Form Heterojunction Structures Research Articles

Efficient degradation of organics by ultrasonic piezoelectric effect on CuO-BTO/AFC composite

The recombination of photoexcited electron–hole pairs greatly limits the degradation performance of photocatalysts. Ultrasonic cavitation and internal electric field induced by the piezoelectric effect are helpful for the separation of electron–hole pairs and degradation efficiency. The activated foam carbon (AFC) owing to its high surface area is often used as the substrate to grow catalysts to provide more reactive active sites. In this work, CuO@BaTiO3 (CuO@BTO) heterostructure is prepared by hydrothermal method on the surface of AFC to investigate the ultrasonic piezoelectric catalysis effect. X-ray diffraction (XRD), Raman spectroscopy, energy dispersive x-ray spectroscopy (EDS) and scanning electron microscopy (SEM) were used to analyze the structure and morphology of CuO-BTO/AFC composite. It is found that the CuO-BTO/AFC composite exhibits excellent piezo-catalytic performance for the degradation of organics promoted by ultrasonic vibration. The CuO-BTO/AFC composite can decompose methyl orange and methylene blue with degradation efficiency as high as 93.9% and 97.6% within 25 min, respectively. The mechanism of piezoelectricity enhanced ultrasound supported catalysis effect of system CuO-BTO/AFC is discussed. The formed heterojunction structure between BTO and CuO promotes the separation of positive and negative charges caused by the piezoelectric effect.

Improved photocatalytic performance of TiO2/carbon photocatalysts: Role of carbon additive

A series of TiO2 – based photocatalysts have been prepared by the incorporation of 10 wt% of various carbon-based nanomaterials as modifying agents to titania. More specifically, commercial TiO2 P25 was modified through a wet impregnation approach with methanol with four different carbon nanostructures: single-walled carbon nanotubes (SWCNTs), partially reduced graphene oxide (prGO), graphite (GI), and graphitic carbon nitride (gCN). Characterization results (XPS and Raman) anticipate the occurrence of important interfacial phenomena, preferentially for samples TiO2/SWCNT and TiO2/prGO, with a binding energy displacement in the Ti 2p contribution of 1.35 eV and 1.54 eV, respectively. These findings could be associated with an improved electron-hole mobility at the carbon/oxide interface. Importantly, these two samples constitute the most promising photocatalysts for Rhodamine B (RhB) photodegradation, with nearly 100% conversion in less than 2 h. These promising results must be associated with intrinsic physicochemical changes at the formed heterojunction structure and the potential dual-role of the composites able to adsorb and degrade RhB simultaneously. Cyclability tests confirm the improved performance of the composites (e.g., TiO2/SWCNT, 100% degradation in 1 h) due to the combined adsorption/degradation ability, although the regeneration after several cycles is not complete due to partial blocking of the inner cavities in the carbon nanotubes by non-reacted RhB. Under these reaction conditions, Rhodamine-B xanthene dye degrades via the de-ethylation route.

One-step synthesis of Bi2O2CO3/Bi2S3 S-scheme heterostructure with enhanced photoactivity towards dibutyl phthalate degradation under visible light

Bi2O2CO3/Bi2S3 heterojunction was prepared by one-step hydrothermal method, where Bi(NO3)3 was employed as Bi source, Na2S was used as a sulfur source, and CO(NH2)2 was adopted as C source. The load of Bi2S3 was adjusted by changing the content of Na2S. The prepared Bi2O2CO3/Bi2S3 illustrated strong photocatalytic activity towards dibutyl phthalate (DBP) degradation. The degradation rate was 73.6% under the irradiation of visible light for 3 h, which were 3.5 and 1.87 times for Bi2O2CO3 and Bi2S3, respectively. In addition, the mechanism for the enhanced photoactivity was investigated. After combined with Bi2S3, the formed heterojunction structure inhibited the recombination of photogenerated electron-hole pair, improved the visible light adsorption, and accelerated the migration rate of the photogenerated electron. As a result, analysis of the radical formation and the energy band structure revealed that Bi2O2CO3/Bi2S3 was consistent with the S-scheme heterojunction model. The S-scheme heterojunction allowed the Bi2O2CO3/Bi2S3 to possess high photocatalytic activity. The prepared photocatalyst presented acceptable cycle application stability. This work not only develops a facile one-step synthesis technique for Bi2O2CO3/Bi2S3, and also provides a good platform for the degradation of DBP.

One-step preparation of MoOx/ZnS/ZnO composite and its excellent performance in piezocatalytic degradation of Rhodamine B under ultrasonic vibration

This paper synthesized a new type of ternary piezoelectric catalyst MoOx/ZnS/ZnO (MZZ) by a one-step method. The catalytic degradation of Rhodamine B (RhB) solution (10 µg/g, pH = 7.0) shows that the composite catalyst has excellent piezoelectric catalytic activity under ultrasonic vibration (40 kHz). The piezoelectric degradation rate of the optimal sample reached 0.054 min−1, which was about 2.5 times that of pure ZnO. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS) technologies were used to analyze the structure, morphology, and interface charge transfer properties of the MZZ piezocatalysts. The results showed that the composite catalyst may have a core-shell structure. ZnS is coated on the surface of ZnO, while MoOx adheres to the surface of ZnS. This structure endowed MZZ larger specific surface area than ZnO, which benefits the RhB adsorption. More importantly, the formed heterojunction structure between ZnS and ZnO promotes the separation of positive and negative charges induced by the piezoelectric effect. MoOx species may act as a charge trap to further promote more carriers to participate in the reaction. In addition, MoOx may also be beneficial in adsorbing dyes. Active species capture experiments show that superoxide radicals and holes are the main active species in piezoelectric catalytic reactions on MZZ catalysts.

Plasmonic Ag/AgCl/NH2-MIL-88B (Fe) inorganic-organic hybridized heterojunction as visible-light-driven photocatalyst for hexavalent chromium reduction

To effectively photocatalytic removal of Cr (VI), this work focuses on constructing the inorganic-organic hybridized heterojunction composite photocatalyst of Ag/AgCl/NH2-MIL-88B (Fe), where the Ag/AgCl nanoparticles were deposited on the surface of mesoporous NH2-MIL-88B (Fe) via a simple photo-assisted reduction method. The formation of the ternary Ag/AgCl/NH2-MIL-88B (Fe) composite presented more stable and higher photocatalytic reduction performance for Cr (VI) removal than that of MIL-88B (Fe) photocatalyst. Its photo-reduction rate reached to 85.7%, which was ascribed to its high separation and migration efficiency of photogenerated electrons and holes pair. The formed heterojunction structure and surface plasmon resonance effect originated from Ag/AgCl could largely enhance the absorption capacity of visible light, which promotes the excitation of photogenerated electrons, accelerates their transfer, and reduces the recombination rate of electrons and holes. Besides, a possible catalytic mechanism of this reaction is proposed by the photo-induced charge transfer path in the Ag/AgCl/NH2-MIL-88B (Fe) inorganic-organic hybridized system.

One step and fast preparation of VOx/g-C3N4 photocatalyst via microwave heating for effective degradation of RhB under visible light

This work reports a microwave heating method for synthesis of a VOx/g-C3N4 complex for applications in photocatalytic degradation of rhodamine B (RhB) under visible light illumination. The facile strategy allows the fabrication of the VOx/g-C3N4 composite in 40 min with NH4VO3 and melamine as precursors. A thorough investigation was performed to investigate the structure, morphology, photoabsorption performance and charge separation capability of the VOx/g-C3N4 composite. XRD, FT-IR, XPS and TEM experiments proved that VOx nanoparticles which have the mixed valence state of V5+ and V4+ were dispersed in the g-C3N4. DRS experiment demonstrated that these introduced VOx nanoparticles improved the light absorption property for visible light. N2-adsorption analysis showed that the surface area of VOx/g-C3N4 sample was enhanced with increasing VOx content. Although the improved photoabsorpion performance and surface area are beneficial to the photocatalytic RhB degradation, the mostly important reason of the enhanced photoactivity was ascribed to the formed heterojunction structure of g-C3N4 and VOx, which triggers electrons migration via a type-II mechanism and elevates the charge separation. This improved charge separation was confirmed by the PL, EIS, and PC experiments, while the type-II mechanism was verified by reactive species trapping experiment and photocatalytic H2 evolution test. Due to the improved efficiency in separating charge carriers, the VOx/g-C3N4 worked very well in RhB degradation via photocatalysis. The best VOx/g-C3N4 sample displayed a RhB degradation velocity of 0.079 min−1 under visible light, which is 3.6 folds faster than that of neat g-C3N4.

Design of visible-light photocatalysts by coupling of inorganic semiconductors

The design of advanced photocatalytic systems, effectively working under visible-light, is essential for practical application of photocatalysts in removing environmental pollutants. To achieve high efficiency, the required properties for the photocatalysts are profound solar light absorption in the visible-range, efficient charge-separation, suitable energy band locations for redox reactions, and extended photostability. As a single semiconductor-based photocatalyst cannot satisfy all of these requirements, a potential strategy will be construction of coupled structures between two or more semiconductors. In the present study, we explore various types of photocatalytic systems constructed by coupling semiconductors and their working mechanisms. When two narrow bandgap semiconductors (NBSs) absorbing visible-light are coupled to form heterojunction structures, they can be classified as p-n junction or Z-scheme systems, according to the charge-flow pathway between the two semiconductors. When a NBS that absorbs visible-light is coupled with a wide bandgap semiconductor (WBS) functioning as the main photocatalyst, the fabricated catalytic systems can be classified as Type-A or Type-B heterojunction systems dependent on their relative energy band locations. Herein various semiconductor composites reported to be visible-light photocatalysts in the literature are classified using these categories, and their photocatalytic mechanisms, including charge-flow pathways, are discussed in depth. In addition, recent progress and future perspectives for heterojunction systems are reviewed and discussed.

A nanostructured CuWO4/Mn3O4 with p/n heterojunction as photoanode toward enhanced water oxidation

In this paper, we report a novel photoanode fabricated by spin-coating n-type CuWO4 nano-ovoids on the surface of fluorine-doped tin oxide (FTO) glass and then modified with p-type Mn3O4 nanospheres using a deposition-annealing method. A novel nanocomposite (CuWO4/Mn3O4) with p/n heterojunction formed on the surface of FTO glass. The characterazation results of the nanocomposite indicate that the two moieties of the nanocomposite intimately contact with each other. The photoelectrochemical tests of the prepared CuWO4/Mn3O4 photoanode reveal that the electrode can produce larger anodic photocurrent and show enhanced incident photon-to-current efficiency (IPCE) than the bare CuWO4 photoanode does. Moreover, the PEC tandem cell assembled by using the CuWO4/Mn3O4 electrode as photoanode and Pt plate as counter electrode demonstrates enhanced photocatalytic activity and stability for photoeletrochemical (PEC) water splitting. The amount of hydrogen and oxygen, collected from the counter chamber and the photoanode chamber, separately, reached to 5.0 μmol and 2.3 μmol, respectively, under 2 h simulated sunlight irradiation at 1.20V vs. RHE bias without any sacrificial agent. The remarkable performance of the nanocomposite anode owe to the formed heterojunction structure in the nanocomposite, leading to high photoexcited charge transfer and separation.

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.

In-situ preparation of Z-scheme AgI/Bi5O7I hybrid and its excellent photocatalytic activity

The aim of this work was to synthesize, characterize and evaluate the photocatalytic activity of AgI/Bi5O7I composite photocatalyst under visible light irradiation. The photocatalyst was prepared by a simple one-step ionic reaction between Bi5O7I microrods and AgNO3 solutions, and was characterized by various techniques including X-ray diffraction (XRD), Raman spectroscopy (Raman), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), and photoluminescence spectroscopy (PL). The characterizations indicate that AgI particles were closely anchored on Bi5O7I micronods. During the photocataytic reaction, the composite was actually an Ag-AgI-Bi5O7I ternary system. The plasmonic effect of the formed Ag nanoparticles improved the visible light absorption performance, which benefits the photocatalytic reaction. However, more important was the formed heterojunction structure in the composite, which efficiently promoted the separation of electron-hole pairs by a plasmonic Z-scheme mechanism, and ultimately enhanced the photocatalytic activity. The optimal AgI/Bi5O7I composite showed a RhB degradation rate of 0.046min−1, which was 3.83 and 6.57 times higher than those of Bi5O7I and AgI, respectively. This work may provide some insight into the design of novel and highly efficient Z-scheme visible-light photocatalysts.

Enhanced photocatalytic performance of g-C3N4 with BiOCl quantum dots modification

Novel BiOCl quantum dots modified g-C3N4 photocatalyst was synthesized by a one-step chemical bath method at low temperature. The photocatalyst was characterized using X-ray diffraction, high-resolution transmission microscopy UV–visible light diffusion reflectance spectrometry, and photoluminescence spectroscopy. The results indicated that BiOCl quantum dots were dispersed on g-C3N4 to form heterojunction structures with high specific surface area. BiOCl-g-C3N4 showed much higher photocatalytic activity than pure g-C3N4 and BiOCl for rhodamine B degradation. The enhanced performance was induced by the high separation efficiency of photoinduced carriers.

Enhanced photocatalytic performance of direct Z-scheme BiOCl–g-C3N4 photocatalysts

Novel BiOCl–g-C3N4 composite photocatalysts were synthesized by a one-step chemical bath method. The composites were characterized using X-ray diffraction (XRD), high-resolution transmission microscopy (HRTEM), UV-visible light diffusion reflectance spectrometry (DRS), Fourier-transform infrared spectroscopy (FT-IR), and photoluminescence spectroscopy (PL). The results indicated that BiOCl was dispersed on g-C3N4 to form heterojunction structures with high specific surface areas. BiOCl–g-C3N4 showed higher photocatalytic activity than pure g-C3N4 and BiOCl for rhodamine B (RhB) degradation. The enhanced performance was induced by the high separation efficiency of photoinduced carriers. The active species trapping and quantification experiments indicated that the photoinduced charges transfer via a direct Z-scheme.

Construction of exfoliated g-C3N4 nanosheets–BiOCl hybrids with enhanced photocatalytic performance

Exfoliated g-C3N4 nanosheets (CNs) were composited with bismuth oxychloride (BiOCl) to fabricate a series of hybrids via a facile chemical deposition–precipitation method in this investigation. The as-synthesized BiOCl–CNs hybrids were then fully characterized by a collection of analytical techniques. It was obviously observed that CNs were in intimate contact with hierarchical BiOCl flowerlike units to form heterojunction structures, which facilitates transfer and efficient separation of photoinduced electron–hole pairs, thus greatly increasing catalytic activity upon visible light irradiation. Together with other merits such as enlarged specific surface area, favorable optical properties, and suitable energy-band structures, these robust BiOCl–CNs hybrids showed significantly enhanced photocatalytic performance towards Rhodamine B (RhB) dye removal. Under identical conditions, the apparent photocatalytic reaction rate of the best hybrid BiOCl–CNs-3% was about 2.1 and 26.6 times as high as those of BiOCl and CNs alone, respectively. A possible photocatalytic mechanism was also proposed by means of active species trapping measurements, revealing that superoxide radicals (˙O2−) played a crucial role during the catalytic process.

Novel BiOCl–C3N4 heterojunction photocatalysts: In situ preparation via an ionic-liquid-assisted solvent-thermal route and their visible-light photocatalytic activities

Novel BiOCl–C3N4 heterojunctions were synthesized through an in situ ionic-liquid-assisted solvent-thermal method, through which a highly dispersed heterointerface was formed with almost no loss of C3N4. The composites were characterized using X-ray diffraction, high-resolution transmission microscopy, scanning electron microscopy, nitrogen adsorption–desorption measurements, UV–visible light diffusion reflectance spectrometry, Fourier-transform infrared spectroscopy, and photoluminescence spectroscopy. The results indicated that BiOCl was dispersed on C3N4 to form heterojunction structures with high specific surface area and the ability to absorb visible light. The photocatalytic results for the degradation of methyl orange (MO) indicated that the most active heterojunction proportion is 1BiOCl:1C3N4, which increased the visible light photocatalytic activity 3.3-fold relative to that of a sample of the mechanically mixed counterpart of this composition. The enhanced performance was induced by the high separation efficiency of photoinduced electrons from the LUMO of C3N4 injected into the CB of BiOCl. We also determined that the enrichment of holes in the HOMO of the C3N4 in the present BiOCl–C3N4 heterojunctions played a dominant role in the oxidation of MO.