Energy shortage and environmental pollution have become prime concerns of modern society. Among all energy sources used in the world, fossil fuel contributes ~80%, which causes environmental pollution and climate change. Fossil fuel is limited and non-renewable. A sustainable and long-term society needs urgent environment-friendly and renewable technologies for green energy production and environmental remediation. TiO2 utilizes solar energy for facilitating environmentally beneficial reactions through photocatalytic activity (PCA) by splitting of water, CO2 reduction to produce green energy (hydrogen and hydrocarbon fuels). It also degrades harmful pollutants (industrial wastes/dyes) to safe and useful products. Other important applications like a photo-anode in a dye-sensitized solar cell, self-cleaning glass coating material, gas sensors, UV absorber, white pigment, etc. add to the list. As TiO2 is non-toxic, low cost and has high chemical stability, it is good for real-world application.
Property of materials is entirely dependent on its crystal structure, surface morphology, grain size, etc.,. TiO2 has three naturally occurring polymorphs (anatase (A), brookite (B), and rutile (R)). Depending on the crystal structure, each polymorph has different types of application. Anatase and brookite are metastable phases. Anatase phase is formed at a lower temperature due to its lower surface free energy. Most of the photocatalytic applications are in this form. Due to complicated crystal structure and stability problems, brookite doesn’t have many applications. With increasing temperature, metastable anatase and brookite phase irreversibly transformed into thermodynamically stable rutile phase. This transition is neither a distortive or a displacive type; it’s a reconstructive process involving breaking and reforming of Ti-O bonds.
In the pigment industry, ~two third of white pigment is rutile TiO2. It has other applications as capacitors, dielectric mirrors, etc. Degussa P25 is a standard titania photocatalyst, where the majority is anatase phase (A: R = 3:1). Hence, stability of the required phase and its phase transition behavior is very crucial for any application. There are many parameters (e.g, synthesis processes, environment of calcinations, temperature and time, pH, applying strain, initial crystallite size, agglomeration or interfaces, doping of foreign elements etc., which can control phase transition, grain growth/size, and surface morphology and thereby tune the properties of TiO2. Ionic radius and valence states of dopants play a very important role in the phase transition from anatase to rutile (A®R) and thereby change properties (optical, electrical, electronic, magnetic, etc.). It modifies the electronic band-structure, charge transport, and surface activity of TiO2. Though anatase TiO2 has many advantages as a photocatalyst due to its lower effective mass and longer charge carrier lifetime. However, wide bandgap (3.2 eV) and metastability are the two obstacles which restrict its widespread application [7]. Only ~5% of solar light can be absorbed due to its wide bandgap, where the rest of the solar radiation remains unutilized.
Researchers are using different dopants (Zn, Fe, Al, Cu, Ni, Mo, Nd, etc., [7, 24, 25]) to control the phase transition (A®R) by modifying the lattice structure and reduce the bandgap of TiO2 to the visible light region. Some dopants are good to stabilize the anatase phase but not effective to reduce the bandgap. Song et al. [19] reported that Al doping could stabilize the anatase phase as well as reduce the bandgap to the visible light region. However, for this to happen a high amount of doping is required. It was noticed that a high amount of doping sometime might create unwanted defects states which increase the recombination rate of photo-generated charge carrier and results in the reduction of photocatalytic activity and photo-electrochemical performance.
On the other hand, doping of certain elements is effective to reduce the bandgap but destabilizes the anatase phase [27]. Oxygen vacancies (OV) and subsequent conversion of Ti4+ to Ti3+ helps to reduce the bandgap and increase the photocatalytic and photoelectrochemical performance. Liu et al.[28] reported that due to the presence of Ti3+ photocatalytic activity of TiO2 increases and hydrogen production rate increases by 12.5 times than P25. Chen et al.[18] reported that TiO2 showed excellent solar-driven photocatalytic properties towards hydrogen production due to the presence of Ti3+ and OV. In most cases, complicated and high cost technique (heating of TiO2 in reducing atmosphere in high pressure [29, 30], bombardment of TiO2 by high energy laser irradiation or Ar+ ion sputtering[31, 32], air plasma treatment [18, 33], chemical vapor deposition [34], etc.,) are used for introduction of Ti3+ and OV in TiO2 lattice. To address these issues, a simple, low-cost sol-gel process where a charge compensat ed (Ga3+: V5+ = 1:1) Ga-V co-doing is used to prepare stable anatase phase with a bandgap in the visible light region. Systemic doping reveals the effect of Ga-V co-doping in phase transition, grain growth process and optical properties.