Pramod H. Borse

Microwave synthesis of single-crystalline perovskite BiFeO3 nanocubes for photoelectrode and photocatalytic applications

A simple microwave synthesis procedure has been developed for the single-crystalline perovskite nanocubes composed of bismuth ferrite (BiFeO3). Typical nanocubes had sizes ranging from 50to200nm. The single-crystalline nature of nanocubes was confirmed by high resolution transmission electron microscopy and selected area electron diffraction pattern. X-ray diffraction pattern showed the rhombohedral phase with R3c space group. The material showed photoinduced water oxidation activity in both photoelectrochemical and photocatalytic modes. It could become a useful material for photoelectrode and photocatalytic applications.

Photocatalytic hydrogen production from water-methanol mixtures using N-doped Sr2Nb2O7 under visible light irradiation: effects of catalyst structure.

Nitrogen-doped perovskite type materials, Sr2Nb2O7-xNx (0, 1.5 < x < 2.8), have been studied as visible light-active photocatalysts for hydrogen production from methanol-water mixtures. Nitrogen doping in Sr2Nb2O7 red-shifted the light absorption edge into the visible light range and induced visible light photocatalytic activity. There existed an optimum amount of nitrogen doping that showed the maximum rate of hydrogen production. Among the potential variables that might cause this activity variation, the crystal structure appeared to be the most important. Thus, as the extent of N-doping increased, the original orthorhombic structure of the layered perovskite was transformed into an unlayered cubic oxynitride structure. The most active catalytic phase was an intermediate phase still maintaining the original layered perovskite structure, but with a part of its oxygen replaced by nitrogen and oxygen vacancy to adjust the charge difference between oxygen and doped nitrogen. These experimental observations were explained by density functional theory calculations. Thus, in Sr2Nb2O7-xNx, N2p orbital was the main contributor to the top of the valence band, causing band gap narrowing while the bottom of conduction band due to Nb 4d orbital remained almost unchanged.

Band gap tuning of lead-substituted BaSnO3 for visible light photocatalysis

The Pb substitution effect was investigated experimentally and theoretically on the crystal structure of BaSnO3 and on the photo-oxidation activity of H2O. The chemically doped Pb in BaSnO3 induced a concentration-dependent redshift of the experimental band gap (BG). The BaPb0.8Sn0.2O3 system produced 32μmol∕h of O2 under λ⩾420nm photons, but no O2 for BaSnO3. The DFT calculations of BaPbxSn1−xO3 (x=0,0.5,1) by using generalized approximation, implying the BG alteration and the photocatalytic activity of BaPbxSn1−xO3, are due to the induced Pb 6s orbital in the BG of BaSnO3. Thus Pb modified the insulating nature of BaSnO3 to semiconducting and semimetallic.

Engineered Nanorod Perovskite Film Photocatalysts to Harvest Visible Light

The recent fl ourishing of nanostructured materials has widened its potential applications in the much-desired effi cient energy materials. Specifi cally, materials for solar hydrogen production by water splitting should possess superior optoelectric properties in addition to suitable band energetics and durability in aqueous solutions. The tunability of the physicochemical properties of nanostructred materials by virtue of their size and shape renders a wider applicability. Since the critical limitation of popular TiO 2 photocatalyst that absorbs only UV light, [ 1 , 2 ] the fi eld of visible light water splitting photocatalyst is currently being nurtured by various kinds of conventional and new single component materials (CdS, WO 3 , Fe 2 O 3 , TaON, etc.), as well as composite materials. [ 3–20 ] However, there is still much potential for nanostructured materials in this area. There have been several reports on the binary [ 3–6 ] and ternary metal oxides, [ 7–13 ]

Dopant dependent band gap tailoring of hydrothermally prepared cubic SrTixM1−xO3 (M=Ru,Rh,Ir,Pt,Pd) nanoparticles as visible light photocatalysts

Nanostructured cubic SrTiO3 particles were hydrothermally synthesized and studied experimentally/theoretically for photoreduction of water. The particles were doped with metal atoms (M=Ru,Rh,Ir,Pt,Pd), which acquired high cyrstallinity after thermal treatment. SrTiO3:Rh showed the highest rate of H2 evolution under λ>420nm photons. The density functional theory calculations of SrTixM1−xO3 (M=Ru,Rh,Ir,Pt) implied that the photocatalytic activity of SrTixRh1−xO3 was due to its suitable band energetics, and the induced hybridized Ti∕Rh orbitals in the bandgap of SrTiO3.

Photocatalytic Ohmic layered nanocomposite for efficient utilization of visible light photons

The WO3∕W∕PbBi2Nb1.9Ti0.1O9 photocatalyst was fabricated by depositing the tungsten clusters over the p-type perovskite base material with the chemical vapor deposition method, and later partly oxidizing the surfaces of these clusters to obtain n-type WO3 overlayers and W metal layer as an Ohmic junction. This NCPC showed unprecedented high activity for the photocatalytic oxidation of water, photocurrent generation, and acetaldehyde decomposition under visible light irradiation (λ⩾420nm).

Synthesis of a hydrogen producing nanocrystalline ZnFe2O4 visible light photocatalyst using a rapid microwave irradiation method

A rapid microwave solid-state synthesis method is systematically investigated to achieve a H2 producing visible light active spinel photocatalyst. ZnFe2O4 nanocrystallites were obtained by microwave irradiation of precursor compacts under optimized conditions. This investigation led to a uniform sized nanocrystalline photocatalyst that yielded a quantum-yield of H2 evolution ∼3.8 times higher than that of conventionally synthesized ZnFe2O4. The synthesis parameters – microwave power, synthesis temperature, and time, were found to control the physico-chemical properties viz phase formation kinetics, phase purity, crystallinity, specific surface area and photochemical efficiency, of the synthesized photocatalyst. The study reveals that the threshold microwave power of ≥3 kW was necessary to obtain a spinel phase structure, while lower power (<3 kW) could not induce the crystallization even after prolonged low-power irradiation of 180 min. At the threshold power, a minimum of 10 min. synthesis time was enough to obtain uniform sized nanocrystallites, indicating that the synthesis method is ∼24 times faster than the solid state reaction method, which needs nearly 4 h. The particle morphology evolution with irradiation time from 10–150 min. exhibited de-crystallization phenomena. Longer irradiation displayed a morphological crystallization probably induced due to the simultaneous area and volumetric heating effect. The possible “formation mechanism” of these uniform nanocrystallites has been presented here for qualitative understanding. Thus synthesized photocatalysts generated hydrogen from a water–methanol mixture even without the co-catalyst loading. The ferrite photocatalyst was found to decolorize methylene blue dye with a maximum decay constant of 0.232 h−1, thereby demonstrating its capability in the pollutant decomposition applications, all under visible light photons.

Phase and photoelectrochemical behavior of solution-processed Fe2O3 nanocrystals for oxidation of water under solar light

Monodispersed iron-oxide nanocrystals of controlled sizes (5–16nm) were prepared by the thermal decomposition of iron-oleate complex to fabricate photoanode films for photoelectrochemical oxidation of water under simulated solar light. The smallest 5nm particles were made mostly of γ-Fe2O3, but other sizes showed mixed phases with Fe3O4, whose fraction increased with size. Size-dependent photoanodic currents were also observed, showing maximum photocurrent density for 12nm particles. The high photocurrents were attributed to the lowered electron-hole recombination rates for the nanocrystals with the sizes comparable to the hole diffusion length.

Indium induced band gap tailoring in AgGa1−xInxS2 chalcopyrite structure for visible light photocatalysis

Indium was substituted at gallium site in chalcopyrite AgGaS(2) structure by using a simple solid solution method. The spectroscopic analysis using extended x-ray absorption fine structure and x-ray photoelectron spectroscopy confirmed the indium substitution in AgGaS(2) lattice. The band gap energy of AgGa(1-x)In(x)S(2) (x=0-1) estimated from the onset of absorption edge was found to be reduced from 2.67 eV (x=0) to 1.9 eV (x=1) by indium substitution. The theoretical and experimental studies showed that the indium s orbitals in AgGa(1-x)In(x)S(2) tailored the band gap energy, thereby modified the photocatalytic activity of the AgGa(1-x)In(x)S(2).

Theoretical band energetics of Ba(M0.5Sn0.5)O3 for solar photoactive applications

We report here a comparative study of the theoretically calculated electronic structures of cubic BaSnO3 and cubic Ba(M0.5Sn0.5)O3 with M=Ti, V, Cr, Zr, Ce, and Pb, the tetravalent metal ions, to explore their possible efficacy for the visible light photocatalysis and solar energy conversion. We performed the calculations within the framework of density functional theory by using WIEN97 code. The 3d orbitals of Ti, V, and Cr, 4d of Zr, and the 4f and 6s orbitals of Ce and Pb, respectively, contributed to the bottom of the conduction band for narrowing of the band gap of cubic BaSnO3. Calculation of the frequency dependent absorption coefficient I(ω) of Ba(M0.5Sn0.5)O3 indicated that among the transition metal (Ti, V, Cr, and Zr) doped systems, Cr has comparatively higher visible absorption efficiency, whereas among other metal (Pb and Ce) systems, Pb showed significant absorption coefficient in low energy range (E⩽2eV). The comparison of the computed optical absorption coefficients shows that the Ba(M0.5S...