Zaiwang Zhao

In Situ Construction of g-C3N4/g-C3N4 Metal-Free Heterojunction for Enhanced Visible-Light Photocatalysis

The photocatalytic performance of the star photocatalyst g-C3N4 was restricted by the low efficiency because of the fast charge recombination. The present work developed a facile in situ method to construct g-C3N4/g-C3N4 metal-free isotype heterojunction with molecular composite precursors with the aim to greatly promote the charge separation. Considering the fact that g-C3N4 samples prepared from urea and thiourea separately have different band structure, the molecular composite precursors of urea and thiourea were treated simultaneously under the same thermal conditions, in situ creating a novel layered g-C3N4/g-C3N4 metal-free heterojunction (g-g CN heterojunction). This synthesis method is facile, economic, and environmentally benign using easily available earth-abundant green precursors. The confirmation of isotype g-g CN heterojunction was based on XRD, HRTEM, valence band XPS, ns-level PL, photocurrent, and EIS measurement. Upon visible-light irradiation, the photogenerated electrons transfer from g-C3N4 (thiourea) to g-C3N4 (urea) driven by the conduction band offset of 0.10 eV, whereas the photogenerated holes transfer from g-C3N4 (urea) to g-C3N4 (thiourea) driven by the valence band offset of 0.40 eV. The potential difference between the two g-C3N4 components in the heterojunction is the main driving force for efficient charge separation and transfer. For the removal of NO in air, the g-g CN heterojunction exhibited significantly enhanced visible light photocatalytic activity over g-C3N4 alone and physical mixture of g-C3N4 samples. The enhanced photocatalytic performance of g-g CN isotype heterojunction can be directly ascribed to efficient charge separation and transfer across the heterojunction interface as well as prolonged lifetime of charge carriers. This work demonstrated that rational design and construction of isotype heterojunction could open up a new avenue for the development of new efficient visible-light photocatalysts.

An Advanced Semimetal-Organic Bi Spheres-g-C3N4 Nanohybrid with SPR-Enhanced Visible-Light Photocatalytic Performance for NO Purification.

To achieve efficient photocatalytic air purification, we constructed an advanced semimetal-organic Bi spheres-g-C3N4 nanohybrid through the in-situ growth of Bi nanospheres on g-C3N4 nanosheets. This Bi-g-C3N4 compound exhibited an exceptionally high and stable visible-light photocatalytic performance for NO removal due to the surface plasmon resonance (SPR) endowed by Bi metal. The SPR property of Bi could conspicuously enhance the visible-light harvesting and the charge separation. The electromagnetic field distribution of Bi spheres involving SPR effect was simulated and reaches its maximum in close proximity to the Bi particle surface. When the Bi metal content was controlled at 25%, the corresponding Bi-g-C3N4 displayed outstanding photocatalytic capability and transcended those of other visible-light photocatalysts. The Bi-g-C3N4 exhibited a high structural stability under repeated photocatalytic runs. A new visible-light-induced SPR-based photocatalysis mechanism with Bi-g-C3N4 was proposed on the basis of the DMPO-ESR spin-trapping. The photoinduced electrons could transfer from g-C3N4 to the Bi metal, as revealed with time-resolved fluorescence spectra. The function of Bi semimetal as a plasmonic cocatalyst for boosting visible light photocatalysis was similar to that of noble metals, which demonstrated a great potential of utilizing the economically feasible Bi element as a substitute for noble metals for the advancement of photocatalysis efficiency.

Growth of BiOBr nanosheets on C3N4 nanosheets to construct two-dimensional nanojunctions with enhanced photoreactivity for NO removal

The development of approaches to effectively separate the photo-induced charge carriers is a key strategy to promote the photocatalytic activity of semiconductor photocatalysts. This work represents the construction of novel two-dimensional (2D) BiOBr/C3N4 nanojunctions by the growth of BiOBr nanosheets on the surface of C3N4 nanosheets at room temperature. The samples were characterized by XRD, FT-IR, TEM, UV-vis DRS and PL. The photocatalytic activity of the samples was evaluated by the removal of NO in air under visible light irradiation. The results indicated that electronic coupling took place between the {001} plane of BiOBr and {002} plane of C3N4. The BiOBr/C3N4 nanojunctions exhibited enhanced visible light photocatalytic activity compared with pure BiOBr and C3N4. The enhanced photoactivity can be mainly ascribed to the efficient separation and transportation of photo-induced electrons and holes due to the well-coupled crystal planes and well-matched band structures. The present work could provide new insights into the design and construction of 2D nanojunctions with well-matched crystal planes and band structures for efficient visible light photocatalysis.

Mass-Controlled Direct Synthesis of Graphene-like Carbon Nitride Nanosheets with Exceptional High Visible Light Activity. Less is Better.

In the present work, it is very surprising to find that the precursors mass, a long overlooked factor for synthesis of 2D g-C3N4, exerts unexpected impact on g-C3N4 fabrication. The nanoarchitecture and photocatalytic capability of g-C3N4 can be well-tailored only by altering the precursors mass. As thiourea mass decreases, thin g-C3N4 nanosheets with higher surface area, elevated conduction band position and enhanced photocatalytic capability was triumphantly achieved. The optimized 2D g-C3N4 (CN-2T) exhibited exceptional high photocatalytic performance with a NO removal ratio of 48.3%, superior to that of BiOBr (21.3%), (BiO)2CO3 (18.6%) and Au/(BiO)2CO3 (33.8%). The excellent activity of CN-2T can be ascribed to the co-contribution of enlarged surface areas, strengthened electron-hole separation efficiency, enhanced electrons reduction capability and prolonged charge carriers lifetime. The DMPO ESR-spin trapping and hole trapping results demonstrate that the superoxide radicals (•O2−) and photogenerated holes are the main reactive species, while hydroxyl radicals (•OH) play a minor role in photocatalysis reaction. By monitoring the reaction intermediate and active species, the reaction mechanism for photocatalytic oxidation of NO by g-C3N4 was proposed. This strategy is novel and facile, which could stimulate numerous attentions in development of high-performance g-C3N4 based functional nanomaterials.

Enhanced visible-light photo-oxidation of nitric oxide using bismuth-coupled graphitic carbon nitride composite heterostructures

Abstract Pure bismuth (Bi) metal-modified graphitic carbon nitride (g-C3N4) composites (Bi-CN) with a pomegranate-like structure were prepared by an in situ method. The Bi-CN composites were used as photocatalysts for the oxidation of nitric oxide (NO) under visible-light irradiation. The inclusion of pure Bi metal in the g-C3N4 layers markedly improved the light absorption of the Bi-CN composites from the ultraviolet to the near-infrared region because of the typical surface plasmon resonance of Bi metal. The separation and transfer of photogenerated charge carriers were greatly accelerated by the presence of built-in Mott–Schottky effects at the interface between Bi metal and g-C3N4. As a result, the Bi-CN composite photocatalysts exhibited considerably enhanced efficiency in the photocatalytic removal of NO compared with that of Bi metal or g-C3N4 alone. The pomegranate-like structure of the Bi-CN composites and an explanation for their improved photocatalytic activity were proposed. This work not only provides a design for highly efficient g-C3N4-based photocatalysts through modification with Bi metal, but also offers new insights into the mechanistic understanding of g-C3N4-based photocatalysis.

溶剂辅助合成介孔g-C 3 N 4 及其增强的可见光催化去除NO性能研究

Abstract Graphitic carbon nitride (g-C3N4) with efficient photocatalytic activity was synthesized through thermal polymerization of thiourea with the addition of water (CN-W) or ethanol (CN-E) at 550 °C for 2 h. The physicochemical properties of the g-C3N4 were investigated by X-ray diffraction, transmission electron microscopy, ultraviolet-visible spectroscopy, photoluminescence spectroscopy, diffuse-reflection spectroscopy, BET and BJH surface area characterization, and elemental analysis. The carbon content was found to have self-doped into the g-C3N4 matrix during the thermal polymerization of thiourea and ethanol. CN-W and CN-E showed considerably enhanced visible-light photocatalytic activity, with NO removal percentages of 37.2% and 48.3%, respectively. Compared with pure g-C3N4, both the short and long lifetimes of the charge carriers in CN-W and CN-E were found to be prolonged. The mechanism of improved visible-light photocatalytic activity was deduced. The present work may provide a facile route to optimize the microstructure of g-C3N4 photocatalysts for high-performance environmental and energy applications.

Template synthesis of carbon self-doped g-C3N4 with enhanced visible to near-infrared absorption and photocatalytic performance

In order to fully address the low surface area, fast charge recombination and limited visible light absorption of pristine g-C3N4, we present a novel and straightforward strategy towards the synthesis of carbon self-doped g-C3N4 by using porous carbon foam as a soft-template. The C-doped g-C3N4 displayed a high BET surface area (65 m2 g−1), extended absorption ranging from visible light to near-infrared (800 nm) and accelerated electron–hole separation. The role of carbon doping on the band structure and electrical conductivity was revealed. The optimized C-doped g-C3N4 demonstrated an exceptionally high photocatalytic performance towards the purification of NO in air, and exceeded other reported visible-light photocatalysts, such as nonmetal-doped TiO2, BiOBr, (BiO)2CO3 and porous g-C3N4. This decent C-doped g-C3N4 photocatalyst also showed good photocatalytic stability for NO removal. The present work could provide new insights into the modification and understanding of self-doped semiconductor photocatalysts.

Ammonia induced formation of N-doped (BiO)2CO3 hierarchical microspheres: the effect of hydrothermal temperature on the morphology and photocatalytic activity

Three types of N-doped (BiO)2CO3 hierarchical microspheres composed of 2D nanosheets were fabricated by a one-pot template-free hydrothermal method from bismuth citrate and an ammonia solution under different hydrothermal temperatures. The as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, nitrogen adsorption–desorption isotherms, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and UV-visible diffuse reflectance spectroscopy. Here, the ammonia worked as a nitrogen resource, doping (BiO)2CO3, and a solvent hydrolyzing the bismuth citrate. The temperature was found to influence the thickness of the nanosheets and the morphology of the N-doped (BiO)2CO3 microspheres. The fascinating morphology of N-doped (BiO)2CO3, including rose-like, hydrangea flower-like and peony flower-like microspheres, can be controllably fabricated by adjusting the hydrothermal temperature. The photocatalytic activities of the as-prepared samples were evaluated towards the removal of NO under visible-light irradiation. Compared to pure (BiO)2CO3, the N-doped (BiO)2CO3 hierarchical microspheres showed enhanced photocatalytic activity. In addition, the hydrangea flower-like and the peony flower-like N-doped (BiO)2CO3 microspheres displayed admirable photocatalytic activity. Finally, a possible formation mechanism of the various morphologies of N-doped (BiO)2CO3 mediated by the reaction temperature was proposed. This work could provide new insights into the controlled synthesis of photocatalytic nano/microstructures for potential environmental applications.

Plasmonic Bi metal as cocatalyst and photocatalyst: The case of Bi/(BiO)2CO3 and Bi particles

Semimetal bismuth with plasmonic properties has triggered increased interests. In this work, a facile strategy was developed to synthesize the Bi/(BiO)2CO3 (Bi-BOC) nanocomposites and Bi elemental photocatalysts. The Bi nanoparticles were produced via the insitu reduction of (BiO)2CO3 by NaBH4. The catalysts were utilized for the photocatalytic NO removal under visible light and UV illumination. Significantly, the photocatalytic capability of the Bi-BOC was highly enhanced with an unprecedented NO removal of 63.6%. The Bi metal demonstrated a direct plasmonic photocatalytic NO removal ratio of 53.6% under UV irradiation. The significantly enhanced photocatalytic capability of Bi-BOC can be ascribed to the synergistic effects of the SPR effect, enhanced visible-light-harvesting and the efficient electron-hole separation induced by Bi nanoparticles. The Bi nanoparticles can perform as a non-noble metal-based plasmonic cocatalyst for advancing photocatalytic ability. The mechanism of photocatalytic NO oxidation was proposed and compared under both visible light and UV illumination. Furthermore, the Bi-BOC photocatalysts showed good photochemical stability under repeated tests. This work could not only offer new insights into in-situ fine-tune reduction strategy for Bi-based photocatalysts, but also proves the potentials of utilizing low cost Bi cocatalysts as a substitute for noble metals to improve other photocatalysts.

Noble metal-free Bi nanoparticles supported on TiO2 with plasmon-enhanced visible light photocatalytic air purification

Semimetal bismuth (Bi) is an emerging non-noble metal-based plasmonic metal, which has demonstrated exceptional behavior as a unique plasmonic photocatalyst/cocatalyst. In the present work, Bi nanoparticles were uniformly deposited on the well-known TiO2 particles (Degussa, P25) with mixed phases of anatase and rutile by a facile eco-friendly synthesis at room temperature. The Bi-deposited TiO2 nanocomposites demonstrated highly enhanced photocatalytic performance for removal of ppb-level NO in air under visible-light irradiation (λ > 420 nm). The improved photocatalytic capability was found to be crucially dependent on the catalyst architecture: Bi nanoparticles with a diameter (dBi) of 5–8 nm deposited on the surface of TiO2 particles acted as active sites for visible-light-driven electron and hole separation. The enhanced charge separation was well supported by photoluminescence, photocurrent generation and Bode-phase spectra. Significantly, the exceptionally high visible-light photocatalytic capability of the optimized Bi–Ti-50 sample (the mass ratio of Bi to TiO2 is 50%) was also superior to that of universally known non-metal-doped TiO2. The photocatalysis was enhanced through SPR-mediated activation of the Bi particles by visible light followed by consecutive electron transfer in Bi/rutile/anatase interfaces, as supported by the action spectra. The electrons produced from the plasmonic activation of Bi particles could transfer to the conduction band of rutile and then to adjacent anatase TiO2 because of the high potential difference between Bi and rutile TiO2. Also, the free electrons could transfer from Bi to the conduction band of anatase and then to rutile TiO2 owing to the high mass ratio of anatase phase in P25 resulting in the direct contact of the Bi nanoparticles and anatase TiO2. A new photocatalysis mechanism of Bi-deposited TiO2 nanocomposites was proposed on the basis of active species trapping. The catalyst architecture elucidated here for promoted plasmonic photocatalysis would be beneficial for the design and development of more effective visible-light-driven photocatalysts for environmental remediation and could open a new avenue for utilization of low-cost Bi nanoparticles as a substitute for noble metals to promote the utilization efficiency of solar energy.