Graphene‐Based Mesoporous SnO2 with Enhanced Electrochemical Performance for Lithium‐Ion Batteries

Graphene‐based metal oxides generally show outstanding electrochemical performance due to the superior properties of graphene. However, the aggregation of active metal oxide nanoparticles on the graphene surface may result in a capacity fading and poor cycle performance. Here, a mesostructured graphene‐based SnO₂ composite is prepared through in situ growth of SnO₂ particles on the graphene surface using cetyltrimethylammonium bromide as the structure‐directing agent. This novel mesoporous composite inherits the advantages of graphene nanosheets and mesoporous materials and exhibits higher reversible capacity, better cycle performance, and better rate capability compared to pure mesoporous SnO₂ and graphene‐based nonporous SnO₂. It is concluded that the synergetic effect between graphene and mesostructure benefits the improvement of the electrochemical properties of the hybrid composites. This facile method may offer an attractive alternative approach for preparation of the graphene‐based mesoporous composites as high‐ performance electrodes for lithium‐ion batteries.     

High Reversible Pseudocapacity in Mesoporous Yolk–Shell Anatase TiO2/TiO2(B) Microspheres Used as Anodes for Li‐Ion Batteries

As an anode material for lithium‐ion batteries, titanium dioxide (TiO₂) shows good gravimetric performance (336 mAh g^?1 for LiTiO₂) and excellent cyclability. To address the poor rate behavior, slow lithium‐ion (Li⁺) diffusion, and high irreversible capacity decay, TiO₂ nanomaterials with tuned phase compositions and morphologies are being investigated. Here, a promising material is prepared that comprises a mesoporous “yolk–shell” spherical morphology in which the core is anatase TiO₂ and the shell is TiO₂(B). The preparation employs a NaCl‐assisted solvothermal process and the electrochemical results indicate that the mesoporous yolk–shell microspheres have high specific reversible capacity at moderate current (330.0 mAh g^?1 at C/5), excellent rate performance (181.8 mAh g^?1 at 40C), and impressive cyclability (98% capacity retention after 500 cycles). The superior properties are attributed to the TiO₂(B) nanosheet shell, which provides additional active area to stabilize the pseudocapacity. In addition, the open mesoporous morphology improves diffusion of electrolyte throughout the electrode, thereby contributing directly to greatly improved rate capacity.     

A High‐Energy Li‐Ion Battery Using a Silicon‐Based Anode and a Nano‐Structured Layered Composite Cathode

High capacity electrodes based on a Si composite anode and a layered composite oxide cathode, Ni‐rich Li[Ni_0.75Co_0.1Mn_0.15]O₂, are evaluated and combined to fabricate a high energy lithium ion battery. The Si composite anode, Si/C‐IWGS (internally wired with graphene sheets), is prepared by a scalable sol–gel process. The Si/C‐IWGS anode delivers a high capacity of >800 mAh g^?1 with an excellent cycling stability of up to 200 cycles, mainly due to the small amount of graphene (～6 wt%). The cathode (Li[Ni_0.75Co_0.1Mn_0.15]O₂) is structurally optimized (Ni‐rich core and a Ni‐depleted shell with a continuous concentration gradient between the core and shell, i.e., a full concentration gradient, FCG, cathode) so as to deliver a high capacity (>200 mAh g^?1) with excellent stability at high voltage (～4.3 V). A novel lithium ion battery system based on the Si/C‐IWGS anode and FCG cathode successfully demonstrates a high energy density (240 Wh kg^?1 at least) as well as an unprecedented excellent cycling stability of up to 750 cycles between 2.7 and 4.2 V at 1C. As a result, the novel battery system is an attractive candidate for energy storage applications demanding a high energy density and long cycle life.     

An In Situ Simultaneous Reduction‐Hydrolysis Technique for Fabrication of TiO2‐Graphene 2D Sandwich‐Like Hybrid Nanosheets: Graphene‐Promoted Selectivity of Photocatalytic‐Driven Hydrogenation and Coupling of CO2 into Methane and Ethane

A novel, in situ simultaneous reduction‐hydrolysis technique (SRH) is developed for fabrication of TiO₂‐‐graphene hybrid nanosheets in a binary ethylenediamine (En)/H₂O solvent. The SRH technique is based on the mechanism of the simultaneous reduction of graphene oxide (GO) into graphene by En and the formation of TiO₂ nanoparticles through hydrolysis of titanium (IV) (ammonium lactato) dihydroxybis, subsequently in situ loading onto graphene through chemical bonds (Ti–O–C bond) to form 2D sandwich‐like nanostructure. The dispersion of TiO₂ hinders the collapse and restacking of exfoliated sheets of graphene during reduction process. In contrast with prevenient G‐TiO₂ nanocomposites, abundant Ti³⁺ is detected on the surface of TiO₂ of the present hybrid, caused by reducing agent En. The Ti³⁺ sites on the surface can serve as sites for trapping photogenerated electrons to prevent recombination of electron–hole pairs. The high photocatalytic activity of G‐TiO₂ hybrid is confirmed by photocatalytic conversion of CO₂ to valuable hydrocarbons (CH₄ and C₂H₆) in the presence of water vapor. The synergistic effect of the surface‐Ti³⁺ sites and graphene favors the generation of C₂H₆, and the yield of the C₂H₆ increases with the content of incorporated graphene. The work may open a new doorway for new significant application of graphene for selectively catalytic C–C coupling reaction     

Mesoporous Anatase Titania Hollow Nanostructures though Silica‐Protected Calcination

The crystallization of nanometer‐scale materials during high‐temperature calcination can be controlled by a thin layer of surface coating. Here, a novel silica‐protected calcination process for preparing mesoporous hollow TiO₂ nanostructures with a high surface area and a controllable crystallinity is presented. This method involves the preparation of uniform silica colloidal templates, sequential deposition of TiO₂ and then SiO₂ layers through sol–gel processes, calcination to transform amorphous TiO₂ to crystalline anatase, and finally etching of the inner and outer silica to produce mesoporous anatase TiO₂ shells. The silica‐protected calcination step allows crystallization of the amorphous TiO₂ layer into anatase nanocrystals, while simultaneously limiting the growth of anatase grains to within several nanometers, eventually producing mesoporous anatase shells with a high surface area (～311 m² g^?1) and good water dispersibility upon chemical etching of the silica. When used as photocatalysts for the degradation of Rhodamine B under UV irradiation, the as‐synthesized mesoporous anatase shells show significantly enhanced photocatalytic activity with greater enhancement for samples calcined at higher temperatures thanks to their improved crystallinity.     

Well‐Ordered Large‐Pore Mesoporous Anatase TiO2 with Remarkably High Thermal Stability and Improved Crystallinity: Preparation,Characterization, and Photocatalytic Performance

Thermally‐stable, ordered mesoporous anatase TiO₂ with large pore size and high crystallinity has been successfully synthesized through an evaporation‐induced self‐assembly technique, combined with encircling ethylenediamine (EN) protectors to maintain the liquid crystal mesophase structure of TiO₂ primary particles, followed by calcination at higher temperature. The structures of the prepared mesoporous TiO₂ are characterized in detail by small‐angle and wide‐angle X‐ray diffraction, Raman spectra, N₂ adsorption/desorption isotherms, and transmission electron microscopy. Experimental results indicate that the well‐ordered mesoporous structure could be maintained up to 700 °C (M700) and also possesses large pore size (10 nm), high specific BET surface area (122 m² g^?1), and high total pore volumes (0.20 cm³ g^?1), which is attributed to encircling EN protectors for maintaining the mesoporous framework against collapsing, inhibiting undesirable grain growth and phase transformation during the calcination process. A possible formation mechanism for the highly stable large‐pore mesoporous anatase TiO₂ is also proposed here, which could be further confirmed by TG/FT‐IR in site analysis and X‐ray photoelectron spectroscopy. The obtained mesoporous TiO₂ of M700 exhibit better photocatalytic activity than that of Degussa P25 TiO₂ for degradation of highly toxic 2,4‐dichlorophenol under UV irradiation. This enhancement is attributed to the well‐ordered large‐pore mesoporous structure, which facilitates mass transport, the large surface area offering more active sites, and high crystallinity that favors the separation of photogenerated electron‐hole pairs, confirmed by surface photovoltage spectra.     

Self‐Assembling TiO2 Nanorods on Large Graphene Oxide Sheets at a Two‐Phase Interface and Their Anti‐Recombination in Photocatalytic Applications

TiO₂ nanorods are self‐assembled on the graphene oxide (GO) sheets at the water/toluene interface. The self‐assembled GO–TiO₂ nanorod composites (GO–TiO₂ NRCs) can be dispersed in water. The effective anchoring of TiO₂ nanorods on the whole GO sheets is confirmed by transmission electron microscopy (TEM), X‐ray diffraction (XRD), Fourier transform IR spectroscopy (FTIR), and thermogravimetric analysis (TGA). The significant increase of photocatalytic activity is confirmed by the degradation of methylene blue (MB) under UV light irridiation. The large enhancement of photocatalytic activity is caused by the effective charge anti‐recombination and the effective absorption of MB on GO. The effective charge transfer from TiO₂ to GO sheets is confirmed by the significant photoluminescence quenching of TiO₂ nanorods, which can effectively prevent the charge recombination during photocatalytic process. The effective absorption of MB on GO is confirmed by the UV‐vis spectra. The degradation rate of MB in the second cycle is faster than that in the first cycle because of the reduction of GO under UV light irradiation.     

Unraveling Metal Oxide Role in Exfoliating Graphite: New Strategy to Construct High‐Performance Graphene‐Modified SiOx‐Based Anode for Lithium‐Ion Batteries

Exfoliating graphite to graphene has attracted great attention due to the fantastic properties of graphene available for designing graphene‐based materials or devices. Besides the classic solution method, herein a unique role of TiO₂ in exfoliating graphite to be graphene layers effectively is reported. As a paradigm, this discovered effect of TiO₂ is significant for preparing high‐performance graphene‐modified SiO_x‐based anode in lithium‐ion batteries (LIBs), in which the graphite is in situ exfoliated mechanically by TiO₂ to be multilayered graphene (i.e., MLG) and then the SiO_x is wrapped by the MLG to construct a SiO_x/TiO₂@MLG. In this case, an extremely high capacity of 1484 mAh g^?1, long lifespan over 1200 cycles at 2 A g^?1, as well as good performance in full LIBs (vs nickel‐rich cathode) are demonstrated. It is confirmed that the MLG can enhance electric conductivity, mitigate electrolyte decomposition, and alleviate volume effect of the SiO_x effectively. This result is hard to be achieved using other kinds of metal oxide besides TiO₂. It is hoped that the SiO_x/TiO₂@MLG is practical for pursuing LIBs with an energy density beyond 300 Wh kg^?1. In addition, it is believed the ingenious strategy is applicable for designing more functional materials with greater capabilities.     

General Synthesis of N‐Doped Macroporous Graphene‐Encapsulated Mesoporous Metal Oxides and Their Application as New Anode Materials for Sodium‐Ion Hybrid Supercapacitors

A general method to synthesize mesoporous metal oxide@N‐doped macroporous graphene composite by heat‐treatment of electrostatically co‐assembled amine‐functionalized mesoporous silica/metal oxide composite and graphene oxide, and subsequent silica removal to produce mesoporous metal oxide and N‐doped macroporous graphene simultaneously is reported. Four mesoporous metal oxides (WO_{3? x} , Co₃O₄, Mn₂O₃, and Fe₃O₄) are encapsulated in N‐doped macroporous graphene. Used as an anode material for sodium‐ion hybrid supercapacitors (Na‐HSCs), mesoporous reduced tungsten oxide@N‐doped macroporous graphene (m‐WO_{3? x} @NM‐rGO) gives outstanding rate capability and stable cycle life. Ex situ analyses suggest that the electrochemical reaction mechanism of m‐WO_{3? x} @NM‐rGO is based on Na⁺ intercalation/de‐intercalation. To the best of knowledge, this is the first report on Na⁺ intercalation/de‐intercalation properties of WO_{3? x} and its application to Na‐HSCs.     

Ionic Liquid‐Assisted Synthesis of TiO2–Carbon Hybrid Nanostructures for Lithium‐Ion Batteries

Building nanocomposite architectures based on nanocarbon materials (such as carbon nanotubes and graphene nanosheets) and metal‐oxide nanoparticles is of great interests for electrochemical energy storage. Here, an ionic‐liquid‐assisted strategy is presented to mediate the in situ growth of TiO₂ nanocrystals with controlled size on carbon nanotubes and graphene, and also reduce the modified carbon supports to recover the graphitic structure simultaneously. The as‐prepared nanocomposites exhibit a highly porous and robust structure with intimate coupling between TiO₂ nanocrystals and carbon supports, which offers facile ion and electron transport pathway as well as high mechanical stability. When evaluated as electrode materials for lithium‐ion batteries, the nanocomposites manifest high specific capacity, long cycling lifetime, and excellent rate capability, showing their promising application in high‐performance energy storage devices.     

Syntheses,Li Insertion,and Photoactivity of Mesoporous Crystalline TiO2

Ordered mesoporous rutile and anatase TiO₂ samples are prepared using mesoporous silica SBA‐15 as template and freshly synthesized titanium nitrate and titanium chloride solutions as precursors. The rutile material formed from the nitrate solution is monocrystalline and contains minimal amounts of Si with a Si:Ti ratio of 0.031(4), whereas the anatase material formed from the chloride solution comprises nanocrystals and contains a higher content of Si with a Si:Ti ratio of 0.18(3). It is found that control of temperature and selection of Ti‐containing precursor play important roles in determining the crystal phase and crystallinity. A possible formation mechanism of porous crystalline TiO₂ is suggested. Characterization of these porous materials is performed by XRD, HRTEM, and nitrogen adsorption/desorption. SBA‐15‐templated mesoporous rutile TiO₂ exhibits a higher Li ion insertion capability than KIT‐6‐templated TiO₂ due to its larger surface area. Likewise mesoporous anatase TiO₂:SiO₂ composite has a better photoactivity than bulk TiO₂ or TiO₂‐loaded SBA‐15 for bleaching methylene blue.     

Recent Progress in the Synthesis of Spherical Titania Nanostructures and Their Applications

Recent progress in the fabrication and application of diverse spherical titania nanostructures, including mesoporous spheres, spherical flaky assemblies, and dendritic particles of variable diameter and monodispersity in size, is summarized in this article. Utilizing different synthesis strategies, spherical titania nanostructures with tailored polymorphs (including amorphous, anatase, rutile, brookite and TiO₂‐B), particle sizes (from tens of nanometers to millimeters), monodispersity, porosity, and variable surface properties have been produced. Such spherical titania nanostructures show realized and potential applications in the areas of chromatographic separation, lithium‐ion batteries, dye‐sensitized solar cells, photocatalytic oxidation and water splitting, photoluminescence, electrorheological fluids, catalysis, gas sensing, and anticancer intracellular drug delivery. Gaining further understanding of both synthesis design and application of these materials will promote the commercialization of such spherical titania nanostructures in the future.     

Highly Ordered Mesoporous Crystalline MoSe2 Material with Efficient Visible‐Light‐Driven Photocatalytic Activity and Enhanced Lithium Storage Performance

Highly ordered mesoporous crystalline MoSe₂ is synthesized using mesoporous silica SBA‐15 as a hard template via a nanocasting strategy. Selenium powder and phosphomolybdic acid (H₃PMo₁₂O₄₀) are used as Se and Mo sources, respectively. The obtained products have a highly ordered hexagonal mesostructure and a rod‐like particle morphology, analogous to the mother template SBA‐15. The UV‐vis‐NIR spectrum of the material shows a strong light absorption throughout the entire visible wavelength region. The direct bandgap is estimated to be 1.37 eV. The high surface area MoSe₂ mesostructure shows remarkable photocatalytic activity for the degradation of rhodamine B, a model organic dye, in aqueous solution under visible light irradiation. In addition, the synthesized mesoporous MoSe₂ possess a reversible lithium storage capacity of 630 mAh g^?1 for at least 35 cycles without any notable decrease. The rate performance of mesoporous MoSe₂ is much better than that of analogously synthesized mesoporous MoS₂, making it a promising anode for the lithium ion battery.     

Optimization of Von Mises Stress Distribution in Mesoporous α‐Fe2O3/C Hollow Bowls Synergistically Boosts Gravimetric/Volumetric Capacity and High‐Rate Stability in Alkali‐Ion Batteries

Hollow structures are often used to relieve the intrinsic strain on metal oxide electrodes in alkali‐ion batteries. Nevertheless, one common drawback is that the large interior space leads to low volumetric energy density and inferior electric conductivity. Here, the von Mises stress distribution on a mesoporous hollow bowl (HB) is simulated via the finite element method, and the vital role of the porous HB structure on strain‐relaxation behavior is confirmed. Then, N‐doped‐C coated mesoporous α‐Fe₂O₃ HBs are designed and synthesized using a multistep soft/hard‐templating strategy. The material has several advantages: (i) there is space to accommodate strains without sacrificing volumetric energy density, unlike with hollow spheres; (ii) the mesoporous hollow structure shortens ion diffusion lengths and allows for high‐rate induced lithiation reactivation; and (iii) the N‐doped carbon nanolayer can enhance conductivity. As an anode in lithium‐ion batteries, the material exhibits a very high reversible capacity of 1452 mAh g^?1 at 0.1 A g^?1, excellent cycling stability of 1600 cycles (964 mAh g^?1 at 2 A g^?1), and outstanding rate performance (609 mAh g^?1 at 8 A g^?1). Notably, the volumetric specific capacity of composite electrode is 42% greater than that of hollow spheres. When used in potassium‐ion batteries, the material also shows high capacity and cycle stability.     

Fe3O4 Nanoparticles Embedded in Uniform Mesoporous Carbon Spheres for Superior High‐Rate Battery Applications

Robust composite structures consisting of Fe₃O₄ nanoparticles (～5 nm) embedded in mesoporous carbon spheres with an average size of about 70 nm (IONP@mC) are synthesized by a facile two‐step method: uniform Fe₃O₄ nanoparticles are first synthesized followed by a post‐synthetic low‐temperature hydrothermal step to encapsulate them in mesoporous carbon spheres. Instead of graphene which has been extensively reported for use in high‐rate battery applications as a carbonaceous material combined with metal oxides mesoporous carbon is chosen to enhance the overall performances. The interconnecting pores facilitate the penetration of electrolyte leading to direct contact between electrochemically active Fe₃O₄ and lithium ion‐carrying electrolyte greatly facilitating lithium ion transportation. The interconnecting carbon framework provides continuous 3D electron transportation routes. The anodes fabricated from IONP@mC are cycled under high current densities ranging from 500 to 10 000 mA g^?1. A high reversible capacity of 271 mAh g^?1 is reached at 10 000 mAh g^?1 demonstrating its superior high rate performance.     

Efficiency and Stability Enhancement in Perovskite Solar Cells by Inserting Lithium‐Neutralized Graphene Oxide as Electron Transporting Layer

This work proposes a new perovskite solar cell structure by including lithium‐neutralized graphene oxide (GO‐Li) as the electron transporting layer (ETL) on top of the mesoporous TiO₂ (m‐TiO₂) substrate. The modified work‐function of GO after the intercalation of Li atoms (4.3 eV) exhibits a good energy matching with the TiO₂ conduction band, leading to a significant enhancement of the electron injection from the perovskite to the m‐TiO₂. The resulting devices exhibit an improved short circuit current and fill factor and a reduced hysteresis. Furthermore, the GO‐Li ETL partially passivates the oxygen vacancies/defects of m‐TiO₂ by resulting in an enhanced stability under prolonged 1 SUN irradiation.     

SiO2/TiO2 Composite Film for High Capacity and Excellent Cycling Stability in Lithium‐Ion Battery Anodes

In this study, partially crystalline anodic TiO₂ with SiO₂ well‐distributed througout the entire oxide film is prepared using plasma electrolytic oxidation (PEO) to obtain a high‐capacity anode with an excellent cycling stability for Li‐ion batteries. The micropore sizes in the anodic film become inhomogeneous as the SiO₂ content is increased from 0% to 25%. The X‐ray diffraction peaks show that the formed oxide contains the anatase and rutile phases of TiO₂. In addition, X‐ray photoelectron spectroscopy and energy‐dispersive X‐ray analyses confirm that TiO₂ contains amorphous SiO₂. Anodic oxides of the SiO₂/TiO₂ composite prepared by PEO in 0.2 m H₂SO₄ and 0.4 m Na₂SiO₃ electrolyte deliver the best performance in Li‐ion batteries, exhibiting a capacity of 240 µAh cm^?2 at a fairly high current density of 500 µA cm^–2. The composite film shows the typical Li–TiO₂ and Li–SiO₂ redox peaks in the cyclic voltammogram and a corresponding plateau in the galvanostatic charge/discharge curves. The as‐prepared SiO₂/TiO₂ composite anode shows at least twice the capacity of other types of binder‐free TiO₂ and TiO₂ composites and very stable cycling stability for more than 250 cycles despite the severe mechanical stress.     

Empowering Metal Phosphides Anode with Catalytic Attribute toward Superior Cyclability for Lithium‐Ion Storage

Due to high capacity, moderate redox voltage, and relatively low polarization, metal phosphides (MPs) attract much attention as viable anode materials for lithium‐ion storage. However, severe capacity decay induced by the poor reversibility of discharge product (Li₃P) in these anodes suppresses their practical applications. Herein, it is first revealed that N‐doped carbon can effectively catalyze the oxidation of Li₃P by density functional theory calculations and activation experiments. By anchoring Ni₂P nanoparticles on N‐doped carbon sheets (Ni₂P@N‐C) via a facile method, an MP‐based anode rendered with a catalytic attribute is successfully fabricated for improving the reversibility of Li₃P during lithium‐ion storage. Benefiting from this design, not only can high capacity and rate performance be reached, but also an extraordinary cyclability and capacity retention be realized, which is the best among all other phosphides reported so far. By employing such a Ni₂P@N‐C composite and a commercialized active carbon as the anode and cathode, respectively, hybrid lithium‐ion capacitors can be fabricated with an ultrahigh energy density of 80 Wh kg^?1 at a power density of 12.5 kW kg^?1. This strategy of designing electrodes may be generalized to other energy storage systems whose cycling performance needs to be improved.     

Layer‐by‐Layer Synthesis of Thick Mesoporous TiO2 Films with Vertically Oriented Accessible Nanopores and Their Application for Lithium‐Ion Battery Negative Electrodes

TiO₂ films of varying thicknesses (up to ≈1.0 µm) with vertically oriented, accessible 7–9 nm nanopores are synthesized using an evaporation‐induced self‐assembly layer‐by‐layer technique. The hypothesis behind the approach is that epitaxial alignment of hydrophobic blocks of surfactant templates induces a consistent, accessible mesophase orientation across a multilayer film, ultimately leading to continuous, vertically aligned pore channels. Characterization using grazing incidence X‐ray scattering, scanning electron microscopy, and impedance spectroscopy indicates that the pores are oriented vertically even in relatively thick films (up to 1 µm). These films contain a combination of amorphous and nanocrystalline anatase titania of value for electrochemical energy storage. When applied as negative electrodes in lithium‐ion batteries, a capacity of 254 mAh g^?1 is obtained after 200 cycles for a single‐layer TiO₂ film prepared on modified substrate, higher than on unmodified substrate or nonporous TiO₂ film, due to the high accessibility of the vertically oriented channels in the films. Thicker films on modified substrate have increased absolute capacity because of higher mass loading but a reduced specific capacity because of transport limitations. These results suggest that the multilayer epitaxial approach is a viable way to prepare high capacity TiO₂ films with vertically oriented continuous nanopores.     

Oxygen Rich Titania: A Dopant Free,High Temperature Stable,and Visible‐Light Active Anatase Photocatalyst

The simultaneous existence of visible light photocatalytic activity and high temperature anatase phase stability up to 900 °C in undoped TiO₂ is reported for the first time. These properties are achieved by the in‐situ generation of oxygen through the thermal decomposition of peroxo‐titania complex (formed by the precursor modification with H₂O₂). Titania containing the highest amount of oxygen (16 H₂O₂‐TiO₂) retains 100% anatase phase even at 900 °C, where as the control sample exists as 100% rutile at this temperature. The same composition exhibits a six‐fold and two‐fold increase in visible light photocatalytic activities in comparison to the control sample and the standard photocatalyst Degussa P‐25 respectively. Among the various para­meters affecting the photocatalytic action, such as band gap narrowing, textural properties, crystallite size, and anatase phase stability, band gap narrowing was identified as the major factor responsible for the visible light photocatalytic activity. Increased Ti–O–Ti bond strength and upward shifting of the valence band (VB) maximum, which is responsible for the high temperature stability and visible light activity respectively, are identified from FT–IR, XPS, and photoluminescence (PL) spectroscopic studies. It is therefore proposed that the oxygen excess defects present in these titania samples are responsible for the high temperature stability and enhanced visible light photocatalytic activities.