Hierarchical nanostructured Au–SnO2 for enhanced energy storage performance

Electrode materials with high energy and power density are mostly essential to overcome traditional fossil fuel use. Herein, we demonstrate the synthesis of Au decorated self-assembled SnO₂ nanoflowers consisting of nanorods by a cost-effective and eco-friendly solvothermal process. The as-synthesized SnO₂ based materials were employed as electrode material in the energy storage system, which delivered considerably high specific capacitance of 634.3 F/g at a current density of 1 A/g. The electrode material also exhibits excellent cycle stability of 83.52% after 4000 galvanostatic charge–discharge (GCD) cycles. The high specific capacitive value is attributed to the hybrid performance of battery and supercapacitor, more active sites, and higher surface area. A solid state asymmetry device was fabricated using Au–SnO₂ and activated carbon (AC) as positive and negative electrodes. The asymmetry device shows an excellent energy density of 168.9 Wh/kg at a power density of 1 kW/kg with an applied current density of 1 A/g.     

Newly designed dye-sensitized solar cells fabricated with double-layered TiO2 (bottom layer)/Bx–TiO2 (top layer) combined electrodes for improved photocurrent

An unusual double-layered TiO₂ (bottom layer)/B_x–TiO₂ (top layer) combined electrode array was investigated to improve the photocurrent in dye-sensitized solar cells (DSSCs). A positive semiconductor, B_x–TiO₂, with nanometer-sized B (1.0, 5.0, and 10.0 mol%)-incorporated TiO₂ prepared using a solvothermal method, was utilized as the working electrode material by coating onto the second level above the TiO₂ electrode. The photocurrent and photovoltaic efficiency of the TiO₂ (bottom)/B_x–TiO₂ (top)-DSSC were 20.5% and 17.3% greater, respectively, than that of the double-layers of anatase TiO₂–DSSC in the photocurrent–voltage (I–V) curve of the optimal electrode. This result was attributed to their energy levels of reduction (LUMO)/oxidation (HOMO) as determined by cyclic voltammetry (CV). As the LUMO level of B_x–TiO₂ was located at a slightly higher level than that of pure anatase TiO₂, the electrons donated from the dye were easily transferred to the surface of the TiO₂ electrode without electron loss. Moreover, the recombination was also much slower in the TiO₂ (bottom)/B_x–TiO₂ (top)-based DSSCs than in the double-layered pure TiO₂ DSSC.     

Synthesis of π-A-porphyrins and their photoelectric performance for dye-sensitized solar cells

Three π-A-porphyrins containing long alkoxyl chains attached to the ortho position of phenyl ring and a phenyl carboxylate acid or acrylic acid at the meso position of porphyrin were synthesized. All compounds were characterized by ¹H NMR and mass spectrometry. Optical and electrochemical properties were also obtained. The photovoltaic properties of these π-A-porphyrins were examined for the first time and sensitizers N-1 and N-3 achieved comparable light to electricity conversion efficiencies: 3.94% for N-1 and 4.14% for N-3. However, preparation of N-1 required simple and cost-effective synthesis which made it a promising candidate for the future practical DSSC applications. The low efficiency conversion of N-2 was well explained by the amount of dye loading, IPCE and EIS.     

Preparation of sol–gel TiO2/purified Na-bentonite composites and their photovoltaic application for natural dye-sensitized solar cells

The sol–gel TiO₂/purified natural clay electrodes having Ti:Si molar ratios of 95:5 and 90:10 were initially prepared, sensitized with natural red cabbage dye, and compared to the sol–gel TiO₂ electrode in terms of physicochemical characteristics and solar cell efficiency. The results showed that the increase in purified Na-bentonite content greatly increased the specific surface area and total pore volume of the prepared sol–gel TiO₂/purified Na-bentonite composites because the clay platelets prevented TiO₂ particle agglomeration. The sol–gel TiO₂/5 mol% Si purified Na-bentonite and sol–gel TiO₂/10 mol% Si purified Na-bentonite composites could increase the film thickness of solar cells without cracking when they were coated as a scattering layer on the TiO₂ semiconductor-based film, leading to increasing the efficiency of the natural dye-sensitized solar cells in this work.     

Optimization of the Pd–Sn–GNS nanocomposite for enhanced electrooxidation of methanol

Highly dispersed Pd nanoparticles with varying loadings (15–40 wt%) and (20 − x)%Pd–x%Sn (where x = 1, 2, 3 and 5) nanocomposites are obtained on graphene nanosheets (GNS) by a microwave-assisted ethylene glycol (EG) reduction method for methanol electrooxidation in alkaline solution. The electrocatalysts were characterized by XRD, SEM, TEM, cyclic voltammetry, and chronoamperometry. The study shows that the Pd nanoparticles on GNS are crystalline and follow the face centered cubic structure. Introduction of a small amount of Sn (1–5 wt%) shifts the characteristic diffraction peaks for Pd slightly to a lower angle. The electrocatalytic performance of the Pd/GNS electrodes has been observed to be the best with 20 wt% Pd loading; a higher or lower loading than 20 wt% Pd produces an electrode with relatively low catalytic activity. The apparent catalytic activity of this active electrode at E = −0.10 V is found to improve further by 79% and CO poisoning tolerance by 40% with introduction of 2 wt% Sn. Among the electrodes investigated, the 18%Pd–2%Sn/GNS exhibited the greatest electrocatalytic activity toward methanol electrooxidation.     

N–TiO2 crystal seeds incorporated in amorphous matrix for enhanced solar hydrogen generation: Experimental & first-principles analysis

We present here a combined study on the photoelectrochemical activity of highly active Nitrogen doped titanium dioxide thin-film using experiments and First principle density (DFT) based calculation. Hybridization of N 2p with O 2p and localized valence band upshifting leads to the reduction in band-gap of N–TiO₂. To validate theoretical findings, the role of nitrogen in TiO₂ is revisited with a focus on partial crystallinity. The best-case photoelectrode, nanostructured partially crystalline nitrogen-doped titanium dioxide (PCNDTO) offered photocurrent density of 24.3 mA/cm² at 1 V versus saturated calomel electrode (SCE). The absence of well-defined peaks and long-range order in XRD pattern and Raman spectrum respectively suggests partially crystallinity. High-resolution transmission electron microscopy (HR-TEM) images confirm the presence of TiO₂ crystals in the amorphous matrix. High photoelectrochemical response can be attributed to the abundance of hydroxyl groups, high electrochemical active surface area, reduced charge transfer resistance, and reduced charge carrier recombination rate.     

Mesoporous plasmonic Au–TiO2 nanocomposites for efficient visible-light-driven photocatalytic water reduction

The mesoporous Au–TiO₂ nanocomposites with different Au concentrations were prepared via a co-polymer assisted sol–gel method. The structures have been characterized by powder X-Ray diffraction, N₂ adsorption–desorption isotherms, diffuse reflectance UV–Vis spectroscopy, X-ray photoemission spectroscopy, transmission electron microscopy. Most generated Au nanoparticles were embedded in the mesoporous TiO₂ matrix. The prepared Au–TiO₂ nanocomposites exhibit remarkable visible-light activity for H₂ evolution from photocatalytic water reduction in the presence of ascorbic acid as the electron donor. By comparing with Pt–TiO₂ samples, we found that the visible-light activity of the Au–TiO₂ nanocomposites could be partially contributed by the defects/impurity states in the TiO₂ matrix, while the gold surface plasmons could significantly enhance the weak visible-light excitation of TiO₂ matrix. In addition, further studies by controlling irradiation wavelengths suggest that some plasmon-excited electrons could transfer from Au nanoparticles to the contacting TiO₂ to reduce water for H₂ generation. We believe that these Au–TiO₂ nanocomposites as well as the mechanistic studies would have considerable impact on future development of metal-semiconductor hybrid photocatalysts for efficient solar hydrogen production.     

One-pot synthesis of Cu–TiO2/CuO nanocomposite: Application to photocatalysis for enhanced H2 production,dye degradation & detoxification of Cr (VI)

Photocatalytic hydrogen production under the visible spectrum of solar light is an important topic of research. To achieve the targeted visible light hydrogen production and improve the charge carrier utilization, bandgap engineering and surface modification of the photocatalyst plays a vital role. Present work reports the one-pot synthesis of Cu–TiO₂/CuO nanocomposite photocatalyst using green surfactant -aided -ultrasonication method. The materials characterization data reveals the TiO₂ particle size of 20–25 nm and the existence of copper in the lattice as well as in the surface of anatase TiO₂. This is expected to facilitate better optical and surface properties. The optimized photocatalyst shows enhanced H₂ production rate of 10,453 μmol h⁻¹ g⁻¹ of the catalyst which is 21 fold higher than pure TiO₂ nanoparticles. The photocatalyst was tested for degradation of methylene blue dye (90% in 4 h) in aqueous solution and photocatalytic reduction of toxic Cr⁶⁺ ions (55% in 4 h) in aqueous solution. A plausible mechanistic pathway is also proposed.     

Cu–MoS2/rGO hybrid material for enhanced hydrogen evolution reaction performance

Cu doped MoS₂ (Cu–MoS₂)/reduced graphene oxide (rGO) (Cu–MoS₂/rGO) hybrid material is fabricated by a facile one-step solvothermal method. The X-ray diffraction (XRD) results suggest that the doping of Cu does not alter the crystal structure of MoS₂. X-ray photoelectron spectroscopy (XPS) analysis reveal that the doping of Cu atoms influences the electronic structure of MoS₂, which is favorable to increase active sites of edges. Electrochemical impedance spectroscopy (EIS) results indicate that Cu–MoS₂/rGO performed a faster charge-transfer in comparison to MoS₂/rGO hybrid. In addition, the resultant Cu–MoS₂/rGO catalyst with Cu/Mo mole ratio of 9% exhibits a lower overpotential of 199 mV at 10 mA cm⁻², small Tafel slop of 44 mV dec⁻¹ and cycling stability, indicative of enhanced electrocatalytic activity towards HER. The improved performance is attributed to the increased active sites and a synergistic effect between copper and molybdenum, leading to electronic structure change and charge redistribution of MoS₂.     

Synthesis of TiO<Subscript>2</Subscript> nanoparticle using sol–gel route and testing its photovoltaic performance in dye-sensitized solar cell

In the present work, sol–gel method is used to synthesize TiO₂ nanoparticle. The characterization of the prepared TiO₂ powder is done using Powder X-ray diffraction (powder XRD), Scanning Electron Microscope (SEM), Energy-Dispersive X-Ray Spectroscopy (EDS) and Ultraviolet-Visible Spectrophotometry (UV-Vis). The XRD pattern reveals formation of anatase phase TiO₂. The SEM images reveal agglomeration of nanoparticles. The absorbance spectrum of TiO₂ nanoparticles was observed with excitonic peaks at 327 nm and the band gap came out to be ~3.2 eV. This prepared TiO₂ was tested for photovoltaic performance by using it in the Dye sensitized solar cell (FTO/TiO₂/N719/KI-I₂/Pt). Conversion of solar light energy to electricity was successfully done using this TiO₂. The fabricated cell showed an open-circuit voltage (V _OC) of 587 mV and short-circuit current density (J _SC) of 5.06 mA/cm². Maximum power (P _max) generated was 1.912 mW/cm² with a fill factor (FF) of 0.644 and a conversion efficiency of 1.91%.     

Ternary TiO2/Ni–Ni(OH)2/NiPi nanotube arrays with synergetic effect for enhanced photoelectrocatalytic H2-evolution

Dual co-catalysts have an essential effect on improving the photoelectrochemical (PEC) performance of semiconductor materials. In this study, Ni–Ni(OH)₂ or/and Nikel phosphate (NiPi) nanocrystals co-catalysts were deposited on the surface of as-prepared TiO₂ nanotube arrays (TNTAs) by a simple electrodeposition route for PEC hydrogen production. The TNTAs loading with Ni and NiPi bifunctional co-catalysts exhibited remarkably enhanced PEC performance, with about an 8.3-fold increase in the photocurrent; and an 11.7-fold improvement in H₂ evolution rate in comparison to bare TNTAs. The constructed ternary TNTAs/Ni–Ni(OH)₂/NiPi comprised the advantage of Ni–Ni(OH)₂ and NiPi nanoparticles to enhance visible-light absorption and promote oxygen evolution reaction (OER) kinetics. The internal electric field is generated between supported p-type Ni(OH)₂ and n-type TiO₂ under light irradiation, which will drive holes to flow to Ni(OH)₂ and oxidize it as the OER active layer. Also, the photo-generated holes remaining in the TiO₂ valence band can migrate to NiPi co-catalysts to cause OER. Therefore, the photogenerated electron-hole pairs can be successfully separated under the synergetic influence of Ni–Ni(OH)₂ and NiPi co-catalyst, leading to a superior PEC water splitting ability.     

Flexible TCO-free counter electrode for dye-sensitized solar cells using graphene nanosheets from a Ti–Ti(III) acid solution

A rapid and simple route to synthesize highly conductive graphene-based nanosheets for use as a flexible counter electrode in dye-sensitized solar cells is presented. The flexible counter electrode is free of transparent conductive oxide layer, i.e., TCO-free. A clean graphene with high quality is obtained by the chemical reduction of graphene oxide (GO) using titanium metallic powders in a hydrochloric acid solution. The Ti⁺³ ions that dissociated from metallic Ti particles in a hydrochloric acid solution result in a clean graphene material with no formation of TiO₂ nanoparticles, which are always present on graphene when only Ti⁺³ ions are used for the reduction, i.e., an anatase TiO₂ nanoparticle by-product will be always left on the graphene product when not using metallic Ti particles. The chemical reaction mechanisms for these differences are revealed in this report. The reduced materials are characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, thermo-gravimetric analysis, Fourier transform infrared spectrometry, UV–vis spectroscopy and X-ray photoelectron spectroscopy. The four-point probe method is also employed to characterize the surface conductivity of the graphene films. This high quality graphene film exhibits comparable or better performance than those obtained using conventional sputtered Pt counter electrode when used as a flexible counter electrode of dye-sensitized solar cells.     

The enhanced hydrogen storage performance of (Mg–B–N–H)-doped Mg(NH2)–2LiH system

Doping Mg(NH₂)₂–2LiH by Mg₂(BH₄)₂(NH₂)₂ compound exhibits enhanced hydrogen de/re-hydrogenation performance. The peak width in temperature-programmed desorption (TPD) profile for the Mg(NH₂)₂–2LiH–0.1Mg₂(BH₄)₂(NH₂)₂ was remarkably shrunk by 60 °C from that of pristine Mg(NH₂)₂–2LiH, and the peak temperature was lowered by 12 °C from the latter. Its isothermal dehydrogenation rate was greatly improved by five times from the latter at 200 °C. XRD, FTIR and NMR analyses revealed that a series of reactions occurred in the dehydrogenation of the composite. The prior interaction between LiH and Mg–B–N–H yielded intermediate LiBH₄, which subsequently reacted with Mg(NH₂)₂ and LiH in molar ratio of 1:6:9 to form Li₂Mg₂(NH)₃ and Li₄BN₃H₁₀ phases. The observed 6Mg(NH₂)₂–9LiH–LiBH₄ combination dominated the hydrogen release and soak in the composite system, and enhanced the kinetics of the system.     

Layered MoS2–graphene composites for supercapacitor applications with enhanced capacitive performance

Layered molybdenum disulfide (MoS₂)–graphene composite is synthesized by a modified l-cysteine-assisted solution-phase method. The structural characterization of the composites by energy dispersive X-ray analysis, X-ray powder diffraction, Fourier transform infrared spectroscopy, XPS, Raman, and transmission electron microscope indicates that layered MoS₂–graphene coalescing into three-dimensional sphere-like architecture. The electrochemical performances of the composites are evaluated by cyclic voltammogram, galvanostatic charge–discharge and electrochemical impedance spectroscopy. Electrochemical measurements reveal that the maximum specific capacitance of the MoS₂–graphene electrodes reaches up to 243 F g⁻¹ at a discharge current density 1 A g⁻¹. The energy density is 73.5 Wh kg⁻¹ at a power density of 19.8 kW kg⁻¹. The MoS₂–graphene composites electrode shows good long-term cyclic stability (only 7.7% decrease in specific capacitance after 1000 cycles at a current density of 1 A g⁻¹). The enhancement in specific capacitance and cycling stability is believed to be due to the 3D MoS₂–graphene interconnected conductive network which promotes not only efficient charge transport and facilitates the electrolyte diffusion, but also prevents effectively the volume expansion/contraction and aggregation of electroactive materials during charge–discharge process. Taken together, this work indicates MoS₂–graphene composites are promising electrode material for high-performance supercapacitors.     

Thick mesoporous TiO2 films through a sol–gel method involving a non-ionic surfactant: Characterization and enhanced performance for water photo-electrolysis

A method for the preparation of TiO₂ thick films made of anatase nanocrystallites and featuring a mesoporous structure is described. Modification of a typical sol–gel synthesis that uses Titanium (IV) isopropoxide (TTIP) as precursor, through both the incorporation of a non-ionic surfactant (Tween 20) and the optimization of thermal treatments, allows to increase the thickness of each spin-coated layer, and to obtain by successive runs porous, transparent, homogeneous and crackless films with thickness up to 1.2 μm. The effect of the changing the Tween 20/TTIP ratio (R, ranging from 0.25 to 1.00) on the crystallographic, morphological and optical properties has been studied by X-ray Diffraction, N₂ adsorption, UV–Visible Spectroscopy and Field Emission Scanning Electron Microscopy, in both powder and film form. The size of particles decreases slightly with increasing R, while the specific surface area increases somewhat. For all the R values, including R = 0, nanoparticles behave as direct semiconductors, in contrast with bulk anatase: changes in the band gap extent caused by the porous structure are negligible. Photo-electrochemical performance and carrier dynamics were studied using the films as anodes for the water photo-electrolysis reaction by means of Linear Sweep Voltammetry, Amperometry and Electrochemical Impedance Spectroscopy. Increasing R values improves the photo-catalytic performance of the TiO₂ films and leads to a comprehensive faster charge transfer at the oxide–solution interface, so that those with R = 1 offer highest performance, due to the combination of both higher thickness and improved quality of the material.     

Evaluation of in-situ formed La2O3–TiO2–La2O2CO3 nanocomposite photocatalyst for H2 production

The photocatalytic evolution of H₂ over La₂O₃ decorated TiO₂ catalyst was examined under solar light. It was observed that during the course of the reaction, the transformation of La₂O₃/TiO₂ into La₂O₃–TiO₂–La₂O₂CO₃ occurred and these species effectively suppressed electron-hole pair recombination by forming electron trapping centres on the surface, resulting in an increased visible light absorption and improved H₂ yield. The 2 wt%La₂O₃/TiO₂ nanocomposite demonstrated better H₂ yield (～8.76 mmol (g_cat)^?1) than the bare TiO₂ (～1.1 mmol (g_cat)^?1). The catalyst was stable even after several consecutive recycles with no substantial loss of hydrogen production rate. The H₂ rates were correlated with the physicochemical characteristics of the catalysts examined by BET–SA, H₂-TPR, XRD, UV-DRS, Raman spectroscopy, FTIR, HRTEM, EPR and PL spectroscopy.     

B–N co-doped black TiO2 synthesized via magnesiothermic reduction for enhanced photocatalytic hydrogen production

In this work, B–N co-doped TiO₂ has been synthesized by a facile fast sol-gel method, and then, a controlled magnesiothermic reduction has been developed to synthesize B–N co-doped black TiO₂ under a N₂ atmosphere and at 580 °C followed by acid treatment. The prepared black TiO₂ samples were characterized by X-ray diffraction, high resolution transmission electron microscopy, Raman spectrameter, photoluminescence emission spectra, X-ray photoelectron spectroscopy, and ultraviolet–visible diffuse reflectance spectra. It shows that the prepared samples possess a unique crystalline core-amorphous shell structure composed of disordered surface and oxygen vacancies, and exhibit enhanced photocatalytic activity in hydrogen production in the methanol-water system in the presence of Pt as a co-catalyst. Under the full solar wavelength range of light, the maximum hydrogen production rate of the B–N co-doped black TiO₂ is 18.8 mmol h⁻¹ g⁻¹, which is almost 4 times higher than that of pure TiO₂.