Surface Heterojunction Research Articles

In-situ construction of high-efficiency phase-transition induced m-Bi2O4/Bi4O7 surface heterojunction photocatalysts and mechanism investigation

In this work, pure m-Bi2O4, Bi4O7 and m-Bi2O4/Bi4O7 heterojunction photocatalysts with improved visible-light-driven photocatalytic performance have been successfully fabricated by a facile one-pot hydrothermal method. With the prolongation of hydrothermal reaction time, m-Bi2O4 microrods gradually transform into Bi4O7 microsheets and m-Bi2O4 can be in-situ grown on the surface of Bi4O7, resulting in an intimate interface. The phase structure, morphology, microstructure, surface chemical state and optical property are characterized by X-ray powder diffraction (XRD), Fourier transform-infrared (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–visible diffuse reflectance spectroscopy (UV–vis DRS) and X-ray photoelectron spectroscopy (XPS). In comparison with pure m-Bi2O4 and Bi4O7, the optimized m-Bi2O4/Bi4O7 composite photocatalysts show higher visible-light-driven photocatalytic performance toward RhB. In view of electronic energy-band structure analysis, photoelectrochemical and photoluminescence spectra (PL) results, we contribute the improved performance to synergetic effects between m-Bi2O4 and Bi4O7 in the composite followed a possible S-scheme heterojunction photocatalytic reaction mechanism, which prolongs pathway and suppress the recombination rate of photogenerated carriers. Furthermore, the mechanism of effect of m-Bi2O4/Bi4O7 mass ratio and geometry architecture on photocatalytic behavior is also investigated in this paper. Our work offers a new insight to design and develop high-active phase-transition induced bismuth mixed-valence surface heterojunction photocatalyst with facile preparation.

Termination dependence and electric field modification of band alignment in a CNT/CH3NH3PbI3 heterojunction.

Carbon nanotube (CNT) and perovskite composite materials possessing the combined advantages of CNTs and perovskites have drawn substantial attention due to their promising applications in photovoltaic and optoelectronic devices. Understanding the band alignment of heterojunctions is crucial for further performance improvement. Here, we systematically investigated the interfacial electronic structure and optical absorption of a semiconducting CNT/CH3NH3PbI3 heterojunction via density functional theory calculations. It was found that the CNT/PbI2-terminated CH3NH3PbI3 (001) surface heterojunction is a type-I band alignment, while the CNT/CH3NH3I-terminated CH3NH3PbI3 (001) surface heterojunction is a type-II band alignment, suggesting the different charge carrier transfer processes as well as termination dependence of band alignment in the CNT/CH3NH3PbI3 heterojunction. Further investigation indicated that applying electric fields can modify the band alignment type in the CNT/CH3NH3PbI3 heterojunction. Our results provide the first insight into the interfacial electronic structure of the CNT/CH3NH3PbI3 heterojunction, which may give a new route for designing optoelectronic devices.

A dual polymer composite of poly(3-hexylthiophene) and poly(3,4-ethylenedioxythiophene) hybrid surface heterojunction with g-C3N4 for enhanced photocatalytic hydrogen evolution

A surface heterojunction catalyst of g-C3N4–PEDOT/P3HT with P3HT and PEDOT as the polymer sensitizer and hole transport pathway is successfully prepared. The as constructed g-C3N4–PEDOT/P3HT composite exhibits a photocatalyst H2 evolution rate up to 427703.3 μmol h−1 g−1 which is 1059 times higher than that of g-C3N4, 118 times higher than that of g-C3N4–PEDOT with ascorbic acid as sacrificial reagents. What's more, the g-C3N4–PEDOT/P3HT can even show an obviously enhanced photocatalytic H2 evolution rate which is 6.1 times higher than that of pure g-C3N4 in pure water without any sacrificial reagent. Combining the experimental results and molecular dynamic (MD) simulation results, a possible mechanism can be drawn that the existed PEDOT possesses relatively higher hole mobility and can be used as a hole conductor between g-C3N4 and P3HT. Then, the photogenerated holes migration can be accelerated by PEDOT from the VB of g-C3N4 to the VB of P3HT. All those factors may benefit the synergy among g-C3N4, PEDOT and P3HT, which finally facilitates the rapid migration of photoinduced electron–hole pairs and eventually improves the photocatalytic H2 activity process of g-C3N4–PEDOT/P3HT with visible light. The present work may provide useful insights for designing a surface heterojunction composite photocatalyst with high photocatalytic activity for H2 production.

In situ synthesis of holey g-C3N4 nanosheets decorated by hydroxyapatite nanospheres as efficient visible light photocatalyst.

The interesting g-C3N4 nanosheet morphology has drawn huge attention in photocatalytic applications because of its special features. Nonetheless, the relative activity of these nanosheets is still controversial due to the low available active sites and the high recombination probability of photo-induced charge carriers. In this work, in situ sol–gel approach was applied to synthesize holey g-C3N4 nanosheets/hydroxyapatite (HAp) nanospheres with plentiful in-plane holes. Herein, the presence of Ca2+ plays a key role in the formation of holey defects on 2D g-C3N4. In-plane holes provide nanosheets with more active edges and diffusion channelsv, resulting in a tremendous enhanced mass and photo-induced charge transfer speed. Moreover, the holes make highly numbered boundaries, which lead to the prevention of aggregation. On the other hand, distributed nano-HAp spheres on these nanosheets can form effective heterojunctions having high photo-degradation ability of pollutants. Intrinsic O-vacancies inside HAp unit cells mainly affect the capture of photogenerated electrons, pollutant molecules, and O2 gas. The synergistic presence of O-vacancies and holey defects (C-vacancies) on 2D g-C3N4 plays a key role in raising the photocatalytic performance of holey g-C3N4/HAp. It can be concluded that the proposed preparation method is a promising approach for simultaneous synthesis of holey g-C3N4 and surface heterojunctions of Ca-based materials. This new structure has shown significant degradation ability of bisphenol A, a prominent pollutant, with a low amount (0.01 g) and short time.

Visible-light-driven cuprous oxide nanomotors with surface-heterojunction-induced propulsion

The controllable synthesis and customized design of micro/nanomotors represents a highly desired paradigm in the field of intelligent nanovehicles. Exploiting asymmetrical structures and geometry-dependent propulsion are the two main strategies for achieving light-driven micro/nanomotors. However, inherent crystal-structure differences in a single colloidal motor have rarely been explored. Here, we propose the first surface-heterojunction-induced propulsion methodology for cuprous oxide (Cu2O) nanomotors, by tailoring the crystal morphology of a Cu2O crystalloid from a sphere into a truncated octahedron and preserving the controllable-index crystal facets of {100} and {111} in a single colloid. Due to the high crystallinity and distinct activity of the exposed crystal facets, a surface heterojunction between the {100} and {111} facets is formed to enhance electron-hole separation, as confirmed by density functional theory (DFT) calculations, thus endowing the truncated octahedral Cu2O nanomotors with autonomous and vigorous movement in biocompatible fuels under visible light. These Cu2O nanomotors can reach a propulsion speed in water of over two times faster than that of polycrystalline spherical motors with low crystallinity. The efficient Cu2O nanomotors offer a promising guideline not only for the synthesis of novel light-driven motors with desired structures, but also for potential applications in biocompatible environments.

Spatial charge separation and high-index facet dependence in polyhedral Cu2O type-II surface heterojunctions for photocatalytic activity enhancement

Polyhedral 30-faceted Cu2O microcrystal exposed with a {332}–{001} type-II surface heterojunction was synthesized, which displayed an efficient spatial charge separation and high-index facet dependence for photocatalytic activity enhancement.

Coexposed TiO2’s (001) and (101) facets in TiO2/BiVO4 photoanodes for an enhanced photocatalytic fuel cell

This paper reports an investigation on the role of coexposed TiO2’s (001) and (101) facets on the performance of TiO2/BiVO4 photoanodes in the photocatalytic fuel cell. Here, the exposure of these facets was obtained by synthesizing TiO2 with different morphologies, i.e. nanospindles, nanocube, nanooctahedra and nano-truncated octahedra. Based on the result, coexposed (001) and (101) facets were found to be responsible for the enhancement of photoelectrochemical response. The highest photocurrent density was achieved when the photoanode was fabricated using TiO2 nano-truncated octahedra/BiVO4 (29.8 μA/cm2 at 0.8 V vs. NHE), which was primarily due to the improvement of charge separation as a result of the synergistic effect between the formation of type-II heterojunction of TiO2/BiVO4 and internal surface heterojunction of (001) and (101) facet. A similar trend was also observed in the PFC system when RhB was used as fuel. Under the illumination of 13 W LED light, the highest electric power (0.00232 mW/cm2) was obtained with TiO2 nano-truncated octahedra/BiVO4 photoanode. However, TiO2 nanospindles/BiVO4 was found to be more effective in removing RhB due to its high surface area. Such variation was believed due to the fact that TiO2 nanospindles had less exposure of (001) facet than TiO2 nano-truncated octahedra.

Direct and alternating electrical performance of TiO2/SiO2/p-Si heterojunction under visible illumination

The titanium dioxide (TiO2) thin film was deposited on the surface of p-type silicon (Si) substrate using spray pyrolysis technique and TiO2/SiO2/p-Si heterojunction was formed. The microstructure of the heterojunction surface is characterized by X-ray diffraction and atomic force microscopy. The energy-band gap (~3.38 eV) and the work function (~5.53 eV) of the TiO2 thin film were calculated by ultraviolet-visible spectrum and ultraviolet photoelectron spectroscopy, respectively. The energy-band structure of the heterojunction is established. The current-voltage characteristics of the heterojunction is investigated, which displays a typical diode rectifying property. The alternating current capacitance and impedance are measured under different visible light intensities, and the heterojunction exhibits good photosensing performance. The resistance and impedance of the heterojunction decrease with increasing of light intensity, while the capacitance value increases. The phenomenon is mainly attributed to the decrease of Fermi level difference between TiO2 thin film and Si with the increase of light intensities, which can be explained by the energy-band diagram of the TiO2/SiO2/Si heterojunction.

Tungsten Oxide/Carbide Surface Heterojunction Catalyst with High Hydrogen Evolution Activity

Tungsten carbide (WC) with imperfect structures determined by phase engineering and heteroatom doping has attracted a great deal of attention with respect to hydrogen evolution reaction (HER). Howe...

The photocatalytic hydrogen evolution enhancement of the MoS2 lamellas modified g-C3N4/SrTiO3 core-shell heterojunction

Abstract The MoS2 lamellas modified g-C3N4/SrTiO3 core-shell heterojunction is prepared via a simple continuous hydrothermal-annealing method. There, the g-C3N4 shell is deposited on the surface of as-prepared SrTiO3 nanosphere, and then the MoS2 lamellas grow on the surface of core-shell heterojunction. Evaluated by the hydrogen evolution, the as-prepared MoS2/g-C3N4/SrTiO3 core-shell heterojunction exhibits an obvious photocatalytic enhancement of about ∼30 folds than single SrTiO3 nanospheres, which can be mainly ascribed to that, the core-shell heterojunction can promote the charge carriers separation efficiently and the Pt-like behavior MoS2 can promote the photo-generated electrons diffusing into water quickly. Additionally, the ultrathin MoS2 lamellas with a mass of edge S atoms can provide sufficient HER active sites and shorten photoelectron transport route to improve photocatalytic stability.

Energy band regulation and heterophase surface heterojunction in B-C-N-TiO2 catalysts for enhanced photocatalytic activity

B-C-N-TiO2 catalysts with intermediate band gap and heterojunction were prepared by sol–gel method. The crystal structure, elemental composition, and absorbance performance were analyzed by XRD, XPS and UV–vis DRS. Due to the formation of the continuous intermediate band gap in the forbidden band, the optical absorption boundary of B-C-N-TiO2 catalyst extends to 412 nm, which shows different red shift compared with that of TiO2 (397 nm), N-TiO2 (400 nm), B-TiO2 (402 nm) and C-TiO2 (403 nm).Meanwhile, the B-C-N-TiO2 catalyst is a uniform spherical solid particle with exposed (101) and (211) planes. The degradation efficiency of B-C-N-TiO2 catalyst to methylene blue(MB) was up to 97.82% after 3 h under visible light, which shows different degree of improvement compared with that of TiO2 (34.8%), N-TiO2 (79.09%), B-TiO2 (87.82%) and C-TiO2 (73.19%). B-C-N-TiO2 has good stability, and there was still more than 95% degradation efficiency after three degradation cycles. It is also revealed that the incorporation of B-C-N element affects the crystal phase of the catalyst and promotes the generation of heterophase surface heterojunction between (101) and (211). It has certain guiding value for co-doping modification of three elements.

Synergistic effects of octahedral TiO2-MIL-101(Cr) with two heterojunctions for enhancing visible-light photocatalytic degradation of liquid tetracycline and gaseous toluene

The photocatalytic performance of TiO2 is severely restricted by its low photocatalytic activity and high charge-carrier recombination rate. In order to overcome these defects, a novel octahedral TiO2-MIL-101(Cr) (T-MCr) photocatalyst with type-II heterojunction and surface heterojunction is prepared by growth of TiO2 particles on MIL-101(Cr) via optimizing the synthesis conditions. The physicochemical, optical and photoelectrochemical properties of the samples are characterized utilizing several techniques, and its photocatalytic activity for liquid tetracycline and gaseous toluene degradation is evaluated under visible light irradiation. Particularly, the T-MCr-60 shows the outstanding photocatalytic activity. The improved photocatalytic performance of T-MCr-60 is ascribed to strong interfacial contact between (0 0 1) facets, (1 0 1) facets, and MIL-101 and the synergistic effects of two heterojunctions promoting efficient charge transfer. The possible photocatalytic mechanisms involving pollutants degradation pathway and charge transfer mechanisms during the process of pollutants degradation is also discussed.

A synergy of CdSe sensitization and exposure of TiO2 (0 0 1) facet in CdSe-TiO2 nanostructures for photoreduction of bicarbonate

This study reports an investigation on the synergistic effect of sensitization by CdSe and exposure of TiO2′s (0 0 1) facet towards the photoactivity of CdSe-TiO2 nanostructures for photoreduction of bicarbonate. Here, TiO2 nanoparticles with spindle-like morphology with high exposure of (0 0 1) facet were successfully synthesized. Whereas, different sizes of CdSe particles were prepared and integrated to TiO2 using 3-mercopatopropionic acid (MPA) as the linker. Based on the spectroscopic analyses, it was found that the as-prepared CdSe-TiO2 were able to absorb a broad spectrum of light energy from UV to visible as the result of band overlap between CdSe and TiO2. Besides, results demonstrated that this band overlap also caused the blueshift in the TiO2 bandgap, where the magnitude of the shift was found to be proportional to CdSe size. Furthermore, it is revealed that the as-prepared CdSe-TiO2 was able to facilitate the photoreduction of bicarbonate efficiently. Interestingly, the photoactivity of the nanocatalysts was also found to be proportional to the size of CdSe. This excellent photoactivity was believed due to the presence of type II band alignment as the result of the conduction band overlap between TiO2 and CdSe. Additionally, such activity could also be attributed to the formation of surface heterojunction of TiO2′s (1 0 1) and (0 0 1) facets, which facilitates the efficient charge separation.

In2O3-x(OH)y/Bi2MoO6 S-scheme heterojunction for enhanced photocatalytic performance

Surface heterojunction engineering has been extensively studied to promote efficient charge separation in semiconductor materials. Designing an effective heterojunction system to optimize the separation and transport of photo-induced charges is an appealing strategy to enhance the photocatalytic efficiency. In this work, In2O3-x(OH)y in situ decorated Bi2MoO6 two-dimensional step-scheme heterojunctions were synthesized through a controlled dehydroxylation process of indium-based precursors. The charge transfer mechanism of this step-scheme heterojunctions was confirmed by the characterization of electron structures, reactive species, photoelectric properties and DFT theoretical calculation. The band bending and the internal electric field caused by the charge transfer upon hybridization can effectively promote the separation of charges and present the optimal redox capacity. In addition, surface residual hydroxyl groups can regulate the surface energy state and optimize the interfacial charge transfer kinetics of the prepared step-scheme heterojunction. Eventually, the step-scheme heterojunction exhibits superior performance in photocatalytic reduction of hexavalent chromium and degradation of organic pollutants under visible light irradiation. This work provides an innovative perspective to construct photocatalyst with superior activity.

Preparation of Cu3N/MoS2 Heterojunction through Magnetron Sputtering and Investigation of Its Structure and Optical Performance.

Cu3N/MoS2 heterojunction was prepared through magnetron sputtering, and its optical band gap was investigated. Results showed that the prepared Cu3N/MoS2 heterojunction had a clear surface heterojunction structure, uniform surface grains, and no evident cracks. The optical band gap (1.98 eV) of Cu3N/MoS2 heterojunction was obtained by analyzing the ultraviolet-visible transmission spectrum. The valence and conduction band offsets of Cu3N/MoS2 heterojunction were 1.42 and 0.82 eV, respectively. The Cu3N film and multilayer MoS2 formed a type-II heterojunction. After the two materials adhered to form the heterojunction, the interface electrons flowed from MoS2 to Cu3N because the latter had higher Fermi level than the former. This behavior caused the formation of additional electrons in the Cu3N and MoS2 layers and the change in optical band gap, which was conducive to the charge separation of electrons in MoS2 or MoS2 holes. The prepared Cu3N/MoS2 heterojunction has potential application in various high-performance photoelectric devices, such as photocatalysts and photodetectors.

Remarkably enhanced visible-light photocatalytic hydrogen evolution and antibiotic degradation over g-C3N4 nanosheets decorated by using nickel phosphide and gold nanoparticles as cocatalysts

In this study, a highly efficient ternary 6.7%Au/Ni2P/g-C3N4 nanocomposite is rationally fabricated through a two-step hydrothermal followed by green photoreduction strategy. The XPS analysis, SEM and HRTEM images reveal that Ni2P and good nanoparticles (AuNPs) are successfully loaded onto the surface of g-C3N4 and heterojunction formed among them. The optimized 6.7%Au/0.5%Ni2P/g-C3N4 demonstrates the highest photocatalytic efficiency towards decomposition of levofloxacin hydrochloride with 88.23% removal rate and hydrogen generation rate of 78.65 μmol·g−1·h−1, in comparison with the binary 0.5%Ni2P/g-C3N4 (76.86%, 47.06 μmol·g−1·h−1) and pristine g-C3N4 (61.26%, 45.61 μmol·g−1·h−1). Notably, this is the first time using Ni2P and AuNPs co-modified g-C3N4-based nanocomposites to evaluate the photocatalytic degradation performance of levofloxacin hydrochloride. The study of the corresponding enhanced mechanism indicates that the dramatically improved photocatalytic performance is originated from the synergistic effects: including (1) the strong optical absorption property of Ni2P and the SPR effect of AuNPs endows the remarkably improved visible-light harvesting for increasing charge carrier production; (2) the co-catalysts of Ni2P and AuNPs play key roles as interfacial electron sinkers and facilitate photogenerated electron-hole pairs separation and migration; (3) the co-integration improves the specific surface area of final product which can provide more active sites for the improvement of photocatalytic performance. Moreover, a plausible degradation pathway for levofloxacin hydrochloride is proposed based on the UPLC-MS analysis. Our work highlights a good example for designing and constructing other highly-efficient g-C3N4-based multicomponent photocatalysts for both solar energy conversion and environmental remediation.

Phase-dependent gas sensitivity of MoS2 chemical sensors investigated with phase-locked MoS2

In the present study, phase-dependent gas sensitivities of MoS2 chemical sensors were examined. While 1T-phase MoS2 (1T-MoS2) has shown better chemical sensitivity than has 2H-phase MoS2 (2H-MoS2), the instability of the 1T phase has been hindering applications of 1T-MoS2 as chemical sensors. Here, the chemical sensitivity of MoS2 locked in its 1T phase by using a ZnO phase lock was investigated. To develop MoS2 chemical sensors locked in the 1T phase, we synthesized a multi-dimensional nanomaterial by growing ZnO nanorods onto MoS2 nanosheets (ZnO@1T-MoS2). Raman spectroscopy and x-ray photoelectron spectroscopy analyses of such phase-locked 1T-MoS2 subjected to flash light irradiation 100 times confirmed its robustness. ZnO nanomaterials hybridized on MoS2 nanosheets not only froze the MoS2 at its 1T phase, but also increased the active surface area for chemical sensing. The resulting hybridized material showed better response, namely better sensitivity, to NO2 gas exposure at room temperature than did 1T-MoS2 and 2H-MoS2. This result indicated that increased surface area and heterojunction formation between MoS2 and ZnO constitute a more promising route for improving sensitivity than using the 1T phase itself.

Electric fields and local magnetic field enhance Ag-BiVO4-MnOx photoelectrochemical and photocatalytic performance

The Ag-BiVO4-MnOx multi-stage interface heterojunction photocatalyst is prepared via photo-deposition method. The oxidation co-catalyst MnOx and reduction co-catalyst Ag are selectively deposited onto (0 1 0) and (1 1 0) facets of BiVO4. Under ultraviolet (UV) light, MnOx is deposited onto the BiVO4 (1 1 0) facet, causing its energy band to move upward and bend at the interface to produce a p-n junction and a built-in electric field that points from BiVO4 towards MnOx. Simultaneously, the difference in energy levels allows the BiVO4 to form surface heterojunction and produce a built-in electric field that points from (0 1 0) facet towards the (1 1 0) facet. A Schottky junction is formed when Ag is deposited onto the BiVO4 (0 1 0) facet to create space charge region without free carriers. A built-in electric field is formed that points from Ag to BiVO4. The electric fields generated via light excitation and a local magnetic field formed by plasmonic Ag improve the photocatalytic activity of the Ag-BiVO4-MnOx photocatalyst, the degradation rate of Ag-BiVO4-MnOx is 5.9 times higher than that of the BiVO4. This work not only promotes the separation efficiency of photogenerated charge carriers, but also pays attention to the synergistic effect of electric fields generated by light excitation and local magnetic field formed by plasmonic Ag, which provides a new idea for photocatalysis research.

One-step synthesis of dandelion-like lanthanum titanate nanostructures for enhanced photocatalytic performance

The rational design of nanomaterials with distinct exposed facets is of great importance for improving the physicochemical properties of these materials and for the study of structure–activity relationships. This work describes the first synthesis of lanthanum titanate (La2Ti2O7, LTO) with dandelion-like nanostructures via the molten salt method. The lowest synthesis temperature of 700 °C is at least 200 °C lower than that required by other methods. The dandelion structure consists of well-crystallized LTO nanorods (NRs) with sizes of less than 100 nm in the radial direction and 300–500 nm in the axial direction, which is different from the widely accepted two-dimensional form. LaOCl microplates were formed as an intermediate substrate for LTO NR growth outwards to the basal surfaces of the LaOCl crystallites. DFT calculation results showed that the strong LiCl adsorption on the (100) surface led to distinct growth of the (100) and (020) planes, thus promoting the rod-like growth of LTO along the [010] axis. In addition, the photocatalytic performance of as-prepared LTO was evaluated by determining the degradation of rhodamine B. The results suggested that the as-prepared LTO could markedly enhance the photocatalytic activity as a result of the surface heterojunction of coexposed {100} and {002} facets in LTO NRs.

Surface engineered 2D materials for photocatalysis.

Benefitting from their unique structure and physicochemical properties, two-dimensional (2D) materials have aroused tremendous interest from academia and industry, being regarded as an important class of photocatalysts. However, their photocatalytic activities still need further improvement to satisfy the requirement of scale-up production. In this regard, the surface engineering strategy is considered as one of the most effective methods for optimizing their photocatalytic performance. This feature article not only classifies the 2D photocatalysts into layered and non-layered 2D photocatalysts and presents their preferred synthesis methods, but also summarizes the advantages of the surface engineering strategy for boosting the photocatalytic performance of 2D materials from the aspects of light absorption, charge carrier separation and surface active sites. Various surface engineering strategies, such as surface decorating, vacancy engineering, element doping, surface heterojunction construction and regulation of facet-dependent sites, have also been presented as advantages of the surface engineering strategy. Eventually, the challenges and future outlook for optimizing the photocatalytic activities of 2D materials through surface engineering are addressed.