Improvement of electrical and optical properties of ZnO thin films prepared by MOCVD using UV light irradiation and in situ H2 post-treatment

We have investigated the effect of ultraviolet (UV) light irradiation during the deposition of nominally undoped ZnO thin films using metalorganic chemical vapor deposition (MOCVD) and/or in situ hydrogen post-treatment. Due to the desorption of oxygen and incorporation of hydrogen as a shallow donor at the surface, the ZnO film prepared by the photo-MOCVD and in situ hydrogen post-treatment shows the highest film quality as a transparent conductive electrode for thin-film solar cells.     

In situ preparation of N doped orthorhombic Nb2O5 nanoplates /rGO composites for photocatalytic hydrogen generation under sunlight

The synthesis of nitrogen doped orthorhombic niobium oxide nanoplates/reduced graphene oxide composites (NNb₂O₅/rGO) and their photocatalytic activity towards hydrogen generation from water and H₂S under natural sunlight has been demonstrated, uniquely. Nanostructured NNb₂O₅/rGO is synthesized by in situ wet chemical method using urea as a source of nitrogen and optimized by varying percentage of graphene oxide (GO). X?ray diffraction (XRD) study reveals that NNb₂O₅ have orthorhombic crystal structure with crystalline size, 35 nm. Further, X?ray photoelectron spectroscopy (XPS) confirm the presence of nitrogen and rGO in NNb₂O₅/rGO nanocomposite. Morphological features of (NNb₂O₅/rGO) were examined by FE?SEM and FE?TEM showed Nb₂O₅ nanoplates of diameter 25–40 nm anchored on 2D rGO. Diffuse reflectance spectra depicts the extended absorbance in the visible region with band gap of 2.2 eV. Considering the band gap in the visible region, the photocatalytic hydrogen generation from water and H₂S has been performed. The 1 wt % rGO hybridized NNb₂O₅ (S2) exhibited superior photocatalytic hydrogen generation (537 μmol/h) from water and (1385 μmol/h) from H₂S under sunlight. The improved photocatalytic activity is attributed due to an extended absorbance in the visible region, modified electronic structure upon doping and formation of well defined NNb₂O₅/rGO interface, provides large surface area, accelerates the supression of electron and hole pairs recombination rate. In our opinion, this works may provides facile route for energy efficient and economic approach for fabrication of NNb₂O₅/rGO nanocomposites as a visible light active photocatalyst.     

In situ produced hydrogen (hydrogen on demand)

Pollution created by the increased number of people and by industrial and domestic activities put pressure in the planet's climate that can result in a catastrophe which may end the humans' life on the planet 6 and 7. Hydrogen is an endless source of energy, clean and efficient, which exist in the Universe in a high proportion, over 88% 1, 2, 3, 4 and 5.     

Plasma-assisted synthesis of Ag/ZnO nanocomposites: First example of photo-induced H2 production and sensing

Ag/ZnO nanocomposites were developed by a plasma-assisted approach. The adopted strategy exploits the advantages of Plasma Enhanced-Chemical Vapor Deposition (PE-CVD) for the growth of columnar ZnO arrays on Si(100) and Al₂O₃ substrates, in synergy with the infiltration power of the Radio Frequency (RF)-sputtering technique for the subsequent dispersion of different amounts of Ag nanoparticles (NPs). The resulting composites, both as-prepared and after annealing in air, were thoroughly characterized with particular attention on their morphological organization, structure and composition. For the first time, the above systems have been used as catalysts in the production of hydrogen by photo-reforming of alcoholic solutions, yielding a stable H₂ evolution even by the sole use of simulated solar radiation. In addition, Ag/ZnO nanocomposites presented an excellent response in the gas-phase detection of H₂, opening attractive perspectives for advanced technological applications.     

In situ X-ray diffraction study of dehydrogenation of MgH2 with Ti-based additives

Synchrotron based in situ x-ray diffraction measurements and an analysis of the dehydrogenation of MgH₂ and MgH₂ with Ti-based additives, including TiH₂ and TiMn₂, are presented. γ-MgH₂ to β-MgH₂ phase transformation in ball milled MgH₂ samples was observed prior to the dehydrogenation reaction. During the dehydrogenation of MgH₂ there were no significant phase transformations observed of the additives. These Ti-based additives functioned as catalysts during the dehydrogenation process resulting in lower temperature of dehydrogenation.     

Investigation of WO3/ZnO thin-film heterojunction-based Schottky diodes for H2 gas sensing

A comparative study of Schottky diode hydrogen gas sensors based on Pd/WO₃/Si and Pd/WO₃/ZnO/Si structure is presented in this work. Atomic force microscopy and X-ray photoelectron spectroscopy reveal that the WO₃ sensing layer grown on ZnO has a rougher surface and better stoichiometric composition than the one grown on the Si substrate. Analysis of the I–V characteristics and dynamic response of the two sensors when exposed to different hydrogen concentrations and various temperatures indicate that with the addition of the ZnO layer, the diode can exhibit a larger voltage shift of 4.0 V, 10 times higher sensitivity, and shorter response and recovery times (105 s and 25 s, respectively) towards 10,000-ppm H₂/air at 423 K. Study on the energy band diagram of the diode suggests that the barrier height is modulated by the WO₃/ZnO heterojunction, which could be verified by the symmetrical sensing properties of the Pd/WO₃/ZnO/Si gas sensor with respect to applied voltage.     

Solar hydrogen generation with H2O/ZnO/MnFe2O4 system

The hydrogen generation reaction in the H₂O/ZnO/MnFe₂O₄ system was studied to clarify the possibility of whether this reaction system can be used for the two-step water splitting to convert concentrated solar heat to chemical energy of H₂. At 1273 K, the mixture of ZnO and MnFe₂O₄ reacted with water to generate H₂ gas in 60% yield. X-ray diffractometry and chemical analysis showed that 48 mol% of Mn^II (divalent manganese ion) in the A-site of MnFe₂O₄ was substituted with Zn^II (divalent zinc ion) and that chemical formula of the solid product was estimated to be Zn_0.58Mn^II_0.42Mn^III_0.39Fe_1.61O₄ (Mn^III: trivalent manganese ion). Its lattice constant was smaller than that of the MnFe₂O₄ (one of the two starting materials). From the chemical composition, the reaction mechanism of the H₂ generation with this system was discussed. Since the Mn ions in the product solid after the H₂ generation reaction are oxidized to Mn³⁺, which can readily release the O²⁻ ions as O₂ gas around 1300 K, the two-step of H₂ generation and O₂ releasing seem to be cyclic.     

Plasma dissociation of H2S with O2 addition

The process of hydrogen sulfide, H₂S, dissociation with oxygen addition was studied in a gliding arc plasma discharge. The goal of the study was to develop a plasma process that could demonstrate increased H₂S conversion, reduced specific energy requirement (SER), and similar H₂ yield when compared with plasma dissociation of pure H₂S. The minimum SER of H₂ production was found to be 1.0 eV/molecule at atmospheric pressure, 14 L/min H₂S and 2.8 L/min O₂ flow rates, and specific energy input (SEI) of 0.29 eV per H₂S molecule. At the same conditions, the minimum SER of H₂S destruction was found to be 0.43 eV/molecule, while hydrogen yield was observed to be 27%, water production was observed to be 38%, H₂S destruction was observed to be 65%, and no measureable SO₂ was detected. These results confirm the ability to reduce energy cost for H₂ production while increasing H₂S destruction in the system without sacrificing production of H₂. This is very desirable from an industrial standpoint, where the processing and recycling of H₂S is a very costly process.     

Porous MgH2/C composite with fast hydrogen storage kinetics

A porous MgH₂/C composite can be synthesized through decomposition of an organo-magnesium precursor under hydrogen pressure. XRD patterns of the porous MgH₂/C composite exhibit a pure MgH₂ phase with a tetragonal structure. The morphology of the resulted samples is significantly dependent on the synthesis temperature and hydrogen pressure. The samples exhibit a rod-like structure and composed of nano-crystallites of MgH₂ with a size of less than 5 nm. TPD spectra of a sample synthesized at 220 °C for 4 h show a remarkable decrease of the onset hydrogen release temperature. Further, this sample also exhibits fast hydrogen adsorption kinetics adsorbing 6 wt % of hydrogen in 3 min at 250 °C. The same amount of hydrogen is adsorbed in 11 min at 200 °C and 40 min at 150 °C, respectively. N₂ physisorption measurements of this sample indicate meso-porosity with a BET surface area of 40.9 m² g⁻¹ and an average pore diameter of 20 nm.     

First-principles study of the H2 splitting processes on pure and transition-metal-doped Al (111) surfaces

By performing first-principles calculations, H₂ splitting processes on pure and transition metal (TM) atom substituted Al (111) surfaces were examined. Corrected reaction pathways with splitting energy barriers (0.99 eV) lower than those in previous studies (1.28 eV) were obtained. By further analyzing the H₂ splitting process on the 3d-TM-atom-doped Al (111) surface, the relationship of the catalysis effect and the electron donation-back donation process on TM 3d orbitals were examined in detail. Finally, to confirm the possibility of reducing the partially oxidized Al (111) surface with an H₂ molecule, the surface reduction process was studied by using the climb-image nudged elastic band (CI-NEB) method systematically.     

Theoretical investigation of adsorption and dissociation of H2 on (ZrO2)n (n=1-6) clusters

The structure, vibration, and electronic structure of H₂ molecule adsorbed on (ZrO₂)_n (n = 1-6) clusters were investigated with density functional theory. We found that H₂ is easily absorbed on the top Zr atoms of (ZrO₂)_n (n = 1-6) clusters. The Zr₅O₁₀H₂ cluster has the lowest binding energies in the Zr_nO_2nH₂ (n = 1-6) clusters. By analyzing vibrational frequency and Mulliken charge, the H-O and Zr-H bonds were found to be formed in different sized Zr_nO_2nH₂ clusters. The dissociation mechanism of H₂ shows that the charge transfers from (ZrO₂)_n cluster to H₂ due to the important role of the orbital hybridization between the cluster and H₂ molecule. With increasing the number of H₂ molecule adsorbed on (ZrO₂)_n clusters, the adsorption favors to the sites with low coordinate number, and these adsorption modes present a symmetrical tendency.     

Hydrogen storage properties of nanostructured MgH2/TiH2 composite prepared by ball milling under high hydrogen pressure

Nanostructured MgH₂/0.1TiH₂ composite was synthesized directly from Mg and Ti metal by ball milling under an initial hydrogen pressure of 30 MPa. The synthesized composite shows interesting hydrogen storage properties. The desorption temperature is more than 100 °C lower compared to commercial MgH₂ from TG-DSC measurements. After desorption, the composite sample absorbs hydrogen at 100 °C to a capacity of 4 mass% in 4 h and may even absorb hydrogen at 40 °C. The improved properties are due to the catalyst and nanostructure introduced during high pressure ball milling. From the PCI results at 269, 280, 289 and 301 °C, the enthalpy change and entropy change during the desorption can be determined according to the van’t Hoff equation. The values for the MgH₂/0.1TiH₂ nano-composite system are 77.4 kJ mol⁻¹ H₂ and 137.5 J K⁻¹ mol⁻¹ H₂, respectively. These values are in agreement with those obtained for a commercial MgH₂ system measured under the same conditions. Nanostructure and catalyst may greatly improve the kinetics, but do not change the thermodynamics of the materials.     

Influence of hydrogen dilution on low-temperature polycrystalline silicon formation using RF excitation SiH4/H2 plasma

The effect of hydrogen dilution was investigated on polycrystalline silicon formation using radio frequency excitation SiH₄/ H₂ plasma. The hydrogen dilution reduces the growth rate of the a-Si : H films. The dark conductivity of the a-Si : H films increases with increasing H₂ dilution. The dark conductivity of the poly-Si films formed by recrystallization annealing of the a-Si : H film decrease with H₂ dilution. The grain size, with X-ray diffraction spectroscopy and scanning electron microscopy images of the poly-Si films, is in reverse ratio to the H₂ dilution.     

Direct synthesis of Mg(AlH4)2 and CaAlH5 crystalline compounds by ball milling and their potential as hydrogen storage materials

Mg(AlH₄)₂ and CaAlH₅ were synthesized by direct ball milling of AlH₃ and MgH₂ or AlH₃ and CaH₂ hydrides. The XRD profiles indicated crystalline compounds. Several ball-milling conditions were studied and the optimum parameters were found. Among these, the key parameter is the pause used to cool down the milling device, which allows reducing the temperature rise during milling. Thus, the maximum yield of complex hydrides was obtained by minimizing the desorbed alane amount. The decomposition properties were studied and were in agreement with those reported for different synthesis methods. Mg(AlH₄)₂ with a good hydrogen capacity and a decomposition reaction enthalpy close to 0 kJ/mol H₂ can be a candidate for one-way storage systems. As for CaAlH₅, it might be suitable for reversible hydrogen storage thanks to its dehydrogenation reaction enthalpy (26 kJ/mol H₂). However, rather high activation energy values were evaluated for both compounds (119 and 161 kJ/mol, respectively).     

Convenient storage of concentrated hydrogen peroxide as a CaO2·2H2O2(s)/H2O2(aq) slurry for energy storage applications

Prior investigations have proposed, and successfully implemented, a stand-alone supply of aqueous hydrogen peroxide for use in fuel cells. An apparent obstacle for considering the use of aqueous hydrogen peroxide as an energy storage compound is the corrosive nature of the nominally required 50 wt.% maximum concentration. Here we propose storage of concentrated hydrogen peroxide in a high weight percent solid slurry, namely the equilibrium system of CaO₂·2H₂O₂(s)/H₂O₂(aq), that mitigates much of the risk associated with the storage of such high concentrations. We have prepared and studied surrogate slurries of calcium hydroxide/water that are assumed to resemble the peroxo compound slurries. These slurries have the consistency of a paste rather than a distinct two-phase (liquid plus solid) system. This paste-like property of the prepared surrogates enable them to be contained within a 200 lines-per-inch. (LPI) nickel mesh screen (33.6% open area) with no solids leakage, and only liquid transport driven by an adsorbent material is placed in physical contact on the exterior of the screen. This hydrogen peroxide slurry approach suggests a convenient and safe mechanism of storing hydrogen peroxide for use in, say, vehicle applications. This is because fuel cell design requires only aqueous hydrogen peroxide use, that can be achieved using the separation approach utilizing the screen material here. This proposed method of storage should mitigate hazards associated with unintentional spills and leakage issues arising from aqueous solution use.     

A simplified method for synthesis of band-structure-controlled (CuIn)xZn2(1-x)S2 solid solution photocatalysts with high activity of photocatalytic H2 evolution under visible-light irradiation

(CuIn)_xZn_2(1-x)S₂ (x = 0.01–0.5) microspheres were prepared by a simple hydrothermal method. They were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Visible diffuse reflectance spectra (UV–Vis), Raman scattering spectra and Brunauer-Emmett-Teller (BET) surface area measurement. It was found that the (CuIn)_xZn_2(1-x)S₂ samples formed solid solution only in the presence of surfactant cetyltrimethylammonium bromide (CTAB). CTAB enabled the increase of the surface area for the (CuIn)_xZn_2(1-x)S₂ solid solutions. Diffuse reflection spectra of the solid solutions shifted monotonically to long wavelength side as the value of x increased. The photocatalytic H₂ evolution activity under visible-light irradiation of the solid solutions was evaluated. The result showed that the activity depended on their corresponding compositions closely. Ru (1.5 wt%)-loaded (CuIn)_0.2Zn_1.6S₂ showed the highest photocatalytic activity of 198.09 μmol h⁻¹ under visible-light irradiation, and the apparent quantum yield amounted to 15.45% at 420 nm. Furthermore, the density functional theory (DFT) calculations showed that the solid solution with the ration of ZnS and CuInS₂, 6:1, was a direct band gap semiconductor. The valence band consisted of the hybrid orbital of S 3p and Cu 3d and the conduction band consisted of In 5s5p orbital mixed with Zn 4s4p. The energy band structure resulted in the visible-light response of the solid solution, and affected its photocatalytic performance.     

Analysis of H2 to CO2 yield and physiological key parameters of Enterobacter aerogenes and Caldicellulosiruptor saccharolyticus

In our study two strains, Enterobacter aerogenes and Caldicellulosiruptor saccharolyticus, were chosen as model microorganisms for investigation of biohydrogen production. By using E. aerogenes, operated in repetitive batch mode, the highest cumulative volumetric hydrogen evolution rate was obtained at an initial glucose concentration of 13.7 g/L. Growing C. saccharolyticus in repetitive batch mode on xylose revealed that complex media resulted in higher volumetric hydrogen productivities but lower hydrogen yields than defined media. Chemostat culture investigations of E. aerogenes and C. saccharolyticus on glucose revealed that higher dilution rates resulted in higher biohydrogen productivities, but also in lower product yields. The highest hydrogen volumetric productivities were obtained with E. aerogenes  , while the highest product to substrate yield (Y₍H₂/s₎)$\left({\text{Y}}_{\left({\text{H}}_2\text{/s}\right)}\right)$ and hydrogen to carbon dioxide yield (Y₍H₂/CO₂₎)$\left({\text{Y}}_{\left({\text{H}}_2{\text{/CO}}_2\right)}\right)$ were obtained with C. saccharolyticus  . Y₍H₂/CO₂₎${\text{Y}}_{\left({\text{H}}_2{\text{/CO}}_2\right)}$ is an important physiological parameter, regarding a future integration of biohydrogen production into the 5th generation of biofuels.     

Preparation of ZnO thin films using MOCVD technique with D2O/H2O gas mixture for use as TCO in silicon-based thin film solar cells

Zinc oxide (ZnO) thin films have been successfully grown by metal organic chemical vapor deposition (MOCVD) technique using deuterium water (D₂O) and water (H₂O) mixtures as oxidants for diethylzinc (DEZ). B₂H₆ was also employed as a dopant gas. It was found that the crystal orientation of ZnO films strongly depends on D₂O/H₂O ratio. As a result, the surface morphology of ZnO changed from textured surface morphology to smooth surface morphology with increase in the ratio of D₂O/H₂O. Moreover, it was also observed that the carrier concentration of ZnO films did not change with the ratio of D₂O/H₂O, while the mobility of these films was strongly dependent on the D₂O/H₂O ratio. Without D₂O addition, the resistivity of films had its lowest value and the minimum sheet resistance was 10 Ω/square. All films showed transmittance higher than 80% in the visible region. Moreover, the haze values of these films could be controlled by the ratio of D₂O/H₂O. These results indicate that the crystal orientation and surface morphology of the low resistivity ZnO films can be modified by using a mixture of D₂O and H₂O without changing the deposition temperature. Thus, the obtained ZnO films are promising for use as a front TCO layer in Si-based thin film solar cells.     

Maximizing the hydrogen photoproduction yields in Chlamydomonas reinhardtii cultures: The effect of the H2 partial pressure

Photoproduction of H₂ gas has been examined in sulfur/phosphorus-deprived Chalmydomonas reinhardtii cultures, placed in photobioreactors (PhBRs) with different gas phase to liquid phase ratios (V_g.p./V_l.p.). The results demonstrate that an increase in the ratio stimulates H₂ photoproduction activity in both algal suspension cultures and in algae entrapped in thin alginate films. In suspension cultures, a 4× increase (from ∼0.5 to ∼2) in V_g.p./V_l.p results in a 2× increase (from 10.8 to 23.1 mmol l⁻¹ or 264–565 ml l⁻¹) in the total yield of H₂ gas. Remarkably, 565 ml of H₂ gas per liter of the suspension culture is the highest yield ever reported for a wild-type strain in a time period of less than 190 h. In immobilized algae, where diffusion of H₂ from the medium to the PhBR gas phase is not affected by mixing, the maximum rate and yield of H₂ photoproduction occur in PhBRs with V_g.p./V_l.p above 7 or in a PhBR with smaller headspace, if the H₂ is effectively removed from the medium by continuous flushing of the headspace with argon. These experiments in combination with studies of the direct inhibitory effect of high H₂ concentrations in the PhBR headspace on H₂ photoproduction activity in algal cultures clearly show that H₂ photoproduction in algae depends significantly on the partial pressure of H₂ (not O₂ as previously thought) in the PhBR gas phase.     

Facile synthesis of noble-metal free polygonal Zn2TiO4 nanostructures for highly efficient photocatalytic hydrogen evolution under solar light irradiation

Designing of noble-metal free and morphologically controlled advanced photocatalysts for photocatalytic water splitting using solar light is of huge interest today. In the present work, novel polygonal Zn₂TiO₄ (ZTO) nanostructures have been synthesized by citricacid assisted solid state method for the first time and synthesized nanostructures were characterized by using various techniques like PXRD, UV-Vis-DRS, PL, FT-IR, BET, FE-SEM and TEM for their structural, optical, chemical, surface and morphological properties. The PXRD and UV-Vis-DRS analysis show the existence of cubic and tetragonal phases. FE-SEM and TEM results confirm the formation of polygonal ZTO nanostructures. Synthesised ZTO nanostructures have been potentially applied for solar light-driven photocatalytic hydrogen evaluation from water splitting and compare the photocatalytic activity with synthesized conventional Zn₂TiO₄ and commercially available TiO₂, ZnO photocatalysts. A high rate of 529 μmolh^?1g^?1 solar light-driven photocatalytic H₂ evolution has been achieved by using a small amount (5 mg) of polygonal Zn₂TiO₄ nanostructures from glycerol-water solution. The enhanced photocatalytic performance of the polygonal Zn₂TiO₄ nanostructures compare to conventional Zn₂TiO₄ under solar light irradiation is due to the large surface area and low recombination rate. However having the same bandgap, the polygonal Zn₂TiO₄ nanostructures have shown enhanced photocatalytic performance than that of commercially available TiO₂, ZnO photocatalysts.