Highly Selective Photocatalytic Conversion of Glucose on Holo-Symmetrically Spherical Three-Dimensionally Ordered Macroporous Heterojunction Photonic Crystal

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES6 Oct 2022Highly Selective Photocatalytic Conversion of Glucose on Holo-Symmetrically Spherical Three-Dimensionally Ordered Macroporous Heterojunction Photonic Crystal Ting-Wei Wang, Zhi-Wen Yin, Yin-Hao Guo, Fang-Yuan Bai, Jun Chen, Wenda Dong, Jing Liu, Zhi-Yi Hu, Lihua Chen, Yu Li and Bao-Lian Su Ting-Wei Wang State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Google Scholar More articles by this author , Zhi-Wen Yin State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Google Scholar More articles by this author , Yin-Hao Guo State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Google Scholar More articles by this author , Fang-Yuan Bai State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Google Scholar More articles by this author , Jun Chen State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Google Scholar More articles by this author , Wenda Dong State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Google Scholar More articles by this author , Jing Liu *Corresponding author: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Google Scholar More articles by this author , Zhi-Yi Hu State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Nanostructure Research Centre (NRC), Wuhan University of Technology, 430070 Wuhan Google Scholar More articles by this author , Lihua Chen State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Google Scholar More articles by this author , Yu Li *Corresponding author: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Nanostructure Research Centre (NRC), Wuhan University of Technology, 430070 Wuhan Google Scholar More articles by this author and Bao-Lian Su *Corresponding author: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, B-5000 Namur Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202213 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Photocatalytic conversion of biomass is considered an effective, clean, and environmentally friendly route to obtain high-valued chemicals and hydrogen. However, the limited conversion efficiency and poor selectivity are still the main bottlenecks for photocatalytic biomass conversion. Herein, we report the highly selective photocatalytic conversion of glucose solution on holo-symmetrically spherical three-dimensionally ordered macroporous TiO2-CdSe heterojunction photonic crystal structure (s-TCS). The obtained s-TCS photocatalysts show excellent stability and strong light harvesting, uniform mass diffusion and exchange, and efficient photogenerated electrons/holes separation and utilization. The optimized s-TCS-4 photocatalyst displays the highest photocatalytic performance for glucose oxidation and hydrogen production. The glucose conversion, lactic acid selectivity, and yield on s-TCS-4 are about 95.9%, 94.3%, and 96.4%, respectively. The photocatalytic production of lactic acid for s-TCS-4 (18.5 g/L) is 2.3 times higher than the pure spherical TiO2 photonic crystal without CdSe (s-TiO2, 8.1 g/L), and the hydrogen production rate of s-TCS-4 is 9.4 times that of s-TiO2. For the first time, we reveal that the photocatalytic conversion of glucose to lactic acid is a third-order and four-electron-involved reaction. This work could shed some new light on the efficient photocatalysis conversion of biomass to highly value-added products with high selectivity and yield, and simultaneously sustainable hydrogen evolution. Download figure Download PowerPoint Introduction Biomass conversion into high-value-added chemicals and fuels provides a viable tactic to lessen the existent stress on global warming, environmental concerns, and fossil energy shortage.1,2 However, the limited conversion efficiency and poor selectivity are still the main bottlenecks in biomass conversion. As one of the most abundant compounds derived from biomass,3,4 glucose can become abundant high-valued commercial chemicals, such as gluconic acid,5–7 glucaric acid,8,9 arabinose,10,11 xylitol,12 formic acid,13 and lactic acid.14 Among these chemicals, lactic acid has been deemed one of the most important hydroxycarboxylic acids, which can be easily converted into other crucial chemicals, for example, acrylic acid, propylene glycol, 2,3-pentanedione, pyruvic acid, and acetaldehyde.15 In addition, lactic acid has also been widely used in food preprocess, polymers, and cosmetics.16 However, the traditional catalytic and biotransformation methods for glucose conversion possess many shortcomings, such as severe reaction conditions, energy-extensive consumption, low-selectivity, separation difficulties, contaminant emissions, and utilization of costly catalysts and oxidant agents.17,18 A new alternative technology for biomass reforming is urgently needed to improve the reactant conversion and product selectivity. Photocatalytic conversion of glucose is a promising technology for utilizing solar energy at room temperature.19 During the photocatalytic conversion process, glucose solution is oxidized and then converted into high-value-added products accompanied by water splitting to produce hydrogen without any sacrificial agent, such as sulfides20 and alcohols.21 In recent years, many researchers have used the traditional material TiO2 to study the photocatalytic glucose conversion. For example, Da Vià et al.6 explored a glucose-TiO2 charge-transfer complex, demonstrating under visible light 42% glucose conversion, 7% selectivity of gluconic acid, and 93% selectivity of other oxidation products. However, the performance of the above material is very poor due to the rapid recombination of the photogenerated carriers. Chong et al.22 reported the conversion of glucose to arabinose and erythrose on rutile TiO2, and the selectivity for arabinose and erythrose reached 91% at 65% conversion, respectively. Roongraung et al.19 designed 1% Ag modified Ag-TiO2 nanofibers, which exhibited a high glucose conversion (85.49%). Besides, the yields of gluconic acid, arabinose, xylitol and formic acid were 8.55%, 44.41%, 21.97% and 24.48%, respectively. These results on TiO2-based photocatalysts have demonstrated the largely enhanced photocatalytic activity for glucose conversion. Nevertheless, glucose conversion and product selectivity remain unsatisfactory owing to the fast recombination of photogenerated carriers and low light-utilization efficiency. Therefore, it is still challenging to explore novel photocatalysts for high-selectivity photocatalytic conversion of glucose by improving light-harvesting, separation, and the utilization efficiency of photogenerated carriers. Photocatalysts with three-dimensionally ordered macroporous (3DOM) structure have been widely used to improve the photocatalytic efficiencies because of the strong photons-harvesting effect,21,23,24 high mass transfer,25,26 and separation efficiency21,27 as well as a large number of reactive sites.25,28 Indeed, our group has studied monadic (TiO2,29–31 ZnO,32,33 etc.), binary (TiO2/Au,34 TiO2/BiVO4,23,35 ZnO/CdS,36 etc.), and ternary (TiO2/BiVO4/ZnO,27,37 TiO2/Au/CdS,21,38–40 etc.) materials with 3DOM structure. Their results reveal that the 3DOM structure is an important factor that affects the photocatalytic performance because of the greatly improved light absorption and utilization. Recently, we have demonstrated that the Au-TiO2 powder photocatalyst with the 3DOM structure can enhance the utilization of light, and the Au nanoparticles had localized the surface plasmon resonance effect to further promote light absorption. This leads to the 3DOM Au-TiO2 exhibiting ∼37% glucose conversion and 80% selectivity.34 Furthermore, we extended the biomass to the conversion of cellobiose by 3DOM TiO2-Au-CdS. The conversion of cellulose is 24.2% in 8 h and the selectivities of main products are ∼28%, ∼25%, and ∼12% for formic acid, glucose, and glucaric acid, respectively. However, the obtained 3DOM photonic crystal still faces the challenges of instability and lower photocatalytic performance owing to the imperfect structural symmetry. Therefore, it is appealing to design a novel photonic crystal structure to achieve stable mass diffusion and exchange, highly efficient light harvesting, and high separation, transportation, and utilization of the photogenerated carriers. Herein, we propose that the holo-symmetrically spherical 3DOM structure is a promising candidate to address these issues. Figure 1a,b illustrate the initial state of the contact angles for the droplets on different shaped materials (traditional 3DOM structure and spherical 3DOM structure) under ideal conditions. At the moment the droplet contacts the structures (the droplet does not enter the 3DOM pores yet), the contact angle only depends on the shape of the materials. Assuming that a drop with the same volume is deposited on the traditional 3DOM structure and spherical 3DOM structure, the contact area of the droplet on the spherical structure is larger than that of the traditional 3DOM structure due to the curvature (Figure 1a,b). In addition, for the spherical photonic crystal structure, the contact angle for the same droplet is smaller on the convex surface. Therefore, the spherical 3DOM structure demonstrates a better wettability compared with the traditional 3DOM photonic crystal.41 Figure 1 | Schematic diagram of contact angle of (a) traditional 3DOM structure and (b) spherical 3DOM surface (γLG is the interfacial tension between liquid and gas, γSL is the interfacial tension between solid and liquid, γSG is the interfacial tension between solid and gas, θe is the contact angle). Schematic diagram of location of material in reaction process and the corresponding irradiation area (assuming the powder photonic crystal is a cube and its side length is a, the radius of the spherical photonic crystal is r) of (c) traditional 3DOM structure, and (d) holo-symmetrically spherical 3DOM structure. Download figure Download PowerPoint In addition, the holo-symmetrical characteristics of spherical photonic crystal structure can facilitate more uniform mass diffusion and exchange due to the equal radius of curvature.42 In particular, such a holo-symmetrically spherical structure with an invariable irradiation area (S) ensures stable and high photon absorption (Figure 1d) under light irradiation, avoiding a random angle of light absorption. This leads to a constantly changing photon absorption number due to the variational irradiation area in the face (Sface), edge (Sedge), or corner (Scorner) of the traditional 3DOM photonic crystal during stirring (Figure 1c). It is then believed that the holo-symmetrically spherical structure can make more stable light absorption and transfer, as well as uniform mass diffusion and exchange, guaranteeing the high light utilization, high glucose conversion, and products selectivity. To solve the bottlenecks of the limited conversion and poor selectivity in the photocatalytic conversion of biomass, we designed a spherical 3DOM TiO2-CdSe photonic crystal photocatalyst (labelled as s-TCS) that combines the advantages of the holo-symmetrically spherical photonic crystal structure and type-II heterojunction using the easy, green, and high-output emulsion shear polymerization method and the simple successive ion layer adsorption reaction (SILAR) method. The spherical 3DOM structure with the advantages of high porosity, large specific surface, and uniform pore-size distribution displays a strong light trapping effect, high-efficiency mass transfer, and many active sites.23,43 Moreover, the designed holo-symmetrically spherical photonic crystal structure promises stable light absorption and transfer, as well as uniform mass diffusion and exchange, leading to stability and high performance. Owing to the appropriate conduction band (CB) position and a befitting band gap, CdSe can form the type-II heterojunction with TiO2, which is favorable for improving the separation of photogenerated carriers and photocatalytic performance.44 The highest glucose conversion by s-TCS-4 reaches ∼95.9% with a lactic acid (oxidation product) selectivity and yield of ∼94.3% and ∼96.4%, respectively. In particular, the s-TCS-4 has the highest lactic acid concentration of 18.5 g/L, which is 2.3 times that of pure s-TiO2 (the spherical pure TiO2 photonic crystal without CdSe) of 8.1 g/L. The photocatalytic production of lactic acid for s-TCS-4 is also 2.3 times that of TCS-4 (non-spherical 3DOM TiO2-CdSe photonic crystal obtained by mechanical crushing of s-TCS-4, 7.9 g/L). The H2 production displayed similar results. The H2 production by s-TCS-4 is 9.4 times and 11.7 times that of s-TiO2 and TCS-4, respectively. Our photocatalytic conversion of glucose utilizes both photogenerated electrons and holes, which make full use of the photogenerated carriers. Specifically, for the first time, we reveal that the photocatalysis conversion of glucose to lactic acid is a third-order reaction with four photogenerated holes involved. This work not only provides a new, simple, green, and high-yield method to obtain the spherical 3DOM TiO2-CdSe photonic crystal, but also obtains a higher photocatalysis conversion of glucose and lactic acid selectivity. Experimental Section Materials All the reagents were analytical grade and commercially available. Styrene (C6H6), sodium hydroxide (NaOH), methyl methacrylate (C5H8O2), acrylic acid (C3H4O2), ammonium persulfate ((NH4)2S2O8), ammonium bicarbonate ((NH4)HCO3), titanium(IV)bis(ammonium lactato)dihydroxide solution (C6H18N2O8Ti, 50% aqueous solution), hexane (C6H14), dimethylfluorinated silicone oil (KF-96-30cs), sodium borohydride (NaBH4), selenium powder, cadmium acetate (Cd(CH3COOH)2), and glucose (C6H12O6) were purchased from the Aladdin Reagent Company (Shanghai, China). Polymer sphere template preparation Polymer spheres were prepared by surfactant-free emulsion polymerization. Typically, 50 mL of C6H6 and 150 mL of NaOH solution (2 M) were mixed and stirred for 40 min. The less dense phase was taken out and labeled as pretreated C6H6. Configuration of solution A: (NH4)2S2O8 (0.4 g) was slowly added to the solution containing (NH4)HCO3 (0.8 g), C3H4O2 (2 mL), and deionized water (10 mL). Then, the pretreated C6H6 (8 mL), C5H8O2 (1 mL), and deionized water (250 mL) were mixed and heated to 70 °C under an inert atmosphere for 30 min. The prepared solution A was quickly added to the above mixed solution. The reaction was maintained at 70 °C and stirred for 10 h under the inert atmosphere. The resulting milky solution was obtained and prepared into a solution with a mass fraction of 15% after removing the unreacted C6H6 monomer by rotary evaporation. Preparation of spherical 3DOM TiO2 photonic crystals Holo-symmetrically s-TiO2 was prepared by the emulsion shear polymerization method. A mixture of the above polymer template (0.87 mL), C6H18N2O8Ti (100 μL), and ethanol (300 μL) were uniformly mixed by ultrasound and taken up into a syringe. The mixture was added by catheter to a beaker of dimethylfluorinated silicone oil rotating at a speed of 6000 r/min. The mixed solution was injected into a beaker at a flow rate of 15 mL/h. Then, the beaker was placed in an oil bath at 55 °C for 12 h, and the rotating speed was controlled at 6000 r/min. After washing the dimethylfluorinated silicone oil with C6H14, the prepared samples were dried at 50 °C and the held at 300, 400, 550, and 700 °C for 2 h using a heating rate 2 °C/min under air condition, respectively. Preparation of spherical 3DOM TiO2-CdSe photonic crystals Holo-symmetrical s-TCS was prepared by loading CdSe nanoparticles on the surface of s-TiO2 by SILAR. For the anionic source, the liquid B was derived from NaBH4 (54 mg) reduction of selenium powder (74 mg) in deionized water (10 mL). For cationic source, the liquid C was derived from the dissolution of Cd(CH3COOH)2 (27 mg) in deionized water (10 mL). The reaction process can be described as follows: Se + 2 BH 4 − → Se 2 − + H 2 + BH 3 (1) Cd ( CH 3 COO ) 2 → Cd 2 + + 2 CH 3 COO − (2) The SILAR method involves the immersion of s-TiO2 in liquid B for 1 min, rinsing in deionized for 10 s, and immersing in liquid C for 1 min. This process of immersion and rinsing was considered as one SILAR cycle. Such cycles were repeated 2, 4, and 8 times to optimize and acquire good coverage of CdSe over the s-TiO2, named s-TCS-2, s-TCS-4, s-TCS-8, respectively. Finally, the as-prepared samples were rinsed in deionized water followed by annealing at 180 °C for 4 h to improve the crystallinity of CdSe. Characterizations The sample morphology was characterized by field-emission scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDX) were acquired on an FEI Talos F200X (Massachusetts, United States). The crystalline phase was examined by powder X-ray diffraction (XRD, D8 ADVANCE, Karlsruhe, Germany) equipped with a Cu anode X-ray tube (Cu Kα X-rays, λ = 1.54056 Å). X-ray photoelectron spectroscopy (XPS) was performed with an X-ray photoelectron spectrometer (Thermo fisher, Alpha equipped with a monochromatic Al Kα source, Waltham, United States), and all the raw data were calibrated by C (1s) at 284.8 eV. Nitrogen adsorption–desorption isotherms were obtained using a Micromeritics Tristar II 3020 at 77 K (Norcross, United States). The specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was performed by Prodigy7, LEEMAN LABS (New Hampshire, United States). UV–vis absorption spectra were collected by SHIMADZU UV–vis spectrophotometer (Kyoto, Japan) in the range of 200–800 nm. Steady-state photoluminescence (PL) spectra of the sample were recorded by Edinburgh FLS980 (Edinburgh, United Kingdom) with the excitation wavelength at 300 nm, which is also conducted for time-resolved photoluminescence (TRPL) spectra excited by a picosecond pulse laser. Photoelectrochemical measurement Fluorine-doped tin oxide (FTO) conductive glass were prewashed with Piranha solution, deionized water, acetone, and ethanol. The Piranha solution was prepared by carefully mixing concentrated sulfuric acid with 30% hydrogen peroxide with the volume ratio of 3:1. The working electrode was obtained by dip-coating 40 μL photocatalyst slurry (5 mg sample and 10 μL of 5 wt % Nafion solution dispersed in 0.2 mL of ethanol solution) onto the prewashed FTO glass with an exposed area of 1 cm2. Photoelectrochemical measurements were performed on an electrochemical workstation (Autolab PGSTAT 302N, Herisau, Switzerland) in a standard three-electrode system. The as-fabricated photocatalyst films were used as the working electrodes, a Pt wire electrode as the counter electrode, and an Ag/AgCl (saturated in KCl) as the reference electrode. The 0.5 M Na2SO4 solution was used as electrolyte. The incident light was provided by a 300W Xenon lamp (PLS-SXE-300D, Beijing Perfectlight Science & Technology Co. Ltd., Beijing, China). The distance between the working electrode and light source was 15 cm and the actual light intensity was 2.1 W/cm2 measured by a digital power and energy meter from Thorlabs PM100D (New Jersey, United States) with a sensor of S322C. Photocatalytic measurement The photocatalytic hydrogen production was performed on a closed circulation system (Labsolar-3A, Beijing Perfectlight Science & Technology Co. Ltd., Beijing, China) under 5 °C using a PLS-SXE-300D lamp. The reaction suspension was prepared by dispersing 10 mg photocatalyst in 100 mL of 20 g/L glucose and 1 mol/L NaOH solution. The amount of evolved H2 was analyzed by gas chromatography (GC 7890B, Agilent, California, United States) equipped with a thermal conductivity detector. The liquid products were analyzed by high-performance liquid chromatography (HPLC, Waters, Massachusetts, United States) with the SH1011 column. Different products were detected by differential detector (RI) and ultraviolet detector (UV). Glucose content in the solution was obtained by RI, and lactic acid and formic acid were detected by UV. The mobile phase was 5 mM H2SO4 liquid with a flow rate of 0.5 mL/min. The glucose conversion, selectivity, and yield of lactic acid were calculated using the following formulas:1,6 Glucose conversion ( % ) = ( ( Glucose ) in − ( Glucose ) out ) / ( Glucose ) in × 100 % Lactic acid selectivity ( % ) = ( Lactic acid ) out / ( ( Lactic acid ) out + ( Formic acid ) out ) × 100 % Lactic acid yield ( % ) = Carbon of lactic acid/Carbon of converted glucose × 100 % * Selectivity and yield values were calculated on a molar basis. * Converted glucose = ( Glucose ) in − ( Glucose ) out Results and Discussion Preparation and characterizations The holo-symmetrically spherical 3DOM structure was produced by the emulsion shear polymerization method (Figure 2a). The morphology and structure of the spherical 3DOM TiO2-polymer opal structures (s-TP) and s-TiO2 were observed by SEM (Figure 2b,c), which clearly show the complete spherical structure of the opal and inverse-opal 3DOM structure. The internal structure of s-TiO2 is presented in Supporting Information Figure S1, displaying that the whole sphere is a good 3DOM structure. After the deposition of CdSe, the structure is maintained (Figure 2d). Figure 2 | (a) Schematic diagram of the manufacture process of s-TP, s-TiO2, and s-TCS. (b–d) SEM images of s-TP, s-TiO2, s-TCS-4, and insets are the corresponding SEM images in low magnification. (e) XRD patterns of s-TCS with different CdSe contents, and (f) UV–vis diffuse absorption spectra (inset is the corresponding plots of (αhν)1/2 versus photon energy (hν)) of s-TiO2, CdSe, and s-TCS-4. Download figure Download PowerPoint The crystallinity and crystal phases for s-TiO2 and s-TCS with different content of CdSe were analyzed by XRD (Figure 2e). It shows the characteristic diffraction peaks of pure anatase phase (JCPDS Card No: 99-0008) with high crystallinity for s-TiO2.45 The average crystallite size is ∼13 nm according to the Scherrer equation. All characteristic diffraction peaks of s-TCS composites are very similar, indicating that the crystal structure of TiO2 was not changed after CdSe deposition. There are no peaks of CdSe in s-TCS-2 and s-TCS-4 because of low content. As the CdSe content increased, the peak of CdSe (JCPDS Card No: 08-0459) at 41.97° appeared in s-TCS-8 with the average crystallite size of about 7 nm according to the Scherrer equation. The precise content of CdSe for 2, 4, and 8 cycles measured by ICP was 1.45%, 3.4%, 5.7%, respectively. The specific surface areas of s-TiO2 and s-TCS-4 were acquired by the BET experiment ( Supporting Information Figure S2). The typical IV-type hysteresis loops indicate mesoporous structures because of the accumulation of nanoparticles. The BET surface area of s-TiO2 and s-TCS-4 was 51 and 81 m2 g−1, respectively. The increase in area can be attributed to the accumulation of additional CdSe nanoparticles with smaller size. The enhanced surface area is beneficial for increasing the number of active sites, thus improving the photocatalytic performance. The light absorption properties of the samples were studied by UV–vis diffuse absorption spectroscopy. As shown in Figure 2f, the s-TiO2 displays the absorption of TiO2 in the range of 300–400 nm. After CdSe deposition, the absorption of s-TCS-4 is broadened to 400–700 nm. The Kubelka–Munk method was conducted to analyze the band gap of these semiconductor samples.46 The Figure 2f inset shows that the band gap of TiO2 and CdSe is ∼3.0 and ∼1.6 eV, respectively. Figure 3a displays the TEM images of s-TCS-4. It obviously demonstrates that the s-TCS-4 is a typical 3DOM structure with uniform size of about 210 nm (left by template removal) and wormlike mesoporous structure (TiO2 and CdSe nanoparticles packing) ( Supporting Information Figure S3), consistent with the SEM observation (Figure 2d) and BET results ( Supporting Information Figure S2). The crystallinity and close contacts of two components can be identified by HRTEM and SAED patterns (Figure 3b). They reveal the good crystallinity of TiO2 and CdSe with close contact, indicating the constitution of the heterojunction. The corresponding EDX elemental mappings (Figure 3c–f) demonstrate that Ti, O, Cd, and Se elementals are uniformly dispersed, indicating the homogeneous distribution of CdSe in s-TCS-4. The homogeneously dispersed heterojunction in s-TCS-4 can effectively separate the photogenerated carriers and improve the utilization efficiency of photogenerated electrons and holes. Figure 3 | (a) TEM images and (b) HRTEM images of the s-TCS-4 (inset is corresponding FFT image in whole area). (c–f) The corresponding EDX elemental mapping results s-TCS-4; Ti (yellow), O (blue), Cd (red), Se (green). Download figure Download PowerPoint Glucose conversion and hydrogen production To characterize the oxidation properties of the obtained materials, liquid chromatography was used to detect the residual liquid for glucose conversion after 5 h photocatalytic reaction. The species and concentration of substance were determined by comparing the peak position with that of the pure material ( Supporting Information Figure S4). Obviously, the photocatalytic conversion of glucose is a complex process with many products. However, the total carbon quantity does not change before and after the reaction. We obtained the carbon balance data by calculating and comparing the total carbon quantity of all carbon-containing substances before and after the reaction (Figure 4a). The carbon balance of s-TCS-4 was 99.6% (almost 100%), obtained by using the products lactic acid and formic acid, indicating that the main products of the photocatalytic glucose reaction are lactic acid and formic acid. This confirms that the formed heterojunction is beneficial for separating the photogenerated carriers, thus improving the performance. In Figure 4b, the lactic acid concentration of s-TCS-4 (18.5 g/L) is 2.3 times higher than that of s-TiO2 (8.1 g/L). And the s-TCS-4 also demonstrates the highest lactic acid selectivity (∼94.3%), lactic acid yield (∼96.4%), and glucose conversion (∼95.9%) and the lowest residual glucose concentration (∼0.8 g/L) (Figure 4c,d). The lactic acid, lactic acid yield, glucose conversion, and residual glucose concentration of s-TiO2 were worse at ∼72.3%, ∼52.3%, 77.4%, and 4.5 g/L, respectively. Figure 4 | (a) Carbon balance, (b) concentrations of lactic acid and formic acid, (c) lactic acid selectivity and yield, (d) glucose conversion and residual glucose concentration of s-TCS with different CdSe content and TCS-4. (e) Hydrogen production, and (f) photocatalytic stability of