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Bismuth vanadate (BiVO₄) is an efficient photocatalyst or photoanode in water oxidation. However, the product selectivity in water oxidation over the most exposed clean and defective (001) and (101) facets is not clear. In this work, we investigated the water oxidation reaction on clean and oxygen deficient (001) and (101) facets and on Mo/W-doped facets using density functional theory. The pure (001) facet, we found, promotes OH radical formation. In the presence of oxygen vacancies, the O₂ evolution is preferred on the (101) facet with an overpotential of 0.52 V. On doping with Mo/W, the H₂O₂ evolution is preferred on the (101) facet with the overpotential of about 0.40 V; while on the (001) facet, the O₂ evolution is preferred with the overpotential of about 0.5 V. We also found that, on the (101) facet, there is a strong charge transfer from Bi atoms to the intermediates of water oxidation; but on the (001) facet, there is not. Our calculations could guide the design of the photocatalyst toward specific products of the water oxidation reaction, such as an OH radical, H₂O₂, or O₂. Moreover, the results show that the preferred sorption on the (101) facet may be a reason for the photoinduced charge separation between (101) and (001) facets of BiVO₄.

Photocatalytic method has been intensively explored for Cr(VI) reduction owing to its efficient and environmentally friendly natures. In order to obtain a high efficiency in practical application, efficient photocatalysts need to be developed. Here, ZnIn2S4/SnS2 with a three-dimensional (3D) heterostructure was prepared by a hydrothermal method and its photocatalytic performance in Cr(VI) reduction was investigated. When the mass ratio of SnS2 to ZnIn2S4 is 1:10, the ZnIn2S4/SnS2 composite exhibits the highest photocatalytic activity with 100% efficiency for Cr(VI) (50 mg/L) reduction within 70 min under visible-light irradiation, which is much higher than those of pure ZnIn2S4 and SnS2. The enhanced charge separation and the light absorption have been confirmed from the photoluminescence and UV-vis absorption spectra to be the two reasons for the increased activity towards photocatalytic Cr(VI) reduction. In addition, after three cycles of testing, no obvious degradation is observed with the 3D heterostructured ZnIn2S4/SnS2, which maintains a good photocatalytic stability.

In this paper, disk-like α-Fe2O3 nanoparticles were synthesized by one-step via an n-octyl alcohol-mediated self-assembly process and were subsequently used as the catalyst for the updating of heavy crude oil. The characterization results show that the diameter of the as-prepared α-Fe2O3 disks is in the range of 150–200 nm and the nanoparticles have a hematite structure with [1 1 0] preferred orientation. The catalyst strongly acts on the asphaltene and causes its lower S content and N content, lower aromaticity and aromatic condensation, and higher branchiness index. Adding tetralin causes the increment of aromatics content, the decrease of resin, and more molecules containing aromatic moieties bearing 2 and 3 rings. The viscosity of the Shengli extra heavy oil reduced substantially from 161180 mPa∙s to 45882 mPa∙s (50 °C) through the combination of the α-Fe2O3 nanoparticles and tetralin (a hydrogen donor).

Three dimensional microporous zeolites have attracted much attention in catalysis because of their large BET surface area and abundant pores. Herein, Ti-MCM-41 zeolites with different molar ratio of Si/Ti were synthesized, and the surface acidity and active sites were adjusted by the modification of basic-earth metal oxides, and noble metal nanoparticles. The photocatalytic activity and selectivity were evaluated by the reduction of CO2 to CH4 and CO. The results indicated that the efficiency and selectivity of CO2 photoreduction to CH4 of Ti-MCM-41 zeolites were 93 ppm g−1 h−1 and 29% in the ratio Si/Ti = 10 under keeping the zeolite structure, and which is much higher than that of P25 with the value of 24 ppm g−1 h−1 and 23%. When the surface of Ti-MCM-41 zeolites was modified by different basic-earth metal oxides, the photo-reduction activity of CO2 to CH4 increased about 1.5 times with MgO modification, and the selectivity of CO2 to CH4 improved to 42%. Moreover, the photoactivity and selectivity enhanced to 8835 ppm g−1 h−1 and 93% when the surface was further decorated by Pt nanoparticles, while the selectivity of CO2 to CO reached 94% when Pd was loaded. Our work reveal reasonably that adjusting the acidity and active site of the microporous zeolites is a highly efficient way to improve the photocatalytic efficiency of CO2 reduction.

In this work, BCN-TiO2(A+R) heterojunctions were constructed via a simple one-step calcination method as efficient photocatalysts for H2 evolution. Simultaneously, an anatase/rutile phase-junction was also obtained due to the crystalline phase transformation that occurred during high-temperature calcination. The unique heterojunction and phase-junction lead to the formation of an intimate interface and large contact area between two semiconductors and extended the visible-light absorption range. Therefore, BCN-TiO2(A+R) heterojunctions with a mixed anatase and rutile phase exhibit enhanced photocarrier separation efficiency, realizing an eleven-fold increase in the electron lifetime compared to that of pure TiO2 and yielding a remarkable visible-light-driven H2 evolution rate nearly 7.5 and 12.2 times higher than that of TiO2(A+R) and pure BCN, respectively. This work provides a new strategy for designing efficient water-splitting photocatalysts with highly efficient charge separation and transfer.

Developing photocatalysts with a high-efficiency charge separation remains a challenge in the solar hydrogen production. Herein, we devised and prepared a unique 2D/2D heterojunction of CuInS2/ZnIn2S4 nanosheets for solar hydrogen evolution. Structural characterizations reveal that the CuInS2/ZnIn2S4 2D/2D heterojunction with lattice match consists of the thin thickness of nanosheets and has a large interface contact area, boosting charges transfer and separation. Benefiting from the favorable 2D/2D heterojunction structure, the CuInS2/ZnIn2S4 2D/2D heterojunction photocatalyst with 5 wt % CuInS2 yields the highest H2 evolution rate of 3430.2 μmol·g–1·h–1. In addition, the apparent quantum efficiency of 5%CuInS2/ZnIn2S4 2D/2D heterojunction reaches 12.4% at 420 nm, which is high among the ZnIn2S4-based 2D/2D heterojunctions. The enhanced photocatalytic H2 evolution comes from the boosting charge separation. This work demonstrates that a 2D/2D heterojunction provides a potential way for significantly impro...

Anatase titanium dioxide (TiO2) nanocrystals with different percentages (up to 95%) of exposed {101} facet and different concentration ratios of bulk single-electron-trapped oxygen vacancies (SETOVs) to surface oxygen vacancies (SOVs) were prepared by alcohol-thermal method with nanotube titanic acid as the precursor in combination with solid-state reduction by NaBH4. The as-prepared TiO2 nanocrystals were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, electron spin resonance spectroscopy, and ultraviolet–visible light spectrometry. The effects of the percentage of crystal facets and the concentration ratio of bulk SETOVs/SOVs on the photocatalytic hydrogen production rate of TiO2 nanocrystals were investigated with positron annihilation lifetime spectroscopy as well as photocurrent test. Findings indicate that the percentage of the exposed {101} facets of the as-prepared TiO2 nanocrystals and their concentration ratios of bulk SETOVs/SOVs can be well tuned by properly adjusting the amount of NaBH4 and the reduction reaction time as well. Increasing percentage of the {101} facet of anatase TiO2 nanocrystals contributes to improving their photocatalytic hydrogen production activity, because the {101} facets of the anatase TiO2 nanocrystals possess enriched electrons and can act as the reduction sites to enhance the reduction reaction of H+ affording H2 in the sacrifice system of splitting water. Both the bulk SETOVs and SOVs contribute to the improvement of the light absorption while SOVs can facilitate the separation of photogenerated charges, thereby adding to the photocatalytic activity. However, the bulk SETOVs and excessive SOVs are also the combination centers of photogenerated charges, which means it is essential to maintain a suitable concentration ratio of the bulk SETOVs/SOVs so as to enhance the light absorption and achieve the best separation efficiency of photogenerated charges and achieve the best photocatalytic activity for hydrogen production. Particularly, when anatase TiO2 nanocrystal with a high percentage (95%) of exposed {101} facet is reduced by NaBH4 at a mass ratio of 2: 1 for 20 min, the resultant reduced H-TiO2 nanocrystal (denoted as H-TiO2-R20(2:1)) provides the highest photocatalytic hydrogen productive rate. Furthermore, the combination of 0.5% Pt/H-TiO2-R20(2:1) with 0.5% Pt/WO3 can split water to simultaneously produce H2 and O2, showing promising potential for splitting water affording hydrogen and oxygen.

The reasons for the difference of visible light activity between the direct contact g-C3N4/TiO2 Z-scheme composites obtained by one-step or two-step calcination process were investigated in detail by XRD, TEM, XPS, Raman and ESR technologies. TEM results showed that only g-C3N4 obtained by two-step calcination (denoted as CN-TSC) presented the porous structure which was in favor of the light absorption. XPS analysis indicated that interstitial doped N species formed in one-step calcination composite (denoted as CT-OSC) while substitutional N would appear in two-step calcination composite (denoted as CT-TSC). Nevertheless, interstitial N located at the higher position in the band gap of TiO2 and usually acted as the strong capture center of holes which was unfavorable to charge transfer. ESR and Raman results indicated that CT-TSC with some concentration of surface Vo and lower concentration of bulk Vo had excellent charge separation efficiency, according to the surface-enhanced Raman scattering (SERS) results of photo-induced charge-transfer (PICT) enhancement mechanism. And as a result, the visible-light activity for propylene oxidation of CT-TSC was twice higher than that of CT-OSC.

Efficient adsorption and utilization of carbon dioxide play an important role in mitigating the greenhouse effect and developing clean energy. Surface functionalization of photocatalyst is an efficient method to promote the adsorption of CO2 and convert it into hydrocarbon fuels. In this work, tetraethylenepentamine (TEPA) is used to functionalize Ti-MCM-41 molecular sieve photocatalyst to enhance the adsorption and activation of CO2. Ti-MCM-41 molecular sieve was modified without any precious metal additives. When the content of TEPA was 1%, the yield of CH4 was 232 ppm g−1 h−1. Furthermore, the results indicate that high impregnation amount of TEPA causes strong and higher adsorption of CO2 but inefficient for its conversion. This is probably due to the blockage of the channel when the impregnation amount of TEPA exceeds the threshold value, which hinders the further conversion of CO2 molecules. Functionalization Ti-MCM-41 zeolite photocatalyst with amino group is the prime reason for its excellent CO2 adsorption followed by activation capacity. In the future, this proposed composite might come up as a perspective photocatalyst in the field of photocatalysis for the purpose of environment remediation.

A new amphiphilic polymer was synthesized by allowing polyethylenimine to react with benzyl bromide, and it was used as an emulsifier for the viscosity reduction of Shengli extra heavy crude oil. T...

On the basis of density functional theory calculations and experimental results, we propose a spin-flip effect in a novel single-electron-trapped oxygen vacancy (SETOV; Vo ) and Fe co-doped TiO2 photocatalyst which improves significantly the propylene photo-oxidation under visible light illumination through the two spin channels photo-absorption and one spin channel recombination congestion. The presence of SETOV can make strongly localized Fe 3d states within the band gap get more delocalized, which not only increases the carrier mobility but also has an effect on restraining carrier recombination, thus resulting in the remarkably improved photocatalytic activity. Here the proposed photocatalytic process referring to spin-flip effect might afford a way for us about how to design a material owning the expected photoactivity in terms of spin selection rules about interband transition, opening the way to ground breaking applications in energy conversion.

Neutral oxygen vacancies (Ovʼs) in semiconductor oxides give rise to excess electrons that have the potential to affect the binding of adsorbates to the surface through surface-to-adsorbate charge transfer, and, as a result, to alter the overpotential (OP) of reactions on oxygen-deficient materials compared to stoichiometric materials. We report a systematic computational investigation of the effects of Ovʼs on the oxygen evolution reaction (OER) overpotential for β-Ga2O3, a d10 semiconductor that has been shown to exhibit high activity for water splitting. We investigated 18 β-Ga2O3 surfaces/slabs, with and without Ovʼs and observed a clear dependence of OER activity on Ovʼs. A general finding emerged that the excess electrons associated with Ovʼs are found to participate in charge transfer to OER intermediates, making their bonds to the surface more ionic and stronger, depending on the amount of charge transfer. The OER reaction step free energies are significantly affected and the ensuing overpotential...

Oil-soluble catalysts could be of special significance for reducing the viscosity of heavy crude oil, because of their good dispersion in crude oil and high catalytic efficiency toward aquathermolysis. Ferric oleate was synthesized and applied as catalyst in the aquathermolysis reaction of Shengli heavy oil. It was found that ferric oleate was more efficient for heavy oil cracking than Co and Ni oleates. Besides, it was superior to oleic acid and inorganic ferric nitrate and achieved the highest viscosity reduction rate of up to 86.1%. In addition, the changes in the components of Shengli heavy oil before and after aquathermolysis were investigated by elemental analysis, Fourier transform infrared spectrometry, and 1H nuclear magnetic resonance spectroscopy. Results indicated that ferric oleate contributed to a significant increase in the content of light components and decrease in the content of resin, N and S. The as-prepared ferric oleate showed good activity for reducing the viscosity and improving the quality of the heavy crude oil, showing promising application potential in aquathermolysis of heavy crude oil.

Constructing a 0D/2D heterojunction is an important way to ameliorate the charge separation for achieving efficient solar hydrogen production. In this study, zero-dimensional (0D) AgIn5S8 nanoparticles/two-dimensional (2D) layered ZnIn2S4 0D/2D heterojunction composite was designed and prepared via a facile two-step hydrothermal process. Anchoring the AgIn5S8 nanoparticles on the 2D layered ZnIn2S4 can shorten the charge-migration distance, provide abundant active sites and extend the visible-light absorption. Benefiting from those favorable properties, the AgIn5S8/ZnIn2S4 0D/2D heterojunction composite exhibits the highest H2 evolution rate of 949.9 μmol g−1 h−1 under visible light irradiation (λ > 420) when the weight ratio of AgIn5S8 to ZnIn2S4 is 30%, which is about 3.6 and 17.6 times higher than pure ZnIn2S4 and AgIn5S8, respectively. Photoluminescence (PL) and photocurrent response results indicate that the improved charge separation efficiency is a critical reason for the enhanced H2 evolution activity. This study suggests that constructing a 0D/2D heterojunction can provide an efficient way to improve the photocatalytic performance of semiconductor photocatalysts for solar hydrogen production.

An oxygen-vacancy-rich Z-scheme g-C3N4/Pd/TiO2 ternary nanocomposite was fabricated using nanotubular titanic acid as precursors via a simple photo-deposition of Pd nanoparticles and calcination process. The prepared nanocomposites were investigated by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-visible diffuse reflectance spectroscopy, respectively. For g-C3N4/TiO2 binary nanocomposites, at the optimal content of g-C3N4 (2%), the apparent photocatalytic activity of 2%g-C3N4/TiO2 was 9 times higher than that of pure TiO2 under visible-light illumination. After deposition of Pd (1 wt%) at the contact interface between g-C3N4 and TiO2, the 2%g-C3N4/Pd/TiO2 ternary nanocomposites demonstrated the highest visible-light-driven photocatalytic activity for the degradation of gaseous propylene, which was 16- and 2-fold higher activities than pure TiO2 and 2%g-C3N4/TiO2, respectively. The mechanism for the enhanced photocatalytic performance of the g-C3N4/Pd/TiO2 photo-catalyst is proposed to be based on the efficient separation of photo-generated electron-hole pairs through Z-scheme system, in which uniform dispersity of Pd nanoparticles at contact interface between g-C3N4 and TiO2 and oxygen vacancies promote charge separation.

All-solid-state Z-scheme BCN–TiO2 binary heterostructures with a surface oxygen vacancy layer as the contact interface are fabricated by NaBH4 reduction and ball milling. The effect of the surface oxygen vacancy layer of BCN–TiO2 binary heterostructures on the charge carrier transfer and photocatalytic hydrogen evolution was thoroughly investigated. UV-vis diffuse reflection spectra results reveal that the surface oxygen vacancy can extend the light absorption from the UV to the visible-light region for BCN–TiO2 binary heterostructures. The steady-state/time-resolved photoluminescence spectra exhibit an increased charge carrier lifetime, improved charge carrier separation efficiency and strengthened direct Z-scheme charge transfer process for BCN–TiO2−x heterostructures. As expected, the BCN–TiO2−x heterostructures exhibited a higher visible-light photocatalytic efficiency, about 7 and 11 times higher than that of the pure TiO2 and BCN samples. Namely, the photocatalytic activity of hydrogen evolution can be greatly promoted by constructing the surface oxygen vacancy layer. This work provides a new pathway to construct an efficient contact layer for the direct Z-scheme system to improve the charge carrier separation and transfer and thereby improve the photocatalytic activity.

We report first principles and mesoscale kinetic simulations of intrinsic electron and hole transport in stoichiometric monoclinic BiVO4 (BVO). A main finding is that hole transport is bimodal: there are fast hole hops within VO4 tetrahedra that are not ‘transport-efficient’ and slower hole hops across VO4 tetrahedra that are ‘transport-efficient’. This bimodality in hole transport may explain the slow transport/high recombination characteristics of BVO. In this work, polaron hops were described via the two-state Marcus/Holstein model of VV–VIV and OI–OII interchanges for electron and hole polarons respectively. The relevant parameters in the theory were obtained using the density functional theory DFT+U method. Hopping activation energies were combined with Einstein diffusion theory to yield mobility as well as with kinetic Monte Carlo (KMC) modeling. All unique nearest neighbor electron and hole hopping processes were characterized. For V-to-V hops of electron polarons, we obtained activation energies of ∼0.37 eV that compare well with the experimental activation energy of ∼0.30 eV for drift mobility. Electron polarons remain localized on single sites along the hopping pathways before hopping via tunneling, suggestive of small non-adiabatic coupling and a diabatic character consistent with V-to-V electron transfer through space (no bridge linkers). These simulations yield a small electron transport anisotropy, with transport in the (a,b) plane being faster than along the c direction by a factor of ∼2. For hole polarons our calculations support the character of small polarons localized on O atoms as well. Some nearest neighbor hole hops have a diabatic character, while other nearest neighbor hops are adiabatic and phonon-assisted with an Arrhenius thermal dependence. Hops through O–V–O bridges and through space have low activation barriers (∼0.17 eV and ∼0.25 eV) while hops through O–Bi–O bridges have higher activation barriers (∼0.37 eV and higher). The mobility is gated by the slower transport across VO4 tetrahedra through O–Bi–O bridges owing to the underlying BVO lattice network. Hole mobility is determined to be about one order of magnitude larger than electron mobility consistent with THz spectroscopy measurements. This work sets the foundation to address facet selectivity of charge carriers in shape and size-tailored crystals.

The surface and bulk oxygen vacancy have a prominent effect on the photocatalytic performance of TiO 2 . In this study, TiO 2 possessing different types and concentration of oxygen vacancies were prepared by annealing nanotube titanic acid (NTA) at various temperatures in air or vacuum atmosphere. TiO 2 with the unitary bulk single-electron-trapped oxygen vacancies (SETOVs) formed when NTA were calcined in air. Whereas, TiO 2 with both bulk and surface oxygen vacancies were obtained when NTA were annealed in vacuum. The series of TiO 2 with different oxygen vacancies were systematically characterized by TEM, XRD, PL, XPS, ESR, and TGA. The PL and ESR analysis verified that surface oxygen vacancies and more bulk oxygen vacancies could form in vacuum atmosphere. Surface oxygen vacancies can trap electron and hinder the recombination of photo-generated charges, while bulk SETOVs act as the recombination center. The surface or bulk oxygen vacancies attributed different roles on the photo-absorbance and activity, leading that the sample of NTA-A400 displayed higher hydrogen evolution rate under UV light, whereas NTA-V400 displayed higher hydrogen evolution rate under visible light because bulk SETOVs can improve visible light absorption because sub-band formed by bulk SETOVs prompted the secondary transition of electron excited.

Molybdenum-doped akaganeite nano-rods were prepared by one step facile synthesis. The formation of akaganeite depends on the molar ratio of hexaammonium molybdate to ferric chloride hexahydrate. The nano-rods have an average particle size of ca. 30 nm and disperse uniformly. As a catalyst, the nano-rods can effectively increase the viscosity reduction rate of Shengli Oilfield extra heavy crude oil (175000 mPa s at 50 °C), especially combined with tetralin (a hydrogen donor). The results of SARA analysis, elemental analysis, and 1H NMR spectra indicate that the increase of light component caused by the denitrification and desulfuration of the catalyst plus tetralin as well as the loosening of heavy component molecules can be responsible for the viscosity reduction of the heavy oil.

The effect of the concentration ratio of bulk single-electron-trapped oxygen vacancies/surface oxygen vacancies on the photocatalytic hydrogen production of TiO2 was investigated. A series of TiO2 materials with a tunable concentration ratio of bulk and surface oxygen vacancies were prepared through a NaBH4 solid-state reduction method. The results of HRTEM, XPS, ESR, PL and UV-vis DRS verify that surface oxygen vacancies can form on the surface of TiO2 and the concentrations of bulk single-electron-trapped oxygen vacancies and surface oxygen vacancies increase with the increase of the reduction degree. Combining the evaluation of photocatalytic hydrogen production with the analysis of positron annihilation lifetime spectra, it is proved that the concentration ratio of bulk single-electron-trapped oxygen vacancies/surface oxygen vacancies plays the key role in the photocatalytic hydrogen production of TiO2, and the optimal ratio of bulk single-electron-trapped oxygen vacancies/surface oxygen vacancies leads to the best photoactivity.

A zero-dimensional (0D)/two-dimensional (2D) heterojunction has an excellent advantage of boosting the photo-generated carrier separation and obtaining enhanced photocatalytic activities. Here, a ZnIn2S4 nanoparticle/MoS2-RGO nanosheet 0D/2D heterojunction was prepared by a rapid and low temperature hydrothermal method. TEM characterization results reveal that ZnIn2S4 nanoparticles are uniformly dispersed on the surface of MoS2-RGO nanosheets, which can provide abundant active sites and shorten the charge-migration distance, while the MoS2-RGO nanosheet acts as a support to avoid the aggregation of 0D ZnIn2S4 nanoparticles and also serves as a low-cost cocatalyst for effective hydrogen evolution. Through optimizing the MoS2-RGO composition and content, the highest hydrogen evolution rate of 425.1 μmol g−1 h−1 was obtained over the ZnIn2S4/MoS2-RGO 0D/2D heterojunction photocatalyst under visible light irradiation (λ > 420 nm), which is about 34.6 times higher than that of pure ZnIn2S4. Efficient charge separation can be attributed to the significantly enhanced photocatalytic performance, which originates from the favorable properties of the ZnIn2S4/MoS2-RGO 0D/2D heterojunction. This study provides an effective method to improve the photocatalytic performance of the ZnIn2S4 photocatalyst based on the 0D/2D heterojunction.

The sandwich-like Z-scheme g-C3N4/reduced graphene oxide@TiO2 (CN/RGO@T) composite with enhanced photocatalytic performance was successfully synthesized by hydrothermal method followed by a calcination process. It is interesting to note that TiO2 completely enclosed by RGO is presented in a nanoscroll morphology. Photocatalytic performance of the composite was evaluated in the propylene degradation. The results revealed that by introducing RGO as an interfacial mediator, Z-scheme g-C3N4-TiO2 nanocomposites could remarkably speed up the interfacial charge transmission and improve the separation efficiency of photogenerated carriers. Especially, 20%CN/RGO@T composite showed the highest visible light activity up to 21.9% for propylene oxidation. A lower photogenerated electron-hole recombination rate and higher carrier separation efficiency of the ternary complex 20%CN/RGO@T were confirmed by electron spin resonance, photoluminescence spectrum, and photocurrent-time characterization. This is due to the fact that strong π-π interaction between g-C3N4 and RGO leads to a carrier transmission channel between N-O and C-O-C bonds of g-C3N4 and RGO. This newly formed Z-scheme composite is having a possibility to be explored as a versatile photocatalyst to conduct various type photoreactions.

In this paper, rivet-like iron oxide nanoparticles were synthesized via a one-pot hydrothermal method for the first time. SEM and TEM results demonstrated that the as-prepared rivet-like nanoparticle consisted of two parts, a head part with an average size of 50 nm, and a tail one with a diameter of 20–40 nm and a length of ca. 100 nm. XRD result revealed that this nanoparticle had a dominant hematite phase and minor goethite phase. The rivet-like iron oxide nanoparticles exhibited higher catalytic activity for the upgrading of Shengli-oilfield extra heavy crude oil than the cubical α-Fe2O3 nanoparticles in the absence of hydrogen donors. In particular, owing to the site-rich surface of the catalyst, the viscosity of the Shengli extra heavy oil decreased from 244520 mPa∙s to 152000 mPa∙s (at 50 °C) in the presence of the 400 °C-treated nanoparticles after the reaction at 200 °C for 24 h. Higher exposure of (1 0 4) facet could possibly also account for the higher catalytic activity.

Z-scheme heterostructure of nitrogen-vacancy rich g-C3N4 nanoparticles (VN-CN) confined in oxygen-vacancy rich TiO2 nanotube arrays (VO-TiO2) were fabricated to enhance CO2 photoreduction under visible light irradiation. Dual vacancies mediated VN-CN/VO-TiO2 heterostructure exhibits excellent photocatalytic CO2 reduction performance, and the CO and CH4 evolution rate are about 1.5 times and 3 times higher than that of pristine CN/TiO2, respectively. Possible reasons for the improvement of photocatalytic activity are as follows: (1) the adsorption and activation capacity of CO2 on the surface of CN/TiO2 are obviously improved due to the construction of nitrogen and oxygen dual vacancies; (2) the construction of dual vacancies are beneficial for enhancing the visible light absorption, promoting separation and transfer of photogenerated carriers due to intimate contact interface between VN-CN and VO-TiO2; (3) Z-scheme heterostructure could boost oxidation ability of VN-CN/VO-TiO2 and promote the production of more hydroxyl radical, which can react with CO2− to form HCO3−, leading to a new pathway to CO2 photoreduction compared to pristine CN/TiO2.

BCN-TiO 2 nanocomposites are obtained by a simple calcination method using a hexagonal boron carbon nitride (BCN) semiconductor and nanotubular titanic acid (NTA) as precursors. The BCN-TiO 2 nanocomposites are characterized systematically by XRD, TEM, XPS, DRS, ESR, BET, I-t and PL techniques. Compared with the novel TiO 2 or single BCN sample, the BCN-TiO 2 nanocomposites prepared by a calcination method show the highest photocatalytic activity for hydrogen production under visible-light irradiation. The photocatalytic activity of BCN-TiO 2 nanocomposites is optimized by changing the amount of BCN. Characterization results confirm that a large amount of single electron oxygen vacancies can be formed when NTA was calcined at the higher temperature, which is beneficial to the enhancement of visible light absorption and the transfer of photogenerated carriers due to the formation of ohmic contact at the interface between BCN and novel TiO 2 . Therefore, the visible light photocatalytic activity for H 2 production of BCN-TiO 2 nanocomposites has been improved significantly by the formation of BCN-TiO 2 Z-scheme photocatalyst, which results in efficient space separation of photo-induced charge carriers.

Exploiting an efficient and noble-metal-free cocatalyst is highly desired for photocatalytic H2 evolution from water. Here, we synthesized the MoS2-RGO/ZnO composite photocatalysts via a facile two-step solvothermal process. Through optimizing of the MoS2 and reduced graphene oxide (RGO) cocatalyst component proportion, the 0.75 wt%5G95 M/ZnO composite photocatalyst shows the highest H2 production rate of 288.4 μmol g−1 h−1 when the amount of MoS2-RGO is 0.75 wt% and the weight ratio of RGO to MoS2 is 5:95, which is about 14.1 times higher than the pure ZnO. More importantly, the ternary MoS2-RGO/ZnO composite photocatalyst exhibits much higher H2 evolution activity than the Pt loaded ZnO under the same condition. The significant improvement in the H2 evolution activity is ascribed to the positive synergetic effect between MoS2 and RGO, which acts as a hydrogen evolution cocatalyst and a robust electron transport pathway, respectively. This study suggests that MoS2-RGO can serves as an efficient and noble-metal-free cocatalyst to improve the H2 production performance of ZnO by suppressing the electron-hole pairs recombination.

Developing an efficient dye-sensitized photocatalyst for photocatalytic H2 evolution from water is highly desired. In this study, a novel ternary g-C3N4/Pt/GO composite was synthesized via a facile liquid-phase sonochemical approach. Pt nanoparticles were firstly deposited on the surfaces and/or interlayer of g-C3N4 by chemical reduction, and then, GO was penetrated through ultrasonic treatment. This preparation method is facilitated to prohibit the agglomeration of g-C3N4 and GO because of their π–π stacking interaction and enhance electrons transport and the adsorption of dye molecules. The Eosin Y-sensitized ternary g-C3N4/Pt/GO composite shows the highest H2 production rate of 3.82 mmol·g−1·h−1 under the optimization conditions, which about 2.1 times and 7.7 times higher than the Eosin Y-sensitized binary g-C3N4/Pt and GO/Pt photocatalysts, respectively. The improved charge separation efficiency is the main reason for the enhancement in H2 evolution activity. The apparent quantum efficiency of hydrogen evolution for the Eosin Y-sensitized ternary g-C3N4/Pt/GO composite is up to 9.7% at 420 nm. Moreover, the sensitized g-C3N4/Pt/GO composite shows stable photocatalytic activity for H2 evolution under visible light irradiation. The effect of pH, g-C3N4/GO weight ratios, Pt loading amounts, dye concentrations, and different type dyes on the H2 evolution activity was also investigated in detail.

Z-scheme g-C3N4-TiO2 heterojunctions containing g-C3N4 nanosheets with different thickness were prepared by sintering the mixture of g-C3N4 and nanotube titanic acid (denoted as NTA) at different temperatures in air. As-prepared Z-scheme g-C3N4-TiO2 heterojunctions were characterized by X-ray diffraction, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible light diffuse reflectance spectrometry, electron spin resonance, and photoluminescence spectrometry. Findings indicate that the annealing temperature has crucial effects on the visible-light photocatalytic activity (λ ≥ 420 nm) of the as-prepared Z-scheme g-C3N4-TiO2 heterojunctions. The Ti3+ and porous g-C3N4 nanosheets formed upon the calcination at 600 °C as well as the low concentration of bulk single-electron trapped oxygen vacancy are favorable to the transport of the photoexcited charge carriers. This, in association with the Z-scheme system, contributes to improving the photocatalytic activity of g-C3N4-TiO2 photocatalysts. As a result, g-C3N4-TiO2 photocatalyst prepared at 600 °C exhibits good photocatalytic activity towards the degradation of propylene and hydrogen generation by water-splitting under visible light irradiation.

Four kinds of nanosilica-supported metallic and metatitanic acid nanocatalysts were synthesised as potential phase transfer catalysts for enhancing heavy oil recovery via a liquid-reduction method. The catalysts were carried by water and transferred into oil phase, and the catalytic performance of catalysts for the aquathermolysis reaction and viscosity reduction of the extra-heavy oil recovered at Shengli Oilfield (Dongying, China) was investigated. At a mass fraction of 1%, an aquathermolysis reaction temperature of 150°C and an aquathermolysis time of 36 h, the as-synthesised silica/zero valence iron and nickel (denoted as SiO2/Fe/Ni) catalyst was able to reduce the apparent viscosity of the two kinds of heavy oils by 79.3 and 77.6%, showing promising application in the industrial production of heavy crude oil.

Oxygen vacancies play an important role in many photocatalytic reaction, and have attracted enormous attention from the scientists and engineers. The surface or bulk oxygen vacancies have a different function in the photo-reaction process. Herein, three different TiO2 nanoparticles possessing surface oxygen vacancies (SO) and/or bulk single-electron-trapped oxygen vacancy (SETOV) were fabricated by dehydration or reduction of different titania precursors. The three kinds of TiO2 nanoparticles were characterized systematically by XRD, TEM, Raman, XPS, ESR, TG, UV–vis DRS, and PL techniques. The photocatalytic reduction results of CO2 indicated that both the bulk SETOVs and surface oxygen vacancies contributed to the enhancement of the light absorption, while the surface vacancies facilitated to the separation of the photo-generated charge carriers, and on the contrast, the bulk SETOVs acted as the recombination center. The co-existence of the surface and bulk oxygen vacancies exhibited a synergistic effect to improve the photoreduction efficiency of CO2 to CH4. Through adjusting the ratio of the surface and bulk oxygen vacancies and analyzing the positron lifetime and relative intensity by positron annihilation, the photoreduction efficiency of CO2 improved with the increase of the ratio of surface oxygen vacancies to bulk SETOVs.

Ni0@graphene nanocomposites were prepared via a solvothermal method and used as the catalysts for the viscosity reduction of extra heavy crude oil. Higher graphene content in Ni0@graphene nanocomposite has an adverse effect on its catalytic activity. The addition of tetralin and higher reaction temperature can obviously promote the catalytic activity. The catalyst accompanied by hydrogen donor can attain a viscosity reduction rate of 84.3% after the catalytic reaction under 280°C for 24 h and reduce the viscosity of crude oil from 174,219 to 27,352 mPa s (measured at 50°C).

Anatase TiO2 nanocrystals exposed with {001} facets were fabricated by solvothermal strategy in HF-C4H9OH mixed solution, using titanic acid nanobelts (TAN) as a precursor. The shape of TAN is a long flat plane with a high aspect ratio, and F− is easily adsorbed on the surface of the nanobelts, inducing a higher exposure of {001} facet of TiO2 nanoparticles during the structure reorganization. The exposed percentage of {001} facets could vary from 40 to 77% by adjusting the amount of HF. The as-prepared samples were characterized by transmission electron microscopy, N2 adsorption-desorption isotherms, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscope. The photocatalytic measurement showed that TiO2 nanocrystals with 77% {001} facets exhibited much superior photocatalytic activity for photodegradation of methyl orange, methylene blue, and rhodamine B. And what’s more, the mineralization rate of methyl orange was as high as 96% within 60 min. The photocatalytic enhancement is due to a large amount of the high energetic {001} facets exposing, the special truncated octahedral morphology and a stronger ability for dyes adsorption.

Direct contact Z-scheme g-C 3 N 4 -TiO 2 nanocomposites without an electron mediator are prepared via simple annealing the mixture of bulk g-C 3 N 4 and nanotube titanic acid (NTA) in air at 600 °C for 2 h. In the process of annealing, the bulk g-C 3 N 4 transformed to ultra-thin g-C 3 N 4 nanosheets, and NTA converted to a novel anatase TiO 2 , then the two components formed a close interaction. The XPS result reveals that some amount of nitrogen is doped into this novel-TiO 2 , and g-C 3 N 4 nanosheets exist in the composites. The results of XRD, TEM and TG indicate that the thickness of g-C 3 N 4 nanosheets is very thin. The ESR spectrum shows the existence of Ti 3+ and single-electron-trapped oxygen vacancy in the 30%g-C 3 N 4 -TiO 2 composites. In photocatalytic activity test, the 30%g-C 3 N 4 -TiO 2 nanocomposites showed an excellent photo-oxidation activity of propylene under visible light irradiation (λ≥ 420 nm), and the removal efficiency of propylene reached as high as 56.6%, and the activity kept nearly 82% after four consecutive recycles. Photoluminescence (PL) result using terephthalic acid (TA) as a probe molecule indicated that the g-C 3 N 4 -TiO 2 nanocomposites displayed a Z-sheme photocatalytic reaction system and this should be the main reason for the high photocatalytic activity. A possible photocatalytic mechanism was proposed on the basis of PL result and transient photocurrent-time curves.

Shape-dependent ultrafine anatase TiO 2 nanocrystals coexposed with (001) and (101) facets are synthesized in isopropanol by the simple solvothermal strategy. Nanotube titanic acid (NTA) was used as the Ti precursor, and hydrofluoric acid was the shape-control reagent. The nanotubular structure of NTA facilitated HF to adsorb on/into NTA, and made the shape-controlled process to be more economical and easier. The ratio of (001) facets of the obtained decahedral TiO 2 nanocrystals can be adjusted from 5 to 51%, and the mean diameter of nanoparticles is about 10–30 nm. The samples are characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscope, ultraviolet-visible diffuse reflectance spectrum, N 2 adsorption-desorption isotherms, fluorescence and Raman spectroscopy. The truncated octahedral bipyramid TiO 2 nanocrystals with 51% (001) and 49% (101) facets exposing showed the highest photoactivity for CO 2 reduction to CH 4 . This remarkable photocatalytic efficiency is attributed to the surface heterojunction between (001) and (101) crystal facets, the small size and large surface area of the truncated octahedral bipyramid TiO 2 nanocrystals.

Iron(II) phthalocyanine (FePc) is immobilized on graphene sheets to form a graphene(G)/iron(II) phthalocyanine hybrid (G/FePc) by the π–π stacking method. The resultant hybrids are characterized by UV–vis absorption spectra, FTIR, Raman spectra, BET, TEM, SEM and XRD. The result suggests that the interaction between graphene and FePc follows a donor-acceptor mode and the loading of FePc on graphene sheets not only facilitates the dispersion of FePc on graphene but also promotes the exfoliation of the graphene sheets. These samples are tested for photocatalytic degradation of phenol under visible light irradiation (λ ≥ 420 nm). And the photocatalytic activity of graphene, which is not presented when graphene individually acts on phenol, is greatly enhanced because of the π–π stacking interaction when FePc loaded on graphene. The G/FePc hybrid containing 25 wt.% FePc (G/FePc-0.25) exhibits the best photoactivity among the different loading content of FePc, whose degradation rate for phenol achieves 77.1% in the presence of H2O2 under visible light irradiation for 3 h.

The controlled modification of the inner and outer surfaces of hollow nanostructures with dual cocatalysts possessing redox ability is a key factor for improving the charge carrier separation efficiency and surface catalytic reaction efficiency of a catalyst. Herein, a novel hollow urchin-like TiO2 nanomaterial with Pt loaded on the inner surface and RuO2 on the outer surface is reported. The Pt acts as the electron capture centre and is responsible for the reduction reaction, whereas the RuO2 acts as the hole-trapping centre for the oxidation reaction. Compared to nanospheres, the urchin-like structures have a larger specific surface area. Moreover, the as-prepared TiO2 contained numerous single-electron oxygen vacancies (Vo·). The synergy between the highly dispersed catalytic material and spatially separated dual cocatalysts, characterised by hollow structures, a large specific surface area, and oxygen vacancies, greatly increased the activity of the catalyst. When Pt (0.5 wt%) is loaded on the inner surface of the nanomaterials, the hydrogen evolution rate is as high as 2045.1 μmolg−1h−1; moreover, it further improves to 3904.0 μmolg−1h−1 when RuO2 is loaded on the outer shell.

Constructing two-dimensional (2D) heterostructure is an effective method to obtain highly efficient photocatalysts. In this work, 2D/2D TiO2/g-C3N4 heterostructures were constructed via in-situ growth of TiO2 nanosheets on the surface of g-C3N4 nanosheets by a facile hydrothermal co-assembly technique. The increased dispersion is favorable to expose more vacancies, which is beneficial to the enhancement of visible light absorption. This unique 2D/2D heterostructure exhibits ultrathin thickness and larger interfacial contact area, which results in shorter migration distance of photo-generated carriers to the surface and faster participation in photocatalytic water splitting. As expected, the optimized TiO2/g-C3N4 nanocomposites showed a higher photocatalytic hydrogen evolution performance under visible light irradiation than that of pristine g-C3N4 and TiO2 nanosheets. The improved performance can be attributed to the ultrathin nanosheet structure, the intimate interfacial contact and the presence of surface oxygen vacancies. More appealingly, this environment-friendly 2D/2D TiO2/g-C3N4 nanohybrid would attract more attention in water splitting.

Noble metal Pt nanoparticles have been considered as the most effective co-catalyst to improve the photocatalytic hydrogen production activity of TiO2. In this study, the effect of the dispersion of Pt nanoparticles on the photoactivity of TiO2 nanotubes was investigated. Compared with the samples that the co-catalyst of Pt nanoparticles agglomerated or freely dispersed, the sample with the uniformly dispersion of Pt nanoparticles showed a higher performance for photocatalytic hydrogen production. The photocatalysts were characterized systematically by TEM, BET, UV-Vis, XPS, and PL techniques, and the relationship between the structure and the photoactivity was investigated in detail. The results demonstrated that the dispersion status of Pt nanoparticles had a crucial effect on the photocatalytic activity.

An economic and effective Pt-based alloy cocatalyst has attracted considerable attention due to their excellent catalytic activity and reducing Pt usage. In this study, PtNi alloy cocatalyst was successfully decorated on the g-C3N4/GO hybrid photocatalyst via a facile chemical reduction method. The Eosin Y-sensitized g-C3N4/PtNi/GO-0.5% composite photocatalyst yields about 1.54 and 1178 times higher hydrogen evolution rate than the Eosin Y-sensitized g-C3N4/Pt/GO-0.5% and g-C3N4/Ni/GO-0.5% samples, respectively. Mechanism of enhanced performance for the g-C3N4/PtNi/GO composite was also investigated by different characterization, such as photoluminescence, transient photocurrent response, and TEM. These results indicated that enhanced charge separation efficiency and more reactive sites are responsible for the improved hydrogen evolution performance due to the positive synergetic effect between Pt and Ni. This study suggests that PtNi alloy can be used as an economic and effective cocatalyst for hydrogen evolution reaction. Graphical abstractA significant enhancement of photocatalytic H2 evolution is realized over the Eosin Y-sensitized g-C3N4/PtNi/GO composite with PtNi alloy as an efficient cocatalyst.

Catalyst is crucially important to reduce the viscosity of heavy oil during the catalytic aquathermolysis. SO4 2−-modified ZrO2-based nanoparticle catalyst is a commonly used catalyst. But less acid sites and poor hydrothermal stability limited further improving its catalytic performance and practical application. In this study, the Zr-doped silicalite zeolite catalysts with large surface area were prepared as a support matrix, and SO4 2−-modified Zr-doped silicalite zeolite (denoted as SO4 2−/Zr-silicalite-1 zeolite) was used as a solid superacid catalyst to crack the heavy oil. A reference catalyst of SO4 2−/Zr-SiO2 nanoparticles (NPs) was also prepared, which has the same composition with the SO4 2−/Zr-silicalite-1 zeolite catalyst. Compared with the SO4 2−/Zr-SiO2 NP catalyst, the amount of acid sites for the SO4 2−/Zr-silicalite-1 zeolite catalyst is significantly increased and the viscosity reduction efficiency is also enhanced by 40%. More importantly, the SO4 2−/Zr-silicalite-1 zeolite catalyst exhibits a high hydrothermal stability. After catalytic aquathermolysis, the quality of the heavy oil was also ameliorated. The heavy resins and asphaltenes reduced, while the light saturated and aromatic hydrocarbon increased. The results suggest metal element-doped silicalite zeolite catalyst is a potential useful way to solving the less acid sites and poor hydrothermal stability for the SO4 2−-modified nanoparticle catalyst.

A conjugation-grafted-TiO2 nanohybrid was synthesized by chemically grafting conjugated structures on the surface of nanotube titanic acid (NTA) precursor-based TiO2 through the controlled thermal degradation of a coacervated polymer layer of polyvinyl alcohol (PVA). The interfacial interactions between the NTA precursor-based TiO2 and conjugated structures were characterized using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Moreover, the effects of the NTA’s pretreatment temperature and the weight ratio of NTA to PVA on the photocatalytic degradation of methyl orange were also investigated. A higher NTA pretreatment temperature and a lower NTA to PVA weight ratio were found to enhance photogenerated electron–hole separation efficiency and photocatalytic activity. Moreover, the conjugation-grafted-TiO2 nanohybrid synthesized from the NTA precursor displayed a much higher visible-light photocatalytic activity than that of the sample obtained from the P25 precursor. The origin of the enhanced photocatalytic activity under visible-light irradiation is also discussed in detail.

Recently, graphitic carbon nitride (g-C3N4) has been investigated as a photocatalyst for water splitting and organic dye degradation. In this study, we have developed a simple soft-chemical method of doping Zn into g-C3N4 to prepare a metal-containing carbon nitride. The doping was confirmed by x-ray photoelectron spectroscopy, and diffusion reflectance spectra revealed a significant red shift in the absorption edge of Zn/g-C3N4. This hybrid material shows high photocatalytic activity and good stability for hydrogen evolution from an aqueous methanol solution under visible light irradiation (λ≥420 nm). The hydrogen evolution rate was more than 10 times higher for a 10%-Zn/g-C3N4 sample (59.5 μmol h−1) than for pure g-C3N4. The maximum quantum yield was 3.2% at 420 nm.

TiO2 nanotubes attract much attention because of their high photoelectron-chemical and photocatalytic efficiency. But their large band gap leads to a low absorption of the solar light and limits the practical application. How to obtain TiO2 nanotubes without any dopant and possessing visible light response is a big challenge nowadays. Orthorhombic titanic acid nanotubes (TAN) are a special precursor of TiO2, which possess large Brunauer-Emmett-Teller (BET) surface areas and strong ion exchange and adsorption capacity. TAN can transform to a novel TiO2 with a large amount of single-electron-trapped oxygen vacancies (SETOV) during calcination, while their nanotubular structure would be destroyed, and a BET surface area would decrease remarkably. And interestingly, SETOV can lead to a visible light response for this kind of TiO2. Herein, glucose was penetrated into TAN by the vacuum inhalation method, and TAN would dehydrate to anatase TiO2, and glucose would undergo thermolysis completely in the calcination process. As a result, the pure TiO2 nanotubes with visible light response and large BET surface areas were obtained. For further improving the photocatalytic activity, Pd nanoparticles were loaded as the foreign electron traps on TiO2 nanotubes and the photocatalytic oxidation efficiency of propylene was as high as 71 % under visible light irradiation, and the photostability of the catalyst kept over 90 % after 4 cyclic tests.