Review articleReview on charge transfer and chemical activity of TiO2: Mechanism and applications
Introduction
Titanium dioxide (TiO2), owing to its nontoxicity, stability and intriguing electronic structure, has been extensively explored for its promising applications in waste-water purification, catalysis and solar energy utilization [1]. TiO2 based materials are of great interests in energy storage and conversion devices such as rechargeable lithium ion batteries [2] and dye-sensitized solar cells [3], [4]. Various methods including metal/nonmetal doping and surface functionalization, have been explored to tailor the band gap of TiO2 (∼3.0 eV for rutile and 3.2 eV for anatase) for improving the light adsorption and charge transport [5], [6], [7], [8], [9].
Amongst the various applications based on TiO2, transport of nuclei and flow of charge frequently occur and greatly affect their overall performance. While the mass transport dominates in the growth of materials and lithium battery application, the charge transfer affects the evolution of morphology, interfacial formation, and efficiency of external field excitation [10]. Moreover, the transfer and flow of energy are normally accompanied by the charge separation, transport and recombination. Charge separation occurring in the dye-sensitized solar cells by electron injection from the excited state of the sensitizer to TiO2 dominates the efficiency of the photoelectric conversion [11]. Interfacial charge redistribution directly leads to the band bending occurring in various heterostructures consisting of TiO2. Poor charge transport is also one of the major limitation for the water oxidation [12]. Charging of the molecular adsorbates above TiO2 surface is highly important in catalytic oxidation reactions, chemical sensing, and photocatalysis [13]. For instance, as shown in Fig. 1, Lee et al. demonstrated the electron-induced dissociation of CO2 due to electron injection from the STM tip into the adsorbed CO2 adsorbed at the oxygen vacancy (VO) defect on the TiO2 (1 1 0).
Noble metals supported by TiO2 are intensively explored as the catalysis for low-temperature CO oxidation [14], [15]. The charge states of the noble metals and the charge redistribution at the perimeter [16] between these nano particles and the supports are major factors affecting the catalytic performance. Understanding the mechanism of charge transfer is of utmost importance for improving the energy transfer efficiency [17]. Through examining the charged states of the Au20 clusters supported on TiO2 (1 1 0) surface for CO oxidation, it is found that the charging/discharging of the Au20 cluster controls the amount of O2 intake at the oxide/cluster interface and strongly influences the energetics of reactions[18], [19], [20].
There is a large body of review papers on the surface, structure, and applications of TiO2 [21], [22], [23], [24], [25]. In this review, we focus on the charge transfer with respect to the mechanisms and correlation with performance. While most of the examples are taken from TiO2, the concepts and discussions for the charge transfer illustrated are valid in general and apply to other oxide surface. We start with the introduction of the methods to measure and quantify the amount of charge transfer in Section 2. In Section 3, several physical and chemical processes in TiO2 affected by charge transfer are given. In Section 4, we list the factors affecting the charge transfer. Typical materials systems based on TiO2 are given in Section 5.
Section snippets
Experimental and theoretical approaches to quantify the charge transfer
The redistribution of the charge between different materials can lead to the variation of the structural and electronic properties [26]. In principle, tracing the changes in quantities like bond length, vibrational frequencies, and oxidative state of typical atoms should facilitate the observation of charge transfer. However, due to the structural complexity in the nanoscale, determination of the degree of charge transfer is far from straight forward and normally the combination of experimental
Factors affecting charge transfer
Charge transfer occurs upon contacting different materials at the interface. Driven by the alignment of the Fermi level in the two materials, in principle, any factors that affect the electronic structure of either component of the material may change the magnitude or even the trend of the charge transfer. In the following, we list several typical factors which are proven to play important roles in modulating the charge transfer.
Band bending
Band bending refers to the phenomena of spatially dependent band energies of electrons in the proximity of the surface or interface of a semiconductor. The transport property of carriers is strongly affected by the barrier height of the space charge region due to band bending [100], [101]. The underlying mechanism of the band bending is due to the charge transfer in the vicinity of the surface or interface of a semiconducting material which forms contacts with other materials. Band bending can
TiO2-two dimensional materials
Two-dimensional (2D) materials have attracted extensive attention owing to their intriguing electronic properties. Although TiO2 has excellent electrical conductance for applications in photocatalysis and photovoltaic, the relatively large band gap of pristine TiO2 (3.2 eV for anatase and 3.0 eV for rutile) limits the lifetime of the generated photoexcited electrons and holes. Interfacing wide-band gap TiO2 with 2D materials may improve the charge separation and increase the light absorption
Conclusion
Charge transfer at TiO2 not only affects the electronic properties, but also phenomena involving coupled electron-ion dynamics such as resistive switching, chemical sensing and catalysis. The efficiencies of photon adsorption and photocatalysis strongly rely on the fast transport of photo excited carriers across the interface between the sensitizer or the reactants to the TiO2. In this review, various techniques including both experimental and theoretical methods to estimate the degree of
Acknowledgement
Cai Y. gratefully acknowledge the financial support from the Agency for Science, Technology and Research (A∗STAR), Singapore.
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