Visible Light Active Nanocomposites for Photocatalytic Applications

Introduction

The “Nanocomposite” is widely illustrated as hybrid material in which at least one of the phases exhibit dimensions in the nanometer range (1nm=10^-9m). The addition of the nanosized second material assists to achieve enhanced unique behaviour created by the synergistic between constituents. The properties of nanocomposites depend on various parameters such as extent of loading and distribution, shape, size and orientation of the nanoscale second phase and interactions within the materials forming the nanocomposites.

In this chapter, further discussion will be restricted to the visible light active nanocomposites utilized for enhanced photocatalytic hydrogen generation and dye degradation. Most of the energy that we come across today is derived from non-renewable fossil fuels such as coal and petroleum. Fossil fuels cannot be considered as ideal due to some reasons:

• 1.

  Evolution of carbon dioxide (CO₂), which is one of the major greenhouse gases

• 2.

  Risk of depletion in future and

• 3.

  Waste of time and money in the import, relocation and distribution of these fuels.

Therefore, there is an immense need of a sustainable low cost and environmental friendly energy source for future.

Hydrogen is considered as an ideal energy carrier because of the abundance, high heat energy (122 kJ/g) compared to other fuels such as gasoline (40 kJ/g) and environmental friendliness as it never produces any greenhouse gases on combustion (Dutta, 2014; Liao et al., 2012). Hydrogen generation methods can be broadly classified into two categories i.e via (i) renewable resources such as water and solar energy through photocatalytic and photoelectrochemical (PEC) water splitting, and (ii) non-renewable resources such as coal and methane through steam reforming and coal gasification. Semiconductor photocatalytic water splitting to produce large-scale clean and recyclable hydrogen (H₂) and oxygen (O₂) using solar energy is a promising technology for energy conservation (Ismail & Bahnemann, 2014; Jing et al., 2010; Maeda, 2011). Photoelectrochemical water splitting was first demonstrated by Fujishima and Honda in 1972 and since then lots of progress have been observed by the across the world in the area of photocatalytic water splitting technology (Fujishima & Honda, 1972).

In addition to that, textile dyes mainly reactive dyes are the largest source of organic compounds that possess enhanced environmental risk and approximately, 1-20% of the world’s total dye production is lost as effluent during the process of dyeing. Semiconductor photocatalytic process such as advanced oxidation technology has been reported as an economic, environmental friendly and sustainable wastewater treatment method. However, the major technical barrier that restricts its commercialisation is the recovery of the photocatalyst particles after water treatment (Chong et al., 2010).

In the subsequent sections, different synthetic routes and properties of TiO₂ nanocomposite will be discussed in details. TiO₂ nanoparticles have been extensively investigated for application including dye degradation and H₂ production as a fuel using solar energy. Though TiO₂ is an efficient, chemically stable and cheap photocatalyst but its activity is restricted to UV radiation, which is only 3-5% of solar radiation. As solar radiation contains ~40% visible light, current research is directed towards the synthesis of visible light active nanocomposites in order to utilize solar energy efficiently for the photocatalyst industrial applications.

Background

Hetrogeneous semiconductor photocatalysis is among the most promising technologies for solar energy conversions and environmental air/water purification applications. Several semiconductors have band gap energies sufficient to be used as photocatalyst including TiO₂, ZnO, Fe₂O₃, WO₃, SnO₂, ZrO₂, CdS, SrTiO₃ and ZnS. Having high chemical stability, enhanced photocatalytic activity, non-toxicity, corrosion resistance and low cost, TiO₂ has become most widely used photocatalyst solar hydrogen generation and photocatalytic water treatment applications (Lee & Park, 2013). Various significant crystalline properties of TiO₂ are listed in Table 1.