Hydrogen (H2) production via photocatalytic water splitting is one of the most promising technologies for clean solar energy conversion to emerge in recent decades. The achievement of energy production from water splitting would mean that we could use water as a fuel for future energy need. Among the various photocatalytic materials, titanium dioxide (TiO2) is the dominant and most widely studied because of its exceptional physico-chemical characteristics. Surface decoration of metal/non-metal on TiO2 nanoparticles is an outstanding technique to revamp its electronic properties and enrich the H2 production efficiency. Metal dopants play a vital role in separation of electron-hole pairs on the TiO2 surface during UV/visible/simulated solar light irradiation. In this paper, the basic principles, photocatalytic-reactor design, kinetics, key findings, and the mechanism of metal-doped TiO2 are comprehensively reviewed. We found that Langmuir-Hinshelwood kinetic model is commonly employed by the researchers to demonstrate the rate of H2 production. Copper (Cu), gold (Au) and platinum (Pt) are the most widely studied dopants for TiO2, owing to their superior work function. The metal dopants can amplify the H2 production efficiency of TiO2 through Schottky barrier formation, surface plasmon resonance (SPR), generation of gap states by interaction with TiO2 VB states. The recent advances and important consequences of 2D materials, perovskites, and other novel photocatalysts for H2 generation have also been reviewed.

Photocatalytic hydrogen production using metal doped TiO2: A review of recent advances

TiO2 nanoparticles acquire complete crystalline anatase phase on thermal treatment of as-prepared anatase TiO2 at 450°C. Anatase-rutile mixed phase and rutile phase are achieved by annealing anatase TiO2 at 700°C and 950°C respectively. The anatase-rutile mixed phase TiO2 has 87.8% rutile phase. This signifies that the percentage of rutile fraction in mixed phase can be tailored by changing the annealing temperature. As-prepared anatase TiO2 with a crystallite size of 5 nm has a positive strain (η) of 0.0345, which is due to the presence of oxygen defects on the surface and on the grain boundary. Removal of defects releases the strain and relaxes the lattice to its normal state, and thus, a negative strain η of (−) 0.0006 is observed in complete rutile phase. The interface between nearest anatase crystallites and between anatase and rutile crystallites contains oxygen vacancies that act as nucleation site for the growth of rutile nuclei. These oxygen defects are responsible for the broadening of the Raman Eg peak of anatase and for the shortening of the phonon lifetime in a 5-nm-sized anatase nanocrystallite. Removal of defects decreases the Raman peak width and increases the phonon lifetime in a larger rutile crystallite. The long lifetime of phonon in a larger rutile crystallite is due to temperature-dependent anharmonic phonon coupling.

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Local structure modification and phase transformation of tio2 nanoparticles initiated by oxygen defects, grain size, and annealing temperature

Optical and magnetic studies on Gd doped ZnO nanoparticles synthesized by co-precipitation method

Mixed phase nanocrystalline titania are prepared by simple sol–gel method. The physico-chemical characteristics of the prepared nanoparticles are studied with X-ray diffraction, high-resolution transmission electron microscopy, RAMAN, BET, UV–Vis, steady state and time resolved photoluminescence. X-ray diffraction and Raman spectra clearly demarcate the anatase and rutile phase as both the phases give different diffraction patterns and Raman peaks. A comparison in the band gap indicates that pure anatase and rutile phase have band gap in the UV region, whereas a mixture of these phases has lower band gap and corresponds to the visible region. Steady state and time resolved photoluminescence are employed to understand the emissivity and carrier lifetime. The photocatalytic activity is evaluated by monitoring the degradation of phenol under visible light illumination. Due to the synergistic effect of mixed anatase and rutile phases, mixed phase nanocrystalline titania exhibit superior photocatalytic activity.

https://link.springer.com/content/pdf/10.1007%2Fs13204-013-0264-3.pdf

Investigation of the optical property and photocatalytic activity of mixed phase nanocrystalline titania

In-situ manganese doping of TiO2 nanostructures via single-step electrochemical anodizing of titanium in an electrolyte containing potassium permanganate: A good visible-light photocatalyst

Magnetic property study of Gd doped TiO2 nanoparticles

Enhanced visible light photocatalytic activity of Gadolinium doped nanocrystalline titania: An experimental and theoretical study

Effect of manganese doping on the optical property and photocatalytic activity of nanocrystalline titania: Experimental and theoretical investigation

Herein, we have examined the role played by non-magnetic Mn on the magnetic properties of TiO2 nanoparticles. The samples display complete paramagnetic behavior at room temperature as well as at 10 K. Paramagnetism might be associated with the presence of isolated magnetic spins of Mn2+ in the system. Observation of negative Curie–Weiss temperature indicates presence of antiferromagnetic interaction in the system. Direct exchange interaction of Mn2+–Mn2+ and antiferromagnetic superexchange interaction of Mn2+ ions via lattice O2− ions contribute to antiferromagnetism. Surprisingly, ferromagnetism appears in pure and Mn doped (0.07 mol) TiO2 nanoparticles on vacuum calcination. Although Mn doped TiO2 displays ferromagnetism, there is a loss in magnetization as compared to the undoped TiO2. It is speculated that low growth temperature during vacuum calcinations might have resulted in amorphous MnO phase separation. This could result in competing ferromagnetic–antiferromagnetic interactions and overall reduction in magnetization. From the results it is evident that TiO2 could be made ferromagnetic by introducing sufficient concentration of oxygen vacancies in the system. Mn doping, however, has an adverse effect on the overall ferromagnetic ordering, as there is possibility of antiferromagnetic d–d exchange interaction of Mn2+ ions as well as possibility of antiferromagnetic manganese oxide phases separation.

Susmita Paul

Papers

Investigation of the optical property and photocatalytic activity of mixed phase nanocrystalline titania

Magnetic property study of Gd doped TiO2 nanoparticles

Enhanced visible light photocatalytic activity of Gadolinium doped nanocrystalline titania: An experimental and theoretical study

Effect of manganese doping on the optical property and photocatalytic activity of nanocrystalline titania: Experimental and theoretical investigation

Adverse effect of Mn doping on the magnetic ordering in Mn doped TiO2 nanoparticles