Band-structure tunability via the modulation of excitons in semiconductor nanostructures: manifestation in photocatalytic fuel generation

Understanding the energetics of electron transfer at the semiconductor interface is crucial for the development of solar harvesting technologies, including photovoltaics, photocatalysis, and solar fuel systems. However, modern artificial photosynthetic materials are not efficient and limited by their fast charge recombination with high binding energy of excitons. Hence, reducing the exciton binding energy can increase the generation of charge carriers, which improve the photocatalytic activities. Extensive research has been dedicated to improving the exciton dissociation efficiency through rational semiconductor design via heteroatom doping, vacancy engineering, the construction of heterostructures, and donor-pi-acceptor (D-pi-A) interfaces to extend the charge carrier migration, promoting the dissociation of excitons. Consequently, functionalized photocatalysts have demonstrated remarkable photocatalytic performances for solar fuel production under visible light irradiation. This review provides the fundamental aspects of excitons in semiconductor nanostructures, having a high binding energy and ultrafast exciton formation together with promising photo-redox properties for solar to fuel conversion application. In particular, this review highlights the significant role of the excitonic effect in the photocatalytic activity of newly developed functional materials and the underlying mechanistic insight for tuning the performance of nanostructured semiconductor photocatalysts for water splitting, CO2 reduction, and N-2 fixation reactions

In Situ Encapsulation and Release Kinetics of pH and Temperature Responsive Nanogels

A facile synthesis of pH and temperature responsive poly(<i>N</i>-isopropylacrylamide) (PNIPAM) nanogels is presented. The scanning electron microscope (SEM) and dynamic light scattering (DLS) measurements indicate the formation of nanospheres of the order of 150 ± 20 and 230 ± 30 nm in case of PNIPAM (referred to as NG) and acid functionalized PNIPAM nanogels (referred to as AFNG), respectively, whereas on drug loading, the size increased to 170 ± 20 and 270 ± 20 nm, respectively at pH 7.4. Both the AFNG and amphotericin B (AmB) drug loaded AFNG (AmB-AFNG) show swelling as the pH changed from 3 to 11, but NG does not show any swelling with the change in pH. The AmB-AFNG exhibits better drug release up to ∼94% at pH 11. The better drug release observed in the case of AmB-AFNG is due to (a) swollen hydrated state of nanogel and (b) the acting repulsive forces between acid group of AFNG and AmB drug

Mechanistic Insight into the Defect-Engineered White Light Emission from the Single-Phase Orthovanadate Phosphor Synthesized by a Facile Rapid Microwave-Assisted Synthesis

The synthesis of single-phase barium orthovanadate phosphors by a one-pot microwave-assisted hydrothermal approach has been reported, wherein the homogeneous thermal zone generated at the molecular level by microwave radiation gives rise to tunable distortion in the tetrahedral VO4–3 and oxygen vacancies, eventually enabling intrinsic white light emission with CIE of 0.31,0.38, high photoluminescence internal quantum efficiency of 35%, and external quantum efficiency of 28% whereas phosphor synthesized by the hydrothermal route exhibits only bluish-green emission (PLQE: 0.5%). The Rietveld refinement confirms the formation of a single trigonal phase having dissimilar V–O bond lengths and bond angles, implying the formation of a distorted phosphor under optimized conditions, and corroborates with Raman and Fourier transform infrared analyses. The X-ray photoelectron spectroscopy and electron paramagnetic resonance analysis reveal that the origin of white light emission is due to short- and long-range defects, in particular the oxygen vacancies, which eventually form an intermediate energy level in the forbidden region between the valence and conduction bands. Lifetime spectra show triexponential fitting, corresponding to two charge transfer blue and green emission bands (3T2, 3T1 to 1A1) and one oxygen vacancy-related red emission at RT. Furthermore, these phosphors are thermally stable, as no change in the structure or emission characteristics are observed. A prototype fabricated using a 365 nm chip exhibits white-light-emission CIE of 0.353,0.392, correlated color temperature of 4867 K, color rendering index of 85, and high luminous efficacy of 102 lm/W at 140 mA operating current, portentous for practical 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^–1g^–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

Ceria Supported Pt/PtO-Nanostructures: Efficient Photocatalyst for Sacrificial Donor Assisted Hydrogen Generation under Visible-NIR Light Irradiation

In photocatalysis, imperative photoredox behavior and narrow band gap are important properties to exploit solar light for water splitting reaction. Nanostructured ceria (cerium dioxide/CeO₂) with Ce³⁺/Ce⁴⁺ (photoredox couple) shows significant enhancement in photocatalytic activity, however, no significant activity for water splitting reaction. The present study mainly focuses on incorporation of Pt on nanostructured mesoporous ceria by wet-impregnation method and its evaluation for donor assisted photocatalytic water splitting reaction. The BET analysis shows much higher surface area (119–131 m² g^–1) for unmodified as well as Pt modified mesoceria samples as compared to commercial ceria (24.4 m² g^–1), although structure was not ordered. The incorporation of Pt on mesoceria shows remarkable influence on photocatalytic hydrogen generation activity, and 1 wt % Pt was found to be optimized content, with broader light absorption. This photocatalyst was optimized with respect to photocatalyst dose, use of different sacrificial donors and their concentrations as well as other experimental parameters, with 34 h time course evaluation, yielding cumulative 1.52 mmol of hydrogen, under visible-NIR light irradiation and using ethanol as a sacrificial donor. The XPS, BET and photoluminescence studies imply that the enhanced photocatalytic hydrogen evolution in the case of mesoceria is due to the unison of high surface area, reduced recombination of photogenerated charge carrier and lower Ce³⁺ concentration in the case of mesoceria

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