Yatendra S. Chaudhary

Enzymes and bio-inspired electrocatalysts in solar fuel devices

The development of robust systems for the conversion of solar energy into chemical fuels is an important subject in renewable energy research. Key aspects are efficient and rapid catalysis of both fuel production (reduction of H2O or CO2), and water oxidation. Enzymes often have extraordinary and unique capabilities as electrocatalysts, and in this Perspective we consider the role that these molecules can play through their incorporation into model systems for solar fuel production, or as inspiration for synthetic catalysts.

How Light-Harvesting Semiconductors Can Alter the Bias of Reversible Electrocatalysts in Favor of H2 Production and CO2 Reduction

The most efficient catalysts for solar fuel production should operate close to reversible potentials, yet possess a bias for the fuel-forming direction. Protein film electrochemical studies of Ni-containing carbon monoxide dehydrogenase and [NiFeSe]-hydrogenase, each a reversible electrocatalyst, show that the electronic state of the electrode strongly biases the direction of electrocatalysis of CO2/CO and H+/H2 interconversions. Attached to graphite electrodes, these enzymes show high activities for both oxidation and reduction, but there is a marked shift in bias, in favor of CO2 or H+ reduction, when the respective enzymes are attached instead to n-type semiconductor electrodes constructed from CdS and TiO2 nanoparticles. This catalytic rectification effect can arise for a reversible electrocatalyst attached to a semiconductor electrode if the electrode transforms between semiconductor- and metallic-like behavior across the same narrow potential range (<0.25 V) that the electrocatalytic current switches between oxidation and reduction.

Facile synthesis of ultra-small monodisperse ceria nanocrystals at room temperature and their catalytic activity under visible light

An optimized facile method has been developed to synthesize ultra-small monodisperse ceria nanocrystals (3 to 4 nm) strictly at RT. The presence of well resolved broad peaks ([111], [200], [220] and [311]) for as-synthesized samples reveal the formation of the cubic phase of ceria. The detailed characterization by TEM exhibited the formation of monodisperse and crystalline ceria nanocrystals. These as-synthesized nanocrystals have shown autocatalytic behaviour and photocatalytic activity under visible sunlight, hence appears to be potential candidates for biomedical (antioxidant), solar cell applications.

Enhanced Photocatalytic Activity and Charge Carrier Dynamics of Hetero-Structured Organic–Inorganic Nano-Photocatalysts

P3HT-coupled CdS heterostructured nanophotocatalysts have been synthesized by an inexpensive and scalable chemical bath deposition approach followed by drop casting. The presence of amorphous regions corresponding to P3HT in addition to the lattice fringes [(002) and (101)] corresponding to hexagonal CdS in the HRTEM image confirm the coupling of P3HT onto CdS. The shift of π* (C═C) and σ* (C-C) peaks toward lower energy losses and prominent presence of σ* (C-H) in the case of P3HT-CdS observed in electron energy loss spectrum implies the formation of heterostructured P3HT-CdS. It was further corroborated by the shifting of S 2p peaks toward higher binding energy (163.8 and 164.8 eV) in the XPS spectrum of P3HT-CdS. The current density recorded under illumination for the 0.2 wt % P3HT-CdS photoelectrode is 3 times higher than that of unmodified CdS and other loading concentration of P3HT coupled CdS photoelectrodes. The solar hydrogen generation studies show drastic enhancement in the hydrogen generation rate i.e. 4108 μmol h(-1)g(-1) in the case of 0.2 wt % P3HT-CdS. The improvement in the photocatalytic activity of 0.2 wt % P3HT-CdS photocatalyst is ascribed to improved charge separation lead by the unison of shorter lifetime (τ1=0.25 ns) of excitons, higher degree of band bending, and increased donor density as revealed by transient photoluminescence studies and Mott-Schottky analysis.

Facile synthesis of single crystalline n-/p-type ZnO nanorods by lithium substitution and their photoluminescence, electrochemical and photocatalytic properties

Both n- and p-type single crystalline ZnO nanorods were successfully synthesized via a facile one step solvothermal method by lithium substitution. It was observed that substitution of Li is critical for the formation of n-type or p-type ZnO nanorods. The detailed formation mechanism indicates that the transition from n-type behavior to p-type behavior is due to the presence of defect states and lithium incorporation in the lattice. Using photoluminescence (PL) measurements, the role of defect states in generating the p-type or n-type nature of ZnO nanorods via Li substitution has been analyzed. Furthermore, the photocatalytic performances of the as-prepared ZnO nanorods have been investigated towards the degradation of rhodamine B and methyl orange, and the results demonstrate that the n-type ZnO nanorods behave as an excellent photocatalyst compared to the p-type ZnO nanorods owing to their higher charge carrier density.

Array of Cu2O nano-columns fabricated by oblique angle sputter deposition and their application in photo-assisted proton reduction

Nano-columnar arrays of Cu2O were grown by the oblique angle sputter deposition technique based on the self-shadowing principle. The as-grown nano-columnar samples are oriented along {111} direction, and they are highly transmitting in the visible range with a low reflectance. In this work, we show the photo-electrochemical activity of nano-columnar array of Cu2O, which shows a higher (∼25%) photocurrent density and a two-fold enhancement in the incident-to-photon conversion efficiency as compared to continuous thin film of Cu2O in photo-assisted proton reduction type reaction. The improvement in electrochemical activity of nano-columnar Cu2O photocathode can be attributed to the change in morphology, crystal structure, as well as electrical property, which shows a higher degree of band bending, increased donor carrier (e−) density and lower width of space charge region as revealed by capacitance measurements and Mott-Schottky analysis.

Facile synthesis of nanostructured hydroxyapatite–titania bio-implant scaffolds with different morphologies: their bioactivity and corrosion behaviour

A facile synthesis to fabricate HAp–titania scaffolds with different morphological features has been presented. The hydrothermal approach explored for HAp growth leads to the formation of HAp coating onto titania substrate with distinct morphological features such as cauliflower, urchin, porous nanofibres network, nanorods, etc. under different reaction microenvironments. The XRD analysis done for all HAp–titania scaffold samples revealed the formation of hydroxyapatite phase. Furthermore, the detailed FTIR, SAED and EDS analysis performed confirmed the formation of hydroxyapatite. The addition of H2O2 to the reaction mixture led to the high degree of self-assembly of the formed nanosheets into urchin or consolidated sphere like structures when NaOH or KOH was used in hydrothermal reaction, respectively. The detailed TEM analysis reveals that the formation of such HAp structures takes place by (i) formation of localized corrosion sites on titanium substrate with NaOH/KOH and (ii) the subsequent recrystallization of HAp sol onto the corrosion sites which act as nucleation site. The Tafel plot measurements and MTT assay test indicate that the HAp–titania scaffold samples are corrosion resistance as compared to bare titanium foil and are biocompatible. The structural characterization, growth mechanism, corrosion behaviour in SBF medium and cellular biocompatibility of these scaffold samples are discussed in detail.

Facile synthesis and the photo-catalytic behavior of core–shell nanorods

Sodium niobate nanorods (SNRs) have been synthesized by a facile surfactant free hydrothermal method. To explore their potential for photoelectrochemical water splitting under visible light, core–shell nanorods were fabricated by grafting CdS on sodium niobate nanorods. The TEM analysis shows the formation of sodium niobate nanorods which are in the order of 40 ± 5 nm in width and 1300 ± 100 nm in length. The presence of a thin layer on nanorods, as observed in a TEM image, and XRD and SAD analysis, reveals the grafting of hexagonal CdS on orthorhombic sodium niobate nanorods. This was further confirmed by dual band gap values (Eg: 3.6 for sodium niobate and 2.59 eV for CdS) determined from diffuse reflectance data of the CdS–sodium niobate nanorod sample. The CdS–sodium niobate nanorods show drastic enhancement in the current density (Jan: 7.6 mA cm−2 at 0.2 V vs. SHE) when irradiated with monochromatic UV light (300 nm), many folds higher than that observed for bare sodium niobate nanorods (Jan: 2.5 mA cm−2 at 0.2 V vs. SHE), bulk sodium niobate (Jan: 0.6 mA cm−2 at 0.2 V vs. SHE) and CdS. The conduction band (CB) minima calculations show a downhill offset of the CB edges of CdS–sodium niobate. Such a downhill staggered band gap and smooth lattice matched interface, as shown by HRTEM, seem to facilitate an efficient charge separation followed by a photo-generated e− transfer from the CdS CB to the sodium niobate CB and, therefore, appear responsible for the enhancement of the photocurrent density of CdS–sodium niobate nanorods. This is further corroborated by the time resolved photoluminescence decay measurements which show a longer average decay time (〈τ〉) for CdS–sodium niobate nanorods in the order of 8.06 ns than that for sodium niobate nanorods (6.45 ns). Furthermore, better light harvesting efficiency and incident to photon conversion efficiency (23.91% at 300 nm) observed for CdS–sodium niobate nanorods imply a better photo-generated charge carrier separation than those observed for bare sodium niobate nanorods and bulk sodium niobate. The synthesis of CdS modified sodium niobate nanorods, detailed results on the photoelectrochemical behaviour of CdS modified sodium niobate nanorods and underlying mechanism are presented.