MIL-101 composite

Materials Science in Semiconductor Processing 36 (2015) 115–123

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Enhanced photodegradation of Rhodamine B under visible light by N-K2Ti4O9/MIL-101 composite Yanli Xu, Qi Chen, Hanbiao Yang, Mengmeng Lv, Qinqin He, Xueting Liu n, Fengyu Wei n School of Chemistry and Chemical Engineering, Hefei University of Technology, Anhui Key Laboratory of Controllable Chemical Reaction & Material Chemical Engineering, Hefei 230009, China

a r t i c l e i n f o

Keywords: N-K2Ti4O9 MIL-101 Synergistic effect Photocatalysis

abstract N-K2Ti4O9/MIL-101 composites were successfully synthesized by a facile hydrothermal method, and were characterized by powder X-ray diffraction, UV–vis diffuse reflectance spectroscopy, the valence band X-ray photoelectron spectroscopy, field emission transmission electron microscopy, photoluminescence emission spectra, N2 adsorption–desorption and thermogravimetric analysis. Photocatalytic activities of N-K2Ti4O9, MIL-101 and the composites were investigated by the degradation of Rhodamine B (RhB) under visible light irradiation. The results show that the composites exhibit higher photocatalytic activity as compared with the pure materials. The synergistically enhanced photocatalytic activity of the composites is due to big adsorption capacity of MIL-101 and high separation efficiency of photogenerated electron-hole pairs through interfaces between N-K2Ti4O9 and MIL-101. & 2015 Elsevier Ltd. All rights reserved.

1. Introduction Recently, dyes are used in many industries, and have caused serious environmental pollution. The use of solar energy and semiconductor catalysts for photocatalytic degradation of organic dyes in water has been intensively investigated as an emerging renewable technology [1–4]. K2Ti4O9 has been studied as a photocatalyst due to its nontoxicity, low cost and physical and chemical stability,[5– 7] however, its band-gap is between 3.2 eV and 3.4 eV [8]. In order to reduce the band-gap and widen the practical applications of K2Ti4O9 related materials, doping of N element into K2Ti4O9 (entitled “N-K2Ti4O9”) is a common method. The narrow band-gap semiconductor not only acts as the sensitizer, but also reduces the recombination rate of n Corresponding authors. Tel.: þ 86 551 62901458; fax: þ 86 551 62901450. E-mail addresses: [email protected] (X. Liu), [email protected] (F. Wei).

http://dx.doi.org/10.1016/j.mssp.2015.03.025 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

the photogenerated electron-hole pairs, and gives rise to an enhancement of the photocatalytic activity [9–11]. Metal-organic frameworks (MOFs) materials are made up of metal clusters linked to each other by organic ligands [12– 16], and have high specific surface area and uniform but tunable pore size. Thus they are considered as potential candidates for adsorption [17], storage [18] and health care applications [19]. O. I. Lebedev et al. was first to synthesize a chromium(III)-based MOF (MIL-101), which is based on a Cr3F (H2O)2O octahedron, forming lattices by 3-periodic connection through a 1,4-benzene-dicarboxylate (bdc) linker [20]. In this paper, owing to superior electron mobility of NK2Ti4O9 and the high adsorption capacity of MIL-101, numerous efforts have been made to combine MIL-101 with N-K2Ti4O9 to enhance the photocatalytic activity. The result shows that N-K2Ti4O9/MIL-101 composites exhibit better photocatlytic activity than pure materials. The synergistic mechanism of N-K2Ti4O9/MIL-101 composites about photodegradation of Rhodamine B (RhB) was investigated.

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2. Methods 2.1. Materials All chemicals including 1,4-benzenedicarboxylic acid, Cr(NO3)3  9H2O, HF(40%), chloroform, K2CO3 and TiO2 were supplied by Sinopharm Chemical Reagent Co., Ltd, and were used as received without further purification. 2.2. Synthesis of N-K2Ti4O9/MIL-101 composites N-K2Ti4O9 was prepared as described previously [21–22]. N-K2Ti4O9/MIL-101 composites were synthesized by the method reported for MIL-101 with differing in N-K2Ti4O9/Cr (NO3)3  9H2O mass ratio [20]. A typical preparation procedure is as follows: N-K2Ti4O9 (2.00 g), Cr(NO3)3  9H2O (2.00 g), 1,4benzenedicarboxylic acid (0.820 g) and HF (0.25 mL) were dispersed in distilled water with ultrasonic vibration for 30 min. Then the mixture was transferred to a stainless steel teflon-lined autoclave of 30 mL capacity, and then maintained at 493 K for 8 h. After the autoclave was cooled to room temperature, the obtained composite was filtered out, washed with CHCl3, and dried at room temperature under reduced pressure. The as-prepared composite was entitled “N-K2Ti4O9/ MIL-101(1:1)” where 1:1 denotes the mass ratio of N-K2Ti4O9 to Cr(NO3)3  9H2O. Similarly, other composites with the mass ratio x:y are entitled “N-K2Ti4O9/MIL-101 (x:y)”. 2.3. Characterization X-ray diffraction (XRD) patterns of the samples were determined in the range of 2θ ¼41–501 by step scanning on a Rigaku D/max-2500V X-ray diffractometer using CuKα (λ ¼0.154nm) radiation. The morphological analysis of the samples was studied using a JEM-2100F field emission transmission electron microscopy (FETEM) equipped with an energy-dispersive X-ray spectrometer (EDS). UV–vis diffuse reflectance spectroscopy (UV–vis spectra) was recorded at a scanning rate of 3600 nm/min in solidstate on a DUV-3700 spectrometer with the double beam in which spectral resolution is 0.1 nm, and measure range varies from 165 nm to 3300 nm depending on the use of an integrating sphere and the optional direct detection unit. The valence band X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB250 spectrometer. Photoluminescence emission spectra (PL) were measured on a PL Measurement system (Fluorolog Tau-3) with the excitation wavelength of 320 nm. N2 adsorption–desorption (BET) was performed on a Tristar II 3020M surface area and porosity analyzer at 77 K. Before measurement, the sample was degassed at 70 1C for 3 h. Thermogravimetric analysis (TGA) of samples was carried out on a Perkin-Elmer Diamond TG thermal analyzer under a flow of air where approximately 10 mg of the sample was loaded into a standard Al2O3 crucible, which was heated from room temperature to 700 1C at a rate of 30 K/min. 2.4. Photocatalytic experiments The photocatalytic degradation of RhB was measured at ambient pressure and 298 K in a set of home-made

photochemical reaction equipment. The light source was a PHILIPS 70 W metal halide lamp (λ o380 nm was filtered out by a cut off filter). 20 mg of photocatalyst was added into 100 mL RhB (initial concentration: C0 ¼5 mg/L) aqueous solution. Before irradiation, the suspension was stirred continuously for 12 h in the dark in order to reach the adsorption–desorption equilibrium between RhB and the photocatalyst. The supernatant liquid was obtained through filtration by 0.22 μm filter, and examined using a Shimadzu UV-722E spectrophotometer. In order to confirm that RhB was not photodegraded by itself, control experiments were carried out under the same condition but without irradiation or photocatalyst. 3. Results and discussion 3.1. Photocatalysts characterization The TEM images for pure MIL-101 and N-K2Ti4O9 are shown in Fig. 1a and b, respectively. MIL-101 behave as irregular plate-shaped aggregates. In contrast, N-K2Ti4O9 in Fig. 1b have the feature of rod-shaped microcrystals with the diameter between 100 nm and 350 nm and the length of a few micrometers. The TEM image of N-K2Ti4O9/ MIL-101(1:1) (Fig. 1c) indicates that the N-K2Ti4O9 rodshaped microcrystal is covered with MIL-101 aggregates in the composite. The HRTEM image (Fig. 1d) shows one lattice structure with d-space of about 0.255 nm corresponding to the (403) plane of N-K2Ti4O9. By comparison, crystal structure of MIL-101 was destroyed by electron beam bombardment. However, the clear hetero-junction interface of N-K2Ti4O9/MIL-101 can be seen in Fig. 1d. The EDS spectra of N-K2Ti4O9/MIL-101(1:1) in Fig. 2a indicate that Ti, O, Cr, and K elements are the major chemical components present in the composite. The Ti distribution by EDS mapping (Fig. 2b) shows that the N-K2Ti4O9 in the composite is a core rod with some parts of it stripped, which can be also observed in Fig. 1c. This means that hydrothermal and acid preparation condition of the composite can cause damage to the structure of N-K2Ti4O9. Comparing Fig. 2b, c and d, we can see that the Ti and O elements have similar distribution shape although some of the O elements are from MIL-101 with the others from N-K2Ti4O9. This indicates that the O element is mainly distributed in core rod region. The Cr element from MIL-101 has even larger distribution area, which suggests that the MIL-101 aggregates are distributed wholly but not evenly around the N-K2Ti4O9 core rod. The N2 adsorption–desorption isotherms of N-K2Ti4O9, NK2Ti4O9/MIL-101 and MIL-101 are shown in Fig. 3a, and the values of their BET surface area are determined to be 5 m2/g, 135 m2/g and 2321 m2/g, respectively. The average pore size of the N-K2Ti4O9/MIL-101 is 3.6 nm with its pore volume at 0.086 cc/g. The N2 isotherm of MIL-101 is categorized as type I [23,24]. This property implies the presence of micropores (less than 2 nm in size). As shown in Fig. 3b, MIL-101 has two types of holes, and pore sizes are about 1.36 nm and 1.97 nm, respectively, corresponding to p/p0 ¼0.1 and p/p0 ¼0.2 adsorption platforms on N2 adsorption–desorption isotherma(Fig. 3a). This is in agreement with the structure of MIL-101 reported in the literature [25]. In contrast, there is no micropores structure in N-K2Ti4O9. When MIL-101 was loaded on the surface of

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Fig. 1. TEM images of (a) MIL-101, (b) N-K2Ti4O9, (c) N-K2Ti4O9/MIL-101(1:1); and HRTEM image of (d) N-K2Ti4O9/ MIL-101(1:1).

N-K2Ti4O9, the BET surface area increased, and the N2 isotherm of N-K2Ti4O9/MIL-101 is categorized as type IV. This means there exists both micropores and macropores in the composite as shown in Fig. 3c. To go further to determine the MIL-101 content in the composites, the thermogravimetric analysis (TGA) for MIL101 and composites was studied. As shown in Fig. 3d. the mass loss of the composites is little during heating to 300 1C, which may be due to the removal of the residual hydroxyl groups on the surface of MIL-101[26]. Within the scope of the 300–500 1C, the mass loss becomes obvious indicating the collapse of the MIL-101 framework, which is mainly caused by decomposition of the linkers [27]. However, MIL-101 and composites have differed on the

residual mass. As there is no change of N-K2Ti4O9 in the process of TGA, the MIL-101 and N-K2Ti4O9 contents with mass percent in the composites can be determined by the following equations: mN  K 2 Ti4 O9 =MIL  101ðx:yÞ  r N  K 2 Ti4 O9 =MIL  101ðx:yÞ ¼ mN  K 2 Ti4 O9  r N  K 2 Ti4 O9 þmMIL  101  r MIL  101 mN  K 2 Ti4 O9 =MIL  101ðx:yÞ ¼ mN  K 2 Ti4 O9 þ mMIL  101

ð1Þ ð2Þ

where mN  K 2 Ti4 O9 =MIL  101ðx:yÞ , mN  K 2 Ti4 O9 , mMIL  101 , r N  K 2 Ti4 O9 =MIL  101ðx: yÞ, r N  K 2 Ti4 O9 and r MIL  101 are the mass (g) and residual mass fraction (%) of N-K2Ti4O9/MIL-101(x:y),

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Fig. 2. (a) the EDS; (b) Ti, (c) O and (d) Cr distribution by EDS mapping of N-K2Ti4O9/ MIL-101(1:1).

N-K2Ti4O9 and MIL-101 (Table 1). As can be seen, the NK2Ti4O9 mass fractions (Y in Table 1) generally increases with the increasing of the residual mass fraction in the composites. The XRD patterns of MIL-101 and N-K2Ti4O9 shown in Fig. 4a are in accordance with those reported in the literature [21,23], and the composite displays the characteristic peaks of both MIL-101 and N-K2Ti4O9. This means the composite consists of MIL-101 and N-K2Ti4O9. The optical and electronic properties of the composites have also been studied, and the UV–vis absorption spectra of the precursors are illustrated in Fig. 4b. N-K2Ti4O9 clearly shows a characteristic absorption of K2Ti4O9 [28]

in the UV region, and a new absorption shoulder at 400– 500 nm (3.10–2.48 eV) can be attributed to the N surface plasmon resonance with the K2Ti4O9 interband transition [29–31]. It means after N doping, beside the main band gap for K2Ti4O9, another new and small band gap derived from N doping appears between 3.10 eV and 2.48 eV, which is beneficial for absorbing visible light, and enhancing the corresponding visible light photocatalytic activities. The steep shape of MIL-101 shown in the UV region is due to the band-gap transition. Near the absorption band edge, the optical absorption has the following behavior:
αhν ¼ A hν  Eg n=2 ð3Þ

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Fig. 3. (a) the N2 adsorption–desorption isotherms of N-K2Ti4O9, N-K2Ti4O9/MIL-101 and MIL-101; (b) pore size distribution curve of MIL-101; (c) pore size distribution curve of N-K2Ti4O9/MIL-101 composite and (d) TGA curves of MIL-101 and N-K2Ti4O9/MIL-101 composites. Table 1 The N-K2Ti4O9 and MIL-101 contents of the composites. Sample

Residual mass

MIL-101 (1Y)

N-K2Ti4O9 (Y)

N-K2Ti4O9 MIL-101 N-K2Ti4O9/MIL-101 (1:2) N-K2Ti4O9/MIL-101 (2:3) N-K2Ti4O9/MIL-101 (1:1) N-K2Ti4O9/MIL-101 (3:2) N-K2Ti4O9/MIL-101 (2:1)

1 0.1862 0.6015

0 1 0.4897

1 0 0.5103

0.6321

0.4521

0.5479

0.7143

0.3511

0.6489

0.7817

0.2682

0.7318

0.8523

0.1815

0.8185

where α, v, Eg, A are absorption coefficient, light frequency, band-gap and a constant, respectively, and n depends on whether the transition is direct (n ¼1) or indirect (n ¼4). [32] For MIL-101 and N-K2Ti4O9, the value of n is 1. The plot of (αhv)2 versus hv is shown in inset of Fig. 4b. Eg is equal to the intercept of linear extrapolation to hv axis, and

the obtained Eg values for N-K2Ti4O9 and MIL-101 are found to be 3.18 and 3.49 eV, respectively. The valence bands of MIL-101 and N-K2Ti4O9 were also measured by valence band XPS, as shown in Fig. 4c [33]. The valence band of N-K2Ti4O9 is at about 2.57 eV, because the band-gap of N-K2Ti4O9 is 3.18 eV from the UV–vis absorption spectra, the conduction band minimum will occur at about  0.61 eV. Likewise, the valence band and conduction band of MIL-101 are at about 2.09 eV and 1.40 eV, respectively. Due to the direct result of the recombination of the free carriers, the photoluminescence (PL) emission spectra can be regarded as an effective approach to understand the separation efficiency of photogenerated electron-hole pairs. As shown in Fig. 4d, the PL emission spectra for samples under excitation at 320 nm were examined in the wavelength range of 340–800 nm, and the order of the PL spectra intensities is as follows: MIL-101 4physical mixture (1:1) 4N-K2Ti4O9/MIL-101 (1:1) 4 N-K2Ti4O9. As indicated, after combination with N-K2Ti4O9, the PL spectra intensity of MIL-101 is greatly decreased, which implies there may exist synergistic effect between them to inhibit the recombination of photogenerated electron-hole pairs.

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Fig. 4. (a) The XRD patterns of MIL-101, N-K2Ti4O9 and N-K2Ti4O9/MIL-101(1:1); (b) the UV–vis absorption spectra of MIL-101 and N-K2Ti4O9 (inset shows the band-gaps of MIL-101 and N-K2Ti4O9 ); (c) the Valence band XPS spectra of MIL-101 and N-K2Ti4O9 and (d) PL spectra of MIL-101, N-K2Ti4O9 and NK2Ti4O9/MIL-101(1:1).

3.2. Adsorption activity of N-K2Ti4O9, MIL-101 and the composites N-K2Ti4O9 displays poor adsorption capacity of RhB in contrast with MIL-101 (Fig. 5). This is due to a large BET surface area and the micropore structure of MIL-101. Meanwhile, the adsorption of organic dyes onto metal-organic frameworks, for instance, MIL-101 has a great advantage as compared with that onto inorganic metal oxides such as NK2Ti4O9, because there exists various interactions like pi–pi stacking, hydrogen bonding, etc., between aromatic rings of RhB and MIL-101. As shown in Fig. 5, the adsorption activities of composites are between those of N-K2Ti4O9 and MIL-101, and increase with the increasing of the MIL-101 content. 3.3. Photocatalytic activity of MIL-101, N-K2Ti4O9 and the composites The photocatalytic activities of MIL-101, N-K2Ti4O9 and the composites were evaluated using the degradation of

Fig. 5. Dependence of adsorption capacity qt of RhB on time t for NK2Ti4O9, MIL-101 and the composites with different mass ratios of NK2Ti4O9 to MIL-101.

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RhB under visible light irradiation. The concentration Ct of RhB was measured at irradiation time t. Owing to small BET surface area of N-K2Ti4O9 and poor separation efficiency of photogenerated electron-hole pairs of MIL-101, both MIL-101 and N-K2Ti4O9 show low photocatalytic activity as shown in Fig. 6a by comparison with the NK2Ti4O9/MIL-101(1:1) composite. The photocatalytic activities of the composites with different mass ratios of N-K2Ti4O9 to MIL-101 were further studied. It can be seen that the mass ratio of 1:1 is the optimum value to achieve high photodegradation activity (Fig. 6b). The pseudo-first-order kinetics can simulate well the photocatalytic reaction process, and the reaction constant fitted (K) is 0.2590 h  1 for N-K2Ti4O9/MIL-101(1:1), whereas for N-K2Ti4O9, it is only 0.1554 h  1. In order to evaluate quantitatively the synergistic effect, the synergistic factor is proposed and calculated using the following equation [34]: SF ¼

K N  K 2 Ti4 O9 =MIL  101ðx:yÞÞ K N  K 2 Ti4 O9  Y þK MIL  101  ð1  Y Þ

ð4Þ

121

Where K N  K 2 Ti4 O9 =MIL  101ðx:yÞÞ ,K N  K 2 Ti4 O9 and K MIL  101 are the photocatalytic reaction constants of N-K2Ti4O9/MIL-101 (x:y), N-K2Ti4O9 and MIL-101, respectively, and Y is the content of N-K2Ti4O9 (Table 1) in the composite. The calculated synergistic factors of the composites are generally greater than 1. In particular, for N-K2Ti4O9/MIL-101(1:1), the synergistic factor is 1.56. The K, t1/2 (half-life) values and synergistic factors of N-K2Ti4O9, MIL-101 and the composites are summarized in Table 2. In order to confirm that RhB was not photodegraded by itself, a controexperiment was carried out, and the result shows that there is no noticeable change in RhB concentration after 2.5 h stirring under visible light irradiation without photocatalyst. Also, another control experiment shows that RhB was not photodegraded on photocatalyst after 2.5 h without visible light irradiation. These indicate that the photobleaching can only happen under the existence of both irradiation and photocatalyst, and is not the result of the autocatalysis. (Fig. 6c).

Fig. 6. Kinetics of RhB photodegradation on: (a) MIL-101, N-K2Ti4O9 and N-K2Ti4O9/MIL-101(1:1), (b) the composites with different mass ratios of NK2Ti4O9 to MIL-101, (c) N-K2Ti4O9/ MIL-101 (1:1) with irradiation and (or) photocatalyst, and (d) the recycled N-K2Ti4O9/MIL-101(1:1).

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The active oxygen species O2∙  , HOO∙ and ∙OH radicals have taken part in the degradation of RhB. Meanwhile, the photogenerated holes in the VB of MIL-101 can directly destroy the adsorbed RhB or react with H2O to yield ∙OH radicals. In summary, a possible mechanism of photocatalysis is proposed. Compositing MIL-101 with N-K2Ti4O9 can enhance the adsorption of RhB from solution. Then the excited state electrons in LUMO of RhB can readily migrate to CB of NK2Ti4O9, and react with dissolved O2 to yield active oxygen species, which will degrade RhB. Meanwhile, N-K2Ti4O9 absorbs visible light through nitrogen doping induced impurity level to produce photogenerated electron-hole pairs, and the holes can migrate to the VB of MIL-101, and destroy the adsorbed RhB or react with H2O to yielded ∙OH radicals.

Table 2 Photocatalytic degradation kinetic values for RhB. Sample

K  102/h

t1/2/h

Synergistic factor

N-K2Ti4O9 MIL-101 N-K2Ti4O9/MIL-101(2:1) N-K2Ti4O9/MIL-101(3:2) N-K2Ti4O9/MIL-101(1:1) N-K2Ti4O9/MIL-101(2:3) N-K2Ti4O9/MIL-101(1:2)

15.537 18.585 21.412 23.564 25.903 20.117 16.732

4.461 3.730 3.237 2.942 2.676 3.446 4.143

/ / 1.26 1.39 1.56 1.23 1.04

4. Conclusions In conclusion, the N-K2Ti4O9/MIL-101 composites with various N-K2Ti4O9/MIL-101 ratios were synthesized by a facile hydrothermal method. Compositing MIL-101 with N-K2Ti4O9 is beneficial for promoting the photodegradation of RhB. This is due to big adsorption capacity of MIL-101 and high separation efficiency of photogenerated electron-hole pairs through interfaces between N-K2Ti4O9 and MIL-101. In particular, N-K2Ti4O9/ MIL-101 (1:1) exhibits the best photocatalytic activity among the composites, and the synergistic factor is 1.56.

Fig. 7. Mechanism diagram of the RhB photodegradation.

The regeneration of the photocatalyst is one of the important steps for practical applications. The stability of NK2Ti4O9/MIL-101(1:1) was investigated, and after each photodegradation, it was separated from solution by centrifuge, and can be reused without considerable amount of mass loss. As shown in Fig. 6d, after three cycles, photodegradation kinetics constants is 92.71% of the first cycle.

Acknowledgments The financial supports by the Anhui Provincival Natural Science Foundation (No. 1508085MB28), the National Natural Science Foundation of China (Grant. 51372062). References

3.4. Photodegradation mechanism of RhB Based on the calculated band-gaps and valence bands of MIL-101 and N-K2Ti4O9, the mechanism of synergistic effect can be drawn in Fig. 7. RhB has charge-transfer excitation-like transition from the HOMO to the LUMO, and its excited photogenerated electrons can transfer from the LUMO to the conduction band (CB) of N-K2Ti4O9. Meanwhile, the holes on valence band(VB) of N-K2Ti4O9 can transfer to VB of MIL-101 after N-K2Ti4O9 was excited by visible light. The impurity level (IL) is introduced from nitrogen doping to facilitate absorption of visible light [35]. These are advantageous for the separation and transferring of photogenerated electronhole pairs through the interface between N-K2Ti4O9 and MIL101, which can increase the photocatalytic activity. Then, the dissolved O2 captures the photogenerated electron at CB of N-K2Ti4O9 to yield first the superoxide radical anion, O2∙  , and then the HOO∙ radical upon protonation. The ∙OH radical can be produced from the trapped electron after formation of the HOO∙ radical by following equations [36]. HOO þ H þ þ e  -H2 O2

ð5Þ

H2 O2 þ e  -OHþ OH 

ð6Þ

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