease by increasing the amount of ZnO except when ZnO from Zn(CH3COO)2·2H2O was supported on TiO2 (rutile). The samples prepared from Zn(CH3COO)2·2H2O were more porous than those prepared from Zn(NO3)2·6H2O. This was confirmed by BET surface area determinations and SEM observations. XPS spectra indicate that the atomic ratio between OH- and O2- on the particles surface is similar for samples with the same ZnO content independent of the precursor used for the samples' preparation. Moreover a much higher segregation of ZnO was found for samples obtained by using the acetate precursor.

TiO2 nanocluster modified-rutile TiO2 photocatalyst: a first principles investigation

“…The shifts in the energy gaps determined from the DOS analysis are from simple Kohn-Sham energy eigenvalue differences, which do not have the same physical meaning as an excitation energy. Therefore, care needs to be taken in 40 making predictions of shifts in the energy gaps as a result of modifying TiO 2 with the nanocluster. To confirm the effect of surface modification on the light absorption properties of rutile (110), we have also computed the optical (Tauc) gap of the heterostructures.…”

“…We also find that the workfunction of the composite is reduced by around 1 eV compared to the bare surface, which will be important for enhanced reactivity. The importance of forming an interface between two materials 20 to enhance photocatalytic activity is discussed in the literature [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] and new materials combinations are being prepared, e.g. Ismail reported a synthesis of PdO-TiO 2 nanocomposite through one-step sol-gel reaction with enhanced photocatalytic activities 78 .…”

“…While DFT calculations are excellent at providing details on 35 the electronic properties of the type of heterostructures studied in this paper, screening many different adsorption structures becomes unfeasible for structures of the size considered herein. Therefore, a realistic atomistic potential based model can be used to determine favourable adsorption 40 structures for further refinement with DFT. There is much interest in using the charge equilibration (QEq) scheme of Rappé and Goddard 54,55 for TiO 2 structures, in which the electronegativity of the ions is used to adjust the charge distribution in an adaptive manner.…”

“…Interestingly, while SnO 2 -modified anatase TiO 2 showed only UV activity 32 , SnO 2 modified rutile displayed some visible light activity and improved UV activity 33 . In all cases, the 40 activity was reported for dye degradation, which is important for environmental applications. The question of the potential utility of these structures for photocatalytic water splitting has not been addressed to date and remains an interesting avenue of exploration.…”

“…Since the band gap of the rutile and anatase forms of TiO 2 lies in the UV region (typically in the range of 3 eV to 3.2 eV) a major activity in 35 delivering widespread photocatalytic technologies is to shift the band gap in order to have a material that will be active in the visible region of the solar spectrum, thus using a larger fraction of the available solar energy than that available in the UV 8,9 . 40 Until recently, most efforts in modifying TiO 2 to enhance visible light absorption have focused on substitutional cation or anion doping at Ti or O sites [10][11][12][13][14][15][16][17][18] to achieve a band gap reduction. Recent work has treated co-doping with compensating cation-anion pairs [19][20][21] .…”