Semiconducting properties of ZnO/TiO2 composites by electrochemical measurements and their relationship with photocatalytic activity

Engineering Ba2 Bi2 O6 Double Perovskite with La3+ for High Current Density Visible Light Photoelectrochemical Hydrogen Evolution

A Lanthanum ion (La3+ ) incorporation strategy is implemented to modify Ba2 Bi2 O6 -based double perovskite photoelectrodes. X-ray diffraction (XRD) characterization shows that highly crystalline Ba2 La0.4 Bi1.6 O6 double perovskites with the space group I2/m are successfully prepared. UV-vis absorption spectra and the Tauc-plot reveal an optical band gap Eg ≈1.57 ± 0.01 eV. A thickness dependence of the photoelectrodes photoelectrochemical (PEC) performance shows that the submicron (≈1 µm) 4-times spin-coated thin film photoelectrode displays strong p-type conductivity, which delivers an encouraging photocurrent density of 0.88 mA cm-2 at 0.25 VRHE under AM 1.5G illumination. 10-times coated and 20-times coated medium thick (125.8-197 µm) photoelectrodes that exhibit moderate p-type conductivity, show further enhanced photocurrent densities of 1.5 mA cm-2 at 0 VRHE . In contrast, charge recombination centers existing in a standard thick pellet (≈500 µm) Ba2 La0.4 Bi1.6 O6 photoelectrode can quench photo-generated charge carriers and greatly undermine PEC activities. The approach to doping at the Bi(III) sites contrasts with earlier efforts that focus on doping at the Bi(V) sites and thus paves the way for further tailoring a family of novel promising photocathode materials for efficient solar-water conversion devices.

Low-cost test rig for characterization of photocatalytic planar materials using photonically sized UV-A LED light sources

In the presented studies, a system for the characterization of planar photocatalysts was developed and tested. In the system, reference substances can be studied online with regard to their degradability and adsorption on photocatalytic materials. In order to perform accurate calculations of the quantum and photon efficiency of the catalysts, the LED arrays used were adjusted in their spacing by simulations so that a homogeneous light field is imaged on the catalysts. The system was tested with respect to measurement accuracy and reproducibility and the photocatalytic degradation of methylene blue, methyl orange and rhodamine B was investigated. Exemplarily, the reaction kinetics, photolysis and adsorption on the tested photocatalysts were determined for these compounds and the calculation was presented in detail. The exact construction plans and circuits as well as the sensors and their programming are presented in detail and should encourage other scientists to replicate the experimental setup, since especially in the field of photocatalysis research, often the results of publications cannot be compared with each other.

Improvement of photocatalytic activity in the degradation of 4-chlorophenol and phenol in aqueous medium using tin-modified TiO2 photocatalysts

In this work, we present the synthesis of TiO₂ photocatalysts modified with different % mol of tin using the sol-gel method. The materials were characterized using different analytical techniques. The Rietveld refinement, XPS, Raman and UV-Vis techniques confirm the substitution of tin in the TiO₂ structural lattice due to changes in crystal lattice parameters, the low-energy shift of the Sn 3d_5/2 orbital, generation of oxygen vacancies and the decreased band gap and increased BET surface area. The material with 1 mol% tin shows superior catalytic activity compared to the references for the degradation of 40 ppm 4-chlorophenol (3 hours of reaction) and 50 ppm phenol (6 hours of reaction). Reactions fit pseudo first order kinetics in both instances. The increase in photodegradation efficiency was attributed to the generation of energy levels below the TiO₂ conduction band caused by the incorporation of 1% mol of tin, oxygen vacancies, and the heterojunction formed between the brookite-anatase-rutile, causing inhibition of the recombination of the electron (e^-) and hole (h⁺) photogenerated species. The easy synthesis, low cost and increased photodegradation efficiency of the photocatalyst with 1 mol% tin have the potential to favor the remediation of recalcitrant compounds in water.

Investigation of Photocatalyst Composites for Pollutant Degradation in a Microslit Reactor Utilizing High Throughput Screening Techniques

The high-throughput screening investigations on TiO₂ based photocatalyst composites presented here have been carried out in a 60-fold parallel photoreactor. Additional catalyst testing was performed in a microslit reactor system with immobilized catalysts. For further enhancing the photocatalytic activity of TiO₂ (P25), composites of P25 and, for example, Bi₂ O₃ , CeO₂ , g-C₃ N₄ , WO₃ or ZnO were formulated in different nominal molar ratios. The catalysts' performances were assessed by their conversion of 17α-ethinyl estradiol (EE2) in aqueous solutions, determined by LC-MS. Findings show rapid EE2 conversions in short residence times. The extensive testing of catalysts led to the conclusion that the photocatalytic conversion is rather a function of residence time than a function of the materials utilized. This makes adequate process development seem more important than material development. The novelty of this contribution lies in the unique combination of testing a wide range of composite catalysts in a unique microreactor geometry.

DFT Investigation of Substitutional and Interstitial Nitrogen-Doping Effects on a ZnO(100)-TiO2(101) Heterojunction

Density Functional Theory (DFT) calculations have been performed to investigate the structural and electronic properties of the ZnO(wurtzite)-ATiO₂(anatase) heterojunction in the absence and presence of substitutional, interstitial nitrogen (N) doping and oxygen vacancies (O_V). We report a detailed study of the interactions between the two nonpolar ZnO and TiO₂ surfaces and on the role of N-doping and oxygen vacancies, which are decisive for improving the photocatalytic activity of the heterojunction. Our calculations show that substitutional N-doping is favored in the ATiO₂ portion, whereas the interstitial one is favored in the ZnO region of the interface. Both substitutional and interstitial N-doped sites (i) induce gap states that act as deep electronic traps improving the charge separation and delaying electron-hole recombination, (ii) facilitate the O_V formation causing a decrease in the formation energy (E _FORM), and (iii) do not affect the band alignment when compared to the undoped analogue system. The presented results shed light on the N-doping effect on the electronic structure of the ZnO(100)-TiO₂(101) heterojunction and how N-doping improves its photocatalytic properties.

Synergistic photocatalytic effect of BiOBr-BiOI heterojunctions due to appropriate layer stacking

The increasing interest in acquiring efficient visible-light active photocatalytic materials has led to the formation of heterojunctions with different combinations of semiconductors. Despite the fact that increasingly more complex structures are proposed, there are still many unclear factors affecting their performance and limiting their prompt implementation. In this work, we used the spray pyrolysis technique to deposit individual visible light-active BiOBr and BiOI films and formed the heterojunctions BiOBr-BiOI and BiOI-BiOBr to determine the effect of the stacking order of semiconductors. These materials were widely characterized; their structural, optical, (photo)electrochemical, and photocatalytic properties were evaluated, revealing that the configuration BiOI-BiOBr boosted the photocatalytic indigo carmine dye removal under simulated sunlight irradiation, but the opposite layout quenched it. The high efficiency is attributed to a better use of the incident radiation and the effective migration of the photogenerated carriers. BiOBr - with a wider band gap and a less negative conduction band with respect to BiOI - provides its good attributes to the heterostructure, such as high stability and low recombination rates, when it is at the surface. We demonstrated that in thin-film heterostructures, the order in which the layers are stacked becomes decisive for the photocatalytic performance and that the energy band gap and the relative band positions of both semiconductors are the principal features that govern the photocatalytic mechanism. These findings provide a key to designing more efficient photocatalysts without several unsuccessful trials.

Ag-Modified ZnO Nanorod Array Fabricated on Polyester Fabric and Its Enhanced Visible-Light Photocatalytic Performance by a Built-in Electric Field and Plasmonic Effect

ZnO nanorod arrays (NRAs) were fabricated on polyester fabrics (PFs) by a two-step method and modified with Ag by magnetron sputtering. The photogenerated charge transport properties of the Ag/ZnO nanorod heterojunctions were studied by a self-made Kelvin probe system and a surface photovoltage (SPV) test system. The measured work functions (WFs) of the deposited Ag and ZnO nanorod are 4.67 and 5.56 eV, respectively. The SPV spectra indicate that the direction of the inner electric field is from the Ag layer to the inner of the ZnO nanorod. The enhancement of light absorption by the local surface plasma resonance (LSPR) effect of Ag/ZnO NRA was observed by Raman microspectroscopy and UV-vis diffuse reflectance spectroscopy. The photocatalytic activity of the Ag/ZnO NRA-functionalized PFs was evaluated by the photocatalytic degradation of Rhodamine B (RB) solution under visible light. The full photo-oxidation of RB and the outperforming ZnO NRA-coated PFs demonstrate that the enhanced photocatalytic performance of Ag/ZnO NRA-coated PFs results from the cooperation of the inner electric field of the Ag/ZnO nanorod heterojunction and Ag LSPR.

Photocatalytic activity and photoelectrochemical properties of Ag/ZnO core/shell nanorods under low-intensity white light irradiation

Zinc oxide (ZnO) nanorod thin films were prepared by CBD onto glass and FTO/glass substrates. Silver (Ag) nanoparticles were synthesized on the surface of the prepared ZnO nanorod thin films using electrochemical methods. The scanning electron microscopy images of the Ag/ZnO/glass core/shell nanostructure confirmed that the average particles size is 20 nm while it was 41 nm for Ag NPs that synthesized onto ZnO/FTO NRs. The photocatalytic activity of the prepared Ag/ZnO core/shell nanostructure was studied by analyzing the degradation of methylene blue (MB) dye under visible light. Various pH values (6 and 10) and exposure time (30-240) min were controlled to investigate the photocatalytic activity of as-prepared Ag/ZnO core/shell nanostructure and that annealed at 200 °C and 300 °C for 1 h. It was observed that when the pH was 6, the degradation rate increased with the annealing temperature and irradiation time reaching 51% at the annealing temperature of 300 °C and exposure time of 240 min. In other hands, when the pH was 10, and the sample was annealed at 200 °C, it showed a good degradation rate of 100% at the irradiation time of 90 min. By contrast, the sample annealed at 300 °C required 180 min to degrade the MB dye completely. The photoelectrochemical cell measurement based on photocurrent density revealed a slight response to light. Cycle voltammetry (CV) measurement was conducted, and the CV curves of the Ag/ZnO core/shell electrodes indicated nonfaradaic and pseudocapacitance behavior. The electrodes showed nearly rectangular CV curves, which indicated the dominance of the nonfaradaic capacitance behavior. The specific capacitance of the electrodes remained at approximately 99%. Mott-Schottky analysis revealed that the semiconductor was an n-type with dependence on flat band potential V _FB deviation in the negative direction.

Photocatalytic behavior for the phenol degradation of ZnAl layered double hydroxide functionalized with SDS

Functionalized ZnAl layered double hydroxide based photocatalyst was obtained by the addition of sodium dodecyl sulfate (SDS) during the synthesis by the coprecipitation method, and further calcination at 400 °C. Bare and modified materials were characterized by X-ray diffraction, nitrogen adsorption-desorption, IR, UV-Vis, EPR and XPS spectroscopies, SEM and HRTEM. The synthesized material was evaluated in the photodegradation of phenol in a 40 ppm aqueous solution (4.25 × 10^-4 mol of phenol/L), under UV light irradiation. An increasing in the degradation of phenol from 62 to 95%, and from 62 to 82% in the mineralization of phenol was obtained using SDS functionalized ZnAl LDH, in comparison with the unmodified material. This increase could be attributed to the presence of sulfate radicals, confirmed by the EPR study.

Design of Hydrothermally Derived Fe2O3 Rods with Enhanced Dual Functionality Via Sputtering Decoration of a Thin ZnO Coverage Layer

The Fe2O3-ZnO composite rods were successfully synthesized by combining hydrothermal growth of Fe2O3 rods and sputtering deposition of a thin ZnO coverage layer. Two types of the Fe2O3 rods with round and rectangular cross-sectional morphologies grown via control of the urea content in hydrothermal growth processes were used as rod templates to fabricate the Fe2O3-ZnO composite rods. The Fe2O3-ZnO composite rods exhibited an improved photoelectric conversion efficiency in the Fe2O3 rods via a construction of a heterogeneous structure. The photocatalytic degradation performance of rhodamine B dyes with Fe2O3 rods was substantially increased via sputtering decoration of a thin ZnO coverage layer on the Fe2O3 rods. Moreover, the Fe2O3-ZnO composite rods exhibited superior acetone vapor-sensing responses than the pristine Fe2O3 rods herein. The extended optical absorption ability together with the enhanced photoinduced charge separation efficiency via construction of the Fe2O3-ZnO heterogeneous system explained the improved photoactivity of the composite rods. Furthermore, the formation of a heterojunction between the Fe2O3 and ZnO increased the interfacial potential barrier height and enhanced the sensor resistance variation size upon exposure to the acetone vapor. This accounted for the improved gas-sensing performance of the Fe2O3-ZnO composite rods. The experimental results herein provide a promising approach to design Fe2O3-based composite rods with desirable photocatalytic and gas-sensing functionalities.

Enhanced Hydrogen Production from Ethanol Photoreforming by Site-Specific Deposition of Au on Cu2O/TiO2 p-n Junction

Hydrogen production by photoreforming of biomass-derived ethanol is a renewable way of obtaining clean fuel. We developed a site-specific deposition strategy to construct supported Au catalysts by rationally constructing Ti3+ defects inTiO2 nanorods and Cu2O-TiO2 p-n junction across the interface of two components. The Au nanoparticles (~2.5 nm) were selectively anchored onto either TiO2 nanorods (Au@TiO2/Cu2O) or Cu2O nanocubes (Au@Cu2O/TiO2) or both TiO2 and Cu2O (Au@TiO2/Cu2O@Au) with the same Au loading. The electronic structure of supported Au species was changed by forming Au@TiO2 interface due to the adjacent Ti3+ defects and the associated oxygen vacancies while unchanged in Au@Cu2O/TiO2 catalyst. The p-n junction of TiO2/Cu2O promoted charge separation and transfer across the junction. During ethanol photoreforming, Au@TiO2/Cu2O catalyst possessing both the Au@TiO2 interface and the p-n junction showed the highest H2 production rate of 8548 μmol gcat−1 h−1 under simulated solar light, apparently superior to both Au@TiO2 and Au@Cu2O/TiO2 catalyst. The acetaldehyde was produced in liquid phase at an almost stoichiometric rate, and C−C cleavage of ethanol molecules to form CH4 or CO2 was greatly inhibited. Extensive spectroscopic results support the claim that Au adjacent to surface Ti3+ defects could be active sites for H2 production and p-n junction of TiO2/Cu2O facilitates photo-generated charge transfer and further dehydrogenation of ethanol to acetaldehyde during the photoreforming.

Si Nanocrystals/ZnO Nanowires Hybrid Structures as Immobilized Photocatalysts for Photodegradation

Numerous semiconductor-based hybrid nanostructures have been studied for improved photodegradation performance resulting from their broadband optical response and enhanced charge separation/transport characteristics. However, these hybrid structures often involve elements that are rare or toxic. Here, we present the synthesis and material characterization of hybrid nanostructures consisting of zinc oxide (ZnO) nanowires (NWs) and silicon nanocrystals (Si-NCs), both abundant and environmentally benign, and evaluate them for photodegradation performance under various illumination conditions. When incorporating Si-NCs into the vertically-aligned ZnO NWs immobilized on substrates, the resulting photocatalysts exhibited a narrowed band gap, i.e., more responsive to visible light, and enhanced charge separation at the interface, i.e., more reactive species produced for degradation. Consequently, the hybrid Si-NCs/ZnO-NWs displayed a superior photodegradability for methylene blue under UV and white light in comparison to the pristine ZnO NWs. Based on the optical measurements, we hypothesize the band structures of Si-NCs/ZnO-NWs and the potential mechanism for the improved photodegradability.

TiO2/ZnO Nanofibers Prepared by Electrospinning and Their Photocatalytic Degradation of Methylene Blue Compared with TiO2 Nanofibers

TiO2 nanofibers have high chemical stability and high strength and are applied to many fields such as air pollution sensors and air pollutant removal filters. ZnO nanofibers also have very high absorptivity in that air and are used as germicides and ceramic brighteners. TiO2/ZnO nanofibers, which have a composite form of TiO2 and ZnO, were fabricated and show higher photocatalytic properties than existing TiO2. The precursor, including zinc nitrate hexahydrate, polyvinyl acetate, and titanium isopropoxide, was used as a spinning solution for TiO2/ZnO nanofibers. Electrospun TiO2/ZnO nanofibers were calcined at 600 °C and analyzed by field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD). The average diameter of TiO2/ZnO nanofibers was controlled in the range of 189 nm to 1025 nm. XRD pattern in TiO2/ZnO nanofibers have a TiO2 anatase, ZnO, Ti2O3, and ZnTiO3 structure. TiO2/ZnO nanofibers with a diameter of 400 nm have the best photocatalytic performance in the methylene blue degradation experiments and an absorbance decrease of 96.4% was observed after ultraviolet (UV) irradiation of 12 h.

Metallurgical Wastes Employed as Catalysts and Photocatalysts for Water Treatment: A Review

Metallurgical slags are a mass-produced industrial solid waste, often destined to landfills; the volumes disposed represent an environmental burden. Over the last three decades, applications have been found for these wastes, mainly as a low-cost additive in building materials. More recently, their unique chemical properties have attracted attention to produce high-added-value materials for environmental applications, to be used as adsorbents, catalysts, or a source of reactive species in environmental engineering. Such uses can be classified as a function of the added value generated, technological complexity, and environmental impact. This review will focus specifically on the modification and use of slags for catalysis, photocatalysis, and photocatalytic production of hydrogen, which have received relatively little attention in literature. A summary will be presented about the general requirements for using unmodified slags as well as slag processed under alkaline or acidic conditions for advanced oxidation processes. Then, an overview will be given of the use of slags as photocatalysts in water treatment, organized according to the origin of the product (steel, copper, magnesium, ferromanganese), as well as emerging reports on the photocatalytic production of hydrogen, in contrast to the use of highly specific titania-based products developed for the same purpose.

How does the Zn-precursor nature impact carrier transfer in ZnO/Zn-TiO2 nanostructures? organic vs. inorganic anions

The carrier transport capability of ZnO/Zn-TiO₂ nanostructures is affected by Zn-precursor anions, generating donor, acceptor and interfacial energy states.

Improving photocatalytic reduction of 4-nitrophenol over ZrO2–TiO2 by synergistic interaction between methanol and sulfite ions

Reaction rate for 4-nitrophenol photoreduction over ZrO₂–TiO₂ increases 19 times due to the synergistic interaction between methanol and sulfite ions.

ZnO based heterojunctions and their application in environmental photocatalysis

As an alternative to TiO2 photocatalysts, ZnO exhibits a large potential for photocatalytic (PC) applications in environmental treatments, such as degradation of wastewater, sterilization of drinking water, and air cleaning. However, the efficiency achieved with ZnO to date is far from that expected for commercialization, due to rapid charge recombination, photo-corrosion as well as poor utilization of solar energy. Fortunately, in recent years, a great number of breakthroughs have been achieved in PC performance (including activity and stability) of micro-/nano- structured ZnO by forming heterojunctions (HJs) with metal nanoparticles (NPs), carbon nanostructures and other semiconductors. In most cases, the improvement of PC performance was ascribed to the better charge separation at the interfaces between ZnO and the other components. Sometimes, the formation of hybrids is also in favor of visible light harvesting. This review summarizes recent advances in the fields of environmental photocatalysis by ZnO based HJs, and especially emphasizes their abilities in degradation of organic pollutants or harmful substances in water. We aim to reveal the mechanism underlying the enhanced PC performance by constructing HJs, and extend the potential of ZnO HJ photocatalysts for future trends, and practical, large-scale applications in environment-related fields.

A Heterojunction Design of Single Layer Hole Tunneling ZnO Passivation Wrapping around TiO2Nanowires for Superior Photocatalytic Performance

Nanostructured hybrid heterojunctions have been studied widely for photocatalytic applications due to their superior optical and structural properties. In this work, the impact of angstrom thick atomic layer deposited (ALD) ZnO shell layer on photocatalytic activity (PCA) of hydrothermal grown single crystalline TiO2 nanowires (NWs) is systematically explored. We showed that a single cycle of ALD ZnO layer wrapped around TiO2 NWs, considerably boosts the PCA of the heterostructure. Subsequent cycles, however, gradually hinder the photocatalytic activity (PCA) of the TiO2 NWs. Various structural, optical, and transient characterizations are employed to scrutinize this unprecedented change. We show that a single atomic layer of ZnO shell not only increases light harvesting capability of the heterostructure via extension of the absorption toward visible wavelengths, but also mitigates recombination probability of carriers through reduction of surface defects density and introduction of proper charge separation along the core-shell interface. Furthermore, the ultrathin ZnO shell layer allows a strong contribution of the core (TiO2) valence band holes through tunneling across the ultrathin interface. All mechanisms responsible for this enhanced PCA of heterostructure are elucidated and corresponding models are proposed.

Improving the photoelectrocatalytic performance of boron-modified TiO2/Ti sol–gel-based electrodes for glycerol oxidation under visible illumination

B-TiO₂ photoelectrodes showed a high photoelectrocatalytic performance under visible illumination due to efficient electron transport and charge carrier separation.