Nitish Roy

Synergy of Low-Energy {101} and High-Energy {001} TiO2 Crystal Facets for Enhanced Photocatalysis

Controlled crystal growth determines the shape, size, and exposed facets of a crystal, which usually has different surface physicochemical properties. Herein we report the size and facet control synthesis of anatase TiO2 nanocrystals (NCs). The exposed facets are found to play a crucial role in the photocatalytic activity of TiO2 NCs. This is due to the known preferential flow of photogenerated carriers to the specific facets. Although, in recent years, the main focus has been on increasing the surface area of high-energy exposed facets such as {001} and {100} to improve the photocatalytic activity, here we demonstrate that the presence of both the high-energy {001} oxidative and low-energy {101} reductive facets in an optimum ratio is necessary to reduce the charge recombination and thereby enhance photocatalytic activity of TiO2 NCs.

Green Synthesis of Anatase TiO2 Nanocrystals with Diverse Shapes and their Exposed Facets-Dependent Photoredox Activity

The exposed facets of a crystal are known to be one of the key factors to its physical, chemical and electronic properties. Herein, we demonstrate the role of amines on the controlled synthesis of TiO2 nanocrystals (NCs) with diverse shapes and different exposed facets. The chemical, physical and electronic properties of the as-synthesized TiO2 NCs were evaluated and their photoredox activity was tested. It was found that the intrinsic photoredox activity of TiO2 NCs can be enhanced by controlling the chemical environment of the surface, i.e.; through morphology evolution. In particular, the rod shape TiO2 NCs with ∼25% of {101} and ∼75% of {100}/{010} exposed facets show 3.7 and 3.1 times higher photocatalytic activity than that of commercial Degussa P25 TiO2 toward the degradation of methyl orange and methylene blue, respectively. The higher activity of the rod shape TiO2 NCs is ascribed to the facetsphilic nature of the photogenerated carriers within the NCs. The photocatalytic activity of TiO2 NCs are found to be in the order of {101}+{100}/{010} (nanorods) > {101}+{001}+{100}/{010} (nanocuboids and nanocapsules) > {101} (nanoellipsoids) > {001} (nanosheets) providing the direct evidence of exposed facets-depended photocatalytic activity.

Boron-doped diamond semiconductor electrodes: Efficient photoelectrochemical CO2 reduction through surface modification

Competitive hydrogen evolution and multiple proton-coupled electron transfer reactions limit photoelectrochemical CO2 reduction in aqueous electrolyte. Here, oxygen-terminated lightly boron-doped diamond (BDDL) thin films were synthesized as a semiconductor electron source to accelerate CO2 reduction. However, BDDL alone could not stabilize the intermediates of CO2 reduction, yielding a negligible amount of reduction products. Silver nanoparticles were then deposited on BDDL because of their selective electrochemical CO2 reduction ability. Excellent selectivity (estimated CO:H2 mass ratio of 318:1) and recyclability (stable for five cycles of 3 h each) for photoelectrochemical CO2 reduction were obtained for the optimum silver nanoparticle-modified BDDL electrode at −1.1 V vs. RHE under 222-nm irradiation. The high efficiency and stability of this catalyst are ascribed to the in situ photoactivation of the BDDL surface during the photoelectrochemical reaction. The present work reveals the potential of BDDL as a high-energy electron source for use with co-catalysts in photochemical conversion.

Room temperature light-induced recrystallization of Cu2O cubes to CuO nanostructures in water

The mechanism of temperature-assisted Cu and Cu2O oxidation in oxygen and subsequent CuO nanowire growth is well known. Here, we report a simple light-induced process for the recrystallization of Cu2O cubes to [010] growth-directed CuO nanoribbons in water at room temperature. This was attributed to the formation and participation of ˙OH and ˙O2− active species in water as well as the outward diffusion of Cu ions leading to the formation of CuO nanoribbons along unstable {010} facets on the surface of Cu2O cubes. The oxidation was significantly suppressed under dark conditions or by active species scavengers, confirming the strong role of visible light. CO oxidation activity of nanoribbons was found to be superior to that of cubes. Our unique light-induced recrystallization of Cu2O to CuO in water provides new insight and better understanding of the oxidation mechanism of Cu2O, enabling tailoring of nanostructures by varying types of light (sun, incandescent and fluorescent) and opening a new strategy for development of energy- and environment-related Cu oxide nanomaterials.

Charge Separation in TiO2/BDD Heterojunction Thin Film for Enhanced Photoelectrochemical Performance

Semiconductor photocatalysis driven by electron/hole has begun a new era in the field of solar energy conversion and storage. Here we report the fabrication and optimization of TiO2/BDD p-n heterojunction photoelectrode using p-type boron doped diamond (BDD) and n-type TiO2 which shows enhanced photoelectrochemical activity. A p-type BDD was first deposited on Si substrate by microwave plasma chemical vapor deposition (MPCVD) method and then n-type TiO2 was sputter coated on top of BDD grains for different durations. The microstructural studies reveal a uniform disposition of anatase TiO2 and its thickness can be tuned by varying the sputtering time. The formation of p-n heterojunction was confirmed through I-V measurement. A remarkable rectification property of 63773 at 5 V with very small leakage current indicates achieving a superior, uniform and precise p-n junction at TiO2 sputtering time of 90 min. This suitably formed p-n heterojunction electrode is found to show 1.6 fold higher photoelectrochemical activity than bare n-type TiO2 electrode at an applied potential of +1.5 V vs SHE. The enhanced photoelectrochemical performance of this TiO2/BDD electrode is ascribed to the injection of hole from p-type BDD to n-type TiO2, which increases carrier separation and thereby enhances the photoelectrochemical performance.

SYNTHESIS AND STRUCTURAL INVESTIGATION OF METAL COMPLEXES OF 1-AMIDINO-2-THIOUREAS

Abstract Some new metal complexes of 1-amidino-2-thioureas† and its S- and N-alkyl derivatives have been isolated and characterized by electronic, vibrational and pmr spectral studies, magnetic and conductance measurements and thermal analysis. The ligands behave as SN or NN donors depending mainly on the pH of the medium and the nature of the metal atom. Protonation at the central nitrogen atom of the chelate ring converts the inner complexes to cationic ones and vice-versa. Electronic and pmr spectral data confirm the presence of strong ring currents in these chelates. Probable structures of the complexes have been suggested on the basis of their physicochemical properties.

Hydrogen and CO2 Reduction Reactions: Mechanisms and Catalysts

The electrocatalytic reduction of water to useful fuel via the hydrogen evolution reaction or CO2 reduction may afford a sustainable energy supply for the future. In this chapter, we discuss the recent developments on electrocatalytic materials for hydrogen (H2) evolution reaction and carbon dioxide (CO2) reduction which have been implemented in photocatalysis or photoelectrochemical reactions. The emerging strategies for replacing expensive platinum catalyst with earth abundant semiconductor materials in view of managing cost and material availability has been exclusively outlined here. Also, this chapter showcases novel materials, coatings and their recent strides to achieve high efficiency solar to fuel conversion from water and other pollutants. In summary, this chapter provides a holistic framework on a wide spectrum of materials, with consistent and meaningful comparisons between catalysts (hydrogen evolution electrocatalyst and photocatalyst) and etching insight into fundamentals and origin of catalysis.

Nitrosyls of cobalt, iron and ruthenium N-amidinothioureas

Diamagnetic cobalt and ruthenium and paramagnetic iron nitrosyls with N-amidinothioureas of general formula [CoL2(NO(H2O)], [Co(L′H)2(NO)(H2O)] X2, [Co(L″H)2(NO)(SO4)Y] where LH = N-amidinothiourea and its N-alkyl derivatives, L′H = N-amidino S-alkylthioureas, L″H = biguanide, X = halogen, 12SO4 or hydroxide and Y = H2O or pyridine) have been synthesized and characterized by elemental and thermal analysis, vibrational and electronic spectral studies, PMR spectra and magnetic measurements. Physico-chemical properties reveal the presence of NO− in cobalt and NO+ in iron and ruthenium nitrosyls.