Nanosized TiO2 Photocatalyst Research Articles

Facile preparation of isocyanate-functionalized poly(styrene-co-maleimide-co-maleic anhydride) particle and its application as a versatile support for nano-sized TiO2 photocatalyst

Polymer-supported catalysts have attracted intensive research attentions due to easy handling, facile separation and outstanding recycling performance. Herein, a simple, efficient and universal strategy was developed to fabricate isocyanate-functionalized polymer microspheres bearing highly reactive surface isocyanate groups, which could serve as a versatile support for the immobilization of photocatalyst. Firstly, self-stabilized precipitation polymerization of maleimide, maleic anhydride and styrene was performed in the presence of divinyl benzene (crosslinker) to prepare imide-functionalized poly(styrene-co-maleimide-co-maleic anhydride) (PSMM) microspheres. By simply adjusting the solvent composition and monomer feed ratio, monodisperse PSMM particles with diameter in the range of 0.52–1.01 μm were successfully obtained, which were further modified with hexamethylene diisocyanate (HDI). As a proof-of-concept, TiO2 nanoparticles were readily and covalently bound to the surface of HDI-functionalized PSMM (PSMM-HDI) particles through reaction with isocyanate groups, leading to the formation of PSMM-HDI supported TiO2 (PSMM-HDI@TiO2). Furthermore, anhydride groups on the surface of PSMM was hydrolyzed to endow PSMM-HDI@TiO2 with better dispersibility and adsorption performance. The degradation of methylene blue under UV-light was performed to evaluate the photocatalytic performance of the resultant PSMM-HDI@TiO2. Compared with pristine TiO2, the PSMM-HDI@TiO2 exhibited not only higher degradation efficiency, but also facile separation and better reusability.

Plasma-catalytic NO x production in a three-level coupled rotating electrodes air plasma combined with nano-sized TiO2

A novel three-level coupled rotating electrodes air plasma with nano-sized TiO2 photocatalysts is developed for plasma-catalytic NO x production. The effects of plasma catalysis on NO x production with different air flow rates, different N2 fractions and different humidity levels are evaluated. Final results show the exceptional synergistic effect between TiO2 and three-level coupled rotating electrodes air plasma significantly increases the NO x concentration by 68.32% (from 4952 to 8335 ppm) and reduces the energy cost by 40.55% (from 2.91 to 1.73 MJ mol−1) at an air flow rate of 12 l min−1 and relative humidity level of 12%, which beats the ideal thermodynamic energy limit ∼2.5 MJ mol−1 for the thermal gas-phase process. A possible mechanism for enhanced NO x production with TiO2 is discussed: Highly energetic electrons in plasma contribute to the formations of the electron–hole pairs and oxygen vacancy (Vo) on the TiO2 catalyst surface, which may facilitate the dissociative adsorption of O2 molecules to form superoxide radical groups (like O2 −), and H2O molecules to form surface hydroxyl groups (like OH·), and thus, improving energy efficiency.

Comparative Photo-Electrochemical and Photocatalytic Studies with Nanosized TiO2 Photocatalysts towards Organic Pollutants Oxidation

The size of TiO2 can significantly affect both its photocatalytic and photo-electrochemical properties, thus altering the photooxidation of organic pollutants in air or water. In this work, we give an account of the photo-electrochemical and photocatalytic features of some nanosized TiO2 commercial powders towards a model reaction, the photooxidation of acetone. Cyclic voltammograms (CV) of TiO2 particulate electrodes under UV illumination experiments were carried out in either saturated O2 or N2 solutions for a direct correlation with the photocatalytic process. In addition, the effect of different reaction conditions on the photocatalytic efficiency under UV light in both aqueous and gaseous phases was also investigated. CV curves with the addition of acetone under UV light showed a negative shift of the photocurrent onset, confirming the efficient transfer of photoproduced reactive oxygen species (ROSs), e.g., hydroxyl radicals or holes to acetone molecules. The photocatalytic experiments showed that the two nano-sized samples exhibit the best photocatalytic performance. The different photoactivity of the larger-sized samples is probably attributed to their morphological differences, affecting both the amount and distribution of free ROSs involved in the photooxidation reaction. Finally, a direct correlation between the photocatalytic measurements in gas phase and the photo-electrochemical measurements in aqueous phase is given, thus evincing the important role of the substrate-surface interaction with similar acetone concentrations.

Tetracycline catalytic photodegradation with mesoporous phosphated TiO2: characterization, process optimization and degradation pathway.

Concern about the effect of tetracycline (TC) on the ecosystem has been increasing due to its endurance and low decomposition. Photocatalysts have attracted extensive interest as alternatives to other ordinary wastewater treatment methods. A nanosized mesoporous phosphated TiO2 (P-TiO2) photocatalyst was fabricated to degrade TC under Xe lamp irradiation. The TC degradation and COD removal rate reached 98.97% and 79.16% within 30 min. Photocatalysts were characterized by SEM, HRTEM, BET, DRS, XRD, XPS and FT-IR techniques. 31 experiments were designed to identify the best conditions for photocatalytic degradation of TC by response surface methodology (RSM) based on a central composite design (CCD). 6 key operating parameters were selected to study their interrelationships by CCD design. Based on the experimental data and ANOVA, the coefficient of determination (R2), the values of “Prob > F” and F-value were determined to be 0.9692, 0.002 and 7.87, respectively, which demonstrated that the model is significant. And the excellent correlation between the predicted and actual values also provided good confidence in the model. To achieve a higher removal rate of TC under appropriate and more economical experimental conditions, the optimum values of P-loading on TiO2, concentration of P-TiO2, irradiation time, photo intensity, pH and concentration of TC should be set to 17.45 wt%, 1.00 g L−1, 40.39 min, 5 A, 7, 29.93 mg L−1, respectively, in which the degradation of TC can reach 99.16%. Furthermore, the intermediates of TC verified by GC-MS analysis were mainly chains and rings. A possible pathway of photodegradation was also proposed.

Mesoporous black TiO2 phase junction@Ni nanosheets: A highly integrated photocatalyst system

Solar fuels production by using TiO2 nanomaterials has attracted much attention to overcome the energy and environmental crisis. However, it is a big challenge to utilize the visible light, promote charge separation and surface reaction, and efficiently recover nanosized TiO2 photocatalysts without secondary contamination. Here, Ni nanosheets as the non-noble metal co-catalysts are coated on the surface of mesoporous black TiO2 phase junction microspheres (Ni/b–TiO2) for highly efficient H2 evolution. The black TiO2 phase junction core can efficiently harvest the visible light and greatly facilitate the charge transfer. The Ni nanosheet shell can not only serve as a highly efficient collaborative co-catalyst to promote the separation and transfer of photogenerated charge carriers, but also enhance the visible light absorption and improve the utilization ratio of sunlight. The obtained Ni/b–TiO2 exhibits a high hydrogen evolution rate of 166.2 µmol h-1 g-1 under simulated sunlight, which is much higher than that of pristine mesoporous black TiO2 microspheres (80.5 μmol h-1 g-1). More importantly, the obtained photocatalysts can be easily separated through magnetic separation. Such a structure design may provide new insights for fabricating low-cost bifunctional photocatalysts with high-performance.

Improved degradation of etodolac in the presence of core-shell ZnFe2O4/SiO2/TiO2 magnetic photocatalyst

In the present study, susceptibility to photocatalytic degradation of etodolac, 1,8-diethyl-1,3,4,9 – tetrahydro pyran - [3,4-b] indole-1-acetic acid, which is a non-steroidal anti-inflammatory drug frequently detected in an aqueous environment, was for the first time investigated. The obtained p-type TiO2-based photocatalyst coupled with zinc ferrite nanoparticles in a core-shell structure improves the separation and recovery of nanosized TiO2 photocatalyst. The characterization of ZnFe2O4/SiO2/TiO2, including XRD, XPS, TEM, BET, DR/UV–Vis, impedance spectroscopy and photocatalytic analysis, showed that magnetic photocatalyst containing anatase phase revealed markedly improved etodolac decomposition and mineralization measured as TOC removal compared to photolysis reaction. The effect of irradiation and pH range on photocatalytic decomposition of etodolac was studied. The most efficient degradation of etodolac was observed under simulated solar light for a core-shell ZnFe2O4/SiO2/TiO2 magnetic photocatalyst at pH above 4 (pKa = 4.7) and below 7. The irradiation of etodolac solution in a broader light range revealed a synergetic effect on its photodegradation performance. After only 20 min of degradation, about 100% of etodolac was degraded. Based on the photocatalytic analysis in the presence of scavengers and HPLC analysis, the transformation intermediates and possible photodegradation pathways of etodolac were studied. It was found that ∙O2− attack on C2-C3 bond inside pyrrole ring results mostly in the hydroxylation of the molecule, which next undergoes -CH2COOH detachment to give 1,9-diethyl-3,4-dihydro-pyrano[3,4-b]indol-4a-ol. The obtained compound should further undergo subsequent hydropyran and pyrrole ring breaking to give a family of benzene derivatives.

UV-Vis-Induced Degradation of Phenol over Magnetic Photocatalysts Modified with Pt, Pd, Cu and Au Nanoparticles.

The combination of TiO2 photocatalyst and magnetic oxide nanoparticles enhances the separation and recoverable properties of nanosized TiO2 photocatalyst. Metal-modified (Me = Pd, Au, Pt, Cu) TiO2/SiO2@Fe3O4 nanocomposites were prepared by an ultrasonic-assisted sol-gel method. All prepared samples were characterized by X-ray powder diffraction (XRD) analysis, Brunauer-Emmett-Teller (BET) method, X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), Mott-Schottky analysis and photoluminescence spectroscopy (PL). Phenol oxidation pathways of magnetic photocatalysts modified with Pt, Pd, Cu and Au nanoparticles proceeded by generation of reactive oxygen species, which oxidized phenol to benzoquinone, hydroquinone and catechol. Benzoquinone and maleic acid were products, which were determined in the hydroquinone oxidation pathway. The highest mineralization rate was observed for Pd-TiO2/SiO2@Fe3O4 and Cu-TiO2/SiO2@Fe3O4 photocatalysts, which produced the highest concentration of catechol during photocatalytic reaction. For Pt-TiO2/SiO2@Fe3O4 nanocomposite, a lack of catechol after 60 min of irradiation resulted in low mineralization rate (CO2 formation). It is proposed that the enhanced photocatalytic activity of palladium and copper-modified photocatalysts is related to an increase in the amount of adsorption sites and efficient charge carrier separation, whereas the keto-enol tautomeric equilibrium retards the rate of phenol photomineralization on Au-TiO2/SiO2@Fe3O4. The magnetization hysteresis loop indicated that the obtained hybrid photocatalyst showed magnetic properties and therefore could be easily separated after treatment process.

Photocatalytic performance of Fe-doped TiO2 nanoparticles under visible-light irradiation

The present work focuses on the synthesis, characterization and photocatalytic activity of a nanosized Fe-doped TiO2 photocatalyst. The samples were synthesized by the sol–gel method and characterized by using techniques such as x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), UV–visible spectroscopy, photoluminescence (PL) spectroscopy, Raman spectroscopy and Fourier-transform infrared (FTIR). The powder XRD spectra revealed that the synthesized samples are pure and crystalline in nature and show a tetragonal anatase phase of TiO2. The Raman spectroscopy also confirmed the formation of an anatase phase structure in both pure and Fe-doped TiO2 nanoparticles (NPs). The UV–visible and PL spectra illustrated the red shift in Fe-doped TiO2 NPs. The FTIR spectra indicated the vibrational band of the Ti–O lattice. The photocatalytic experimental results demonstrate that Fe-doped TiO2 NPs effectively degrade MB under visible-light illumination. Interestingly, the prepared TiO2 NPs with a dopant concentration of 3.0 mole% showed the maximum photocatalytic activity under investigation.

Magnetic semiconductor photocatalysts for the degradation of recalcitrant chemicals from flow back water

In the present study treatability of persistent organic compounds from the flow back water after hydrauling fracturing was investigated. The combination of TiO2 photocatalyst and magnetic oxide nanoparticles enhance the separation and recoverable property of nanosized TiO2 photocatalyst. Fe3O4/TiO2 and Fe3O4@SiO2/TiO2 nanocomposites were prepared by heteroagglomeration. The photocatalysts’ characteristics by X-ray diffractometry (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS) showed that sample with the mass ratio of Fe3O4 to TiO2 equal 1:4 and molar ratio of TEOS:Fe3O4 = 8:1 and NH4OH:TEOS = 16:1 obtained by deposition TiO2 P25 (Evonik) on magnetite core had about 124 m2 g−1 specific surface area and superparamagnetic properties. The prepared composites contained TiO2 and Fe3O4 crystal phases. The photocatalytic activity was estimated by measuring the decomposition rate of three model pollutants identified in the flow back water from one of the Baltic Shale Basin. Regarding flow back water treatment after shale gas exploration, the progress of photocatalytic degradation of organic compounds was measured by chemical oxygen demand (COD) concentration. The Fe3O4@SiO2/TiO2_P25 composite nanoparticles were recovered and re-used without significant reduction of efficiency.

Preparation, characterization and photocatalytic application of TiO2/Fe-ZSM-5 nanocomposite for the treatment of petroleum refinery wastewater: Optimization of process parameters by response surface methodology

Photocatalytic degradation of organic contaminants from petroleum refinery wastewater under UV and sunlight was investigated by immobilizing nanosized TiO2 photocatalyst into the structure of as-synthesized Fe-ZSM-5 zeolite via sol-gel method. Pure phase of TiO2/Fe-ZSM-5 photocatalyst with specific surface area of 304.6 m2 g−1 and loaded TiO2 of 29.28% was successfully synthesized. Effects of various operational parameters on treatment process were investigated by use of Response Surface Methodology (RSM). Maximum reduction of 80% COD was achieved at pH of 4, a photocatalyst concentration of 2.1 g l−1, temperature of 45 °C and UV exposure time of 240 min. Gas chromatography-mass demonstrated an apparent shift in molecular weight from a higher fraction to a lower fraction even under sunlight. It is expected that the prepared photocatalyst is able to use ultraviolet and visible light energy. Results indicated that removal of COD degradation did not decrease as the reuse cycle of photocatalyst increased. Moreover, the potential to use sunlight energy and the simplicity of operation make photocatalysis an attractive prospect in terms of petroleum refinery wastewater treatment.

Cement Bonded Sol-Gel TiO<sub>2</sub> Powder Photocatalysis for Phenol Removal

Photocatalysis has been proven effective in controlling various environmental problems originated by pollutions both in liquid and gaseous phases. Titanium dioxide (TiO2) is well known the most practical photocatalyst as it has high photocatalytic efficiency, low band gap energy, and no toxicity. Various chemical methods have been tried to produce TiO2 photocatalyst powder with high activity. In this study, sol-gel method has been employed to produce nanosized TiO2 photocatalyst particles and its physical properties and photocatalytic activity in phenol degradation test were compared with the commercial TiO2 powder, Degussa P25. The synthesis process was carried out through hydrolysis of titanium tetraisopropoxide (TPT) and methanol where the molar ratio of water to TPT was monitored to control the hydrolysis rate. From X-Ray Diffraction (XRD) analysis, the sol-gel TiO2 powder obtained was fully in anatase structure with high crystallinity. Scanning Electron Microscope (SEM) measurement showed that the powder was in nanoto sub-micron size, spherical in shape, and tightly agglomerated. Thermal analysis confirmed that sol-gel derived amorphous TiO2 powder transformed to anatase phase after 400°C calcination. The test on photocatalytic performance conducted using aqueous solution of phenol as the representative of water pollutant examined in this study showed that the sol-gel TiO2 powder is more efficient in degrading phenol compared to one of the most active photocatalysts commercially available, Degussa P25.

Super-hydrophobic fluorination mesoporous MCF/TiO2 composite as a high-performance photocatalyst

NH4F was used instead of conventional organic silylation agent as the hydrophobic modifier to synthesize the super-hydrophobic mesocellular foams (MCF) loaded with nano-sized TiO2 photocatalysts in its pore channels, which could be considered as an extractant for organics. Compared to organosilane modified catalysts, NH4F-modified MCF/TiO2 has a more stable super-hydrophobic property and much higher photocatalytic activity. It was found that only using isopropanol as the solvent, the NH4F-modified catalyst showed super-hydrophobic property. It is believed that the solvent plays a role in controlling the exchange between surface OH groups and F ions. The special structure of supported mesoporous catalyst greatly facilitated the surface fluorination, which together with the Ti3+ generation led to its excellent adsorption capacity and UV/visible light photocatalytic activity. This novel super-hydrophobic mesoporous photocatalyst has a large application potential in the field of photocatalysis, shipbuilding, and other industries.

Improving the visible light photocatalytic activity of nano-sized titanium dioxide via the synergistic effects between sulfur doping and sulfation

S-doped and sulfated nano-sized TiO2 photocatalysts were prepared by a low-temperature solvothermal method, using potassium persulfate as sulfur source. The obtained samples were characterized by XRD, UV–vis DRS, FTIR, FTIR-pyridine, XPS, and nitrogen adsorption–desorption methods. Compared with undoped TiO2, the S-doped and sulfated TiO2 photocatalysts exhibited much higher photocatalytic activity for phenol degradation under visible light irradiation. It was revealed that S was not only incorporated into the crystal lattice of TiO2 replacing Ti4+ (as S4+) but also anchored on its surface in the form of SO42−. Further study indicated that there are synergistic effects between doped S and anchored SO42− on improving the visible light photocatalytic activity of TiO2. The doped S can form an impurity level below the conduction band of TiO2, extending the light response of TiO2 into visible light region, while SO42− anchored on TiO2 surface is favorable for trapping photo-induced electrons, suppressing the recombination of electrons and holes and consequently promoting the formation of hydroxyl radical.

Synthesis and photocatalytic activity of TiO2 nanoparticles prepared by chemical vapor condensation method with different precursor concentration and residence time

Nanosized TiO2 photocatalysts were synthesized using a chemical vapor condensation method under a range of synthesis conditions (precursor vapor concentration and residence time in a tubular electric furnace). X-ray diffraction showed that the prepared TiO2 powders consisted mainly of anatase (>94%) with a small amount of rutile. The mean particle diameter from the Brunauer–Emmett–Teller surface area and transmission electron microscopy measurements ranged from 9.4 to 16.6nm. The specific surface area (92.5–163.5m2g−1) of the prepared TiO2 powders was found to be dependent on the synthesis conditions. The content of hydroxyl groups on the surface of the prepared TiO2 sample was higher than those on commercial TiO2, resulting in increased photocatalytic oxidation. The photocatalytic activity of the TiO2 samples prepared in a methylene blue solution was strongly dependent on the crystallinity and specific surface area, which were affected by the TTIP vapor concentration and residence time.

Interface junction at anatase/rutile in mixed-phase TiO2: Formation and photo-generated charge carriers properties

Nanosized TiO2 photocatalysts of anatase, rutile and mixed-phase were synthesized through hydrolysis method. The influence of the interface junction between anatase and rutile TiO2 on the photo-generated charge carriers properties were studied by Kelvin probe (KP), surface photovoltage (SPV) and transient photovoltage (TPV) techniques. The KP results revealed that the difference of the surface work function between anatase and rutile might cause the formation of built-in field at the interface in mixed-phase TiO2. The role of the interface junction was further testified through the characteristics of photo-generated charge carriers by SPV and TPV.

Synthesis of Fe3+-doped Nanosized TiO2 Photocatalyst with Sol-gel Based Self-propagating High-temperature Synthesis Process

A Fe3+-doped nanosized TiO2 photocatalyst was synthesized from a sol-gel based self-propagating high-temperature synthesis process(SHS) with TiCl4 as the raw material.Its photocatalytic activity was studied by probing the degradation of methylene blue(MB) under different light irradiations including sunlight,low and high pressure mercury lamps.XRD and SEM results indicate that the as-prepared nanoscale TiO2 powder is a porous anatase material after SHS at 350 ℃ and heat-treatment at 500 ℃.The optimal precursor molar ratio of n(Ti):n(citric acid):n(NH4NO3) is 1:3:5,the particle size is ～ 20 nm.Experimental results show that the highest photocatalytic activity of Fe3 +-doped TiO2 could be achieved when the molar ratio of Fe3 + to TiO2is 0.02%.Removing rate of MB was 96.1% with Fe3 +-doped TiO2 as catalyst and sunlight as light source,which is 1.78 times of that of pure TiO2.

The effect of particle shape on the activity of nanocrystalline TiO2 photocatalysts in phenol decomposition. Part 2: The key synthesis parameters influencing the particle shape and activity

Nanosized TiO2 photocatalysts were synthesized via hydrogen–air flame hydrolysis by using two slightly different, home made diffusion flame burners (Burners A and B). Titanium(IV) chloride vapor was introduced into the flame via bubbling dry air throughout the precursor liquid. X-ray diffraction (XRD) measurements revealed that the particles are anatase–rutile mixtures with a phase composition ranging from 98:2 to 57:43 anatase to rutile weight ratio. The hydrogen–oxygen molar ratio in the flame, as well as the precursor vapor feeding rate were found to be the key parameters which determine both particle structure and morphology. Spherical and polyhedral particles with significantly varying photocatalytic activity were produced in Burner A by increasing the precursor vapor feeding rate at a constant hydrogen–oxygen molar ratio, the photocatalytic activity increased for samples synthesized. The differences in particle size, specific surface area and anatase–rutile composition were found to be marginal but the particle shape changed from spherical to polyhedral by increasing that parameter. From this, it was concluded that particle shape plays an important role in photocatalytic activity, the faceted particles are better photocatalysts than spherical ones. With Burner B, predominantly polyhedral particles were obtained, some of them with further improved photocatalytic activity. Synthesis parameters for obtaining maximum activity have been established for both burners. Our results prove, that bare (undoped) TiO2 photocatalysts with photocatalytic activity significantly better than that of P25 can routinely be prepared and their performance optimized via the fine-tuning of the synthesis parameters employed.

S–N Co-Doped TiO2 Photocatalysts with Visible-Light Activity Prepared by Sol–Gel Method

S–N co-doped anatase nanosized TiO2 photocatalyst was successfully prepared by simple sol–gel method. The samples were characterized by XRD, XPS, UV–Vis. From the results of UV–Vis, a red shift of the absorption edge was brought out owing to the S and N codoping. XPS and UV–Vis studies revealed that N and S were in situ codoped in the lattice of TiO2 and the absorbance in visible light region decreased with the calcination temperature increased. The photocatalytic activity was evaluated by the photocatalytic oxidation of penicillin solution under visible light irradiation. The results show that visible-light induced photocatalytic activities of the as-prepared TiO2 powders were improved by S–N copoing. The high activity of S–N co-doped TiO2 can be related to the results of the synergetic effects of strong absorption in the UV–Vis region, red shift in adsorption edge.

Preparation and characterization of N–S-codoped TiO 2 photocatalyst and its photocatalytic activity

N–S-codoped anatase nanosized TiO 2 photocatalyst (NSTO) was successfully prepared by one-step hydrothermal method from a mixed aqueous solution of Ti(SO 4) 2 and thiourea. The samples were characterized by XRD, UV–vis, XPS, FT-IR and EA. From results of UV–vis, a red shift of the absorption edge was brought out owing to N and S codoping, and the extension for photoabsorption range of NSTO occurred. XRD, XPS, EA and FT-IR studies revealed that N and S were in situ codoped in the lattice of TiO 2 and N concentration decreased from the surface to the center of NSTO. Especially, the photocatalytic tests indicated that NSTO exhibited a high activity for decompositions of methyl orange both under UV-light and vis-light irradiation comparing to S-doped TiO 2 (STO) and undoped TiO 2 (TO). The high activity of NSTO can be related to the results of the synergetic effects of strong absorption in the UV–vis region, red shift in adsorption edge, oxygen vacancies and the enhancement of surface acidity induced by N and S codoping.

Synthesis of visible-light active TiO2 photocatalyst with Pt-modification: Role of TiO2 substrate for high photocatalytic activity

Nano-sized TiO2 photocatalysts were prepared by hydrolysis of TiCl4 followed by calcination at different temperatures. X-ray diffraction (XRD) measurements revealed that all TiO2 powders crystallized in rutile even before calcination. The photocatalytic activity of the raw TiO2 powder clearly decreased with increasing calcination temperature. Pt-modification of the surface of TiO2 increased photocatalytic activity under visible-light irradiation, depending on the calcination temperature. Maximal photocatalytic activity was obtained at calcination temperatures of 300–450°C, which was correlated with the light absorption properties of the Pt complex on TiO2 nano-particles. It was found that TiO2 surface structure plays an important role in the formation of TiOPt bonds, resulting in a large visible-light absorbance and high photocatalytic activity under visible-light irradiation.