Fabrication of Hollow Mesoporous CdS@TiO2@Au Microspheres with

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Fabrication of Hollow Mesoporous CdS@TiO@Au Microspheres with High Photocatalytic Activity for Hydrogen Evolution from Water under Visible Light Wei Yuan, Zhen Zhang, Xiaoling Cui, Huarong Liu, Chen Tai, and Yuanrui Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01787 • Publication Date (Web): 15 Sep 2018 Downloaded from http://pubs.acs.org on September 16, 2018

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ACS Sustainable Chemistry & Engineering

Fabrication of Hollow Mesoporous CdS@TiO2@Au Microspheres with High Photocatalytic Activity for Hydrogen Evolution from Water under Visible Light Wei Yuan†, Zhen Zhang†, Xiaoling Cui, Huarong Liu*, Chen Tai, Yuanrui Song

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230026, P. R. China.

Corresponding Author. *Huarong Liu, E-mail: [email protected]; Tel: +86 551 63601586

ACS Paragon Plus Environment

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ABSTRACT. Hollow mesoporous CdS@TiO2@Au microspheres as a photocatalyst have been successfully synthesized by γ-irradiation deposition of Au nanoparticles (NPs) on the double shell of hollow CdS@TiO2 microspheres, which are prepared via “hard core template” approach combining with calcination and solvothermal reaction. The morphology, structure and properties of the as-prepared hybrid photocatalyst have been characterized by TEM, XRD, XPS, N2 adsorption−desorption, UV−vis DRS, PL and photoelectrochemical measurements. The results show that hollow mesoporous double-shell structure is formed with fluffy TiO2 coating on the surface of CdS shell composed of stacked CdS NPs. This ternary hybrid photocatalyst exhibits excellent photocatalytic activity for hydrogen evolution and stability under visible-light illumination, which is attributed to the unique ternary hollow mesoporous double-shell heterostructure that may extend the light responsive region, enhance the light-harvesting efficiency, prevent the direct contact between the CdS shell and O2 and H2O, facilitate mass transfer, reduce the recombination rate of charge carriers and promote the efficiency of water splitting H2 evolution ultimately. The photocatalytic hydrogen generation rate (1720 µmol·g-1·h1

) of the hollow CdS@TiO2@Au microspheres is 2.37 and 12.7 times higher than those of

hollow CdS@TiO2 and CdS microspheres, respectively. The loading amount of Au NPs also influences the photocatalytic hydrogen evolution activity of ternary photocatalyst. Our work expands the application of hybrid hollow microsphere photocatalysts in photocatalytic hydrogen generation by the rational design for enhancing photocatalytic activity under visible light.

KEYWORDS: Hollow microspheres, Hard core template, Photocatalysts, H2 evolution, γIrradiation


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INTRODUCTION In recent years, an increasingly great deal of attention has been devoted to seeking new energy sources, which were promoted to replace the conventional fossil fuels. Hydrogen, an ideal clean energy, has long been recognized to be the most promising next generation energy.[1] Since Fujishima[2] firstly reported the semiconductor photoelectrode to be used in photocatalytic water splitting into H2 in 1972, much progress has been achieved in TiO2 photocatalyst,[3-8] which was regarded as one of reliable semiconductor photocatalyst for hydrogen production due to its low cost, high physical and chemical stability, and non-toxicity.[9-13] However, there are two great difficulties lying in front of us for TiO2 as an ideal photocatalyst. On the one hand, the wide gap (3.2 eV) limits the absorption spectrum of TiO2 under ultraviolet region[14-17] that only accounts for about 5% of the whole solar spectrum, imposing restriction on full utilization of visible light of solar energy. On the other hand, the fast recombination of photogenerated electrons and holes results in poor quantum yield,[18,19] hindering the development of TiO2 as an efficient photocatalyst. It is well established that no single semiconductor can meet all demands of photocatalytic water splitting into H2.[20-24] So in fact, many strategies have been explored to improve TiO2 photocatalytic activity. For example, bandgap engineering was used to extend the visible light absorption of TiO2 by doping metallic or nonmetallic elements and/or combining with other narrow bandgap semiconductors such as CdS, ZnS and WO3.[25-29] Among them, CdS possesses the narrow band gap (2.4 eV) matching with the visible light absorption and the relatively negative conduction band, which was widely investigated in hydrogen evolution.[30-35] The coupled CdS/TiO2 heterostructure, such as the reported 0D or 1D core/shell CdS@TiO2,[36,37] TiO2 nanosheet/CdS nanorods,[38] CdS quantum dots-TiO2 heterostructures,[3941]

and TiO2/CdS hierarchical photocatalyst,[42, 43] displaying superior photocatalytic activities.


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Compared with any single component of TiO2 or CdS, the favorable photocatalytic activity of these CdS/TiO2 composites are attributed to both the wide light absorption to visible light and the efficient limit of the rapid recombination of photogenerated electron-hole pairs. To improve the generation efficiency of H2, some noble metal (such as Au or Pt) cocatalysts can be introduced as electron reservoirs to prolong the lifetime of photogenerated electron-hole pairs, thus efficiently enhancing the photocatalytic activity for hydrogen production.[44-46] For example, Xue group[47] reported a new type of Au@TiO2−CdS ternary nanostructure by decorating CdS NPs onto Au@TiO2 core−shell structures, which exhibited a remarkably high photocatalytic H2 generation rate compared to CdS-TiO2 and Au@TiO2 binary structures. For better light harvesting efficiency, Fan group[48,49] fabricated butterfly wing architecture and leaf– inspired hierarchical heterostructures of ternary CdS/Au/TiO2, and found that these unique architectures also contributed to the great enhancement of light harvesting ability and the improvement of water-splitting efficiency. Recently, three-dimensionally ordered macroporous structured TiO2-Au-CdS was prepared which demonstrated the synergistic effect of the light absorption enhancement by 3D macroporous structure, the photosensitizing and electron reservoir effect of Au NPs, and the reduction of recombination rate of charge carriers by CdS.[50] However, CdS NPs as visible light absorption photocatalyst in all the above nanostructures were just deposited on the surface of TiO2, these unprotected CdS NPs are prone to photocorrosion caused by the oxidation reaction between CdS and O2 and H2O assisted with photogenerated holes[51] and falling off. On the other hand, the above studies show that the architecture optimization of photocatalysts is an effective strategy to promote the light harvesting efficiency for better photocatalytic activity. Hollow structure as a typical one can also enhance the light harvesting efficiency, which


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is attributed to relatively long light residence time due to the multiple reflections of visible light within the internal cavity. Meanwhile, both the inner and outer surfaces can contact with the reactant molecules, improving the molecular diffusion and transfer.[52,53] Some hollow CdS/TiO2 sphere photocatalysts had been reported, however, they were just used in degradation of RhB dyes.[54, 55] It is inspired by the above works, we report herein a hollow mesoporous CdS@TiO2@Au microsphere photocatalyst which is prepared via “hard core template” approach combining with solvothermal reaction and γ-irradiation reduction method. The γ-irradiation reduction method is chosen to prepare Au NPs because the contamination of chemical reducing agents can be avoided and Au NPs may be evenly distributed on the surface of the hollow mesoporous CdS@TiO2 microspheres,[56,57] which is beneficial to improve the photocatalytic performance of the final product. This ternary photocatalyst exhibits excellent photocatalytic activity of water splitting into H2 and stability under visible light, which is attributed to the unique ternary hollow mesoporous double-shell heterostructure. On the one hand, this unique structure both enhances light-harvesting efficiency by extending the light residence time owing to the hollow mesoporous structure with fluffy TiO2 surface, and suppresses the recombination of photogenerated electrons and holes due to the synergistic effect of ternary components and mass transfer facilitation of hollow mesoporous structure. On the other hand, the TiO2 coating effectively prevents the direct contact between the CdS shell and O2 and H2O, thus considerably retarding the photocorrosion of CdS upon visible-light exposure.[51] Moreover, the sacrificial agents can quickly eliminate the photogenerated holes in the VB of CdS due to mass transfer facilitation of hollow mesoporous structure, leading to the enhanced photostability of photocatalysts. Possible photocatalytic


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hydrogen evolution mechanism of hollow CdS@TiO2@Au microspheres under visible light illumination will be discussed. EXPERIMENTAL SECTION Materials. Styrene (St, 99%) was bought from Sinopharm Chemical Regent Company (SCRC) and purified by passing through columns packed with basic aluminum oxide before use. Acrylic acid (AA, 98%, SCRC) was purified by distillation under reduced pressure. Potassium persulfate (KPS, 99.5%, SCRC) was recrystallized from distilled water. Gold potassium chloride (KAuCl4, Aladdin), titanium (IV) isopropoxide (TIP, 97%, Alfa Aesar), TiO2 P25 powders (99%, Aladdin), divinylbenzene (DVB, 80%, Aldrich), cadmium sulfate octahydrate (CdSO4·8H2O, 99%, Energy Chemical), thioacetamide (TAA, 99%, Energy Chemical), 2-(dimethylamino) ethyl methacrylate (DMEMA, 99.5%, Energy Chemical), diethylene triamine (DETA, 99%, SigmaAldrich), isopropanol (IPA, 99%, SCRC), sodium sulfite (Na2SO3, 98%, SCRC) and Na2S·9H2O (98%, SCRC) were all used as received. Deionized water was used in all experiments. Preparation of hollow CdS microspheres. “Hard core template” method was used in our experiment to prepare hollow CdS microspheres. PS microspheres with carboxyl groups on the surface were firstly synthesized by soap-free emulsion polymerization according to our previous literature.[58] Then, 0.5 g of as-prepared PS microspheres was redispersed with 60 ml of deionized water in 100 ml round-bottom flask, 1 ml of DMEMA and 3 g of CdSO4·8H2O were then added under the magnetic stirring. After ultrasonic dispersion for 15 min and another 1 h magnetic stirring for the full absorption of Cd2+ onto the surface of PS microspheres, 1.7 g of TAA were added into this dispersion system. Finally, this mixed system was heated to 55ºC under continuous magnetic stirring for 3 h. The yellow nanocomposite products were collected


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by centrifugation and washed several times with distilled water and ethyl alcohol alternately, and then dried in the vacuum oven at 50ºC for 12 h. The hollow CdS microspheres packed with nanocrystals were finally obtained by calcining the as-prepared powder at 550ºC under N2 atmosphere for 3 h with the ramping rate of 10ºC·min-1 and then at 400ºC in air for 2 h. Synthesis of hollow CdS@TiO2 microspheres. Hollow CdS@TiO2 microspheres were synthesized according to the previous literature with a little modification.[59] Typically, 0.2 g of as-prepared hollow CdS microspheres was dispersed in 45 ml of IPA before 50 µl of DETA was added. After magnetic stirring and ultrasonic dispersion for 15 min, respectively, 0.8 ml of TIP was added into the dispersion. The reaction mixture was transferred into 100 ml Teflon-lined stainless steel autoclave and kept at 200ºC for 24 h. The product was collected by centrifugation and dried in the vacuum oven at 50ºC for 12 h. Finally, the yellow powder was calcined at 400ºC in air for 2 h to obtain highly crystalline anatase. As a control, TiO2 NPs photocatalyst was synthesized similar to the above steps, just that 0.2 g of CdS was replaced by 0.2 g of PS microspheres. Preparation

of

hybrid

CdS@TiO2@Au

photocatalyst.

Hybrid

CdS@TiO2@Au

photocatalyst was synthesized with in-situ deposition of Au NPs on the surface of hollow CdS@TiO2 microspheres by γ-irradiation. To be specific, 20 mg of hollow CdS@TiO2 microspheres was dispersed in 15 ml of distilled water and then 200 µl of KAuCl4 solution (8 g/L) was added. After magnetic stirring for 15 min, 0.2 ml of IPA was added as a scavenger of free radicals to remove oxidative hydroxyl radicals. The obtained mixture was irradiated by γ-ray in

60

Co source at the dose rate of 118 Gy·min-1 for 3 h. The final products were collected by

centrifugation and washed with distilled water for several times, and then dried at 50ºC in


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vacuum oven. As a control, CdS@Au and TiO2@Au photocatalysts were synthesized with the same steps except that 20 mg of CdS@TiO2 was replaced by 20 mg of CdS or TiO2. Characterization. The morphology of the products were examined by transmission electron microscopy (TEM, Hitachi H-7650). High-resolution TEM (HRTEM) was conducted on a JEM2100F transmission electron microscope equipped with an energy-dispersive X-ray spectroscopic analysis (EDS) system. The crystalline phases of the samples were examined by powder X-ray diffraction (XRD) on a Japan Rigaku D/max A X-ray diffractometer equipped with graphite monochromatized Cu Kα irradiation (λ = 0.154178 nm). UV–vis diffuse reflectance spectra (DRS) were measured with Japan Shimadzu SOLID-3700 UV/Vis/NIR spectrophotometer using BaSO4 as reference. X-ray photoelectron spectroscopy (XPS) analysis was performed on the Thermo ESCALAB 250 X-ray photoelectron spectrometer with Al Kα X-ray as the exciting source. Photoluminescence (PL) spectra were recorded on fluorescence spectrometer (JY Fluorolog-3-Tou) with a Xe lamp at room temperature (λex = 410 nm). The N2 adsorption−desorption measurements were carried out using Tristar II 3020 instrument at 77.3 K. Prior to the measurements, samples were degassed at 150°C for 10 h under vacuum. The obtained specific surface areas and pore size distribution plots were calculated by the BrunauerEmmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method, respectively. Inductively coupled plasma mass spectra (ICP-MS) were carried out on a Thermo scientific VG Plasma Quad 3. Photocurrent measurement. All photoelectrochemical measurements were performed on a PGSTAT302N electrochemical workstation (Metrohm Autolab) with a three-electrode system using Pt wire as the counter electrode and Ag/AgCl as a reference electrode at a 0.5 V potential bias under the visible light of a 300 W Xe-lamp (light on/off at regular intervals of 20 s)


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equipped with a 420 nm cut-off filter. The working electrodes were prepared by dropping the photocatalyst suspensions in 5% nafion solution/ethanol (v/v =10%) onto a cleaning fluorinedoped tin oxide (FTO) glass surface using a pipette, and dried at room temperature. Sodium sulfate (Na2SO4) aqueous solution (0.5 M) was used as supporting electrolyte. Photocatalytic water splitting for hydrogen evolution. Typically, 20 mg of the photocatalyst was dispersed in 40 ml of aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3 as the sacrificial agents. The mixture was sealed in a quartz cylindrical vessel after degassing residual oxygen with argon for 30 min under magnetic stirring. Then, the suspension was illuminated under 300 W Xe lamp (100 mW·cm-2, Perfect Light PLS-SXE 300) coupled with a UV cut-off filter (λ≥420 nm) to ensure that the photocatalytic reaction was conducted under visible light. The produced hydrogen was periodically analyzed by Agilent SHIMADZUGC-2014 gas chromatograph (GC) with a thermal conductivity detector (TCD). RESULTS AND DISCUSSION Morphology and structure of hollow CdS@TiO2@Au microspheres. The synthetic procedure of hollow CdS@TiO2@Au microspheres is illustrated in Scheme 1. Carboxylfunctionalized PS microspheres synthesized by soap-free emulsion polymerization were used as “hard core template” to load CdS NPs. After calcination to remove the PS core, hollow CdS microspheres were obtained. Then fluffy TiO2 was coated on the surface of hollow CdS microspheres via solvothermal reaction using TIP as a precursor, followed by calcination to make the crystalline anatase more and better. Finally, Au NPs were deposited on double-shell CdS@TiO2 hollow microspheres by an effective γ-irradiation reduction of KAuCl4 aqueous solution, resulting in the formation of hollow CdS@TiO2@Au microspheres.


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Scheme 1. Synthetic procedure of hollow CdS@TiO2@Au microspheres using the “hard core template” method. TEM images (Figure 1) present the morphological evolution during the synthetic process of hollow CdS@TiO2 microspheres. PS microspheres are monodisperse with smooth surface, and the average diameter is 285 nm (Figure 1(a)). The surface of PS@CdS microspheres (Figure 1(b)) becomes rough, which indicates that CdS NPs are successfully stacked on the surface of PS microspheres. After removing PS core from PS@CdS microspheres by calcination, well-defined hollow mesoporous CdS microspheres with average diameter about 325 nm can be evidenced (Figure 1(c)), which is slight larger than that of PS microspheres. The thickness of CdS shell is about 20 nm, which is enough to make the morphology of hollow microspheres complete. From Figure 1(d), it can be clearly seen that fluffy TiO2 layer is coated on the surface of hollow CdS microspheres, demonstrating that the double-shell CdS@TiO2 hollow mesoporous microspheres were successfully prepared. Fluffy TiO2 particles can be obtained by the similar method (Figure S1(a) in Supporting Information). Due to large surface area and abundant surface hydroxyl groups of fluffy TiO2 layer or particles, Au NPs can be easily deposited onto the surface of hollow CdS@TiO2 microspheres or TiO2 particles by γ-irradiation method.


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Figure 1. TEM images of (a) PS, (b) PS@CdS, (c) hollow CdS and (d) hollow CdS@TiO2 microspheres. TEM images (Figure 2(a, b)) with different magnification exhibit the morphology of hollow mesoporous CdS@TiO2@Au microspheres, and fluffy TiO2 is more clearly visible; however, Au NPs are indistinguishable, which also happens in both TiO2@Au and CdS@Au binary systems (Figure S1(b, c) in Supporting Information). The selected area electron diffraction (SAED) indicates polycrystalline structure of hollow CdS@TiO2@Au microspheres (Figure 2(c)). HRTEM (Figure 2(d)) image clearly reveals the boundary between TiO2 of low crystallinity and CdS of high crystallinity, which is further evidenced by the following XRD patterns. The lattice spacing of 0.331 and 0.324 nm can be ascribed to the (002) and (101) facets of CdS, respectively, while the lattice spacing of 0.221 nm in the indistinct lattice fringes may correspond to the (001) plane of anatase TiO2. Au NPs with the size about 5 nm show a relatively dark contrast compared with CdS and TiO2 as shown in the circles of Figure 2d, where the lattice spacing of 0.257 nm can be indexed as that of the (111) plane of Au. The high-angle annular


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dark field scanning transmission electron microscopy (HAADF-STEM) image as shown in Figure 2(e), together with the energy-dispersive X-ray spectroscopic (EDS) elemental mappings (Figure 2(f−j)) further confirm the hollow structure of hybrid CdS@TiO2@Au microspheres. The S and Cd element mappings (Figure 2(f, g)) clearly distinguish the hollow interior and the CdS shell, while homogeneous distribution of Ti and O elements (Figure 2(h, i)) beyond the size range of the CdS shell indicates that TiO2 is coated on the CdS shell to form a double shell. It is interesting to note that the distribution range of Au element is little bigger than the size of the CdS shell but smaller than that of TiO2 layer, demonstrating that the Au NPs are deposited inside fluffy TiO2 shell. In combination with the former HRTEM analysis, we can believe that γirradiation deposition of Au NPs has been successfully achieved.

Figure 2. (a, b) TEM images with different magnification, (c) SAED, (d) HRTEM image and (e) HAADF-STEM image of CdS@TiO2@Au. (f-j) EDS elemental mappings of (f) S, (g) Cd, (h) Ti, (i) O, and (j) Au from panel (e).


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XRD patterns of various samples are demonstrated in Figure 3. Compared with PS@CdS microspheres, hollow CdS microspheres show better crystalline structure after calcination and can be assigned to hexagonal CdS (JCPDS card No. 80-0006). The diffraction peaks of binary CdS@TiO2 and CdS@Au as well as ternary CdS@TiO2@Au are all similar to those of the hollow CdS microspheres, but the peak intensities decrease. The diffraction peaks of TiO2 phase cannot be found due to the amorphous TiO2 obtained mainly, which is consistent with the HRTEM observation result. No apparent diffraction peak of Au appears after the deposition of Au NPs on the surface of hollow CdS or CdS@TiO2 microspheres, which could be attributed to the low content and high dispersion. For TiO2@Au nanocomposite, we can observe the characteristic diffraction peaks of anatase TiO2 (JCPDS card No. 83-2243) and face-centered cubic Au (JCPDS card No. 01-1174). Note that the TiO2 (004) peak overlaps with the Au (111) peak at 2θ=38.2°.

Figure 3. XRD patterns of (a) PS@CdS, (b) hollow CdS, (c) CdS@TiO2, (d) CdS@Au, and (e) CdS@TiO2@Au microspheres and (f) TiO2@Au particles.


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The XPS spectra of hollow CdS, CdS@TiO2 and CdS@TiO2@Au microspheres are shown in Figure 4 and Figure S2 (Supporting Information) to confirm the chemical composition and the surface states of the as-prepared photocatalyst. Comparing the typical survey spectra of these three samples, we can clearly see that the strong double sharp peaks of Cd3d and the peak of S2p in Figure S2(a, b) and Figure 4(a) indicate the existence of CdS for all samples, while the existence of inconspicuous C1s peak might be caused by gas molecules such as CO2 absorbed by the surface of the samples and the remaining carbonaceous species coming from the PS template which was not completely removed. The peak of O1s in Figure S2(a) is weak which might be caused by gas molecules such as O2 and CO2 absorbed by the surface of the sample, while those in Figure S2(b) and Figure 4(a) increase a lot due to the coating of TiO2 accompanying with the appearance of Ti2p double peaks. Furthermore, the double peaks of Au4f appear in Figure 4(a). High-resolution XPS spectra of Au4f, Cd3d, S2p, Ti2p and O1s in Figure 4(b-f) are exhibited to further determine elemental chemical states. The peaks of Au4f at 83.9 eV and 87.6 eV (Figure 4(b)) are attributed to metallic gold,[60] which further confirms the existence of Au NPs on the surface of hollow hybrid microspheres. The two sharp peaks of Cd3d observed at 405.0 eV and 411.8 eV (Figure 4(c)) can be assigned to the characteristic peaks of Cd3d5/2 and Cd3d3/2, respectively.[42,

50]

Moreover, a spin-orbit separation of 6.8 eV between Cd3d3/2 and Cd3d5/2

further confirms the existence of Cd2+ from hollow CdS microspheres. The double peaks of S2p located at 161.2 eV and 162.3 eV (Figure 4(d)) could be ascribed to the characteristic doublets of S2p3/2 and S2p1/2,[55] indicating that S element mainly exists in the form of S2−. The splitting doublet of Ti2p at 458.9 eV (Ti2p3/2) and 464.7 eV (Ti2p1/2) with a splitting energy of 5.8 eV (Figure 4(e)) indicates the existence of Ti4+ corresponding to the TiO2.[61] The two asymmetric peaks of O1s centred at 530.3 eV and 531.6 eV (Figure 4(f)) are associated with the lattice


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oxygen of TiO2 and the surface hydroxyl group (–OH), respectively.[62] All the results from TEM observation, EDS elemental mappings, XRD patterns and XPS analysis demonstrate the successful formation of hollow mesoporous CdS@TiO2@Au microspheres.

Figure 4. (a) XPS survey scan of hybrid CdS@TiO2@Au hollow microspheres and Highresolution XPS spectra of hybrid CdS@TiO2@Au hollow microspheres: (b) Au 4f; (c) Cd 3d; (d) S 2p ; (e) Ti 2p; (f) O 1s.

Figure 5(a) displays the N2 adsorption−desorption isotherms of hollow CdS, CdS@TiO2 and CdS@TiO2@Au microspheres. According to the IUPAC classification, all of them exhibit a type IV isotherm with a type H3 hysteresis loop, indicating the presence of mesopores,[55] which is further confirmed by the pore size distribution curves (Figure 5(b)) calculated from the nitrogen desorption isotherms by the BJH method. The specific surface area calculated by the BET


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method, average pore size and pore volume of these samples are summarized in Table S1 (Supporting Information). The BET surface area of hollow CdS@TiO2 microspheres is 215.5 m2·g-1, which is much higher than that of hollow CdS microspheres (129.1 m2·g-1) owing to the decoration of fluffy TiO2 layer with a high specific surface area. After the further deposition of Au NPs on the surface of hollow CdS@TiO2 microspheres, the BET surface area of hollow CdS@TiO2@Au microspheres (200.5 m2·g-1) just slightly decreases compared with that of hollow CdS@TiO2 microspheres. Meanwhile, the average pore size of hollow CdS@TiO2@Au microspheres (5.42 nm) is smaller than that of hollow CdS@TiO2 microspheres (5.67 nm), which could be attributed to the deposition of Au NPs in some mesopores. However, both of them are larger than the average pore size of hollow CdS microspheres (5.13 nm) due to the mesoporous structure of fluffy TiO2 layer. The above analysis is consistent with the TEM results.

Figure 5. Nitrogen adsorption−desorption isotherms (a) and the corresponding pore size distribution curves (b) of hollow CdS, CdS@TiO2 and CdS@TiO2@Au microspheres.

The UV-vis diffuse reflectance spectra of CdS, TiO2, CdS@TiO2, CdS@Au, TiO2@Au and CdS@TiO2@Au are given in Figure 6(a) to investigate optical properties. It is observed that the absorption onsets of anatase TiO2 and hollow CdS microspheres are below 400 nm in UV absorption region and about 550 nm in visible light, corresponding to bandgap energy of 3.2 eV


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and 2.3 eV, respectively. It is obvious that the introduction of Au NPs enhances the visible light absorption of both TiO2@Au and CdS@Au which may be ascribed to the weak surface plasmon resonance (SPR) effect of Au NPs[9, 21] (though there are no obvious SPR peaks due to the small size of Au NPs[63] and the overlapped characteristic absorption peaks of CdS and Au NPs) and the light scattering of as-prepared materials at long wavelengths (from 550 to 800 nm).[64,65] Compared with the hollow CdS microspheres, hollow CdS@TiO2 microspheres exhibit significantly enhanced light absorption capacity in the UV region, while hollow CdS@TiO2@Au microspheres further promotes the light absorption capacity from UV to visible light owing to the synergistic effect of ternary components and unique structure, which is responsible for the enhanced photocatalytic activity of the ternary system under visible light. The color of hollow CdS@TiO2@Au microspheres becomes dark yellow from the original light yellow of hollow CdS or CdS@TiO2 microspheres (Figure S3 in Supporting Information), also suggesting that it can absorb a considerable portion of visible light. To illustrate the recombination circumstance of photo-generated electrons and holes, which enormously influence the photocatalytic activity of water splitting, photoluminescence (PL) spectra are further tested. As shown in Figure 6(b), all the spectra have same emission band from 500 to 650 nm. The PL intensity of double-shell CdS@TiO2 hollow microspheres is much weaker than that of hollow CdS microspheres, suggesting the longer lifetime of photogenerated charge carriers for hollow CdS@TiO2 microspheres. It is possibly attributed to the less negative conduction band (CB) of TiO2 than that of CdS, so that the photogenerated electrons would transfer from the CB of CdS to that of TiO2, prolonging the lifetime of electron-hole pairs. The PL intensity of ternary CdS@TiO2@Au hollow microspheres is further reduced, which may be due to that the Au NPs act as electron reservoirs[47] to further trap the electrons in the CB of TiO2,


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and hence the charge separation is further accelerated. This unique internal electron-transfer of ternary photocatalyst would promote efficient separation of photogenerated electron-hole pairs, thus exhibit the enhanced photocatalytic H2 production.

Figure 6. (a) UV-vis diffuse reflectance spectra of CdS, TiO2, CdS@TiO2, CdS@Au, TiO2@Au and CdS@TiO2@Au; (b) Photoluminescence (PL) spectra of CdS, CdS@TiO2 and CdS@TiO2@Au using the excitation wave length of 410 nm.

Photocurrent measurement is another strategy to investigate the ability of photogenerated electron-hole pairs and the efficiencies of charge carrier migration and separation. As it is shown in Figure 7(a), all samples exhibit a great increase of photocurrent densities when the light is on, but a sharp decrease when the Xe-lamp turns off. Using Iphoto and Idark to represent the currents in the presence and absence of light illumination, respectively, the photogenerated currents (IphotoIdark) of the three samples are illustrated in Figure 7(b). It can be seen that the ternary CdS@TiO2@Au hollow microsphere photocatalyst shows the highest current density, which is nearly 1.6 and 4.1 times higher than hollow CdS@TiO2 and CdS microspheres, respectively. It is demonstrated that the introduction of Au NPs and TiO2 coating can promote efficient separation of photogenerated charge carriers, which is consistent with the results of PL spectra.


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Figure 7. Transient photocurrent responses (a) and photogenerated currents (b) of three typical as-prepared samples in a 0.5 M Na2SO4 aqueous solution at a potential bias of 0.5 V vs. Ag/AgCl under visible light illumination (λ≥420 nm).

Evaluation of photocatalytic performance of hydrogen production. Photocatalytic activities of water splitting for hydrogen evolution of the as-prepared samples are evaluated under the visible light (≥420 nm) illumination in an aqueous solution containing 0.1 M Na2SO3 and 0.1 M Na2S used as the sacrificial agents to quench the holes. The average hydrogen evolution rates for all the samples are illustrated in Figure 8(a) comparing with the commercial TiO2 P25 powders. It is apparent that very little H2 is generated under the visible light using either TiO2 P25 powders or our own synthetic TiO2 particles as a photocatalyst due to its inherent bandgap, and the hydrogen evolution rate is increased to 52.8 µmol·g-1·h-1 by the decoration of Au NPs on the surface of TiO2 particles owing to that the plasmonic hot electrons from Au NPs could be captured by the CB of TiO2.[9, 21] However, this production mechanism and transfer path of photoexcited electrons in the TiO2@Au binary photocatalyst are quite different from those of CdS-based photocatalysts discussed later in our work. As TiO2 can be barely photoexcited in visible light region, the plasmonic hot electrons transferred from Au NPs to the CB of TiO2 are responsible for the increased hydrogen evolution rate in the TiO2@Au


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binary photocatalyst system. In comparison, CdS can be well photoexcited under visible light, while the SPR effect of Au in our work is extraordinarily weak. For this reason, the SPR effect of Au is negligible in our CdS-based system. It is noted that the hydrogen evolution rate of hollow CdS microspheres is 135.7 µmol·g-1·h-1, which corresponds to its intrinsic bandgap matching the visible light. Meanwhile, the hollow mesoporous structure can enhance the light-harvesting efficiency owing to the multiple reflections of light within the internal cavity and promote molecular diffusion to eliminate quickly the photogenerated holes in the valence band (VB) of CdS. The incorporation of either Au NPs or fluffy TiO2 layer increases the hydrogen production rate (523.8 µmol·g-1·h-1 for hollow CdS@Au microspheres and 726 µmol·g-1·h-1 for hollow CdS@TiO2 microspheres). This is mainly attributed to the restricted recombination of photogenerated electrons and holes because the photogenerated electrons in the CB of CdS are attracted to Au NPs or transferred to that of TiO2 as discussed above. Moreover, it is established that the conduction bands of both CdS and TiO2 are more negative than hydrogen production potential (-0.41 V),[23] which is an indispensable requirement for hydrogen evolution reaction. Compared with hollow CdS@Au microspheres where the Au NPs are interspersed on the surface of CdS shell, the heterostructure of hollow CdS@TiO2 microspheres may exhibit higher electron transfer efficiency. Therefore, the photocatalytic activity of the latter is better than that of the former. In addition, the increased specific surface area owing to the coating of fluffy TiO2 (as shown in Figure 5(a) and Table S1 in Supporting Information) enhances light-harvesting efficiency, which is attributed to the multiple reflections and scattering of light within the fluffy TiO2 layer. By the deposition of Au NPs on the hollow CdS@TiO2 microspheres, the H2 production rate is further increased to 1720 µmol·g-1·h-1, which is 2.37 times as high as that of hollow CdS@TiO2 microspheres. As shown in Figure 6(a), the absence of a clear SPR peak of


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Au NPs in the UV-vis spectrum of ternary hollow CdS@TiO2@Au microspheres is an indication that the SPR effect of Au may not be evoked in the mechanism of our ternary photocatalyst system, although many other reported Au-based catalysts are known to rely on the SPR effect.[9,24,28,60,66] We propose alternatively that Au NPs act as electron reservoirs according to the reported literatures.[46,47] Thus, the electrons in the CB of TiO2 transferred from the CB of CdS are attracted to Au NPs, further promoting the separation of electrons and holes. This prolonged lifetime of the photogenerated charge carriers has been confirmed by the PL spectra in Figure 6(b). This unique ternary hollow mesoporous heterostructure is responsible for the excellent photocatalytic activity with the synergistic effect of the enhanced light-harvesting efficiency owing to the hollow mesoporous structure with fluffy TiO2 surface, and the reduced recombination rate of photogenerated electrons and holes due to the heterostructure of thin CdSTiO2 shells and the electron reservoir effect of Au NPs as well as mass transfer facilitation of hollow mesoporous structure. Moreover, the apparent quantum efficiencies (AQE) of hollow CdS, CdS@TiO2 and CdS@TiO2@Au microspheres for hydrogen evolution were tested, and the results were listed in Table S2. It can be seen that the AQE of hollow CdS@TiO2@Au is much higher than those of hollow CdS and CdS@TiO2 microspheres, demonstrating that the addition of Au NPs can effectively improve the hydrogen evolution rate. This is consistent with the corresponding hydrogen production results. We also investigate the influence of the Au loading amount on the photocatalytic hydrogen evolution rate of ternary photocatalyst. By increasing the amount of KAuCl4 aqueous solution, the loading amount of Au in hollow CdS@TiO2@Au microspheres also increases (Table S3 in Supporting Information). However, the hydrogen production rate of hollow CdS@TiO2@Au microspheres first increases with the increase of the loading amount of Au NPs, and then


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decreases after reaching the maximum value of 1720 µmol·g-1·h-1 at the Au content of 2.36% (Figure S4 in Supporting Information). It demonstrates that the loading amount of Au NPs should be kept within a reasonable range. The stabilities of the photocatalysts including hollow CdS, CdS@TiO2 and CdS@TiO2@Au microspheres were evaluated through a cycling test of hydrogen evolution. As shown in Figure 8(b), the photocatalytic activities of both hollow CdS@TiO2 and CdS@TiO2@Au microspheres show no obvious decrease after five cycles in total of 25 h visible-light illumination, while that of hollow CdS microspheres decreases significantly from the average hydrogen generation rate of 137 µmol·g-1·h-1 to 71 µmol·g-1·h-1. The significantly decrease may be ascribed to the photocorrosion of hollow CdS microspheres with the extension of light illumination time, leading to the oxidization of S2− in CdS into elemental sulphur accumulating on the surface of CdS microspheres, such that the oxidized CdS would lose its photoreactivity. The coating of fluffy TiO2 can not only enhance the separation of electron-hole pairs via electron transfer and light harvesting efficiency by the multiple light reflections and the scattering inside the abundant pore channels of fluffy TiO2 coating, but also reduce the photocorrosion effect of CdS by preventing the direct contact between the CdS shell and O2 and H2O,[51] leading to an improved stability and a increased hydrogen generation rate of 726 µmol·g-1·h-1 for hollow CdS@TiO2 microspheres. The further promoted photocatalytic activity of hollow CdS@TiO2@Au microspheres demonstrate that Au NPs play an important role, which mainly act as electron reservoirs[47] due to the lower Fermi levels that could trap the electrons in the CB of TiO2 to further prolong the lifetime of photogenerated charge carriers. At the same time, the fluffy TiO2 coating as the substrate for loading Au NPs can prevent them from aggregation. From Figure S5 and S6 (Supporting Information), we can see that the morphology and the structure of hollow


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CdS@TiO2@Au microspheres did not change significantly after five cycling tests, demonstrating an excellent stability of hollow CdS@TiO2@Au microspheres as the photocatalyst. In addition, it is worth noting that the hollow mesoporous CdS@TiO2@Au microspheres exhibit higher H2 generation rate and stability than most of the other similar reported photocatalyts.[48-50] Although Au@TiO2−CdS ternary nanostructures[47] and 3D ordered macroporous structured TiO2-Au-CdS[50] showed slightly higher rates of H2 production, the former was more expensive due to the high content of Au NPs and the latter was difficult and tedious to be prepared with poor stability. Therefore, the comprehensive performance of our ternary photocatalyst is excellent based on preparation, cost, photocatalytic activity and stability.

Figure 8. Photocatalytic hydrogen evolution activities: (a) Histogram of the hydrogen production rates of various samples under visible light illumination; (b) Recycling tests of photocatalytic H2 evolution of hollow mesoporous CdS, CdS@TiO2 and CdS@TiO2@Au microspheres.

According to the above results, a possible mechanism of photocatalytic hydrogen generation for hollow mesoporous CdS@TiO2@Au microspheres is proposed in Scheme 2. Under visible light illumination, most of photogenerated electrons of CdS are quickly transferred from the CB of CdS to that of TiO2 and then partly to Au NPs further, while holes accumulating in the VB of CdS are quenched by the Na2S and Na2SO3, hence the fast recombination of photogenerated


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electron-hole pairs is effectively inhibited, which has been proved by the results of PL spectra shown in Figure 6(b) and photoelectrochemical measurements shown in Figure 7. Moreover, the abundant pore channels of fluffy TiO2 coating and the cavities of hollow mesoporous microspheres not only greatly enhance the absorption of light, but also facilitate mass transfer, which allow S2− and SO32− to quickly eliminate the photogenerated holes in the VB of CdS, and simultaneously make water accessible to the active sites for H2 evolution. On the other hand, the TiO2 coating effectively prevents the direct contact between the CdS shell and O2 and H2O, thus considerably retarding the photocorrosion of CdS upon visible-light exposure. Therefore, hollow mesoporous CdS@TiO2@Au microspheres exhibit excellent photocatalytic activity and stability.

Scheme 2. Photocatalytic hydrogen evolution mechanism of hollow CdS@TiO2@Au microspheres under visible light

CONCLUSIONS In summary, we had successfully synthesized ternary CdS@TiO2@Au hollow mesoporous microspheres using “hard core template” approach combined with solvothermal reaction and γirradiation reduction. This unique ternary hollow mesoporous heterostructure not only possesses


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excellent stability, but also remarkably promotes hydrogen generation rate by enhancing lightharvesting efficiency, facilitating mass transfer and restricting the recombination of photogenerated electrons and holes. The photocatalytic hydrogen generation rate (1720 µmol·g1

·h-1) of the ternary hollow CdS@TiO2@Au microspheres under visible light illumination was

2.37 and 12.7 times higher than those of hollow CdS@TiO2 and CdS microspheres, respectively. It was found that the photogenerated electrons might transfer from the CB of CdS to that of TiO2 then to Au NPs, highly promoting the separation of photogenerated charge carriers. Au NPs mainly acted as electron reservoirs to further trap photogenerated electrons. Our ternary hollow mesoporous heterostructure microsphere photocatalysts should have promising applications in environmental and energy fields.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Figure S1: TEM images of (a) TiO2, (b) TiO2@Au and (c) CdS@Au; Figure S2: XPS survey spectra of (a) CdS and (b) CdS@TiO2; Figure S3: Photos of (a) CdS, (b) CdS@TiO2, (c) CdS@TiO2@Au; Figures S4: Curve of hydrogen production rates (a) and histogram of the hydrogen production rates (b) of CdS@TiO2@Au microspheres with different amount of Au NPs; Figure S5: TEM image of hollow mesoporous CdS@TiO2@Au microspheres after five cycling tests; Figure S6: XRD patterns of hollow mesoporous CdS@TiO2@Au microspheres before and after five cycling photocatalytic reactions; Table S1: Specific surface area, average pore size and pore volume of hollow CdS, CdS@TiO2 and CdS@TiO2@Au microspheres; Table S2: The apparent quantum efficiency (AQE) of hollow CdS,


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CdS@TiO2 and CdS@TiO2@Au microspheres for hydrogen evolution; Table S3: The content of Au in hollow CdS@TiO2@Au microspheres synthesized by adding different amount of KAuCl4 aqueous solution (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Huarong Liu). ORCID Huarong Liu: 0000-0002-8066-1241 Author Contributions †

(W. Yuan, Z. Zhang) These authors equally contributed to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (Project No. 51373160 and 21074122) for financial support

REFERENCES (1) Muradov, N.; Veziroglu, T. “Green” path from fossil-based to hydrogen economy: An overview of carbon-neutral technologies. Int. J. Hydrogen Energy 2008, 33 (23), 6804-6839. DOI 10.1016/j.ijhydene. 2008.08.054.


26

Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(2) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238 (5358), 37-38. DOI 10.1038/238037a0. (3) Xiang, Q.; Yu, J.; Jaroniec, M. Tunable photocatalytic selectivity of TiO2 films consisted of flower-like microspheres with exposed {001} facets. Chem. Commun. 2011, 47 (15), 45324534. DOI 10.1039/c1cc10501a. (4) Lee, J. S.; You, K. H.; Park, C. B. Highly photoactive, low bandgap TiO2 nanoparticles wrapped by graphene. Adv. Mater., 2012, 24 (8), 1084-1088. DOI 10.1002/adma.201104110. (5) Yang, Q.; Li, M.; Liu, J.; Shen, W.; Ye, C.; Shi, X.; Jiang, L.; Song, Y. Hierarchical TiO2 photonic crystal spheres prepared by spray drying for highly efficient photocatalysis. J. Mater. Chem. A 2013, 1 (7), 541-547. DOI 10.1039/c2ta00060aa. (6) Wang, W.; Dong, J.; Ye, X.; Li, Y.; Ma, Y.; Qi, L. Heterostructured TiO2 nanorod@nanobowl arrays for efficient photoelectrochemical water splitting. Small 2016, 12 (11), 1469-1478. DOI 10.1002/smll.201503553. (7) Qi, D.; Xing, M.; Zhang, J. Hydrophobic carbon-doped TiO2/MCF-F composite as a high performance

photocatalyst.

J.

Phys.

Chem.

C

2014,

118

(14),

7329-7336.

DOI

10.1021/jp4123979. (8) Qiu, B.; Xing, M.; Zhang, J. Mesoporous TiO2 nanocrystals grown in situ on graphene aerogels for high photocatalysis and lithium-ion batteries. J. Am. Chem. Soc. 2014, 136 (16), 5852-5855. DOI 10.1021/ja500873u. (9) Fang, J.; Cao, S.W.; Wang, Z.; Shahjamali, M. M.; Loo, S. C. J.; Barber, J.; Xue, C. Mesoporous plasmonic Au–TiO2 nanocomposites for efficient visible-light-driven photocatalytic water

reduction.

Int.

J.

Hydrogen

Energy

2012,

37

(23),

17853-17861.

DOI

10.1016/j.ijhydene.2012.09.023.


27


Page 28 of 37

(10) Wang, M.; Han, J.; Hu, Y.; Guo, R.; Yin, Y. Carbon-incorporated NiO/TiO2 mesoporous shells with p-n heterojunctions for efficient visible light photocatalysis. ACS Appl. Mater. Interfaces 2016, 8 (43), 29511-25921. DOI 10.1021/acsami.6b10480. (11) Jing, L.; Zhou, W.; Tian, G.; Fu, H. Surface tuning for oxide-based nanomaterials as efficient photocatalysts. Chem. Soc. Rev. 2013, 42 (24), 9509-9549. DOI 10.1039/c3cs60176e. (12) Wang, M.; Hu, Y.; Han, J.; Guo, R.; Xiong, H.; Yin, Y. TiO2/NiO hybrid shells: p-n junction photocatalysts with enhanced activity under visible light. J. Mater. Chem. A 2015, 3 (41), 20727-20735. DOI 10.1039/c5ta05839b. (13) Li, W.; Wu, Z.; Wang, J.; Elzatahry, A. A.; Zhao, D. A perspective on mesoporous TiO2 materials. Chem. Mater. 2013, 26 (1), 287-298. DOI 10.1021/cm4014859. (14) Pan, J. H.; Han, G.; Zhou, R.; Zhao, X. S. Hierarchical N-doped TiO2 hollow microspheres consisting of nanothorns with exposed anatase {101} facets. Chem. Commun. 2011, 47 (24), 6942-6944. DOI 10.1039/c1cc11796c. (15) Wang, J. D.; Tafen, N. ; Lewis, J. P.; Hong, Z.; Manivannan, A.; Zhi, M.; Li, M.; Wu, N. Origin of photocatalytic activity of nitrogen-doped TiO2 nanobelts. J. Am. Chem. Soc. 2009, 131 (34), 12290-12297. DOI 10.1021/ja903781h. (16) Batzill, M. Fundamental aspects of surface engineering of transition metal oxide photocatalysts. Energy Environ. Sci. 2011, 4 (9), 3275-3286. DOI 10.1039/c1ee01577j. (17) Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C. Titanium dioxide-based nanomaterials for photocatalytic fuel generations. Chem. Rev. 2014, 114 (19), 9987-10043. DOI 10.1021/cr500008u.


28

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Butburee, T.; Bai, Y.; Pan, J.; Zong, X.; Sun, C.; Liu, G.; Wang, L. Step-wise controlled growth of metal@TiO2 core-shells with plasmonic hot spots and their photocatalytic properties. J. Mater. Chem. A 2014, 2 (32), 12776-12784. DOI 10.1039/c4ta01120a. (19) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-light photocatalysis in nitrogen-doped

titanium

oxides.

Science

2001,

293

(5528),

269-271.

DOI

10.1126/science.1061051. (20) Li, X.; Yu, J.; Low, J.; Fang, Y.; Xiao, J.; Chen, X. Engineering heterogeneous semiconductors for solar water splitting. J. Mater. Chem. A 2015, 3 (6), 2485-2534. DOI 10.1039/c4ta04461d. (21) Cai, J.; Wu, X.; Li, S.; Zheng, F.; Zhu, L.; Lai, Z. Synergistic effect of double-shelled and sandwiched TiO2@Au@C hollow spheres with enhanced visible-light-driven photocatalytic activity. ACS Appl. Mater. Interfaces 2015, 7 (6), 3764-3772. DOI 10.1021/am508554. (22) Lian, Z.; Xu, P.; Wang, W.; Zhang, D.; Xiao, S.; Li, X.; Li, G. C60-decorated CdS/TiO2 mesoporous architectures with enhanced photostability and photocatalytic activity for H2 evolution. ACS Appl. Mater. Interfaces 2015, 7 (8), 4533-4540. DOI 10.1021/am5088665. (23) Li, X.; Yu, J.; Jaroniec, M. Hierarchical photocatalysts. Chem. Soc. Rev. 2016, 45 (9), 2603-2636. DOI 10.1039/c5cs00838g. (24) Wang, M.; Han, J.; Xiong, H.; Guo, R.; Yin, Y. Nanostructured hybrid shells of rGO/AuNP/m-TiO2 as highly active photocatalysts. ACS Appl. Mater. Interfaces 2015, 7 (12), 6909-6918. DOI 10.1021/acsami.5b00663. (25) Yun, H. J.; Lee, H.; Kim, N. D.; Lee, D. M.; Yu, S.; Yi, J. A combination of two visiblelight responsive photocatalysts for achieving the Z-scheme in the solid state. ACS Nano 2011, 5 (5), 4084-4090. DOI 10.1021/nn2006738.


29


Page 30 of 37

(26) Jiang, Z.; Qian, K.; Zhu, C.; Sun, H.; Wan, W.; Xie, J.; Li, H.; Wong, P. K.; Yuan, S. Carbon nitride coupled with CdS-TiO2 nanodots as 2D/0D ternary composite with enhanced photocatalytic H2 evolution: A novel efficient three-level electron transfer process. Appl. Catal. B 2017, 210 (5), 194-204. DOI 10.1016/j.apcatb.2017.03.069. (27) Nguyen, C.C.; Vu, N.N.; Do, T.O. Efficient hollow double-shell photocatalysts for the degradation of organic pollutants under visible light and in darkness. J. Mater. Chem. A 2016, 4 (12), 4413-4419. DOI 10.1039/c5ta09016d. (28) Cai, J.; Wu, X.; Li, S.; Zheng, F. Synthesis of TiO2@WO3/Au nanocomposite hollow spheres with controllable size and high visible-light-driven photocatalytic activity. ACS Sustain. Chem. Eng. 2016, 4 (3), 1581-1590. DOI 10.1021/acssuschemeng.5b01511. (29) Park, C. Y.; Choi, J. G.; Ghosh, T.; Meng, Z. D.; Zhu, L.; Oh, W. C. Preparation of ZnSgraphene/TiO2 composites designed for their high photonic effect and photocatalytic activity under visible light. Fuller. Nanotub. Car. Nanostruct. 2014, 22 (7), 630-642. DOI 10.1080/1536383x.2012.717556. (30) Wang, P.; Wu, T.; Wang, C.; Hou, J.; Qian, J.; Ao, Y. Combining heterojunction engineering with surface cocatalyst modification to synergistically enhance the photocatalytic hydrogen evolution performance of cadmium sulfide nanorods. ACS Sustain. Chem. Eng. 2017, 5 (9), 7670-7677. DOI 10.1021/acssuschemeng.7b01043. (31) Yuan, J.; Wen, J.; Zhong, Y.; Li, X.; Fang, Y.; Zhang, S.; Liu, W.

Enhanced

photocatalytic H2 evolution over noble-metal-free NiS cocatalyst modified CdS nanorods/g-C3N4 heterojunctions. J. Mater. Chem. A 2015, 3 (35), 18244-18255. DOI 10.1039/c5ta04573h.


30

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32) Yu, H.; Zhong, W.; Huang, X.; Wang, P.; Yu, J. Suspensible cubic-phase CdS nanocrystal photocatalyst: Facile synthesis and highly efficient H2-evolution performance in a sulfur-rich system. ACS Sustain. Chem. Eng. 2018, 6 (4), 5513-5523. DOI 10.1021/acssuschemeng.8b00398. (33) Yu, H.; Huang, X.; Wang, P.; Yu, J. Enhanced photoinduced-stability and photocatalytic activity of CdS by dual amorphous cocatalysts: Synergistic effect of Ti (IV)-hole cocatalyst and Ni

(II)-electron

cocatalyst.

J.

Phys.

Chem.

C

2016,

120

(7),

3722-3730.

DOI

10.1021/acs.jpcc.6b00126. (34) Wu, T.; Wang, P.; Qian, J.; Ao, Y.; Wang, C.; Hou, J. Noble-metal-free nickel phosphide modified CdS/C3N4 nanorods for dramatically enhanced photocatalytic hydrogen evolution under visible light irradiation. Dalton Trans. 2017, 46 (40),

13793-13801. DOI

10.1039/c7dt02929b. (35) Xiang, Q.; Cheng, F.; Lang, D. Hierarchical layered WS2/graphene-modified CdS nanorods for efficient photocatalytic hydrogen evolution. ChemSusChem 2016, 9 (9), 996-1002. DOI 10.1002/cssc.201501702. (36) Liu, S.; Zhang, N.; Tang, Z. R.; Xu, Y. J. Synthesis of one-dimensional CdS@TiO2 core– shell nanocomposites photocatalyst for selective redox: The dual role of TiO2 shell. ACS Appl. Mater. Interfaces 2012, 4 (11), 6378-6385. DOI 10.1021/am302074p. (37) El-Maghrabi, H. H.; Barhoum, A.; Nada, A. A.; Moustafa, Y. M.; Seliman, S. M.; Youssef, A. M.; Bechelany, M. Synthesis of mesoporous core-shell CdS@TiO2 (0D and 1D) photocatalysts for solar-driven hydrogen fuel production. J. Photochem. Photobio. A 2018, 351, 261-270 (2018, 357, 213 Corrigendum). DOI.org/10.1016/j.jphotochem.2017.10.048. (38) Wang, J.; Wang, Z.; Qu, P.; Xu, Q.; Zheng, J.; Jia, S.; Zhu, Z. A 2D/1D TiO2 nanosheet/CdS nanorods heterostructure with enhanced photocatalytic water splitting


31


Page 32 of 37

performance for H2 evolution. Int. J. Hydrogen Energy 2018, 43 (15), 7388-7396. DOI.org/10.1016/j.ijhydene.2018.02.191. (39) Li, G. S.; Zhang, D. Q.; Yu, J. C. A new visible-light photocatalyst: CdS quantum dots embedded mesoporous TiO2. Environ. Sci. Technol. 2009, 43 (18), 7079-7085. DOI 10.1021/es9011993. (40) Xie, Y.; Ali, G.; Yoo, S. H.; Cho, S. O. Sonication-assisted synthesis of CdS quantumdot-sensitized TiO2 nanotube arrays with enhanced photoelectrochemical and photocatalytic activity. ACS Appl. Mater. Interfaces 2010, 2 (10), 2910-2914. DOI 10.1021/am100605a. (41) Xiao, F. X.; Miao, J.; Wang, H. Y.; Liu, B.; Self-assembly of hierarchically ordered CdS quantum dots–TiO2 nanotube array heterostructures as efficient visible light photocatalysts for photoredox

applications.

J.

Mater.

Chem.

A

2013,

1

(39),

12229-12238.

DOI

10.1039/c3ta12856c. (42) Meng, A.; Zhu, B.; Zhong, B.; Zhang, L.; Cheng, B. Direct Z-scheme TiO2/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity. Appl. Surf. Sci. 2017, 422, 518-527. DOI 10.1016/j.apsusc.2017.06.028. (43) Tian, F.; Hou, D.; Hu, F.; Xie, K.; Qiao, X.; Li, D. Porous TiO2 nanofibers decorated CdS nanoparticles by SILAR method for enhanced visible-light-driven photocatalytic activity. Appl. Surf. Sci. 2017, 391, 295-302. DOI.org/10.1016/j.apsusc.2016.07.010. (44) Jang, J. S.; Choi, S. H.; Kim, H. G.; Lee, J. S. Location and state of Pt in platinized CdS/TiO2 photocatalysts for hydrogen production from water under visible light. J. Phy. Chem. C 2008, 112 (44), 17200-17205. DOI 10.1021/jp804699c.


32

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(45) Park, H.; Kim, Y. K.; Choi, W. Reversing CdS preparation order and its effects on photocatalytic hydrogen production of CdS/Pt-TiO2 hybrids under visible light. J. Phy. Chem. C 2011, 115 (13), 6141-6148. DOI 10.1021/jp2015319. (46) Wang, P.; Sheng, Y.; Wang, F.; Yu, H. Synergistic effect of electron-transfer mediator and interfacial catalytic active-site for the enhanced H2-evolution performance: A case study of CdS-Au photocatalyst. Appl. Catal. B 2018, 220, 561-569. DOI 10.1016/j.apcatb.2017.08.080. (47) Fang, J.; Xu, L.; Zhang, Z.; Yuan, Y.; Cao, S.; Wang, Z.; Yin, L.; Liao, Y.; Xue, C. Au@TiO2–CdS ternary nanostructures for efficient visible-light-driven hydrogen generation. ACS Appl. Mater. Interfaces 2013, 5 (16), 8088-8092. DOI 10.1021/am4021654. (48) Ding, L.; Zhou, H.; Lou, S.; Ding, J.; Zhang, D.; Zhu, H.; Fan, T. Butterfly wing architecture assisted CdS/Au/TiO2 Z-scheme type photocatalytic water splitting. Int. J. Hydrogen Energy 2013, 38 (20), 8244-8253. DOI 10.1016/j.ijhydene.2013.04.093. (49) Zhou, H.; Ding, L.; Fan, T.; Ding, J.; Zhang, D.; Guo, Q. Leaf-inspired hierarchical porous CdS/Au/N-TiO2 heterostructures for visible light photocatalytic hydrogen evolution. Appl. Catal. B 2014, 147, 221-228. DOI 10.1016/j.apcatb.2013.08.025. (50) Zhao, H.; Wu, M.; Liu, J.; Deng, Z.; Li, Y.; Su, B. L. Synergistic promotion of solardriven H2 generation by three-dimensionally ordered macroporous structured TiO2-Au-CdS ternary photocatalyst. Appl. Catal. B 2016, 184, 182-190. DOI 10.1016/j.apcatb.2015.11.018. (51) Wang, M.; Cai, L.; Wang, Y.; Zhou, F.; Xu, K.; Tao, X.; Chai, Y. Graphene-draped semiconductors for enhanced photocorrosion resistance and photocatalytic properties. J. Am. Chem. Soc. 2017, 139, 4144−4151. DOI 10.1021/jacs.7b00341.


33


Page 34 of 37

(52) Luo, M.; Liu, Y.; Hu, J.; Liu, H.; Li, J. One-pot synthesis of CdS and Ni-doped CdS hollow spheres with enhanced photocatalytic activity and durability. ACS Appl. Mater. Interfaces 2012, 4 (3), 1813-1821. DOI 10.1021/am3000903. (53) Nguyen, C. C.; Vu, N. N.; Do, T. O. Recent advances in the development of sunlightdriven hollow structure photocatalysts and their applications. J. Mater. Chem. A 2015, 3 (36), 18345-18359. DOI 10.1039/c5ta04326c. (54) Meng, H. L.; Cui, C.; Shen, H. L.; Liang, D. Y.; Xue, Y. Z.; Li, P. G.; Tang, W. H. Synthesis and photocatalytic activity of TiO2@CdS and CdS@TiO2 double-shelled hollow spheres. J. Alloy. Compd. 2012, 527, 30-35. DOI 10.1016/j.jallcom.2012.02.043. (55) Xue, C.; Wang, T.; Yang, G.; Yang, B.; Ding, S. A facile strategy for the synthesis of hierarchical TiO2/CdS hollow sphere heterostructures with excellent visible light activity. J. Mater. Chem. A 2014, 2 (21), 7674-7679. DOI 10.1039/c4ta01190b. (56) Yang, S.; Liu, H. A novel approach to hollow superparamagnetic magnetite/polystyrene nanocomposite microspheres via interfacial polymerization. J. Mater. Chem. 2006, 16, 44804487. DOI 10.1039/b612013j. (57) Liu, H.; Du, J. Synthesis and characterization of MoO2/P(St-co-MMA-co-AA) microspheres via microemulsion by γ-ray radiation. Solid State Sci. 2006, 8, 526-530. DOI 10.1016/j.solidstatesciences.2005.12.002. (58) Wang, F. W.; Liu, H. R.; Zhang, Y.; Liu, H. W.; Ge X. W.; Zhang, X. Y. Synthesis of snowman-like polymer-silica asymmetric particles by combination of hydrolytic condensation process with γ-ray radiation initiated seeded emulsion polymerization. J. Polym. Sci. Part A: Polym Chem. 2014, 52 (3), 339-348. DOI 10.1002/pola.27006.


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Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(59) Chen, J. S.; Luan, D.; Li, C. M.; Boey, F. Y.; Qiao, S.; Lou, X. W. TiO2 and SnO2@TiO2 hollow spheres assembled from anatase TiO2 nanosheets with enhanced lithium storage properties. Chem .Commun. 2010, 46 (43), 8252-8254. DOI 10.1039/c0cc02973d. (60) Wang, M.; Han, J.; Xiong, H.; Guo, R. Yolk@shell nanoarchitecture of Au@r-GO/TiO2 hybrids as powerful visible light photocatalysts. Langmuir 2015, 31 (22), 6220-6228. DOI 10.1021/acs.langmuir.5b01099. (61) Zhang, Y. C.; Li, J.; Xu, H. Y. One-step in situ solvothermal synthesis of SnS2/TiO2 nanocomposites with high performance in visible light-driven photocatalytic reduction of aqueous Cr(VI). Appl. Catal. B 2012, 123, 18-26. DOI 10.1016/j.apcatb.2012.04.018. (62) Qi, L.; Cheng, B.; Yu, J.; Ho, W. High-surface area mesoporous Pt/TiO2 hollow chains for efficient formaldehyde decomposition at ambient temperature. J. Hazard. Mater. 2016, 301, 522-530. DOI 10.1016/j.jhazmat.2015.09.026. (63) Govorov, A.; Zhang, H.; Gun’ko, Y. Theory of photoinjection of hot plasmonic carriers from metal nanostructures into semiconductors and surface molecules. J. Phys. Chem. C 2013, 117, 16616-16631. DOI 10.1021/jp405430m. (64) Sarina, S.; Zhu, H. Y.; Xiao, Q.; Jaatinen, E.; Jia, J.; Huang, Y.; Zheng, Z.; Wu, H. Viable photocatalysts under solar-spectrum irradiation: Nonplasmonic metal nanoparticles. Angew. Chem. Int. Ed. 2014, 53, 2935-2940. DOI 10.1002/anie.201308145. (65) Bai, S.; Ge, J.; Wang, L.; Gong, M.; Deng, M.; Kong, Q.; Song, L.; Jiang, J.; Zhang, Q.; Luo, Y.; Xie, Y.; Xiong, Y. A unique semiconductor–metal–graphene stack design to harness charge flow for photocatalysis. Adv. Mater. 2014, 26, 5689-5695. DOI 10.1002/adma.201401817.


35


Page 36 of 37

(66) Seh, Z. W.; Liu, S.; Low, M.; Zhang, S. Y.; Liu, Z.; Mlayah, A.; Han, M. Y.; Janus AuTiO2 photocatalysts with strong localization of plasmonic near-fields for efficient visible-light hydrogen generation. Adv. Mater. 2012, 24, 2310-2314. DOI 10.1002/adma.201104241.


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For Table of Contents Use Only

Graphic for Table of Contents

Hollow mesoporous CdS@TiO2@Au microspheres synthesized by “hard core template” combined with solvothermal reaction and γ-irradiation reduction exhibit excellent photocatalytic activity and photostability for hydrogen evolution under visible light.


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Fabrication of Hollow Mesoporous CdS@TiO2@Au Microspheres with

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