Photocatalytic Activity For Hydrogen Generation Research Articles

New evidence for the regulation of photogenerated electron transfer on surface potential energy controlled co-catalyst on TiO2 – The investigation of hydrogen production over selectively exposed Au facet on Au/TiO2

In this study, Au/TiO2 samples with different exposed facets ({100}, {100/111}, and {111}) of Au were employed as catalysts for the examination of facet-dependent catalytic activity toward photocatalytic hydrogen evolution from water. By photosensitized using Eosin Y as antenna molecule, Au/TiO2 series photocatalysts exhibited different photocatalytic hydrogen evolution performances under visible light irradiation. Au{111}/TiO2 photocatalyst presented the highest photocatalytic hydrogen generation activity among Au/TiO2 series samples. As evidenced by photoluminescence spectra, photocurrent, electrochemical impedance spectra, and Mott–Schottky characterizations, the difference in photocatalytic activities resulted from the different electron transfer rates from the conduction band of TiO2 to Au nanoparticles. Au nanoparticles with exposed {111} facets were more effective in trapping electrons due to their higher Fermi level. In addition, the apparent activation energy of Au{111}/TiO2 sample was the lowest, resulted from the biggest uncoordinated numbers of Au atoms on Au{111} nanoparticles, which was favor in forming the hydrogen–metal bond. This study discloses the facet-dependent effect of noble-metal cocatalyst on semiconductor photocatalysts in photocatalytic water reduction, and will give an insight into design and synthesis of high-efficient noble metal/semiconductor hybrid photocatalysts.

Synergy of the Combination of Titanate Nanotubes with Titania Nanoparticles for the Photocatalytic Hydrogen Generation from Water-Methanol Mixture Using Simulated Sunlight

Alkali digestion of titanium nanoparticles leads, after neutralization, to the formation of titanate nanotubes with long aspect ratio. One salient change in the formation of titanate nanotubes is the observation of an extended visible absorption band up to 550 nm, responsible for their brown colour. Combination of titanate nanotubes with commercial titanium dioxide nanoparticles, either Evonik P25 or Millennium PC500, results in an enhanced photocatalytic activity for hydrogen generation from water-methanol mixtures. This synergy between the two titanium semiconductors has an optimum for a certain proportion of the two components and is observed in both the absence and the presence of platinum or gold nanoparticles. The best efficiency under simulated sunlight irradiation was for a combination of 12 wt.% titanate nanotubes containing 0.32 wt.% platinum in 88 wt.% Millennium PC500, where a two-time increase in the hydrogen generation is observed versus the activity of Millennium PC500 containing platinum. This synergy is proposed to derive from the interfacial electron transfer from titanate nanotubes undergoing photoexcitation at wavelengths in which Millennium PC500 does not absorb this form of titania nanoparticles. Our results illustrate how the combination of several titanium semiconductors can result in an enhancement efficiency with respect to their individual components.

Quantum confinement controlled solar hydrogen production from hydrogen sulfide using a highly stable CdS0.5Se0.5/CdSe quantum dot–glass nanosystem

We have demonstrated unique CdS0.5Se0.5 and CdSe quantum dot-glass nanosystems with quantum confinement effect. The stable, monodispersed CdS0.5Se0.5 and CdSe quantum dots (QDs) of size 2 to 12 nm have been grown in a germanate glass matrix by a simple melt quench technique at moderate temperature. XRD and Raman studies show formation of hexagonal CdS0.5Se0.5 and CdSe in the glass matrix. The quantum confinement of CdS0.5Se0.5 and CdSe was studied using TEM and UV-Vis spectroscopy. The band gap of the glass nanosystem was tuned from 3.6 to 1.8 eV by controlling the CdS0.5Se0.5 quantum dot size in the glass matrix. It can be further tuned to 1.68 eV using growth of CdSe quantum dots in the glass matrix. Considering the tuneable band gap of the CdS0.5Se0.5 and CdSe quantum dot-glass nanosystem for the visible light absorption, a study of size tuneable photocatalytic activity for hydrogen generation from hydrogen sulfide splitting was performed under visible light irradiation for the first time. The utmost hydrogen evolution, i.e. 8164.53 and 7257.36 μmol h(-1) g(-1) was obtained for the CdS0.5Se0.5 and CdSe quantum dot-glass nanosystems, respectively. The apparent quantum yield (AQY) was observed to be 26% and 21% for the CdS0.5Se0.5 and CdSe quantum dot-glass nanosystems, respectively. It is noteworthy that the present glass nanosystem as a photocatalyst was found to be very stable as compared to naked powder photocatalysts.

Spatial engineering of photo-active sites on g-C3N4 for efficient solar hydrogen generation

ZnFe2O4 modified g-C3N4 was successfully synthesized by a simple one-pot method. The visible-light-driven photocatalytic hydrogen production activity of g-C3N4 was significantly enhanced due to spatial engineering of the photo-active sites via ZnFe2O4 modification and Pt loading. It is proposed that ZnFe2O4 does not function as visible light sensitizer but as oxidation active sites. In the present ZnFe2O4/g-C3N4 photocatalysts, the photo-induced holes in g-C3N4 tend to transfer to ZnFe2O4 due to the straddling band structures (Type I band alignment), while the photo-induced electrons in g-C3N4 prefer to transfer to the loaded Pt cocatalysts, which can function as reduction active sites for hydrogen production. As a result, the photoinduced electrons and holes in g-C3N4 are efficiently separated by spatial engineering of the photo-active sites, and hence enhanced photocatalytic hydrogen generation activity is obtained.

Band gap engineering in huge-gap semiconductor SrZrO3 for visible-light photocatalysis

Using SrZrO3 (SZO, the intrinsic band gap being 5.6 eV) as an example, we have investigated the design principles for huge-gap semiconductors with band gap larger than 5 eV for the application of efficient visible-light driven photocatalysts for splitting water into hydrogen. Based on the hybrid density function calculations, the electronic structures of mono-doped and co-doped SZO are investigated to obtain design principles for improving their photocatalytic activity in hydrogen generation. The cationic–anionic co-doping in SZO could reduce the band gap significantly and its electronic band position is excellent for the visible-light photocatalysis. This work reports a new type of candidate material for visible-light driven photocatalysis, i.e., huge-gap semiconductors with band gap larger than 5 eV. Furthermore, based on the present results we have proposed the design principles for band gap engineering that provides general guideline for other huge-gap semiconductors.

Modulating Photogenerated Electron Transfer and Hydrogen Production Rate by Controlling Surface Potential Energy on a Selectively Exposed Pt Facet on Pt/TiO2 for Enhancing Hydrogen Production

We reported the results of the modulation of photogenerated electrons transfer and photocatalytic hydrogen evolution behaviors of Pt/TiO2 photocatalyst via controlling surface potential energy on a selectively exposed Pt facet for a highly efficient photocatalytic hydrogen generation from water. By photosensitization using Eosin Y as an antenna molecule, distinct differences in photocatalytic hydrogen evolution performances over Pt/TiO2 with different exposed facets ({100}, {100/111}, and {111}) of Pt under visible light irradiation were observed. Pt{111}/TiO2 photocatalyst exhibited a much higher photocatalytic hydrogen generation activity than those of Pt{100}/TiO2 and Pt{100/111}/TiO2. As evidenced by photoluminescence spectra, photoelectrochemical characterizations, electrochemical impedance spectra (EIS) measurements, and Mott–Schottky measurements, Pt nanoparticles with exposed {111} facets were more effective in trapping the electrons from the conduction band of TiO2 than that of {100} facets due t...

CdO–CdS nanocomposites with enhanced photocatalytic activity for hydrogen generation from water

Nanocomposites of CdO–CdS have been prepared in ethylene glycol water mixture followed by heating at 300 °C. TEM and XRD studies confirmed the atomic scale mixing of CdO and CdS nanoparticles, leading to the formation of CdSO3 phase at the interfacial region between CdO and CdS. Photocatalytic studies for hydrogen generation from water show an enhanced activity for CdO–CdS composites compared to individual components namely CdO or CdS nanoparticles. Based on optical absorption, surface area measurements, steady state and time resolved fluorescence studies, it is established that, enhanced absorption in the visible region, higher surface area and increase in lifetime of the charge carriers are responsible for the observed increase in hydrogen yield from water when composite sample was used as the photocatalyst compared to individual components. The composite sample when combined with Pt as co-catalyst exhibit a large increase in the photocatalytic activity.

High visible-light photocatalytic hydrogen evolution of C,N-codoped mesoporous TiO2 nanoparticles prepared via an ionic-liquid-template approach

A simple and novel method was developed to fabricate carbon and nitrogen codoping mesoporous TiO2 (CNMT-x) through evaporation induced self-assembly by using an ionic liquid simultaneously as carbon, nitrogen sources and a mesopore creator. These CNMT-x samples were fully characterized by a series of spectroscopic and analytical techniques, such as small- and large-angle X-ray diffraction (XRD), N2 adsorption–desorption isotherms, transmission electron microscopy (TEM), Raman, ultraviolet–visible (UV–vis) diffuse reflectance and X-ray photoelectron (XPS) spectroscopies. The obtained results showed that CNMT-0.75 catalysts exhibited the optimal photocatalytic hydrogen generation activity in water/methanol sacrificial reagent system under visible light irradiation, which was highly superior to those of conventionally prepared C-doped and commercial TiO2. The superior photocatalytic performances observed for CNMT-x could be attributed to the synergistic effects of C,N-codoping with high surface area of mesostructured TiO2.

Enhanced photocatalytic hydrogen generation over ZrO2–TiO2–CdS hybrid structure

Hybrid photocatalysts with suitable band structures are expected to show enhanced photocatalytic activity as compared to their constituent single phase compounds due to their improved physico-chemical properties. Here, we report an enhanced photocatalytic activity of a new composite photocatalyst comprising of ZrO2, TiO2, and CdS. This hybrid catalyst exhibits increased photocatalytic activity for hydrogen generation from water as compared to their constituent compounds. The photocatalytic activity decreases in the order: ZrO2–TiO2–CdS>TiO2–CdS>ZrO2–CdS>CdS>ZrO2–TiO2≈TiO2>ZrO2. An apparent quantum efficiency of 11.5% is obtained for ZrO2–TiO2–CdS with Pd as co-catalyst. Absorption edge of the composite is slightly blue shifted compared with that of pure CdS. Photoluminescence lifetime studies indicate an increased lifetime for the charge carriers in the composite sample as compared to that of pure CdS. Transmission electron microscopy images reveal that the particle size of the composite is much less than that of single phase CdS. The enhanced photocatalytic activity of the composite is attributed to the decreased particle size of CdS and increased lifetime of the charge carriers resulting from the efficient interfacial transfer of photogenerated electrons at the CdS/TiO2 and CdS/ZrO2 interface.

Synthesis of Cu/S-TiO2 photocatalysts for hydrogen generation

S-doped TiO2 was modified by Cu with methods of electroless plating, wet impregnation and chemical reduction respectively. The chemical properties of the prepared Cu/S-TiO2 were characterised by ultraviolet visible absorption spectrum (UV-Vis), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy analysis (XPS) techniques. The results showed that copper species loaded by electroless copper plating and wet impregnation method were Cu2O/CuO, while Cu was detected in the sample prepared by chemical reduction method. The Cu/S-TiO2 prepared by electroless plating method showed excellent visible light absorption ability, and the catalyst exhibited the highest photocatalytic activity for hydrogen generation in methanol solution under simulated sunlight (420–500 nm).

Photocatalytic water splitting on Au/TiO2 nanocomposites synthesized through various routes: Enhancement in photocatalytic activity due to SPR effect

In the present investigation, Au/TiO2 nanocomposites were prepared by in situ, photodeposition (PD) and SMAD method. The synthesized photocatalysts were evaluated for their photocatalytic activity in hydrogen generation. The hydrogen evolution rate was observed to be 1200μmolsh−1, 920μmolsh−1 and 1600μmolsh−1 for in situ, photodeposition (PD) and SMAD, respectively under UV–vis light illumination. However, under purely visible light illumination the highest hydrogen evolution of 32.4μmolsh−1 was observed in case of Au/TiO2 prepared by PD followed by SMAD (6.9μmolsh−1). The significantly high photocatalytic activity as demonstrated by Au/TiO2 synthesized by photodeposition method under purely visible light irradiation proves the potential of photocatalyst in efficient solar energy conversion. The poly disperse particles of AuNPs on TiO2 by photodeposition method enhances plasmonic effects. The catalyst was thoroughly characterized by powdered X-ray diffraction, UV–vis DRS and TEM for understanding the plasmonic properties. TEM images further substantiated the deposition of gold on TiO2 matrix of heterogeneous particle size with an average size of 8–10nm (smaller particles) and 53–70nm (bigger particles). The photodeposition of AuNPs on different titania supports was also studied.

Nonaqueous synthesis of CoOx/TiO2 nanocomposites showing high photocatalytic activity of hydrogen generation

This paper describes a nonaqueous sol–gel preparation of p–n diode-type CoOx/TiO2 nanocomposites for photocatalytic hydrogen generation. Characterization results suggested that the nanocomposites consisted of highly crystalline anatase nanoparticles and Co3O4-like species. They showed composition-dependent activity for hydrogen production under ultraviolet light irradiation with methanol as a sacrificial agent. The nanocomposite with the optimum composition exhibited superior photocatalytic activity and a hydrogen evolution rate of 2.17mmolg−1h−1. In comparison, the hydrogen evolution rates for the reference photocatalysts Degussa P25 and sodium tantalite measured under the same conditions were 20μmolg−1 and 410μmolg−1, respectively. The stability and the concentration-dependent hydrogen production with methanol or ethanol as sacrificial agent were also studied. In addition, the nanocomposite was capable of producing hydrogen, albeit in small amount, in the sacrificial agent-free conditions in the phosphate-buffered solutions. The results show not only the uniqueness of the nonaqueous synthesis for highly active CoOx/TiO2 heterojunction photocatalysts, but also the possibility to further improve the photocatalytic activity of CoOx/TiO2 through surface engineering.

Cubic phase indium doped cadmium sulfide dispersed on zinc oxide: Enhanced photocatalytic activity for hydrogen generation from water

In this study, we report the photocatalytic activity of cubic phase indium doped cadmium sulfide dispersed on zinc oxide having wurtzite structure (CdInS-ZnO). Enhanced photocatalytic activity for hydrogen generation is observed for CdInS-ZnO compared to CdS dispersed on ZnO. Photocatalytic activity of CdInS-ZnO is more than double when Pd was used as co-catalyst. Detailed characterization of the composite catalysts revealed that the dispersed phases of CdS/CdInS contain zinc dissolved in it. The particle size of CdS and indium doped CdS decreases significantly when dispersed on ZnO. Improved optical absorption is observed as a result of indium doping in CdS. Fluorescence lifetime studies indicated an increased lifetime for the charge carriers in the composites as compared to pure CdS. Surface analysis of Pd-CdInS-ZnO sample by XPS before and after photocatalysis work indicates a partial conversion of sulfide to sulfate species on the surface during photocatalytic process whereas no considerable change in the bulk occurs after the experiment as seen from the XRD pattern. The enhanced photocatalytic activity of CdInS-ZnO composite is attributed to a synergistic effect of improved visible light absorption and increased lifetime of the charge carriers.

Preparation, characterization and photocatalytic activity of Co3O4/LiNbO3 composite

Abstract The Co3O4/LiNbO3 composites were synthesized by impregnation of LiNbO3 in an aqueous solution of cobalt nitrate and next by calcination at 400°C. The activity of produced samples has been investigated in the reaction of photocatalytic hydrogen generation. The crystallographic phases, optical and vibronic properties were studied using X-ray diffraction (XRD), diffuse reflectance (DR) UV-vis and resonance Raman spectroscopic techniques, respectively. The influence of cobalt content (range from 0.5 wt.% to 4 wt.%) on the photocatalytic activity of Co3O4/LiNbO3 composites for photocatalytic hydrogen generation has been investigated. Co3O4/LiNbO3 composites exhibited higher than LiNbO3 photocatalytic activity for hydrogen generation. The highest H2 evolution efficiency was observed for Co3O4/LiNbO3 composite with 3 wt.% cobalt content. The amount of H2 obtained in the presence of LiNbO3 and Co3O4/LiNbO3 (3 wt.% of cobalt content) was 1.38 µmol/min and 2.59 µmol min−1, respectively.

Photocatalytic hydrogen generation from water–methanol mixtures using halogenated reconstituted graphenes

Graphene oxide (GO) obtained by Hummers oxidation of graphite and subsequent exfoliation was modified by treatment with aqueous solutions of HX at 150 °C for 30 h. This procedure introduces a percentage of the corresponding halogen, accompanied by reconstitution of G. The samples were characterized by spectroscopic techniques, including: (i) IR spectroscopy that shows the reconstitution of G, (ii) Raman spectroscopy that shows a high density of defects in the G layer, (iii) microscopy showing the typical 2D morphology, and (iv) XPS measurements to quantify the percentage of halogen in the samples. Graphene-based materials show promise as solar light photocatalysts for hydrogen generation from water, the target being improvement of the efficiency of the process. The halogenated reconstituted Gs exhibit upon 355 nm irradiation higher photocatalytic activity for hydrogen generation than GO, the most efficient sample in the series being the chlorinated one that is about seven times more active than conventional GO. This enhanced efficiency is probably the result of the influence of the nature and percentage of the halogen present in the G layer. The materials maintained the morphology after irradiation, with some variations in Raman spectroscopy compatible with reduction of the (X)G sheet as indicated by the appearance of C–H vibration peaks at 2950 cm−1. Time resolved transient spectroscopy allows monitoring of a transient signal decaying in the microsecond time scale, attributable to electrons in the conduction band of the semiconductor as supported by quenching with oxygen and signal enhancement and prolongation of lifetime by methanol.

Influence of the irradiation wavelength on the photocatalytic activity of Au–Pt nanoalloys supported on TiO2 for hydrogen generation from water

Seven samples of P25 titania containing Au–Pt nanoalloys at constant total metal loading (0.25% wt), very similar particle size (1.5 nm) and different proportions of Pt (0.0, 22.8, 27.8, 35.0, 50.0, 69.0 and 100.0% wt) have been prepared and characterized. Formation of real nanoalloys was supported by the shifts in the binding energy values of Au and Pt 4f7/2 peaks in XPS. Using 532 nm laser excitation it was observed that the six samples exhibit similar photocatalytic activity for hydrogen generation in the presence of methanol as a sacrificial electron donor. In contrast, in pure water remarkable differences in efficiency for hydrogen generation between the sample containing exclusively Au that was the most active and the others containing Pt were observed. In addition, oxygen evolution was also observed in the absence of methanol under visible light irradiation. Our results illustrate the potential that nanoalloys offer to optimize the photocatalytic activity of TiO2.

Alginate as Template in the Preparation of Active Titania Photocatalysts

AbstractA simple and reliable procedure to prepare TiO2 and Au/TiO2 samples with high photocatalytic activity for hydrogen generation from water/methanol mixtures is reported, which uses natural alginate as the templating agent. Aqueous solutions of sodium alginate are flocculated as beads by TiO2+ ions in the presence or absence of AuCl4−. The resulting alginate beads containing approximately 25 wt % of Ti and various Au contents are dehydrated by ethanol washings before drying under supercritical CO2 conditions. The key step in the preparation method is to obtain Au/Ti‐containing alginate aerogels of approximately 700 m2 g−1 Brunauer–Emmett–Teller surface area. The surface area of the TiO2 and Au/TiO2 samples obtained after calcination of the organic biopolymer at 450 °C under air ranges from 187 to 136 m2 g−1, and the Au content has been varied from 1.3 to 0.05 wt % to optimize the photocatalytic activity of the samples. TiO2 forms in the anatase phase according to XRD and Raman spectroscopy. The highest activity Au/TiO2 sample (containing 0.556 wt % of Au) prepared by means of the biopolymer templating method was approximately eight times more active for hydrogen generation using a solar simulator than was an analogous Au‐containing TiO2 P25 sample prepared by means of the conventional deposition–precipitation method.

First-principles calculations of Cd1−xZnxS doped with alkaline earth metals for photocatalytic hydrogen generation

We have investigated the structure parameters and electronic properties of Cd1−xZnxS doped with alkaline earth metals by first-principles calculations on the basis of density functional theory to predict the photocatalytic activity for hydrogen generation. The optimized structure parameters show that the smaller dopants tend to pull inward their surrounding atoms and the larger ones tend to push them outward. The negative defect formation energies indicate that almost all of the doped Cd1−xZnxS are stable. Be or Mg doping raises the conduction band edge, and the d orbital of Ca, Sr, Ba interacts with the S3p orbital leading to the changes of valence band. The band gap of the doped systems decreases with the increase of atomic number of dopant, and so alkaline earth metal doping acts as an effective method to adjust the optical properties.

The design of a hierarchical photocatalyst inspired by natural forest and its usage on hydrogen generation

A novel photocatalyst was designed from the inspiration of natural forest's high efficient on light harvesting and energy conversion. This novel “forest-like” photocatalyst was successfully synthesized by a facile continuously-conducted three steps methods: electrospinning TiO2 nanofiber acts as the trunks, hydrothermal growth ZnO nanorods on the surface of TiO2 nanofiber acts as the branches, while photodeposition of Cu nanoparticles on the surface of TiO2 nanofiber and ZnO nanorods act as the leaves. This novel photocatalyst demonstrated higher photocatalytic hydrogen generation rate than most of semiconductor catalysts and many newly developed catalysts such as Pt/TiO2 catalyst and artificial leaves Pt/N–TiO2 catalyst in a water/methanol sacrificial reagent system under the light irradiation as a result of its enhanced light absorption ability, enlarged specific surface area promoting mass transfer and providing more reaction sites and its potential on anti-recombination of electrons and holes. Meanwhile, it is interesting to note that the photocatalytic hydrogen generation activity has a liner relationship with the hierarchy of materials, which means higher hierarchy materials display higher photocatalytic hydrogen generation activity. It is reasonable to believe that this natural mimic photocatalyst without noble metals will benefit the energy generation and novel materials development.

Enhanced Photocatalytic Hydrogen Generation Using Polymorphic Macroporous TaON

Macroporous TaON (mac-TaON) is prepared using polymer sphere templating and controlled ammonolysis. In contrast to typical powder synthesis, which gives the β polymorph, mac-TaON is a mixture of β and γ polymorphs. mac-TaON shows twice the activity for photocatalytic hydrogen generation in comparison to mac-TaON when normalised for surface area.