Conduction Band Potential Research Articles

Synthesis, structure, and properties of new Dion-Jacobson compounds A’LnNaNb3O10 (A’ = Cs, Rb, H; Ln = Nd, Pr)

A new series of Dion-Jacobson layered perovskite compounds A’LnNaNb3O10 (A’ = Cs, Rb, H; Ln = Nd, Pr) was synthesized for the first time. Phase and chemical purity of compounds were proved using XRD, SEM, and EDX. Compounds RbNdNaNb3O10, RbPrNaNb3O10, CsNdNaNb3O10 and CsPrNaNb3O10 were synthesized using solid-state reaction, whereas HNdNaNb3O10, HPrNaNb3O10 were obtained by ion-exchange reaction. Crystal structures of solid-state reaction products were refined using the Rietveld method. The plausibility of obtained crystal structures was studied using the bond valence sum (BVS) method. The thermal behavior of compounds was established using DTA. Ion-exchange products were found to be metastable hydrates. The presence of the crystallization water was confirmed by DTA and IR spectroscopy. Optical band gap, electronegativity, valence band (EVB), and conduction band (ECB) potentials of obtained compounds were calculated from diffuse reflectance spectroscopy data. A previously proposed correlation between band gap and structural distortions was confirmed.

Floating ZnO QDs-Modified TiO2/LLDPE Hybrid Polymer Film for the Effective Photodegradation of Tetracycline under Fluorescent Light Irradiation: Synthesis and Characterisation

In this work, mesoporous TiO2-modified ZnO quantum dots (QDs) were immobilised on a linear low-density polyethylene (LLDPE) polymer using a solution casting method for the photodegradation of tetracycline (TC) antibiotics under fluorescent light irradiation. Various spectroscopic and microscopic techniques were used to investigate the physicochemical properties of the floating hybrid polymer film catalyst (8%-ZT@LLDPE). The highest removal (89.5%) of TC (40 mg/L) was achieved within 90 min at pH 9 due to enhanced water uptake by the LDDPE film and the surface roughness of the hybrid film. The formation of heterojunctions increased the separation of photogenerated electron-hole pairs. The QDs size-dependent quantum confinement effect leads to the displacement of the conduction band potential of ZnO QDs to more negative energy values than TiO2. The displacement generates more reactive species with higher oxidation ability. The highly stable film photocatalyst can be separated easily and can be repeatedly used up to 8 cycles without significant loss in the photocatalytic ability. The scavenging test indicates that the main species responsible for the photodegradation was O2●−. The proposed photodegradation mechanism of TC was demonstrated in further detail based on the intermediates detected by LC-time-of-flight/mass spectrometry (LC/TOF-MS).

Nickel interlayer enables indirect corrosion protection of magnesium alloy by photoelectrochemical cathodic protection

As a green and sustainable corrosion protection technology, photoelectrochemical cathodic protection (PECCP) cannot be applied to protect metals having low corrosion potential (Ec) due to the insufficient negative conduction band (CB) potential of the present semiconductor materials. Herein, for the first time, PECCP is reported to indirectly protect metals with low Ec from corrosion by introduction of an interlayer with high Ec between the substrate and semiconductor. As an example, a nickel interlayer was firstly deposited on magnesium alloy substrate by chemical deposition and electrodeposition to provide the high Ec, followed by electrodeposition of Cu2O semiconductor layer on the nickel layer. A series of characterizations, including high-resolution transmission electron microscope and X-ray photoelectron spectroscopy, indicate that the resultant Cu2O semiconductor has an octahedral structure and an edge length of ca. 514 nm. The n-type property of the semiconductor is determined by the Mott-Schottky curve, and the photo-response behavior, such as obvious photo-induced potential drop and stable photocurrent density as high as 747 μA cm−2 under visible light illumination, is confirmed by electrochemical and photoelectrochemical measurements. The new proposed methodology described in this study is not limited to magnesium alloy but also can be possibly applied in other structural metals having low Ec.

In-situ fabrication of Cu3(MoO4)2(OH)2 films decorated with MO (M=Zn, Cu, and Ni) for CO2 photoconversion into value-added compounds

CO2 photoreduction is an emerging technology to produce value-added compounds using abundant resources such as CO2, H2O, sunlight, and photocatalyst materials. Here, Cu3(MoO4)2(OH)2 (CMO) films were proposed as photocatalysts for CO2 photoconversion into value-added compounds, such as methanol, formic acid, and hydrogen. The CMO films were decorated with simple oxides MO (M=Zn, Cu, and Ni) by an in-situ microwave-hydrothermal method. The CMO films exhibited a bar morphology, high crystallinity, band gap suitable for solar-light activation (2.8 eV), and valence and conduction band potential of 1.3 eV and 1.5 eV vs NHE, respectively. After decoration with the simple oxides, the films retained the bar morphology and resulted in a more efficient charge transfer by forming type-I heterostructure between Cu3(MoO4)2(OH)2 and CuO; while, type-II heterostructures were produced between CMO and NiO and ZnO. The CO2 was photoconverted to HCOOH and CH3OH in the liquid phase using CMO reference and the decorated films as photocatalysts. Cu3(MoO4)2(OH)2 decorated with CuO (CMO/Cu) favored the production of 155 µmol g−1 HCOOH, 109 µmol g−1 for CH3OH, and 127 µmol g−1 H2 after 1 h under visible-light irradiation. The yields obtained were up to 8 times higher than other conventional photocatalysts. The stability and recyclability of the films were demonstrated after three consecutive cycles. The photocatalytic activity for value-added compounds production increased as follows: CMO/Cu > CMO/Ni > CMO/Zn > CMO, associated to a higher CO2 adsorption, 2D morphology, adequate conduction band potential, and a more efficient charge transfer.

Metal-free tellurene cocatalyst with tunable bandgap for enhanced photocatalytic hydrogen production

The discovery of metal-free elemental cocatalysts has spurred dramatic advances in the field of photocatalysis. Bandgap engineering has always been used as an effective approach to make these cocatalysts form favorable band alignment with photocatalysts. Here, we develop metal-free tellurene (Te) with largely tunable bandgap as a highly efficient cocatalyst and investigate its thickness-dependent catalytic activity. Our experimental and theoretical studies reveal that the increased reduction potential of conduction band together with the decreased thickness, abundant active sites, and high charge mobility of Te, as well as the strong electronic coupling effects between Te and metal sulfides enable the enhanced photocatalytic hydrogen evolution rate and high quantum efficiency of 11.4% under the light illumination with the wavelength of 420 nm. This work enriches our understanding of the structure–activity correlations for Te and opens a path for solar energy harvesting from artificial photosynthesis by robust and cost-effective photocatalysts.

Solid solutions of CdS and ZnS: Comparing photocatalytic activity and photocurrent generation

The series of Cd1-xZnxS (x = 0–1.0) photocatalysts and Cd1-xZnxS/FTO thin film photoelectrodes were prepared. The obtained samples were studied by X-ray diffraction method (XRD), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and N2 low temperature adsorption. The photocatalysts were tested in the reaction of hydrogen production from Na2S/Na2SO3 solution under visible light irradiation. The photocurrents were measured in aqueous solution of Na2Sn and NaCl. It was shown that the target properties in the hydrogen evolution and photocurrent production are changed differently dependent on the composition of the mixed solid solutions. The highest photocatalytic hydrogen production rate was observed over Cd0.3Zn0.7S while the most effective photoelectrode was Cd0.8Zn0.2S/FTO. The Cd1-xZnxS/FTO samples were studied by the electrochemical methods in details. The factors affecting the photocatalytic activity and the photocurrent generation were found and listed for the first time. Conduction band potential (or flat band potential) and electron lifetime play a crucial role for effective photocatalytic hydrogen production over Cd1-xZnxS. Optimal photoelectrochemical characteristics were obtained in case of high values of electron lifetime. If the electron lifetimes of the tested samples have similar values, the high concentration of charge carriers is required for high photocurrent generation over Cd1-xZnxS/FTO photoelectrodes.

Thiosulfate enhanced degradation of organic pollutants in aqueous solution with g-C3N4 under visible light irradiation

Developing new strategies to design more practicable and efficient g-C3N4 based photocatalysts is important to solve the environmental issues. Thiosulfate (STS) is a common residual product found in wastewater and removal of STS remains a matter of great environmental concern. In this work, however, STS is activated by g-C3N4 under visible light irradiation, resulting in a fast degradation of Rhodamine B (RhB) and other pollutants. The performance of g-C3N4 prepared from urea was much higher than that from melamine, due to the higher surface area and more negative conduction band potential of the former catalyst. In addition, comparison with other oxidants and reductants such as peroxymonosulfate, peroxydisulfate, hydrogen peroxide and sulfite, the use of STS in g-C3N4/Vis system showed the highest efficiency for RhB degradation. During ten successive cycles, the excellent reusability of the catalyst was also obtained. The effect of different concentrations of STS and g-C3N4, and initial solution pH on the performance of the system were also studied. The mechanism study suggests that STS is first oxidized to S2O3− radicals by photohole, which will be transformed to other oxysulfur radicals such as SO3− and finally to SO42− ions. At the same time, the rate of O2 reduction by photoelectrons to O2− radicals as well as RhB degradation increases. The finding of this study provides a promising advanced oxidation process for organic pollutants degradation via STS activation.

Regulation of band edge and specific surface area of BixInyOCl microsphere for excellent photocatalytic performance

In order to solve the problem of low efficiency of existing photocatalyst in wastewater treatment, BiOCl and BixInyOCl solid solution photocatalysts with different indium element content were prepared by one-step solvothermal method. The solid solution of indium element greatly affects the porous structure and specific surface area of BiOCl, and the specific surface area of BiInOCl-0.5 is 9.26 times that of BiOCl, which provides abundant active sites for photocatalytic reaction. Furthermore, after indium element solid solution, the conduction band and valence band potentials of BiOCl shift towards negative and positive, respectively, enhancing the redox capacity of the photogenerated electrons and holes. Thus, BiInOCl shows excellent photocatalytic performances for reduction of Cr(VI), degradation of norfloxacin, dyes and even industrial cotton pulp wastewater. This provides a possibility for the practical application of photocatalytic technology in industrial wastewater treatment.

Ultrathin carbon-coated Zr3+-ZrO2 nanostructures for efficient visible light photocatalytic antibiotic elimination

Broadband solar light absorption and visible light photocatalysis of surface modified ultrawide bandgap semiconductors are receiving wide attentions because of their more negative conduction band potential and positive valence band potential for refractory organics degradation. Here, we reported a novel modified solvothermal strategy by the introduction of distilled water into small molecular alcohols to synthesize crystallized carbon-coated Zr3+-ZrO2 composites for enhanced visible light absorption and visible light photocatalytic tetracycline degradation, in which the distilled water favored for the crystallization of Zr3+-ZrO2 and the formed ultrathin carbon layers derived from the polymerization of small molecular alcohols were contributing to the generation of Zr3+ species and well dispersion of Zr3+ self-doped ZrO2 nanoparticles. The experimental results also showed that carbon-coated Zr3+-ZrO2 composites exhibited enhanced visible light absorption, long-lived charges, efficient visible light photocatalytic performance and good recyclable stability. Besides, possible photocatalytic mechanism and antibiotic degradation pathways over carbon-coated Zr3+-ZrO2 composites under visible light irradiation were also proposed in detail.

Indium sulfide-based photocatalysts for hydrogen production and water cleaning: a review

Solar illumination is a promising source of primary energy to reduce global warming and to clean polluted waters, thus fostering research of the design of efficient photocatalysts for hydrogen production by water splitting and for contaminant degradation. In particular, photocatalysis by indium sulfide (In2S3) is drawing attention due to its suitable narrow bandgap of 2.0–2.3 eV for visible light harnessing, yet large-scale application of unmodified In2S3 is limited. Here we review the photocatalyst criteria for water splitting, the synthesis and morphological manipulations of In2S3, the synthesis of heterojunctions by coupling semiconductors to increase performance, and doping In2S3. In2S3-based heterojunctions, i.e., traditional type II, all-solid-state, and direct Z-scheme photocatalytic systems show benefits such as larger charge separation, broad solar spectrum absorption, and amended conduction band and valence band edge potentials for maximum pollutant removal and H2 production. The effect of dopant incorporation on electronic modulations of In2S3 is explained by the density functional theory.

Encapsulated anion-dominated photocatalytic and adsorption performances for organic dye degradation and oxoanion pollutant capture over cationic Cu(i)-organic framework semiconductors.

Decontamination of industrial wastewater containing toxic organic dye molecules and oxoanions is urgently desirable for environmental sustainability and human health. Water-stable porous metal-organic frameworks (MOFs) have emerged as highly efficient photocatalysts and/or adsorbents for water purification through controllable integration of the constitutive requirements. To reveal the inclusion anion effect of microporous MOFs on wastewater treatment, two isostructural MOFs incorporating positive charge and semiconductive characteristics, {[Cu(tpt)]·3H2O·0.5SO4}n (1) and {[Cu(tpt)]·2H2O·ClO4}n (2, tpt = 2,4,6-tris(4-pyridyl)-1,3,5-triazine), have been synthesized and employed as dual-functional materials for both dye photodegradation and oxoanion removal. The two MOFs possess the same 3-fold interpenetrating cationic backbones but are encapsulated by highly disordered sulfate or perchlorate in the open channels. These included anions have significantly tuned the hydrophilicity of the channels, extended the visible-light absorption, optimized the bandgap and decreased the conduction band potential. Under the low-energy irradiation of a 30 W LED lamp, MOF 1 has selectively and efficiently degraded rhodamine B compared to 2 with accelerated kinetics, resulting from the stronger reduction ability and less migration resistance of the photogenerated electrons. Instead, MOF 2 can quickly capture harmful MnO4- and Cr2O72- by exchanging with the entrapped ClO4-, with maximum adsorption amounts of 557 and 168 mg g-1, respectively, under ambient conditions. The improved decolorization of the aqueous solution over 2 benefits essentially from the shape and charge memory effect and the smaller hydration energy of ClO4- than SO42-. These interesting observations highlight the importance of the included anions inside the porous MOF semiconductors on wastewater treatment.

Refining the band structure of BiOBr nanosheets through the synergetic effect of VO43− ions replacement and oxygen vacancies for promoted visible-light-driven photocatalysis

The band structure of BiOBr nanosheets is regulated by the oxygen vacancies and VO43− ions replacement. The BiOBr nanosheets possess defective states and a more negative conduction band potential, promoting visible-light photocatalytic activity.

Tuning the Conduction Band Potential of Bi-based Semiconductors Using a Combination of Organic Ligands.

Most Bi-based semiconductors are incapable of photocatalytic reduction reaction from a thermodynamic view, owing to relatively positive conduction band potentials (ECB ). Here, a novel Bi-based metal-organic framework (Bi-MBA, MBA=4-mercaptobenzoic acid) with excellent photocatalytic reduction activities is developed. The ECB of Bi-MBA locates at -1.38 eV, which is able to efficiently reduce O2 , CrVI and CO2 . Theoretical calculations reveal the significant contribution of organic ligand (MBA) to the conduction band. Our results provide an effective route to improve the photocatalytic reduction activities of Bi-based photocatalysts.

Crystal Facet-Dependent CO2 Photoreduction over Porous ZnO Nanocatalysts.

Crystal facet engineering provides a promising approach to tailor the performance of catalysts because of the close relationship between the photocatalytic activity and the surface atomic and electronic structures. An in-depth understanding mechanism of crystal facet-dependent CO2 photoreduction is still an open question. Herein, two different types of porous ZnO nanocatalysts are used as model photocatalysts for the investigation, which are, respectively, with exposed {110} and {001} facets. The porous ZnO with an exposed {110} facet exhibits superior photocatalytic activity to the one with the {001} facet. Various influencing factors have been thoroughly studied both theoretically and/or experimentally, including light harvesting (i.e., band gap), reduction capability (potential of conduction band), crystallinity, CO2 adsorption ability, CO2 activation, and charge separation. The major influencing factors are eventually figured out based on the experimental and calculation results. The product selectivity and the influence of the hole scavenger can be explained too. Our work may pave a way for directing the future rational design of efficient photocatalysts for CO2 reduction.

Online image monitoring and kinetics study on photocathodic protection of carbon steel using α-Fe2O3 photoanode

Although the photocathodic protection technique is a green and effective metal protection method, the corrosion images monitoring of protected metals and the carrier dynamics of photoanode materials are rarely studied. In this work, we successfully collected the real-time images of carbon steel (CS) by using a particle video microscope probe and verified that the use of a hematite (α-Fe2O3) photoanode could realize the long-term protection of CS under visible-light illumination in a Na2S-containning alkaline solution. In the absence of Na2S, the irradiated α-Fe2O3 electrode could delay but not prevent CS corrosion. Time-resolved spectroscopy investigation revealed that rapid electron/hole recombination via a trapping-detrapping model and slow water oxidation kinetics with a rate constant of 10 s−1 were the main factors limiting the photocathodic protection efficiency of α-Fe2O3. Trap-state-mediated recombination could be reduced by accelerating interfacial hole transfer with Na2S as the hole scavenger. This effect reduced charge transfer resistance by three orders of magnitude relative to water oxidation. Our work showed that photocathodic protection behavior was determined not only by the relative position between the conduction band potential of α-Fe2O3 and the corrosion potential of CS but also by the reaction dynamics on the electrode surface. The acceleration of the interfacial charge transfer of α-Fe2O3 was proposed to be the key to effective photocathodic protection.

Efficient TiO2/AgInS2/ZnS Nanoarchitecture Photoelectrode for the Photoelectrochemical Cathodic Protection of Copper in NaCl Solution

An environmentally friendly AgInS2/ZnS nanoparticles (NPs) co-sensitized TiO2 ultrafine branched nanolawn photoelectrode was constructed by depositing AgInS2 sensitizer and ZnS passivator on TiO2 ultrafine nanostructure. This architecture significantly improved the photoelectrochemical conversion efficiency and photoelectrochemical cathodic protection performance for pure copper with a more negative self-corrosion potential under simulated sunlight illumination and in NaCl solution containing no additional hole scavengers. Compared with single AgInS2 NPs sensitized TiO2, the design of ZnS passivation layer on TiO2/AgInS2 significantly improves the charge generation and separation efficiency, and promotes the consumption of the photogenerated holes. The three-dimensional TiO2 with porous ultrafine nanobranched structure possesses high-speed electron transmission pathways and facilitates the rapid collection and migration of the photogenerated electrons. Since the conduction band potentials of AgInS2 and ZnS are more negative, the photogenerated electrons generated by TiO2/AgInS2/ZnS will be maintained at a more negative quasi-Fermi level. This ensures that the photogenerated electrons can be transferred to pure copper. The present work provides new ideas for designing nanoheterojunction materials for the protection of pure copper and other metallic materials with more negative self-corrosion potentials exposed in marine environment under simulated sunlight illumination.

Effects of adsorption capacity and activity site on Fenton-like catalytic performance for Na and Fe co-doped g-C3N4

Na and Fe co-doped g-C3N4 was selected as a Fenton-like catalyst to degrade Methylene blue (MB). The properties of the catalyst changed with different contents of Fe and Na. The adsorption capacity and degradation ratio for MB increased from 1.16% to 56.7% and 89.6% to 98.4%, respectively, when partial Fe was replaced by Na. And the rate constants increased from 0.0886 min−1 to 0.1702 min−1. The experimental results indicate that the adsorption capacity and the activity site of the catalyst are the key effects on the catalytic activity. The adsorption capacity was affected by the zeta potential of the catalyst. It was almost not effects of between specific surface area, electron transfer, band gap, and positions of the valence band potential and conduction band potential on the catalytic activity. This study may give a sustainable strategy to synthesize catalysts with enhanced activity for the Fenton reaction.

Construction of novel direct Z-scheme AgIO4-g-C3N4 heterojunction for photocatalytic hydrogen production and photodegradation of fluorescein dye

Highly mesoporous AgIO4/g-C3N4 heterojunctions were synthesized by sonochemical route for exceptional photocatalytic degradation of fluorescein dye (FLU) and hydrogen production. AgIO4 nanoparticles are deposited on the corners and edges of the wrinkled sheets of g-C3N4. Mass ratio of AgIO4 acted a pivotal part in enhancing the photocatalytic reactivity. Due to the small difference between the valence band potential of g-C3N4 (EVB = +1.4 eV) and conduction band potential of AgIO4 (ECB = +1.08 eV), a perfect direct Z-scheme mechanism is constructed. Noticeably, the removal of fluorescein dye reach 98% and the rate of hydrogen produced is 23 mmolh−1 g−1. Taking the benefits of matching the band energy structure, AgIO4/g-C3N4 heterojunction enhances the recombination of the useless positive hole and electron in the inferior valence and conduction band of g-C3N4 and AgIO4, respectively through Z-scheme aspects. However, the holes and electrons in the higher valence and conduction band are preserved with strong oxidation and reduction power. This Z-scheme mechanism not only enhances the quantum efficiency of charge separation, but also, increase the oxidative and reductive power of the charge carrier. Positive holes and the superoxide radicals are the main reactive species confirmed the Z-scheme mechanism. This research work give a hot spot in synthesis of low cost novel photocatalyst for hydrogen production and wastewater treatment.

Interplay between Ruthenium Sensitizer and Ruthenium Catalyst in Photoelectrochemical Cells with Different Water-Based Electrolytes

Popular ruthenium sensitizer (RuP) and ruthenium catalyst (RuOEC) are studied in the cosensitized configurations of a dye-sensitized photoelectrochemical cell with several frequently used electrolytes. Using transient absorption techniques, we have observed that the initial steps of electron transfer from the catalyst to the oxidized sensitizer occur on a very fast time scale (below 100 ps in most of the electrolytes), comparable to that of previously reported supramolecular systems. Electron injection from RuP to TiO2 occurs over a broad span of time scales ranging from tens of picoseconds to hundreds of nanoseconds. Its average rate constant strongly depends on the TiO2 conduction band potential, which varies in different electrolytes and does not follow a simple pH dependence. On the contrary, the excited state of the catalyst deactivates very fast, mostly within 1 ps in all electrolytes. Also, the ratios of initial to stable photocurrents and dark to stable photocurrents, recorded in the operating photoelectrochemical cell, have been found to depend on the conduction band potential. Both sensitizer and catalyst show different resistances to desorption in various environments, e.g., RuP is the most stable in Na2SO4 as well as HCl and RuOEC in phosphate buffer and ethylphosphonic acid. Thus, finding the optimum conditions ensuring the best stability of both compounds is required. The studies prove the potential of cosensitized configurations for efficient water-splitting systems and give more insight into further ways of optimization of such systems.

Tribo-catalytic degradation of organic pollutants through bismuth oxyiodate triboelectrically harvesting mechanical energy

As an old physical concept, triboelectrification is a common phenomenon in different mechanical movements and always plays a negative effect on our daily life. Collection of triboelectricity from ambient mechanical energy recently has been realized via a triboelectric nanogenerator (TENG). Herein, we successfully used the triboelectricity from the magnetic stirring induced friction to catalytic degradation of organic dyes, therefore realizing the conversion of mechanical energy to electric energy and finally to chemical energy. The tribo-catalyst of BiOIO 3 exhibited an efficient catalytic activity where the dye degradation reached >90% after 6 h magnetic stirring, and meanwhile showed a superior durability. The charge exchange during the friction between catalyst and magnetic bar or beaker and the subsequent reaction of charges with aqueous solution to produce reactive oxygen species are considered to be responsible for the tribo-catalysis. However, due to the positive edge potential of conduction band (CB) of BiOIO 3 , superoxide radicals cannot be generated by the reduction of dissolved oxygen with the electrons on CB. As a result, the holes on valence band (VB) and the hydroxyl radicals are mainly active species for dye degradation. This finding well demonstrates that catalysis reactions can be initiated by triboelectric charges on catalyst during harvesting small ambient mechanical energy. • Mechanical energy of friction was harvested and converted to degrade organic dyes. • Tribo-catalytic effect originated from the friction of magnetic bar/BiOIO 3 catalyst. • The BiOIO 3 showed efficient tribo-catalytic activity and superior catalytic durability. • Tribo-catalysis can be improved by optimizing magnetic bar size and beaker material.