BiVO4 and WO3 nanophotocatalysts: water-splitting and environmental applications Dr. Chandrappa G T Department of Chemistry, Bangalore University Bangalore E-mail: gtchandrappa@yahoo.co.in 6th International Conference and Exhibition on Materials Science and Engineering, Atlanta, September 12 -14, 2016

Objectives Materials Investigated BiVO4 WO3 Materials synthesis: Solution combustion method Materials characterization: PXRD, TGA, UV-Vis, SEM, EDX and TEM Properties study: Photocatalytic activity of materials for hydrogen evolution and degradation of dyes

Synthesis methods Building-up process Breaking-down process Hydrothermal synthesis Inert gas condensation Chemical vapor deposition Ion beam technique Combustion synthesis Laser ablation Co-precipitation Lithography Micelles Mechanical attrition Microwave technique Plasma pyrolysis Sol-gel method Sputtering

Advantages of combustion synthesis It is a self-sustained, short duration exothermic reaction Volatalize low boiling point impurities and, therefore, result in higher purity products High temperatures results in the direct formation of phases No need of expensive processing facilities and equipment The high thermal gradients and rapid cooling rates can give rise to new non-equilibrium or metastable phases

Methodology of solution combustion synthesis Fuel Oxidizer Water Aqueous redox mixture in a pre-heated muffle furnace (350 ⁰C - 500 ⁰C) Combustion Nanocrystalline powder

Bismuth vanadate (BiVO4) Bismuth vanadate (BiVO4) is an n-type semiconductor and has been identified as one of the most promising photoanode materials. It crystallizes either in a scheelite or a zircon-type structure. The scheelite phase has either tetragonal crystal structure or a monoclinic crystal structure while the zircon-type has tetragonal structure. (a)Monoclinic scheelite (b) Scheelite tetragonal (c) Zircon-tetragonal

Monoclinic phase shows better photocatalytic activity compared to tetragonal phase. Synthesis Solution A: Bi(NO3)3 + 0.5 ml HNO3 + 1.5 g MA Solution B: NH4VO3 + 1.5 g MA + 5ml Water + Solution B was mixed with solution A with vigorous stirring to avoid precipitation of Bi2O3. MA-Malic acid

Small volume combustion (SVC-BiVO4) : 2. 0-2 Small volume combustion (SVC-BiVO4) : 2.0-2.5 ml precursor solution in 100 ml beaker Bulk volume combustion (BVC-BiVO4) : more than 2.5 ml precursor solution in 100 ml beaker Solid combustion (SC-BiVO4) : wet powder/solid obtained after evaporation of aqueous medium over hot plate

PXRD patterns of (a) SVC-BiVO4, (b) BVC-BiVO4 and (c) SC-BiVO4

TEM images of SVC-BiVO4 EDX pattern of SVC-BiVO4 d c 0.26nm (200) 0.254 nm (002) a b Lattice spacing of 0.26 nm and 0.254 nm correspond to (2 0 0) and (0 0 2) crystalline planes respectively for monoclinic scheelite BiVO4.

Nitrogen adsorption– desorption isotherms and the corresponding pore-size distribution curves (inset) of SVC-BiVO4 surface area of SVC-BiVO4 (13.86 m2 g−1) is nearly 20 times as larger as that of conventional solid state synthesised BiVO4 (0.7m2 g−1). (a) Band gap for SVC-BiVO4 and (b) Potential energy diagram for photochemical reaction of SVC-BiVO4. a b There is a shift in CB to more negative redox potential of H+/H2 (0 V vs NHE)

Time course of H2 evolution under UV-light irradiation. 489 μ mole/ 2.5 h of H2 evolution from UV photolysis of water-ethanol in the presence of SVC-BiVO4 was determined by gas chromatography

Degradation of MB under solar light At various concentration At various catalyst loading

MB absorption at 664 nm.

Carbon source for combustion Photocatalytic activity Synthesis of BiVO4 by different carbon source and their physicochemical properties Carbon source for combustion Particle size in nm surface area in m2/g Photocatalytic activity Urea 200-300 3.1 Phenol and Cr(VI) Urea and citric acid 400-600 2.8 Methylene Blue Sodium carboxymethylcellulose 50-200 3.0 Rhodamine B DL-Malic acid 10-20 13.86 H2 generation and Methylene Blue Comparison of hydrogen generation by BiVO4  No Material Irradiation conditions Reactant solution Activity (μmol/h) H2 O2   1 BiVO4 300 W Xe at >520 nm aqueous AgNO3 ----- 31 2 BiV0.98Mo0.02O4 Xe lamp (l>420 nm) ------ 370 3 BiVO4-RGO visible light,0.8 external bias 0.1M Na2SO4 0.75 0.21 4 PO4-doped BiVO4 50 5 UV light Water- ethanol 195.6

Conclusion We demonstrated a novel synthetic strategy to produce highly effective visible-light-driven photocatalyst m-BiVO4 and investigated H2 evolution under UV-light in water/ethanol system. Control over the solution preparation, amalgamation and the quantity of precursor used for the reaction. The amount of precursor solution used for combustion reaction plays an important role in achieving the impurity free product G. P. Nagabhushana, G.Nagaraju, G. T. Chandrappa , Journal of Materials Chemistry A, 2013, 1, 388-394.

Tungsten trioxide (WO3) WO3 is a material of great interest, due to its applications in electrochromic, photocatalytic, photoluminescent, and gas sensing materials. Synthesis 0.2 g of tungsten metal + 3 ml H2O2 1:1 mol ratio of sucrose was added + hot plate was maintained at 170±5 °C. Vigorous flammable reaction with bluish black product

(b) calcined at 500 °C for 30 min and PXRD patterns of WO3 a b c (a) as synthesised WO3 (b) calcined at 500 °C for 30 min and (c) calcined at 500 °C for 2 hours Phase : Monoclinic

TGA of as synthesised WO3 Weight loss of 3.03 wt % up to 150 °C due to adsorbed water Weight loss of 12.34 wt % in the range of 150 °C and 470 °C is due to the oxidation of carbon to carbon dioxide.

SEM images of (a) as synthesized WO3 and (b) WO3 sample in muffle furnace after 30 min at 500 °C The porous nature is very much evident from the SEM images Nitrogen adsorption-desorption isotherms (inset is the corresponding pore-size distribution curve) of WO3 BET surface area = 34.28 M2/g

Band gap for WO3 calcined sample at 500 °C for 30 min The band gap calculated for WO3 using Mott–Schottky plot is found to be 2.845 eV. CB and the VB are found to be at 0.1125 eV and 2.7325 eV respectively.

TEM images of WO3 (a and b) WO3 sample calcined at 500 °C for 30 min HRTEM of the same sample ( inset -SAED pattern) Size distribution histogram

Irradiation conditons Time course of Hydrogen evolution under UV-light irradiation WO3 nanoparticles showing hydrogen evolution of about 457 μ mol per 2.5 h, in the absence of either coupling oxides or doping metals. No Materials Irradiation conditons Reactant solution Activity (μ mol/h) H2 References 1 Bi2W2O9; Pt as Co-catalyst 400–450W Hg lamp Aq.Methanol 18 26 2 (NaBi)0.5WO4; Pt as Co-catalyst 300 W Xe lamp 7 27 3 (AgBi)0.5WO4; Pt as Co-catalyst 0.1 4 WO3;Pt as Co-catalyst 300–500 W Xe lamp with a cut-off filter (L42) 28-29 5 PbWO4;RuO2 as Co-catalyst 200 W Hg–Xe lamp Water 24 30-31 6 WO3 240 W Hg-Xe lamp Water-Ethanol 182.8 Present work Comparison of hydrogen generation by tungsten trioxide and other tungsten based oxides reported in the literature

Degradation of MB under UV-light At various catalyst loading At various concentration The highest photocatalytic activity for the degradation of 10 PPM MB of about 97% within 60 min was exhibited by 100 mg of the photocatalyst.

Conclusion We have demonstrated a simple green process for the synthesis of highly crystalline tungsten oxide. The high surface area and the average particle size, ~5 nm, of WO3 has shown the excellent photocatalytic activity under UV-light. The reactants as well as the process is eco-friendly. The present method is the most cost effective compared to any of the reported ones till date. Chosing suitable oxidant & fuel in combustion is boundless and hence one can synthesize nanomaterials with unexpected properties.

Acknowledgements University Grants Commission for financial support

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