This study was mainly aimed at designing molecular p–n heterojunctions on the surface of N-doped TiO2 for visible-light hydrogen evolution. A series of NiO/N-TiO2 samples were prepared via NH3-postnitridation of nickel oxide-modified TiO2. H2 production from ethanol/water solution was utilized as a model reaction to evaluate the photocatalytic properties of the catalysts. Compared to N-TiO2, a 90-fold-enhanced H2 evolution rate was achieved over the optimal NiO/N-TiO2 catalyst with a 0.5 wt% Ni content. Detailed characterizations clearly showed that loading NiO via wet impregnation leads to high dispersion of Ni2+ species on the surface oxygen vacancy (Vo) sites of the anatase TiO2 nanoparticles, predominantly presenting mononuclear Ti–O–Ni heteroatomic clusters on the surface of TiO2 in the case of low Ni content. These surface heteroatomic clusters can speed up the transfer and separation of photogenerated carriers of N-TiO2. It can thus be established that the molecular Ti–O–Ni heterojunctions are the main contributors to the synergistic enhancement of H2 evolution, whereas NiO nanoclusters are not responsible for the photoactivity. Only one of the N species, N–Ti–Vo, was active and effective for this cooperation with the hydrogen-releasing Ni sites, which can induce ca. 20-fold improvement of hydrogen production over NiO/TiO2 under visible light irradiation.

For a long time, there has been global concern over the environment and energy problems. Recently, the problems, which have brought about serious effect on the global living condition, have been in the “spotlight” and given impetus to the universal’s efforts to head for the same direction: stem the worst warming and strive for the renewable energy source. Hydrogen peroxide (H2O2) is undoubtedly a good choice, which holds the promise as a clean, efficient, safe and transferrable energy carrier. Octahedral coordination polymer, Cd3(C3N3S3)2, was found to be a robust photocatalyst for H2O2 generation under visible light irradiation. To further improve the H2O2 generation efficiency, adhering the octahedron to reduced graphene (rGO) was applied as the strategy herein. The study shows that by adhering Cd3(C3N3S3)2 to rGO, the formation of H2O2 is 2.5-fold enhanced and its deformation is concurrently suppressed. This work not only demonstrates the effectiveness of adhering Cd3(C3N3S3)2 polymer to rGO for the improvement of the polymer’s photocatalytic performance, but also proposes a general way for the fabrication of graphene/coordination compound hybrids for maximizing their synergy.Download high-res image (202KB)Download full-size image

NO2 sensor has attracted extensive attention due to its important application in environment monitor. Conventional NO2 sensors based on the yttria-stabilized zirconia (YSZ) electrolyte possess fast response, high sensitivity and good stability, whereas its ionic conductivity decreases significantly at low- and intermediate-temperature, limiting the practical application in motor vehicles. In this work, a pyrochlore-phase A2B2O7 solid electrolyte, Pr2Zr2−xCexO7+δ (PZC), was applied for the first time to construct the amperometric-type NO2 sensor. Ce incorporated significantly improved the sensing performance of the PZC NO2 sensor with NiO as the sensing electrode. The effect of Ce-doped concentration and operating temperature on the sensitivity, selectivity, stability, response and recovery characteristics were investigated in detail. The results showed that the optimal sensor based on the Pr2Zr1.9Ce0.1O7+δ substrate gave high sensitivity, excellent selectivity and quick response-recovery behavior to NO2 gas. The gas-sensing mechanism was also discussed. The PZC sensors are well established effective for sensing NO2 at mild-temperature working window of 500–700 °C, and thus exhibit the promising application in motor vehicles.

This paper mainly focuses on the synergistic effect of Sn and N dopants to enhance the photocatalytic performance of anatase TiO2 under visible light or simulated solar light irradiation. The Sn and N co-doped TiO2 (SNT-x) photocatalysts were successfully prepared by the facile sol–gel method and the post-nitridation route in the temperature range of 400–550 °C. All the as-prepared samples were characterized in detail by X-ray diffraction, UV-vis diffuse reflectance spectroscopy, transmission electron microscopy, X-ray photoelectron and electron spin resonance spectroscopy and photoelectrochemical measurements. The characterization results reveal that the co-incorporation of Sn and N atoms remarkably modifies the electronic structure of TiO2, which gives rise to a prominent separation of photogenerated charge carriers and more efficient interfacial charge-transfer reactions in a photocatalytic process. The enhanced photocatalytic activity is attributed to the intensified active oxygen species including hydroxyl radicals (˙OH) and superoxide anion radicals (O2˙−) for degradation of organic pollutants. And the result of photocatalytic hydrogen production further confirms the existence of the synergistic effect in the SNT-x samples, because they exhibit higher photocatalytic activity than the sum of N/TiO2 and Sn/TiO2. This work provides a paradigm to consolidate the understanding of the synergistic effect of metal and non-metal co-doped TiO2 in domains of photocatalysis and photoelectrochemistry.

The alkaline earth metal stannates MSnO3 (M = Ca, Sr, and Ba) photocatalysts with different morphologies are successfully prepared by hydrothermal method and their photocatalytic activities are evaluated by photocatalytic reforming of ethanol/water solution to hydrogen. All of the as-prepared samples are characterized in detail by X-ray diffraction (XRD), ultraviolet-visible diffuse reflectance (UV-vis DRS), N2 physical adsorption, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) before and after the photocatalytic hydrogen production to illustrate the effect of the photoreaction on the surface structure, and thus to make the photocatalysis clear. The results reveal that the greatest photocorrosion occurs on the surface of CaSnO3, SrSnO3, and BaSnO3 samples. And the formed surface species have great influence on H2 production from ethanol/water solution. The photocatalytic reaction can transform CaSnO3 into CaSn(OH)6, producing CaSn(OH)6/CaSnO3 composite where the photogenerated charges can be more efficiently separated and transferred, consequently enhancing the hydrogen evolution. As for SrSnO3, the photocorrosion can cause the formation of Sn2+ self-doped SrSnO3 nanoparticles on the surface to increase the hydrogen production efficiency. Unlike CaSnO3 and SrSnO3, the photocatalytic activity of BaSnO3 is gradually decreased due to the conversion of BaSnO3 to BaCO3. As expected, the H2 evolution rate decreased in the order of CaSnO3 > SrSnO3 > BaSnO3 under UV light irradiation. It is well demonstrated in the present work that CaSnO3 is a potential photocatalyst for the photocatalytic reforming of ethanol/water solution to hydrogen.

A layered nanoarchitecture composed of photoactive MOFs of UiO-66-NH2 and graphene was facilely fabricated herein by an innovative strategy, which utilizes a noncovalent methodology for graphene functionalization combined with in situ self-assembling and a solvothermal synthesis technique. The fabricated hybrids were characterized and evaluated in detail by the selective photocatalytic oxidation of benzyl alcohol under visible light. The hybrid displayed improved efficiency with high selectivity, compared with the parent MOF. The characterization results clearly demonstrate that this originates from the sandwich-like hierarchical nanoarchitecture formed by the compact interaction between UiO-66-NH2 and graphene via the adopted mediator. The synthesis strategy was also proven effective in building the rGO/NH2-MIL-125(Ti) hierarchical nanoarchitecture. Thus, this work offers a general strategy for constructing MOF/graphene sandwich heterostructures, which have great potential in the fields of electronics, optics, optoelectronics, and photoconversion.

The role of polymers in artificial photosystems has been studied in detail. The photosystems were composed of tris(2,2′-bipyridyl) ruthenium(II) chloride as a photosensitizer (PS), colloidal Pt stabilized by polymer as a hydrogen-evolving catalyst and sodium ascorbate as an electron donor, without the addition of a traditional molecular electron mediator. Comprehensive insights into the production of hydrogen on irradiation with visible light were achieved. Several polymers, including neutral polyvinyl pyrrolidone, anionic poly(sodium 4-styrene sulfonate) and poly(acrylic acid) not only stabilized the nanoparticles, but were also effective in the production of hydrogen. Under the optimum conditions, an outstanding apparent quantum efficiency of 12.8% for the evolution of hydrogen was achieved. The formation of self-assembled and spatially separated donor–acceptor complexes via the non-covalent intermolecular interaction between PS and the polymer–Pt was pivotal in the efficient conversion of solar energy to hydrogen fuel. Important details of the photo-induced electron and energy transfer processes in the self-assembled artificial photosystems were determined by nanosecond transient absorption spectrometry and time-resolved fluorescence spectrometry. The initial step in the photo-catalytic production of hydrogen was a reductive quenching of the triplet excited state of the PS by sodium ascorbate, leading to a reduced form of PS, which could then be quickly quenched by the polymer. The rate-determining step was the electron transfer from PS to the catalyst via the polymer bridge.

Metal-free carbon-based hybrids composed of reduced graphene oxide (rGO) and C3N3S3 polymers with a layered “sandwich” structure were fabricated by an in situ two-step polymerization strategy and used as visible light photocatalysts for selective aerobic oxygenation of benzylic alcohols to the corresponding aldehydes, at good efficiency and high selectivity. This work shows the great potential and prospective application of the metal-free C3N3S3 polymer in the photocatalyzed synthesis of fine chemicals.Keywords: C3N3S3 polymer; graphene; hybrids; metal-free photocatalysis; selective oxidation

A variety of ternary nanoheterostructures composed of Pt nanoparticles (NPs), SnOx species, and anatase TiO2 are designed elaborately to explore the effect of interfacial electron transfer on photocatalytic H2 evolution from a biofuel–water solution. Among numerous factors controlling the H2 evolution, the significance of Pt sites for the H2 evolution is highlighted by tuning the loading procedure of Pt NPs and SnOx species over TiO2. A synergistic enhancement of H2 evolution can be achieved over the Pt/SnOx/TiO2 heterostructures formed by anchoring Pt NPs at atomically-isolated Sn-oxo sites, whereas the Pt/TiO2/SnOx counterparts prepared by grafting single-site Sn-oxo species on Pt/TiO2 show a marked decrease in the rate of H2 evolution. The characterization results clearly reveal that the synergy of Pt NPs and SnOx species originates from the vectorial electron transfer of TiO2 → SnOx → Pt occurring on the former, while the latter results from the competitive electron transfer from TiO2 to SnOx and to Pt NPs.

This work shows the efficient degradation of benzene over robust Sn2+-doped TiO2 nanoparticles prepared by a facile sol–gel route under visible light irradiation. The structure, optical properties and chemical states of Sn species incorporated into anatase TiO2 were carefully characterized by X-ray diffraction, transmission electron microscopy, Raman, UV-vis diffuse reflectance, X-ray photoelectron, X-ray absorption, and electron spin resonance (ESR) spectroscopy. The maximal conversion rate of benzene achieved is up to 27% over the Sn/TiO2 with a Ti/Sn atomic ratio of 40:1 and remains constant for a cyclic run of six days, indicating the high photo-stability for the decomposition of benzene. The characterization results reveal that the Sn2+-doping narrows the band gap energy of anatase TiO2, leading to a visible-light response. The photocatalytic degradation pathway of benzene was proposed based on the results of ESR and Fourier transform infrared spectra. These results offer a full comprehension of the visible light photocatalysis of Sn2+-doped TiO2 for degradation of volatile organic pollutants.

This work demonstrates the molecular engineering of active sites on a graphene scaffold. It was found that the N-doped graphene nanosheets prepared by a high-temperature nitridation procedure represent a novel chemical function of efficiently catalyzing aerobic alcohol oxidation. Among three types of nitrogen species doped into the graphene lattice—pyridinic N, pyrrolic N, and graphitic N—the graphitic sp2 N species were established to be catalytically active centers for the aerobic oxidation reaction based on good linear correlation with the activity results. Kinetic analysis showed that the N-doped graphene-catalyzed aerobic alcohol oxidation proceeds via a Langmuir–Hinshelwood pathway and has moderate activation energy (56.1 ± 3.5 kJ·mol–1 for the benzyl alcohol oxidation) close to that (51.4 kJ·mol–1) proceeding on the catalyst Ru/Al2O3 reported in literature. An adduct mechanism was proposed to be different remarkably from that occurring on the noble metal catalyst. The possible formation of a sp2 N–O2 adduct transition state, which can oxidize alcohols directly to aldehydes without any byproduct, including H2O2 and carboxylic acids, may be a key element step. Our results advance graphene chemistry and open a window to study the graphitic sp2 nitrogen catalysis.Keywords: alcohols; aldehydes; graphene; nitrogen doping; nonmetal catalysis; selective oxidation;

An amine-functionalized zirconium metal–organic framework (MOF) was used as a visible-light photocatalyst for selective aerobic oxygenation of various organic compounds including alcohols, olefins and cyclic alkanes, at high efficiency and high selectivity. This study shows the great potential for design and application of MOF-based photocatalysts.

A novel organic semiconductor photocatalyst mimicking natural light-harvesting antenna complexes in photosynthetic organisms, a disulfide (–S–S–) bridged C3N3S3polymer, was designed and developed to generate hydrogen from water under visible light irradiation. The artificial conjugated polymer shows high H2-producing activity from the half-reaction of water splitting without the aid of a sacrificial electron donor. The H2-producing efficiency and photo-stability of the catalyst could be improved greatly using Ru and single-wall carbon nanotubes as cocatalysts or by adding a sacrificial donor. The results represent a potential and prospective application of the C3N3S3polymer in solar energy conversion and offer significant guidance to develop more stable and efficient photocatalytic systems based on organic semiconductors.

This work focuses on the nature of enhanced H2 production over SnO2/TiO2 composite photocatalysts. Three kinds of Sn-modified TiO2 materials were prepared by different methods. Photocatalytic H2 production from methanol/water solution as a model reaction was used to evaluate the photocatalytic properties of the materials. The chemical states of Sn were characterized in detail by transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption fine structure spectroscopy. The results reveal a Sn nuclearity-dependent enhancement of H2 production over Sn/TiO2 photocatalysts. According to the characterization results, it was well established that the number of TiIV–O–SnIV linkages formed at the TiO2–SnO2 interface determines the enhancement amplitude of H2 production. The photogenerated electrons of TiO2 across the interfacial linkages are transferred into the mononuclear Sn moieties grafted on the (1 0 1) and (0 0 1) planes, where H2 is presumably released. An interatomic electron transfer pathway for accelerating photocatalytic H2 production over Sn/TiO2 catalysts was proposed based on this work.Graphical abstractInterfacial Ti–O–Sn linkages are proposed as the key bridge to accelerate electron transfer from TiO2 to the mononuclear Sn moiety, where H2 is released. The enhanced H2 production of photocatalytic methanol reforming on tin-modified anatase originates from efficient charge separation at the molecular junction.Download high-res image (85KB)Download full-size imageHighlights► Preparation of single-site tin-grafted anatase TiO2 photocatalysts by a SOMC route. ► A nuclearity-depended enhancement of H2 production over Sn/TiO2 photocatalysts. ► The molecular junction consists of the interfacial TiIV–O–SnIV linkages. ► The molecular junction controls the enhancement amplitude of H2 production. ► H2 release appears at the Sn moieties trapping photogenerated electrons of TiO2.

This paper focuses on the photoactive centers of nitrogen-doped TiO2 visible light photocatalyst. A series of N-doped TiO2 materials were prepared by a post-nitridation route at the temperature range of 400–800 °C. The photocatalytic oxidation of acetone as a model reaction was used to evaluate the photocatalytic properties of the materials. The chemical states of doped nitrogen species were characterized by near-edge X-ray absorption fine structure, X-ray photoelectron, and electron paramagnetic resonance spectroscopies. The results reveal that four types of N species exist alone or together in TiO2 depending on nitridation temperature. The photoactive centers of the materials are a diamagnetic [O–Ti4+–N3−–Ti4+–V0-] cluster containing an oxygen vacancy and a nitrogen anion. The visible light photocatalysis of N-doped TiO2 is proposed to be initiated by an excited state of the surface [Ti4+–N3−] unit.The nitridation of TiO2 by NH3-generated substitutional N species with a diamagnetic [O–Ti4+–N3−–Ti4+] core stabilized by a neighboring oxygen vacancy that can work under visible light.Download high-res image (106KB)Download full-size image

A layered nanoarchitecture composed of photoactive MOFs of UiO-66-NH2 and graphene was facilely fabricated herein by an innovative strategy, which utilizes a noncovalent methodology for graphene functionalization combined with in situ self-assembling and a solvothermal synthesis technique. The fabricated hybrids were characterized and evaluated in detail by the selective photocatalytic oxidation of benzyl alcohol under visible light. The hybrid displayed improved efficiency with high selectivity, compared with the parent MOF. The characterization results clearly demonstrate that this originates from the sandwich-like hierarchical nanoarchitecture formed by the compact interaction between UiO-66-NH2 and graphene via the adopted mediator. The synthesis strategy was also proven effective in building the rGO/NH2-MIL-125(Ti) hierarchical nanoarchitecture. Thus, this work offers a general strategy for constructing MOF/graphene sandwich heterostructures, which have great potential in the fields of electronics, optics, optoelectronics, and photoconversion.

A novel organic semiconductor photocatalyst mimicking natural light-harvesting antenna complexes in photosynthetic organisms, a disulfide (–S–S–) bridged C3N3S3polymer, was designed and developed to generate hydrogen from water under visible light irradiation. The artificial conjugated polymer shows high H2-producing activity from the half-reaction of water splitting without the aid of a sacrificial electron donor. The H2-producing efficiency and photo-stability of the catalyst could be improved greatly using Ru and single-wall carbon nanotubes as cocatalysts or by adding a sacrificial donor. The results represent a potential and prospective application of the C3N3S3polymer in solar energy conversion and offer significant guidance to develop more stable and efficient photocatalytic systems based on organic semiconductors.

This paper mainly focuses on the synergistic effect of Sn and N dopants to enhance the photocatalytic performance of anatase TiO2 under visible light or simulated solar light irradiation. The Sn and N co-doped TiO2 (SNT-x) photocatalysts were successfully prepared by the facile sol–gel method and the post-nitridation route in the temperature range of 400–550 °C. All the as-prepared samples were characterized in detail by X-ray diffraction, UV-vis diffuse reflectance spectroscopy, transmission electron microscopy, X-ray photoelectron and electron spin resonance spectroscopy and photoelectrochemical measurements. The characterization results reveal that the co-incorporation of Sn and N atoms remarkably modifies the electronic structure of TiO2, which gives rise to a prominent separation of photogenerated charge carriers and more efficient interfacial charge-transfer reactions in a photocatalytic process. The enhanced photocatalytic activity is attributed to the intensified active oxygen species including hydroxyl radicals (˙OH) and superoxide anion radicals (O2˙−) for degradation of organic pollutants. And the result of photocatalytic hydrogen production further confirms the existence of the synergistic effect in the SNT-x samples, because they exhibit higher photocatalytic activity than the sum of N/TiO2 and Sn/TiO2. This work provides a paradigm to consolidate the understanding of the synergistic effect of metal and non-metal co-doped TiO2 in domains of photocatalysis and photoelectrochemistry.

This study was mainly aimed at designing molecular p–n heterojunctions on the surface of N-doped TiO2 for visible-light hydrogen evolution. A series of NiO/N-TiO2 samples were prepared via NH3-postnitridation of nickel oxide-modified TiO2. H2 production from ethanol/water solution was utilized as a model reaction to evaluate the photocatalytic properties of the catalysts. Compared to N-TiO2, a 90-fold-enhanced H2 evolution rate was achieved over the optimal NiO/N-TiO2 catalyst with a 0.5 wt% Ni content. Detailed characterizations clearly showed that loading NiO via wet impregnation leads to high dispersion of Ni2+ species on the surface oxygen vacancy (Vo) sites of the anatase TiO2 nanoparticles, predominantly presenting mononuclear Ti–O–Ni heteroatomic clusters on the surface of TiO2 in the case of low Ni content. These surface heteroatomic clusters can speed up the transfer and separation of photogenerated carriers of N-TiO2. It can thus be established that the molecular Ti–O–Ni heterojunctions are the main contributors to the synergistic enhancement of H2 evolution, whereas NiO nanoclusters are not responsible for the photoactivity. Only one of the N species, N–Ti–Vo, was active and effective for this cooperation with the hydrogen-releasing Ni sites, which can induce ca. 20-fold improvement of hydrogen production over NiO/TiO2 under visible light irradiation.

The role of polymers in artificial photosystems has been studied in detail. The photosystems were composed of tris(2,2′-bipyridyl) ruthenium(II) chloride as a photosensitizer (PS), colloidal Pt stabilized by polymer as a hydrogen-evolving catalyst and sodium ascorbate as an electron donor, without the addition of a traditional molecular electron mediator. Comprehensive insights into the production of hydrogen on irradiation with visible light were achieved. Several polymers, including neutral polyvinyl pyrrolidone, anionic poly(sodium 4-styrene sulfonate) and poly(acrylic acid) not only stabilized the nanoparticles, but were also effective in the production of hydrogen. Under the optimum conditions, an outstanding apparent quantum efficiency of 12.8% for the evolution of hydrogen was achieved. The formation of self-assembled and spatially separated donor–acceptor complexes via the non-covalent intermolecular interaction between PS and the polymer–Pt was pivotal in the efficient conversion of solar energy to hydrogen fuel. Important details of the photo-induced electron and energy transfer processes in the self-assembled artificial photosystems were determined by nanosecond transient absorption spectrometry and time-resolved fluorescence spectrometry. The initial step in the photo-catalytic production of hydrogen was a reductive quenching of the triplet excited state of the PS by sodium ascorbate, leading to a reduced form of PS, which could then be quickly quenched by the polymer. The rate-determining step was the electron transfer from PS to the catalyst via the polymer bridge.

An amine-functionalized zirconium metal–organic framework (MOF) was used as a visible-light photocatalyst for selective aerobic oxygenation of various organic compounds including alcohols, olefins and cyclic alkanes, at high efficiency and high selectivity. This study shows the great potential for design and application of MOF-based photocatalysts.

A variety of ternary nanoheterostructures composed of Pt nanoparticles (NPs), SnOx species, and anatase TiO2 are designed elaborately to explore the effect of interfacial electron transfer on photocatalytic H2 evolution from a biofuel–water solution. Among numerous factors controlling the H2 evolution, the significance of Pt sites for the H2 evolution is highlighted by tuning the loading procedure of Pt NPs and SnOx species over TiO2. A synergistic enhancement of H2 evolution can be achieved over the Pt/SnOx/TiO2 heterostructures formed by anchoring Pt NPs at atomically-isolated Sn-oxo sites, whereas the Pt/TiO2/SnOx counterparts prepared by grafting single-site Sn-oxo species on Pt/TiO2 show a marked decrease in the rate of H2 evolution. The characterization results clearly reveal that the synergy of Pt NPs and SnOx species originates from the vectorial electron transfer of TiO2 → SnOx → Pt occurring on the former, while the latter results from the competitive electron transfer from TiO2 to SnOx and to Pt NPs.