Photocatalytic Redox Reactions Research Articles

Essential Roles of Proton Transfer in Photocatalytic Redox Reactions

AbstractNowadays, semiconductor‐based heterogeneous photocatalysis is receiving much more attention than ever before owing to the increasing challenge of environmental pollution and worldwide demand for clean energy. The stoichiometry of the photocatalytic reactions usually includes the simultaneous gain or loss of electrons and protons. However, not enough emphasis has been placed on the effect of proton transfer on the photocatalytic reactions, particularly on the kinetics of interfacial redox steps. In this article, we highlight the effects of proton on the interfacial electron transfer during the photocatalytic redox reactions. We try to emphasize that the proton transfer can largely determine the energetic profile and reaction pathway by participating in the interfacial redox reactions. Better understanding on the roles and the detailed mechanisms of proton transfer in the photocatalytic surface reactions is required for the further improvement of photocatalytic efficiency.

Visible light induced photocatalytic reduction of metals (Cr, Cu, Ni, and Zn) and its synergism with different pH, TiO2, and H2O2 doses in simulated wastewater

An attempt has been made to treat the wastewater containing a combination of inorganic contaminants to take the advantage of solar photocatalytic redox reactions. Synthetic wastewater is prepared containing substantial heavy metals to obtain a simultaneous reduction of both the wastewater components. The reaction is also attempted at different pH values to optimize the same. The metals in the waste combination are Cr, Ni, Zn, and Cu. Dark adsorption reactions are also performed along with the sunlight irradiation, in order to understand the process in detail. Results show that significant chromium and copper are the metals which are significantly reduced as compared to the other two metals studied. Chromium showed maximum reduced in the acidic pH range whereas the other metals were reduced in the alkaline and neutral pH ranges. The results provide a cost-effective method to remove organic and inorganic pollutants simultaneously in the simulated wastewater through photocatalytic.

Defect-Mediated Growth of Noble-Metal (Ag, Pt, and Pd) Nanoparticles on TiO2 with Oxygen Vacancies for Photocatalytic Redox Reactions under Visible Light

A synergistic strategy involving oxygen-vacancy generation and noble-metal deposition is developed to improve the photocatalytic performance of TiO2 under visible-light irradiation. Through a redox reaction between the reductive TiO2 with oxygen vacancies (TiO2-OV) and metal salt precursors, noble-metal nanoparticles (Ag, Pt, and Pd) are uniformly deposited on the defective TiO2-OV surface in the absence of any reducing agents or stabilizing ligands. The resulting M-TiO2-OV (M = Ag, Pt, and Pd) nanocomposites are used as visible-light-driven photocatalysts for selective oxidation of benzyl alcohol and reduction of heavy metal ions Cr(VI). The results show that the oxygen vacancy creation obviously enhances the visible-light absorption of semiconductor TiO2. Meanwhile, the noble-metal deposition can effectively improve charge-separation efficiency of TiO2-OV under visible-light irradiation, thereby enhancing the photoactivity. In particular, Pd-TiO2-OV, having the average Pd particle size of 2 nm, shows th...

CdS–graphene nanocomposites as visible light photocatalyst for redox reactions in water: A green route for selective transformation and environmental remediation

A series of CdS–graphene (GR) nanocomposites with different weight addition ratios of GR have been fabricated via a facile one-step solvothermal approach. CdS–GR nanocomposites are proven to serve as selective visible light photocatalysts toward aerobic selective oxidation of alcohols and reduction of heavy ions Cr(VI), instead of being nonselective in water. Furthermore, we find that decreasing the defect density of GR by using the solvent-exfoliated graphene (SEG) instead of graphene oxide (GO) as the precursor of GR can efficiently enhance the photocatalytic activity of CdS–GR nanocomposites due to its improved electron conductivity as compared to reduced GO (RGO). In addition, the hybridization of CdS with GR (RGO, SEG) via an intimate interfacial interaction can also effectively inhibit the photocorrosion of CdS during the photocatalytic redox reaction. It is hoped that this work can draw attention to making better use of graphene to synthesize more efficient GR-based nanocomposite photocatalysts for solar energy conversion, especially in the field of diverse redox processes in water in the framework of green chemistry.

The photocatalytic inactivation effect of Ag–TiO2 on β-amyloid peptide (1–42)

Abstract Due to its low toxicity and excellent biocompatibility, titanium dioxide and its alloyed are widely used in biomedical applications. Furthermore, TiO2 can be excited by UV light to create charge carriers giving rise to photocatalytic redox reactions at its surface and photo-induced super-hydrophilicity. In this work, TiO2 films were modified with silver (Ag–TiO2) by the photocatalytic reduction of Ag+ from solution. The films and the adsorption of β-amyloid (1–42), were studied using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and atomic force microscopy (AFM). These films were confirmed to contain mainly anatase crystallites and the photo-reduced Ag was predominantly Ag0 (>0%). Ag loading of the TiO2 markedly enhanced the Raman signal (ca. 15-fold), but caused significant changes to the protein spectrum indicating non-specific binding of β-amyloid side chain residues to the silver. The amide modes remained well-resolved and were used to estimate the conformational change induced in the protein by the silver. Raman analysis showed an increase in the intensity of the band at ∼1665 cm−1 assigned to the disordered conformation of the β-amyloid, suggesting that the adsorption to the silver sites induces conformational changes. Contaminated surfaces were exposed to UVB irradiation, caused further conformational changes in the β-amyloid, which mildly inhibited amyloid fibril formation, thought to be induced through a photocatalytic mechanism.

Adsorption and photocatalytic degradation of human serum albumin on TiO 2 and Ag–TiO 2 films

Titanium dioxide is a useful material in the biomedical field as it has excellent biocompatibility based on its non-toxicity and non-inflammatory properties. Furthermore, TiO 2 can be excited by UV light to create charge carriers giving rise to photocatalytic redox reactions at the surface and photo-induced super-hydrophilicity. These properties might be exploited for surface decontamination of medical devices and implants. With this in mind, titanium dioxide TiO 2 films were prepared on stainless steel substrates using magnetron sputtering. Silver loaded (Ag–TiO 2) films were prepared by the photocatalytic reduction of Ag + from solution. The adsorption of human serum albumin (HSA) was studied. Surface analysis methods used included X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and atomic force microscopy (AFM). The TiO 2 films were predominantly anatase crystal phase and the photoreduced Ag was present at greater than 90% of the silver content as Ag 0 on the surface. Ag loading of the TiO 2 markedly enhanced the Raman signal (ca. 15-fold), but caused significant changes to the spectrum indicating non-specific binding of protein side chain residues to the Ag. The amide I and III modes remained well-resolved and were used to estimate the conformational change induced by the Ag. Raman analysis showed an increase in the intensity of the band at ∼1665 cm −1 assigned to the disordered conformation, suggesting that the adsorption to the Ag sites induces conformational changes in the protein. UVB irradiation of the protein contaminated surfaces caused further changes in the protein conformation, consistent with denaturation and enhanced binding and oxidation, thought to be induced through a photocatalytic mechanism.

Photogenerated charge carriers in semiconductor nanomaterials for solar energy conversion

Semiconducting nanomaterials possess unique optical and electronic properties ideally suited for solar energy harvesting applications, including tuneable optical absorption due to quantum confinement effects. With above bandgap light excitation, photogenerated charge carriers (PCCs) can be collected as electricity in photovoltaic solar cells, used directly in photocatalytic redox reactions, or utilised to drive chemical reactions in photoelectrochemical (PEC) devices. Inherent electron hole recombination and charge carrier trapping usually compete with and limit the efficiency of charge separation, collection, transport, or reaction. The advantage of using semiconductor nanomaterials for solar devices is the ability to manipulate their electronic band structures and thereby alter their opto-electronic properties and functionalities. To understand the fundamental charge carrier dynamics, various time-resolved techniques that directly probe the carrier lifetime and behaviour have been employed. This review focuses on new strategies to increase the separation of charge carriers using methods such as quantum dot (QD)-sensitisation of metal oxides (MOs), elemental doping, or combined approaches for achieving synergistic effects.

Synthesis, structures and photocatalytic activities of microcrystalline ABi 2Nb 2O 9 ( A=Sr, Ba) powders

Microcrystalline ABi 2Nb 2O 9 ( A=Sr, Ba) photocatalysts were successfully synthesized by a citrate complex method. The as-prepared samples were characterized by the X-ray diffraction technique, BET surface area analysis, UV–vis diffuse reflectance spectrum, transmission electron microscopy, X-ray photoelectron spectroscopy and inductively coupled plasma-atomic emission spectrometry. The results indicated that single-phase orthorhombic SrBi 2Nb 2O 9 could be obtained after being calcined above 650 °C, while BaBi 2Nb 2O 9 was tetragonal. Based on the diffuse reflectance spectra, the band gaps of the obtained samples were calculated to be around 3.34–3.54 eV. For the photocatalytic redox reaction of methyl orange under UV-light irradiation, SrBi 2Nb 2O 9 exhibited higher photocatalytic activity than that of BaBi 2Nb 2O 9. The effects of the crystallinities, BET surface areas and crystal structures of the samples on the photocatalytic activities were discussed in detail.

Highly Efficient Photocatalysts from Nanoscale Metal/Semiconductor/Metal Heterojunctions

Photocatalyst-mediated photoelectrochemical processes can utilize photogenerated electrons and holes onsite for photocatalytic redox reactions, and enable the harness and conversion of solar energy into chemical energy, in analogy to natural photosynthesis. However, the photocatalysts available to date are limited by either poor efficiency in visible light range, or insufficient photoelectrochemical stability. Metal/semiconductor/metal heterostructures are designed and synthesized as highly efficient and stable photocatalysts by integrating a nanoscale Schottky photodiode encased in a protective insulating shell with two exposed metal catalysts. As an example, Pt/Si/Ag nanowire heterostructures are synthesized to exhibit efficient photocatalytic activity for a wide range of thermodynamically downhill and uphill reactions, including photocatalytic degradation of organic dyes and toxic pollutants, reduction of metal ions and carbon dioxide, as well as hydrogen generation.

Heterointegration of Pt/Si/Ag Nanowire Photodiodes and Their Photocatalytic Properties

AbstractPhotocatalyst mediated photoelectrochemical processes can make use of the photogenerated electrons and holes onsite for photocatalytic redox reactions, and enable the harness and conversion of solar energy into chemical energy, in analogy to natural photosynthesis. However, the photocatalysts available to date are limited by either poor efficiency in the visible light range or insufficient photoelectrochemical stability. Here, it is shown that a Pt/Si/Ag nanowire heterostructure can be rationally synthesized to integrate a nanoscale metal‐semiconductor Schottky diode encased in a protective insulating shell with two exposed metal catalysts. The synthesis of Pt/Si/Ag nanowire diodes involves a scalable process including the formation of silicon nanowire array through wet chemical etching, electrodeposition of platinum and photoreduction of silver. The Pt/Si/Ag diodes exhibit highly efficient photocatalytic activity for a wide range of applications including environmental remediation and solar fuel production in the visible range. In this article, photodegradation of indigo carmine and 4‐nitrophenol are used to evaluate the photoactivity of Pt/Si/Ag diodes. The Pt/Si/Ag diodes also show high activity for photoconversion of formic acid into carbon dioxide and hydrogen.

Heterointegration of Pt/Si/Ag Nanowire Photodiodes and Their Photocatalytic Properties.

Photocatalyst mediated photoelectrochemical processes can make use of the photogenerated electrons and holes onsite for photocatalytic redox reactions, and enable the harness and conversion of solar energy into chemical energy, in analogy to natural photosynthesis. However, the photocatalysts available to date are limited by either poor efficiency in the visible light range or insufficient photoelectrochemical stability. We show that a Pt/Si/Ag nanowire heterostructure can be rationally synthesized to integrate a nanoscale metal-semiconductor Schottky diode encased in a protective insulating shell with two exposed metal catalysts. The synthesis of Pt/Si/Ag nanowire diodes involves a scalable process including the formation of silicon nanowire array through wet chemical etching, electrodeposition of platinum and photoreduction of silver. We further demonstrated that the Pt/Si/Ag diodes exhibited highly efficient photocatalytic activity for a wide range of applications including environmental remediation and solar fuel production in the visible range. In this article, photodegradation of indigo carmine and 4-nitrophenol were used to evaluate the photoactivity of Pt/Si/Ag diodes. The Pt/Si/Ag diodes also show high activity for photoconversion of formic acid into carbon dioxide and hydrogen.

Photocatalytic Redox Reactions for In‐Source Peptide Fragmentation

In-source photocatalytic redox reactions based on a photosensitive target plate have been developed to realize peptide fragmentation during laser desorption ionization. Sample peptides and glucose are simply deposited on a spot of sintered TiO2 nanoparticles. With the irradiation of UV laser on TiO2, electrons are excited from the valence to the conduction band, leaving oxidative holes and reductive electrons to drive various in-source redox reactions. Glucose, working here as a hole scavenger and conductor, can favor both on-surface reduction and long distance in-plume oxidation, therefore inducing peptide fragmentation. C(alpha)-C backbone cleavage was observed to generate a,x fragment decay, while the N-C(alpha) bond cleavage was also sometimes obtained to induce c,z fragmentation, but was rather weaker. The former dissociation is believed to originate from oxidative routes induced by the valence band holes, based on the oxidation of nitrogen atom at the peptide backbone, including hydrogen-radical abstraction and electron transfer. In contrast, the latter dissociation is supposed to be the result of reductive processes by the conduction band electrons, which are then rather similar to electron capture dissociation in tandem mass spectrometry.

Chlorophyll and Metal Porphyrins in Photocatalytic Redox Reactions of Hydrogen Peroxide

The photocatalytic decomposition of Н 2 О 2 in the presence of chlorophyll and metal complexes of tetraphenylporphyrin and phthalocyanine immobilized on silica was studied. The value of adsorption of porphyrins is shown to influence kinetics of Н 2 О 2 decomposition. Processes of reduction of some electron acceptors photocatalyzed by chlorophyll in Н 2 О 2 solution are demonstrated.

Methane generated during photocatalytic redox reaction of alcohols on TiO2 suspension in aqueous solutions

Studies on the photo-catalytic redox reaction of C1–C3 alcohols such as methanol, ethanol and 2-propanol were carried out in aqueous solution containing TiO2 photocatalyst (0.1% w/v) as suspension using 350 nm light. Other hydrocarbons such as ethane and ethene in the case of ethanol, and propene in the case of 2-propanol with low yields were produced along with the major photolytic products methane and carbon dioxide. The yields of methane and CO2 were found to be dependent on the light exposure time and ambient conditions. Methane yields were higher in 2-propanol and ethanol systems than in methanol system, showing their better hole-scavenging properties. In the aerated condition, methane was produced during photolysis of all alcohols in the presence of TiO2 and the yield was comparable to those observed in the corresponding CO2-saturated systems. The overall results reveal that the surface adsorbed, as well as in-situ-generated CO2 from photo-oxidation of alcohols are equally responsible for methane formation through photo-reduction in presence of TiO2. In the O2-saturated system, the methane yield was lower as compared to that in aerated system, in contrast to the CO2 yield. In N2O-and N2-purged systems, the yield of methane was observed to be low, inferring that the methane generation has not taken place through photodecomposition/photodissociation of alcohols. Again, photolysis of alcohols without TiO2 did not generate any methane.

Criteria for the Efficiency, Stability, and Capacity of Abiotic Photochemical Solar Energy Storage Systems

AbstractThe utilization of simple photochemical reactions for the storage of solar energy in the form of chemical energy in energy‐rich products has often been considered in the further development and improvement of e. g. simple thermosolar techniques. The hitherto proposed criteria for the qualification of an abiotic photochemical system are, however, mostly of a qualitative nature, so a mutal comparison of the systems is not precise enough. In this article it is shown how a useful correlation on the basis of time‐independent experimental data can be achieved and how, from the viewpoint of photochemistry, a comparative classification of known reactions is possible. The following reactions are compared: the [2 + 2]‐photocycloadditions of norbornadiene, dimethyl 2,3‐norbornadienedicarboxylate, and dicyclopentadienone, the photoisomerization of trans‐ to cis‐diacetylindigo, the photodissociation of nitrosyl chloride as well as a photocatalytic redox reaction. The quantity of material required and storage efficiency are by far the most favorable in the case of trans‐diacetylindigo. The main disadvantage of the latter however, is that the energy‐rich cis‐from rapidly reverts to the stable trans‐form at elevated temperatures.

Photochemical evolution of hydrogen from aqueous solutions of europium ions at .lambda. >310 nm. The role of europium(II)

The absorption spectrum of Eu/sup 2 +//sub aq/ was determined in the range lambda 200 to 400 nm. Aqueous solutions of Eu/sup 3 +/ containing organic H and OH scavengers, formate or 2-propanol, undergo on illumination with lambda 254 nm photocatalytic free radical redox reactions, in which Eu/sup 2 +/ is formed and becomes the main light absorbing species. Steady state phi/sub H/sub 2// ..-->.. 9% is obtained. Once Eu/sup 2 +/ is present, light of lambda 300 to 400 nm also becomes photoactive and steady state phi/sub H/sub 2// ..-->.. 5% obtained at 365 nm. 3 figures.