Elevated Conduction Band Research Articles

Facile fabrication of heavily N-doped Zn0.67Cd0.33S nanocatalyst with congenital sulfur vacancies for efficient photocatalytic reduction of water and hexavalent chromium

Twinned zinc cadmium sulfide (ZnxCd1-xS) has been widely investigated for efficient Cr(VI) reduction and H2 production under light irradiation due to its inherent homojunctions. However, long-distance migration of charge carriers and relatively low conduction band level inevitably cause low photocatalytic reduction ability. Herein, we developed a DMF-involved one-step solvothermal strategy to introduce N heteroatoms into Zn0.67Cd0.33S crystals incorporated with dopant-induced S vacancies. The synergetic promotion of the elevated conduction band, coupled with the robust aggregation of interactants, effectively impeded the recombination behavior of charge carriers, leading to a notable increase in photocurrent density (∼2.4 times), electron density (∼2.8 times), and a higher photo-reducing potential. The optimal NZCS-10 demonstrated a superior photocatalytic Cr(VI) reduction efficiency of 99.32 % within 30 min, and rendered ca. 5.8- and 7.6-fold enhancement for H2 evolution rate under alkaline and acidic conditions, respectively. This study provided a universal strategy for gaining highly reductive transition metal sulfides with more active sites.

In situ fabrication of N-doped Ti3C2Tx-MXene-modified BiOBr Schottky heterojunction with high photoelectron separation efficiency for enhanced photocatalytic ammonia synthesis

NH3 synthesis under ambient conditions remained great challenge owing to the strong NN bond energy, high ionization energy, and poor proton affinity of NH3. Herein, dopamine hydrochloride was selected as a N doping source to in-situ polymerization on the Ti3C2Tx MXenes surface forming a highly efficient co-catalyst, and then integrated into BiOBr to induce a super high photocatalysis for ammonia synthesis. As expected, BiOBr was evenly dispersed on the N-Ti3C2Tx MXene surface. The photocatalysis possessed a high degree of photocatalytic ammonia evolution (270.73 μmol/g·h and 3.27 times higher than that of BiOBr) and retained about 60 % of its ammonia evolution after five consecutive cycles. The photocatalyst mechanism is high dispersion and elevated conduction band potential of N-Ti3C2Tx MXene nanosheets facilitated charge transfer to the active sites of the nanosheets. This study proved the potential use of dopamine hydrochloride in MXene photocatalytic materials for solar energy utilization ammonia synthesis.

Ag-Doped CuV2O6 Nanowires for Enhanced Visible-Light Photocatalytic CO2 Reduction

Photocatalytic CO2 reduction into chemical fuels is a promising strategy to realize the CO2 conversion and solar energy utilization, yet there remains a great challenge in conversion efficiency. Herein, Ag-ion-doped CuV2O6 nanowires (NWs) with abundant oxygen vacancies were prepared by a simple hydrothermal method and utilized as catalysts for CO2 photoreduction with H2O vapor. It was demonstrated that Ag-ion doping in CuV2O6 resulted in a narrow band gap, enhanced visible-light absorption ability, elevated conduction band, abundant oxygen vacancies, strong CO2 surface-absorption capacity, superior hydrophilicity, and high separation/transfer efficiency of the photoinduced electron–hole pairs. Consequently, the Ag-doped CuV2O6 NWs exhibit excellent photocatalytic performance for CO2 reduction as compared to the pristine CuV2O6 NWs. Remarkably, the optimized Ag-doped CuV2O6 NWs exhibited a significantly enhanced photocatalytic activity with a CO evolution rate of 6.95 μmol g–1 h–1 under visible-light irradiation, which is 8.9 times higher than those of pure CuV2O6 NWs. The present work may provide a valid strategy to engineer CuV2O6 NWs for boosting visible-light photocatalytic CO2 reduction.

Synthesis and Characterization of Zn-doped CuWO4Nanoparticles and Their Opto-structural Properties

Abstract: CuWO4 and Zn-doped CuWO4 nanoparticles were prepared by a solid-state reaction method. The XRD study confirms the triclinic crystal structure for both samples and the peak shift is noticed for Zn-doped CuWO4 particles with high crystallinity. The FTIR spectra show metal oxide vibration which arose from the CuWO4 and Zn-doped CuWO4 particles. The optical absorption spectra exhibit strong absorption in the visible region and the band gap of Zn-doped CuWO4 is found to be increased to 2.44 eV compared to that of CuWO4 (2.36 eV), which is due to the elevated conduction band levels after Zn-doping. The SEM images of both CuWO4 and Zn-doped CuWO4 nanoparticles show densely aggregated particles. Keywords: Copper tungstate, Zn-doped CuWO4, Absorption, Nanoparticles.

Toward photocatalytic hydrogen generation over BiVO4 by controlling particle size

Owing to excellent light absorption and high activity for oxygen evolution, monoclinic bismuth vanadate (BiVO4) is regarded as an ideal candidate for photocatalytic water splitting. However, its application is limited by the large particle size in micrometer scale, as well as the slightly positive conduction band. In this work, we successfully synthesized nano-BiVO4 with particle size ranged from 27 nm to 57 nm by wet chemical method based on electrostatic spinning method. Unlike bulk BiVO4, the nano-sized BiVO4 possesses the ability to generate hydrogen by water splitting, and the activity could reach up to 1.66 μmol h−1 g−1 with the assistance of Pt. The enhanced activity is mainly attributed to the improvements resulted from reduced particle size, which includes elevated conduction band, enlarged specific surface area and promoted charge separation. This work provides a simple method for synthesizing photocatalyst with small particle size and high yield.

Green synthesis of boron and nitrogen co-doped TiO2 with rich B-N motifs as Lewis acid-base couples for the effective artificial CO2 photoreduction under simulated sunlight

Boron and nitrogen co-doped Titanium dioxide (TiO2) nanosheets (BNT) with high surface area of 136.5 m2 g−1 were synthesized using ammonia borane as the green and triple-functional regent, which avoids the harmful and explosive reducing regents commonly used to create surface defects on TiO2. The decomposition of ammonia borane could incorporate reactive Lewis acid-base (B, N) pairs, together with the as-generated H2 to create mesoporous structure and rich oxygen vacancies in pristine TiO2. The BNTs prepared from various ammonia borane loading are evaluated in photoreduction of carbon dioxide (CO2) with steam under simulated sunlight, achieving about 3.5 times higher carbon monoxide (CO) production than pristine TiO2 under the same conditions. Steady state and transient optical measurements indicated BNT with reduced band gap, rich defect states and elevated conduction band position could enhance the light harvesting efficiency and promote the charge transfer at the catalyst/CO2 interface. Density functional theory simulation and in situ FTIR suggest that the Lewis acid-base (B, N) pairs on BNT may very substantially increase the activation of inert CO2 which facilitates their photoreduction with the hydrogen from the water splitting at the surface defects on TiO2. Finally, a reaction mechanism of Lewis acid-base assisted CO2 photoreduction leading to substantially improved performance is proposed.

In situ synthesis of exfoliation TiO2@C hybrids with enhanced photocatalytic hydrogen evolution activity

TiO2 is an ideal semiconductor material for photocatalytic applications. However, inadequate visible light response and fast recombination of charge carriers restrict its photocatalytic performance. Constructing a junction structure between TiO2 and carbon nanomaterials can solve the above shortcomings. In the present work, two-dimensional (2D) transition metal carbides (Ti3C2) were synthesized from the Ti3AlC2 MAX phase firstly. Then the porous carbon-supported TiO2 (TiO2@C) hybrids were in-situ synthesized by calcination in air. Also, the pristine Ti3C2 was exfoliated by intercalation with dimethyl sulfoxide (DMSO). The exfoliation TiO2@C catalyst was in-situ prepared by freeze-drying and calcining, and it delivered an enhanced visible-light-driven photocatalytic H2 evolution rate of 160.42 µmol·h−1·g−1, 13 times higher than the primary TiO2@C catalyst. The enhanced photocatalytic activity results from the increased specific surface area, facilitating charge separation and elevated conduction band. The paper indicates that MXene oxidation can further apply in the visible light photocatalysis.

Photocatalytic reduction of Cr(VI) by WO3@PVP with elevated conduction band level and improved charge carrier separation property

Photocatalytic reduction of heavy metal ions is a green and promising technology which requires electrons with enough negative energy levels as well as efficient separation property from photo-generated holes of photocatalysts. For WO3, the low conduction band edge and the severe photo-generated charge carrier recombination limited its application in photocatalytic reduction of pollutants. In this work, we prepared WO3@PVP with PVP capped WO3 by a simple one-step hydrothermal method, which showed an elevated energy band structure and improved charge carrier separation property. XRD, SEM, TEM, XPS, DRS, and the photocurrent density test were carried out to study the properties of the composite. Results demonstrated monoclinic WO3 with a size of ∼100–250 nm capped by PVP was obtained, which possessed fewer lattice defects inside but more defects (W5+) on the surface. Moreover, the results of the photocatalytic experiment showed the kinetic constant of Cr(VI) reduction process on WO3@PVP was 0.532 h−1, which was 3.1 times higher than that on WO3 (0.174 h−1), demonstrating WO3@PVP with good photocatalytic capability for Cr(VI) reduction. This can be attributed to the improved charge carrier separation performance, the improved adsorption capacity and the elevated conduction band edge of WO3@PVP. More importantly, the energy band structure of WO3@PVP was proved elevated with a value as high as 1.14 eV than that of WO3 nanoparticles, which enables WO3@PVP a promising material in the photocatalytic reduction reaction of heavy metal ions from wastewater.

Ag-Bi/BiVO4 chain-like hollow microstructures with enhanced photocatalytic activity for CO2 conversion

Developing semiconductor-based photocatalysts with high activity and stability to reduce CO2 into valuable solar fuels is of great significance because of the increasingly serious energy crisis and global warming. Here, metallic Ag and Bi nanoparticles co-decorated BiVO4 (Ag-Bi/BiVO4) chain-like hollow microstructures were prepared via construction of Bi/BiVO4 hollow microstructures followed by coupling with Ag through a galvanic replacement reaction between Bi and AgNO3. The characterization results demonstrated that as-fabricated Ag-Bi/BiVO4 composites possess unique features such as extended visible-light response, rich oxygen vacancies (OVs), elevated conduction band, and high separation efficiency of photoexcited electron-hole pairs, which endow the Ag-Bi/BiVO4 composites with a surprising photocatalytic performance toward CO2 conversion to CO under visible light irradiation. The present work develops a feasible and effective strategy for developing BiVO4-based photocatalysts with suitable electronic band structure to improve the efficiency of CO2 conversion under the irradiation of visible-light.

A bismuth rich hollow Bi4O5Br2 photocatalyst enables dramatic CO2 reduction activity

The insufficient separation of photogenerated charge carriers and faint CO2 capture remains the major obstacles for photocatalytic conversion of CO2 into solar fuel. Rational design of semiconductor photocatalysts with unique structures may be promising to break this bottleneck. Herein, bismuth rich Bi4O5B2 hollow microspheres are designed as a robust photocatalyst for efficient CO2 reduction. Thanks to the bismuth rich strategy, the highly dispersed band structure and the elevated conduction band (CB) potential facilitate the charge transfer and photoreduction ability. Meanwhile, hollow structure provides the large specific area and creates a resonance in its interior to enhance the CO2 adsorption and activation. Benefiting from the collaborative promotion effect, the local charge arrangement and electronic structure are tuned so as to an exceptional efficiency of photocatalytic CO2 conversion into CO (3.16 μmol g−1 h−1) and CH4 (0.5 μmol g−1 h−1) is attained over hollow Bi4O5Br2, which is superior to that of solid Bi4O5Br2 and BiOBr, as well as other reported Bi-based photocatalysts. This work paves new opportunities for exploring high-efficiency CO2 photoreduction catalysts.

Synthesis of SPR Au/BiVO4 quantum dot/rutile-TiO2 nanorod array composites as efficient visible-light photocatalysts to convert CO2 and mechanism insight

It is highly desired to develop efficient visible-light-driven BiVO4-based nano-photocatalysts with recyclable feature by modulating photogenerated electrons and extending visible-light responses. Herein, a novel nanocomposite array has been successfully fabricated by uniformly decorating BiVO4 quantum dot (BiVO4-QD) with an average size of ∼5 nm onto rutile-TiO2 nanorod array (RTA) through the successive ionic layer adsorption and reaction processes, and exhibited high photocatalytic activity for converting CO2 to CO and CH4 under visible-light irradiation. The exceptional photoactivity is mainly attributed to the greatly-enhanced charge separation and prolonged charge lifetime by effectively transferring photogenerated electrons at the elevated conduction band bottom of BiVO4 to rutile-TiO2 nanorods as favorable electron acceptors and transport mediums by means of surface photovoltage technique, Mott-Schottky analyses and EIS measurements. Moreover, the photoactivity of optimized BiVO4-QD/RTA nanocomposite is further improved after loading SPR Au nanoparticles by promoting charge separation and extending visible-light responses, about 6.5-time higher than that of BiVO4-QD decorated anatase nanoparticle film. This work provides a feasible strategy to prepare easily reused and efficient BiVO4-based photocatalyst, which is also applicable to other semiconductors with low CB.

Simultaneous enhancement in light absorption and charge transportation of bismuth vanadate (BiVO4) photoanode via microwave annealing

We report the crystallization of electrodeposited BiVO4 photoanode by deploying conventional furnace annealing and hybrid microwave annealing, with the latter proving to possess higher crystallinity, charge carrier mobility, light absorption and conduction band level. The crystallization of BiVO4 was improved by microwave annealing, yielding higher charge carrier density. Higher morphological compactness and crystallinity for microwave annealed sample enhanced its light absorption properties. The smaller crystallite sizes upon microwave annealing resulted in band gap augmentation due to quantum confinement effect and manifested itself in its more elevated conduction band. The enhanced intrinsic properties of BiVO4 increased photoelectrochemical performance of microwave annealed sample by approximately two times compared with that of furnace annealed sample. The ultrafast synthesis as well as the possibilities of morphological, optical and electronics tunability motivates further research into devices and photoelectrodes based on microwave-annealed BiVO4.

Synergetic tuning charge dynamics and potentials of Ag3PO4 photocatalysts with boosting activity and stability by facile in-situ fluorination

Exploiting advanced silver phosphate (Ag3PO4) photocatalyst with excellent activity and durability is highly desirable for developing practical environmental photocatalytic techniques. In this study, fluorine (F)-doped Ag3PO4 photocatalysts with significantly improved activity and stability are successfully synthesized by a facile in-situ fluorine ion mediated co-precipitation route. The microstructures and physicochemical properties of as prepared samples are characterized by a series of advanced optical and electronic spectroscopy together with systematic photoelectrochemical measurements (including transient photocurrent responses, electrochemical impedance spectroscopy and Mott-Schottky plots). All the F-doped Ag3PO4 photocatalysts prepared with different initial F/Ag molar ratio (RF) exhibit higher efficiency than pure Ag3PO4 for degrading rhodamine B (RhB) as a typical organic pollutant. In particular, F-doped Ag3PO4 prepared with RF = 0.5 exhibits the best photocatalytic activity and stability. Most fluorine ions were doped into the lattice of Ag3PO4 by substituting O, as suggested by the XRD and XPS results. Significantly, the in-situ F-doping enhances the degree of crystallinity and conductivity, introduces more surface oxygen vacancies and elevates the conduction band levels, which together significantly increase the charge separation and transfer efficiency, as supported by systematic photoelectrochemical measurements. Scavenger studies suggest that photogenerated hole (h+) and H2O2 are the dominant reactive oxidation species (ROSs) responsible for the elimination of organic pollutants. After F-doping in Ag3PO4, the elevated conduction band position endows the photogenerated e− with stronger reduction ability, and the enriched surface oxygen vacancies favor for surface O2 capture, together facilitating two-electron mediated surface O2 activation to yield more H2O2. More photogenerated electrons playing a role in photocatalytic reactions in F-doped Ag3PO4 not only enhance the photocatalytic activity, but also promote photocatalytic durability by inhibiting electron-induced self-corrosion. This study demonstrates that F-doping is a facile and feasible method to enhance the photocatalytic activity and stability of Ag3PO4 by synergetic tuning the charge potentials and dynamics.

In-situ synthesis of sulfur doped carbon nitride microsphere for outstanding visible light photocatalytic Cr(VI) reduction

Abstract Sulfur doped graphitic carbon nitride microspheres were synthesized from in situ solvothermal condensation process using trithiocyanuric acid as sulfur source. The photocatalytic activity for Cr(VI) reduction and mechanism on sulfur doped carbon nitride microspheres were investigated in detail. Structure characterizations reveal that the as-prepared sample has incomplete heptazine heterocyclic ring structure. The sulfur species doped into the lattice of carbon nitride was identified as formation of C S and N S in the hybrid framework. Improved conjugated structure, expanding visible light harvesting and elevated conduction band reduction potential were realized by sulfur doping. The photocatalytic activities of the sulfur-doped carbon nitride were evaluated by Cr(VI) reduction under visible light in neutral aqueous solution. After sulfur doping, the catalysts showed much enhanced Cr(VI) reduction rate, about 24 times higher than un-doped sample. This work provides a new strategy for the preparation of sulfur doped polymer semiconductor from mild solvothermal route for the promising application in toxic heavy metal removal in neutral condition.

Significantly enhanced photocatalytic hydrogen generation over graphitic carbon nitride with carefully modified intralayer structures

Graphitic carbon nitride has been widely investigated as promising photocatalyst for solar-driven photocatalytic hydrogen production. In this study, the intralayer properties of graphitic carbon nitride was modified by a facile hydrothermal treatment with appropriate amount of ammonium hydroxide to promote its photocatalytic H2 production performance. Transition from intralayer modification to bulk structural transformation occurred once the concentration of NH3·H2O is over certain concentration. The destruction of heptazine units resulted in the partial oxidation of C atoms in g-C3N4 during the hydrothermal treatment process and the introduction of OH and CO can effectively enhance the interaction between g-C3N4 with reactants. Band structure measurement revealed that the thus obtained g-C3N4 possessed elevated conduction band bottom indicative of improved reduction capacities. The significantly enhanced redox ability on the surface reaction sites overcame the deceased light absorption and higher charge recombination efficiency of charge carriers, and finally induced a significantly improved photocatalytic H2 evolution (707.58μmol·h−1·g−1). The highest visible light photocatalytic H2 evolution rate was 7.5 times higher than that of pristine graphitic carbon nitride (94.46μmol·h−1·g−1), with an apparent quantum yield of 5.18% at 420nm. This study demonstrates a facile and effective strategy to tune the electronic and photocatalytic properties of carbon nitride for improved solar energy conversion efficiency which are expected to be extended to the modifications of other photocatalytic materials.

Efficient hydrogen evolution under visible light irradiation over BiVO4 quantum dot decorated screw-like SnO2 nanostructures

Elevated conduction band level and prolonged photogenerated carrier lifetime remarkably enhance the visible-light catalytic activity of BiVO4 in hydrogen evolution.

Graphene-like sulfur-doped g-C3N4 for photocatalytic reduction elimination of UO22+ under visible Light

Owing to the unique electronic and optical properties, the graphene-like carbon nitride (C3N4) materials have attracted widespread interest. However, the exfoliation of bulk C3N4 into graphene-like structure is inhibited by the planar hydrogen bond between strands of polymeric melon units with NH/NH2, due to the higher electronegativity of N with respect to C atoms. Herein, graphene-like sulfur-doped C3N4 (SC3N4) samples were successfully prepared by introducing sulfur into C3N4 to weaken the planar hydrogen bonding, and investigated as catalysts to photoreductively eliminate uranyl ion in aqueous media. Both experimental and computational perspectives confirmed that S-doping in SC3N4 can modify its electronic structure, reflected by the elevated conduction and valence band levels, as well as the improved transportation capability of photogenerated electrons. As a result, the photoactivity for UO22+ photocatalytic reduction over the optimal SC3N4 was significantly improved, with apparent rate value reaching 0.16min−1 under visible-light irradiation, which was 2.28 times that over C3N4. This study provides an effective strategy for designing efficient visible-light-responsive photocatalysts for environmental remediation.

Mass-Controlled Direct Synthesis of Graphene-like Carbon Nitride Nanosheets with Exceptional High Visible Light Activity. Less is Better.

In the present work, it is very surprising to find that the precursors mass, a long overlooked factor for synthesis of 2D g-C3N4, exerts unexpected impact on g-C3N4 fabrication. The nanoarchitecture and photocatalytic capability of g-C3N4 can be well-tailored only by altering the precursors mass. As thiourea mass decreases, thin g-C3N4 nanosheets with higher surface area, elevated conduction band position and enhanced photocatalytic capability was triumphantly achieved. The optimized 2D g-C3N4 (CN-2T) exhibited exceptional high photocatalytic performance with a NO removal ratio of 48.3%, superior to that of BiOBr (21.3%), (BiO)2CO3 (18.6%) and Au/(BiO)2CO3 (33.8%). The excellent activity of CN-2T can be ascribed to the co-contribution of enlarged surface areas, strengthened electron-hole separation efficiency, enhanced electrons reduction capability and prolonged charge carriers lifetime. The DMPO ESR-spin trapping and hole trapping results demonstrate that the superoxide radicals (•O2−) and photogenerated holes are the main reactive species, while hydroxyl radicals (•OH) play a minor role in photocatalysis reaction. By monitoring the reaction intermediate and active species, the reaction mechanism for photocatalytic oxidation of NO by g-C3N4 was proposed. This strategy is novel and facile, which could stimulate numerous attentions in development of high-performance g-C3N4 based functional nanomaterials.

Efficiency improvement by polarization-reversed electron blocking structure in GaN-based Light-emitting diodes.

Polarization-reversed electron-blocking structure, which had negative polarization charges localized at the interface between the last quantum barrier (LQB) and electron-blocking layer (EBL), was demonstrated to remarkably improve the light-emitting efficiency of GaN-based blue light-emitting diodes (LEDs) numerically and experimentally. The improvement was attributed to the enhanced electron-blocking effectiveness by the elevated conduction band nearby the LQB/EBL interface. Nevertheless, the efficiency droop was not mitigated because the decrease of electron-leakage was accompanied by the increase of Auger recombination.

Mg-doped TiO2 nanorods improving open-circuit voltages of ammonium lead halide perovskite solar cells

Mg-doped TiO2 nanorods were synthesized from colloidal titanate by a microwave hydrothermal reaction. Use of such TiO2 having an elevated conduction band edge as an electron extracting material for ammonium lead halide perovskite solar cells resulted in a marked improvement of Voc.