Highly selective photocatalytic reduction of CO_(2) to CH_(4) on electron-rich Fe species cocatalyst under visible light irradiation

Efficient photocatalytic reduction of CO_(2) to high-calorific-value CH4,an ideal target product,is a blueprint for C_(1)industry relevance and carbon neutrality,but it also faces great challenges.Herein,we demonstrate unprecedented hybrid SiC photocatalysts modified by Fe-based cocatalyst,which are prepared via a facile impregnation-reduction method,featuring an optimized local electronic structure.It exhibits a superior photocatalytic carbon-based products yield of 30.0μmol g^(−1) h^(−1) and achieves a record CH_(4) selectivity of up to 94.3%,which highlights the effectiveness of electron-rich Fe cocatalyst for boosting photocatalytic performance and selectivity.Specifically,the synergistic effects of directional migration of photogenerated electrons and strongπ-back bonding on low-valence Fe effectively strengthen the adsorption and activation of reactants and intermediates in the CO_(2)→CH_(4) pathway.This study inspires an effective strategy for enhancing the multielectron reduction capacity of semiconductor photocatalysts with low-cost Fe instead of noble metals as cocatalysts.

Photocatalytic reduction of carbon dioxide to methanol by Cu_2O/SiC nanocrystallite under visible light irradiation

The Cu2O/SiC photocatalyst was obtained from SiC nanoparticles （NPs） modified by Cu2O. Their photocatalytic activities for reducing CO2 to CH3OH under visible light irradiation have been investigated. The results indicated that besides a small quantity of 6H-SiC, SiC NPs mainly consisted of 3C-SiC. The band gaps of SiC and Cu2O were estimated to be about 1.95 and 2.23 eV from UV-Vis spectra, respectively. The Cu2O modification can enhance the photocatalytic performance of SiC NPs, and the largest yields of methanol on SiC, Cu2O and Cu2O/SiC photocatalysts under visible light irradiation were 153, 104 and 191μmol/g, respectively.

Room-temperature solid phase surface engineering of BiOI sheets stacking g-C_(3)N_(4) boosts photocatalytic reduction of Cr(Ⅵ)

Cr(Ⅵ)-based compounds pollution have attracted global concern due to serious harm to humans and environment.Hence,it is crucial to exploit an effective technique to eliminate Cr(Ⅵ)in water.Herein,we in-situ grown BiOI on graphitic carbon nitride to prepare the BiOI/g-C_(3)N_(4)(BCN)direct Z-scheme heterojunction by solid phase engineering method at room temperature.Experimental result shown the photocatalytic activity of pure BiOI were obviously enhanced by constructing Z-scheme BCN heterostructure,and BCN-3 heterostructure exhibited the optimal photocatalytic degradation of RhB with 98%yield for 2.5 h and reduction of Cr(Ⅵ)with more than 99%yield for 1.5 h at pH=2.Stability test shows BCN-3 still kept more than 98%reduction efficiency after 6 cycles.In addition,we also studied the reduction mechanism that shown the.O_(2)^(-)radicals essentially helped to reduce the Cr(Ⅵ)in aqueous solution under illumination,verified the direct Z-scheme charge transfer path by X-ray photoelectron spectroscopy(XPS)and the free radical trapping experiment.The work open a new way for rationally designing photocatalyst heterostructure to reduce Cr(Ⅵ)to Cr(Ⅲ).

One-step synthesis of defected Bi_(2)Al_(4)O_(9)/β-Bi_(2)O_(3) heterojunctions for photocatalytic reduction of CO_(2) to CO

Defect and charge transfer efficiency of nano-photocatalysts are important factors which influence their photocatalytic performance.In this work,oxygen vacancies are successfully introduced in the synthesis process of Bi_(2)Al_(4)O_(9)/β-Bi_(2)O_(3)heterojunctions through one-step in situ selfcombustion method.High-resolution transmission electron microscopy (HRTEM),UV-Vis diffuse reflectance spectra (UV-Vis DRS),and electron spin resonance (ESR) measurements confirm the existence of oxygen vacancies.In addition,by controlling the ratio of reactants of Bi(NO_(3))_(3)to Al(NO_(3))_(3),the ratio of Bi_(2)Al_(4)O_(9)and β-Bi_(2)O_(3)in the heterojunction can be easily adjusted.Photocurrent responses and surface photovoltage spectroscopy (SPV) indicate that the construction of the Bi_(2)Al_(4)O_(9)/β-Bi_(2)O_(3)heterostructure improves the separation efficiency of the photo-generated electrons and holes.CO_(2)-TPD results imply that the amounts and stability of heterojunctions are enhanced compared with their counterparts.The Bi_(2)Al_(4)O_(9)/β-Bi_(2)O_(3)heterojunction with 14 mol%Bi_(2)Al_(4)O_(9)shows the highest photocatalytic ability for reduction of CO_(2)into CO.The enhanced photoreduction of CO_(2)performance can be ascribed to the synergistic effects of the heterojunction for electron separation and oxygen vacancies for CO_(2)activation.

The photo-decomposition and self-restructuring dynamic equilibrium mechanism of Cu_(2)(OH)_(2)CO_(3)for stable photocatalytic CO_(2)reduction

Developing suitable photocatalysts and understanding their intrinsic catalytic mechanism remain key challenges in the pursuit of highly active,good selective,and long-term stable photocatalytic CO_(2)reduction(PCO_(2)R)systems.Herein,monoclinic Cu_(2)(OH)_(2)CO_(3)is firstly proven to be a new class of photocatalyst,which has excellent catalytic stability and selectivity for PCO_(2)R in the absence of any sacrificial agent and cocatalysts.Based on a Cu_(2)(OH)_(2)^(13)CO_(3)photocatalyst and 13CO_(2)two-sided^(13)C isotopic tracer strategy,and combined with in situ diffused reflectance infrared Fourier transform spectroscopy(DRIFTS)analysis and density functional theory(DFT)calculations,two main CO_(2)transformation routes,and the photo-decomposition and self-restructuring dynamic equilibrium mechanism of Cu_(2)(OH)_(2)CO_(3)are definitely revealed.The PCO_(2)R activity of Cu_(2)(OH)_(2)CO_(3)is comparable to some of state-of-the-art novel photocatalysts.Significantly,the PCO_(2)R properties can be further greatly enhanced by simply combining Cu_(2)(OH)_(2)CO_(3)with typical TiO_(2)to construct composites photocatalyst.The highest CO_(2)and CH_(4)production rates by 7.5 wt%Cu_(2)(OH)_(2)CO_(3)-TiO_(2)reach 16.4μmol g^(-1)h^(-1)and 116.0μmol g^(-1)h^(-1),respectively,which are even higher than that of some of PCO_(2)R systems containing sacrificial agents or precious metals modified photocatalysts.This work provides a better understanding for the PCO_(2)R mechanism at the atomic levels,and also indicates that basic carbonate photocatalysts have broad application potential in the future.

Regulating^(*)COOH intermediate via amino alkylation engineering for exceptionally effective photocatalytic CO_(2) reduction

Photocatalytic reduction of CO_(2) into fuel represents a promising approach for achieving carbon neutrality,while realizing high selectivity in this process is challenging due to uncontrollable reaction intermediate and retarded desorption of target products.Engineering the interface microenvironment of catalysts has been proposed as a strategy to exert a significant influence on reaction outcomes,yet it remains a significant challenge.In this study,amino alkylation was successfully integrated into the melem unit of polymeric carbon nitrides(PCN),which could efficiently drive the photocatalytic CO_(2) reduction.Experimental characterization and theoretical calculations revealed that the introduction of amino alkylation lowers the energy barrier for CO_(2) reduction into^(*)COOH intermediate,transforming the adsorption of^(*)COOH intermediate from the endothermic to an exothermic process.Notably,the as-prepared materials demonstrated outstanding performance in photocatalytic CO_(2) reduction,yielding CO_(2)at a rate of 152.8μmol h^(-1) with a high selectivity of 95.4%and a quantum efficiency of 6.6%.

Effects of surface chlorine atoms on charge distribution and reaction barriers for photocatalytic CO_(2)reduction

Photocatalytic CO_(2)reduction to produce high value-added carbon-based fuel has been proposed as a promising approach to mitigate global warming issues.However,the conversion efficiency and product selectivity are still low due to the sluggish dynamics of transfer processes involved in proton-assisted multi-electron reactions.Lowering the formation energy barriers of intermediate products is an effective method to enhance the selectivity and productivity of final products.In this study,we aim to regulate the surface electronic structure of Bi_(2)WO_(6)by doping surface chlorine atoms to achieve effective photocatalytic CO_(2)reduction.Surface Cl atoms can enhance the absorption ability of light,affect its energy band structure and promote charge separation.Combined with DFT calculations,it is revealed that surface Cl atoms can not only change the surface charge distribution which affects the competitive adsorption of H_(2)O and CO_(2),but also lower the formation energy barrier of intermediate products to generate more intermediate*COOH,thus facilitating CO production.Overall,this study demonstrates a promising surface halogenation strategy to enhance the photocatalytic CO_(2)reduction activity of a layered structure Bi-based catalyst.

C-doped BiOCl/Bi_(2)S_(3) heterojunction for highly efficient photoelectrochemical detection and photocatalytic reduction of Cr(VI)

Novel C-BiOCl/Bi_(2)S_(3) composites are prepared by hydrothermal C doping in BiOCl and in-situ growth of Bi_(2)S_(3) on C-BiOCl.Compared with BiOCl,C-BiOCl has a larger exposed surface area and can effectively absorb visible light.The construction of a heterojunction in C-BiOCl/Bi_(2)S_(3) further promotes the separation and transfer of photogenerated carriers.With improved photoelectric properties,the optimized 5C-BiOCl/5Bi_(2)S_(3) is applied as a dual-functional composite for photoelectrochemical(PEC)detection and photocatalytic(PC)reduction of Cr(VI).The 5C-BiOCl/5Bi_(2)S_(3) shows a linear range of 0.02-80μM for PEC cathodic detection of Cr(VI)with a detection limit of 0.01628μM.Additionally,99.5%of Cr(VI)can be removed via absorption and PC reduction by 5C-BiOCl/5Bi_(2)S_(3),with the reduction rate constant(k)336 times higher than that of BiOCl.

Development strategies and improved photocatalytic CO_(2) reduction performance of metal halide perovskite nanocrystals

In recent years,photocatalytic CO_(2)reduction reaction(CRR) has attracted much scientific attention to overcome energy and environmental issues by converting CO_(2)into high-value-added chemicals utilizing solar energy.Metal halide perovskite(MHP) nanocrystals(NCs) are recognized as an ideal choice for CRR owing to their outstanding optoelectronic properties.Although great efforts have been devoted to designing more effective photocatalysts to optimize CRR performance,severe charge recombination,instability,and unsatisfactory activity have become major bottlenecks in developing perovskite-based photocatalysts.In this review,we mainly focus on the recent research progress in the areas of relevance.First,a brief insight into reaction mechanisms for CRR and structural features of MHPs are introduced.Second,efficient modification approaches for the improvement of the photocatalytic activity and stability of the perovskite-based catalysts are comprehensively reviewed.Third,the state-of-the-art achievements of perovskite-based photocatalysts for CRR are systematically summarized and discussed,which are focused on the modification approaches,structure design,and the mechanism of the CO_(2)reduction process.Lastly,the current challenges and future research perspectives in the design and application of perovskite materials are highlighted from our point of view to provide helpful insights for seeking breakthroughs in the field of CRR.This review may provide a guide for scientists interested in applying perovskite-based catalysts for solar-to-chemical energy conversion.

Cu-Based Materials for Enhanced C_(2+) Product Selectivity in Photo-/Electro-Catalytic CO_(2) Reduction: Challenges and Prospects

Carbon dioxide conversion into valuable products using photocatalysis and electrocatalysis is an effective approach to mitigate global environmental issues and the energy shortages. Among the materials utilized for catalytic reduction of CO_(2), Cu-based materials are highly advantageous owing to their widespread availability, cost-effectiveness, and environmental sustainability. Furthermore, Cu-based materials demonstrate interesting abilities in the adsorption and activation of carbon dioxide, allowing the formation of C_(2+) compounds through C–C coupling process. Herein, the basic principles of photocatalytic CO_(2) reduction reactions(PCO_(2)RR) and electrocatalytic CO_(2) reduction reaction(ECO_(2)RR) and the pathways for the generation C_(2+) products are introduced. This review categorizes Cu-based materials into different groups including Cu metal, Cu oxides, Cu alloys, and Cu SACs, Cu heterojunctions based on their catalytic applications. The relationship between the Cu surfaces and their efficiency in both PCO_(2)RR and ECO_(2)RR is emphasized. Through a review of recent studies on PCO_(2)RR and ECO_(2)RR using Cu-based catalysts, the focus is on understanding the underlying reasons for the enhanced selectivity toward C_(2+) products. Finally, the opportunities and challenges associated with Cu-based materials in the CO_(2) catalytic reduction applications are presented, along with research directions that can guide for the design of highly active and selective Cu-based materials for CO_(2) reduction processes in the future.

Engineering of oxygen vacancy and bismuth cluster assisted ultrathin Bi_(12)O_(17)Cl_(2)nanosheets with efficient and selective photoreduction of CO_(2)to CO

The photocatalytic conversion of CO_(2)into solar‐powered fuels is viewed as a forward‐looking strategy to address energy scarcity and global warming.This work demonstrated the selective photoreduction of CO_(2)to CO using ultrathin Bi_(12)O_(17)Cl_(2)nanosheets decorated with hydrothermally synthesized bismuth clusters and oxygen vacancies(OVs).The characterizations revealed that the coexistences of OVs and Bi clusters generated in situ contributed to the high efficiency of CO_(2)–CO conversion(64.3μmol g^(−1)h^(−1))and perfect selectivity.The OVs on the facet(001)of the ultrathin Bi_(12)O_(17)Cl_(2)nanosheets serve as sites for CO_(2)adsorption and activation sites,capturing photoexcited electrons and prolonging light absorption due to defect states.In addition,the Bi‐cluster generated in situ offers the ability to trap holes and the surface plasmonic resonance effect.This study offers great potential for the construction of semiconductor hybrids as multiphotocatalysts,capable of being used for the elimination and conversion of CO_(2)in terms of energy and environment.

Bi-, Y-Codoped TiO2 for Carbon Dioxide Photocatalytic Reduction to Formic Acid under Visible Light Irradiation

Bi- and Y-codoped TiO2 photocatalysts were synthesized through a sol-gel method, and they were applied in the photocatalytic reduction of CO2 to formic acid under visible light irradiation. The results revealed that, after doping Bi and Y, the surface area of TiO2 was increased from 5.4 to 93.1 m2/g when the mole fractions of doping Bi and Y were 1.0% and 0.5%, respectively, and the lattice structures of the photocatalysts changed and the oxygen vacancies on the surface of the photocatalysts formed, which would act as the electron capture centers and slow down the recombination of pho- to-induced electron and hole. The photocurrent spectra also proved that the photocatalysts had better electronic transmission capacities. The HCOOH yield in CO2 photocatalytic reduction was 747.82 μmol/gcat by using 1% Bi-0.5% Y-TiO2 as a photocatalyst. The HCOOH yield was 1.17 times higher than that by using 1% Bi-TiO2, and 2.23 times higher than that by using pure TiO2. Furthermore, the 1% Bi-0.5% Y-TiO2 showed the highest apparent quantum efficiency （AQE） of 4.45%.

Formation of willow leaf-like structures composed of NH2-MIL68（In） on a multifunctional multiwalled carbon nanotube backbone for enhanced photocatalytic reduction of Cr（VI）

Efficient separation and transfer of photogenerated electron/hole as well as enhanced visible light absorption play essential roles in photocatalytic reactions. To promote the photocatalytic reduction of Cr（VI）, a toxic heavy metal ion, multiwalled carbon nanotube （MWCNT） was introduced as an electron acceptor into NH2-MIL-68（In）. This led to the growth of a willow leaf-like metal-organic framework （MOF） on an MWCNT backbone forming MWCNT/NH2-MIL-68（In） （PL-1）, which showed a highly efficient transfer of photogenerated carriers. Moreover, MWCNT incorporation introduced more mesopores for Cr（VI） diffusion and enhanced the visible light adsorption without lowering the conduction band position. As a result, the photocatalytic kinetic constant of PL-1 was found to be almost three times higher than that of the parent NH2-MIL-68（In）. Thus, growing MOFs on MWCNTs provides a facile and promising solution for effective remediation of environmental pollution by utilizing solar energy. This work provides the first example of using MWCNT/MOF composites for photocatalytic reactions.

Synergy of photocatalytic reduction and adsorption for boosting uranium removal with PMo_(12)/UiO-66 heterojunction

In this work,we proposed a new U(Ⅵ)removal strategy combining adsorption and photocatalytic reduction by the PMo_(12)/UiO-66 heterojunctions.The PMo_(12)has been encapsulated in the cavities of Ui O-66 by a one-step hydrothermal method,and the PMo_(12)/UiO-66 exhibited high adsorption capacity and photocatalytic activity.The maximal theoretical sorption capacity of U(Ⅵ)on 15%PMo_(12)/UiO-66 reached225.36 mg/g and the photoreduction rate of 15%PMo_(12)/UiO-66 is about thirty times as much as UiO-66.Under the light irradiation,the photogenerated electrons rapidly transport from UiO-66 to PMo_(12),and the photo-generated electrons could efficiently reduce the pre-enriched U(Ⅵ)to U(IV).This work provides new insights into remediation of the radioactive environment.

Efficient photocatalytic reduction of chromium(Ⅵ) using photoreduced graphene oxide as photocatalyst under visible light irradiation

Graphene oxide(GO),a new and promising material,has been widely used as a co-catalyst in photocatalytic reactions;however,its capacity as a sole photocatalyst has rarely been investigated.In this study,ultraviolet(UV) light irradiation was used as a modification method to obtain reduced GO(rGO) samples.The samples were used as photocatalysts to examine their visible light photocatalytic activity toward hexavalent chromium(Cr(Ⅵ)) removal.Atomic force microscopy(AFM),X-ray diffraction(XRD),UV-vis spectrophotometry,Raman spectroscopy,X-ray photoelectron spectroscopy(XPS),and electron spin resonance(ESR) spectroscopy were applied to interpret the surface and structure changes with UV irradiation.The oxygen-containing functional groups(OFGs) on the GO surface were reduced to defective carbons andπ-conjugated C=C(sp^(2) domains) under UV light;this led to a decrease in the interlayer distance between GO sheets,GO fragmentation,and increased disorder on the GO surface.The restoration of sp^(2) domains led to a narrower band gap of GO,which favored the rGO excitation by visible light to generate electron-hole pairs.The rGO pre-irradiated with UV for 1 h(rGO-1),possessing the highest defect density and electron generation efficiency,exhibited the best Cr(Ⅵ) reduction efficiency,which was about three times that of the GO sample;moreover,it outperformed most of the reported GO-based nanomaterials.In addition,low pH and the addition of citric acid as a hole scavenger could further improve the photocatalytic activity.This study proves that GO or rGO can be used as a sole photocatalyst under visible light to remove environmental pollutants such as heavy-metal ions,and it paves the way for the development of this kind of material and its UV-irradiation modification for further applications.

Nb_2O_5 nanowires in-situ grown on carbon fiber: A high-efficiency material for the photocatalytic reduction of Cr(Ⅵ)

Niobium oxide nanowire-deposited carbon fiber（CF） samples were prepared using a hydrothermal method with amorphous Nb2O5·nH2O as precursor. The physical properties of the samples were characterized by means of numerous techniques, including X-ray diffraction（XRD）, energy-dispersive spectroscopy（EDS）, scanning electron microscopy（SEM）, transmission electron microscopy（TEM）, selected-area electron diffraction（SAED）, UV–visible spectroscopy（UV–vis）, N2 adsorption–desorption, Fourier transform infrared spectroscopy（FT-IR）, and X-ray photoelectron spectroscopy. The efficiency for the removal of Cr（VI） was determined.Parameters such as pH value and initial Cr（VI） concentration could influence the Cr（VI） removal efficiency or adsorption capacity of the Nb2O5/carbon fiber sample obtained after hydrothermal treatment at 160°C for 14 hr. The maximal Cr（VI） adsorption capacity of the Nb2O5 nanowire/CF sample was 115 mg/g. This Nb2O5/CF sample also showed excellent photocatalytic activity and stability for the reduction of Cr（Ⅵ） under UV-light irradiation： the Cr（VI） removal efficiency reached 99.9% after UV-light irradiation for 1 hr and there was no significant decrease in photocatalytic performance after the use of the sample for 10 repeated cycles. Such excellent Cr（VI） adsorption capacity and photocatalytic performance was related to its high surface area,abundant surface hydroxyl groups, and good UV-light absorption ability.

Effect of Ti（lll） Surface Defects on the Process of Photocatalytic Reduction of Hexavalent Chromium

In this paper, the process of photocatalytic reduction of hexavalent chromium was investigated over Ti3＋- modified TiO2 photocatalysts. The Ti3＋ surface defects were repaired by annealing as-prepared sample at different temperatures to control the amount of Ti3＋ sites. The samples were characterized by SEM, XRD, BET, UV-Vis absorption, EPR and XPS. The results showed Ti3＋ defects were successfully doped in TiO2. The surface selective adsorption of hexavalent chromium [Cr2072 （Cr（VI））] and the desorption of trivalent chromium [Cr3＋ （Cr（III））] were investigated during the process ofphotocatalytic reduction positive charges due to more Ti3＋ defects on the surface show a Accordingly, the surface positive reduction of Cr（VI）. charges controlled by the Ti3＋ Zeta potential results presented that the increased significant improvement for adsorption of Cr（VI）. defects play important roles in the photocatalytic

Photocatalytic reduction of Cr(Ⅵ)by WO_(3)@PVP with elevated conduction band level and improved charge carrier separation property

Photocatalytic reduction of heavy metal ions is a green and promising technology which requires electrons with enough negative energy levels as well as efficient separation property from photo-generated holes of photocatalysts.For WO_(3),the low conduction band edge and the severe photo-generated charge carrier recombination limited its application in photocatalytic reduction of pollutants.In this work,we prepared WO_(3)@PVP with PVP capped WO_(3) by a simple one-step hydrothermal method,which showed an elevated energy band structure and improved charge carrier separation property.XRD,SEM,TEM,XPS,DRS,and the photocurrent density test were carried out to study the properties of the composite.Results demonstrated monoclinic WO_(3) with a size of~100-250 nm capped by PVP was obtained,which possessed fewer lattice defects inside but more defects(W^(5+))on the surface.Moreover,the results of the photocatalytic experiment showed the kinetic constant of Cr(Ⅵ)reduction process on WO_(3)@PVP was 0.532 h^(-1),which was 3.1 times higher than that onWO_(3)(0.174 h^(-1)),demonstratingWO_(3)@PVP with good photocatalytic capability for Cr(Ⅵ)reduction.This can be attributed to the improved charge carrier separation performance,the improved adsorption capacity and the elevated conduction band edge of WO_(3)@PVP.More importantly,the energy band structure of WO_(3)@PVP was proved elevated with a value as high as 1.14 eV than that of WO_(3) nanoparticles,which enables WO_(3)@PVP a promising material in the photocatalytic reduction reaction of heavy metal ions from wastewater.

Photocatalytic reduction of dichromate by titanium dioxide supported on hollow glass microbeads

The photocatalytic reduction of dichromate using TiO2/beads as a photocatalyst was studied. The results showed hat 3.8 × 104 mol/dm3 of dichromate can be completely reduced into Cr3+ after illumination far 35 min with a 375 W medium pressure mercury lamp. The effects of factors such as the amount of TiO2/beads, initial concentration of dichromate, initial pH, nitrogen and oxygen atmosphere, and concentration of Fe3+ on the photocatalytic reduction of dichromate were investigated. A possible mechanism of the photocatalytic reaction was proposed. After illumination for 200 h, no significant loss of photocatalytic activity of TiO2/beads was observed.

Fabrication of ultrafine ZnFe2O4 nanoparticles for efficient photocatalytic reduction CO2 under visible light illumination

A one-pot, solvent-thermal process was used to create the ultrafine ZnFe2O4nanoparticles photocatalyst.During the solvent-thermal process, the in situ self-forming NaCl not only served as a "cage" to confine the ion diffusion, but also acted as a microreactor for nanocrystallite growth. An average particle size of ～10 nm and a high-specific surface area of～112.9 m2/g were observed for the ultrafine ZnFe2O4nanoparticles Owing to the synergistic effect of ultrafine particle size, the full utilization of the visible light region and high conduction band(CB) position, ultrafine ZnFe2O4photocatalyst displayed an efficient photocatalytic CO2reduction under visible light illumination. Besides, the ultrafine ZnFe2O4photocatalyst showed high production selectivity for CH3CHO and C2H5OH generation in aqueous CO2/NaHCO3solution. This work may provide a new idea for the synthesis of new high-efficiency photocatalysts.