Photoelectrochemical Solar water splitting: the role of the carbon nano materials in bismuth vanadate composite photoanodes towards efficient charge separation and transport.

Photoelectrochemical performance of bismuth vanadate (BiVO4) photoanode is limited by poor charge separation and transport properties. The role of carbon nanotube, reduced graphene oxide or graphitic carbon nitride in BiVO4 composite photoanode were investigated towards enhancing light absorption and reducing overall impedance during photo electrochemical water oxidation process. The XRD and Tauc analysis showed that the BiVO¬4 retains its monoclinic phase, n type semiconductor nature and band gap in all carbon nano materials incorporated composite photoanodes. It was observed that the carbon nano materials incorporation in BiVO4 film increases its surface porosity and ultimately leading to enhanced light absorption. The BiVO4 photoanode with reduced graphene oxide and graphitic carbon nitride showed same bulk charge separation efficiency, whereas later one has showed better charge transfer. It was found that the graphitic carbon nitride formed composite with BiVO4 to provide enhanced light absorption efficiency i.e. 89% in 350 to 505 nm range. The BiVO4 with graphitic carbon nitride photoanode has shown best performance with photocurrent of 2.2 mA·cm-2, charge separation efficiency of 67% and photocurrent reaches to 4.0 mA·cm-2 with cobalt-phosphate surface catalyst at 1.23 VRHE for water oxidation under one sun illumination. The Mott- Schottky and impedance measurements confirm the shift of conduction band position towards hydrogen reduction potential and reduction in film resistance, respectively with carbon nano materials addition and the shift was most significant for graphitic carbon nitride. It is concluded that by concomitantly formation of junction during photoanode fabrication between carbon nano materials, BiVO4 and fluorine doped tin oxide glass substrate, better charge separation, transport and light absorption can be achieved.

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