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Carbon-Incorporated NiO/TiO2 Mesoporous Shells with pn Heterojunctions for Efficient Visible Light Photocatalysis Minggui Wang, Jie Han, Yimin Hu, Rong Guo, and Yadong Yin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10480 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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ACS Applied Materials & Interfaces

Carbon-Incorporated NiO/TiO2 Mesoporous Shells with p-n Heterojunctions for Efficient Visible Light Photocatalysis Minggui Wang,a Jie Han,a,b* Yimin Hu,a Rong Guoa* and Yadong Yinb*

a

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu,

225002, P. R. China. E-mail: [email protected]; [email protected] b

Department of Chemistry, University of California, Riverside, CA 92521, United States. E-mail:

[email protected] KEYWORDS: TiO2; NiO; carbon; mesoporous shells; photocatalyst ABSTRACT: Carbon-incorporated mesoporous NiO/TiO2 (NiO/TiO2/C) hybrid shells as lowcost and highly efficient visible light photocatalysts have been developed. The NiO/TiO2/C hybrid shells were synthesized by choosing polystyrene nanospheres as templates, followed by TiO2 and NiO coating, and finally the calcination post-treatment to carbonize PS with the aid of metal oxides. Polystyrene nanospheres serve dual purposes as both a template to ensure the hollow structure and the electrically conductive graphite carbon source. Evaluation of their photocatalytic activity by organic pollutes (Rhodamine B, Methylene Blue and phenol) degradation and H2 production under visible light demonstrated the superior photocatalytic performance, thanks to the enhanced visible-light absorption and exciton separation associated with the incorporation of electrically conductive graphite carbon.

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1. INTRODUCTION As a promising metal oxide semiconducting material, titania (TiO2) has been extensively studied as a typical photocatalyst for environment treatment and chemical energy conversion1-7 due to its advantages, such as low toxicity, high stability, and of particular high photocatalytic activity.8-11 However, its photocatalytic application is still limited owing to the wide band gap and rapid recombination of photogenerated electron-hole pairs.12-14 Therefore, taking full advantage of solar energy and improving the exciton separation efficiency have become the most important goals in photocatalytic applications. As reported, non-metallic element doping15-19 (such as C, N, S, etc.) and noble metal nanoparticle loading/depositing20-26 (such as Au, Ag, Pt etc.) can largely improve the photocatalytic activity of TiO2 under visible light. Nevertheless, the non-metallic element doping processes are usually complex under harsh conditions, and noble metals are of high-price that restricts its practical applications. Consequently, developing lowcost and noble metal-free strategies towards powerful TiO2-based visible light photocatalysts are highly desirable. Creating p-n heterojunctions has been attested as one of the most intriguing strategies to develop low-cost and efficient visible light photocatalysts.27-34 On one hand, the altered band gaps of the hybrids may enhance visible light adsorption in the case of rational combination of TiO2 with other metal oxide semiconductors. On the other hand, the exciton separation efficiency can be improved. When an n-type TiO2 and p-type metal oxide semiconductor forms a p-n junction, an inner electric field is then established at the p-n junction interface, where negative charges accumulate at the p-type metal oxide region, and positive charges at the n-type TiO2 region. Under light irradiation, the photogenerated holes will transfer to the valence band of ptype metal oxide and the photogenerated electrons will transfer to the conduction band of TiO2.


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The establishment of the inner electric field will undoubtedly contribute to the exciton separation efficiency improvement.35, 36 Among those TiO2-based p-n heterojunctions, NiO/TiO2 hybrids have attracted more attention because NiO possesses unique properties of typically high hole mobility.37-39 Previous studies have indicated that NiO can be considered as one of the most effective co-catalysts to enhance the photocatalytic activity of TiO2,40-42 and NiO/TiO2 p-n heterojunctions with mesoporous hollow nanostructures have shown high photocatalytic activity due to their high specific surface area with more accessible active sites.43 Nevertheless, the inherent poor conductivity of metal oxide semiconductors hinders the efficient transfer of excitons, leaving room for further improvement of the photocatalytic activity of NiO/TiO2 p-n heterojunctions. Herein, we demonstrated that the incorporation of carbon into NiO/TiO2 mesoporous hollow shells (denoted as NiO/TiO2/C) could significantly improve the electrical conductivity and therefore the photocatalytic activity of the p-n heterojunctions. Specifically, polystyrene (PS) nanospheres were used as templates, coated sequentially with layers of TiO2 and NiO, and calcined to carbonize the core and enhance the crystallinity of the shells, finally producing NiO/TiO2/C ternary hybrid shells. In comparison with conventional synthesis of mesoporous TiO244-47 and NiO/TiO2 hybrid shells43 using SiO2 nanospheres as templates, PS nanospheres act as not only templates to create void interiors but more importantly carbon source to enhance the electrical conductivity,15 as PS can be transformed into electrically conductive carbon in the presence of metal oxides under inert atmosphere.29,48-51 Therefore, the formation of mesoporous hollow structures and incorporation of electrically conductive carbon can be accomplished in one step of calcination, producing NiO/TiO2/C hybrid shells with significantly enhanced photocatalytic performances, which have been established by photocatalytic tests


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organic pollutes (Rhodamine B, Methylene Blue and phenol) degradation and H2 production under visible.

2. EXPERIMENTAL METHODS 2.1. Materials: Tetrabutyl orthotitanate (TBOT, 97%) was obtained from Fluka. NiCl2·6H2O, styrene monomer, Rhodamine B (RhB), Methylene Blue (MB), phenol and all other reagents were obtained from Sinopharm Chemical Reagent Co. Ltd (China). 2.2. Synthesis of PS nanospheres At first, 35.0 mL styrene monomer was mixed with 150.0 mL deionized water containing 1.0 mL methacrylic acid and 1.0 mL ammonium peroxydisulfate aqueous solution (0.1575 g mL-1). After stirring for 3 h under 95 oC, the PS nanoparticles were separated by centrifugation, washed with ethanol three times, and then dried under vacuum at 80 oC for 6 h. 2.3. Synthesis of PS/TiO2 core/shell hybrids Firstly, 50.0 mg PS nanospheres were added in a solution containing 28.0 mL ethanol and 8.0 mL acetonitrile with vigorous stirring for 3 h to form a colloidal solution. After that, 0.2 mL ammonia (25-28%) was added, followed by addition of 0.4 mL TBOT. Then the solution was vigorously stirred for another 3 h. Finally, the PS/TiO2 core/shell particles were isolated by centrifugation and washed with deionized water three times, and then dried under vacuum at 80 o

C for 6 h. 2.4. Synthesis of NiO/TiO2/C hybrid shells Firstly, 100.0 mg PS/TiO2 core/shell particles were added in 25.0 mL NiCl2·6H2O aqueous

solution (0.015 mol L-1), followed by 30 min ultrasonication. Then the colloidal solution was transferred to a Teflon-lined stainless steel autoclave, which was then heated in an oven at 150


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o

C for 5 h. After cooling down to room temperature, the PS/TiO2/NiO core/shell particles were

collected by centrifugation, washed with deionized water three times, and then dried under vacuum at 80 oC for 6 h. Finally, the PS/TiO2/NiO core/shell particles were calcined under N2 protection at different temperature (500-900 oC) for 4 h to carbonize PS through pyrolysis. By controlling the calcination temperature at 500, 600, 700, 800 and 900 oC, the as-prepared NiO/TiO2/C hybrid shells were denoted as S1, S2, S3, S4 and S5, respectively. 2.5. Photocatalytic activity tests: (1) Photocatalytic degradation of organic pollutes. The photocatalytic performance of NiO/TiO2/C hybrid shells was firstly investigated in degradation of organic pollutes (RhB, MB and phenol). In a typical photocatalytic degradation experiment, 5.0 mg catalyst was firstly dispersed in 25.0 mL RhB (or MB) aqueous solution (2.0×10-5 mol L-1) or 25.0 mL phenol aqueous solution (10 mg L-1) in a quartz cell and then stirred in dark for 30 min to ensure equilibrium adsorption. The light source of the visible lamp (400 W metal halide) with a UVcutoff filter (400 nm) filter was used in a commercial photoreactor system (Xujiang XPA-7). The concentration of organic pollutes was measured with a UV-vis spectrophotometer (HR2000CGUV-NIR, Ocean Optics). (2) Photocatalytic H2 production. Firstly, 10.0 mg catalyst was dispersed in a solution containing 7.5 mL water and 2.5 mL methanol, followed by 20 min stirring. Then the colloidal solution was bubbled with nitrogen for 30 min to remove the dissolved O2, and then sealed with a parafilm. The light source of the visible lamp (350 W Xe light with the power density of 100 mW cm−2) with a UV-cutoff filter (400 nm) filter was used. The produced gas was withdrawn with a syringe and examined by a gas chromatography (GC-7900, China). 2.6. Characterization


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The morphology was obtained with a transmission electron microscopy (TEM, JEM-2100, Japan) and a high-resolution TEM (HRTEM, Tecnai G2 F30 S-Twin, FEI USA). X-ray diffraction (XRD) spectrum was recorded in the 2θ range from 10° to 80° in steps of 0.04° with a count time of 1 s each time using a Bruker AXS D8 ADVANCE X-ray diffractometer. UV/Vis diffuse reflectance spectrum was measured by UV/Vis spectrophotometer (Cary 5000, Varian, USA) using Barium sulfate as the reflectance standard. The N2 adsorption-desorption curves were conducted using a Beishide 3H-2000PS2 analysis instrument (3H-2000PS2, China). The specific surface area was determined from the adsorption isotherm by the multi-point Brunauer-EmmetTeller (BET) method, and the pore size distribution was determined from the desorption isotherm by the Barrett-Joyner-Halenda (BJH) method. The X-ray photoelectron spectroscope (XPS, ESCALAB 250Xi, Thermo Scientific, USA) was conducted with Al Kα source as radiation. Fluorescence spectrum was obtained by a fluorescence spectrophotometer (Hitachi F-7000, Japan) with the excitation wavelength at 315 nm. The photoluminescence (PL) spectrum was detected by time resolved fluorescence spectra analytical instruments (FLSP20, lhstrcamenss, Edinburgh). Thermogravimetric (TG) analysis was measured with a Perkin-Elmer instrument (Pyris 1 TGA, USA) at heating rate of 10 oC min-1 up to 900 oC in O2 or N2 atmosphere. Raman spectrum was studied using a Raman spectrographer (In Via, Renishaw, UK) with visible light excitation at 532 nm. The electrochemical impedance spectrum (EIS) was carried out on an electrochemical workstation (VMP3, Biologic, France) by using three-electrode cells with a platinum wire as the counter electrode and an Ag/AgCl (saturated KCl) electrode as the reference electrode. The measurements were carried out in the presence of 0.5 mol L-1 Na2SO4 solution.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of NiO/TiO2/C Hybrid Shells.


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Scheme 1 illustrates the synthesis route for NiO/TiO2/C hybrid shells. Monodispersed PS nanoparticles were chosen as templates, followed by TiO2 and NiO coating. The morphology evolution from PS to PS/TiO2/NiO were monitored as shown in Figure S1. Finally, the PS/TiO2/NiO core/shell hybrids were calcined under an inert atmosphere, realizing the successful formation of NiO/TiO2/C hybrid shells. Figure 1a shows the TEM image of NiO/TiO2/C ternary hybrid shells, indicating the welldefined hollow structures. The inner diameter of NiO/TiO2/C hybrid shells as shown in Figure 1b is similar to the size of PS template (~290 nm in diameter, Figure S1a) and the shell thickness is ~50 nm. Figure 1c gives the HRTEM image of NiO/TiO2/C hybrid shells, which clearly demonstrates atomically connected TiO2 and NiO nanocrystals which produce well defined p-n heterojunctions. More interestingly, there are numerous p-n heterojunctions in a single NiO/TiO2/C hybrid shell, which are believed to supply abundant catalytic sites for photocatalysis. The lattice spacing of 0.35 nm and 0.209 nm should be ascribed to the (101) plane of anatase TiO2 and the (200) plane of NiO, respectively. The high-angle annular darkfield scanning transmission electron microscope (HAADF-STEM) image in Figure 1d further prove the hollow interior. The energy dispersive X-ray spectroscopic (EDS) mapping of Ti, Ni and C elements within a single particle is shown in Figures 1e-g, indicating their homogeneous distribution in the shell and confirming the expected ternary hybrid shell structure. In combination with the HRTEM analysis, it is believed that NiO and TiO2 are dispersed at the crystalline grain sale, and carbon is believed to locate on surfaces of crystalline grains as carbonization centers. The morphology of NiO/TiO2/C hybrids calcined at different temperature in N2 atmosphere was examined for further understanding the formation mechanism. The intact hollow structures


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can be retained at the calcination temperature below 800 oC (Figure 2). However, it should be noted that the outer diameter of NiO/TiO2/C hybrid shells decreases from 390, 365, 345 to 325 nm with calcination temperature increasing from 500, 600, 700 to 800 oC. At calcination temperature of 900 oC, it is hard to maintain the intact hollow structure. Instead, solid aggregation is the dominant morphology (Figure S2). The TG analysis as shown in Figure 3 can give more information about the transformation from PS/TiO2/NiO core/shell hybrids into NiO/TiO2/C hybrid shells during calcination process. The following conclusions can be obtained: 1) PS can be completely degraded below 450 oC under either O2 or N2 atmospheres for pure PS nanospheres; 2) PS in PS/TiO2/NiO core/shell hybrids is also completely degraded below 450 oC under O2 atmosphere, however, it can be carbonized under N2 atmosphere, and the remaining carbon decreases with increasing calcination temperature. The shell containing metal oxides should act as activation agents that favor the carbonization of PS.52 This can also explain why carbon is incorporated into NiO/TiO2 hybrid shells (Figure 1g). During calcination process of PS/TiO2/NiO core/shell hybrids under N2 atmosphere, the NiO/TiO2 hybrid shells can be well crystallized and the carbonization of PS in the presence of NiO/TiO2 produces carbon-incorporated NiO/TiO2 hybrid shells. The NiO/TiO2 hybrid shells shrink as the inner templates are degraded/carbonized, leading to smaller inner diameter of hollow spheres at a higher calcination temperature. If the calcination temperature further increases, extreme grain overgrowth of NiO/TiO2 results in the collapse of the shells. The morphology of PS/TiO2/NiO core/shell hybrids calcined at 800 oC under O2 atmosphere has also been examined, where broken pieces of NiO/TiO2 hybrids are the dominant structure (Figure S1). At the same calcination temperature but under N2 atmosphere, intact shell structure can be maintained (Figure 2e), suggesting that the incorporated graphite carbon may bridge


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NiO/TiO2 crystalline grains and improve shell integrity. The crystal and phase structures of NiO/TiO2/C hybrid shells were then investigated by XRD measurements (Figure 4a). The intensity and sharpness of the peaks corresponding to anatase TiO2 and NiO were considerably strengthened with increasing calcination temperature, indicating improved crystallinity of both TiO2 and NiO. However, when the calcination temperature reached above 800 oC, new peaks at 2θ = 27.4 and 38.0°, corresponding to the (110) and (111) planes of rutile TiO2, respectively, could be found. The XPS spectrum of NiO/TiO2/C hybrid shells (Figure S4a) shows peaks of Ti, Ni and C, where the binding energy of C 1s XPS spectrum with a peak at 284.2 eV (Figure S4b) is attributed to graphite carbon. Figure 4b shows the high resolution Ti 2p spectra of TiO2 hollow spheres, NiO/TiO2 and NiO/TiO2/C hybrid shells. In each sample, the splitting between the two Ti 2p doublets is 5.7 eV, implying a normal state of Ti4+.25,53,54 In contrast to TiO2 hollow spheres, the Ti 2p binding energy of NiO/TiO2 and NiO/TiO2/C hybrid shells display a higher value, indicating that TiO2 has interaction with not only NiO but also carbon. Figure 5b shows the high resolution Ni 2p peak from the NiO/TiO2 and NiO/TiO2/C hybrid shells, suggesting no obvious change of the NiO peaks after introduction of carbon. Result suggests that the incorporated carbon show more affinity towards TiO2 in the NiO/TiO2/C hybrid shells rather than NiO. The graphite carbon in NiO/TiO2/C hybrid shells was further confirmed by Raman spectroscopy. The typical peaks at about 1360 cm-1 (D band) and 1580 cm-1 (G band) for graphite carbon can be clearly found (Figure 4d). The D/G intensity ratios are increased with calcination temperature, indicating high degree of carbonization. The carbon content of NiO/TiO2/C hybrid shells as measured by TG analysis together with D/G intensity ratio (degree


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of carbonization) are presented in Table 1. Figure 4e shows the N2 adsorption-desorption isotherms of NiO/TiO2/C hybrid shells. The pore size distribution profiles are shown as inset in Figure 4e, which indicate the mesoporous nature of NiO/TiO2/C hybrid shells.55 Table 2 summaries the specific surface area, pore size and pore volume of NiO/TiO2/C hybrid shells, where the corresponding data for TiO2 hollow spheres and NiO/TiO2 hybrid shells are also given for comparison. It can be concluded that sample S4 shows the highest specific surface area, pore size and pore volume. After introduction of graphite carbon in NiO/TiO2 hybrid shells, the powder changed from dark yellow to black in appearance. Figure S5 shows the UV/Vis diffuse reflectance spectra of TiO2, NiO/TiO2 and NiO/TiO2/C, where the absorption of TiO2 shifted from UV to the visible region after combined with NiO, and the introduction of graphite carbon further enhanced the visible absorption. As shown in Figure 4f, the visible light absorption for NiO/TiO2/C hybrid shells increases from S1 to S4, but drops down for S5, which can be attributed to the breakdown of mesoporous hollow structures.43 It is believed that the broad absorption in visible light region enhances the effective utilization of solar energy, which is beneficial for improved photocatalytic activity.42, 56 The PL spectra of TiO2 hollow spheres, NiO/TiO2 and NiO/TiO2/C hybrid shells excited at 360 nm (Figure 5a) were used to characterize the recombination rate of photogenerated electronhole pairs.26, 57-60 TiO2 hollow spheres and NiO/TiO2/C hybrid shells show the highest and lowest emission peak intensity, respectively, indicating that NiO/TiO2/C hybrid shells have the lowest recombination rate of electrons and holes. Results confirm efficient inhibition of exciton recombination due to the presence of graphite carbon. As for NiO/TiO2/C hybrid shells, the photogenerated electrons and holes are effectively separated due to well-defined NiO/TiO2 p-n


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junctions, and the photogenerated electrons in the conduction band of TiO2 will be transferred by strongly interacted graphite carbon (as confirmed by XPS), which greatly enhances the separation efficiency. As shown in Figure 5b, the PL intensity decreases in the order of S5 < S1 < S2 < S3 < S4. To better understand the exciton behavior, time-resolved fluorescence decay spectra of TiO2, NiO/TiO2 and NiO/TiO2/C were also given. In contrast to TiO2 and NiO/TiO2, NiO/TiO2/C shows the slowest decay kinetics (Figure 5c). Fitting the decay spectra gives average lifetime values of 0.58 ns for TiO2 hollow spheres, 3.91 ns for NiO/TiO2 hybrid shells, and 11.2 ns for NiO/TiO2/C hybrid shells. The life times of NiO/TiO2/C hybrid shells for S1 to S5 (Figure 5d) were also studied, where S4 showed the slowest decay kinetics. Fitting the decay spectra gave average lifetime values of 7.31, 7.55, 8.96, 11.2 and 5.13 ns for S1, S2, S3, S4, and S5, respectively. More efficient exciton separation is favorable for improvement in photocatalytic activity. 3.2. Photocatalytic Performances of NiO/TiO2/C Hybrid Shells With remarkable visible light absorption, NiO/TiO2/C hybrid shells are expected to display improved photocatalytic activity under visible light illumination.61-63 Two different organic dyes of RhB (a negatively charged dye) and MB (a positively charged dye) as degradation targets were chosen to investigate the photocatalytic activity. In a blank test, RhB and MB were not degraded over 75 min. As shown in Figure 6a and Figure S6a, NiO/TiO2/C hybrid shells show the best photocatalytic activity for the degradation of RhB with 6% remained within 75 min. The apparent reaction rate (k) using NiO/TiO2/C hybrid shells is calculated to be 0.035 min-1, whereas that using TiO2 hollow spheres and NiO/TiO2 hybrid shells is 0.0061 min-1 and 0.018 min-1, respectively. The results confirm that the incorporation of graphite carbon into NiO/TiO2 hybrid shells can greatly enhance their photocatalytic activities. Figure 6b and Figure S6b show


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the RhB degradation profiles using NiO/TiO2/C hybrid shells (S1-S5), where the photocatalytic activity continuously increases from S1 to S4 and then decreases for S5. The photocatalytic activity of NiO/TiO2/C hybrid shells follows the following order: S4 > S3 > S2 > S1 > S5. The apparent quantum efficiency for NiO/TiO2/C hybrid shells S4 photocatalyst is calculated to be 1.5%. As for degradation of MB, NiO/TiO2/C hybrid shells also exhibit the highest photocatalytic activity as compared with TiO2 hollow spheres and NiO/TiO2 hybrid shells (Figure 6d and Figure S6c), where 98% of MB was degraded within 100 min with 1.3% apparent quantum efficiency. The photocatalytic activity of NiO/TiO2/C hybrid shells toward MB degradation follows the same order as degradation on RhB (Figure 6e and Figure S6d). In addition, recycled tests of NiO/TiO2/C hybrid shells toward RhB and MB degradation (Figures 6c and 6f) suggest high stability of catalysts. In order to further confirm the mineralization of RhB, TOC values of the reaction solution during the photocatalytic process were recorded (Figure S7). The TOC values decrease after visible light irradiation, indicating the degradation of RhB into CO2. Besides, the mineralization process is obviously slower than the decolorization process. In comparison with reported TiO2/NiO-based photocatalyst,32,

40, 42, 43

NiO/TiO2/C

hybrid shells show remarkably enhanced catalytic performance in degradation of organic pollutes. It is recognized that the dye decolorization as an activity test method is not perfect. It has been reported that organic dye itself may absorb visible light and therefore the degradation can be caused by the excited dye rather than the excited photocatalyst.64 In order to eliminate this doubt, other organic pollute, such as phenol, which cannot absorb visible light, was then chosen. Figure 6g and Figure S6e present the comparison of the activity in photodegradation of phenol using TiO2 hollow spheres, NiO/TiO2 or NiO/TiO2/C hybrid shells. It is undoubted that the the


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photocatalytic activity of NiO/TiO2 p-n junction photocatalyst can be improved after introduction of graphite carbon. As shown in Figure 6h and Figure S6f, NiO/TiO2/C hybrid shells of S4 show the best photocatalytic activity with 0.56% apparent quantum efficiency. The photocatalytic activities are in the same order as well as mentioned in the cases of RhB and MB. In addition, the cycled photocatalytic tests towards phenol degradation also evidence high stability of NiO/TiO2/C hybrid shells (Figure 6i). The photocatalytic H2 production using methanol as a scavenger was also investigated. Control experiments showed no H2 production without visible light irradiation or without photocatalysts. Figure 7a gives the photocatalytic activity of H2 production using TiO2, NiO/TiO2 and NiO/TiO2/C. The highest H2 production activity, as observed for NiO/TiO2/C hybrid shells, is 356 µmol h-1 g-1, together with the highest apparent quantum efficiency of 1.1%. Figure 7b compares the photocatalytic activity of NiO/TiO2/C hybrid shells (S1-S5) in H2 production, which follows the same order as degradation on organic dyes. In addition, the photocatalytic activity shows no obvious decay (Figure 7c), and the original morphology can be remained after six cycles (Figure S8), suggesting high stability of NiO/TiO2/C hybrid shells. The photocatalytic results of organic pollutes degradation and H2 production all indicate that NiO/TiO2/C hybrid shells show the best photocatalytic activity, which clearly evidences the vital function of graphite carbon in NiO/TiO2 p-n heterojunctions. As known, the photocatalytic activity of TiO2 mainly lies on the utilization of incident light and exciton separation efficiency.21,40,65,66 The incorporation of graphite carbon in NiO/TiO2 hybrid shells contributes to not only enhanced visible light absorption as confirmed by UV/Vis diffuse reflectance spectra (Figure 4f), but also improved exciton separation efficiency as verified by PL (Figure 5a) and time-resolved fluorescence decay spectra (Figure 5c). The EIS of TiO2/NiO and NiO/TiO2/C


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hybrid shells that presented as Nyquist plots in Figure 8a attests that the incorporation of graphite carbon can significantly improve the electrical conductivity of the TiO2/NiO hybrid shells and thus make fast interfacial electron transfer. As the graphite carbon shows more affinity towards TiO2 rather than NiO as indicated by XPS analysis (Figure 4b), it can function as electron collector and transporter which favors the charge transfer and suppresses the exciton recombination. In addition, for NiO/TiO2/C hybrid shells of S1 to S5, S4 shows the smallest semicircle (Figure 8b). Combining with XRD, Raman and BET results, the photocatalytic activity of NiO/TiO2/C hybrid shells is mainly determined by hybrid nanostructure and carbonization degree of incorporated graphite carbon. Intact mesoporous shells with high specific surface area and large pore volume can guarantee enhanced harvesting of incident light, provide more active sites for fast transportation of reactant molecules and products,67,

68

meanwhile, high carbonization degree of graphite carbon can ensure improved exciton separation efficiency. The photocatalytic mechanism of NiO/TiO2/C hybrid shells was then given in Scheme 2. As soon as NiO loads on surface of TiO2, abundant p-n heterojunctions are formed at the interface. Once excited by visible light for NiO/TiO2/C hybrid shells, electron-hole pairs are created. Thanks to the internal electric field formed by p-n heterojunctions, the holes will transfer to the negative side and the electrons will transfer to the positive side. As a result, the electron-hole pairs are separated induced by the formation of p-n heterojunctions. As the graphite carbon contacts closely with TiO2 and shows greatly improved electronic conductivity than TiO2, the electrons in the conduction band of TiO2 can further transfer to graphite carbon. Hence the incorporation of graphite carbon in NiO/TiO2 hybrid shells can effectively suppress electron-hole recombination. During the photocatalytic reactions, the conduction band electrons on the hybrid


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shells not only can be trapped by O2, producing oxygen free radicals (·O2-), but also can reduce H+ ions to produce H2. Meanwhile, the valence band holes react with H2O or OH- to produce ·OH. The fluorescence method was applied to detect the produced hydroxyl radicals.69,70 Commonly, the fluorescence intensity is in related to the amount of produced hydroxyl radicals. Figure S9a gives the change of fluorescence intensity with visible light irradiation time in the presence of NiO/TiO2/C hybrid shells. The peak intensity at about 425 nm increases with irradiation time, indicating that more hydroxyl radicals are produced on the surface of photocatalysts. Figure S9b compares the fluorescence intensity of NiO/TiO2/C hybrid shells (S1S5), where S4 produces the most hydroxyl radicals. Results agree well with the photocatalytic tests where S4 shows the highest photocatalytic activity than others.

4. CONCLUSION In summary, mesoporous NiO/TiO2/C hybrid shells were synthesized by choosing polystyrene nanospheres as templates, followed by TiO2 and NiO coating, and finally the calcination posttreatment to carbonize PS with the aid of metal oxides. PS nanospheres show dual functionalities as template to shell structure and graphite carbon source. The formation of NiO/TiO2 p-n heterojunctions with graphite carbon contacting close with TiO2 grains have been disclosed. Photocatalytic results in organic pollutes (RhB, MB and phenol) degradation and H2 production reveal that NiO/TiO2/C hybrid shells as calcined at 800 oC show the best visible light photocatalytic activities. The method developed here may be instructive for construction other graphite carbon-incorporated heterojunctions for broader applications.

ASSOCIATED CONTENT


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Supporting Information. Additional figures about material and photocatalytic characterizations. This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Han); [email protected] (R. Guo); [email protected] (Y. Yin) ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21673202 and 21273004), Innovation Program for Graduate Students in Universities of Jiangsu Province (No. KYZZ15_0361), Excellent Doctoral Dissertation of Yangzhou University, Qing Lan Project and the Priority Academic Program Development of Jiangsu Higher Education Institutions. We would also like to acknowledge the technical support received at the Testing Center of Yangzhou University. Yin also thanks the financial support by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, & Biosciences (CSGB) Division (Award No. DE-SC0002247).

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FIGURE CAPTIONS Scheme 1 Schematic representation of the synthesis processes of NiO/TiO2/C hybrid shells. Scheme 2 Photocatalytic mechanism of NiO/TiO2/C hybrid shells under light irradiation. Figure 1 (a, b) TEM and (c) HRTEM images of NiO/TiO2/C hybrid shells (S1) calcined at 500 o

C in N2 atmosphere. (d) HAADF-STEM image of NiO/TiO2/C hybrid shells and (e, f) EDS

maps of (e) Ti, (f) Ni, and (g) C from a single NiO/TiO2/C particle given in Figure 1d. Figure 2 TEM images of NiO/TiO2/C hybrid shells: (a, b) S2; (c, d) S3; (e, f) S4. Figure 3 TG profiles of PS nanospheres and PS/TiO2/NiO core/shell hybrids calcined under N2 and O2 atmospheres. Figure 4 (a) XRD patterns of NiO/TiO2/C hybrid shells. XPS spectra of (b) Ti 2p and (c) Ni 2p of TiO2 hollow nanospheres, NiO/TiO2 and NiO/TiO2/C hybrid shells. (d) Raman spectra and (e) N2 adsorption-desorption isotherms of NiO/TiO2/C hybrid shells. Inset in Figure 4e shows the pore size distribution. (f) UV/Vis diffuse reflectance spectra of NiO/TiO2/C hybrid shells. Figure 5 PL spectra of (a) TiO2 hollow spheres, NiO/TiO2 and NiO/TiO2/C hybrid shells, and (b) NiO/TiO2/C hybrid shells (S1-S5). (c) Time-resolved fluorescence decay spectra of TiO2 hollow spheres, NiO/TiO2 and NiO/TiO2/C hybrid shells, and (d) NiO/TiO2/C hybrid shells (S1-S5) excited at 360 nm. Figure 6 Apparent reaction rates of photocatalytic degradation of (a) RhB, (d) MB and (g) phenol under visible light using TiO2 hollow spheres, NiO/TiO2 and NiO/TiO2/C hybrid shells (S4) as photocatalysts. Apparent reaction rates of photocatalytic degradation of (b) RhB (e) MB and (h) phenol under visible light using NiO/TiO2/C hybrid shells (S1-S5) as photocatalysts. Recyclability of NiO/TiO2/C hybrid shells (S4) as photocatalysts in degradation of (c) RhB, (f) MB and (i) phenol under visible light irradiation.


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Figure 7 Comparison of the photocatalytic H2 production using (a) TiO2 hollow spheres, NiO/TiO2 and NiO/TiO2/C hybrid shells and (b) NiO/TiO2/C hybrid shells (S1-S5) as photocatalysts under visible light irradiation. (c) The recyclability of NiO/TiO2/C hybrid shells (S4) as photocatalysts in H2 production under visible light irradiation. Figure 8 Nyquist plots of (a) NiO/TiO2 and NiO/TiO2/C hybrid shells, and (b) NiO/TiO2/C hybrid shells (S1-S5).


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Scheme 1

Scheme 2


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Figure 1


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Figure 2


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100

Weight loss / %

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


PS/TiO2/NiO

in N2 atmosphere

80 60

PS/TiO2/NiO

in O2 atmosphere

40 PS in N2 atmosphere

20 0 PS in O2 atmosphere 100

200

300

400

500

600

700 0

800

900

Temperature / C Figure 3


31


(b)

A(101) R(110) A(004)

S5

Intensity / a. u.

Intensity / a. u.

(a)

A(200) A(105) A(211)

NiO(200)

S4 S3 S2

459.1 eV

458.1 eV 458.5 eV

TiO2

NiO/TiO2/C

20

30

40

50

60

70

80

456

460

2 Theta / degree

Intensity / a. u.

NiO 2p3/2

NiO/TiO2 NiO

NiO

2p3/2 sat

2p1/2

NiO 2p1/2sat

NiO/TiO2/C

850 855 860 865 870 875 880 885

(e)

D

800

1.6

0.03

Absorbance / a. u.

200

dV/dogD / cm3/nm/g

3

300

0.04

0.02 0.01 0.00 0

100 0 0.0

5

10

15

20

25

30

Pore Diameter D / nm

0.2

0.4

0.6

S1 S2 S3 S4 S5

G

1200

1600

Wavenumber / cm

S1 S2 S3 S4 S5

500 400

468

(d)

Binding energy / eV

600

464

Binding energy / eV

Raman signal / a. u.

(c)

-1

NiO/TiO2

S1

10

Volume Adsorbed / cm g STP

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

Page 32 of 38

0.8

2000

(f)

1.5 1.4 1.3 1.2 1.1 200

1.0

-1

S1 S2 S3 S4 S5

300

400

500

600

700

800

Wavelength / nm

Relative Pressure P/P0 Figure 4


32

Page 33 of 38

(a)

(b)

NiO/TiO2 NiO/TiO2/C

20k

10k

0 380

390

400

410

380

Wavelength / nm

(c)

NiO/TiO2/C

8

12

400

410

(d)

TiO2

4

390

Wavelength / nm

NiO/TiO2

0

S1 S2 S3 S4 S5

Intensity / a. u.

TiO2

S1 S2 S3 S4 S5

Intensity / a. u.

Intensity / a. u.

30k

Intensity / a. u.

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


16

0

4

8

12

16

t / ns

t / ns Figure 5


33

(a)

R

3.0

2

0.00614 0.9849 0.01856 0.9897 0.03523 0.9879

1.5

NiO/TiO2

NiO/TiO2/C

1.0

TiO2

2.0 1.5

0.5

0.0

0.0 15

30

45

60

R

0.9832 0.9768 0.9784 0.9879 0.9628

1.0

0.5 0

2

k 0.01511 0.02083 0.02682 0.03523 0.01122

75

0

15

30

3.5

-ln(ct/c0)

2

k

R

0.00539 0.9868 0.01584 0.9887 0.03045 0.9798

3.0 2.5 2.0

1.5 NiO/TiO2/C

3.0

NiO/TiO2

1.5

0.5 0.0 60

80

k 0.01347 0.01873 0.02442 0.03045 0.00821

2.0

0.0 40

(e)

2.5

0.5 20

100

0

20

(g)

NiO/TiO2

1.0 NiO/TiO /C 2

TiO2

-ln(ct/c0)

1.5

2.0

R

0.00346 0.9869 0.00715 0.9927 0.01233 0.9858

80

(h)

R2

k 0.00594 0.00697 0.00884 0.01233 0.00304

1.5 1.0

0.9888 0.9925 0.9902 0.9858 0.9879

0.0

0.0 30

60

90

t / min

40 20 0 1

100

120

150

0

30

60

90

2

3

4

5

6

120

150

t / min

(f)

80 60 40 20 0

100

0.5

0.5

0

60

1

t / min

2

k

60

(c)

80

Cycle

R2 0.9804 0.9774 0.9868 0.9798 0.9769

40

t / min

2.0

100

75

1.0

TiO2

1.0

0

60

3.5

(d)

-ln(ct/c0)

4.0

45

Page 34 of 38

t / min

t / min

Degradation ratio / %

2.0

(b)

2.5


2.5

k

-ln(ct/c0)

-ln(ct/c0)

3.0

-ln(ct/co)

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



100

2

3

4

5

6

3

4

5

6

Cycle

(i)

80 60 40 20 0 1

2

Cycle

Figure 6


34

-1 -1 g )

(a)

300

(b)

300

200 100

400

H2 production (µmol h

400

H2 production (µ mol h

200

Trace

100

0

0 NiO/TiO2 NiO/TiO2/C

-1 -1 g )

TiO2

400

S1

S2

S3

S4

S5

(c)

300

H2 production (µmol h

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


-1 -1 g )

Page 35 of 38

200 100 0 0

4

8

12

16

20

24

t/h Figure 7


35


(b)

(a) -Z'' / ohm

120

-Z'' / ohm

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

Page 36 of 38

80

NiO/TiO2

40

S1 S2 S3

4

S4 S5

NiO/TiO2/C

0

0 0

40

80

120

160

200

0

Z' / ohm

1

2

3

Z' / ohm Figure 8


36

Page 37 of 38

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


Table 1 Carbon content and carbonization degree of NiO/TiO2/C hybrid shells. Sample

S1

S2

S3

S4

S5

Carbon content ID/IG

10.3 0.57

9.17 0.78

7.73 0.91

5.40 0.94

2.11 0.96

Table 2 Specific surface area, pore size and pore volume of TiO2 hollow spheres, NiO/TiO2 and NiO/TiO2/C hybrid shells.

S1 S2 S3 S4 S5

Surface area (m2 g-1) 111.7 130.5 138.2 165.8 99.7

Pore size (nm) 3.84 3.34 3.30 4.59 3.33

Pore volume (cm3 g-1) 0.4660 0.6037 0.6040 0.8534 0.5006

TiO2

89.1

4.16

0.3283

NiO/TiO2

119.6

3.69

0.5864

Catalysts


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

Page 38 of 38

TOC Graphical


38

TiO2 Mesoporous Shells ... - ACS Publications

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