Indranil Mondal

Co-MOF as a sacrificial template: manifesting a new Co3O4/TiO2 system with a p–n heterojunction for photocatalytic hydrogen evolution

The present work reports a novel method for preparing an elusive Co3O4/TiO2 p–n heterojunction using Co-based metal organic frameworks (Co-MOFs) as a TiO2-absorbent cum sacrificial template for nanocomposite formation. Four new Co-MOFs based on a bispyrazole ligand and different carboxylic acids, with a wide variety of dimensionality, porosity and surface characteristics, were exploited for this purpose. We detail here the synthesis of cobalt MOFs using the hydro(solvo)thermal method and structural characterization by single crystal X-ray diffraction (XRD). We have also successfully demonstrated our strategy of using MOFs for fabricating superior p–n diode-type Co3O4/TiO2 hetero-nanocomposites for photocatalytic hydrogen production. The characterization results suggested that the nanocomposites consisted of highly crystalline desired anatase TiO2 nanoparticles and spinel Co3O4-like species. The nanocomposite with 2 wt% Co loading exhibited the maximum photoactivity with a hydrogen evolution rate of ∼7 mmol g−1 h−1 under UV-vis light irradiation. The above results indicate that the present preparative strategy is an amenable route for the synthesis of desirable synergistic photocatalysts combining a remarkable reactivity relevant to solar energy conversion.

Visible light induced hydrogen production over thiophenothiazine-based dye sensitized TiO2 photocatalyst in neutral water

In this study, we report the design and synthesis of a series of organic sensitizers, namely, (E)-3-(10-butyl-8-(methylthio)-10H-phenothiazin-3-yl)-2-cyanoacrylic acid (UP1), (E)-3-(10-pentyl-8-(methylthio)-10H-phenothiazin-3-yl)-2-cyanoacrylic acid (UP2) and (E)-3-(10-hexyl-8-(methylthio)-10H-phenothiazin-3-yl)-2-cyanoacrylic acid (UP3), and their application for photocatalytic hydrogen production. The as-prepared sensitizers show pronounced light harvesting capability under a wide visible region (400–650 nm) and exhibit relatively strong fluorescence at 625–628 nm with lifetimes of 0.59–0.62 ns. The extinction coefficient of the dyes increases by increasing the different substituents of methine chain on the nitrogen of the thiophenothiazine ring. Alkyl substitution in the sensitizers led to desirable electrochemical behaviour, which facilitates the effective electron injection from the dye to TiO2. The resulting thiophenothiazine@TiO2–Pt composites exhibited high hydrogen production efficiency from water splitting at neutral conditions. The UP3 sensitized Pt–TiO2 (UP3@PT) photocatalyst showed hydrogen evolution up to 1048 μmol (TON 1397) with an excellent apparent quantum yield, ∼50%, which is much higher than the other photocatalysts such as UP1@PT, UP2@PT and pure TiO2. Operational parameters, such as the effects of the substitution in dye molecules, dye concentration, activation temperature of TiO2 and pH of the reaction medium, were explored. Theoretical studies and experimental measurements corroborated that, the addition of methine unit in the sensitizer enhanced the efficiency in dye@TiO2 composite via reduced charge recombination and increased light capture.

A simple carbazole based sensitizer attached to a Nafion-coated-TiO2 photocatalyst: the impact of controlling parameters towards visible light driven H2 production

Here we report a new composite material consisting of a simple and cost effective carbazole based photosensitizer attached onto a Nafion/Pt/TiO2 (NPT) hybrid as a high-performance photocatalyst for H2 evolution under visible light. The engineered composite of dye with NPT is shown to render improvement in its photocatalytic activity. We have spectroscopically observed that the spacer free simple carbazole based dye was perfectly attached onto TiO2 surfaces, which also remained unaffected in alkaline medium. The development of the photocatalyst was confirmed by different morphological characterization procedures. Under optimum conditions (pH 10, 1.0 × 10−4 mol/10 mg AM dye, 0.5 wt% Pt), the maximal apparent quantum yield (AQY%) of D1@PT and D1@NPT for hydrogen evolution increases up to 16.5% and 19.16% under ∼2-sun-intensity of visible light irradiation with triethanolamine (TEOA) as a sacrificial agent. The combined effects of several factors may contribute to the remarkably enhanced photocatalytic activity for the D1@NPT sample including dye concentration, pH and catalyst amount variation, and the spacer effect of the sensitizers (with and without oligothiophene moieties).

Modulated Binary–Ternary Dual Semiconductor Heterostructures

A generic modular synthetic strategy for the fabrication of a series of binary-ternary group II-VI and group I-III-VI coupled semiconductor nano-heterostructures is reported. Using Ag2 Se nanocrystals first as a catalyst and then as sacrificial seeds, four dual semiconductor heterostructures were designed with similar shapes: CdSe-AgInSe2 , CdSe-AgGaSe2 , ZnSe-AgInSe2 , and ZnSe-AgGaSe2 . Among these, dispersive type-II heterostructures are further explored for photocatalytic hydrogen evolution from water and these are observed to be superior catalysts than the binary or ternary semi-conductors. Details of the chemistry of this modular synthesis have been studied and the photophysical processes involved in catalysis are investigated.

First Study on Phosphonite-Coordinated Ruthenium Sensitizers for Efficient Photocatalytic Hydrogen Evolution

For the first time we report the design and syntheses of phosphonite coordinated ruthenium(II) sensitizers bearing ĈN̂N ligand and/or terpyridine derivatives carboxylate anchor (GS11, GS12. and GS13) and its application for hydrogen production over Pt-TiO2 system. These heteroleptic complexes exhibit broad metal-to-ligand charge transfer transition band over the whole visible regime extending up to 900 nm. DFT calculations of these complexes show that the HOMO is distributed over the Ru and Cl atom whereas; LUMO is localized on the polypyridile ligand, which are anchored on TiO2 surface. Among the sensitizers tested for photocatalytic hydrogen evolution, GS12 exhibited a maximum turnover number (TON) 8605 (for 8 h), which is very high compared to the reference sensitizer (N719) with TON 163 under similar evaluation condition. The dependence of the hydrogen evolution rate at different pH using GS11, GS12, GS13, and DX-1-sensitized Pt-TiO2 has been studied and the maximum H2 production yield was obtained at pH 7 for all sensitizers.

Noble metal free naphthylbisimide/TiO2/graphene: an efficient H2 evolution photocatalyst

Current photocatalytic systems must meet numerous very demanding features such as being non-noble, high-performance, cost-effective and environmentally friendly. Herein, an active and cheap photocatalyst was constructed by combining graphene with a naphthylbisimide-core based organic photosensitizer on TiO2. This synergistic photocatalyst displayed enhanced photocatalytic hydrogen generation by splitting water in the presence of methanol as the electron donor. A one-step soft template directed solvothermal synthesis process has been developed to prepare the crystalline mesoporous TiO2 materials. Electrochemical characterization of the sensitizer showed that naphthyl core has the ability to act as an electron acceptor, which has been further supported by the density functional theory (DFT) calculation. The as-prepared photocatalysts were thoroughly characterized by FESEM, TEM, AFM, XPS, UV-vis DRS, Raman and FTIR spectroscopic techniques. Graphene sheets, with desirable electronic properties, acted as good support for TiO2 to enhance the photocatalytic activity. Surface adsorption of the sensitizer on TiO2 extended the light absorption over the visible region (400–510 nm). High surface area (76.66 m2 g−1) and morphological changes in the as-synthesized TiO2 and graphene reinforcement play a vital role in the enhancement of photocatalytic activity. Hydrogen generation reached up to TON 4790 with the rate of 10 mmol g−1 h−1 after 6 h irradiation with a light intensity of ∼0.2 W cm−2 under the conditions of 10% v/v CH3OH, sensitizer concentration of 2.5 × 10−5 mol g−1, 5 wt% graphene and pH 7.0. The photocatalyst showed remarkable long term stability and reproducibility of H2 evolution without any appreciable deactivation. This study provides an inexpensive means of harnessing solar energy to achieve highly efficient H2 evolution without noble metals.

Shedding light on hydroxyquinoline-based ruthenium sensitizers with a long-lived charge carrier to boost photocatalytic H2 evolution

Three new hydroxyquinoline-coordinated ruthenium(II) sensitizers bearing a polypyridyl derivative anchor (R1, R2 and R3) were synthesized that could tether to visible light sensitization over a Pt–TiO2 system for hydrogen production. The highest hydrogen production yield of 225.2 μmol was obtained and a turnover number (TON) of 1734 (for 6 h) over R1/Pt–TiO2. The photoactivity of the as-prepared sensitizers was found to be predominantly influenced by the longer lifetime of the metal-to-ligand-charge transfer (MLCT) states and suitable redox properties of Ru(II) species in the heteroleptic complexes. Measurement of the binding kinetics revealed a comprehensive relationship between the adsorption behaviour of dyes onto a TiO2 surface and ligand influence on the hydrogen evolution activity. The dependence of the hydrogen generation rate was systematically screened at different dye concentrations (0.05–1.0 μmol/10 mg TiO2) and the results were correlated with their pertinent adsorption properties. The operational parameters like, pH of the medium and noble metal loading were taken into account for optimal photocatalytic performance.