Heterophase Junction Research Articles

Defective Grey TiO2 with Minuscule Anatase–Rutile Heterophase Junctions for Hydroxyl Radicals Formation in a Visible Light-Triggered Photocatalysis

The novelty of this work was to prepare a series of defect-rich colored TiO2 nanostructures, using a peroxo solvothermal-assisted, high-pressure nitrogenation method. Among these solids, certain TiO2 materials possessed a trace quantity of anatase–rutile heterojunctions, which are beneficial in obtaining high reaction rates in photocatalytic reactions. In addition, high surface area (above 100 m2/g), even when utilizing a high calcination temperature (500 °C), and absorption of light at higher wavelengths, due to the grey color of the synthesized titania, were observed as an added advantage for photocatalytic hydroxyl radical formation. In this work, we adopted a photoluminescent probe method to monitor the temporal evolution of hydroxyl radicals. As a result, promising hydroxyl radical formations were observed for all the colored samples synthesized at 400 and 500 °C, irrespective of the duration of calcination.

Crystal phase transition of β-Bi2O3 and its enhanced photocatalytic activities for tetracycline hydrochloride

The crystal phase has an important effect on the photocatalytic activity of Bi2O3. However, due to the high recombination of photo-induced charge carriers in single crystal phase Bi2O3, the photocatalytic activity was still unsatisfied. Herein, we prepared a novel α/β-Bi2O3 hetero-phase junction (xB) by a simple solvothermal-calcination method, the photocatalyst was well characterized by XRD, SEM, TEM, UV–vis DRS, XPS, photoelectrochemical technology, etc. The characterizations indicated that the crystal phase of Bi2O3 gradually transformed from β phase to α phase as the calcination temperature increased, and the Bi2O3 of different crystal phases were in close contact with a clear interface. With the optimum calcination temperature at 400 ℃, 400B sample showed the highest photocatalytic activity for tetracycline hydrochloride (TC-HCl) degradation, 91.4% removal within 120 min, the kinetic rate (0.0188 min−1) was 3.2 and 3.1 times higher than that of β-Bi2O3 (0.0058 min−1) and α-Bi2O3 (0.0061 min−1), respectively. This work provides an ideal strategy to synthesize hetero-phase junction photocatalysts with the crystal phase transition of semiconductors.

Enhanced Photocatalytic Antibacterial Properties of TiO2 Nanospheres with Rutile/Anatase Heterophase Junctions and the Archival Paper Protection Application.

TiO2 has been generally studied for photocatalytic sterilization, but its antibacterial activities are limited. Herein, TiO2 nanospheres with rutile/anatase heterophase junctions are prepared by a wet chemical/annealing method. The large BET surface area and pore size are beneficial for the absorption of bacteria. The rutile/anatase heterojunctions narrow the bandgap, which enhances light absorption. The rutile/anatase heterojunctions also efficiently promote the photogenerated carriers’ separation, finally producing a high yield of radical oxygen species, such as •O2− and •OH, to sterilize bacteria. As a consequence, the obtained TiO2 nanospheres with rutile/anatase heterojunctions present an improved antibacterial performance against E. coli (98%) within 3 h of simulated solar light irradiation, exceeding that of TiO2 nanospheres without annealing (amorphous) and TiO2 nanospheres annealing at 350 and 550 °C (pure anatase). Furthermore, we design a photocatalytic antibacterial spray to protect the file paper. Our study reveals that the TiO2 nanospheres with rutile/anatase heterojunctions are a potential candidate for maintaining the durability of paper in the process of archival protection.

Study of synthesis and photocatalytic performance of the monoclinic/cubic heterophase junction of rare earth doped zirconia

Eu and Y co-doped ZrO2 is fabricated by solid state reaction method. ZrO2/Eu/Y products evolve from the monoclinic phase to the monoclinic/cubic phase in structure as the Eu ion content increases. Monoclinic/cubic heterophase junction is obtained during structural evolution, which develops a new route to fabricate ZrO2 heterophase junction. Photocatalytic property measurement is carried out for ZrO2/Eu/Y materials. The ZrO2/Eu0.1/Y monoclinic/cubic heterophase junction promotes photogenerated electron-hole separation effectively and hence exhibits better photocatalytic performance.

Unraveling the Lattice Matching Effect in Surface Phase Junctions for Interfacial Charge Separation

Exquisite control over the interface structure is highly desired for the successful fabrication of a semiconductor heterojunction, which has been confirmed to effectively promote charge separation and transfer. However, the influences of the interface microstructure in the heterojunction on the charge separation and transfer efficiency are still unclear. Herein, taking the TiO2 heterophase junction (A/R) between anatase and rutile phases and the TiO2 homophase junction (Rs/R) between large rutile rodlike particles and small rutile nanoparticles (∼20 nm) as a prototypical model, it is found that the lattice match at the interface is a key factor determining the charge separation and transfer efficiency and the photocatalytic activity. As compared with A/R, the atomically smooth interface with a highly matched lattice in Rs/R leads to a less-defective and abrupt interface and provides a smooth interfacial charge separation and transfer path, leading to improved charge separation and transfer efficiency and a great enhancement in photocatalytic activity. This study provides a novel insight into optimizing the heterojunction structure by designing a highly matched lattice interface between two components.

Achieving high performance anticorrosive nanosheet photo-catalysts via a metal, surfactant co-modification approach

Though TiO2 was famous for good chemical stability, as an ultrathin flaky structure, TiO2 nanosheet also got mired in photo-corrosion. Herein, we explored the inner mechanism of photo-corrosion phenomenon among nanosheet structure, and demonstrated the vital role of high specific surface area and perfect dispersity of nanosheet photo-catalyst in H2 evolution reaction, rather than the heterophase junction of TiO2(B)/λ-Ti3O5. Through exploring four different Conditions, we have confirmed the important modification of highly active metal co-catalyst and surfactant PVP for greatly promoted catalytic efficiency and excellent morphology, efficiency stability. In especial, sole addition of PVP not only stabilized the nanosheet morphology, also tripled the original TiO2(B) efficiency, while TiO2-Au-4h(PVP) displayed the highest QE of 30.9 %, and best stability. This work developed an excellent stable TiO2(B) nanosheet based catalyst, also established a possible strategy to settle photocorrosion for other nanosheet mateials under illuminated applications.

Introduction of crystalline hexagonal-C3N4 into g-C3N4 with enhanced charge separation efficiency

Polymeric carbon nitride is a promising candidate for photocatalytic hydrogen evolution but mostly just shows moderate activity because of its inefficient charge separation and sluggish surface reaction kinetics. Phase-based heterostructures, especially crystal-phase heterostructures that are composed of identical compositions with different crystal phases, can endow nanomaterials with promising properties and efficient photocatalysis applications. In this study, a novel crystalline hexagonal-C3N4/g-C3N4 hetero-phase junction was synthesized using a direct in situ alkali salt template coupled with organic solvents strategy. High hydrophilicity was obtained because of the synergistic effect between alkali salt and organic solvents in the polycondensation process. The greatly increased separation and transfer efficiency of charge carriers and the improved proton absorption in the surface reaction of the photocatalyst were achieved by the constructed phase junctions between the crystalline hexagonal-C3N4 and amorphous g-C3N4 decorated with numerous hydrophilic groups. Thus, the carbon nitride hetero-phase junction exhibited dramatically enhanced photocatalytic performance in hydrogen evolution and excellent cycling stability under visible light irradiation. This work presents a novel insight into phase engineering based on carbon nitride materials with surface functional modification for an efficient photocatalytic activity.

Engineering of anatase/rutile TiO2 heterophase junction via in-situ phase transformation for enhanced photocatalytic hydrogen evolution

Constructing effective interphase boundary is one of the efficient approaches for improving photocatalytic performances of semiconductor materials. In this work, an anatase/rutile-TiO2 (AR-TiO2) heterophase junction with appropriate carbon content was successfully fabricated via an in-situ phase transformation process. The phase transformation started from the inner core of the nanoparticles and the area of phase interface between anatase and rutile was carefully controlled by regulating the activation temperature. The well-established type-II band alignment between two TiO2 phases with residual carbon as additional charge transfer intermediary which significantly improved the light-harvesting and photoinduced electron-hole pair separation. As a result, the optimal AR-TiO2-550 catalyst (without adding commonly used Pt as co-catalyst) remarkably enhanced photocatalytic H2 generation (201 μmol h−1 g−1), which was about 12-fold to that of P25. The AR-TiO2-550 heterophase junction also showed long-term stability under simulated solar light irradiation. This research provides a new phase engineering route for developing high-efficient photocatalysts.

Recent Advances in TiO2-Based Heterojunctions for Photocatalytic CO2 Reduction With Water Oxidation: A Review.

Photocatalytic conversion of CO2 into solar fuels has gained increasing attention due to its great potential for alleviating the energy and environmental crisis at the same time. The low-cost TiO2 with suitable band structure and high resistibility to light corrosion has proven to be very promising for photoreduction of CO2 using water as the source of electrons and protons. However, the narrow spectral response range (ultraviolet region only) as well as the rapid recombination of photo-induced electron-hole pairs within pristine TiO2 results in the low utilization of solar energy and limited photocatalytic efficiency. Besides, its low selectivity toward photoreduction products of CO2 should also be improved. Combination of TiO2 with other photoelectric active materials, such as metal oxide/sulfide semiconductors, metal nanoparticles and carbon-based nanostructures, for the construction of well-defined heterostructures can enhance the quantum efficiency significantly by promoting visible light adsorption, facilitating charge transfer and suppressing the recombination of charge carriers, resulting in the enhanced photocatalytic performance of the composite photocatalytic system. In addition, the adsorption and activation of CO2 on these heterojunctions are also promoted, therefore enhancing the turnover frequency (TOF) of CO2 molecules, so as to the improved selectivity of photoreduction products. This review focus on the recent advances of photocatalytic CO2 reduction via TiO2-based heterojunctions with water oxidation. The rational design, fabrication, photocatalytic performance and CO2 photoreduction mechanisms of typical TiO2-based heterojunctions, including semiconductor-semiconductor (S-S), semiconductor-metal (S-M), semiconductor-carbon group (S-C) and multicomponent heterojunction are reviewed and discussed. Moreover, the TiO2-based phase heterojunction and facet heterojunction are also summarized and analyzed. In the end, the current challenges and future prospects of the TiO2-based heterostructures for photoreduction of CO2 with high efficiency, even for practical application are discussed.

Portable wastewater treatment system based on synergistic photocatalytic and persulphate degradation under visible light

Highly efficient, low-cost, and portable wastewater treatment and purification solutions are urgently needed for aqueous pollution removal, especially at remote sites. Synergistic photocatalytic (PC) and persulphate (PS) degradation under visible light offers an exceptional alternative for this purpose. In this work, we coupled a TiO2-based PC system with a PS oxidation system into a portable advanced oxidation device for rapid and deep degradation of organic contaminants in wastewater. Using hydrogenation, we fabricated hydrogenated anatase branched-rutile TiO2 nanorod (H-AB@RTNR) photocatalysts which enable the PC degradation to occur under visible light and improve the utilization of solar energy. We also discovered that the addition of PS resulted in the synergistic degradation of tenacious and persistent organics, dramatically improving the extent and kinetics of the degradation. A degradation rate of 100% and a reaction rate constant of 0.0221 min−1 for degrading 1 L rhodamine B (20 mg L−1) were achieved in 120 min in a specially designed thin-layer cell under visible light irradiation. The superior performance of the synergistic PC and PS degradation system was also demonstrated in the degradation of real industrial wastewater. Both remarkable performances can be attributed to the heterophase junction and oxygen vacancies in the photocatalyst that facilitate the catalytic conversion of PS anions into highly active radicals (·SO4− and ·OH). This work suggests that the as proposed synergistic degradation design is a promising solution for building a portable wastewater treatment system.

An intriguing case of morphology control and phase transitions in TiO2 nanostructures with enhanced photocatalytic activity

The control over crystal structure and morphology is crucial to design an active catalyst, particularly where several accessible morphologies and phases are possible. This work presents a strategy of how TiO2 transforms from anatase phase, passing through mixed phase transitions, to pure rutile phase. The XRD result show phase transformations, that are accompanied with the morphology changes from anatase particulates to well-defined rod-shaped rutile characterized through TEM. While the steady state photoluminance (SSPL) spectra indicate the mixed phase display enhanced photoluminescence (PL) as compared to individual anatase or rutile phase. The Time-resolved photoluminescence (TRPL) measurements also demonstrate enhanced PL lifetime in anatase-rutile mixed phase sample, whereas valence band XPS results confirm the existence of energy differences in band edge positions of pure anatase, (3.31 eV) anatase-rutile phase heterojunction (3.15 eV) and rutile TiO2 (3.08 eV). These differences are evident as photocatalytic activity results reveal that the mixed phase sample exhibits superior performance (8 mmol g−1h−1 of hydrogen evolution from water under UV illumination) than pristine phases. We attribute this higher hydrogen evolution rate to the built-in electric field (type-II heterojunctions) that facilitates charge transfer across the interface in mixed phase sample. This example highlights the scope of fine-tuning structure-property relationships in a myriad of materials in a similar fashion.

Photocatalytic reduction of Cr(VI) using a wurtzite/natural sphalerite heterostructure: Synergistic effects of exposed active facets, vacancies and a heterophase junction

The development of stable and efficient photocatalysts is a basic requirement for photocatalytic treatment of heavy metal pollutants. For this study, a composite photocatalyst (WxS1-x) consisting of hierarchical nano-sphere wurtzite with exposed active facets and natural sphalerite was constructed. Optimized as W0.4S0.6, it was able to reduce Cr (VI) at an efficiency of 97.11% in 30 min via photocatalysis, which was 4.20 times that of natural sphalerite. The key factors enhancing the photocatalytic performance of WS are the exposed active facets of wurtzite, the heterogeneous transition layer and the inherent mineralization defects of natural sphalerite, all combining to increase vacancies. Furthermore, the separation of photogenerated electrons and holes is more efficient thanks to the built-in electric field induced by the active facets and the heterophase junction, which contributes to the material’s overall photocatalytic efficiency. Multiple characterizations were done, revealing the photocatalytic mechanism that was just outlined. This study provides a new approach to constructing high-efficiency and low-cost photocatalysts by combining defect engineering and heterojunction design.

Facile synthesis of α/β-Bi2O3 hetero-phase junction by a solvothermal method for enhanced photocatalytic activities

A novel α/β-Bi2O3 hetero-phase junction (xBB) was successfully prepared by a simple in-situ growth method without further calcination, in which α-Bi2O3 served as the substrate for the growth of β-Bi2O3 nanosheets. The nanocomposites were well characterized by XRD, SEM, TEM, UV–vis DRS, XPS, EIS, etc. The characterizations testified that the bismuth oxides of two crystal phases were in close contact, and the hetero-phase junction was formed. The photocatalytic performance was evaluated by degradation of rhodamine B (RhB) and tetracycline hydrochloride (TC-HCl) under visible light illumination. With the optimum mass ratio, 0.5BB sample exhibited the highest photocatalytic activity, the removal rates were 82.7 % for TC-HCl in 120 min and 94.9 % for RhB in 90 min, and a rate constant of 0.0297min−1 was evaluated for RhB degradation, which were higher than that of pure α-Bi2O3 (0.0019 min−1) and β-Bi2O3 (0.0028 min−1) by a factor of 15.6 and 11.9, respectively. The synthetic method provides a promising route for the in-situ construction of hetero-phase junction nanocomposites.

In Situ Induced Crystalline-Amorphous Heterophase Junction by K+ to Improve Photoelectrochemical Water Oxidation of BiVO4.

Solar water splitting is one of the most efficient technologies to produce H2, which is a clean and renewable energy carrier. Photoanodes for water oxidation play the determining roles in solar water splitting, while its photoelectrochemical (PEC) performance is severely limited by the hole injection efficiency at the interface of semiconductor/electrolyte. To address this problem, in this research, by employing BiVO4 as the model semiconductor for photoanodes, we develop a novel, facile, and efficient method, which simply applies K cations in the preparation process of BiVO4 photoanodes, to in situ induce a crystalline-amorphous heterophase junction by the formation of an amorphous BiVO4 layer (a-BiVO4) on the surface of the crystalline BiVO4 (c-BiVO4) film for PEC water oxidation. The K cation is the key to stimulate the formation of the heterophase, but not incorporated in the final photoelectrodes. Without sacrificing the light absorption, the in situ formed a-BiVO4 layer accelerates the kinetics of the hole transfer at the photoanode/electrolyte interface, leading to the significantly increased efficiency of the surface hole injection to water molecules. Consequently, the BiVO4 photoanode with the crystalline-amorphous heterophase junction (a-BiVO4/c-BiVO4) exhibits almost twice the photocurrent density at 1.23 V (vs reversible hydrogen electrode) for water oxidation than the bare c-BiVO4 ones. Such advantages from the crystalline-amorphous heterophase junction are still effective even when the a-BiVO4/c-BiVO4 is coated by the cocatalyst of FeOOH, reflecting its broad applications in PEC devices. We believe this study can supply an efficient and simple protocol to enhance the PEC water oxidation performance of photoanodes, and provide a new strategy for the potential large-scale application of the solar energy-conversion related devices.

Investigation of photo-electrochemical response of iron oxide/mixed-phase titanium oxide heterojunction toward possible solar energy conversion

Photocatalysts are part of key strategies to enable green fuel. Photocatalysis and water splitting could be a promising solution to challenges associated with the intermittent nature of sunlight as a huge energy source on Earth. In this study, photo-electrochemical performance and behavior of mixed-phase titanium oxide and iron oxide heterojunction (Ti-TiOx (High-voltage)-FeOx electrode) are compared to the photo-electrochemical performance and behavior of titanium oxide nanotubes with the rutile phase and iron oxide heterojunction (TiOx-nanotubes (H2SO4/KF)-FeOx electrode). The results of photo-electrochemical experiments show that the application of stabilization potential and the presence/absence of dissolved oxygen could not be considered as significant factors affecting the photo-electrochemical properties of the Ti-TiOx (High-voltage)-FeOx and TiOx-nanotubes (H2SO4/KF)-FeOx electrodes. The Ti-TiOx (High-voltage)-FeOx electrode shows an anodic photo-electrochemical response in wavelengths shorter than 530 nm and cathodic photo-electrochemical response in wavelengths longer than 530 nm. However, the Ti-nanotubes (H2SO4/KF)-FeOx electrode consistently exhibits the anodic photo-electrochemical response. Both of the prepared heterojunctions are further characterized through Scanning Electron Microscopy, Energy-dispersive X-ray Spectroscopy, Diffuse Reflectance UV–Vis Spectroscopy, X-ray Diffraction, and Attenuated Total Reflectance Spectroscopy methods. These experiments show that despite different morphologies observed in SEM imaging data, the deposited iron oxide layers on both mixed-phase titanium oxide and titanium oxide nanotubes share the same hematite phase structure. However, only iron oxide electro-deposited on the surface of the mixed-phase titanium oxide, which contains both anatase and rutile phases, with vacant sites of oxygen, exhibits un-expected anodic and cathodic photo-electrochemical responses. Furthermore, according to the results of the characterization and photo-electrochemical investigations, the different chemical environment of mixed-phase titanium oxide, and the possible formation of different types of heterojunction structures in mixed-phase titanium oxide and iron oxide, in contrast to the titanium oxide nanotubes and iron oxide, might be considered the possible discernible reasons for the observed different photo-electrochemical responses. This paper sheds new light on photo-electrochemistry of iron oxide/mixed-phase titanium oxide heterojunction for possible solar energy conversion.

Effect of interfacial defects on the electronic properties of MoS2 based lateral T-H heterophase junctions.

The coexistence of semiconducting (2H) and metallic (1T) phases of MoS2 monolayers has further pushed their strong potential for applications in the next generation of electronic devices based on two-dimensional lateral heterojunctions. Structural defects have considerable effects on the properties of these 2D devices. In particular, the interfaces of two phases are often imperfect and may contain numerous vacancies created by phase engineering techniques, e.g. under an electron beam. Here, the transport behaviors of the heterojunctions with the existence of point defects are explored by means of first-principles calculations and non-equilibrium Green's function approach. While vacancies in semiconducting MoS2 act as scattering centers, their presence at the interface improves the flow of the charge carriers. In the case of VMo, the current has been increased by two orders of magnitude in comparison to the perfect device. The enhancement of transmission was explained by changes in the electronic densities at the T–H interface, which open new transport channels for electron conduction.

Controlling 1T/2H heterophase junctions in the MoS2 microsphere for the highly efficient photocatalytic hydrogen evolution

The 1T/2H heterophase junction MoS2 microspheres were fabricated for a highly efficient photocatalytic hydrogen evolution.

Phase transformation and heterojunction construction of bismuth oxyiodides by grinding-assisted calcination in the presence of thiourea and their photoactivity.

Bismuth-rich oxyhalides are promising photocatalysts due to their special layered structure and adjustable band gap energy. In this work, a series of bismuth oxyiodides were fabricated by grinding-assisted calcination in the presence of thiourea, where grinding-induced mechanical force could accelerate the decomposition reaction and thiourea could prohibit the crystal particles from growing due to coordination action. The combined effect of grinding and thiourea could decrease the temperature of phase transformation of bismuth oxyiodides. Among these, heterojunction Bi4O5I2/Bi5O7I containing uniform flower-like microspheres assembled by ultra-thin nanosheets exhibited the highest photocatalytic activity and favorable stability for the degradation of the antibiotic tetracycline under visible light irradiation. This work could provide a good reference for the design of bismuth-rich oxyhalide heterojunction for photocatalytic applications.

Enhanced photocatalytic degradation of gaseous toluene and liquidus tetracycline by anatase/rutile titanium dioxide with heterophase junction derived from materials of Institut Lavoisier-125(Ti): Degradation pathway and mechanism studies

Anatase/rutile titanium dioxide (TiO2) with heterophase junction and large Brunauer-Emmett-Teller (BET) specific surface area (50.1 m2 g−1) is successfully synthesized by calcinating Materials of Institut Lavoisier-125(Ti) (MIL-125(Ti)) with 30% O2/Ar at the temperature of 600 °C (M−O−600). Several techniques are used to examine the physicochemical, photoelectrochemical and optical properties of samples, and their photocatalytic performances are evaluated by photodegradation of gaseous toluene and liquidus tetracycline (TC) under visible light illumination. It is found that the calcination temperature has significant influence on the crystal structure and physicochemical parameters of TiO2. The weight fractions of rutile and anatase TiO2 of M−O−600 are approximately 0.7 and 0.3, which displays outstanding photocatalytic activity. Through the construction of heterophase junction, M−O−600 has better oxygen adsorption and higher density of localized states, which effectively promotes the generation of superoxide radical (·O2–) and hydroxyl radical (·OH) species. In-situ infrared spectra indicate that toluene is oxidized to benzyl alcohol, benzaldehyde and benzoic acid in turn and then oxidized to formic acid and acetic acid before eventually degraded into H2O and CO2. Gas chromatography-mass spectrometry (GC–MS) is also used to further investigate the degradation pathway of toluene. Degradation pathway and mechanism of TC are studied by liquid chromatography-tandem mass spectrometry (LC-MS). Moreover, three-dimensional excitation-emission matrix fluorescence spectroscopy (3D EEMs) and total organic carbon (TOC) show that TC can be effectively mineralized through a series of reactions by M−O−600 during photocatalysis.

Ta2O5 NTs-TiO2 nanodots heterostructure photocatalyst material for enhanced photodegradation and photoelectrochemical performance under simulated solar light

Ta2O5-TiO2 heterojunction photocatalysts with a structure of TiO2 nanodots deposited on Ta2O5 nanotube (NTs) were prepared by anodization and in situ growth methods following annealing treatment. The heterojunction photocatalytic material exhibited enhanced photodegradation behavior of methylene blue (MB) and photoelectrochemical water splitting performance under simulated solar light. The degradation rate of MB and photocurrent density of heterojunction is 3.99 and 8.33 times higher than that of pure Ta2O5 NTs respectively. The synergetic effect of the phase heterojunction and highly ordered structure increased the specific surface area and improved the separation and transmission efficiency of photogenerated electrons and holes, thereby enhancing the photocatalytic activities. Hence, Ta2O5 TNs-TiO2 nanodots heterojunction has promising application in the fields of photodegradation, photoelectrochemical water splitting, solar cells, and other photocatalytic devices.