Separation Efficiency Of Photocarriers Research Articles

An unusal strategy of Ca2+ heterovalent doping enabled upconversion enhancement of Er3+ in bismuth oxychloride layered semiconducting crystals

Substitutional heterovalent doping has been considered as an effective method to control optical properties of lanthanide doped phosphors in the past, but it focuses only on three-dimensional and insulating crystal hosts. This study aimed to report the efficient upconversion enhancement of Er3+ induced by a new strategy of Ca2+ heterovalent doping in the layered semiconducting host based on BiOCl crystals concentrated by van der Waals force. The surface photovoltage (SPV) tests, theoretical calculations and optical probing effect of Eu3+ ions provided evidence that the replacement of Bi3+ with Ca2+ strengthened the macroscopic polarization and the corresponding spontaneous internal electric field (IEF) along the [001] of the BiOCl crystal sheet. The upconversion (UC) red and green emission intensities of the sample co-doped with 10 mol% Ca2+ ions were enhanced 39 and 12 times, respectively, compared with those of the Ca2+-free sample. Moreover, this study showed that the higher separation efficiency of photocarriers led to the prolongation of the decay time of excited energy levels under the strengthened intrinsic IEF by Ca2+-heterovalent doping, which was partially responsible for the UC photoluminescence (PL) enhancement of Er3+. This work not only provided new insight into the method for the engineering of UC emissions but also enhanced the understanding of layered semiconducting materials to modify the transition of lanthanide ions.

Preparation of g-C3N4/vermiculite composite with improved visible light photocatalytic activity

Composites of g-C3N4 and vermiculite (gCNV–X, X = 10, 8, 6, 4, 2) with expanded vermiculite (VE) contents of 15.4%, 18.5%, 23.2%, 31.2%, and 47.6% were synthesized by a wet chemical method and provided confined space due to VE. Nanosheets or nanoparticles of g-C3N4 were homogeneously distributed in the gCNV–X composites. With increasing VE content, the photocatalytic activity of gCNV–X improved first, reached the optimum at a VE content of 18.5% (gCNV-8), and then decreased gradually. Composite gCNV-2, with a high VE content of 47.6%, exhibited a similar photocatalytic performance as pure g-C3N4 (gCN). After photocatalysis for 70 min, composite gCNV-8 had degraded 98.07% of RhB with a reaction rate constant that was approximately three times larger than that of gCN. Furthermore, gCNV-8 showed excellent stability and reusability. The composite g-C3N4/vermiculite exhibited an excellent photocatalytic performance, which was ascribed to the reduced particle size of g-C3N4, close interfacial contact, electrostatic interactions between g-C3N4 and vermiculite, enhanced photocarrier separation efficiency, improved photocurrent and light absorption ability, as well as a narrow band gap.

Fabrication of Ag/RP composite with excellent photocatalytic activity for degrading high concentration of methyl orange solution

A series of ultra-low content of Ag decorated red phosphorus (Ag/RP) composites were fabricated by a simple photo-deposition method and were characterized by XRD, UV–Vis, SEM, TEM, BET, EDS and XPS. The results showed that Ag/RP displayed excellent photocatalytic activity for degrading high concentration of methyl orange solution (MO, 50 mL, 50 mg/L) under visible light (λ > 420 nm). The ultra-low content of Ag largely enhanced the activity of RP due to the strong separation efficiency of photocarriers, ensured by the transient photocurrent responses (TPR) and electrochemical impedance spectra (EIS). This result guides us to design highly efficient photocatalyst for removal of high concentrations of waste waters under visible light.

Enhancing photoelectrochemical performance of ZnIn2S4 by phosphorus doping for sensitive detection of miRNA-155.

We report the synthesis of phosphorus-doped ZnIn2S4 (ZnIn2S4-xPx) materials through a solid/gas-phase reaction. ZnIn2S4-xPx shows enhanced photoelectrochemical (PEC) performance due to the improved photo-carrier separation efficiency achieved by substituting some of the sulfur with phosphorus. A PEC biosensor was further developed based on ZnIn2S4-xPx, which exhibits excellent analytical performance for miRNA-155.

Synthesis of Ni−MoSx/g‐C3N4 for Photocatalytic Hydrogen Evolution under Visible Light

AbstractGraphitic carbon nitride loaded with MoSx nanodots (MoSx/g‐C3N4) was synthesized through a sonochemical method. MoSx nanodots (3∼20 nm) are shown to have the same excellent co‐catalytic H2 evolution performance as that of Pt. Nevertheless, the photocatalytic activity of MoSx/g‐C3N4 decreased with time. To overcome this instability, we show that co‐incorporation of Ni can significantly reduce the photo‐corrosion. The rate of H2 evolution of Ni−MoSx/g‐C3N4 photocatalyst reaches a maximum of 5968 μmol h−1 g−1, which is 89 % higher than that of Pt/g‐C3N4. The element mapping images show that Ni is deposited on the surface of MoSx. The co‐presence of Ni and MoSx not only enhance optical light absorption by g‐C3N4, but also significantly improve the separation efficiency of photocarriers and thus the photocatalytic hydrogen evolution efficiency.

Rational modulation of p-n homojunction in P-doped g-C3N4 decorated with Ti3C2 for photocatalytic overall water splitting

As a metal-free polymeric semiconductor, graphitic carbon nitride (g-C3N4) is a promising candidate for photocatalytic water splitting but suffers from the insufficient optical absorption and sluggish utilization of photocarriers. Herein, based on the energy band engineering theory, a new design philosophy of tunable p-n homojunction is proposed and realized in a novel P-doped g-C3N4 (PCN) material. Through regulating the tendency of transformation in p-n homojunction, a strong internal electric field is built and integrated with two secondary internal electric fields by further decorating with Ti3C2, leading to the formation of a triple-internal electric field system. Thus the spatial separation efficiency of photocarriers is improved and their lifetime is prolonged. The specific photocatalyst exhibits excellent overall water splitting performance in pure water with a notable quantum yield. This work presents a peculiar concept of adjustable p-n homojunction and holds great significance for the rational design of advanced photocatalyst in water splitting.

A nanoporous GaN photoelectrode on patterned sapphire substrates for high-efficiency photoelectrochemical water splitting

The photoelectrode of highly ordered nanoporous GaN on patterned sapphire substrate (PSS) is exploited to address issues of optical absorption and photocarrier separation efficiency in solar water splitting. The introduced PSS reduces the threading dislocation (TD) density and defects of the GaN epilayer, resulting in the improved crystalline quality, which suppresses recombination of electron-hole pairs. A single-step top-down etching approach is developed to fabricate the nanoporous GaN using an anodic aluminum oxide (AAO) mask conveniently and economically. The highly ordered nanoporous morphologies of AAO membrane are well transferred into the GaN surface grown on PSS via inductively coupled plasma (ICP) dry etching. Surface nanostructuring significantly increases surface-volume ratio of GaN, more incident light is trapped and absorbed by nanoporous structure, and the unabsorbed light scattered by PSS upwards is re-absorbed in nanoporous GaN. The ultraviolet light absorptance and reflectance of nanoporous GaN grown on PSS were improved significantly, close to 89% and 9% respectively. The resulting improved absorption in the nanoporous GaN further enhances the generation of photocarriers. The increasing surface-volume ratio also contributes to increasing of photoelectrochemical (PEC) reaction area and photocarrier separation efficiency, decrease of photocarrier migration distance towards the GaN-electrolyte interface, more holes participate in the PEC reaction, leading to an improved PEC efficiency and photocurrent density by 470% times with respect to planar counterpart. This work will pave the way towards low-cost and mass production of nanoporous GaN photoelectrode for efficient solar water splitting.

Enhancement of photocatalytic activity of g-C3N4 by hydrochloric acid treatment of melamine

Modified g-C3N4 samples (g-X, where X corresponds to the number of hours of acid treatment of the melamine) with outstanding photocatalytic performance were prepared by using hydrochloric acid-treated melamine as a precursor and calcining at 550 °C for 2 h. An x-ray diffractometer, field-emission scanning electron microscope, infrared spectrometer, N2 adsorption–desorption test, x-ray photoelectron spectroscopy, and ultraviolet–visible diffuse–reflectance spectroscopy analysis were carried out to characterize the phase composition, microstructure, chemical structure, specific surface area (SSA), chemical states, elemental composition and optical properties of the samples, respectively. The photocatalytic performance of the samples was evaluated by degrading the Rhodamine B (RhB) aqueous solution. The results showed that the crystal structure and vibration bands of melamine changed due to the reaction with hydrochloric acid. The crystallinity and grain size of g-C3N4 in g-X (X = 1, 2, 4, 6, 8, 10) reduced, and the SSA values of g-X increased compared to that of the g-0 sample, which was synthesized from pristine melamine. The g-X samples exhibited excellent photocatalytic activity towards degradation of RhB compared to g-0. The photocatalytic activity of the g-X samples increased gradually as the acid treatment time of the melamine increased from 1 h to 2 h, and then decreased gradually with the extension of the acid treatment time. The rate constant (k) values of g-X are higher than that of g-0. g-2 presented the highest rate constant (k = 0.052 min−1), which was 5.5 times higher than that of g-0. The improved photocatalytic activity of the g-X samples was attributed to the higher SSA value, the appearance of surface defects, the outstanding photo-carrier separation efficiency and stronger light harvesting ability of g-X, with the last two factors being more significant. Acid treatment of melamine is helpful in the preparation of high performance g-C3N4 photocatalyst, and the microstructure and photocatalytic performance of g-C3N4 were affected significantly by the acid treatment time.

Effect of heterojunctions and phase-junctions on visible-light photocatalytic hydrogen evolution in BCN-TiO2 photocatalysts

A significant enhancement of photocatalytic H 2 evolution performance was realized over the BCN-TiO 2 (A+R) heterojunctions. The synergistic effect of heterojunction and phase-junction increases carrier lifetime and photocatalytic activity due to the formation of intimate interfacial structure and large contact area. • BCN-TiO2(A+R) heterojunctions was constructed via a calcination method. • Intimate interfacial structure with large contact area was obtained. • BCN-TiO2(A+R) exhibited the highest photocatalytic activity of H 2 evolution. • Synergistic effect of heterojunction and phase-junction increases carrier lifetime. In this work, BCN-TiO 2 (A+R) heterojunctions were constructed via a simple one-step calcination method as efficient photocatalysts for H 2 evolution. Simultaneously, an anatase/rutile phase-junction was also obtained due to the crystalline phase transformation that occurred during high-temperature calcination. The unique heterojunction and phase-junction lead to the formation of an intimate interface and large contact area between two semiconductors and extended the visible-light absorption range. Therefore, BCN-TiO 2 (A+R) heterojunctions with a mixed anatase and rutile phase exhibit enhanced photocarrier separation efficiency, realizing an eleven-fold increase in the electron lifetime compared to that of pure TiO 2 and yielding a remarkable visible-light-driven H 2 evolution rate nearly 7.5 and 12.2 times higher than that of TiO 2 (A+R) and pure BCN, respectively. This work provides a new strategy for designing efficient water-splitting photocatalysts with highly efficient charge separation and transfer.

Distinctive ternary CdS/Ni2P/g-C3N4 composite for overall water splitting: Ni2P accelerating separation of photocarriers

Efficient overall water splitting over semiconductor photocatalyst is vital but difficult for resolving the energy and environmental crises. In this report, a novel transition metal phosphide Ni2P-based ternary CdS/Ni2P/g-C3N4 composite was constructed for achieving efficient overall water splitting activity under visible light (λ > 420 nm). The H2 and O2 evolution ratios are 15.56 and 7.75 μmol·g–1 h–1 over CdS/Ni2P/g-C3N4 with 3 wt% Ni2P, respectively, which is 4.02 times higher than that of binary type II CdS/g-C3N4. The separation efficiency of photocarriers of CdS/Ni2P/g-C3N4 is enhanced through greatly speeding up the transfer efficiency of electrons from the conduction band of g-C3N4 to that of CdS using Ni2P as electron-bridge, proved by the photoluminescence, transient photocurrent measurements and electrochemical impendence spectroscopy. The significant finding of this paper sheds light on the important role of transition metal phosphide as electron-bridge to connect the two conduction bands of type II semiconductor heterojuction for strengthening the separation efficiency of photocarriers.

Novel broad spectrum light responsive PPy/hexagonal-SnS2 photocatalyst for efficient photoreduction of Cr(VI)

Broad spectrum light response and high photo-carrier separation efficiency are two key postulates for promising photocatalysts. In this perspective, polypyrrole (PPy) modified hexagonal SnS2 nanosheet composites were firstly designed via in-situ polymerization method and used for Cr(VI) reduction with broad spectrum light (UV–vis-NIR) driven. Results indicated that 15-PPy/SnS2 composite material not only has good photocatalytic performance in the visible region (λ > 420 nm), but also has a great improvement in the visible (λ > 550 nm) and near-infrared (λ > 760 nm). With the analysis of EIS, PL and transient photocurrent responses, PPy can observably accelerate the separation of electrons-holes pair in the photocatalytic reaction. Trapping experiments indicated that e− and O2− all played master roles in the photoreduction process. Based on the trapping experiments and the Mott-Schottky analysis, a possible mechanism is proposed. This work will provide a thinking to construct other SnS2-based photocatalysts for efficient Cr(VI) reduction with full spectrum light utilization.

Surface Nonpolarization of g-C3 N4 by Decoration with Sensitized Quantum Dots for Improved CO2 Photoreduction.

Photocatalytic conversion of CO2 can provide a solution for simultaneously addressing global warming and solar fuel generation. However, its applicability is presently limited by the unsatisfactory photoconversion efficiency of the state-of-art photocatalysts. In this regard, enhancing CO2 adsorption through surface modification could be an efficient way to improve the photoconversion efficiency. Herein, doping of nonpolar carbon quantum dots (CQDs) onto g-C3 N4 is reported for the construction of a metal-free heterojunction photocatalyst (CQDs/g-C3 N4 ). CQDs offer several advantages such as band-gap reduction and electron-withdrawing effect to improve light absorption and photocarrier separation efficiency. However, this study reveals that nonpolar CQDs could also improve CO2 adsorption, photoinduced H2 production, reaction kinetics, and alter CO2 photoreduction pathways to generate CH4 . Consequently, the CQDs/g-C3 N4 could generate six times more CO and CH4 without detectable H2 compared to pristine g-C3 N4 , under similar conditions. Therefore, this study demonstrates a promising strategy for efficient adsorption, activation, and subsequent photoreduction of CO2 by nonpolar surface modification of g-C3 N4 .

Excellent visible light photocatalytic H2 evolution activity of novel noble-metal-free Ni12P5/CdS composite

Novel noble-metal-free Ni12P5/CdS composite was designed and synthesized by a hydrothermal method. The photocatalytic H2 generation activity of the as-prepared Ni12P5/CdS is 1.93 fold of that of pure CdS under visible light (λ > 420 nm) irradiation. The enhanced activity of Ni12P5/CdS is ascribed to the improved separation efficiency of photocarriers over a direct Z-scheme mechanism. It provides a potential strategy to improve the photocatalytic H2 evolution activity of CdS greatly by loading noble-metal-free Ni12P5 as electron co-catalyst via direct Z-scheme mechanism.

Preparation and photocathodic protection property of ZnIn2S4/RGO/TiO2 composites for Q235 carbon steel under visible light

For improving the photocathodic protection performance of TiO2 photoanode, ZnIn2S4/reduced graphene oxide (RGO)/TiO2 nanotubes (NTs) composite was synthesized by electrochemical anodizing combined with hydrothermal method. The photoelectrochemical and photocathodic protection properties of the ZnIn2S4/RGO/TiO2 composite were investigated in detail. Compared with pure TiO2 NTs, the ZnIn2S4/RGO/TiO2 composite showed a significant improvement in photocathodic protection performances. The photogenerated open circuit potential drop of ZnIn2S4/RGO/TiO2 coupled with Q235 carbon steel in 3.5 wt% NaCl solution under visible light could reach 420 mV versus saturated calomel electrode, which was more than twice as much as that of pure TiO2. The photogenerated current density of ZnIn2S4/RGO/TiO2 could reach 5.6 mA cm−2, which was twice as much as that of pure TiO2. The appearance of the adsorbed oxygen (OA) enabled the ZnIn2S4/RGO/TiO2 to have better photo-carrier separation efficiency and better photogenerated cathodic protection than pure TiO2. The present work demonstrated ZnIn2S4/RGO/TiO2 could be a potential material for photoanodes because of its better visible-light response, higher electrical conductivity and smaller charge transfer resistance.

Enhancement of acid treated g-C3N4[sbnd]Cu2O photocatalytic activity by PEG under visible light irradiation

In this study, g-C3N4Cu2O was successfully synthesized in the presence of PEG-400 surfactant via an acid treatment hydrothermal method and a high-temperature calcination method. The structures and properties of as-synthesized samples were characterized using a range of techniques, such as XPS, TEM, PL and BET. The g-C3N4Cu2O heterojunction exhibits the enhanced photocatalytic performance and high stability. It is revealed that the addition of PEG can promote the heterojunction effect of g-C3N4Cu2O, effectively improving the crystallinity and specific surface area of the photocatalyst, separation efficiency of photocarriers, and light absorption, thus enhancing the photocatalytic performance.