B. S. Naidu

Importance of trivalency and the e(g)(1) configuration in the photocatalytic oxidation of water by Mn and Co oxides.

Prompted by the early results on the catalytic activity of LiMn2O4 and related oxides in the photochemical oxidation of water, our detailed study of several manganese oxides has shown that trivalency of Mn is an important factor in determining the catalytic activity. Thus, Mn2O3, LaMnO3, and MgMn2O4 are found to be very good catalysts with turnover frequencies of 5 × 10−4 s−1, 4.8 × 10−4 s−1, and 0.8 ×10−4 s−1, respectively. Among the cobalt oxides, Li2Co2O4 and LaCoO3—especially the latter—exhibit excellent catalytic activity, with the turnover frequencies being 9 × 10−4 s−1 and 1.4 × 10−3 s−1, respectively. The common feature among the catalytic Mn and Co oxides is not only that Mn and Co are in the trivalent state, but Co3+ in the Co oxides is in the intermediate t2g5eg1 state whereas Mn3+ is in the t2g3eg1 state. The presence of the eg1 electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.

Characterization of few-layer 1T-MoSe2 and its superior performance in the visible-light induced hydrogen evolution reaction

Based on earlier results on the photocatalytic properties of MoS2, the 1T form of MoSe2, prepared by lithium intercalation and exfoliation of bulk MoSe2, has been employed for the visible-light induced generation of hydrogen. 1T-MoSe2 is found to be superior to both 2H and 1T MoS2 as well as 2H-MoSe2 in producing hydrogen from water, the yield being in the 60–75 mmol h−1 g−1 range with a turn over frequency of 15–19 h−1. First principles calculations reveal that 1T-MoSe2 has a lower work function than 2H-MoSe2 as well as 1T and 2H-MoS2, making it easier to transfer an electron from 1T-MoSe2 for the production of H2.

Liquid Metal/Metal Oxide Frameworks with Incorporated Ga2O3 for Photocatalysis

Solvothermally synthesized Ga2O3 nanoparticles are incorporated into liquid metal/metal oxide (LM/MO) frameworks in order to form enhanced photocatalytic systems. The LM/MO frameworks, both with and without incorporated Ga2O3 nanoparticles, show photocatalytic activity due to a plasmonic effect where performance is related to the loading of Ga2O3 nanoparticles. Optimum photocatalytic efficiency is obtained with 1 wt % incorporation of Ga2O3 nanoparticles. This can be attributed to the sub-bandgap states of LM/MO frameworks, contributing to pseudo-ohmic contacts which reduce the free carrier injection barrier to Ga2O3.

Ln0.5 A0.5 MnO3 (Ln=Lanthanide, A= Ca, Sr) Perovskites Exhibiting Remarkable Performance in the Thermochemical Generation of CO and H2 from CO2 and H2 O.

Perovskite oxides of the Ln0.5 A0.5 MnO3 (Ln=lanthanide, A=Sr, Ca) family have been investigated for the thermochemical splitting of H2 O and CO2 to produce H2 and CO respectively. The amounts of O2 and CO produced strongly depend on the size of the rare earth ions and alkaline earth ions. The manganite with the smallest rare earth possessing the highest distortion and size disorder as well as the smallest tolerance factor, gives out the maximum amount of O2 , and, hence, the maximum amount of CO. Thus, the best results are found with Y0.5 Sr0.5 MnO3 , which possesses the highest distortion and size disorder. Y0.5 Sr0.5 MnO3 shows remarkable fuel production activity even at the reduction and oxidation temperatures as low as 1200 °C and 900 °C, respectively.

Lanthanide-ion-assisted structural collapse of layered GaOOH lattice.

GaOOH nanorods were prepared by hydrolysis of Ga(NO(3))(3)·xH(2)O by urea at ~100 °C in the presence of different amounts of lanthanide ions like Eu(3+), Tb(3+), and Dy(3+). On the basis of X-ray diffraction and vibrational studies, it is confirmed that layered structure of GaOOH collapses even when very small amounts of lanthanide ions (1 atom % and more) are present in the reaction medium during the synthesis of GaOOH nanorods. The incorporation of lanthanide ions at the interlayer spacing of the GaOOH lattice, followed by its reaction with OH groups that connect the layers containing edge-shared GaO(6) in GaOOH, is the reason for the collapse of the layered structure and associated amorphization. This leads to the formation of finely mixed hydroxides of lanthanide and gallium ions. These results are further confirmed by steady-state luminescence and excited-state lifetime measurements carried out on the samples. The morphology of the nanorods is maintained upon heat treatment at high temperatures like 500 and 900 °C, and during this process, the finely mixed lanthanide and gallium hydroxides facilitate diffusion of lanthanide ions into the Ga(2)O(3) lattice, as revealed by the existence of strong energy transfer with an efficiency of more than 90% between the host and lanthanide ions.

Nature of WO4 tetrahedra in blue light emitting CaWO4 probed through the EXAFS technique

Blue emission due to self trapped exciton recombination in CaWO4 is believed to be very sensitive to the nature of the WO4 structural units. The present manuscript deals with the probing of structural differences existing in WO4 tetrahedra of CaWO4 particles having varying average crystallite sizes and different blue light emission characteristics. Based on XRD, W L1 edge XANES and W L3 edge EXAFS studies, it is inferred that factors like average W–O and Ca–O bond lengths, average number of oxygen atoms around W6+ ions and disorder in WO4 tetrahedra do not have any effect on the blue luminescence intensity from the samples. The lifetime value of excitons is lower for the nanocrystals/nanoparticles of CaWO4 compared to the bulk sample. The lower lifetime of self trapped excitons and the associated decrease in blue luminescence intensity for nanoparticles/nanocrystals (compared to bulk) has been explained based on competing non-radiative processes involving the interaction of holes with surface hydroxyl groups and an associated decrease in the extent of radiative exciton recombination.

Synthesis and characterization of Ga2O3:Eu nanorods

Eu3+ doped α-Ga2O3 and β-Ga2O3 nanorods were prepared by heating Eu3+ doped GaOOH nano-rods at 500 and 900 °C respectively. XRD patterns and infrared spectra revealed that the sample are crystalline upto 5 at % Eu3+ incorporation in β-Ga2O3 Unlike this in the case of α-Ga2O3, incorporation of Eu above 0.5 at % resulted in the amorphisation of crystalline Ga2O3 phase. This is explained in terms of the difference in the coordination of Ga in α and β forms of Ga2O3.

Structural differences in the luminescence properties of lanthanide doped orthorhombic and monoclinic phases of Y2GeO5

Stable monoclinic and metastable orthorhombic phases of Y2GeO5 have been prepared and characterized by XRD and FT-IR. Changes in the local environment around the Y3+ ions in both phases have been probed by Eu3+ doping of the lattices and subsequent measurement of their luminescence properties. The XRD and FT-IR studies revealed that Y2O2 chains exist in the metastable orthorhombic Y2GeO5 phase, while edge-shared distorted YO6 octahedra and YO7 pentagonal bi-pyramids constitute the monoclinic Y2GeO5 lattice. Detailed luminescence studies, including the experimentally evaluated Judd–Ofelt parameters (Ω2 and Ω4 values), have confirmed that the polarisability around the Y3+/Eu3+ ions and the associated electric dipole transition probabilities are higher in the metastable orthorhombic Y2GeO5 phase in comparison to the thermodynamically stable monoclinic Y2GeO5. This has been attributed to the lower symmetry of the electronic environment around the Y3+/Eu3+ ions, and the larger Y⋯Y average separation in the metastable form than that of stable monoclinic Y2GeO5.