Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

UV-Visible ار راد ن .د TiO2 ( تیفرظ راون مان هب نورتکلا یاراد یژرنا زارت لماش VB و ) رگید زارت ی یژرنا اب ( ییاناسر راون مان هب نورتکلا زا یلاخ و رتلااب VB یم ) .دشاب ت ود نیا نیب یژرنا توافت یژرنا فاکش زار ، پگ دناب هدیمان یم .دوش هک ینامز زا نورتکلا لاقتنا VB هب VB یم ماجنا دریگ ، TiO2 اب ودح یژرنا بذج د ev 2 / 3 ، نورتکلا تفج کی دیلوت یم هرفح .دیامن و نورتکلا هرفح ی نا اب هدش دیلوت یم کرتشم حطس هب لاقت ثعاب دناوت شنکاو ماجنا اه یی ددرگ . TiO2 دربراک ،دراد یدایز یاه هلمج زا یم ناوت اوه یگدولآ هیفصت یارب (CO2) و بآ و ... نآ زا هدافتسا درک .

Advanced Nanoarchitectures for Solar Photocatalytic Applications

A surface science perspective on TiO2 photocatalysis

To solve the problem of the global energy shortage and the pollution of the environment, in recent years, semiconductor photocatalytic technology that converts solar energy into chemical fuel has been widely studied. Regarding semiconductor-based photocatalysts, CdS has attracted extensive attention due to its relatively narrow bandgap for visible-light response and sufficiently negative potential of the conduction band edge for the reduction of protons. Studies have shown that CdS-based photocatalysts possess excellent photocatalytic performance in terms of solar-fuel generation and environmental purification. This critical review presents the recent advances and progress in the design and synthesis of various CdS and CdS-based photocatalysts. The basic physical and chemical properties of CdS and the related growth mechanism have been briefly summarized. Moreover, the applications of CdS-based photocatalysts have been discussed such as in photocatalytic hydrogen production, reduction of CO2 to hydrocarbon fuels and degradation of pollutants. Finally, a brief perspective on the challenges and future directions for the development of CdS and CdS-based photocatalysts are also presented.

CdS-Based photocatalysts

This feature article summarizes recent research on and development of semiconductor-based photocatalyst materials that are applicable to environmental remediation and/or chemical synthesis purposes. A wide variety of TiO2 particles and/or films have been studied during the past 30 years because they are the most stable and powerful photocatalysts leading to the degradation of various organic pollutants. The photocatalytic performance of other semiconductor materials such as ZnO, SnO2, WO3, Fe2O3 and CdS has also been intensively investigated. A general limitation in the efficiency of any photocatalytic process is the recombination of the photogenerated charge carriers, i.e., of electrons and holes, following bandgap illumination. Considerable efforts have been made to suppress this recombination and hence to enhance the charge carrier separation and the overall efficiency by means of coupling of different semiconductors with desirable matching of their electronic band structures, or incorporation of noble metal nanoclusters onto the surface of semiconductor photocatalyst particles. Modification of the physicochemical properties, such as particle size, surface area, porosity and/or crystallinity of the semiconductor materials, and optimization of the experimental conditions, such as pH, illumination conditions and/or catalyst loading, during photocatalytic reactions have also been carefully addressed to achieve high reaction rates or yields. To utilize solar energy more efficiently, i.e., to extend the optical absorption of the mostly UV-sensitive photocatalysts into the visible light range, numerous research groups have contributed to the development of novel visible light active photocatalysts. With the application of semiconductors with narrower bandgaps such as CdS, Fe2O3 and WO3 being straightforward choices, doping of wide bandgap semiconductors like TiO2 has been the most popular technique to enhance the catalysts' optical absorption abilities. Research on mixed-oxide-based semiconductor photocatalysts with deliberately modulated band structures has also attracted tremendous attention in the past decade, concentrating on, for example, the generation of H2 and/or O2 from H2O splitting, and the degradation of organic pollutants under visible light irradiation. Both theoretical calculations and experimental results have convincingly shown that the developed materials can serve as highly efficient photocatalysts that are both environmentally and economically significant.

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Photoelectrocatalytic materials for environmental applications

Mn+−TiO2 thin films (M:  Cr, V, Fe, Co) with different M/Ti ratios have been prepared by ion beam induced CVD (IBICVD) with the aim of investigating their structural, optical, and photocatalytic properties. These samples can be considered as model systems to describe the photocatalytic behavior of other Mn+−TiO2 materials reported in the literature. In their “as prepared” state the samples were amorphous and the cations were homogeneously distributed within the TiO2 layer. Crystallization of the thin films upon annealing has been investigated with X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). This latter technique has proved to be a suitable and rapid method of crystal phase assessment of TiO2. After annealing at T > 573 K, TiO2 crystallizes into the anatase or rutile structures depending on the type and amount of cations present in the film. The local environment around the different cations has been ascertained by X-ray absorption spectroscopy (XAS). It has been shown that...

Structural, Optical, and Photoelectrochemical Properties of Mn+−TiO2 Model Thin Film Photocatalysts

This paper reports an analysis of the changes in the photoemission parameters of copper in small particles of copper oxides deposited on silicon dioxide. This study is of relevance for investigations in the fields of heterogeneous catalysis and coordination chemistry. Copper oxides (Cu2O and CuO) have been deposited on the surface of a flat SiO2 substrate by evaporation of copper and subsequent oxidization of the deposited particles. XPS has been used to analyze the chemical and coordination state of copper. Large variations in the Cu 2p(3/2) binding energy (BE) and Auger parameter (alpha') have been found as a function of the type and amount of deposited copper oxide. The differences in BE calculated from the values of the lowest amount of deposited material and those of the bulk compounds were -0.4 eV (Cu2O) and -1.9 eV (CuO), while those in alpha' amounted to 2.9 (Cu2O) and 1.6 eV (CuO). The observed changes have been described in terms of the chemical state vector (CSV) concept in a Wagner plot and rationalized by considering the characteristics of bonding and electronic interactions that occur at a given oxide/oxide interface. These interactions have been modeled by means of quantum mechanical calculations with cluster models simulating the Cu-O-Si bonding at the interface. The effect of the polarization of the surrounding media around the copper cations has been also estimated for both the dispersed clusters supported on the SiO2 substrate and for the copper oxide materials in bulk form. A change in the values of alpha' and BE of copper (ie., delta alpha' = 1.1 eV, deltaBE = 0.1 eV) upon adsorption on the Cu+ species of Cu2O moieties dispersed on SiO2 of a phenyl-acetylene molecule illustrates the use of XPS to study the formation of cation-ligand complexes in heterogeneous systems. A detailed description of the bonding interactions of these coordinated Cu+ species in terms of initial and final state effects of the photoemission process has been also carried out by means of quantum mechanical calculations and cluster models.

XPS Study of Interface and Ligand Effects in Supported Cu2O and CuO Nanometric Particles

SiOx thin films have been prepared by evaporation of silicon monoxide powder in an ultrahigh vacuum chamber. The films are characterized by x-ray photoelectron spectroscopy (XPS), synchrotron photoemission, and x-ray absorption spectroscopy at the Si K edge. XPS shows that the films prepared by evaporation in ultrahigh vacuum have a SiO1.3 stoichiometry and are formed by Si3+(∼77%) and Si+(∼23%) species. Based on extended x-ray absorption fine structure analysis, the structure of these films has been described as formed by tetrahedra of the type Si–(Si, O3) and Si–(Si3, O), in agreement with the Si 2p photoelectron spectra. No significant amount of Si2+ species [i.e., Si–(Si2, O2)] tetrahedra) or elemental silicon were detected in these films. When SiOx thin films are prepared by evaporation of silicon monoxide in O2 atmosphere, the oxygen content in the film increases with the partial pressure of this gas. Under these conditions, Si4+ species are formed in detriment of the Si+ and Si3+ oxidation states. ...

Chemical stability of Sin+ species in SiOx (x<2) thin films

Structure and chemistry of SiOx (x<2) systems

TiO2 thin films have been prepared and their structure analyzed by XRD at grazing angles and x‐ray absorption spectroscopies (EXAFS/XANES). The deposition method is an ion beam induced chemical vapor deposition procedure (IBICVD) consisting of the bombardment of a substrate with accelerated O+2 ions while the vapor of a volatile metallic precursor is directed on its surface. The structure of the films was dependent on the type of substrate (glass, fused quartz, and sapphire) and on the conditions used for their preparation (temperature of substrate and beam energy of 1 or 10 keV). Thus, while on glass and silica/sapphire, an amorphous structure is formed when the substrate is at 300 K during preparation, a rutile‐rich structure is obtained if the substrate is at 573 K during deposition or, even to a larger degree, when using ions of 10 keV. The amorphous structure of the films grown on glass yield 100% anatase after calcining at 773 K. On sapphire, the crystallization was less perfect and besides anatase,...

A. Caballero

Papers

Structural, Optical, and Photoelectrochemical Properties of Mn+−TiO2 Model Thin Film Photocatalysts

XPS Study of Interface and Ligand Effects in Supported Cu2O and CuO Nanometric Particles

Chemical stability of Sin+ species in SiOx (x<2) thin films

Structure and chemistry of SiOx (x<2) systems

Contribution of the x‐ray absorption spectroscopy to study TiO2 thin films prepared by ion beam induced chemical vapor deposition