Hollow Sphere TiO2âZrO2 Prepared by Self-Assembly with

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Hollow spheres TiO2-ZrO2 prepared by self-assembly polystyrene colloidal crystal template for both photocatalytic degradation and H2 evolution from water splitting Jianqi Zhang, Li Li, Zhixin Xiao, Di Liu, Shuang Wang, Jingjing Zhang, Yuting Hao, and Wenzhi Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01359 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 24, 2016

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ACS Sustainable Chemistry & Engineering

Hollow spheres TiO2−ZrO2 prepared by self-assembly polystyrene colloidal template for both photocatalytic degradation and H2 evolution from water splitting Jianqi Zhang a,b, Li Li a,b,c,*, Zhixin Xiao b, Di Liu a, Shuang Wang a, Jingjing Zhang b, Yuting Hao b, Wenzhi Zhang b a. College of Materials Science and Engineering, Qiqihar University, Qiqihar 161006, PR China b. College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar 161006, PR China c. College of Heilongjang Province Key Laboratory of Fine Chemicals, Qiqihar University, Qiqihar 161006, PR China

ABSTRACT: Based on self-assembly technique through polystyrene (PS) spheres as colloidal template, the composite TiO2−ZrO2 hollow spheres are synthesized by combining water bath with calcining post-processing method. XRD, UV–vis/DRS, XPS, SEM-EDS, TEM, HR-TEM and N2 adsorption–desorption measurements are employed to characterize the composition, structure and morphology of TiO2−ZrO2 hollow spheres. The results exhibit that TiO2−ZrO2 hollow spheres are mainly TiO2 anatase and well retain the spherical structure of the PS crystal template, whose shell is closely packed by TiO2−ZrO2 nanoparticles, with the thickness is ca. 24 nm. The combining of TiO2 and ZrO2 and the special hollow structure are beneficial to improve the photocatalytic activity. TiO2−ZrO2 hollow spheres have remarkable photocatalytic properties under UV light, simulated sunlight, and microwave-assisted three different modes, which can also degrade organic pollutants of different structures. In addition, the H2 evolution quantity in 8 h, which is produced by the photolysis of water, is 23.7 µmol, indicating that TiO2−ZrO2 hollow spheres have a certain hydrogen production performance. Moreover, the results of trapping experiment indicate that the active radicals ·O2-, h+ and ·OH- are responsible for the photocatalytic reaction, and the possible reaction mechanism of TiO2−ZrO2 hollow spheres in both photocatalytic degradation and photocatalytic H2 evolution from splitting water is also proposed. Keywords: Polystyrene, TiO2−ZrO2, hollow spheres, multi-modes photocatalysis, H2 evolution. In recent years, the researches about exploring new nano-structure have gradually become the hotspot in the fields of material, physics and chemistry etc. At *

Corresponding author. E-mail address: [email protected](L.Li); [email protected] 1

ACS Paragon Plus Environment


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present, the morphology of nanocomposite materials has been extended from one dimension to two and three dimensional spaces, and the nanocomposites with controllable morphology are endowed with various new properties [1-4]. As a kind of structure with special morphology, the most obvious feature of hollow spheres is that they have large internal space and the shell thickness in nano-scale. Compared with full particles, the special structure of the hollow spheres is often expressed different chemical physical properties, such as light, electricity and magnetism, etc. As it can be used as the material carrier to object, the hollow spheres can effectively overcome the problem of electron−hole separating, and control the retention time of the photocatalyst about the light [5]. At present, due to the unique physical and chemical properties, the application of nano-hollow spheres taking on potential application value is constantly expanding to catalysis, chromatography, photonic crystals, nanoelectronic devices, waste removal, biomedical and other fields [6, 7]. So far, the synthesis methods of nano-hollow spheres can be divided into chemical and physicochemical routes, including heterophase polymerization combined with sol-gel progress, emulsion/interfacial polymerization strategies, ultrasonic chemical method, hydrothermal method, nozzle reactor approach, self-assembly technique, surface living polymerization process, etc [8]. Among them, based on the template method, the hollow spheres can be prepared with uniform size and larger surface area, which is the one of the most classical synthetic mean [9]. The synthesis process as described below, the template particles are coated in solution either by controlling surface precipitation of inorganic molecule precursors or by direct surface reactions utilizing specific functional groups on the cores to create core-shell composite material. Then the template particles are subsequently removed from the hollow structure by calcination at high temperature or by selective dissolution in an appropriate solvent to generate ceramic hollow spheres [7, 10]. Ippei Y. et al prepared single layer polystyrene (PS)/CeO2 core/shell nanostructures by using electrophoretic deposition and potentiostatic electrodeposition method on glass substrates, taking polystyrene as template. In the synthetic process, the obtained morphology of the core/shell structures was changed drastically depending on the different concentrations of precursor solution [11]. Cai et al proposed a novel approach for the fabrication of double-shelled and sandwiched nano-structure TiO2@Au@C hollow spheres by hydrothermal reaction and calcination, and the obtained composite showed excellent activity for the photocatalytic degradation of 4-nitraniline [12]. Zhang et al reported on the light-induced photochemical synthesis 2


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of Cu2O microncubes utilizing CuWO4 as the precursor, and found that under light irradiation, in the presence of glucose CuWO4 can be reduced in situ into Cu2O with its morphology reassembled from irregular bulk particles to hollow microcubes. And the photocatalytic performance for hydrogen production of the hollow structure was remarkably enhanced when 0.1wt % Zn was doped [13]. Recently, photocatalytic technology has become mature gradually, and turned into the effective means to govern the environmental pollution and to solve the energy problem [14, 15]. As a kind of advanced technology with wide application prospect, photocatalytic technology can completely degrade the organic pollutants in the environment and mineralize them into H2O, CO2 and inorganic acid at low temperature; meanwhile, it can utilize solar energy to produce renewable materials such as hydrogen just taking water or biomass as raw materials [16]. Hydrogen evolution from water splitting is a thermodynamic reaction, which makes use of the reduction of the photo-generated electron is one of the most challenging reactions in the chemical field because of the need of multiple electron transfer in the reaction process. The reaction involves three major steps: (i) absorption of light by semiconductor to generate electron-hole pairs, (ii) charge separation and migration to the surface of semiconductor, and (iii) surface reactions for water reduction or oxidation [17, 18]. If the three conditions cannot be met at the same time, then the efficiency of hydrogen production will be very low. In general, the efficiency of photocatalytic decomposition of water can be effectively improved by modifying the catalyst such as supported catalyst, transition metal ion or nonmetal ion doping, and the addition of sacrificial agent to the reaction system [19]. At present, there are various semiconductor photocatalysts used in photocatalytic degradation of organic pollutants such as TiO2，ZnO, CdS and Fe2O3, and the typical catalysts for hydrogen production are TiO2, ZrO2, IrO2, and γ-Bi2O3 etc[20]. Among them, TiO2 is more attractive in the environment treatment because of its various advantages, such as high chemical stability, against photocorrosion, high oxidation reduction potential, powerful photocatalytic reaction driving force, favorable photocatalytic activity, and TiO2 can realize and accelerate the chemical reactions on its surface under the light illuminated, therefore, the study on photocatalytic activity of TiO2 is the most active task [21]. However, TiO2 also has shortcomings including the wide band gap, the small photoluminescence efficiency and the absorption of light only at UV wavelengths etc, which are hindering its further applications. Due to the existence of these defects, the practical application of TiO2 photocatalytic technology has been 3



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restricted to some extent [22, 23]. And the other kind of photocatalytic material ZrO2 is an excellent amphoteric catalyst, of which the surface has two properties of oxidation and reduction, and owns both acid and base sites. But the shortcomings of wider band gap and smaller surface area also limit its application to a certain extent. Therefore, there are some researchers using the synergy to compensate the defects of wide band gap of monomer based on the combination between TiO2 and ZrO2, this also can make the catalyst have the similar amphoteric feature of ZrO2, so as to further expand their application range [24, 25]. According to this, the main ideas of this paper are: (1) By combining TiO2 and ZrO2, utilize the synergistic effect of the two materials to remedy the defects of the wide monomer band gap, to improve the quantum efficiency and to enhance the photocatalytic properties; (2) By preparing TiO2−ZrO2 composite owning hollow structure, improve the light trapping ability and enhance the absorption or utilization of light; (3) By carrying out degradation of organic pollutants under the multi-modes photocatalytic conditions and photocatalytic H2 evolution from water splitting experiment, further investigate the photocatalytic activities and the practical application of TiO2−ZrO2 hollow spheres, so as to broaden its application range.

Experimental section Materials Zirconium n-butoxide (C16H36O4Zr) was purchased from Meryer (Shanghai) Chemical Technology Co. Ltd. The following reagents and samples were all of analytical grade and were used without further purification: Tetra-n-butyl Titanate (C16H36O4Ti), Styrene, Sodium sulfide(Na2S) and Potassium persulfate (K2S2O8) were purchased from Tianjin KaiTong chemical reagent co., LTD. Methyl orange (MO), Congo red (CR), crystal violet (CV), Methylene blue (MB), Malachite green oxalate (MG), Salicylic acid (SA), Phenol (PH), p-benzoquinone (BQ), tert-butyl alcohol (TBA) and triethanolamine (TEA) were purchased from Beijing Chemical Inc., China. Deionized and doubly distilled water was used in all the experiments. Synthesis of TiO2 hollow spheres PS spheres were synthesized by an emulsifier-free emulsion polymerization method [26]. Tetrabutyl titanate was dissolved in 10 mL anhydrous ethanol, in which 1 g PS colloidal template dispersed in the mixture of water and ethanol was added, then, the mixture was stirred to completely homogeneous mixing. After the whole progress was kept at 80℃for 4 h during the water bath, the obtained PS@TiO2 4


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nanocomposite was washed 3 times with ethanol, dried in vacuum at 60℃ for 12 h, and calcined at 500℃ for 7 h to remove the PS core. The TiO2 hollow spheres were obtained finally, marked as TiO2-S. In addition, under the same experimental conditions, the sample without the function of PS colloidal template was labeled as TiO2 and ZrO2. Synthesis of TiO2−ZrO2 hollow spheres The TiO2−ZrO2 hollow spheres were synthesized by the same method as TiO2-S, while the precursor was tetrabutyl titanate and zirconium n-butoxide with a volume ratio of 4:1. The obtained TiO2−ZrO2 hollow spheres were marked as TiO2−ZrO2-S. Moreover, the sample without the function of PS colloidal template was labeled as TiO2−ZrO2-P. Characterization The X-ray diffraction (XRD) patterns were obtained on German Bruker-AXS (D8) X-ray diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was performed using ESCALAB 250Xi spectrometer equipped with Al Kα radiation.

Scanning

spectrometer

electron

(SEM-EDS)

microscopy assisted

and

X-ray

energy

transmission electron microscopy

dispersive

(TEM)

was

performed on a Hitachi S-4700 and H-7650 microscope. The images of HR-TEM were obtained by JEOL JEM-2100F High Resolution Transmission Electron Microscopy. Brunauer-Emmet-Teller (BET) specific surface area, pore sizes and pore volumes were measured on 3H-2000PS2 model surface analyzer produced from Beishide Instrumentation Technologies Ltd., via determination of nitrogen adsorption isotherm at 77 K. The UV–vis/DRS of the composites and the absorbance of the sample solution were tested by TU-1901 UV visible spectrophotometer purchased from Beijing Puxi. The LabSolar-IIIAG photocatalytic online analysis system from Beijing Bofeilai Company combines GC7900 chromatograph from Shanghai Tianmei Company were used to carry out the experiments of photocatalytic H2 evolution from water slpitting. Multi-mode photoactivity test The

photocatalytic

microwave-assisted,

experiment

simulated

solar

includes: irradiation

ultraviolet photocatalytic

light

(UV),

reaction,

the

experimental conditions as follows: A 125 W high pressure mercury lamp was used (λ= 313.2 nm) as a UV source in the photocatalytic experiment. The microwave-assisted irradiation was performed 5



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using a microwave electrodeless lamp as a light source (UV emission wavelength mainly located at 280 nm), having a power of 15 W and a microwave reactor output power of 600 W. A photochemical reaction instrument equipped with a 1000 W xenon lamp (Shanghai bilon Instruments Co., Ltd) was used for simulated solar irradiation displaying strong consecutive spectra from UV to the near IR region. The concentration of MO which was used as a model dye and other organic pollutants used in the experiment were all 50 mg L-1. Moreover, the amounts of the catalyst were 0.09, 0.50 and 0.15 g, respectively, the corresponding reaction liquid volumes for the three modes (UV, microwave-assisted irradiation and simulated solar light) were 90, 500 and 90 mL. The UV, microwave-assisted and simulated solar irradiation photocatalytic reaction were performed as describes below: a certain amount of photocatalyst was dispersed into MO solution by ultrasound for 10 min. Before photoreaction, the suspensions were magnetically stirred for 30 min in dark to ensure the adsorption–desorption equilibrium between the MO solution and the photocatalyst. The catalyst was removed by centrifugation after the completion of the reaction. After the light source was excited, at a definite time interval, the concentration of MO in the photocatalytic reaction was analyzed using a UV–vis spectrophotometer at 463 nm. The photocatalysis procedure of the other organic pollution was in same condition with MO, and the concentration was also analyzed by UV–vis spectrophotometer at their λmax. The recycled catalysis was washed by deionized water and ethanol after irradiation, dried and set aside. Photocatalytic H2 evolution from water splitting The photocatalytic splitting of water to generate hydrogen was carried out in a vacuum reactor. 0.1 g photocatalyst was dispersed in 50 mL deionized water, and Na2S was used as the sacrificial agent, and the reaction was pursued under the stirring condition after the system was vacuumized. The photocatalyst was irradiated with UV and visible light from 300 W Xe lamp (external type, model PLS-SXE300), with high purity nitrogen as the carrier gas, the output pressure of the device was 0.4~0.5 MPa, the operating voltage was about 20 mV, and the operating current was 50 mA. In the reaction process, the temperature was controlled at 5℃ by using low and constant temperature circulating water device. The reaction cell was connected to a gas circulation system, and the hydrogen evolved was analyzed by an online TCD detector chromatograph assisted with a 5Å molecular sieve column. The hydrogen production was calculated by the peak area which measured under the different 6


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reaction time, and the total hydrogen production in 8 h was used to judge the photocatalytic activity of catalysts.

Results and discussion XRD analysis Fig. 1 is the XRD patterns of TiO2, ZrO2, TiO2−ZrO2 and TiO2−ZrO2 hollow spheres. As can be seen from Fig. 1, TiO2 is mainly composed of anatase structure and its diffraction peaks mainly located in the diffraction 2 theta angle of 25.31°, 37.90°, 48.02°, 54.64° and 62.83° (JCPDS 21-1272) [1]. The tetragonal phase diffraction peaks of the pure ZrO2 are appeared at 2θ = 30.1°, 34.5°, 50.4° and 60.1° (JCPDS 50-1089) [27]. Both TiO2−ZrO2 and TiO2−ZrO2 hollow spheres have a strong characteristic diffraction peak of anatase TiO2, while the difference is, the peaks become narrower and sharper than that of the monomer TiO2, indicating that the as-composites have better crystallinity and more perfect crystal structure.

--- TiO2

--- ZrO2

* --- (Ti-Zr)O2

**

**

TiO2-ZrO2-S

* *

**

TiO2-ZrO2-P

ZrO2 TiO2

20

30

40

50

60

70

80

2θ / degree

Fig. 1 XRD patterns of TiO2, ZrO2, TiO2−ZrO2-P and TiO2−ZrO2-S.

The grain sizes of the samples estimated by the Scherrer equation are listed in Table 1. As seen in Table 1, the grain sizes of TiO2−ZrO2 composites are obviously larger than that of the monomer, which indicates that the addition of zirconia promotes the growth of TiO2 crystallinity to a certain extent [28]. However, no characteristic diffraction peaks of ZrO2 are observed in TiO2−ZrO2-S and TiO2−ZrO2-P, possibly due to the small amount of zirconia fed into the system, or ZrO2 has combined with TiO2 to form titanium zirconium oxide. Compared with pure TiO2 and ZrO2, some new characteristic peaks of the composite material are found. The weak diffraction peaks signed as * in Fig. 1 are corresponded to the (Ti−Zr)O2, indicating that there is Ti−O−Zr bond in the composites [29]. Comparing each set of data, there is no obvious change in the diffraction angles of a set of synthesized 7



catalyst, demonstrating that the composition of catalyst is relatively stable, and has no change in the preparation process. UV–visible diffuse reflectance spectroscopy In order to investigate the optical absorption of TiO2, ZrO2 and two kinds of TiO2−ZrO2 composites, as-materials are studied by UV−vis diffuse reflectance spectra, the results are shown in Fig. 2. From Fig. 2 (a) we can see that, the prepared TiO2−ZrO2-S and TiO2−ZrO2-P composites have strong absorption in the wavelength range below 400 nm in the UV region. Compared with the TiO2, ZrO2, and TiO2−ZrO2-P, the absorption peak of TiO2−ZrO2-S has slightly red shift, which illustrates that the optical absorption property is a little increase due to the combination of TiO2 and ZrO2, and this is also related to the light harvesting properties of hollow spherical structures [30]. Different materials have different band gap energy (Eg), the smaller the material Eg, the greater light wavelength is required, meanwhile, the lower energy is needed, the more easily electron transited, and the higher photocatalytic activity is obtained. So according to the Kubelka-Munk energy curve in Fig. 2 (b), the Eg values of TiO2−ZrO2-S, TiO2−ZrO2-P, TiO2 and ZrO2 can be obtained (see Table 1). From the data in Table 1, we can see that the band gap of TiO2−ZrO2-S is slightly smaller than that of TiO2−ZrO2-P, but being no obvious difference, suggesting that the morphology changes of two composites haven’t influenced their light absorption properties. TiO2

a 0.6

TiO2-ZrO2 -S

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Abs

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TiO2

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ZrO2

λ / nm

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ZrO2

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TiO2-ZrO2 -S

2

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λ / nm

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hν / eV

Fig. 2 The UV–vis/DRS spectra (a) and plot of transformed Kubelka–Munk function versus the energy of light for different samples (b).

SEM and EDS analysis Fig. 3 is SEM images of PS (a, c) and TiO2−ZrO2 hollow spheres (b, d) in different scales. From Fig. 3 (a, c), it can be seen the size of the PS spheres is uniform 8


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and the PS spheres exhibit monodisperse state with smooth surface. From the TiO2−ZrO2 hollow spheres SEM images in Fig. 3 (b, d), we can find that, during the mild water bath process, the amorphous TiO2−ZrO2 nanoparticles formed after the hydrolysis of zirconium and titanium precursor evenly coated on the surface of the PS spheres, then, the core of PS is removed via high temperature calcination, at the same time, the amorphous structure is known to experience a phase change to form TiO2−ZrO2 nanoparticles with anatase phase, to be the constitution of the rough shell with the thickness reaching to ca. 24 nm. Fig.4. shows the elemental maps of Ti, O, Zr in TiO2−ZrO2 hollow spheres, confirming that the TiO2−ZrO2-S shell is composed of uniform distribution of the three elements.

Fig. 3 SEM images of PS (a, c) and TiO2−ZrO2 hollow spheres (b, d) at different magnifications.

Fig.4 Elemental maps of Ti, O, Zr in TiO2−ZrO2 hollow spheres.

In addition, the obtained sample still retains the appearance of PS colloidal template. But compared with the monodisperse PS colloid spheres, the monodisperse state of TiO2−ZrO2 hollow spheres is decreased along with some adhesion 9



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phenomenon, attributed to the coating progress and self-assembly process taking place at the same time during the synthesis. Either the coating is not uniform in individual regions or the PS spheres are squeezed each other, resulting in a slight deformation of the spheres. Although the shell of TiO2−ZrO2 hollow spheres is thicker, some of the spherical walls in the composite materials are still broken, this is mainly ascribed to the gas release made by the local sphere internal pressure rising rapidly when part of PS colloidal template was calcined in high temperature, leading to the rupture of spheres wall [31]. TEM and HR-TEM analysis The characters of morphological and crystal structure of TiO2−ZrO2 hollow spheres are further investigated by TEM and HR-TEM, the results illustrated as Fig. 5. Fig. 5 (a-c) is the TEM images of TiO2−ZrO2 hollow spheres under different scales, in each image, the dark area corresponds to the shell layer of hollow spheres, and the bright area represents the void space. These images indicate that the TiO2−ZrO2 hollow spheres are monodisperse in the three-dimensional space, and their integrity is higher. But, similar to the SEM images, some of the TiO2−ZrO2 hollow spheres have been adhered to each other. The HR-TEM images from the spherical shell display two types of clear lattice fringes as shown in Fig. 5(d), and the two sets of fringes spacing are ca. 0.353 nm and 0.236 nm, corresponding to the anatase phase TiO2 (101) and (001), respectively. It is worth noting that, the anatase TiO2 (001) crystal surface with high reduction ability also has a strong hydrogen production activity, and the (101) crystal surface has a high oxidation ability together with the favorable degradation performance of organic pollutants [32, 33]. The fast fourier transform (FFT) (Figure 5d illustration) shows that the adjacent quantum dots have a clear particle spacing, and the TiO2−ZrO2 hollow spheres present a mixed crystal structure, but most of the small grains are still in the same orientation in the crystal growth process [34]. In the HR-TEM images, the apparent ZrO2 crystal surface is not observed in the large region, only a very small area of the ZrO2 (100) crystal surface is detected in Fig. 5(e) [35], which may be due to the quantity of ZrO2 precursor invested into the system is less, or the uniform distribution of ZrO2 occurred in the formation process, the results are consistent with the XRD analysis.

10


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Fig. 5 TEM images (a-c) and HR-TEM images (d,e) of TiO2−ZrO2 hollow spheres.

XPS analysis The surface chemical compositions and the valence state of surface elements of TiO2−ZrO2 hollow spheres are measured by X-ray photoelectron spectroscopy (XPS). From Fig. 6 (a), it can be seen that there are C, O, Ti and Zr four elements on the surface of the TiO2−ZrO2 hollow spheres. Among them, the presence of the C element may be due to the presence of CO2, which the sample absorbed in the air or the hydrocarbon impurities occasionally stuck with apparatus itself [22]. Fig. 6 (b) is a XPS pattern of O1s, the characteristic peak at binding energy of 530.3 and 532.2 eV ascribed to the lattice oxygen and hydroxyl oxygen, respectively [21]. Fig. 6 (c) is the XPS spectra of Ti 2P, of which the peak could be fitted with two peaks at binding energies of 459.1 and 464.9 eV, assigned to the spin orbit of Ti 2P3/2 and Ti 2P1/2 of Ti4+ chemical state, respectively [36]. In Fig. 6(d), the binding energies of Zr 3d5/2 and Zr 3d3/2 are 182.4 and 184.8 eV, respectively, which is found to be that the presence of zirconium in the composite is Zr4+ in agreement with the values of ZrO2 reported in literature [23]. 80000

600000

O 1s

530.3

a Intensity/a.n.

450000

Intensity/a.u.

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Ti 2p 300000

150000

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40000 30000

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Intensiyy/a.n.

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182.4

9000

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455

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Eb/eV

Fig. 6 XPS results showing the survey spectra of TiO2−ZrO2 hollow spheres (a) and spectra of O1s (b), Ti 2p (c) and Zr 3d (d).

N2 adsorption–desorption measurement In order to study the surface physical and chemical properties of TiO2−ZrO2 nanocomposite, N2 adsorption−desorption was determined to test the TiO2−ZrO2-P and TiO2−ZrO2-S, respectively, the results shown as Fig. 7. According to the definition of IUPAC, the N2 adsorption−desorption results for both the TiO2−ZrO2-S and TiO2−ZrO2-P show type-IV isotherms, indicating a typical mesoporous structure in the two kinds of composite materials. At the same time, the two composites have different types of hysteresis loop, wherein, of which TiO2−ZrO2 is type H2 mainly due to the capillary condensation in the mesoporous. While TiO2−ZrO2-S has two kinds of hysteresis loops, which is H2 in the relatively low pressure range of 0.40-0.80, and the hysteresis loop formed in the pressure range of 0.8-1.0 is H3, due to the intergranular pores containing in the samples [1]. The each illustrations in Fig. 7 are the distribution of the pore size belonging to the composite material, as indicated by the graph illustrated in Fig. 7, the aperture distribution of TiO2−ZrO2-P is widely, while that of TiO2−ZrO2-S is relatively concentrated. This is due to the prepared hollow spheres having the characteristics of structural integrity in highly degree [26].

100 80

a

(a)

60 3 -1

0.020 0.015

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[dV/d(lgd)] / (cm3/g)

3 -1

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0.010 0.005 0.000 0 10 20 30 40 50 60 70 80

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b

(b)

0.03 0.02 0.01 0.00 0

20

30

40

60

80

D / nm

20 10 TiO2-ZrO2 -S

TiO2-ZrO2-P

0.2

0.4

0.6

0.8

1.0

Relative presure p / p0

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative presure p / p0

Fig. 7. Nitrogen adsorption–desorption isotherms and BJH pore size distributions (insets) of TiO2−ZrO2-P(a) andTiO2−ZrO2-S(b). 12


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In addition, the BET specific surface area, average pore size and pore volume of the samples are listed in Table 1. From the above SEM images, it can be known that the integrity of hollow spheres is relatively high, and there is nearly no broken hollow spheres, and the hollow spherical wall is closely packed with nanoparticles, all leading to the various data of TiO2−ZrO2 hollow spheres with dense and thick spherical shell are lower than that of the powder TiO2−ZrO2 composite. Table 1 The band gap energy (Eg), average crystallite size (D*), BET surface area (SBET), average pore diameter (D) and pore volume (Vtotal) of TiO2, ZrO2, TiO2−ZrO2-P and TiO2−ZrO2-S Sample

SBET/(m2·g-1)

D/nm

Vtotal/(cm3· g-1)

D*/nm

Eg/eV

TiO2

125.1

4.5

0.16

6.0

3.20

ZrO2

130.4

3.7

0.07

14.0

3.45

TiO2−ZrO2-P

86.0

8.6

0.26

30.3

3.19

TiO2−ZrO2-S

53.6

3.8

0.11

40.9

3.18

D*: Average crystallite sizes of samples were calculated using the Scherrer equation.

Photocatalytic activity In order to investigate the photocatalytic activity and the practical application of TiO2−ZrO2 composites, a series of photocatalytic degradation experiments under multi-modes, including UV, simulated solar and microwave-assisted irradiation, and photocatalytic H2 evolution and trapping experiments are carried out, the related results shown as Fig. 8. The photocatalytic activity of TiO2−ZrO2 hollow spheres was tentatively studied under UV irradiation, and the results are shown in Fig. 8(a). The preliminary understandings about the photocatalytic activity of the prepared catalysts can be obtained by UV photocatalytic experiment. So from Fig. 8 (a) we can see that, the activity of TiO2−ZrO2-S is remarkably higher than that corresponding to direct photolysis, monomer TiO2, ZrO2, TiO2-S and TiO2−ZrO2-P. The addition of ZrO2 could improve the photocatalytic activity of the pure TiO2 hollow spheres. In the photocatalytic experiment of methyl orange, TiO2−ZrO2-S can essentially degrade methyl orange to colorless within 60 min, the absorption curve as shown in Fig. 7b. At the same time, the photocatalytic degradation reaction basically follows pseudo first-order kinetics (Fig. 8c), and the apparent reaction rate constants of TiO2−ZrO2-S, TiO2−ZrO2-P, P25, ZrO2, TiO2-S and direct photolysis are 0.05186、0.04097、 0.01399、0.01259, 0.01063 and 0.00372 min-1, respectively. Fig. 8 (d, e) is the experimental results of TiO2−ZrO2-S under simulated solar and photocatalysis microwave-assisted, as well as the degradation of various organic pollutants under UV light. In the microwave-assisted photocatalysis experiment (Fig. 8d), since the microwave can produce the additional defect sites, the polarization 13



effect of the photocatalysts can increase the transfer possibility of the photo-electron, so under the double effects of microwave field and ultraviolet light, the activity of TiO2−ZrO2-S should be further improved. But owing to the low density of the prepared hollow spheres, which can be attached to the surface of the microwave electrodeless lamp in the photocatalytic experiment, affecting the light transmittance. Moreover, since the reactant needs external circulation cooling to ensure the reaction carried out smoothly, which will limit the duration of catalyst to absorb light. As well as, the different light source adopted in the three modes has its own light intensity; the reaction efficiency is less than that of the photocatalysis under ultraviolet light in the same reaction time [37]. And the results of the simulation photocatalysis and the photocatalytic degradation of organic pollutants with different structure under UV irradiation show that TiO2−ZrO2-S has the strong practical application value and a certain degree of usability for the degradation of organic pollutants. 1.0

3.5

a

ZrO2

2.5

-ln( Ct/C0)

Ct / C0

0.6 direct photolysis TiO2

0.4

ZrO2 TiO2-S

0.2

TiO2-ZrO2 -S

0

10

TiO2-S TiO2-ZrO2-P

2.0

TiO2-ZrO2 -S 1.5 1.0 0.5

TiO2-ZrO2-P

0.0

0.0

20

30

40

50

60

0

10

20

30

t / min 1.0

d

0.8

Ct / C0

TiO2-ZrO2-S

0.6

60

0.8

f

0.8

direct photolysis TiO2 ZrO2

0.4

0.6

Ct / C0

TiO2-ZrO2-P TiO2-ZrO2-S 0.2 0

4

50

1.0

e

20

direct photolysis

40

60

t / min

TiO2

80

100

120

ZrO2

TiO2-ZrO2-P

0.6

MO CV CR MG SA PH

0.4

0.2

2 0.0

Ct/C0

6

40

t / min

1.0

8

c

direct photolysis TiO2

3.0

0.8

degration percentage(%)

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

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2

3

4

0

10

TiO2-ZrO2-S

0.2

20

0 1

0.4

30

40

50

60

t / min

5

0.0 0

BQ TEA TBA 10

20

30

40

50

60

t / min

Fig. 8 (a) The UV photocatalytic degradation MO profiles with different photocatalysts; (b) The UV−vis spectra at different reaction time in the UV photocatalytic degradation MO profiles with TiO2−ZrO2-S(inset is the photograph of the MO solution at different reaction time); (c) The kinetics of UV photocatalytic degradation MO with different photocatalysts; (d) The results of mircowave-assisted (t = 120 min) and solar light photocatalytic degradation (t = 240 min) for MO profiles with different photocatalysts; (e) The degradation results of different organic pollutant with TiO2−ZrO2-S under UV light irradiation; (f) The results of the photo-induced carrier capture experiment during different TiO2−ZrO2-S photocatalytic degradations for the MO system.

In the present study, we detect the possible active groups of TiO2−ZrO2-S in the photocatalytic process by adding different free radical trapping agents, as shown in 14


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Fig. 8 (f). From Fig. 8 (f), obviously seen that, the degradation rate of methyl orange in TiO2−ZrO2-S is intensively declined in 90 min when benzoquinone (BQ) and triethanolamine (TEA) are put into the system as scavengers, of which the activity is obviously inhibited, meaning the ·O2- and the hole are the active group in the system as reported by the relevant literature [38]. While methyl orange still has a certain degree of degradation when tert butyl alcohol (TBA) is joined as a capture agent within 90 min, but the degradation efficiency is slightly lower than that of without trapping agent, indicating that there is a small amount of active groups ·OH free radical in the TiO2−ZrO2 hollow spheres system [39]. The being of different active groups is the theoretical basis on the better degradation of methyl orange by TiO2−ZrO2 hollow spheres. Photocatalytic H2 evolution from water splitting Fig. 9 is the experimental results of photocatalytic H2 evolution from different catalysts under the same experimental conditions. As shown in Fig. 9, after 8 h of reaction, total hydrogen production amounts of TiO2−ZrO2-S is 23.7 µmol, which is obviously higher than that of TiO2−ZrO2-P, monomer P25 and ZrO2, and the excellent hydrogen production activity of TiO2−ZrO2-S can be attributed to: (1) Unique hollow spheres structure. The hollow structure with high-efficiency light harvesting ability can effectively utilize light energy and convert it into chemical energy. (2) The combination of TiO2 and ZrO2. By combining the TiO2 with strong light stability and the ZrO2 with redox property, the obtained composite material will have the advantages of the both, and the different energy level between the two semiconductors can provide a way for enhancing the separation yield of photogenerated charges, which is beneficial to photogenerated carriers inject from the level of ZrO2 into TiO2 effectively. (3) TiO2 (001) and ZrO2 (100) crystal surfaces of the composites have

Fig. 9 Experimental results of photocatalytic H2 evolution from different catalysts. 15



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strong reducibility and can promote to carry out the reaction of 2H2O→2H2+O2 [33, 35]. Photocatalytic cycle experiment The cycle stability complementary experiments both for photocatalytic degradation and H2 evolution from water splitting are carried out, and the detailed experimental results are shown in Fig. 10. The experimental results show that the photocatalytic degradation of organic pollutants decreased slightly after three cycles, which may be due to some hollow spheres broken in the recycling process, the specific surface area and the ability to capture light both decreased. At the same time, compared with the initial hydrogen production rate, the photocatalytic H2 evolution from water splitting activity of TiO2−ZrO2 hollow spheres was slightly lower, the second and third hydrogen production rates were 97.6% and 91.4%, respectively. In spite of this, the TiO2−ZrO2 hollow spheres prepared in this paper still have some recycling value. 100% 97.6%

98.0%

100

91.4%

89.8%

80 60

71.9% 40 20 0

1 st 2 nd 3 rd Photocatalytic Degradation Percentage (%) Photocatalytic H2 Evolution Efficiency (%)

Fig. 10 Experimental results of photocatalytic cycle experiment.

Possible photocatalytic reaction mechanism According to the characterization results, combined with the relevant results of the capture assay, the possible mechanism of the phtotcatalytic H2 evolution from water splitting and photocatalytic degradation of TiO2−ZrO2-S is proposed, as shown in Fig. 11. What the conditions of phtotcatalytic H2 evolution from water splitting need to meet are: firstly, the semiconductor conduction band position should be negetive than the hydrogen electrode reaction potential (0 eV) to achieve the requirements of the hydrogen evolution reaction; secondly, the location of valence 16


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band should be positive than that of potential oxygen electrode (1.23 eV) in the reaction, so as to the photogenerated holes can effectively oxidize water. To this end, the minimum values of conduction band of TiO2 and ZrO2 are -0.29 and -0.32 eV, and the maximum values of valence band are 2.91 and 3.13 eV, respectively [40], which are calculated according to E(CB)=χ-4.5-0.5Eg formula, and the results show that the TiO2−ZrO2-S system meets the requirement of hydrogen production. In the photocatalytic reaction process, the reaction system of TiO2−ZrO2-S can lead to the generation of ·O2-, ·OH and H+ on the catalyst surface, and some of these groups can be combined with the H+ in the water and produce the clean energy H2O and H2[41]. In the process of photocatalytic degradation of organic pollutants, TiO2−ZrO2-S is excited by the light irradiation, resulting in the production of photo electron−hole pairs. Since the bottom of the conduction band edge of ZrO2 is higher than TiO2, upon UV irradiation, there may be a fast transfer of the photo-generated electron from the CB of ZrO2 to that of TiO2, and the unique hollow structure of the TiO2−ZrO2-S can prevent the recombination of the photo-formed electron−hole, therefore, the electrons can be better combined with the O2 in the solution to generate the ·O2- ion. While the photo-generated hole combines with the H2O to gernerate ·OH, all these active groups can finish the degradation completely by mineralizing the organic pollutants into oxidized products, like CO2 and H2O and inorganic salts [42]. calcining

packing

PS

O2

TiO2-ZrO2 hν

O2- MO

-1

Potential(eV)

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


0

-0.32 -0.29

CB

-

H+

CB -

-

H+/H

3.20 eV

3.45 eV

H2 2

H2O→H2+1/2 O2 O2/H2O

1 1.23

CO2↑+H2O

2 TiO2

ZrO2 3

2.91 3.13

VB

+

+

VB

H 2O O2

+

OHCO2↑+H2O

MO

·OH

Fig. 11 Possible mechanism of both photocatalytic H2 evolution and photocatalytic degradation with TiO2−ZrO2 hollow spheres

17



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Conclusions Under the action of self-assembly PS colloidal crystal template, TiO2−ZrO2 composite with favorable hollow structure was prepared through water bath method combined with calcination post treatment method. SEM and TEM images showed that the TiO2−ZrO2 hollow spheres retained the integrity of PS colloidal template, whose compact and rough shell with the thickness about 24 nm was composed of TiO2−ZrO2 nanoparticles. The HR-TEM analysis results of the hollow shell showed the anatase TiO2 (001) and (101) crystal surface and a little region of ZrO2 (100) crystal surface, which brought about strong photocatalytic degradation activity of organic pollutants and photocatalytic hydrogen evolution from water splitting. Simultaneously, the addition of ZrO2 can effectively inhibit the recombination of photo-generated electron−hole pairs and lengthen the lifetime of the carrier being conducive to the efficient photocatalytic reaction. In addition, the TiO2−ZrO2 hollow spheres exhibit excellent

photocatalytic

degradation

properties

in the

microwave

assisted

photocatalysis, simulated sunlight photocatalysis, and UV photocatalytic degradation of different types of organic pollutants. It is worth noting that the TiO2−ZrO2 hollow spheres in the photocatalytic H2 evolution from water splitting also reveal prominent performance, the total hydrogen yield of 8 h being 23.7 µmol, which is significantly higher than that of TiO2−ZrO2, ZrO2 and P25. The favorable morphology reinvests TiO2−ZrO2 hollow spheres with the more excellent photocatalytic properties. The high activities of TiO2−ZrO2 hollow spheres of photocatalytic degradation and photocatalytic H2 evolution from water splitting greatly expand the application of TiO2−ZrO2 in the field of photocatalysis. Acknowledgements

This study are supported by the National Natural Science Foundation of China (21376126), Natural Science Foundation of Heilongjiang Province, China (B201106), Scientific Research of Heilongjiang Province Education Department (12511592), Government of Heilongjiang Province Postdoctoral Grants, China (LBH-Z11108), Open Project of Green Chemical Technology Key Laboratory of Heilongjiang Province College, China (2013 year), Postdoctoral Researchers in Heilongjiang Province of China Research Initiation Grant Project (LBH-Q13172), Innovation Project of Qiqihar University Graduate Education (YJSCX2014-009X), College Students' Innovative Entrepreneurial Training Program Funded Projects of Qiqihar University (201510221077) and Qiqihar University in 2015 College Students Academic Innovation Team Funded Projects. 18


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Graphical abstract

The TiO2−ZrO2 hollow spheres are synthesized basing on self-assembly polystyrene (PS) spheres as colloidal crystal template, which well retain the spherical structure of the PS crystal template with uniform dispersion. Its shell is closely packed by TiO2−ZrO2 nanoparticles and the thickness is ca. 24 nm. The as-composites have remarkable degradation activity under multi-modes and favorable photocatalytic capacity of H2 evolution from water splitting.

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Hollow Sphere TiO2âZrO2 Prepared by Self-Assembly with

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