BiOCl composites with enhanced photocatalytic activity

Materials Characterization 99 (2015) 8–16

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Low temperature one-step synthesis of rutile TiO2/BiOCl composites with enhanced photocatalytic activity Fangfang Duo, Yawen Wang ⁎, Caimei Fan ⁎, Xiaoming Mao, Xiaochao Zhang, Yunfang Wang, Jianxin Liu College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China

a r t i c l e

i n f o

Article history: Received 24 July 2014 Received in revised form 26 October 2014 Accepted 2 November 2014 Available online 4 November 2014 Keywords: Rutile TiO2/BiOCl Low temperature Photocatalyst Composites

a b s t r a c t The rutile TiO2/BiOCl composites were successfully fabricated by a facile one-step hydrolysis method at low temperature (50 °C). The X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV–vis diffuse reflectance spectra, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller, and X-ray photoelectron spectroscopy measurements were employed to characterize the phase structures, morphologies, optical properties, surface areas, and electronic state of the samples. The rutile TiO2/BiOCl composites exhibited higher photocatalytic activity than pure BiOCl and rutile TiO2 for the degradation of phenol under artificial solar light irradiation. In addition, the photocatalytic mechanism has also been investigated and discussed. The enhanced photocatalytic performance of rutile TiO2/BiOCl composites is closely related to the heterojunctions between BiOCl and rutile TiO2, which can not only broaden the light adsorption range of BiOCl but also improve the electron–hole separation efficiency under artificial solar light irradiation. © 2014 Elsevier Inc. All rights reserved.

1. Introduction With the ever-increasing interest in environmentally friendly processes and renewable energy sources, photocatalysis has recently attracted a great deal of attention [1]. Bismuth-based photocatalytic materials have recently aroused great interest in the scientific community due to their intriguing electronic structures, such as BiOX (X = Cl, Br, I) [2–5], Bi2WO6 [6,7], and BiVO4 [8,9]. Among these compounds, BiOX (X = Cl, Br, I) with layered structures shows great promise owing to its mechanical robustness, and outstanding photocatalytic activities. In particular, BiOCl which has the internal structure of [Bi2O2]2+ layers interleaved by double slabs of Cl ions has been demonstrated to show remarkable photocatalytic performance under ultraviolet light irradiation [10–12]. However, due to its large indirect band gap of about 3.40 eV, the pure BiOCl has limited photocatalytic activity under sunlight. Moreover, the rapid recombination of photoinduced electrons and holes greatly lowers the quantum efficiency [13]. Therefore, it is of great need to develop effective ways to broaden the light adsorption region and improve the charge separation efficiency of BiOCl. Many studies have highlighted the way of coupling a narrow band-gap semiconductor with a wide band-gap semiconductor (with the proper band positions) to enhance the charge separation efficiency [14–20]. There have been several types of composite semiconductors about BiOCl, such as BiOI/BiOCl [21], BiOCl/Bi2O2CO3 [22], BiOCl/SrFe12O19 [23], In2O3/BiOCl [24], and TiO2/BiOCl [25–28]. Among them, the composites about TiO2/BiOCl are all between anatase TiO2 and BiOCl. It is well ⁎ Corresponding authors. E-mail address: [email protected] (Y. Wang).

http://dx.doi.org/10.1016/j.matchar.2014.11.002 1044-5803/© 2014 Elsevier Inc. All rights reserved.

known that anatase TiO2 as one of the most promising photocatalyst has been widely investigated for its excellent photocatalytic performance under UV light and attractive applications to environmental problems [29]. In fact, TiO2 have three phases: anatase, rutile and brookite. Among them, rutile is the most stable phase even in strongly acidic or basic conditions [30–32]. It is generally accepted that anatase TiO2 exhibits higher photocatalytic activity than rutile TiO2. The main reason is that most of the rutile TiO2 is fabricated by calcining anatase TiO2 over 700 °C. The high temperature calcination usually leads to large particle size and low surface area, which is disadvantageous for the photocatalytic performance of rutile TiO2 [33]. In recent studies, rutile fabricated at low temperature (150 °C) was proven to have higher photocatalytic activity than anatase TiO2 [34]. Rutile TiO2 (2.9 eV) has more narrow band gap than anatase TiO2 (3.2 eV), which is beneficial for absorbing more visible light in sunlight [28]. However, there has no report about rutile TiO2/BiOCl composites yet. In this study, we present a facile one-step hydrolysis method to synthesize rutile TiO2/BiOCl composites at a low temperature (50 °C). The method is quite convenient, energy-saving and self-sustainable for the large-scale synthesis. The rutile TiO2/BiOCl composites exhibited superior photocatalytic activity for the decomposition of phenol than pure BiOCl and rutile TiO2. 2. Experimental sections 2.1. Materials and preparation All the reagents were of analytical grade and used as received without further purification. In a typical synthesis, different stoichiometric

F. Duo et al. / Materials Characterization 99 (2015) 8–16

amounts of bismuth chloride (BiCl3) were added slowly into 5 mL of absolute ethanol solution containing a certain amount of titanium tetrachloride (TiCl4). The obtained mixtures were stirred for about 0.5 h at room temperature in air until receiving clarifying solution. Then 20 mL of deionized water was added to the mixture drop by drop. After being stirred for another 0.5 h, the resulting solution was maintained in a closed system at 50 °C in an oven for 24 h, resulting in a white precipitate. Finally, the precipitates were collected and washed with distilled water thoroughly and dried in an oven at 50 °C. The obtained sample were labeled as x% TiO2/BiOCl, where x% is the mole percent of rutile TiO2 to BiOCl. For comparison, pure BiOCl and rutile TiO2 powders were also prepared by using the same procedure. 2.2. Characterization Material characterizations including powder X-ray diffraction (XRD) patterns were measured on a D/max-2500 diffractometer with Cu Kα radiation. Fourier transform infrared spectroscopy (FT-IR) analysis was carried out on a Shimadzu 8400 spectrophotometer. The morphology was characterized using a 430 field emission scanning electron microscope (SEM, America). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were recorded on a JEOL-2011 microscope (Japan, 200 kV). A NOVA 2000 e (Quantachrome Instruments, USA) instrument was used to measure the Brunauer–Emmett–Teller (BET) surface areas of the samples at liquid nitrogen temperature (77.3 K). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 Xi XPS system with an Al Ka (hv = 1486.6 eV) 150 W, 500 μm beam spot source. All the binding energies were referenced to the adventitious C1s peak at 284.8 eV. UV–visible diffused reflectance spectra (DRS) of the samples were obtained for the dry-pressed film samples using a UV–visible spectrophotometer (UV-3600, Shimadzu, Japan), and BaSO4 was used as a reflectance standard in a UV–visible diffuse reflectance experiment. The photoluminescence (PL) spectra were measured using a Cary-300 spectrofluorometer (Agilent). The thermal stability of the photocatalyst was tested on a Netzsch-STA-409 C thermogravimetric analysis meter. Sample was heated from room temperature to 800 °C at a rate of 10 °C min− 1 with the nitrogen flowing of 80 mL min−1. 2.3. Photocatalytic activity measurements Photocatalytic activities of the samples were evaluated by the degradation of phenol under artificial solar light irradiation. The optical system used for the photocatalytic reaction consisted of a 500 W Xe lamp. The distance between the light and the center of the beaker was 20 cm. The intensity of light was 119.2 klx measured by a digital light meter. Prior to illumination, 100 mL of 10 mg/L phenol solution containing 0.10 g of photocatalyst was magnetically stirred at 25 °C in a water bath for 30 min to establish an adsorption–desorption equilibrium. At irradiation time intervals of 5 min, 4 mL of the suspension was collected, then centrifuged (7000 rpm, 2 min) to remove the photocatalyst particles. A Varian Cary 50 probe UV–vis spectrometer was used to record the change of the concentration of phenol during light irradiation. 3. Results and discussion 3.1. XRD patterns Fig. 1a shows the XRD patterns of BiOCl, rutile TiO2, and rutile TiO2/ BiOCl composites. The XRD patterns for BiOCl and rutile TiO2 show that all the diffraction peaks can be well indexed to tetragonal structure of BiOCl (JCPDS file No. 82–485) and tetragonal rutile TiO2 (JCPDS file No. 86–147), respectively. For sample 15% TiO2/BiOCl, 25% TiO2/BiOCl and 50% TiO2/BiOCl, only 50% TiO2/BiOCl contains both BiOCl (JCPDS

9

file No. 82–485) and rutile (JCPDS file No. 86–147), indicating the coexistence of BiOCl and rutile TiO2, as shown in Fig. 1b. But the intensity of rutile TiO2 is quite low, which may be derived from the weak crystallinity intensity of the rutile TiO2 with small particles which was prepared at a low temperature (50 °C) [28]. For sample 15% TiO2/BiOCl and 25% TiO2/BiOCl, there are only characteristic peaks of BiOCl (JCPDS file No. 82–485), and the peaks belonging to rutile TiO2 can't be seen. 3.2. Fourier transformation infrared spectroscopy (FT-IR) FT-IR spectra of BiOCl, rutile TiO2, and rutile TiO2/BiOCl composites are shown in Fig. 2. In the spectrum of pure BiOCl, the peak at 519 cm−1 can be assigned to the Bi–O bond stretching vibration [1–3]. The peak at 620 cm−1 is attributed to the Ti–O bond vibrations in rutile TiO2 [35]. Obviously, for the rutile TiO2/BiOCl composites, the peak intensity of Ti–O bond increased slightly with the increase of rutile TiO2 content from 15 to 50% [36], indicating the presence of rutile TiO2 in rutile TiO2/BiOCl composites. The wide band at 3440 cm−1 should originate from the absorption of water or O–H groups [37]. It appears that the O–H stretching vibration became shifted to relatively lower wavenumber in rutile TiO2/BiOCl composites, which could be attributed to the replacement of water molecules into rutile TiO2 on the surface of BiOCl [35]. Therefore, although any characteristic peaks of rutile TiO2 can't be seen from the XRD results for sample 15% TiO2/BiOCl and 25% TiO2/BiOCl, the FT-IR data suggests the coexistence of BiOCl and rutile TiO2 in them. The conclusion is further demonstrated via XPS measurements. 3.3. X-ray photoelectron spectroscopy (XPS) Fig. 3 shows the corresponding high-resolution spectra of the binding energy peaks of Bi, Ti, and O. From Fig. 3a, it can be seen that the bands located at binding energies of 159.6 eV and 164.9 eV are assigned to the Bi (4f5/2) and Bi (4f7/2) spin-orbital splitting photoelectrons in the BiOCl chemical state, respectively [38]. For 25% TiO2/BiOCl, the two peaks of Bi (4f5/2) and Bi (4f7/2) were moved to 159.3 eV and 164.6 eV with an energy spacing of 5.3 eV over the entire range of compounds, which indicate the change of chemical surroundings for Bi3+ [39,40]. The binding energies of Ti 2p3/2 and Ti 2p1/2 peaks in sample rutile TiO2 are 458.3 eV and 464.2 eV, respectively, which are attributed to the Ti–O bands in rutile TiO2 as shown in Fig. 3b [41]. However, the Ti 2p peak of 25% TiO2/BiOCl can be divided into four peaks. The binding energies of Ti 2p3/2 and Ti 2p1/2 peaks are 458.9 eV and 465.0 eV, shifting to higher binding energies than those of the rutile TiO2. The shift of the peaks for Ti 2p3/2 and Ti 2p1/2 probably originates from the formation of Ti–O–Bi bond. The other two peaks at 461.8 eV and 466.7 eV can be attributed to the Ti–Cl bond [42,43]. The shift of the peaks for Bi 4f and Ti 2p and the presence of Ti–Cl indicate that the rutile TiO2 and BiOCl is connected by strong chemical bond, rather than a simple mixture by two materials in rutile TiO2/BiOCl composites. The O 1 s peaks located at 529.6 eV should be ascribed to the Ti–O bond of TiO2 in sample rutile TiO2, as displayed in Fig. 3c [41]. For sample BiOCl, the two peaks located at 530.4 eV and 532.7 eV correspond to the Bi–O bonds in [Bi2O2]2 + slabs of the BiOCl layered structure and the O–H bonds of the surface adsorbed water, respectively [44–47]. As for the O 1 s peaks located at 530.0 eV and 529.4 eV in sample 25% TiO2/BiOCl, it may be the effect of both Bi–O and Ti–O bonds among BiOCl and rutile TiO2, respectively. From the above analysis, we can conclude that the compositions of the synthesized rutile TiO2/BiOCl composites were determined by surface analysis means. 3.4. SEM and TEM images The scanning electronic microscopy (SEM) images of the asprepared products are shown in Fig. 4a–e. The pure BiOCl (Fig. 4a) is composed of bulk plates with lengths of 0.5–2 μm, thickness of

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F. Duo et al. / Materials Characterization 99 (2015) 8–16

6000

202 211 104

103 201

003 112

110

002 101

102

* rutile TiO2

BiOCl

3000

15% TiO2/BiOCl

2000 1000

25% TiO2/BiOCl

211

rutile TiO2

002

50

111

110

100

50% TiO2/BiOCl

*

0

10

20

30

40

50

221 301

*

0 150

101

Intensity (a.u.)

4000

001

(a) 5000

60

70

80

2 Theta (degrees) 1500

(b)

1000

BiOCl

500

Intensity (a.u.)

0 300

50% TiO2/BiOCl

150

* 0 100

*

*

rutile TiO2

50

*

0 26

28

30

32

34

36

38

2 Theta (degree) Fig. 1. X-ray diffraction (XRD) patterns of the rutile TiO2/BiOCl samples (a), and the XRD diffraction peaks from 25° to 38° of sample BiOCl, rutile TiO2 and 50% TiO2/BiOCl (b).

91–273 nm, while for the pure rutile TiO2, it consists of irregularly sized plush pellets with the diameters between 0.07 μm and 0.87 μm (Fig. 4e). For the composites, there are both plates and plush pellets, and the plush pellets were apparently increased with the increase of rutile TiO2. Even to the sample 50% TiO2/BiOCl, the plush pellets are too many to see the plates. In addition, the specific surface areas of the obtained samples were measured, and the surface area values are 2.18, 12.03, 25.30, 124.62, and 148.65 m2 g−1 for BiOCl, 15% TiO2/BiOCl, 25% TiO2/BiOCl, 50% TiO2/BiOCl, and rutile TiO2, respectively, as shown in Table 1. Obviously, the rutile TiO2/BiOCl composites possess the higher surface areas than individual BiOCl. The transmission electron microscopy (TEM) image of 25% TiO2/BiOCl (Fig. 4f) reveals that the sample is composed of plates and plush pellets. The inserted image also suggests that the lattice spacing of 0.344 nm for plates is corresponding to the [101] plane of BiOCl, while the lattice spacing of 0.325 nm for plush pellets match the [110] plane of tetragonal

rutile TiO2. The SEM and TEM results further demonstrate the coexistence of BiOCl and rutile TiO2 in rutile TiO2 and BiOCl composites. 3.5. UV–vis DRS analysis Fig. 5a shows a comparison of UV–vis diffuse absorption spectra of BiOCl, rutile TiO2, and rutile TiO2/BiOCl composites. From the inserted image of Fig. 5a, it can be clearly seen that pure BiOCl and rutile TiO2 have an absorption edge at about 364 nm and 403 nm, respectively. Obviously, the rutile TiO2/BiOCl composites exhibit two absorption edges at both 405 nm and 410 nm (Table 1), implying a combination of the optical absorption property of BiOCl and rutile TiO2 [48,49]. The band gap energy of a semiconductor could be calculated by the following formula:  n=2 αhν ¼ A hν−Eg

ð1Þ

F. Duo et al. / Materials Characterization 99 (2015) 8–16

11

can be inferred that the introduction of rutile TiO2 is beneficial to improve the absorption of visible light of BiOCl under artificial solar light irradiation.

BiOCl 15% TiO2/BiOCl 25% TiO2/BiOCl

Transmittance

3.6. TG-DTG analysis 50% TiO2/BiOCl

Results of thermogravimetric (TG) and differential thermogravimetric (DTG) analyses are shown in Fig. 6, which represent dynamic weight loss profile of sample 25% TiO2/BiOCl from room temperature to 800 °C. From the TG curve, two different sharp decreases of 5.6 and 7.8 wt.% can be observed and they occur in the temperature range of 25–100 °C and 700–800 °C, respectively. The first weight loss step can be explained by the release of physisorbed water on the surface of the catalyst. The second one may originate from the decomposition of BiOCl [52]. In addition, there is a slight loss between 100 and 200 °C in the TG curve, which may be caused by the evaporation of chemisorbed water and alcohol [53]. Another weight loss, with a gentle decline over weight loss of 10.8 wt.% is observed in the range of 100–700 °C. The DTG curve indicates that two main weight losses are centered at 31 and 759 °C.

TiO2

Bi-O

O-H

Ti-O

400

800 1200 1600 2000 2400 2800 3200 3600 4000 Wavenumber (cm)-1

Fig. 2. FT-IR spectra of BiOCl, 15% TiO2/BiOCl, 25% TiO2/BiOCl, 50% TiO2/BiOCl, and rutile TiO2.

where α, hν, Eg, and A are the absorption coefficient, the photon energy, the band gap, and a constant, respectively. Among them, n depends on the characteristics of the transition in a semiconductor, namely, direct transition (n = 1) or indirect transition (n = 4). For TiO2 and BiOCl, the value of n is 4 for the indirect transition [50]. Therefore, the band gap energy (Eg value) of the resulting samples can be estimated from a plot of (αhν)1/2 versus photon energy (hν) [51]. The intercept of the tangent to the X axis would give a good approximation of the band gap energy of the samples (Fig. 5b). The band gap of pure BiOCl and rutile TiO2 is roughly estimated to be 3.29 eV and 2.96 eV, respectively, and the other estimated band gap energies are 2.91, 2.92, 2.90 eV of 15%, 25%, 50% TiO2/BiOCl, respectively, as summarized in Table 1. So it

Bi 4f

Bi 4f7/2

The photocatalytic activity and adsorption property of the asprepared samples were investigated by the degradation of phenol under artificial solar light irradiation. Prior to light irradiation, the adsorption property was conducted in darkness until adsorption–desorption equilibrium was achieved between phenol and the catalyst. As shown in Fig. 7a, the adsorption efficiency of phenol in darkness is 0.33%, 0.38%, 0.32%, 0.76%, and 0.68% for BiOCl, 15% TiO2/BiOCl, 25% TiO2/BiOCl, 50% TiO2/BiOCl and rutile TiO2, respectively, indicating the removal of phenol could be negligible in the dark.

(b)

Bi 4f5/2

159.3 eV

Relative intensity (a.u.)

164.6 eV 25% TiO2/BiOCl

159.6 eV 164.9 eV

BiOCl 158

160

162

164

166

168

(c)

465.0 eV

466.7 eV

458.9 eV 461.8 eV 25% TiO2/BiOCl

458.3 eV 464.2 eV rutile TiO2

450

455

Binding energy (eV)

460

465

Binding energy (eV) 530.4 eV

O1s

Relative intensity (a.u.)

156

Ti 2p

Relative intensity (a.u.)

(a)

3.7. Enhanced photocatalytic activity of rutile TiO2/BiOCl composites

BiOCl 532.7 eV

529.4 eV

530.0 eV 25% TiO2/BiOCl

529.6 eV

rutile TiO2

526

528

530

532

534

536

538

540

Binding energy (eV) Fig. 3. XPS high resolution spectra of BiOCl, 25% TiO2/BiOCl, and rutile TiO2: (a) Bi 4f (b) Ti 2p, and (c) O 1 s.

470

475

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F. Duo et al. / Materials Characterization 99 (2015) 8–16

(a)

(b)

(c)

(d)

(e)

(f)

BiOCl [101] d=0.344 nm

TiO2 [110] d=0.325 nm2 nm

Fig. 4. The SEM images of (a) BiOCl, (b) 15% TiO2/BiOCl, (c) 25% TiO2/BiOCl, (d) 50% TiO2/BiOCl, (e) rutile TiO2 and the TEM image of sample (f) 25% TiO2/BiOCl.

Fig. 7b reveals the variation of phenol concentration (C/C0) with photo-degradation time over different photocatalysts. It is observed that no phenol molecule in the absence of catalyst could be decomposed due to its chemical stability. 37%, 50%, 93%, 44% and 22% degradation of phenol were observed after 30 min irradiation for sample BiOCl, 15% TiO2/BiOCl, 25% TiO2/BiOCl, 50% TiO2/BiOCl and rutile TiO2, respectively. It can be seen that all the rutile TiO2/BiOCl composites exhibite higher photocatalytic activity than pure BiOCl and rutile TiO2. With increasing the percentage of rutile TiO2, the photocatalytic performance enhanced gradually and then decreased, and the optimum mole percentage of rutile TiO2 in composites is 25%. These results suggest that the rutile TiO2/

BiOCl composites with suitable proportion can improve the photocatalytic activity of pure BiOCl and rutile TiO2. Reusability is one of the most important factors in researching catalysts. Sample 25% TiO 2 /BiOCl was prepared for reuse experiments by washing in distilled water and drying at 50 °C. When 25% TiO2/BiOCl is applied to decomposition of phenol with initial concentration of 10 mg/L, removal percentage reaches 93% for the first time, after reusing the same 25% TiO2/BiOCl sample five times, removal percentage is about 87% (Fig. 7c). These results indicate that this photocatalyst has excellent stability and maintains high photocatalytic efficiency in the fifth cycle.

Table 1 Solid and optical properties of rutile TiO2/BiOCl composites: specific surface areas, absorption band-edge and energy band gap (Eg). Samples

BiOCl

15% TiO2/BiOCl

25% TiO2/BiOCl

50% TiO2/BiOCl

Rutile TiO2

Specific surface areas (m2 g−1) Absorption band-edge (nm) Energy band gap Eg (eV)

2.18 364 3.29

12.03 407 2.91

25.30 405 2.92

124.62 410 2.90

148.65 403 2.96

F. Duo et al. / Materials Characterization 99 (2015) 8–16

BiOCl 15% TiO2/BiOCl

1.4

25% TiO2/BiOCl

rutileTiO2

50% TiO2/BiOCl

0.5

rutile TiO2

0.8

1/2

0.8

BiOCl 15% TiO2/BiOCl

1.2

50% TiO2/BiOCl

(eV)

1.0

1.6

25% TiO2/BiOCl

1.0

1.2

Absorbanc (a.u.)

0.6 0.4

(ah ν)

Absorbanc（ a.u.）

(b)

1.6

1/2

(a)

13

0.0 340

360

380

400

420

0.4

Wavelength (nm)

0.2 0.0 200

0.0 300

400

500

600

700

800

2.8

3.0

3.2

3.4

3.6

Photo energy (eV)

Wavelength (nm)

Fig. 5. (a) UV–vis diffuse absorption spectra and (b) the plots of (ahν)1/2 vs. photo energy of rutile TiO2/BiOCl composites.

3.8. Reactive oxygen species Reactive species trapping experiments were performed to investigate the reactive oxygen species in the photocatalytic process as shown in Fig. 8. In this study, three different chemicals, sodium hydrogen carbonate (SHC, a hole scavenger, 10 mM), isopropyl alcohol (IPA, a •OH radical scavenger, 10 mM) [54], and N2 (sweeping away the dissolved oxygen) are employed. The experimental results show that the addition of SHC obviously reduced the photocatalytic activity for the degradation of phenol from 93.1% to 36.6% in 30 min, demonstrating that holes may be the primary reactive species during the photocatalytic process. The marked reduced photocatalytic activity of 25% TiO2/BiOCl by the introduction of IPA also indicates that •OH radical primarily acts as an efficient reactive species. The presence of N2 existing condition shows a marked inhibition in the photocatalytic process. Namely, the dissolved oxygen also plays a comparatively pivotal role for phenol degradation. Therefore, the results indicate that •OH radical originated by holes plays an important role and electrons also have a function in the photocatalytic experiment.

BiOCl, rutile TiO2, and rutile TiO2/BiOCl composites with an excitation wavelength of 384 nm. The main emission peak center at visible light area about 597 nm can be ascribed to the band gap recombination of electron–hole pairs [55,56]. The emission intensity of PL spectra for the rutile TiO2/BiOCl composites weakens with increasing the content of rutile TiO2 up to 25% then enhances with further increasing the amount of the rutile TiO2 in rutile TiO2/BiOCl composites. It is found that the PL data of the rutile TiO2/BiOCl composites with suitable proportion is very effective for the suppression of electron–hole pair recombination, which is a necessary factor for improving the degradation of phenol. 3.10. Possible mechanism of the improved photocatalytic activity of the rutile TiO2/BiOCl composites

3.9. PL spectra

It is well-known that the photocatalytic activities of photocatalysts depend on the generation, transfer and separation of photo-generated electron–hole pairs. In order to clearly understand the formation of rutile TiO2/BiOCl composites, the initial energy band structures of BiOCl and rutile TiO2 were provided. The band positions of BiOCl and rutile TiO2 were calculated by the following empirical formulas [57]:

To further support the proposed photocatalytic mechanism, Fig. 9 shows the photoluminescence emission spectra (PL) spectra for pure

EVB ¼ X−E þ 1=2 Eg

e

ð2Þ

Thermogravimetry (%)

100

Thermogravimetry 0.000 90

80 -0.001

Differential thermogravimetric 70

Differential thermogravimetric (%/min)

0.001

110

-0.002

60 100

200

300

400

500

600

700

800

Temperature ( C) Fig. 6. Thermogravimetric (TG) and differential thermogravimetric (DTG) curve of sample 25% TiO2/BiOCl.

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(a)

1.000

1.0

0.995

0.8

0.990

Ct/Ce

C/C0

0.6 0.985

BiOCl 15% TiO2/BiOCl

0.980

SHC + 25% TiO2/BiOCl IPA + 25% TiO2/BiOCl

25% TiO2/BiOCl 0.975

0.4

0.2

50% TiO2/BiOCl

N2 + 25% TiO2/BiOCl 25% TiO2/BiOCl

rutile TiO2 0.970

0.0 0

5

10

15

20

25

30

35

40

45

0

6

Time (min)

(b)

12

18

24

30

Time (min)

1.0 Fig. 8. Effects of sodium hydrogen carbonate (SHC), isopropyl alcohol (IPA), and N2 on the degradation of phenol over 25% TiO2/BiOCl under artificial solar light irradiation (Ct and Ce are the concentration of the reactant after adsorption equilibrium and illumination time t, respectively).

0.8

Ct/Ce

0.6

ECB of BiOCl were separately calculated to be 3.49 and 0.20 eV. Accordingly, the EVB and ECB of BiOCl and rutile TiO2 were obtained to be 2.88 and − 0.08 eV, respectively. The band position calculations suggest that pure BiOCl and rutile TiO2 have the stagger band structure before contact, which is advantageous to the separation of the photogenerated carriers, as shown in Fig. 10a. Fig. 10b is proposed to show the charge separation processes between BiOCl and rutile TiO2. It is speculated that both BiOCl and rutile TiO2 could be activated when the rutile TiO2/BiOCl composites are exposed to artificial solar light. The photo-generated electrons on the surface of rutile TiO2 can migrate easily to the CB of BiOCl by the interface, and are further trapped by molecular oxygen to produce reactive •O− 2 or •OH [58]. Meanwhile, the photoinduced holes can migrate from the VB of BiOCl to that of rutile TiO2 across the interface, and then the holes remaining on the VB of rutile TiO2 would react with the reactants adsorbed on the surface of the photocatalyst to form •OH [59]. Therefore, rutile TiO2/BiOCl composites effectively reduce the recombination of photo-generated electrons and holes, and thus enhance the photocatalytic degradation efficiency. The analysis results are in good agreement with the above experiment phenomena of reactive species scavengers and PL results. Therefore, the improved photocatalytic activity of rutile TiO2/BiOCl composites may

BiOCl 15% TiO2/BiOCl

0.4

25% TiO2/BiOCl 50% TiO2/BiOCl 0.2

rutile TiO2 Photolysis

0.0 0

6

12

18

24

30

Time (min)

(c)

1.0

1 st

2 nd

3 rd

4 th

5 th

90

120

150

0.8

Ct/Ce

0.6

0.4

0.2

0.0 30

60

BiOCl 15% TiO2/BiOCl

Time (min)

ECB ¼ EVB −Eg

25% TiO2/BiOCl 50% TiO2/BiOCl

Intensity (a.u.)

Fig. 7. (a) The adsorption rate C/Co (Co is the original concentration of the reactant, C is the concentration of the reactant after adsorption) for phenol of rutile TiO2/BiOCl composites in the dark, (b) photocatalytic degradation rate Ct/Ce of phenol over as-synthesized samples under artificial solar light irradiation, and (c) the column dependence of photocatalytic degradation rate Ct/Ce on cycling time was obtained of sample 25% TiO2/BiOCl (Ct and Ce are the concentration of the reactant after adsorption equilibrium and illumination time t, respectively).

TiO2

ð3Þ

where EVB is the valence band (VB) potential, ECB is the conduction band (CB) potential; X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms; Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV); and Eg is the band gap energy of the semiconductor. The X values for TiO2 and BiOCl are 5.90 eV and 6.34 eV, respectively. Herein, the EVB and

580

590

600

610

620

Wavelength (nm) Fig. 9. Photoluminescence spectra of rutile TiO2/BiOCl composites with the excitation and emission wavelength of 384 and 597 nm, respectively.

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15

Fig. 10. Schematic diagrams for (a) energy bands of BiOCl and rutile TiO2 (b) the formation of a heterojunction and transfer of photo-induced electrons from rutile TiO2 to BiOCl.

mainly be caused by constructing a heterojunction between BiOCl and rutile TiO2, which can not only broaden the light adsorption range of BiOCl but also improve the electron–hole separation efficiency. 4. Conclusions The rutile TiO2/BiOCl composites were successfully fabricated by a facile one-step hydrolysis method at a low temperature (50 °C). The photocatalytic activities of rutile TiO2/BiOCl composites for decomposing phenol are obviously superior to pure BiOCl and rutile TiO2 under artificial solar light irradiation, and the phenol degradation percentage of 25% TiO2/BiOCl is 93%, obviously higher than 37% and 22% of BiOCl and rutile TiO2 after 30 min, respectively. In addition, reactive species trapping experiments reveals that the •OH originated by holes played a crucial part and the electrons also have a function during the photocatalytic process. The photoluminescence emission spectra results also suggest that the rutile TiO2/BiOCl composite with suitable proportion is very effective for the suppression of electron–hole pair recombination. The enhanced photocatalytic performance of rutile TiO2/BiOCl composites is closely related to the heterojunctions between BiOCl and rutile TiO2, which can not only broaden the light adsorption range of BiOCl but also improve the electron–hole pair separation efficiency under artificial solar light irradiation. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21176168, 21303116), and the Shanxi Province Science Foundation (2012081017). References [1] A. Kubacka, M. Fernández-García, G. Colón, Chem. Rev. 112 (2011) 1555–1614. [2] C.F. Guo, S. Cao, J. Zhang, H. Tang, S. Guo, Y. Tian, Q. Liu, J. Am. Chem. Soc. 133 (2011) 8211–8215. [3] H. Hamaed, M.W. Laschuk, V.V. Terskikh, R.W. Schurko, J. Am. Chem. Soc. 131 (2009) 8271–8279. [4] Z. Ai, W. Ho, S. Lee, J. Phys. Chem. C 115 (2011) 25330–25337. [5] X. Xiao, W.D. Zhang, J. Mater. Chem. 20 (2010) 5866–5870. [6] M. Shang, W. Wang, L. Zhang, S. Sun, L. Wang, L. Zhou, J. Phys. Chem. C 113 (2009) 14727–14731. [7] Y. Li, J. Liu, X. Huang, Nanoscale Res. Lett. 3 (2008) 365–371. [8] S. Kohtani, M. Koshiko, A. Kudo, K. Tokumura, Y. Ishigaki, A. Toriba, K. Hayakawa, R. Nakagaki, Appl. Catal. B Environ. 46 (2003) 573–586. [9] A. Iwase, A. Kudo, J. Mater. Chem. 20 (2010) 7536–7542.

[10] L.P. Zhu, G.H. Liao, N.C. Bing, L.L. Wang, Y. Yang, H.Y. Xie, CrystEngComm 12 (2010) 3791–3796. [11] J. Yu, B. Wei, L. Zhu, H. Gao, W. Sun, L. Xu, Appl. Surf. Sci. 284 (2013) 497–502. [12] X. Zhang, X. Liu, C. Fan, Y. Wang, Y. Wang, Z. Liang, Appl. Catal. B Environ. 132–133 (2013) 332–341. [13] S.Y. Chai, Y.J. Kim, M.H. Jung, A.K. Chakraborty, D. Jung, W.I. Lee, J. Catal. 262 (2009) 144–149. [14] U. Bach, D. Lupo, P. Comte, J.E. Moser, F. Weissortel, J. Salbeck, H. Spreitzer, M. Gratzel, Nature 395 (1998) 583–585. [15] A. Tang, Y. Jia, S. Zhang, Q. Yu, X. Zhang, Catal. Commun. 50 (2014) 1–4. [16] F.T. Li, Q. Wang, X.J. Wang, B. Li, Y.J. Hao, R.H. Liu, D.S. Zhao, Appl. Catal. B Environ. 150–151 (2014) 574–584. [17] C. Chen, W. Cai, M. Long, B. Zhou, Y. Wu, D. Wu, Y. Feng, ACS Nano 4 (2010) 6425–6432. [18] M. Lu, C. Shao, K. Wang, N. Lu, X. Zhang, P. Zhang, M. Zhang, X. Li, Y. Liu, ACS Appl. Mater. Interfaces 6 (2014) 9004–9012. [19] Y.C. Zhang, L. Yao, G. Zhang, D.D. Dionysiou, J. Li, X. Du, Appl. Catal. B Environ. 144 (2014) 730–738. [20] S. Bera, S.B. Rawal, H.J. Kim, W.I. Lee, ACS Appl. Mater. Interfaces 6 (2014) 9654–9663. [21] F. Dong, Y. Sun, M. Fu, Z. Wu, S. Lee, J. Hazard. Mater. 219 (2012) 26–34. [22] X. Zhang, T. Guo, X. Wang, Y. Wang, C. Fan, H. Zhang, Appl. Catal. B Environ. 150–151 (2014) 486–495. [23] T. Xie, L. Xu, C. Liu, J. Yang, M. Wang, Dalton Trans. 43 (2014) 2211–2220. [24] G.Q. Zhu, M. Hojamberdiev, K.I. Katsumata, N. Matsushita, K. Okada, P. Liu, J.P. Zhou, Adv. Mater. Res. 747 (2013) 635–638. [25] Y. Cai, P. Wang, Y. Ye, J. Liu, Z. Tian, Y. Liu, C. Liang, RSC Adv. 3 (2013) 19064–19069. [26] D. Sun, J. Li, L. He, B. Zhao, T. Wang, R. Ti, S. Yin, Z. Feng, T. Sato, CrystEngComm 16 (2014) 7564–7574. [27] W.Q. Fan, X.Q. Yu, S.Y. Song, H.Y. Bai, C. Zhang, D. Yan, C.B. Liu, Q. Wang, W.D. Shi, CrystEngComm 16 (2014) 820–825. [28] Z. Liu, X. Xu, J. Fang, X. Zhu, B. Li, Water Air Soil Pollut. 223 (2012) 2783–2798. [29] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C Photochem. Rev. 1 (2000) 1–21. [30] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. Dunlop, J.W.J. Hamilton, J.A. Byrne, K. O'Shea, M.H. Entezari, D.D. Dionysiou, Appl. Catal. B Environ. 125 (2012) 331–349. [31] E. Hosono, S. Fujihara, K. Kakiuchi, H. Imai, J. Am. Chem. Soc. 126 (2004) 7790–7791. [32] Y. Li, J. Liu, Z. Jia, Mater. Lett. 60 (2006) 1753–1757. [33] H. Zhang, X. Liu, Y. Li, Q. Sun, Y. Wang, B.J. Wood, P. Liu, D. Yang, H. Zhao, J. Mater. Chem. 22 (2012) 2465–2472. [34] M. Nag, P. Basak, S.V. Manorama, Mater. Res. Bull. 42 (2007) 1691–1704. [35] Z. Sun, C. Bai, S. Zheng, X. Yang, R.L. Frost, Appl. Catal. A Gen. 458 (2013) 103–110. [36] A. Adamczyk, E. Długoń, Spectrochim. Acta A Mol. Biomol. Spectrosc. 89 (2012) 11–17. [37] J. Wu, S. Bai, X. Shen, L. Jiang, Appl. Surf. Sci. 257 (2010) 747–751. [38] H. Gan, G. Zhang, Y. Guo, J. Colloid Interface Sci. 386 (2012) 373–380. [39] W. Yang, B. Ma, W. Wang, Y. Wen, D. Zeng, B. Shan, Phys. Chem. Chem. Phys. 15 (2013) 19387–19394. [40] H. Xu, H. Li, G. Sun, J. Xia, C. Wu, Z. Ye, Q. Zhang, Chem. Eng. J. 160 (2010) 33–41. [41] M. Ren, H. Yin, Z. Lu, A. Wang, L. Yu, T. Jiang, Powder Technol. 204 (2010) 249–254. [42] O. Akhavan, E. Ghaderi, J. Phys. Chem. C 113 (2009) 20214–20220. [43] S. Ntais, V. Dracopoulos, A. Siokou, J. Mol. Catal. A Chem. 220 (2004) 199–205. [44] J.M. Song, C.J. Mao, H.L. Niu, Y.H. Shen, S.Y. Zhang, CrystEngComm 12 (2010) 3875–3881.

16

F. Duo et al. / Materials Characterization 99 (2015) 8–16

[45] M.J. Clarke, M. Kastner, L. Podbielski, P. Fackler, J. Schreifels, G. Meinken, S. Srivastava, J. Am. Chem. Soc. 110 (1988) 1818–1827. [46] J. Shi, X. Wang, Energy Environ. Sci. 5 (2012) 7918–7922. [47] J. Jiang, X. Zhang, P. Sun, L. Zhang, J. Phys. Chem. C 115 (2011) 20555–20564. [48] F.J. Zhang, F.Z. Xie, S.F. Zhu, J. Liu, J. Zhang, S.F. Mei, W. Zhao, Chem. Eng. J. 228 (2013) 435–441. [49] Z. Chen, W. Wang, Z. Zhang, X. Fang, J. Phys. Chem. C 117 (2013) 19346–19352. [50] K.L. Zhang, C.M. Liu, F.Q. Huang, C. Zheng, W.D. Wang, Appl. Catal. B Environ. 68 (2006) 125–129. [51] M. Butler, J. Appl. Phys. 48 (1977) 1914. [52] Z.Q. Shi, Y. Wang, C.M. Fan, Y.F. Wang, G.Y. Ding, Trans. Nonferrous Metals Soc. China 21 (2011) 2254–2258.

[53] [54] [55] [56]

Y.H. Lin, T.C. Chiu, H.T. Hsueh, H. Chu, Appl. Surf. Sci. 258 (2011) 1581–1586. I.K. Konstantinou, T.A. Albanis, Appl. Catal. B Environ. 49 (2004) 1–14. L. Ge, C. Han, X. Xiao, L. Guo, Appl. Catal. B Environ. 142–143 (2013) 414–422. P. Gomathisankar, K. Hachisuka, H. Katsumata, T. Suzuki, K. Funasaka, S. Kaneco, ACS Sustain. Chem. Eng. 1 (2013) 982–988. [57] G. Dai, J. Yu, G. Liu, J. Phys. Chem. C 115 (2011) 7339–7346. [58] H. Cheng, B. Huang, X. Qin, X. Zhang, Y. Dai, Chem. Commun. 48 (2012) 97–99. [59] X. Zhang, T. Guo, X. Wang, Y. Wang, C. Fan, H. Zhang, Appl. Catal. B Environ. 150 (2014) 486–495.