Effective Electron-hole Separation Research Articles

Hierarchical core-shell tungsten oxide/TiO2 nanowires as an effective photocatalyst

In this letter, tungsten oxide(WO3)/TiO2 core-shell nanowires were prepared by depositing a layer of TiO2 nanoparticles on the surface of WO3 nanowires via a pulse laser deposition method. These TiO2 nanoparticles are qusi-aligned wire-like structure on the surface of WO3 nanowires. The number of laser pulses during the deposition adjusts the TiO2 thickness. When used as photocatalyst, these core-shell nanowires have photocatalytic activity in the degradation of rhodamine B (RhB). The photo-induced electron-hole separation effect between WO3 and TiO2 contributes to the improvement of the photocatalystic activity. This makes these nanowire arrays having good potential in wastewater treatment.

A sea cucumber-like nanoporous TiO2 modified by bimetal Pt[sbnd]Au through the dealloying for water splitting

Bimetal PtAu modifying nanoporous TiO2 composites are prepared by dealloying. X-ray diffraction is used to determine the phase constitution. The specific surface area and mesoporous pores are measured by Brunauer-Emmett-Teller. X-ray photoelectron spectroscopy, photoluminescence spectroscopy, UV–Vis diffuse reflectance spectroscopy and Raman spectroscopy are employed to analyse the mutual synergy between Pt and Au. The hydrogen generation rate is measured to determine the photocatalytic performance. The results reveal that TiO2 is the anatase structure and possess a sea cucumber-like morphology with a large specific surface area. The hydrogen generation rate is 1.745 mmol h−1 g−1 for the dealloyed Al92Ti7.86Pt0.04Au0.1 ribbons under the full spectrum irradiation. This value is 29.6 times, 4.4 times and 1.8 times those of the dealloyed Al92Ti8, Al92Ti7.9Au0.1, and Al92Ti7.96Pt0.04 ribbons, respectively. The above results indicate that the addition of a small amount of Au and Pt significantly enhances the photocatalytic activity for H2 production. The Au promotes the absorption of visible light, and Pt serve as an electron sink for effective electron-hole pairs separation during the photocatalytic process. It is a feasible and an effective strategy to take full advantages of respective virtues of noble metals and their strong interactions to enhance photocatalytic activity.

TiO2 nanorod array film decorated with rGO nanosheets for enhancing photocatalytic and photoelectrochemical properties

A well-aligned TiO2 nanorod array film decorated with reduced graphene oxide(rGO/TNR) was constructed on fluorine-doped tin oxide (FTO) via a one-pot hydrothermal route with the aim of improving the photocatalytic and photoelectrochemical performances. The optimal ratio of components was investigated by changing the GO content that was added to the TiO2 precursor solution. The structure, morphology, composition, optical, photocatalytic and photoelectrochemical properties of the rGO/TNR films were investigated in detail. The resulting rGO/TNR films had a unique microstructure, apparent red shift of the absorption edge and low recombination rate of photoinduced charge carriers. For the degradation of methylene orange (MO) under UV light, the 5 wt%-rGO/TNR film exhibited the best photogradation efficiency of 95.8% in 120 min compared to bare TNR, which had a photogradation efficiency of 41.4%. Moreover, the 5 wt%-rGO/TNR film also showed the highest photocurrent of 110 μA/cm2, which was 6 times higher than that of bare TNR (16 μA/cm2). The excellent performance of the rGO/TNR film may be caused by the synergistic effect of the extended photoresponse range, high adsorption of target molecules and effective photogenerated electron-hole separation via rGO nanosheets as electron collectors and transporters.

Insights into the Photothermal Hydrogen Production from Glycerol Aqueous Solutions over Noble Metal‐Free Ti@TiO2 Core–Shell Nanoparticles

AbstractPhotocatalytic production of hydrogen from renewable sources with platinum group metal‐free catalysts is of great importance for environmentally friendly energetics. Herein, Ti@TiO2 core–shell nanoparticles prepared by sonohydrothermal treatment of titanium metal nanoparticles are examined for hydrogen generation from aqueous glycerol solutions under vis/NIR light irradiation. It is shown that photocatalytic reforming of glycerol in the presence of Ti@TiO2 exhibits strong photothermal effect allowing more efficient use of solar energy. Apparent activation energy equal to E act = 27 ± 2 kJ mol−1 indicates that the thermal effect is related to the diffusion of reaction intermediates at catalyst surface rather than to the activation of chemical bonds. The photoexcitation mechanism of Ti@TiO2 particles involves interband transitions in nonplasmonic Ti metal core followed by nonradiative Landau damping. Effective electron–hole separation between Ti core and nanocrystalline TiO2 anatase shell in Ti@TiO2 nanoparticles is confirmed by photoluminescence spectroscopy. In studied system, photothermal hydrogen production is not accompanied by CO2 emission indicating that glycerol is oxidized to glyceric acid without further decarboxylation.

High visible light-driven photocatalytic activity of large surface area Cu doped SnO2 nanorods synthesized by novel one-step microwave irradiation method

The paper investigates the structural, optical and photocatalytic activity of large surface area single crystalline copper (Cu) doped SnO2 nanorods (NRs) synthesized by a novel one-step microwave irradiation method. Powder X-ray diffraction (XRD) analysis confirms that both pure and Cu doped SnO2 are tetragonal rutile type structure (space group P42/mnm) formed during the microwave process within 10 min without any post annealing treatment. Transmission electron microscopy (TEM) reveals that the as synthesized Cu doped SnO2 samples exhibited rod-like shape and the length was less than 80 nm and diameter was about few nanometers. Typical selected-area electron diffraction (SAED) pattern indicates that, the growth direction of Cu–SnO2 nanorod is along [110] direction. The variety of phonon interaction in the pure and Cu doped SnO2 is observed by Raman spectroscopy. Electron paramagnetic resonance and X-ray photoelectron spectroscopy (XPS) confirms that the presence of copper and tin as Cu2+ and Sn4+ in state, respectively. The photocatalytic activity was monitored via the degradation of methylene blue (MB) and Rhodamine B (RhB) dyes and the Cu–SnO2 showed better photocatalytic activity than that of pure SnO2. This could be attributed to the effective electron–hole separation by surface modification.

A novel electrostatic self-assembly method for preparation of TiO2@BiOI photocatalyst

In this work, TiO2@BiOI photocatalyst was successfully fabricated by a novel electrostatic self-assembly method. A series of characterizations were employed to characterize the as-prepared photocatalyst. The potential application and mechanism of this photocatalyst were evaluated and discussed by photocatalytic degradation of RhB. The results showed that the TiO2@BiOI photocatalyst exhibited more excellent photocatalytic performance than pure TiO2. The remarkable enhancement in the photocatalytic activities of the TiO2@BiOI photocatalyst could be attributed to the effective electron–hole separations at the interfaces of the two semiconductors, which facilitated the transfer of the photoinduced carriers. In addition, relative large specific surface area, and appropriate energy band gap have great contribution to the enhancement of photocatalytic performance. In this paper, a new type of photocatalyst was prepared by electrostatic self-assembly method. The photocatalytic performance of the sample was evaluated by various characterization and degradation of RhB. The experimental results show that the photocatalytic activity of the sample is obviously improved.

The triple-component Ag3PO4-CoFe2O4-GO synthesis and visible light photocatalytic performance

This paper introduces a novel triple-component silver phosphate-cobalt ferrite-graphene oxide (Ag3PO4-CoFe2O4-GO) photocatalyst, illustrates its synthetic principle of adjusting the pH value, elaborates on its dual-channel reaction mechanism and reveals its advantages. Morphology and elementary analyses revealed that the magnetic CoFe2O4 nanoparticles (NPs) and the GO were uniformly adorned on the Ag3PO4 particles’ surface, forming a spherical structure. Because of this particular structure, not only could the Ag3PO4-CoFe2O4-GO composite be thoroughly removed with magnet field from treated water, but the photocatalytic activity and stability had been greatly improved to pure Ag3PO4. The effects of different samples were also evaluated, in terms of the efficiencies in inactivation and degradation. The reactive oxygen species (ROSs) yield measurements and photoluminescence spectra analysis indicated that O2 adsorption could be promoted by the CoFe2O4 and the GO. Upon visible light irradiation, a part of motivated electrons of the Ag3PO4 were consumed by the CoFe2O4, and others were transferred to the GO. The effective electron-hole separation is due to the dual transfer channel existing in the Ag3PO4-CoFe2O4-GO composite. Hence, the dual transfer channel is the major reason for enhancing photocatalytic activity and stability.

Fabricating direct Z-scheme PTCDA/g-C3N4 photocatalyst based on interfacial strong interaction for efficient photooxidation of benzylamine

Constructing Z-scheme system is a promising way to fabricate efficient photocatalysts for solar light utilization. In this study, a novel all-solid direct Z-scheme photocatalyst PTCDA-C3N4 was synthesized and characterized by several methods. XRD and HRTEM confirmed the strong π-π interaction between PTCDA(Perylene-3,4,9,10-tetracarboxylic dianhydride) and g-C3N4 induced the crystal transformation of PTCDA from β-phase to α-phase, self-assembling and reconstructing into nanocrystalline of 5 nm. UV–vis and photoelectronic measurements illustrated that the coupling of PTCDA on g-C3N4 not only dramatically broadened visible light absorption, but also accelerated the charge transfer of g-C3N4. Both the theoretical calculation and active species trapping experiments verified the Z-scheme mechanism of PTCDA-C3N4 heterostructure, which endows the hole of PTCDA-C3N4 much strong oxidizability and effective electron-hole separation, thus eight times higher activity than that of pristine g-C3N4 for benzylamine oxidation under solar driven. Our current work paves a facile way for synthesis efficient Z-scheme photocatalysts.

Facile fabrication of hierarchical porous ZnO/Fe3O4 composites with enhanced magnetic, photocatalytic and antibacterial properties

Hierarchical porous ZnO/Fe3O4 composites were fabricated through a facile surfactant-free method. The as-prepared samples were characterized using different instrumental techniques. The X-ray diffraction and energy dispersive X-ray results showed that the as-prepared samples were two-phase photocatalysts consisted of ZnO and Fe3O4. The UV–vis diffuse reflectance spectroscopy result indicated that the addition of Fe3O4 modified the optical properties of the photocatalysts. Under visible light irradiation, the ZnO/Fe3O4 exhibited enhanced photocatalytic performance for phenol degradation and Escherichia coli inactivation compared to ZnO. The photocatalytic enhancement of ZnO/Fe3O4 was attributed to the effective electron-hole pairs separation as verified by the photoluminescence spectra. The ZnO/Fe3O4 also exhibited magnetic properties, which can easily be recovered using an external magnet along with good reusability.

Vanadium disulfide decorated graphitic carbon nitride for super-efficient solar-driven hydrogen evolution

Highly efficient, earth-abundant, and low-cost photocatalysts are widely pursued for solar-driven hydrogen generation from water. Herein, we first report vanadium disulfide (VS2) with high hydrogen evolution reaction (HER) activity both in basal and edges to be the co-catalyst of graphitic carbon nitride (g-C3N4) for ultrahigh solar-driven hydrogen production. VS2-decorated g-C3N4 shows an impressing photocatalytic hydrogen evolution with a rate of 87.4 μmol/h, 26 times higher than pristine g-C3N4. Our combined experimental and computational studies reveal that the excellent efficiency of the composite is attributed to: (1) effective electron-hole separation and electron transfer from g-C3N4 to VS2, resulting from the optimal band alignment between VS2 and g-C3N4 and metallic characteristic of VS2; (2) fast hydrogen generation on the surface due to the high surface area and excellent HER activity of VS2. Our findings demonstrate that VS2/g-C3N4 may be applicable in solar-driven water splitting, and the design principle can be applied to search for novel photocatalysts.

Free-standing CuS–ZnS decorated carbon nanotube films as immobilized photocatalysts for hydrogen production

Free-standing carbon nanotube films (CNTF) with entangled carbon nanotubes (CNT) were used as conductive supports for the preparation of CuS–ZnS/CNTF composite as immobilized photocatalysts for H2 production. The surface morphology, crystalline property, surface chemistry, and optical properties of the CuS–ZnS/CNTF photocatalysts were investigated. The effects of forming CuS–ZnS heterojunction and conductive CNTF on the separation of photogenerated charges and photocatalytic hydrogen production activity of CuS–ZnS/CNTF photocatalysts were evaluated by the photocatalytic hydrogen production tests. Conductive CNT films can prevent the recombination of photogenerated electron–hole pairs. The deposition of CuS nanoparticles on the ZnS/CNTF leads to higher photocatalytic activity which can be attributed to the effective electron–hole separation. Introducing ZnS and CuS makes the photocatalyst surface more hydrophilic. The porous structure contributed to the effective contact between the sacrificing agents and the photocatalysts, leading to enhanced H2 production activity.

Evidence of a strong electron–hole separation effect in ZnO@TiO2 core/shell nanowires

In this work, a strong electron-hole separation effect in ZnO@TiO2 core/shell nanowire arrays was investigated. On the surface of well-aligned ZnO nanowires, a high-quality TiO2 shell was grown in a range of thicknesses through the deposition of TiO2 nanoparticles on ZnO nanowire arrays by pulsed laser deposition (PLD). In the photoluminescence (PL) spectra of the ZnO@TiO2 core/shell nanowires, a strong and broad visible emission band from 450 to 650 nm appears, which is related to surface defects; meanwhile, the ultraviolet (UV) emission due to exciton recombination simultaneously disappears. These changes in the PL spectra originate from a highly efficient electron-hole separation effect. This work presents evidence of this strong electron-hole separation effect in these ZnO@TiO2 core/shell nanowire arrays, which makes them promising candidates for use in optoelectronic devices for dye-sensitized solar cells and ultraviolet photodetectors.

Theoretical design of sandwich two-dimensional structures for photocatalysts and nano-optoelectronic devices

In this paper, three series of heterostructures of 2D transition metal dichalcogenides (including MoS2, MoSe2, MoTe2, WS2, WSe2, and WTe2) sandwiched by graphene (Gr), h-BN, and g-C3N4, which will be referred to as Gr sandwich heterostructures (SHS), h-BN-SHS, and g-C3N4-SHS in the rest of this paper, have been systematically studied firstly by using first-principle calculations. Gr-SHS are found to hold stable structures with an expanded band gap when MoSe2 and WSe2 are used as interlayers, indicating potential applications to Hall switch sensors. With the same type of interlayers, the stable h-BN-SHS shows additionally good response to visible light. In addition to good response to the visible light, effective electron–hole separation is also observed in g-C3N4-SHS. Such properties suggest structures in this sandwich type may be applied in plentiful areas, such as photocatalyst and solar cells. Moreover, the number and density of catalytically active sites in SHS may be dramatically increased. Consequently, we hope our findings could provide guidance for both the design of advanced materials and the corresponding nano-devices.

Increases in the Charge Separation Barrier in Organic Solar Cells Due to Delocalization.

Because of the low dielectric constant, charges in organic solar cells must overcome a strong Coulomb attraction in order to separate. It has been widely argued that intermolecular delocalization would assist charge separation by increasing the effective initial electron-hole separation in a charge-transfer state, thus decreasing their barrier to separation. Here we show that this is not the case: including more than a small amount of delocalization in models of organic solar cells leads to an increase in the free-energy barrier to charge separation. Therefore, if delocalization were to improve the charge separation efficiency, it would have to do so through nonequilibrium kinetic effects that are not captured by a thermodynamic treatment of the barrier height.

Mesoporous tungsten oxide modified by nanolayered manganese-calcium oxide as robust photoanode for solar water splitting

Described herein is the synthesis, characterization and photoelectrochemical behavior of a novel composite consisting of nanolayered manganese-calcium oxide (MCO) and mesoporous tungsten trioxide (WO3). The samples were characterized by transmission electron microscopy (TEM), and X-ray diffraction (XRD). The results demonstrated that superior interfacial contacts had been formed between WO3 and MCO. UV–vis diffuse reflectance spectroscopy (DRS), photoelectrochemical characterization, and incident photon-to-current efficiency (IPCE) revealed an enhanced light harvesting and effective electron-hole separation. A photoelectrochemical (PEC) cell composed of the n-type MCO/WO3 as a photoanode and platinum sheet as a counter electrode was assembled to estimate the feasibility for overall water splitting under a solar simulator illumination. The photocatalytic hydrogen and oxygen production from the photochemical cell with optimized photocatalyst (MCO/WO3-9) under 2 h simulated solar light irradiation was 1.9 μmol and 0.7 μmol, respectively, at low extra bias (0.90 V vs. RHE). Our investigation suggests that coupling MCO with n-type semiconductor WO3 as photoanode is a promising method to improve the activity of overall water splitting to generate oxygen and hydrogen.

Ag@AgCl decorated graphene-like TiO2 nanosheets with nearly 100% exposed (0 0 1) facets for efficient solar light photocatalysis

The large scale, ultrathin and (0 0 1) facet exposed anatase two-dimensional (2D) graphene-like TiO2 nanosheets were synthesized by hydrogen-bonding-mediated. The products were employed as scaffolding and deposited Ag@AgCl nanoparticles. The crystalline phase, morphologies and optical properties of as-prepared sample were surveyed. The results indicated that the as-prepared TiO2 nanosheets only have few nanometers thick. The Ag@AgCl nanoparticles were also successfully introduced into the surface of TiO2 nanosheets. By the modified of Ag@AgCl and significant enhanced the absorption spectrum in the visible light region. To assess the photocatalytic activities of as-prepared samples, the methylene blue (MB), methyl orange (MO) and rhodamine B (RhB) were employed to simulate the pollutants. The Ag@AgCl/TiO2 exhibited excellent photocatalysis under solar irradiation. There results suggested that, Ag@AgCl/TiO2-0.6 photocatalyst has suitable size of Ag@AgCl particles, large surface areas, abundant surface active sites and effective photogenerated electron-hole pair's separation, which significant improve the photocatalytic activities.

Photoelectron Transfer at ZnTPyP Self-Assembly/TiO2 Interfaces for Enhanced Two-Photon Photodynamic Therapy

Two-photon (TP) absorption nanomaterials are highly desirable for deep-tissue clinical diagnostics and orthotopic disease treatment. Here, a well-designed core/shell nanostructure was successfully synthesized with a ZnTPyP self-assembly nanocrystal (ZSN) inner core coated by a homogeneous TiO2 layer outside (ZSN-TO). The ZSN is a good photosemiconductor, showing both one-photon (OP) and TP absorption properties for red fluorescence emission and electron-hole pair generation; TiO2 with good biocompatibility acts as the electron acceptor, which can transfer photoelectron from ZSN to TiO2 for highly effective electron-hole separation, favoring the production of long-life superoxide anion (O2•-) by electrons and oxygen and strong oxidizing hydroxyl radical (•OH) by holes and surrounding H2O. Once pretreated with ZSN-TO, the simultaneous OP-405 nm or TP-800 nm laser stimulation and fluorescent imaging of reactive oxygen species (ROS) showed dynamical and continuous generation of ROS in HeLa cells, with cytotoxicity significantly increasing via the type-1-like photodynamic therapy process. The results demonstrated that the combination of organic ZSN with inorganic TiO2 has great applications as an excellent photosensitizer for deep-tissue fluorescent imaging and noninvasive disease treatment via TP photodynamic therapy.

Cellular heterojunctions fabricated through the sulfurization of MOFs onto ZnO for high-efficient photoelectrochemical water oxidation

Photoelectrochemical (PEC) water splitting offers a promising way for producing clean and renewable hydrogen fuel, but how to improve the solar conversion efficiency is still a challenge. Herein, the sulfurization of metal-organic frameworks (MOFs) onto the semiconductor ZnO to fabricate heterojunctions for PEC water oxidation was proposed. Cellular ZnO@ZnS, ZnO@CoS, and ZnO@ZnS/CoS heterostructured catalysts thus obtained through the direct sulfurization of ZnO@zeolitic-imidazolate-frameworks (ZnO@ZIFs) composites exhibit excellent PEC performances. Particularly, the ZnO@ZnS/CoS represents a largely improved photoconversion efficiency (0.65% at 0.14 V) and photocurrent density (2.46 mA cm−2 at 0.6 V) under full spectrum illumination, outperforming those of previously reported ZnO-based catalysts in neutral medium. Clearly, their special cellular structure affords rich exposed active sites and long incident photon transport pathway for well light absorption. And, the suitable band matching with strong electronic coupling in the integrated heterojunctions facilitates the effective electron-hole separation. Furthermore, CoS acting as co-catalyst can speed up the hole transfer and simultaneously participate in the surface water oxidation, which leads to the lower oxygen evolution barrier for ZnO@CoS and ZnO@ZnS/CoS. This MOF-derivative decoration approach could be expanded to fabricate other composite photoanodes for a variety of energy storage and conversion applications.

Hydrothermal synthesis of CdS nanorods anchored on α-Fe2O3 nanotube arrays with enhanced visible-light-driven photocatalytic properties

As an n-type semiconductor with an excellent physicochemical properties, iron oxide (Fe2O3) has been extensively used in the fields of environmental pollution control and solar energy conversion. However, the high recombination rate of the photoinduced electron-hole pairs and poor charge mobility for Fe2O3 nanomaterial generally result in low photocatalytic efficiency. Herein, an uniform CdS nanorods grown directly on one-dimensional α-Fe2O3 nanotube arrays (NTAs) are successfully synthesized by a facile hydrothermal method and the constructed heterojunction can be a kind of efficient and recyclable photocatalysts. Successful deposition of CdS nanorods onto the α-Fe2O3 NTAs is verified by field emission scanning electron microscopy(FESEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy (EDS). UV–Vis diffuse reflectance spectroscopy indicates that α-Fe2O3/CdS NTAs possess the intense visible light absorption and also display a red-shift of the band-edge compared with the pure α-Fe2O3 NTAs. The as-obtained α-Fe2O3/CdS NTAs display excellent photocatalytic activity for decomposition of methylene blue (MB), methyl orange (MO), and phenol under visible light illumination. Among all the tested photocatalysts, the film synthesized for 3h with good stability exhibits the best photocatalytic properties and produces the highest photocurrent of 1.43 mA/cm2 at 0.8 V vs. Ag/AgCl electrode, owing to its well formed heterojunction structure, effective electron-hole pair separation and direct electron transfer pathway along the CdS nanorods and α-Fe2O3 NTAs. Besides, the photogenerated holes (h+) and superoxide radicals (O2−) play dominant roles in the photocatalytic process. On the basis of the photocatalytic results and energy band diagram, the photocatalytic process mechanism is proposed. Considering the easy preparation and excellent performance, α-Fe2O3/CdS NTAs could be a promising and competitive visible-light-driven photocatalyst in the field of environment remediation.

TiO2 Photonic Crystal Sensitized with Mn3O4 Nanoparticles and Porphine Manganese(III) as Efficient Photoanode for Photoelectrochemical Water Splitting

We report the synthesis and photoelectrochemical characterization of a novel composite consisting of Mn3O4 nanoparticles, porphine manganese(III) (PMA), and TiO2 photonic crystal (TPC). The prepared composite (Mn3O4/PMA/TPC) was used for fabricating the photoanode of a photoelectrochemical tandem cell. The obtained Mn3O4/PMA/TPC composite was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and UV–vis diffuse reflectance spectroscopy (DRS). The results demonstrated that PMA and Mn3O4 nanoparticles had been loaded in the hole of TPC successfully. The photoelectrochemical characterization of the Mn3O4/PMA/TPC electrode revealed an enhanced light harvesting and effective electron–hole separation. The photoelectrochemical tandem cell, of which Mn3O4/PMA/TPC electrode acted as a photoanode and a Pt plate as counter electrode, was used to evaluate the feasibility for water splitting to produce H2 and O2 under a 300 W solar simulator irradiation. The gases evolved from...