As-synthesized Heterojunction Research Articles

Visible light-induced H2 production and pollutant degradation by copper oxide nanosphere embedded zinc-cadmium-sulfide composite

AbstractGreen hydrogen production using solar water splitting and solving water pollution issues are intricately intertwined global goals which are hindered by the scarcity of highly active photocatalytic materials. Herein, we have presented a simple strategy to couple two semiconductors (Cu2O and ZnCdS) to form a type-I heterojunction with high visible light response. The as-synthesized heterojunction was well characterized by the battery techniques, such as TEM, HAADF-STEM elemental mapping, XRD and XPS. The visible light response was higher for composite than individual components, as was also supported by UV–vis DRS. The Cu2O-ZnCdS composite showed a higher visible light-driven photocatalytic H2 production rate (78.5 µmol g–1 h–1). The catalyst was also active for photocatalytic degradation of a model dye-methylene blue (MB)-with a degradation rate constant of 0.079 min−1. The enhanced performance of the Cu2O-loaded ZnCdS catalysts can be ascribed to both factors, such as enhancement of the visible light absorption and the growth of Cu2O-ZnCdS heterojunction. The heterojunction formation facilitates efficient charge separation with smaller charge resistance, as evidenced by transient photocurrent response and electrochemical impedance spectroscopy (EIS) studies. This study strongly indicates that the photocatalytic reactions with this catalyst material are kinetically favoured by coupling the two semiconductors. Graphical abstract

Combustion-synthesized ZnO-CeO2 heterojunctions for advanced photocatalytic dye degradation

ZnO-CeO2 heterojunctions were successfully synthesized utilizing a simple combustion approach employing equimolar solutions of the precursor chemicals. The morphology, content, crystallinity, purity, crystal size and bandgap energy features of the synthesized heterojunctions were carefully investigated utilizing various analytical methods. FESEM investigation indicated a heterogeneous flake or sheet-like appearance, as well as a rough and granular surface for the heterojunctions. XRD study found that the average crystallite size for ZnO-CeO2 heterojunctions was 5.38 nm. The absorbance spectrum showed a peak absorption at 372 nm and a band gap energy value of 2.1 eV. The photocatalytic activity of the synthesized ZnO-CeO2 heterojunctions towards three model dyes, CR, RhB, and MB dyes, were investigated. Percent degradations of 96.61 % and 98.11 %, respectively were achieved for CR and MB dyes within 30 min, while 98.25 % photodegradation was observed for RhB dye within 60 min of UV irradiation. The rapid degradation of CR and MB dyes suggests their high susceptibility to the photocatalytic action of ZnO-CeO2 heterojunctions as compared to RhB dye. Kinetic analysis indicated that the degradation of dyes followed a pseudo-first-order kinetic model. Finally, for the photocatalytic activity of the as-synthesized heterojunctions, a photodegradation mechanism was also postulated.

S-scheme AgIO4/CeO2 heterojunction nanocomposite photocatalyst for degradation of rhodamine B dye

Photocatalysis is auspicious technology for transferring inexhaustible solar energy into storable chemical energy. In this novel research, an auspicious S-scheme heterojunction is constructed by hybridizing CeO2 nanoparticles with band gap energy [2.9 eV], surface area [75 m2/g], and negative conduction band potential (ECB = -0.38 eV) with AgIO4 nanorods with 22 m2/g surface area, band gap energy of 1.98 eV and valence band potential of + 3.27 eV. The novel AgIO4/CeO2 heterojunctions containing different compositions of CeO2 and AgIO4 are fabricated through coupled sol–gel and sonochemical route. SEM, XRD, BET, XPS, DRS, PL and HRTEM investigate the morphological, structural, textural and optical features of the solid specimens. Combined PL and DRS analysis have pointed out that AgIO4 prohibit the as-synthesized heterojunctions with strong visible light absorbability and highly separation efficiency of photogenerated electrons and holes. CeAg15 optimal catalyst decompose RhB dye with pseudo-first order rate of 0.024 min−1 which is tenfold higher than that of pristine CeO2. In the light of scrubber experimental results and PL analysis of terephthalic acid, S-scheme is proposed for analyzing the route of charge transportation and generation of charge carriers with strong redox power. The construction of S-scheme mechanism reveal the preservation of the positive holes of AgIO4 oxidative photocatalyst and negative electrons of CeO2 reductive photocatalyst for consuming in destruction of RhB dye into eco-friendly species as manifested from TOC analysis. This novel research work provide a clear insight on fabrication of S-scheme heterojunction with strong oxidative and reductive power for eradicate water pollutants.

Oxygen vacancy configuration in confined BiVO4-Bi2S3 heterostructures promotes photocatalytic oxidation of NO

Oxygen vacancy configuration is a promising means for tuning the electronic structure and determining the adsorption/desorption mode of the reactants in photocatalytic NOx oxidation. We report here that the formation of oxygen vacancies (OVs) can activate free NO and O2 molecules via a changed oxidation route, leading to fast conversion of NO into NO3-. The boosted NO oxidation reaction was conducted on novel OVs-induced BiVO4-Bi2S3 heterojunctions with the shape of double-deck hollow nanospheres assembled by a hard-templating treatment, followed by an annealing process. Abundant OVs were incorporated into the lattice of the as-synthesized heterojunctions, which significantly decreased the recombination level of the photo-exited carriers, leading to the promotion of the exchange of adsorbed ambiance oxygen and lattice oxygen, and the formation of reactive oxygen species and the final highly-efficient removal of NO under visible-light illumination. This work verifies the role of defective and hierarchical interface configuration on catalytic NOx oxidation and provides a viable methodology for the discovery of advanced photocatalysts via surface engineering.

Fabrication of novel AgIO4/SnO2 heterojunction for photocatalytic hydrogen production through direct Z-scheme mechanism

Direct Z-scheme photocatalysts exhibits many merits as enhanced light harvesting, spatially separated reductive and oxidative active sites, and well-maintained strong redox ability, which improve the performance of various photocatalytic processes. In this research, SnO2 nanoparticles were fabricated by sol-gel route using pluronic as templating agent. However, various proportions of AgIO4 nanoparticles were incorporated on SnO2 surface by sonochemical route. The complete profile of the as-synthesized heterojunctions was assessed by XRD, N2-adsorption-desorption isotherm, DRS, PL, XPS and HRTEM. Among incorporation of AgIO4 nanoparticles, the photocatalytic hydrogen rate soar rapidly indicating that AgIO4 plays a pivotal role in light harvesting and photocatalytic reactivity. The photocatalytic hydrogen production was estimated using methanol as scavenger is 10.7 mmolh−1 g−1 which is 50 fold higher than that of bare SnO2. The pronounced photocatalytic hydrogen rate can ascribed to the production of AgIO4/SnO2 heterojunction through which charge migration was proceeds adopting direct Z-scheme mechanism. The Z-scheme mechanism not only enhances the efficiency of charge carriers separation, but, also, increases the oxidative and reductive power of the photogenerated holes and electrons. The as-synthesized heterojunction retains 85 % its photocatalytic reactivity after five consecutive cycles. XRD and XPS analysis indicates that the heterojunction emerges after cyclic process exhibiting the same crystalline and oxidation state features. The novel photocatalyst enhances the photocatalytic hydrogen production in various industrial applications.

Enhanced photoactivity of cerium tungstate-modified graphitic carbon nitride heterojunction photocatalyst for the photodegradation of moxifloxacin

Design and optimization of visible-light-driven photocatalysts for degradation of organic pollutants is an important step towards environmental decontamination. In this study, wolframite cerium tungstate (Ce2(WO4)3, (CW)) hybridized with g-C3N4 (CN) nanosheets was synthesized via a simple hydrothermal route followed by an ultrasound-assisted synthesis method. The prepared Ce2(WO4)3@ g-C3N4 (CW@CN) heterojunction was investigated for photocatalytic degradation of the antibiotic moxifloxacin (MXF) under visible light irradiation. Structural, morphological, and optical properties as well as chemical composition of the as-synthesized heterojunction were investigated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS) and photoluminescence (PL). MXF photocatalytic degradation by the binary nanostructure (Ce2(WO4)3@ g-C3N4) (94.1%) was the highest compared to g-C3N4 (53.6%) and Ce2(WO4)3 (46.4%). Such enhanced activity could be ascribed to efficient suppression of the charge carriers’ recombination, leading to adequate formation of the reactive species responsible for MXF degradation. Furthermore, the Ce2(WO4)3@ g-C3N4 heterojunction showed remarkable stability over five consecutive cycles, with only 11.5% reduction after the 5th cycle. This work established the potential applicability of Ce2(WO4)3@ g-C3N4 nanostructures towards photocatalytic removal of MXF.

A composite of platelet-like orientated BiVO4 fused with MIL-125(Ti): Synthesis and characterization

The development of heterojunctions is the current focus of the scientific community as these materials are visible light active and the staggered positioning of their band edges combats electron-hole recombination which is the downside of most photocatalysts. In this work, a two- step hydrothermal synthesis protocol was utilized to fabricate a novel observable-light active material, composed of platelet-like BiVO4 and a titanium-based metal organic framework (MOF) called MIL-125(Ti). The tuning of specific morphologies, such as platelet-like in BiVO4, provides the exposure of most reactive facets which are more reactive towards photooxidation of organics in water, thus increasing their efficiency. The as-synthesized heterojunction was characterized by Transmission electron microscopy (TEM), scanning transmission microscopy (SEM), X-Ray diffraction (XRD), Raman spectroscopy, ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS), X-Ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectra. The formation of the heterojunction lead to a positive shift of the 3-2 Bi:Ti valence band (VB) (1.78 eV) when compared to 1.27 eV VB position of BiVO4. The PL and photoelectrochemical measurements revealed that the heterojunction photocatalyst designated 3-2 Bi-Ti demonstrated inhibited recombination rate (platelet-like BiVO4 > 3-2 Bi:Ti (PM) > MIL-125 > 1–1 Bi:Ti > 2–3 Bi:Ti > 3-2 Bi:Ti) and highly efficient interfacial charge shuttle between platelet-like BiVO4 and MIL-125(Ti) through the formed n-n junction.

Simple solid-state synthesis of BiOCl/Bi2O2CO3 heterojunction and its excellent photocatalytic degradation of RhB

The composition and microstructure of heterojunctions could remarkably affect the photo-induced charge separation of semiconductors, thus changing their photocatalytic activity. In this work, a simple solid-state chemical reaction route was developed to fabricate BiOCl/Bi2O2CO3 p-n heterojunctions hierarchical nanostructures self-assembled by thin nanoplates. The as-synthesized heterojunctions significantly enhanced photocatalytic activity in comparison with individual Bi2O2CO3 and BiOCl. In particularly, the 60% BiOCl/Bi2O2CO3 heterojunction exhibited the highest photocatalytic activity, which was around 188 times and 17 times higher than that of pristine Bi2O2CO3 under visible and ultraviolet light, respectively. The remarkable enhanced photodegradation performance of heterojunction is ascribed to its higher specific surface area and the efficient separation of photo-generated carriers. Combining with the theoretical and experimental results, the transition paths of electrons and holes between BiOCl and Bi2O2CO3 was investigated, therefore the self-photosensitized photocatalytic mechanism of BiOCl/Bi2O2CO3 composite has also been well explained.

Electrical and Optoelectronic Properties of Chemically Prepared PbS/MnS Heterojunction

Lead sulfide (PbS)/manganese sulfide (MnS) heterojunction is synthesized by a simple two-step chemical bath deposition technique. The as-synthesized heterojunction is studied based on the optical and electrical analyses. The optical absorbance spectrum of the test structure confirms dual peaks at two different wavelengths, which has prompted consideration of the two band gaps of the structure. The origin of dual peaks in the absorbance plot is attributed to the existence of the PbS base layer and the MnS window layer. The band gaps are estimated using the conventional Tauc’s method (Eg = 1.75 eV and 2.9 eV) and also by the absorbance spectrum fitting method (Eg = 1.82 eV and 2.97 eV). In the dc analyses, the PbS/MnS heterojunction possesses low dark current and high dynamic resistance. However, under illumination (400–700 nm) a distinct photo-response is observed, showing a substantial rise in device current with the reduction of dynamic resistance. The photo-response of the heterojunction is examined at 635–699 nm, 514–538 nm and 410–452 nm wavelengths by analyzing the photo-current and photo-resistance. The device is found to be more photosensitive at lower wavelength illuminations compared to the higher wavelength exposures.

Visible light responsive Bi2WO6/BiOCl heterojunction with enhanced photocatalytic activity for degradation of tetracycline and rohdamine B

Visible light responsive Bi2WO6/BiOCl heterojunctions with varying molar ratio of WO66− to Cl− were successfully fabricated by a facile one-step hydrothermal method. The photocatalytic performance of as-synthesized heterojunctions was investigated through degradation of rohdamine B (RhB) and tetracycline (TC). Results showed that the Bi2WO6/BiOCl heterojunction exhibited markedly improved degradation efficiency under visible light irradiation compared with bare BiOCl and Bi2WO6, which can be attributed to the synergistic effect between Bi2WO6 and BiOCl. The heterojunction formed at the interface of Bi2WO6 and BiOCl results in the effective separation of photo-generated electron-hole pairs and contributes to the improved photocatalytic activity.

Perovskite-structured CaTiO3 coupled with g-C3N4 as a heterojunction photocatalyst for organic pollutant degradation.

A novel graphitic carbon nitride (g-C3N4)–CaTiO3 (CTCN) organic–inorganic heterojunction photocatalyst was synthesized by a facile mixing method, resulting in the deposition of CaTiO3 (CT) nanoflakes onto the surface of g-C3N4 nanosheets. The photocatalytic activity of the as-synthesized heterojunction (along with the controls) was evaluated by studying the degradation of an aqueous solution of rhodamine B (RhB) under UV, visible and natural sunlight irradiation. The CTCN heterojunction with 1:1 ratio of g-C3N4/CT showed the highest photocatalytic activity under sunlight irradiation and was also demonstrated to be effective for the degradation of a colorless, non-photosensitizing pollutant, bisphenol A (BPA). The superior photocatalytic performance of the CTCN heterojunction could be attributed to the appropriate band positions, close interfacial contact between the constituents and extended light absorption (both UV and visible region), all of which greatly facilitate the transfer of photogenerated charges across the heterojunction and inhibit their fast recombination. In addition, the two-dimensional (2D) morphology of g-C3N4nanosheets and CT nanoflakes provides enough reaction sites due to their larger surface area and enhances the overall photocatalytic activity. Furthermore, the active species trapping experiments validate the major role played by superoxide radicals (O2−•) in the degradation of pollutants. Based on scavenger studies and theoretically calculated band positions, a plausible mechanism for the photocatalytic degradation of pollutants has been proposed and discussed.

The synthesis of a Bi2MoO6/Bi4V2O11 heterojunction photocatalyst with enhanced visible-light-driven photocatalytic activity.

A novel Bi2MoO6/Bi4V2O11 heterostructured photocatalyst was successfully fabricated using a facile one-pot solvothermal method. This heterojunction consists of homogeneous dispersed Bi4V2O11 nanocrystals anchored on the surface of Bi2MoO6 nanoflakes, endowing the heterojunction with nanosized interfacial contact. Based on the favorable interfacial contact, the band alignment at the heterojunction effectively facilitated photo-generated carrier transfer, which was verified by photoelectrochemical and photoluminescence measurements. Thereby, in contrast with pristine Bi2MoO6 and Bi4V2O11, the as-synthesized heterojunction with nanoscale contact exhibited significantly enhanced photocatalytic activity towards the degradation of MB and the reduction of Cr(vi). In addition, the as-fabricated Bi2MoO6/Bi4V2O11 heterojunction exhibited good cycling stability for MB degradation after 4 cycles. Finally, a plausible photocatalytic mechanism for MB degradation over the Bi2MoO6/Bi4V2O11 heterojunction was discussed in detail. This work not only reports a highly efficient photocatalyst but also sheds light on the design and optimization of a heterojunction.

In-situ synthesis of direct solid-state Z-scheme V2O5/g-C3N4 heterojunctions with enhanced visible light efficiency in photocatalytic degradation of pollutants

The constructing of direct solid-state Z-scheme heterojunction photocatalytic system has received much attention in environmental purification and hydrogen generation from water. In this study, a novel direct solid-state Z-scheme V2O5/g-C3N4 heterojunctions were synthesized via a facile in-situ growth strategy for the first time. The photocatalytic performance was evaluated by the degradation of rhodamine B (RhB) and tetracycline (TC) under visible light irradiation (λ>420nm). Results show that the as-synthesized heterojunctions can significantly enhance photocatalytic activity in comparison with pure g-C3N4 and V2O5. The optimum photocatalytic efficiency of VC1.0% sample for the degradation RhB was about 7.3 and 13.0 times higher than that of individual g-C3N4 and V2O5, respectively. In addition, the VC1.0% sample as well as can efficiently degrade methyl orange (MO) and methylene blue (MB) under visible light. By further experimental study, the possible for the enhancing photocatalytic mechanism was found to be a direct solid-state Z-scheme heterojunction system based on the active species trapping and electron spin resonance (ESR) experiments, which not only can improve the photogenerated electron–hole pair’s separation but also exhibit a strong oxidation and reduction ability for efficiency degradation of organic pollutants. This work will be useful for the design of other direct solid-state Z-scheme photocatalytic systems for application in energy conversion and environmental remediation.

Structure Modification Function of g-C3 N4 for Al2 O3 in the In Situ Hydrothermal Process for Enhanced Photocatalytic Activity.

Heterojunctions of g-C3 N4 /Al2 O3 (g-C3 N4 =graphitic carbon nitride) are constructed by an in situ one-pot hydrothermal route based on the development of photoactive γ-Al2 O3 semiconductor with a mesoporous structure and a high surface area (188 m(2) g(-1) ) acting as electron acceptor. A structure modification function of g-C3 N4 for Al2 O3 in the hydrothermal process is found, which can be attributed to the coordination between unoccupied orbitals of the Al ions and lone-pair electrons of the N atoms. The as-synthesized heterojunctions exhibit much higher photocatalytic activity than pure g-C3 N4 . The hydrogen generation rate and the reaction rate constant for the degradation of methyl orange over 50 % g-C3 N4 /Al2 O3 under visible-light irradiation (λ>420 nm) are 2.5 and 7.3 times, respectively, higher than those over pristine g-C3 N4 . The enhanced activity of the heterojunctions is attributed to their large specific surface areas, their close contact, and the high interfacial areas between the components as well as their excellent adsorption performance, and efficient charge transfer ability.