Mohammad Qureshi

Visible light assisted photocatalytic hydrogen generation and organic dye degradation by CdS–metal oxide hybrids in presence of graphene oxide

Single step hydrothermal synthesis of CdS/Oxide (Oxide = ZnO, Al2O3) was demonstrated and examines their photocatalytic activity in presence of graphene oxide (GO). CdS/Al2O3/GO and CdS/ZnO/GO both exhibits enhanced photocatalytic activity for hydrogen generation with apparent quantum yields (AQY) of 14% and 30% respectively. Moreover, CdS/Oxide/GO displayed efficient photodegradation of an organic dye; ∼90% for CdS/Al2O3/GO and ∼99% for CdS/ZnO/GO within 60 min of time interval. Superior photocatalytic properties are attributed to the enhanced surface area and effective separation of photoinduced charge carriers due to the presence of GO. The present study highlights the potential application of graphene based materials in the field of energy conversion and environment remediation.

Hierarchical 3D NiO–CdS heteroarchitecture for efficient visible light photocatalytic hydrogen generation

A novel hierarchical three dimensional (3D) NiO–CdS heteroarchitecture was successfully synthesized by a facile single pot hydrothermal route and characterized by powder X-ray diffractogram, UV-Vis spectroscopy, BET, FE-SEM and TEM. FE-SEM and TEM analyses revealed the formation of the hierarchical 3D structure of the as-prepared NiO–CdS. The purity and crystalline phase of the individual component in 3D NiO–CdS were determined by a powder X-ray diffractogram. The hierarchical 3D NiO–CdS exhibited enhanced photocatalytic activity for hydrogen generation by water reduction with an AQY value of 6%. In 3D NiO–CdS, the high hydrogen production rate in comparison to 1D CdS NWs (AQY 2%) is mainly attributed to the enhanced specific surface area and facile charge transfer from the conduction band of highly crystalline CdS to the conduction band of NiO, which reduces the recombination rate of reductive electrons and oxidative holes for efficient hydrogen production.

Enhanced Photovoltaic Performance of Semiconductor-Sensitized ZnO–CdS Coupled with Graphene Oxide as a Novel Photoactive Material

We report, for the first time, a ternary hybrid composite of ZnO, CdS, and graphene oxide (GO) as a one-coat paintable solution in performing the role of a photoanode for the semiconductor-sensitized solar cell, wherein hierarchical ZnO-CdS heteroarrays are embedded onto the GO sheets. The photoconversion properties of the hybrid ternary-system-based photoanodes are evaluated in the photovoltaic devices having Pt and Ag as the counter electrodes with sulfide/polysulfide redox couple as the electrolyte. Power conversion efficiency (PCE) of ~2.82% has been achieved with a short-circuit current density (Jsc) of ~7.3 mA/cm(2), a maximum open-circuit voltage (Voc) of 703 mV, and a fill factor (FF) of 54% for the photovoltaic cell with Pt as a counter electrode. The identical hybrid photoanode against the Ag counter electrode resulted in the following values: PCE ≈ 1.96%, Jsc ≈ 5.7 mA/cm(2), Voc ≈ 565 mV, and 63% FF. The band position proximity of CdS, ZnO, and GO in the proposed ternary system facilitates an efficient electronic interactions thereby promoting the electron transport within CdS-ZnO-GO. The hierarchically grown CdS nanorods over ZnO nanoparticle act as the sensitizer for ZnO, enhancing the visible light harvesting ability. The loading of 1.0 wt% of GO to ZnO-CdS results in enhanced separation of photogenerated electrons and holes within the photoactive layer, thereby improving the photovoltaic performance. The electronic interactions of GO to ZnO-CdS is evident from the drastic quenching of fluorescence, reduced exciton lifetime and Raman scattering measurements. In order to study the effect of GO in the photovoltaic performance, we have compared our result with the photoelectrical parameters of the devices fabricated using the binary ZnO-CdS composite as GO-free photoanodes.

White organic light-emitting diodes based on spectral broadening in electroluminescence due to formation of interfacial exciplexes

We demonstrate white organic light-emitting diodes (OLEDs) having spectral width of approximately 260 nm in electroluminescence (EL) in a simple bilayer structure, consisting of TPD and zinc benzothiazole, without taking recourse to complex strategies such as blending and doping. The EL is broader than the corresponding photoluminescence (PL) of both component materials and their structures. A deconvolution of PL and EL spectra shows that as large as 60% of the broad EL emission originates from multiple exciplexes formed at the interface during electrical excitation.

Noble metal-free counter electrodes utilizing Cu2ZnSnS4 loaded with MoS2 for efficient solar cells based on ZnO nanowires co-sensitized with CuInS2–CdSe quantum dots

Noble metal-free counter electrodes using a co-catalytic loading of MoS2 nanosheets onto Cu2ZnSnS4 microspheres are investigated by means of CuInS2–CdSe quantum dot (QD) co-sensitized solar cells fabricated with single crystalline ZnO nanowires. An ex situ electrophoretic deposition route is employed to deposit QDs onto ZnO nanowires that are epitaxially grown on ZnO seed layered FTO substrates. Hydrothermally grown ZnO nanowires have also established an excellent stability against the bias conditions applied to fabricate the photoanodes of the devices. The superior photovoltaic performance of CuInS2–CdSe QD co-sensitized cells is compared with that of bare CuInS2 and CdSe counterparts, using current–voltage and incident photon-to-current conversion efficiency measurements. The present study also demonstrates an enhanced performance of the devices fabricated with 1.0 wt% of MoS2 loaded Cu2ZnSnS4 based counter electrodes in contrast to bare Cu2ZnSnS4. Hydrothermal loading of MoS2 onto Cu2ZnSnS4 generates an electrically interconnected network of Cu2ZnSnS4 microspheres, leading to facile charge transport in the counter electrode of the devices. MoS2 in its nanosheet form acts as an electrical bridge that interlinks the Cu2ZnSnS4 microspheres. Additionally, a favorable band energy alignment of Cu2ZnSnS4 and MoS2 stimulates the charge transfer dynamics in the Cu2ZnSnS4–MoS2 composite. Electron transport and recombination kinetics of the devices are measured using electrochemical impedance spectroscopy.

Graphitic carbon nitride as a photovoltaic booster in quantum dot sensitized solar cells: a synergistic approach for enhanced charge separation and injection

A ∼70% improvement in power conversion efficiency (PCE, η) is observed for the devices fabricated with a binary hybrid composite of graphitic carbon nitride and zinc oxide nanorods, i.e., (g-C3N4–ZnO NR) [η ≈ 2.43%, for an optimized weight ratio (0.5 : 1)] as compared to the pristine ZnO NR device (η ≈ 0.65%). Systematic investigations reveal that g-C3N4 boosts the light harvesting ability of the photovoltaic devices primarily by impeding photo-induced electron interception to the redox couple and injecting electrons into the conduction band of the semiconductor. Electrochemical impedance spectroscopy (EIS) analysis shows a reduced tunneling of photo-induced electrons to the sulfide–polysulfide (S2−/Sn2−) redox shuttle in the case of (g-C3N4–ZnO NR) composite devices. Higher recombination resistance (Rk) indicates that the g-C3N4 sheet acts as a barrier for photo-induced electron interception at the working electrode/electrolyte interface. Preliminary investigation using steady state and dynamic photoluminescence analyses suggest a similar fact about the photo-induced electron injection from g-C3N4 sheets to ZnO, contributing to the enhanced light harvesting ability of (g-C3N4–ZnO NR) composite devices.

Ethyl Cellulose and Cetrimonium Bromide Assisted Synthesis of Mesoporous, Hexagon Shaped ZnO Nanodisks with Exposed ±{0001} Polar Facets for Enhanced Photovoltaic Performance in Quantum Dot Sensitized Solar Cells

Hexagon shaped mesoporous zinc oxide nanodisks (ZnO NDs) with exposed ±{0001} polar facets have been synthesized by using ethyl cellulose (EC) and cetrimonium bromide (CTAB) as the capping and structure directing agents. We have characterized ZnO NDs using analytical techniques, such as powder X-ray diffraction (PXRD), diffuse reflectance UV-visible (UV-vis) spectroscopy, photoluminescence (PL) spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) surface area analysis and proposed a plausible mechanism for the formation of ZnO NDs. EC molecules form a colloidal solution in a 1-butanol:water (3:1) solvent system having a negative zeta potential (ζ ≈ -32 mV) value which can inhibit CTAB assisted c-axis growth of ZnO crystal and encourage the formation of ZnO NDs. In the control reactions carried out in presence of only CTAB and only EC, formation of hexagonal ZnO nanorods (NRs) and ZnO nanosheets (NSs) composed of numerous ZnO nanoparticles are observed, respectively. Photovoltaic properties of ZnO NDs as compared to ZnO NRs, ZnO NSs, and conventional ZnO nanoparticles (NPs) are investigated by co-sensitizing with CdS/CdSe quantum dots (QDs). An ∼35% increase in power conversion efficiency (PCE, η) is observed in ZnO NDs (η ≈ 4.86%) as compared to ZnO NPs (η ≈ 3.14%) while the values of PCE for ZnO NR and ZnO NS based devices are found to be ∼2.52% and ∼1.64%, respectively. Enhanced photovoltaic performance of the ZnO NDs based solar cell is attributed to an efficient charge separation and collection, boosted by the exposed ±(0001) facets apart from the single crystalline nature, better light-scattering effects, and high BET surface area for sensitizer particle adsorption. Electrochemical impedance spectroscopy (EIS) analysis further reveals that the charge recombination resistance and photoinduced electron lifetime are substantially higher in the ZnO ND based device than in ZnO NR, ZnO NP, and ZnO NS based devices, which demonstrates a slower electron-hole (e(-)-h(+)) recombination rate and faster charge migration through the single crystalline ZnO NDs.

Luminescence and Energy Transfer Characteristics of Tb3+- and Ce3+-Codoped BaBPO5 Phosphors

The luminescence properties of barium borate phosphate phosphors doped with trivalent terbium ions and the effects of codoping cerium and sodium ions on their emission characteristics were investigated. BaBPO5:Tb3+ phosphors exhibit intense green emissions upon excitation at 230 nm. The emissions can be assigned to various 5D4→7FJ (J=3, 4, 5, and 6) transitions of Tb3+ ions. Excitation in the 200–230 nm range in BaBPO5:Tb3+ is ascribed to spin-allowed transitions. The relatively weak excitation band at 268 nm is attributed to spin-forbidden transitions. The codoping of Ce3+ ions along with Tb3+ in BaBPO5 results in Tb3+ emissions and the quenching of Ce3+ emissions. Energy transfer from Ce3+ ions to Tb3+ ions increases with an increase in Ce3+ content. The codoping of sodium ions in terbium and cerium ions-doped BaBPO5 phosphors leads to luminescence enhancement. This enhancement is attributed to the reduction in non-radiative losses. These investigations reveal that the phosphors containing Tb3+, Ce3+, and Na+ ions in the BaBPO5 host have potential for applications as green phosphors for tricolor lamps.

Enhanced photovoltaic performance of meso-porous SnO2 based solar cells utilizing 2D MgO nanosheets sensitized by a metal-free carbazole derivative

Herein, we report the power conversion efficiency (PCE) of ∼3.71%, achieved in a mesoporous SnO2 based solar cell by introducing 15 wt% of 3D porous hierarchical MgO composed of 2D nanosheets by a simple sonochemical route followed by a mixing process, sensitized with a metal-free carbazole dye, namely, 2-cyano-3-(4-(2-(9-p-tolyl-9H-fluoren-6-yl)vinyl)phenyl)acrylic acid (i.e. SK1 dye). We have optimized the performance of solar cell devices with the addition of MgO and performed a comparative study on photovoltaic performances of the fabricated devices, such as SnO2–MgO with pristine SnO2, by employing two different redox mediators, namely, (I−/I3−) and [Co(bpy)3]2+/3+. We observed a significant improvement in the open circuit voltage (Voc) and fill factor (FF) for the SnO2–MgO based dye sensitized solar cell (DSSC) over the pristine SnO2 device, i.e. from 357 mV to 550 mV and from ∼38% to ∼50% respectively, wherein an ∼60% enhancement in PCE for the SnO2–MgO device as compared to bare SnO2 device using the redox mediator (I−/I3−) is achieved. Interestingly, in the case of cobalt tris(2,2′-bipyridyl) redox shuttle, we observed further improvement in the PCE value of the photovoltaic device by 74%, i.e. from ∼1% (pristine SnO2 device, redox mediator I−/I3−) to ∼3.71% (SnO2–MgO device). From electrochemical impedance spectroscopy (EIS) of the devices, we conclude that the charge transfer resistance at the SnO2–MgO/SK1/electrolyte interfaces is lower as compared to SnO2/SK1/electrolyte interfaces, which demonstrates faster charge migration across the interfaces and a slower electron–hole (e−–h+) recombination rate. It was observed that in the presence of MgO, the life time of photoinduced electrons in the conduction band (CB) of SnO2 microspheres increases to τe = 15.9 ms from 7.1 ms (for pristine SnO2 device), by employing [Co(bpy)3]2+/3+ as a redox shuttle, and thus resulting in higher values of open circuit voltage (Voc) and short circuit current density (Jsc). Therefore, hierarchical MgO not only improves the PCE of the photovoltaic devices by minimizing the leakage of trapped electrons in the conduction band level of SnO2/electrolyte interface but also provides more surface area for the adsorption of dye molecules.

Quaternary semiconductor Cu2ZnSnS4 loaded with MoS2 as a co-catalyst for enhanced photo-catalytic activity

Quaternary Cu2ZnSnS4 (CZTS) loaded with 1% MoS2 shows excellent photo-catalytic activity for water oxidation, leading to efficient H2 generation (AQY 22.67%), as well as in the degradation of an organic pollutant. The photo-catalysts were characterized using powder X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, UV-Vis absorbance and fluorescence spectroscopy. Taking Rhodamine B as a model system, the apparent rate constant for CZTS–MoS2 (i.e. kapp ∼ 0.066 min−1) is nearly two-fold higher in comparison to CZTS (kapp ∼ 0.032 min−1). Various scavenger tests have been performed to establish the role of O2˙− and ˙OH scavengers in the photo-degradation of a pollutant.