Recombination Of Photoinduced Carriers Research Articles

Room temperature synthesized BaTiO3 for photocatalytic hydrogen evolution

As a photocatalyst, barium titanate (BaTiO3) has shown a great potential in photocatalytic water splitting for hydrogen evolution. In this work, BaTiO3 nanoparticles were synthesized at room temperature conditions under ambient pressure. The small particle size below 10 nm plays a key role in suppressing the recombination of photo-induced carriers, and thus promoting the photocatalytic activity. The photocatalytic hydrogen evolution rates of BaTiO3 (BTO-R) synthesized at room temperature are 8 and 2.9 times that of commercial BaTiO3 (BTO-C) in the presence and absence of triethanolamine (TEOA) as a sacrificial agent, respectively. This work provides a good example on size control, low-cost synthesis and photocatalysts' design.

Killing two birds with one stone: To eliminate the toxicity and enhance the photocatalytic property of CdS nanobelts by assembling ultrafine TiO2 nanowires on them

Although CdS has been proven to be one of the best photocatalysts in visible light region, its photocorrosion characteristics and toxicity prohibit its practical applications in photocatalytic hydrogen generation and photodegradation of organic pollutants in waste water. In addition, the photocatalytic efficiency of single phase CdS nanostructure is still low due to the recombination of photo-induced carriers. In this paper, a facile modification of CdS nanobelts approach is proposed by assembling TiO2 nanowire on the surface of CdS nanobelts to form TiO2 nanowire/CdS nanobelt hybrid nanstructures. This approach can kill two birds with one stone, that is, to eliminate the photocorrosion induced nontoxicity, and enhance photocatalytic properties by inducing UV light active TiO2 nanowires, and enhance the photo-induced carrier separation ability. The cell cyto-compatibility experiments on TiO2 nanowire/CdS nanobelt hybrid photocatalyst proved that the toxicity of CdS nanobelts can be totally eliminated. At the same time, photodegradation performance of for CdS/TiO2 is over 3 times of that for CdS nanobelts and10 times of that for TiO2. This work gives a new insight in the field of photocatalytic water treatment.

Synthesis of synergetic phosphorus and cyano groups ([sbnd]C[tbnd]N) modified g-C3N4 for enhanced photocatalytic H2 production and CO2 reduction under visible light irradiation

Graphitic carbon nitride (g-C3N4) has become an attractive photocatalyst for solar energy conversion owing to its numerous advantages. Due to insufficient solar-light absorption and fast photogenerated carriers recombination, the photocatalytic activity of traditional g-C3N4 material is unsatisfactory. In this work, P element and cyano groups (CN) incorporated into the g-C3N4 framework is successfully fabricated by heat treatment of the mixed g-C3N4 and NaH2PO2. The cyano groups (CN) originated from the de-protonation of CNH2 can enhance the light absorption and act as strong electron-withdrawing groups, which can promote the efficient separation of photo-generated electron-hole pairs. The P element doping can enhance the visible light absorption, shorten the band gap and suppress the recombination of photo-induced carriers. The synergistic effect of CN functional groups and P element doping results in a 6.7 times enhanced photocatalytic H2 production activity and 1.58 times enhanced photocatalytic CO2 reduction activity than that of pristine g-C3N4. Experimental analysis indicates that the enhanced photocatalytic performance is mainly attributed to the enhanced light absorption and charge separation. Our work provides a new thought to design other high performance and low-cost g-C3N4-based photocatalytic materials for solar energy conversion.

Tunable Ultraviolet Photorefractive Effect in LiNbO3/TiO2‐X (X = Pd or N): Theoretically Prediction and Experimental Validation

In this study, the introduction of unoccupied or partially occupied surface states energy levels into TiO2 upon Pd or N modification, respectively are theoretically predicted, suggesting a tunable carriers behavior in LiNbO3/TiO2‐X (X = Pd or N) composites. Inspired by the theoretical simulation, a new low‐cost method of compositing LiNbO3 with TiO2‐X (X = Pd or N) to mediate the ultraviolet photorefraction of LiNbO3 is proposed. The experimental results indicate that LiNbO3/TiO2 and LiNbO3/TiO2‐Pd show enhanced photorefraction due to reduced recombination of photoinduced carriers or increased free holes in LiNbO3. While LiNbO3/TiO2‐N exhibits photorefraction resistance since the photoinduced holes in LiNbO3 would get trapped by NOX species at the interface. This study will not only provide a simple way to fabricate tailored photorefractive material, but also offer new insights into the mechanism of photorefraction and photorefractive material design method.

Carbothermal Reduction Induced Ti3+ Self-Doped TiO2 /GQD Nanohybrids for High-Performance Visible Light Photocatalysis.

A facile calcination method is developed for the in situ synthesis of nanohybrids of Ti3+ self-doped TiO2 /graphene quantum dot nanosheets (Ti3+ -TiO2 /GQD NSs). Ti3+ sites are formed on the surface of the TiO2 nanosheets through carbothermal reduction by GQDs, using citric acid as a carbon source. Such heterojunctions exhibit enhanced visible-light absorption properties, large photocurrent current densities, and low recombination of photoinduced carriers. The methylene blue (MB) and rhodamine B (RhB) photodegradation result demonstrates a higher visible-light photocatalysis performance than that of the original TiO2 . On one hand, inducing Ti3+ sites is efficient for the separation of photogenerated charge carriers and for reducing electron-hole pair recombination. On the other hand, GQDs are beneficial for generating more photocurrent carriers and facilitating the charge transfer across the TiO2 surface. It is proposed that Ti3+ sites and GQDs induced in TiO2 nanosheets have a synergistic effect, leading to excellent photocatalysis properties. Finally, a theoretical calculation is provided of the carbothermal reduction for the formation mechanism of the Ti3+ defect sites.

Constructing 2D BiOCl/C3N4 layered composite with large contact surface for visible-light-driven photocatalytic degradation

The design and construction of a two-dimensional (2D) layered composite with large contact surface provide an efficient way for solving detrimental photoinduced carriers recombination. In this work, 2D layered composite coupling (001) facet of BiOCl nanoplates and (002) facet of C3N4 nanosheets was reasonable designed and successful constructed. In comparison with pure C3N4, the BiOCl/C3N4 hybrid structure with loading of 70% BiOCl exhibits the highest methyl orange photodegradation performance although BiOCl/C3N4 hybrid photocatalyst harvest less visible light. Obviously, enhanced photocatalytic performance is mainly ascribed to large contact surface of 2D layered hybrid structure, which is favorable for the interface electrons transfer and the separation of carriers between C3N4 and BiOCl. A probable degradation mechanism based on trapping experiments of active species, transient photocurrents, photoluminescence spectra, electrochemical impedance spectroscopy and energy band structures is proposed. This work may provide a further insight into the rational construction composites with large interface contact for high-efficiency light utilization.

One-pot electrochemical preparation of BiOCl/BiPO4 double-layer heterojunction film with efficient photocatalytic performance

A BiOCl/BiPO4 double-layer heterojunction film on metal Bi plate with reduced recombination of photo-induced carriers has been successfully prepared by one-step electrochemical anodization process. The morphology characterizations displayed that the double-layer film consisted of BiPO4 rods array at the bottom and BiOCl nanosheets on the top. Meanwhile, the control experiments indicated that the BiPO4 rods array was transformed by BiOCl rather than Bi3+ reacting with PO43− directly. In addition, the resulting BiOCl/BiPO4 heterojunction film exhibited excellent photocatalytic activity and stability on the degradation of phenol comparing with pure BiOCl film. Transient photocurrent measurements and PL spectra proved that the enhanced photocatalytic performance of BiOCl/BiPO4 heterojunction film was attributed to the effective separation of photo-generated electron-hole pairs excited on BiOCl. Moreover, O2− and OH played a crucial role in the photodegradation process of phenol. Based on these analytical results, a plausible photocatalytic mechanism has been proposed.

Analysis of Photoinduced Carrier Recombination Kinetics in Flat and Mesoporous Lead Perovskite Solar Cells

In this work, we analyze the carrier recombination kinetics and the associated charge carrier density in methylammonium lead iodide perovskite (MAPI) solar cells that use mesoporous TiO2 as selective contact (m-MAPI) and flat solar cells (without the mesoporous TiO2, f-MAPI), which are the most common device architectures for perovskite solar cells. The use of PIT-PV (photoinduced transient photovoltage) and L-TAS (laser transient absorption spectroscopy) showed that for devices that cannot reach efficiencies close to 19% there is a slow component of the photovoltage decay that corresponds to a charge recombination pathway for carrier losses responsible for the lower device efficiency. Moreover, we have also identified a primary interfacial charge recombination pathway for carrier losses that is common in all devices studied, independent of their efficiency or their device structure, which we have associated with the recombination reaction between electrons in the perovskite and holes in the organic semic...

PdO/TiO2 nanobelt heterostructures with high photocatalytic activities based on an exposed highly active facet on ultrathin TiO2 nanobelts

Combining the high photocatalytic activity of P25 (anatase/rutile bi-phase nanoparticle) and the good recycling property of TiO2 nanobelts is a promising approach for the preparation of high performance photocatalysts. In this work, through controlling the volume ratio of the component of organic solvent, bi-phase ultrathin TiO2 nanobelts consisting of both anatase and rutile phases have been successfully synthesized via a one-step solvothermal process by using tetrabutyl titanate as the Ti source and acetic acid/N,N-dimethylformamide (DMF) as the blended organic solvent. Compared with single phase TiO2, the bi-phase TiO2 nanobelts display a superior photocatalytic activity under ultraviolet light irradiation. Based on the refinement result of the X-ray diffraction (XRD) patterns of TiO2 nanobelts, the ratio of anatase and rutile phases in the as-synthesized TiO2 nanobelts were calculated to be 90.8wt% and 9.2wt%, respectively. Scanning electron microscope and high resolution transmission electron microscope images demonstrated that the thickness of the nanobelts is only 3–5nm, and the main exposed facet is (010), a photocatalytically active facet of anatase. The bi-phase and active facet exposure characteristics endow the ultrathin TiO2 nanobelt with high photocatalytic activity, which is superior to P25. The PdO/TiO2 ultrathin nanobelt heterostructures were synthesized at ambient temperature by using ultrathin nanobelts as substrates, which exhibited outstanding photodegradation activity with methyl orange, excellent hydrogen generation, and observably enhanced photoelectrochemical activity under simulated solar irradiation. The efficient suppression of the recombination of photo-induced carriers was caused by the bi-phase characteristics of the ultrathin TiO2 nanobelts. These nanobelts will have great potential in applications for photocatalytic water treatment and hydrogen generation because of the facile synthesis process and superior photocatalytic activity.

Grafting Fe(III) species on carbon nanodots/Fe-doped g-C3N4 via interfacial charge transfer effect for highly improved photocatalytic performance

Carbon quantum dots (CQDs) are coupled with Fe-doped g-C3N4 to prepare a binary composite photocatalyst, which shows retarded recombination of photo-induced carriers and enhanced surface absorption active sites. Then, interfacial charge transfer effect (IFCT) is introduced the binary composite by grafting Fe(III) species on the surface of CQDs/Fe-doped g-C3N4 via a facile impregnation method. The obtained ternary Fe(III)/CQDs/Fe-doped g-C3N4 composite photocatalyst exhibits largely increased optical absorption with its absorption region extended to 1200nm and efficiently inhibited charge recombination through the IFCT effect. It is revealed that the increased surface absorption active sites provided by CQDs lead to more amount of Fe(III) species anchored in the ternary photocatalyst than in the binary Fe(III)/Fe-doped g-C3N4 composite, which favors the photocatalytic activity. The EPR probe experiments elucidate that OH active species could be generated from the ternary Fe(III)/CQDs/Fe-doped g-C3N4 photocatalyst under visible light irritation, originating from the cooperation between the IFCT effect and the photo-Fenton reaction due to the existence of the Fe(III) species, which accounts for its highly enhanced photocatalytic performance on the degradation of MO and phenol.

Enhanced photocatalytic activity of ZnS nanoparticles loaded with MoS2 nanoflakes by self-assembly approach

A hybrid consisting of ZnS nanoparticles supported on layered MoS2−ZnS was synthesized by a hydrothermal method based on self-assembly technique without using a template. XRD, SEM-EDX, TEM, HR-TEM, TG-DTA, XPS, N2 adsorption-desorption, and UV–Vis spectroscopies were used to characterize the structural features, morphology, and composition of the MoS2–ZnS hybrid. The results show that the MoS2–ZnS hybrid is mainly ZnS nanoparticles on layered MoS2 with a thickness of ca. 5–20nm. The combination of the MoS2 and ZnS hybrid structure is beneficial for enhancing the photocatalytic degradation of rhodamine B (RhB) under visible light irradiation. A possible photoreaction mechanism of the MoS2–ZnS hybrid in the degradation is proposed. The photoexcited electrons from the ZnS could easily transfer to the conduction band of MoS2, thus decreasing the recombination of photoinduced carriers and enabling the degradation of RhB under visible light irradiation.

Ultrathin g-C3N4 nanosheets coupled with carbon nanodots as 2D/0D composites for efficient photocatalytic H2 evolution

Carbon nanodots synthesized from rapeseed flower bee pollens were used to couple with ultrathin g-C3N4 nanosheets for preparing the 2D/0D type photocatalysts by a hydrothermal method. The g-C3N4 nanosheets (UCN) obtained from ammonium chloride blowing dicyandiamide methods exhibit ultrathin two-dimension structure with a series of hollow spherical structures, and CQDs are well dispersed and uniformly anchored into the UCN network via π-π stacking interactions. The incorporation of CQDs can effectively expand visible light absorption regions via photosensitization and suppress the recombination of photo-induced carriers. Time-resolved fluorescence spectroscopy, electron paramagnetic resonance technology and photoelectrochemical measurements together reveal that CQDs serve as electron transfer mediation to facilitate charge separation and extend the lifetime of photo-induced carriers. The UCN/CQDs-0.2% composite has the optimal H2 evolution of 88.1μmol/h, 9.79, 3.02 and 1.91 folds of bulk g-C3N4 (BCN), pristine UCN and BCN/CQDs-0.2%, respectively. Consequently, the UCN/CQDs composites exhibit high photocatalytic activity of hydrogen release under the visible light irradiation.

In-Situ Probing Plasmonic Energy Transfer in Cu(In, Ga)Se2 Solar Cells by Ultrabroadband Femtosecond Pump-Probe Spectroscopy.

In this work, we demonstrated a viable experimental scheme for in-situ probing the effects of Au nanoparticles (NPs) incorporation on plasmonic energy transfer in Cu(In, Ga)Se2 (CIGS) solar cells by elaborately analyzing the lifetimes and zero moment for hot carrier relaxation with ultrabroadband femtosecond pump-probe spectroscopy. The signals of enhanced photobleach (PB) and waned photoinduced absorption (PIA) attributable to surface plasmon resonance (SPR) of Au NPs were in-situ probed in transient differential absorption spectra. The results suggested that substantial carriers can be excited from ground state to lower excitation energy levels, which can reach thermalization much faster with the existence of SPR. Thus, direct electron transfer (DET) could be implemented to enhance the photocurrent of CIGS solar cells. Furthermore, based on the extracted hot carrier lifetimes, it was confirmed that the improved electrical transport might have been resulted primarily from the reduction in the surface recombination of photoinduced carriers through enhanced local electromagnetic field (LEMF). Finally, theoretical calculation for resonant energy transfer (RET)-induced enhancement in the probability of exciting electron-hole pairs was conducted and the results agreed well with the enhanced PB peak of transient differential absorption in plasmonic CIGS film. These results indicate that plasmonic energy transfer is a viable approach to boost high-efficiency CIGS solar cells.

Impact of Oxygen Vacancy on Band Structure Engineering of n-p Codoped Anatase TiO2

Doping with various impurities is an effective approach to improve the photoelectrochemical properties of TiO2. Here, we explore the effect of oxygen vacancy on geometric and electronic properties of compensated (i.e. V-N and Cr-C) and non-compensated (i.e. V-C and Cr-N) codoped anatase TiO2 by performing extensive density functional theory calculations. Theoretical results show that oxygen vacancy prefers to the neighboring site of metal dopant (i.e. V or Cr atom). After introduction of oxygen vacancy, the unoccupied impurity bands located within band gap of these codoped TiO2 will be filled with electrons, and the position of conduction band offset does not change obviously, which result in the reduction of photoinduced carrier recombination and the good performance for hydrogen production via water splitting. Moreover, we find that oxygen vacancy is easily introduced in V-N codoped TiO2 under O-poor condition. These theoretical insights are helpful for designing codoped TiO2 with high photoelectrochemical performance.

Surface defects control for ZnO nanorods synthesized by quenching and their anti-recombination in photocatalysis

ZnO nanorods with controllable surface defects was synthesized by high-temperature quenching method, and the recombination of photogenerated electron–hole pairs had been drastically suppressed, thus significantly improving the photocatalytic reactivity. The as-prepared samples were characterized for the surface structure, chemical state, phase structure as well as optical absorption using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), fluorescent spectrophotometer (PL), diffuse reflectance UV–visible spectroscopy (DRS) and zeta potential. With XPS valence band spectra characterization, its light absorption enhancement in UV–vis range was found due to induced additional electronic states above the valence band edge. Specific types of defects related to the quenching process were further investigated. Moreover, the concentration of surface defects and the recombination of carriers were controllable by quenching temperature, also affected by cooling rates. It provides a time-saving and straightforward method to suppressed recombination of photo-induced carriers and increased UV–vis light absorption for highly efficient ZnO-based photocatalyst applied to environmental remediation.

Temporal evolution of photothermal-induced Rayleigh wave and plate deformation as an interference in the transient kinetics of photoinduced carrier recombination of a rutile titanium dioxide single crystal.

The dynamic properties of photothermal processes occurring after the initial photogenerated carrier recombination in the flash photolysis of a semiconductor plate sample probed with a reflected infrared beam are reported in this work. Transient kinetics pertaining to the photothermal processes always appear as interfering signals in that of the photogenerated carrier recombination and should be distinguished and excluded. We observed that the photothermal-induced Rayleigh wave occurs immediately after the photogenerated carrier recombination that is then followed by the photothermal-induced flexural vibration of the sample substrate with a set of intrinsic frequencies as reported in our previous work (Appl. Spectrosc. 2013. 67(5): 506-512). When these two faster types of waves decay, the transient decaying signal from the plate deformation due to the inhomogeneous temperature field remains for much longer than 22 ms. Thus, all three types of the photothermal dynamic processes of different temporal properties induced by the pulsed laser on an absorbing semiconductor thin plate are clearly identified.

Enhanced photoelectrochemical performance and charge transfer properties in self-organized NiOx-doped TiO2 nanotubes

In this present work, 5 wt% NiOx-doped TiO2 nanotubes with 8μm thickness were fabricated by anodization from Ti-Ni alloy. After the characterization of SEM, XRD, XPS and electrochemical measurements, the doped nanotube structure shows a strongly enhanced electrochemical performances in conductivity, donor concentration and OH− ion intercalation ability. In order to better understand the effects of NiOx doping on mechanism of photoinduced carriers’ transfer, the photovoltaic performances were investigated by Transient Photovoltage (TPV) and Surface Photovoltage (SPV) techniques. It is interesting to find that the doped one exhibits a strong photoresponse in visible region and high photovoltage intensity. These results indicated that the incorporation of NiOx promoted the charge separation and inhibited recombination of photoinduced carriers in TiO2 nanotube.

High photocatalytic performance of BiOI/Bi2WO6 toward toluene and Reactive Brilliant Red

BiOI sensitized nano-Bi2WO6 photocatalysts with different BiOI contents were successfully synthesized by a facile deposition method at room temperature, and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) high-resolution transmission electron microscopy (HR-TEM), photoluminescence (PL) spectra, UV–vis diffuse reflection spectroscopy (UV–vis DRS) and Brunauer–Emmett–Teller (BET) surface area measurements. The photocatalytic activity of BiOI/Bi2WO6 was evaluated by the photo-degradation of Reactive Brilliant Red (X-3B) in suspended solution and toluene in gas phase. It has been shown that the BiOI/Bi2WO6 catalysts exhibit a coexistence of both tetragonal BiOI and orthorhombic Bi2WO6 phases. With increasing BiOI content, the absorption intensity of BiOI/Bi2WO6 catalysts increases in the 380–600nm region and the absorption edge shifts significantly to longer wavelengths as compared to pure Bi2WO6. The 13.2% BiOI/Bi2WO6 catalyst exhibits obviously higher UV and visible light photocatalytic activities than commercial P25, pure Bi2WO6 and BiOI, for the photodegradation of toluene and X-3B. The remarkably enhanced photocatalytic activities can be attributed to the fact that the proper BiOI sensitized nano-Bi2WO6 increase its BET surface area, decrease band-gap energy, enhance absorption in the 380–600nm region and inhibit the recombination of photo-induced carriers.

Microemulsion synthesis, characterization of bismuth oxyiodine/titanium dioxide hybrid nanoparticles with outstanding photocatalytic performance under visible light irradiation

Reverse microemulsions, consisting of n-hexanol, Triton X-100, Cyclohexane and aqueous salt solutions, were used to synthesize BiOI, TiO2 and BiOI/TiO2 hybrid nanoparticles at room temperature. The particles had been characterized by X-ray powder diffraction, FT-IR spectra, TG-DSC analysis, nitrogen sorption, electron microscopy, and UV–vis diffuse reflectance spectroscopy. The photocatalytic properties of those particles were evaluated by degradation of methyl orange under visible light irradiation. The BiOI/TiO2 composites showed about 5 times higher photocatalytic performances than BiOI when the mole ratio of BiOI to TiO2 was 75%. The remarkable enhancement in the visible light photocatalytic activities of the BiOI/TiO2 heterostructures could be first attributed to the effective electron–hole separations at the interfaces of the two semiconductors, which facilitated the transfer of the photoinduced carriers. Meanwhile, the heterojunction formed between BiOI and TiO2 would further retard the recombination of photoinduced carriers. In addition, high degree of crystallization, bimodal porous structure, relative large specific surface area, and appropriate energy band gap have great contribution to the enhancement of photocatalytic performance.

Characterization of Plasma-Irradiated SiO2/Si Interface Properties by Photoinduced-Carrier Microwave Absorption Method

We report photoinduced carrier recombination induced by 13.56 MHz radio frequency argon plasma treatment for samples of silicon substrates coated with thermally grown SiO2 layers. The transmissivity of a 9.35 GHz microwave was measured and analyzed under illumination of 532 nm green light to the top or rear surface. Irradiation of argon plasma at 100 W for 5 min caused substantial carrier recombination. The recombination velocity of the plasma-irradiated surface was estimated to be 6000 cm/s. Investigation of 100 kHz capacitance response characteristics as a function of bias voltage for metal oxide p-type silicon structures revealed that irradiation of argon plasma increased the fixed charge density from 3.2×1011 to 3.0×1012 cm-2. Those changes were restored completely by heat treatment at 300 °C in air for 60 min.