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.

Efficient Photocatalytic Reduction of Hexavalent Chromium by BiVO4-Decorated MXene Photocatalysts and Their Charge Carrier Dynamics.

The synergistically MXene (Ti3C2Tx) co-catalyst-decorated BiVO4-based heterostructured photocatalysts have been synthesized by a hydrothermal approach with varied loading concentrations of MXene (Ti3C2Tx) to drive the hexavalent chromium reduction efficiently. The formation of the heterostructured photocatalyst was confirmed by the appearance of X-ray diffraction (XRD) peaks corresponding to the monoclinic BiVO4 phase and MXene (Ti3C2Tx) and also the antisymmetric (834 cm-1) and symmetric stretching (715 cm-1) of tetrahedral VO4 and D (1330 cm-1) and G (1570 cm-1) bands corresponding to MXene (Ti3C2Tx) in the Raman spectrum. The worm-like structures of BiVO4 nanocrystals grew onto the lamellar sheets of MXene (Ti3C2Tx), as shown by field emission scanning electron microscopy (FESEM), and has an increased surface area of 15.62 m2g-1 in the case of BVO-20-TC. X-ray photoelectron spectroscopy (XPS) analysis confirms the presence of V5+ and Ti3+states, and the uniform distribution of BiVO4 nanocrystals over lamellar sheets of MXene (Ti3C2Tx) is evident from energy-dispersive X-ray (EDX) analysis. The ultraviolet-diffuse reflectance spectroscopy (UV-DRS) spectra suggest a decrease in the band gap energy of BVO-20-TC to 2.335 eV, promoting a higher degree of visible light harvesting. Upon optimization, by varying the pH, the amount of the photocatalyst, and the concentration of Cr(IV), BVO-20-TC exhibits the highest photocatalytic efficiency (96.39%) while using a Cr(VI) concentration of 10 ppm at pH 2 and 15 mg of the photocatalyst, and the photoreduction of Cr(VI) to Cr(III) follows the pseudo-first-order reaction. The decrease in the PL intensity in BVO-20-TC reveals a faster transfer of electrons from MXene (Ti3C2Tx) to BiVO4. Further, the higher degree of band bending at the BiVO4/MXene (Ti3C2Tx) heterojunction, revealed from the Mott-Schottky analysis, facilitates efficient charge transfer and eventually faster and efficient photoreduction of Cr(VI) to Cr(III). The reusability and stability test undertaken for BVO-20-TC reveals that even after five cycles, the Cr (VI) photoreduction efficacy is retained. This work provides insights into photoreduction of Cr (VI) by using such heterostructures.

Efficient Photocatalytic CO2 Reduction with High Selectivity for Ethanol by Synergistically Coupled MXene-Ceria and the Charge Carrier Dynamics.

The primary factors that govern the selectivity and efficacy of CO2 photoreduction are the degree of activation of CO2 on the active surface sites of photocatalysts and charge separation/transfer kinetics. In this context, the rational synthesis of heterostructured MXene-coupled CeO2-based photocatalysts with different loading concentrations of Ti3C2MXene via a one-step hydrothermal approach has been undertaken. These photocatalysts exhibit a shift in X-ray diffraction peaks to higher 2θ values and changes in stretching vibrations of 5 wt % Ti3C2MXene/CeO2(5-TC/Ce) that indicate interaction between Ti3C2MXene and CeO2. Moreover, XPS analysis confirms the presence of the Ce3+/Ce4+ states. A sharp band at 2335 cm-1 observed during the CO2 photoreduction process corresponds to bidentate b-CO32-, which facilitates the adsorption of CO2 at the surface of the catalyst as revealed by the TPD analysis. Furthermore, the Schryvers test and NMR analysis were undertaken to confirm the formaldehyde intermediate formation during CO2 photoreduction to C2H5OH. The decrease in emission intensity, reduced lifetimes (2.68 ns), and lower interfacial resistance, as revealed by PL, TR-PL, and EIS analysis, imply an efficient charge separation and charge transfer in the case of the Ti3C2MXene/CeO2 heterojunction. The decrease in the intensity of peaks in the EPR spectrum in the case of 5-TC/Ce further confirms efficient charge transfer kinetics across the interface. The optimized 5-TC/Ce shows CO2 reduction with a drastically enhanced yield of ethanol on the order of 6127 µmol g-1 at 5 h with 98% selectivity and 7.54% apparent quantum efficiency, which is 6-fold higher than that of ethanol produced by bare CeO2. Herein, CeO2 that acts as a redox couple (Ce3+/Ce4+) when coupled with MXene having a metallic nature that reduces the electron transfer resistance is in unison, enabling an enhanced mobilization of electrons. Thereby, the synergistic coupling of Ti3C2MXene with CeO2 leads to an efficient photoreduction of CO2 under visible light illumination.

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-π-acceptor (D-π-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 N2 fixation reactions.

Enhanced Nitrogen Reduction to Ammonia by Surface- and Defect-Engineered Co-catalyst-Modified Perovskite Catalysts under Ambient Conditions and Their Charge Carrier Dynamics.

An electrocatalytic nitrogen reduction reaction is considered a potential approach for green ammonia production─a zero-carbon fertilizer, fuel, and energy storage for renewable energy. To harness the synergistic properties of perovskites, the inherent dipole moment due to their non-centrosymmetric structure (that facilitates better charge separation), oxygen vacancies, and the presence of Ni metal sites that permit activation and reduction of N2 efficiently, the NiTiO3-based nanoelectrocatalysts have been synthesized. Further, these catalysts have been modified with ultra-small metal nanocrystal co-catalysts to form heterointerfaces that not only aid to improve the charge separation but also activate N2 and lower overpotential requirements. The appearance of peaks corresponding to (012), (104), (110), (11-3), (024), (11-6), (018), (027), and (300) confirms the formation of rhombohedral NiTiO3. The shift in the XRD peak corresponding to the (104) plane to a smaller 2θ value and peak shifting and widening of Raman spectra imply the lattice distortion that signifies the formation of Pd-NiTiO3 and Pt-NiTiO3 heterojunction electrocatalysts with the loadings of 0.4 and 0.3 wt % of Pd and Pt, respectively, as confirmed by ICP-OES analysis. The detailed XPS analysis reveals the presence of Pd (0), Pd (II), and Pt (0), Pt (II) in respective electrocatalysts. The appearance of XPS peaks at 528.7 and 531.1 eV suggests the presence of oxidative oxygen species (O2-/O-) and the presence of oxygen defects due to oxygen vacancy. The detailed nitrogen reduction (NRR) investigation exhibits a 5-fold enhancement in ammonia yield rate (∼14.28 µg h-1 mg-1 at -0.003 V vs RHE), a faradic efficiency of 27% (at 0.097 V vs RHE) for Pd-NiTiO3 electrocatalysts than that for bare NiTiO3 (3.08 µg h-1 mg-1), and 9-folds higher than that of the activity shown by the commercial TiO2 (P25) (1.52 µg h-1mg-1). The formation of ammonia was further confirmed by using isotopic nitrogen as the feeding gas. Furthermore, the highest NRR is observed at lower cathodic potential (-0.003 V vs RHE) in the case of the Pd-NiTiO3 electrocatalyst than that of the Pt-NiTiO3 electrocatalyst (-0.203 V vs RHE), implying significantly reduced overpotential requirement. Such enhanced NRR activity with lower overpotential requirement in the case of the Pd-NiTiO3 electrocatalyst is due to efficient charge separation as shown by the semicircle Nyquist plot, decreased photoluminescence emission intensity, shorter average lifetime (∼29 ns) of excitons, appropriate band bending, and improved activation of N2 by the oxygen vacancies and heterointerface formed between Pd nanocrystals and NiTiO3. Furthermore, no change is observed in the current density, after stabilization in the initial few seconds, even up to 2 h, which signifies that these electrocatalysts are stable. The structural and morphological integrity of the optimized catalyst remained even after the nitrogen reduction reactions, as revealed by no significant change observed in FESEM, elemental mapping, and EDS analysis.

Tuning the Red Emission to Instigate Intrinsic White-Light Emission in Single-Phase Phosphor with Excellent Color Rendering Index.

There is ever-growing interest to develop intrinsic white light emitting single-phase phosphors that have high CRI, devoid of bluish tinge, ease of synthesis and are scalable. Herein, manipulating vacuum pressure to instigate white light emission in Cu2+ -doped-ZnS phosphors is reported. The detailed X-ray diffraction and electron microscopy confirm the cubic phase of Cu2+ -doped-ZnS phosphor having agglomerated particles (∼130-150 nm). The incorporation of Cu2+ in the ZnS lattice is substantiated by the anti-Stokes shift of Raman peaks and shifting of XRD peaks to higher 2θ values. Upon increasing Cu2+ doping concentration, the resulted decrease in the FWHM of XRD peaks implies shrinkage of the ZnS lattice. Interestingly, by tailoring the excitation wavelength, the stoichiometry of dopant ion, and defect states by varying the vacuum pressure, the optimized ZSC-3 (3% Cu2+ -doped-ZnS) displays the origin of clear blue, green and red emission bands, consequently giving rise to white light emission (CIE values: 0.345:0398). The PLQY and average lifetime calculated for ZSC-3 are 5.98% and 1.5 ms, respectively. Such intense white light emission prompted to fabricate a prototype using a 310 nm UV LED. It exhibits high CRI (97) and warm CCT (4538 K), meeting highly desired values for a white light-emitting phosphor for different lighting and electroluminescence applications.

Depletion Driven Assembly of Ultrasmall Metal Nanoclusters: From Kinetically Arrested Assemblies to Thermodynamically Stable, Spherical Superclusters.

Nanoscale assembly of ultrasmall metal nanoclusters (MNCs) by means of molecular forces has proven to be a powerful strategy to engineer their molecule-like properties in multiscale dimensions. By leveraging depletion attraction as the guiding force, herein, we demonstrate the formation of kinetically trapped NCs assemblies with enhanced photoluminescence (PL) and excited state lifetimes and extend the principle to cluster impregnated cationic nanogels, nonluminescent Au(I)-thiolate complexes, and weakly luminescent CuNCs. We further demonstrate a thermal energy driven kinetic barrier breaking process to isolate these assemblies. These isolated assemblies are thermodynamically stable, built from a strong network among several discrete, ultrasmall AuNCs and exhibit several unusual properties such as high stability in various pH, strong PL, microsecond lifetimes, large Stocks shifts, and higher accumulation in the lysosome of cancer cells. We anticipate our strategy may find wider use in creating a large variety of MNC-based assemblies with many unforeseen arrangements, properties, and applications.

Engineering the Surface of a Polymeric Photocatalyst for Stable Solar-to-Chemical Fuel Conversion from Seawater.

The design of an efficient and highly selective organic polymeric semiconductor photocatalyst consisting of Earth-abundant elements for solar fuel generation using seawater, and also deionized water, as a proton source is reported. The mesoporous g-C3 N4 synthesized using a conventional precursor (urea) shows significant H2 generation (ca. 33 000 µmol h-1 g-1 ) and drives the photoreduction of CO2 to CH4 , along with trace amount of methanol. However, when the chosen precursor cyanamide is used, drastic improvement in H2 generation (ca. 41 600 µmol h-1 g-1 ) and CO2 photoreduction is observed. The introduction of a surface nitrogen deficiency and modification of the surface with Cu0 further enhances solar H2 generation (ca. 50 000 µmol h-1 g-1 ) and CO2 photoreduction (3.12 µmol h-1 g-1 ) activity, respectively, owing to improvement in light harvesting and charge separation, as revealed by a shorter average lifetime of 3.52 ns and higher Stern-Volmer quenching constant value of approximately 11.2 m-1 . In addition, improved selectivity in CO2 photoreduction to only CH4 is also observed. The designed photocatalytic system is stable, with the solar H2 generation rate increasing even after 20 h under continuous illumination with a turnover number of 6500. When seawater used instead of deionized water, the overall solar fuel generation efficiencies of all photocatalysts marginally decreased owing to a decrease in the photogenerated charge-carrier separation efficacy.

Interaction of Lysozyme with Rhodamine B: A combined analysis of spectroscopic & molecular docking.

The interaction of Rhodamine B (RB) with Lysozyme (Lys) was investigated by different optical spectroscopic techniques such as absorption, fluorescence, and circular-dichroism (CD), along with molecular docking studies. The fluorescence results (including steady-state and time-resolved mode) revealed that the addition of RB effectively causes strong quenching of intrinsic fluorescence in Lysozyme and mostly, by the static quenching mechanism. Different binding and thermodynamic parameters were calculated at different temperatures and the binding constant value was found to be 2963.54Lmol(-1) at 25°C. The average distance (r0) was found to be 3.31nm according to Förster's theory of non-radiative energy transfer between Lysozyme and RB. The conformational change in Lysozyme during interaction with RB was confirmed from absorbance, synchronous fluorescence, and circular dichroism measurements. Finally, molecular docking studies were done to confirm that the dye binds with Lysozyme.

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.

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.

Visible light-driven CO2 reduction by enzyme coupled CdS nanocrystals.

Assemblies of carbon monoxide dehydrogenase molecules with CdS nanocrystals show fast CO(2) reduction driven by visible light. Activity is strongly influenced by size and shape of nanocrystals, and by the nature of the electron donor.