Enhanced chemosensing of ammonia based on the novel molecular semiconductor-doped insulator (MSDI) heterojunctions

A series of new molecular semiconductor-doped insulator (MSDI) heterojunctions as conductimetric transducers to NH₃ sensing were fabricated based on a novel semiconducting molecular material, an amphiphilic tris(phthalocyaninato) rare earth triple-decker complex, Eu₂[Pc(15C5)₄]₂[Pc(OC₁₀H₂₁)₈], quasi-Langmuir-Shäfer (QLS) film, as a top-layer, and vacuum-deposited and cast film of CuPc as well as copper tetra-tert-butyl phthalocyanine (CuTTBPc) QLS film as a sub-layer, named as MSDIs 1, 2 and 3, respectively. MSDIs 1-3 and respective sub-layers prepared from three different methods were characterized by X-ray diffraction, electronic absorption spectra and current-voltage (I-V) measurements. Depending on the sub-layer film-forming method used, α-phase CuPc film structure, β-phase CuPc crystallites and H-type aggregates of CuTTBPc have been obtained, respectively. An increasing sensitivity to NH₃ at varied concentrations in the range of 15-800 ppm, follows the order MSDI 2 < MSDI 3 < MSDI 1, revealing the effect of sub-layer film structures on sensing performance of the MSDIs. In particular, the time-dependent current plot of the MSDI 1, with α-phase CuPc film as a sub-layer, clearly shows an excellent separation of the different ammonia concentration levels and nearly complete reversibility and reproducibility even at room temperature, which is unique among the phthalocyanine-based ammonia sensors thus far reported in the literature. This provides a general method to improve sensor response of organic heterojunctions by controlling and tuning the film structure of sub-layer with appropriate fabrication techniques. On the other hand, the enhanced sensitivity, stability and reproducible response of the MSDI 1 heterostructure in comparison with the respective single-layer films have also been obtained. A judicious combination of materials and molecular architectures has led to enhanced sensing properties of the MSDI 1, in which control at the molecular level can be achieved.     

Controlled surface doping for operating stability enhancement in organic field-effect transistors

The introduction of an inorganic/organic or organic/organic heterojunction in the pentacene-based organic field-effect transistors is demonstrated to be in favor of improving their operating stability. The heterojunction-induced p-type doping of pentacene is nondestructive, and it can be controlled by varying the adlayer thickness. The bias stress effects are compared at similar surface carrier density for the doped and undoped devices, and the current flow in the pentacene bulk is found to be more stable than that in the conducting channel close to the gate dielectric. In the initial stage of the bias stress characteristics, the carrier trapping associated with the gate dielectric is mainly responsible for the current instability. On the other hand, in the prolonged stage, the carrier trapping in the active layer may become dominant.     

Enhanced photocatalytic activity for hydrogen evolution from water by Zn0.5Cd0.5S/WS2 heterostructure

Zn_0.5Cd_0.5S/WS₂ nanocomposites with different amount of Zn_0.5Cd_0.5S solid solution deposited onto the surface of WS₂ were prepared by a one-step hydrothermal method with thioacetamide as sulfur source. Intimate heterojunctions between Zn_0.5Cd_0.5S and WS₂ were identified by TEM and HRTEM technologies in these materials. The chemical composition and valence of the elements were characterized by XPS experiments. The band energies were characterized by UV–vis diffusive reflectance spectroscopy (DRS). The photocatalytic properties of Zn_0.5Cd_0.5S/WS₂ nanocomposites for H₂ evolution from water in the presence of sacrificial reagents were tested. The highest rate of H₂ evolution achieved for these hybrid materials under visible-light irradiation (λ≥420 nm) is about 6 times higher than that of pristine Zn_0.5Cd_0.5S. The promoted catalytic activity of this hybrid material can be ascribed to the formation of the heterojunctions between Zn_0.5Cd_0.5S and WS₂, which enhanced the separation of the photogenerated hole-electron pairs.     

Bulk and interface properties of (Zn,Mg)O buffer layers sputtered in hydrogen-containing atmosphere

Sputtered (Zn,Mg)O buffer layers are one of the few promising options for completely dry and cadmium-free manufacturing of chalcopyrite-based solar cells. The performance of a heterojunction solar cell depends critically on the electrostatic charge contained in the interface. In chalcopyrite-based cells this charge can be influenced by redox treatments. In this work we have added hydrogen to the working gas when sputtering the buffer layer in an effort to optimize the interface charge. We report on the unexpectedly complex effects even at low hydrogen flow. They include a reduced deposition rate, significantly altered diffractograms and higher Mg/Zn-ratio in the film. X-ray photoelectron spectroscopy reveals the deposition of a thin layer of metallic zinc in the initial deposition stage which is not observed in the absence of hydrogen. Small hydrogen concentrations appear to be beneficial in terms of cell performance and reproducibility. However, higher concentrations typically cause a loss in blue response which indicates the formation of a homojunction buried within the chalcopyrite absorber.     

Enhanced triethylamine sensing performance of superfine NiO nanoparticles decoration by two-dimensional hexagonal boron nitride

The novel two-dimensional hexagonal boron nitride (h-BN) decorated nickel oxide (NiO) heterojunction was successfully synthesized by a facile solvothermal precipitation method combining with heat treatment. SEM and TEM analysis were used to corroborate the average size (~8 nm) and overall distribution of superfine NiO nanoparticles on h-BN. XRD, FT-IR and XPS characterization confirmed the configuration of highly crystallinity and p-n heterojunction as well as the presence of surface oxygen vacancy defects. Gas sensing test results revealed that the decoration of h-BN could significantly enhanced triethylamine (TEA) sensing property of NiO. The main contribution of such remarkable results lies in NiO nanoparticles that are close to Debye length scale were embedded on vacancy defects of functionalized h-BN nanosheets, which can optimize sensitivity and selectivity by taming two-dimensional (2D) interfacial electronic effects that strongly affect nonmetal-support interaction between grain boundaries. Meanwhile, the formation of p-n Schottky nanoscale heterojunction between NiO and h-BN can significantly enlarge resistance variation and efficiently promoted the adsorbed triethylamine molecules to oxidize into NO₂, H₂O, and CO₂. Our work highlights the important role of coupling functionalized h-BN in gas sensors, which can also provide a valuable avenue in boosting the sensing performance.     

Construction of Au/g-C3N4/ZnIn2S4 plasma photocatalyst heterojunction composite with 3D hierarchical microarchitecture for visible-light-driven hydrogen production

In this paper, a novel Au/g-C₃N₄/ZnIn₂S₄ plasma photocatalyst heterojunction composite with 3D hierarchical microarchitecture has been successfully constructed by integrating Au/g-C₃N₄ plasmonic photocatalyst composite with 3D ZnIn₂S₄ nanosheet through a simple hydrothermal process. The Au nanoparticles were firstly anchored on the surface of pristine g-C₃N₄ material to get Au/g-C₃N₄ plasmonic photocatalyst. Ascribing to the surface plasmon resonance of Au nanoparticles, the obtained Au/g-C₃N₄ plasmonic photocatalyst shows a significant improved photocatalytic activity toward hydrogen production from water with visible light response comparing with pristine g-C₃N₄. Further combining Au/g-C₃N₄ plasmonic photocatalyst with 3D ZnIn₂S₄ nanosheet to construct a heterojunction composite. Owing to the synergistic effect of the surface plasmon resonance of Au nanoparticles in Au/g-C₃N₄ and the heterojunction structure in the interface of Au/g-C₃N₄ and ZnIn₂S₄, the prepared Au/g-C₃N₄/ZnIn₂S₄ plasma photocatalyst heterojunction composite shows an excellent photocatalytic activity toward hydrogen production from water with visible light response, which is around 7.0 and 6.3 times higher than that of the pristine C₃N₄ and Znln₂S₄ nanosheet, respectively. The present work might provide some insights for exploring other efficient heterojunction photocatalysts with excellent properties.     

Construction of heterojunction and homojunction to improve the photocatalytic performance of ZnO quantum dots sensitization three-dimensional ordered hollow sphere ZrO2–TiO2 arrays

The synergy of multiple engineering strategies is an important method for constructing highly efficient photocatalytic materials. In this work, a novel three-dimensional ordered hollow spheres (3DOHs) array TiO2 was prepared by a simple colloidal template method. The crystal structure, morphology, composition, and surface physicochemical properties of the composites were characterized by various analytical methods. The results showed that the synergy between ZnO QDs and ZrO2 was used to construct a homojunction between anatase and rutile and the heterojunction between TiO2 and ZrO2, which promoted the formation of a double-junction structure in the composite. The characterization results illustrate the existence of homojunction and heterojunction in detail, and the results of photocatalytic experiments further confirm that the 3DOHs array 0.5 ZnO QDs@ZrO2–TiO2 with homojunction and heterojunction exhibits the best photodegradation and hydrogen evolution properties. The hydrogen evolution amount is 1530 times of P25, which reaches 2573 μmol g−1 in 8 h. In summary, this work provides some new insights into the construction of highly efficient photocatalytic materials and demonstrates the generality of constructing a stable three-dimensional ordered hollow sphere array structure.     

Hetero-epitaxy growth of cobalt oxide/nickel oxide nanowire arrays on alumina substrates for enhanced ethanol sensing characteristics

In this work, Co₃O₄ nanowire arrays are prepared in-situ on the flat alumina substrates via a simple hetero-epitaxial growth without adding seed layers. It is found that the density of Co₃O₄ nanowires can be regulated by varying the concentration of NH₄F that acts as substrate activation promoting the formation of nuclei on alumina substrate. In order to improve the gas-sensing performance, porous NiO nanosheets are anchored on the surface of Co₃O₄ nanowire arrays to form a novel heterostructure (Co₃O₄/NiO). Gas-sensing tests indicate a higher response value of these array composites towards ethanol (R_g/R_a = 4.26) than that of pristine Co₃O₄ nanowire arrays (R_g/R_a = 1.27) and NiO nanosheets (R_g/R_a = 1.89). The improved gas-sensing performances resulting from the special array structures and novel heterojunctions can provide abundant diffusion channels for gas molecules as well as a synergistic effect between Co₃O₄ and NiO.     

Type-II CeO2(111)/hBN vdW heterojunction for enhanced photocatalytic hydrogen evolution: A first principles study

Researches on environmentally friendly semiconductor photocatalysts for efficient photocatalytic hydrogen evolution have important practical significance. Here, using first-principles calculations, the CeO₂(111)/hBN heterojunction was conceived. The influence of the interface effect on the structural, electronic and optical properties of the heterojunction was investigated in detail. The band gap of the heterojunction is smaller than the two individual components and forms a type-II heterojunction, improving the photocatalytic activity. Furthermore, by doping two C atoms, the band gap of heterojunction was further narrowed. Both the oxidation and reduction potential of CeO₂(111)/hBN heterojunction meet the requirements of water splitting and has certain advantages over other photocatalysts in the ability for photocatalytic hydrogen evolution. The study revealed the possible mechanism of CeO₂(111) and hBN monolayers compositing to facilitate photocatalysis and hydrogen evolution ability, which may provide a possible reference direction for the practical design of more high-quality semiconductor photocatalysts.     

Insight into NiCo-based nanosheets modified MnS/Mn0·2Cd0·8S hybrids for enhanced visible-light photocatalytic H2 evolution

Bimetallic compounds nanocrystals exhibited great potential in catalysis due to the synergistic effects and encouraging performance. Herein, a series of NiCo-based nanosheets, including NiCo LDH/NiCo(OH)₂, NiCo, and NiCo₂O₄, have been developed to modify MnS/Mn_0·2Cd_0·8S (MMCS) nanoparticles for photocatalytic H₂ production under visible light (λ > 420 nm). The two-dimensional (2D) NiCo₂O₄ and NiCo were derived from the oxidation and reduction process of the as-prepared NiCo LDH/NiCo(OH)₂ nanosheets, respectively. MMCS nanoparticles were prepared using a one-pot solvothermal method and then integrated into three different NiCo-based nanosheets through a simple hybridization approach. Compared to pure MMCS, the resultant NiCo-based nanosheets/MMCS hybrids show dramatically improved visible-light photocatalytic activities. Moreover, among the three types of composites, NiCo₂O₄-MMCS (7%NiCo₂O₄-MMCS) displays the highest H₂ production rate of 3.31 mmol g^?1 h^?1 with the apparent quantum efficiency of 6.42% at 420 nm, approximately 22 and 5 times that of pure MMCS (0.15 mmol g^?1 h^?1) and Pt/MMCS (0.67 mmol g^?1 h^?1), respectively. The remarkably enhanced photocatalytic activities of the NiCo LDH/NiCo(OH)₂-MMCS, NiCo-MMCS, and NiCo₂O₄-MMCS are mainly ascribed to the formed type-II, Schottky, and p-n heterojunctions, respectively, which efficiently boost photogenerated charge carrier separation and migration. In this paper, we intensively investigate the roles of three different NiCo-based nanosheets in the Mn_xCd_1-xS-based system. This work provides an effective strategy to design and construct the innovative 2D bimetallic compounds-based catalysts for high-efficiency photocatalytic H₂ production.