Improved solar-driven photocatalytic performance of BiOI decorated TiO2 benefiting from the separation properties of photo-induced charge carriers

Solid State Sciences 52 (2016) 106e111

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Improved solar-driven photocatalytic performance of BiOI decorated TiO2 benefiting from the separation properties of photo-induced charge carriers Jianzhang Li a, **, Junbo Zhong a, *, Yujun Si a, Shengtian Huang a, Lin Dou a, Minjiao Li a, b, Yinping Liu a, Jie Ding a a

Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Pharmaceutical Engineering, Sichuan University of Science and Engineering, Zigong 643000, PR China Sichuan Provincial Academician (Expert) Workstation, Sichuan University of Science and Engineering, Zigong 643000, PR China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 October 2015 Received in revised form 20 December 2015 Accepted 22 December 2015 Available online 24 December 2015

In this work, BiOI decorated TiO2 photocatalysts were prepared in-situ by a facile hydrothermal method and characterized by X-ray diffraction (XRD), UV/Vis diffuse reflectance spectroscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and surface photovoltage (SPV) spectroscopy. The reactive radicals during the photocatalytic reaction were detected by scavenger experiments. BiOI/TiO2 composites exhibit higher performance than the pure TiO2 towards photocatalytic decolorization of methyl orange (MO) aqueous solution, when the molar ratio of Bi/Ti is 2%, the sample has the highest photocatalytic activity. The enhanced photocatalytic performance of BiOI/TiO2 could be ascribed to the separation properties of photo-induced charge carriers and strong interaction between BiOI and TiO2. Based on the observations, a Z-scheme charge separation mechanism was proposed. © 2015 Elsevier Masson SAS. All rights reserved.

Keywords: Semiconductor TiO2 Surface Photocatalytic performance Charge separation

1. Introduction The emission of dye wastewater has become an important environmental danger and initiated increasing attention [1e3]. Most of dyes and their degradation products may be carcinogenic and toxic to mammals [4,5], thus it is urgent to effectively treat the dye wastewater before discharging into environment. To settle this issue, various treatment approaches have been developed to remove dyes from aqueous solution. However, these processes are not effective to total removal of dyes and their intermediates. Therefore, developing efficient methods is a hotpot in environmental science and technology. Recently, semiconductor-based photocatalysis is considered to be a promising and prospective method [6,7]. Among the photocatalysts developed, TiO2 is one of the most widely used photocatalysts due to its high chemical stability, low cost, low toxicity,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (J. Zhong). http://dx.doi.org/10.1016/j.solidstatesciences.2015.12.020 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.

and excellent oxidation properties [6e8]. However, the potential application of TiO2-based photocatalysis has been limited by the high recombination of photo-induced charge carriers and wide bandgap [9,10]. Various approaches have been applied to boost the photocatalytic performance under sun light illumination. Construction of heterojuctions is a simple and effective method to promote the photocatalytic activity. Usually, construction of heterojuctions needs two different types of semiconductors with matched band structures. Bi-based visible light driven-photocatalysts have attracted increasing interesting [11e13]. Many Bi3þ-containing compounds hold a narrow band gap and exhibit high visible light photocatalytic activity because of the hybridized O 2p and Bi 6s2 valence bands. Among the Bi-based visible light driven-photocatalysts, BiOI has received special attention due to its outstanding visible light response. BiOI is a p-type semiconductor; the valence band (VB) and conduction band (CB) is 2.38 eV and 0.52 eV vs. normal hydrogen electrode (NHE), respectively. TiO2 is an n-type semiconductor, the VB and CB of TiO2 is 2.91 eV and 0.29 eV vs NHE, respectively [14e18]. It is clear that these two semiconductors have matched band structures, thus it is feasible to prepare BiOI/TiO2

J. Li et al. / Solid State Sciences 52 (2016) 106e111

hetero-structures. In fact, BiOI/TiO2 hetero-structures have been fabricated by many groups and the corresponding properties were investigated [14,19e23]. According to the principle of photocatalysis, if the photoinduced charges can be separated effectively before recombination, then charges migrate to the surface of semiconductor, which initiates the photocatalytic reactions. The holes can oxidize the organic pollutants or react with surface adsorbed water to generate hydroxyl radicals (OH), the electrons can either reduce the organic pollutants directly or can react with surface adsorbed O2 to produce superoxide radicals ð$O2  Þ. It is clear that the separation process of photo-induced charges can greatly influence the photocatalytic performance. The knowledge on the separation properties of photocatalysts is pivotal to understand the mechanism of photocatalysis, especially for heterojuctions. Although the preparation and photocatalytic activity of BiOI/ TiO2 hetero-structures have been investigated as mentioned above, the charge separation properties of BiOI/TiO2 photocatalysts have seldom concerned. The intent of this paper is study the charge separation properties of BiOI/TiO2 composites, the photocatalytic activities of composites were evaluated by decolorization of methyl orange (MO) aqueous solution. Based on the observations, the charge separation mechanism with a Z-scheme was suggested. 2. Experimental section 2.1. Preparation of photocatalysts All chemicals with analytical grade were supplied from Chengdu Kelong Chemical Reagent Factory and used as received. TiO2 was prepared as the method described in the reference [24], the powder was annealed at 723 K. BiOI/TiO2 hetero-structures were fabricated by a hydrothermal method. 2 g TiO2 was dispersed in 20 mL glacial acetic acid, then desired Bi(NO3)3$5H2O was added into the suspension system above under intensely stirring, the molar ratio of Bi/Ti was 1%, 2%, 3% and 4%, respectively. After Bi(NO3)3$5H2O was totally dissolved, 10 mL KI aqueous solution with different concentration was added dropwise to the above suspension system under stirring, forming brown precipitate. The above mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and maintained at 453 K for 24 h, then cooled to room temperature naturally. The resulting sample was collected by filtration, washed several times with distilled water and absolute ethanol, and then dispersed in absolute ethanol and dried at 333 K in air overnight. The sample with different molar ratios of Bi/Ti (1%, 2%, 3% and 4%, respectively) was named as 1%, 2%, 3% and 4%, respectively. BiOI was prepared as the procedure mentioned above without the presence of TiO2. TiO2 also was hydrothermally treated without the presence of Bi(NO3)3$5H2O and KI, and labeled as 0%. 2.2. Characterization of photocatalysts The BrunauereEmmetteTeller (BET) specific surface area parameters were analyzed on a SSA-4200 automatic surface analyzer. The X-ray diffractometer (XRD, DX-2600) was applied to study the crystal structure of the samples. The X-ray tube was operated at 40 kV and 25 mA. UVeVis diffuse reflectance spectra (DRS) were carried out on a spectrometer (TU-1907). The surface photovoltage (SPV) spectroscopy was a home-made instrument as described in reference [25]. Scanning electron microscope (SEM) images were taken with a Hitachis 4880 scanning electron microscope, using an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) (Tecnai TEM G2) was applied to study the microstructure of the samples using an accelerating voltage of 300 kV. Samples for TEM analysis were prepared by drying a drop of sample dispersion

107

in absolute EtOH on carbon-coated copper grids. X-ray photoelectron spectroscopy (XPS) measurements were performed on a XSAM 800 using Mg Ka at 12 kV and 12 mA. The X-ray photoelectron spectra were referenced to the C1s peak (BE ¼ 284.80 eV) resulting from adventitious hydrocarbon (i.e. from the XPS instrument itself) present on the sample surface. 2.3. Measurements of photocatalytic activity The photocatalytic experiments and scavenger experiments were followed the procedure described in reference [26]. In typical photocatalytic experiments, 50 mg of photocatalyst was added into 50 mL of MO solution (10 mg L1). The light source was a 500 W Xe lamp (simulated sun light) and the initial pH of MO solution was 7.0. After 25 min, the suspension was centrifuged to remove the photocatalyst particles for analysis. 3. Results and discussion Table 1 shows the specific surface area parameters. Considering the measurement error (±10 m2/g), the specific surface area parameters have no obvious difference, which may due to the low loading of BiOI. Fig. 1is the XRD patterns of the photocatalysts. As shown in Fig. 1, for the pure TiO2, the peaks can be readily assigned to the anatase phase (JCPDS No. 21-1272). For the pure BiOI, all strong peaks can be indexed to tetragonal phase (JCPDS No.10-0445). For the 1% sample, besides the peaks for TiO2, no other peaks were detected, which can be assigned to the low loading of BiOI and high dispersion of BiOI on the surface of TiO2. However, for the 2%, 3% and 4% samples, some strong peaks were observed, compared with the peaks of the pure BiOI, these peaks shift to lower angle, which demonstrates that there exists a strong interaction between TiO2 and BiOI. The results here demonstrate that the presence of TiO2 inhibits the growth of BiOI during the hydrothermal treatment, resulting in the shift of diffraction angles and disappear of some peaks; the detail mechanism needs to be investigated in the near future. The results of XRD indicate the coexistence of BiOI and TiO2 in the sample. Due to the partially overlap of diffuse reflectance spectra of composites, only the UVeVis diffuse reflectance spectra of 0% and 2% are shown in Fig. 2. The two spectra appear similar characteristics, indicating that the presence of BiOI has little effect on the spectrum of TiO2, which may due to the relative low loading of BiOI. From the results, it is reasonable to speculate that the response to light is not the primary factor to determine the difference in photocatalytic activity. Fig. 3a and Fig. 3b show that the 2% sample consists of irregular lump and sheet-shaped structures. Fig. 3c is a HRTEM image recorded from the white framed area indicated in Fig. 3b. The lattice fringes of 0.284 nm and 0.357 nm agree well with the spacing of the (102) plane of the BiOI and (101) plane of the TiO2, respectively. The results further demonstrate that the coexistence of BiOI and TiO2 in the sample, which fits well with the results of XRD. The X-ray photoelectron spectroscopy (XPS) was carried out to

Table 1 Specific surface parameter of photocatalysts. Photocatalyst

SBET (m2/g)

Pore volumes (cc/g)

Pore size(nm)

0% 1% 2% 3% 4%

51.4 50.6 49.5 45.1 43.4

0.0281 0.0276 0.0264 0.0249 0.0230

10.9 10.9 11.5 11.1 10.6

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J. Li et al. / Solid State Sciences 52 (2016) 106e111

(004)

Intensity (a.u.)

9000

6000

(002)

(102) (110) (013) BiOI

4% 3%

3000

2% 1%

0

0% 20

30

40

50

60

2 Theta (degree) Fig. 1. XRD patterns of photocatalysts.

80

Reflectance (%)

60

2% 0%

40

20

0

300

400

500

600

700

800

Wavelength (nm) Fig. 2. UVeVis diffuse reflectance spectra of photocatalysts.

investigate the surface chemical composition of TiO2 and BiOI/TiO2 composites as well as the valence states of various species. As shown in Fig. 4a, the spectrum of the 2% sample exhibits characteristic peaks for Ti, O, Bi, I and C. The Ti element resulted from the TiO2. The O element was assigned to the TiO2 and BiOI. The C element probably came from the organic precursors which were not completely burnt out during heat-treatment and adventitious hydrocarbon (i.e. from the XPS instrument itself) present on the sample surface. Bi and I elements were assigned to the BiOI. The XPS results further demonstrate that the composites consist of BiOI and TiO2, which agrees well with the results of XRD and HRTEM. Fig. 4b shows the high resolution XPS spectra of the Ti2p3/2 region taken on the surface of TiO2 and BiOI/TiO2 composites. For the pure TiO2, the peak located at 458.3 eV is assigned to Ti 2p3/2, suggesting a normal state of Ti4þ. Considering the measurement error (±0.1 eV), the peak for Ti 2p3/2 of the 1% sample has no obvious difference compared with TiO2, which indicates that the effects of BiOI on the chemical environments of the Ti element is so small that can be ignored due to the low BiOI content. However, for the 2%, 3% and 4% samples, there is a detectable shift of the Ti 2p value, the Ti 2p value shifts to lower value, demonstrating that the binding energy and chemical environments of Ti element have been altered. When these composites are irradiation by solar, photoelectrons are injected to Ti4þ, so Ti3þ centers can be easily formed on the surface of the photocatalysts, improving the

photocatalytic activities of the photocatalysts [27]. Moreover, the shift of Ti 2p3/2 further confirms that a strong interaction between BiOI and TiO2 exists, which accords well with the results of XRD. In Fig. 4c, the binding energies are 458.3, 248.1, and 457.8 eV for Bi4f 7/ 3þ assigned to 2, respectively, suggesting that Bi is in the form of Bi BiOI [28]. However, compared to 1%, Bi 4f of 2%, 3% and 4% shifts to lower value, indicating an interaction between BiOI and TiO2 exists, which fits well with the shift of Ti 2p. Due to the weak XPS of I3d of 1%, no I 3d binding energy of 1% was provided in Fig. 4d. The I 3d core level spectrum from Fig. 4d could be observed at the binding energies of around 630.0 eV (I3d3/2) and 618.4 eV (I3d5/2), according well with that in BiOI [29]. The SPV responses of photocatalysts are shown in Fig. 5. As illustrated in Fig. 5, TiO2 displays obvious SPV response from 300 to 400 nm and all the BiOI/TiO2 hetero-structures display much stronger SPV response than that of the pure TiO2.Intrestingly, BiOI/ TiO2 composites have detectable SPV response from 400 to 450 nm, which is assigned to the presence of BiOI, since BiOI is a narrowband semiconductor. The intensity of the SPV response gradually strengthens as the loading of BiOI increasing, and reaches a maximum when the molar ration of Bi/Ti is 2% and drops notably at 3%. Increasing the amount of BiOI, the SPV response becomes worsen but is still higher than that of BiOI. The results show that loading BiOI onto the surface of TiO2 can remarkably increase the SPV response of composites; however, there is a maximal loading of BiOI. Commonly, the strong SPV response corresponds to high separation rate of charge carriers generated under illumination. High separation rate of charge carriers is conducive to the photocatalytic activity. The phase values of photocatalysts are presented in Fig. 6. From 400 to 450 nm, the phase values of TiO2 and BiOI are positive, which suggests that the electrons transfer to the top electrode from which light is incident [30]. After constructing of BiOI/TiO2 composites, when the wavelength is above 430 nm, the phase values of 2% and 4% are in -90-0 , which means that the photo-generated holes generally transport to the top electrode, albeit the phase value of pure BiOI and TiO2 are positive from 400 to 450 nm. The results here demonstrate that coupling of BiOI with TiO2 results in different electronic transfer properties when the BiOI/TiO2 photocatalysts are irradiated by different wavelengths of light. To detect the active species during the photocatalytic reaction, benzoquinone (BQ), isopropanol (IPA) and ammonium oxalate (AO) were added into the reaction system, respectively. The effects of scavengers on the decolorization of MO are shown in Fig. 7. After adding BQ, AO and IPA, the photocatalytic decolorization of MO drops from 85.0% to 15.3, 61.3% and 78.2%, accordingly, implying that $O2  is the main active species, while hþ and OH are the minor active species in the photocatalytic decoloration of MO. The photolysis of MO solution (10 mg/L) under simulated sun light exposure without photocatalyst after 25 min can be totally neglected; the adsorption of MO on different photocatalysts after 25 min in dark is less than 4%. The catalytic performance of photocatalysts towards decolorization of MO is displayed in Fig. 8. All composites appear higher photocatalytic performance than TiO2 under simulated solar light irradiation and 2% holds the highest photocatalytic activity, even the 4% sample exhibits higher activity than TiO2. The results certify that loading BiOI onto the surface of TiO2 can greatly boost the photocatalytic activity of TiO2, when the molar ratio of Bi/Ti is 2%, the photocatalyst possesses the best photocatalytic performance. Based on all the evidences, it is reasonable to point out that the enhanced photocatalytic performance is assigned to the improved photo-induced charge separation and strong interaction between BiOI and TiO2. To better understand the improvement of photocatalytic performance, a charge separation mechanism was proposed in Fig. 9.

J. Li et al. / Solid State Sciences 52 (2016) 106e111

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Fig. 3. SEM and TEM of 2%, the HRTEM image recorded from the white framed area indicated in B.

180000

O1s

90000

(a)

4%

Ti 2p

60000

Bi 4f

Bi 4p I 3d Ti 2s

1000

800

600

400

Ti 3p

15000

C1s

Bi 4d

30000

0

Intenstity (a.u.)

150000

O KLL

Intensity (a.u.)

75000

45000

200

3% 120000 2%

90000

458.1

60000 30000

0% 0 470

0

468

30000

Intensity (a.u.)

( a . u .) Intensity

35000

Bi 4f 7/2

Bi 4f 5/2

80000

158.5

60000

466

464

462

460

458

456

454

452

Binding energy (eV)

158.5

(c)

1%

458.3

Binding energy (eV) 100000

457.8

(b)

40000

(d)

618.4

630.0 4%

25000 20000

3%

158.4

15000

20000

2%

158.8 0 170

168

166

164

162

160

158

Bingding energy (eV)

156

154

152

10000 636 634 632 630 628 626 624 622 620 618 616

Binding energy (eV)

Fig. 4. (a) Survey XPS spectrum of 2%; and high-resolution XPS spectra of phototocatalyst; (b)Ti 2p; (c)Bi 4f; (d)I 3d.

110

J. Li et al. / Solid State Sciences 52 (2016) 106e111

2.5

85.0

2%

80 69.0

Decolorization (%)

Photovoltage (mV)

2.0 3%

1.5

4%

1.0

1%

0.5

0%

0.0 300

350

400

450

60

51.5

40 20 0

0%

1%

Wavelength (nm)

2%

3%

4%

Photocatalyst

Fig. 5. SPV responses of photocatalysts.

Fig. 8. Catalytic activity of photocatalysts; the concentration of MO was 10 mg L1, the initial pH of MO was 7.0, the concentration of photocatalysts was 1 g L1; the irradiation time was 25 min.

150 O2

Phase (degree)

100 4% 50

0%

e- e- e- e-0.56 eV

BiOI 2%

420 430

300

350

400

450

MO

Products

Wavelength (nm)

Decolorization(%)

80

78.2 61.3

60 40 20 0

15.3

Blank

IPA

AO

MO

2.38 eV h+ h+ h+ h+ BiOI

2.91 eV h + h + h+ h + TiO2

Products

Fig. 9. Schematic diagram of photo-excited electronehole separation process.

Fig. 6. Phase spectra of the as-prepared photocatalysts.

85.0

-0.29 eV

0.52 eV

0 -50

e- e- e- e-

BQ

Scavenger Fig. 7. Effects of scavengers on the MO conversion over 2% (Illumination time ¼ 25 min, Scavenger dosage ¼ 0.2 mmol/L).

in the VB of BiOI could be excited up to a higher potential edge (0.56 eV) [31,32]. At pH 7.0, the standard redox potential of H2O/ $OH is about 2.7e2.8 eV, so hþVB on VB of BiOI cannot oxidize H2O to generate OH, the standard redox potential of O2/$O2  is 0.33 eV vs. NHE, so the photo-induced electrons on CB of TiO2 cannot reduce O2 to form $O2  , while the photo-induced electrons on CB of BiOI can. Under simulated sun light illumination, the photo-induced electrons with low reductive power in the CB of TiO2 transfer to the VB of BiOI, leading to recombination of electrons and holes, while the accumulated holes in the VB of TiO2 and electrons in the CB of BiOI can initiate the photocatalytic reactions. According to Fig. 9, a Z-scheme mechanism can be applied to explain the effective separation of photo-generated charges. In fact, lots of heterojuctions with a Z-scheme charge-separation mechanism have been constructed and applied in various fields. For heterogeneous photocatalytic systems with a Z-scheme chargeseparation mechanism, the photo-generated electrons and holes are spatially isolated, which greatly inhibits their undesirable recombination, resulting in relative high photocatalytic activity [33]. 4. Conclusions

The VB and CB of TiO2 is 2.91 eV and 0.29 eV vs. NHE [14e18], the VB and CB of BiOI is 2.38 eV and 0.52 eV vs. NHE, respectively [14]. However, under visible-light irradiation (l > 420 nm), the electrons

In summary, BiOI decorated TiO2 photocatalysts were successfully prepared in-situ by a facile hydrothermal method.

J. Li et al. / Solid State Sciences 52 (2016) 106e111

Modification of TiO2 by BiOI results in promoted photocatalytic activity of TiO2 due to the photo-induced charge separation properties induced by the strong interaction between BiOI and TiO2. When the molar ratio of Bi/Ti is 2%, the photocatalyst exhibits the highest photocatalytic activity under simulated sun light irradiation. Modification of TiO2 by BiOI is an effective and simple approach to promote the photocatalytic performance of TiO2. Acknowledgments This project was supported financially by the Research Fund Projects of Sichuan University of Science and Engineering (No. 2013PY03), the Project of Zigong city (2015HX20), Construct Program of the Discipline in Sichuan University of Science and Engineering, the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No. LZJ1301, No. LZY1101, No. LYJ14203), and Sichuan Provincial Academician (Expert) Workstation (No. 2015YSGZZ03). References [1] E.T. Acar, S. Ortaboy, G. Atun, Adsorptive removal of thiazine dyes from aqueous solutions by oil shale and its oil processing residues: characterization, equilibrium, kinetics and modeling studies, Chem. Eng. J. 276 (2015) 340e348. [2] I.A. Aguayo-Villarreal, V. Hernandez-Montoya, A. Bonilla-Petriciolet, R. TovarGomez, E.M. Ramirez-Lopez, M.A. Montes-Moran, Role of acid blue 25 dye as active site for the adsorption of Cd2þ and Zn2þ using activated carbons, Dyes Pigm. 96 (2013) 459e466. [3] M. Arshadi, F. SalimiVahid, J. Salvacion, M. Soleymanzadeh, Adsorption studies of methyl orange on an immobilized Mn-nanoparticle: kinetic and thermodynamic, RSC Adv. 4 (2014) 16005e16017. [4] Y.C. Hsiao, T.F. Wu, Y.S. Wang, C.C. Hu, C. Huang, Evaluating the sensitizing effect on the photocatalytic decoloration of dyes using anatase-TiO2, Appl. Catal. B Environ. 148e149 (2014) 250e257. [5] S. Zhang, Preparation of controlled-shape ZnS microcrystals and photocatalytic property, l, Ceram. Int. 40 (2014) 4553e4557. [6] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Environmental applications of semiconductor Photocatalysis, Chem. Rev. 95 (1995) 69e96. [7] M.L. Marin, L. Santos-Juanes, A. Arques, A.M. Amat, Miguel A. Miranda, Organic photocatalysts for the oxidation of pollutants and model compounds, Chem. Rev. 112 (2012) 1710e1750. [8] J. Schneider, M. Matsuoka, M. Takeuchi, J.L. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Understanding TiO2 photocatalysis: mechanisms and materials, Chem. Rev. 114 (2014) 9919e9986. [9] N. Venkatachalam, M. Palanichamy, B. Arabindoo, V. Murugesan, Enhanced photocatalytic degradation of 4-chlorophenol by Zr4þ doped nano TiO2, J. Mol. Catal. A Chem. 266 (2007) 158e165. [10] V.B.R. Boppana, R.F. Lobo, Photocatalytic degradation of organic molecules on mesoporous visible-light-active Sn(II)-doped titania, J. Catal. 281 (2011) 156e168. [11] J. Yang, L.J. Xu, C.L. Liu, T.P. Xie, Preparation and photocatalytic activity of porous Bi5O7I nanosheets, Appl. Surf. Sci. 319 (2014) 265e271. [12] R.A. He, S.W. Cao, P. Zhou, J.G. Yu, Recent advances in visible light Bi-based photocatalysts, Chin. J. Catal. 35 (2014) 989e1007. [13] X. Lin, L.L. Yu, L. Yan, H.J. Li, Y.S. Yan, C.B. Liu, H.J. Zhai, Visible light

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24] [25]

[26]

[27] [28]

[29]

[30]

[31]

[32]

[33]

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photocatalytic activity of BiVO4 particles with different morphologies, Solid State Sci. 32 (2014) 61e66. C.X. Liao, Z.J. Ma, G.P. Dong, J.R. Qiu, BiOI nanosheets decorated TiO2 nanofiber: tailoring water purification performance of photocatalyst in structural and photo-responsivity aspects, Appl. Surf. Sci. 314 (2014) 481e489. W.K. Jo, T.S. Natarajan, Influence of TiO2 morphology on the photocatalytic efficiency of direct Z-scheme g-C3N4/TiO2 photocatalysts for isoniazid degradation, Chem. Eng. J. 281 (2015) 549e565. J.H. Li, Y.L. Liu, H.M. Li, C. Chen, Fabrication of g-C3N4/TiO2 composite photocatalyst with extended absorption wavelength range and enhanced photocatalytic performance, J. Photoch. Photobio. A 317 (2016) 151e160. J. Zhou, L. Yin, H.R. Li, Z.Y. Liu, J.X. Wang, K. Duan, S.X. Qu, J. Weng, B. Feng, Heterojunction of SrTiO3/TiO2 nanotubes with dominant (001) facets: synthesis, formation mechanism and photoelectrochemical properties, Mat. Sci. Semicon. Proc. 40 (2015) 107e116. Y.H. Ao, J.L. Xu, P.F. Wang, C. Wang, J. Hou, J. Qian, Y. Li, Bi2MoO6 nanosheets deposited TiO2 nanobelts with spatially branched hierarchical heterostructure for enhanced photocatalytic activity under visible light irradiation, Colloid. Surf. A 487 (2015) 66e74. Y.Y. Li, J.S. Wang, B. Liu, L.Y. Dang, H.C. Yao, Z.J. Li, BiOI-sensitized TiO2 in phenol degradation: a novel efficient semiconductor sensitizer, Chem. Phys. Lett. 508 (2011) 102e106. L.Y. Wang, W.A. Daoud, BiOI/TiO2-nanorod array heterojunction solar cell: growth, charge transport kinetics and photoelectrochemical properties, Appl. Surf. Sci. 324 (2015) 532e537. D.Y. Wu, H.Y. Wang, C.L. Li, J. Xia, X.J. Song, W.S. Huang, Photocatalytic selfcleaning properties of cotton fabrics functionalized with p-BiOI/n-TiO2 heterojunction, Surf. Coat. Tech. 258 (2014) 672e676. G.P. Dai, J.G. Yu, G. Liu, Synthesis and enhanced visible-light photoelectrocatalytic activity of p-n junction BiOI/TiO2 nanotube arrays, J. Phys. Chem. C 115 (2011) 7339e7346. Y. Zhang, Q. Pei, J.C. Liang, T. Feng, X. Zhou, H. Mao, W. Zhang, Y. Hisaeda, X.M. Song, Mesoporous TiO2-based photoanode sensitized by BiOI and investigation of its photovoltaic behavior, Langmuir 31 (2015) 10279e10284. J.G. Yu, X.J. Zhao, Q.N. Zhao, J.C. Du, XPS of study of TiO2 photocatalytic thin film prepared by the sol-gel method, Chin, J. Materi. Res. 14 (2000) 203e209. Q.D. Zhao, D.J. Wang, L.L. Peng, Y.H. Lin, M. Yang, T.F. Xie, Surface photovoltage study of photogenerated charges in ZnO nanorods array grown on ITO, Chem. Phys. Lett. 434 (2007) 96e100. X.L. Liu, J.B. Zhong, J.Z. Li, S.T. Huang, W. Song, PEG-assisted hydrothermal synthesis of BiOCl with enhanced photocatalytic performance, Appl. Phys. A 119 (2015) 1203e1208. J.M. Coronado, J. Soria, ESR study of the initial stages of the photocatalytic oxidation of toluene over TiO2 powders, Catal. Today 123 (2007) 37e41. J. Jiang, X. Zhang, P. Sun, L. Zhang, ZnO/BiOI heterostructures: photoinduced charge-transfer property and enhanced visible-light photocatalytic activity, J. Mater. Chem. C 115 (2011) 20555e20564. Y.Y. Li, J.S. Wang, B. Liu, L.Y. Dang, H.C. Yao, Z.J. Li, BiOI-sensitized TiO2 in phenol degradation: a novel efficient semiconductor sensitizer, Chem. Phy. Lett. 508 (2011) 102e106. Q. Zhao, T. Xie, L. Peng, Y. Lin, P. Wang, L. Peng, D. Wang, Size- and orientation-dependent photovoltaic properties of ZnO nanorods, J. Phys. Chem. C 111 (2007) 17136e17145. D. Hou, X. Hu, P. Hu, W. Zhang, M. Zhang, Y. Huang, Bi4Ti3O12 nanofiberseBiOI nanosheets pen junction: facile synthesis and enhanced visible-light photocatalytic activity, Nanoscale 5 (2013) 9764e9772. T. Cao, Y. Li, C. Wang, Z. Zhang, M. Zhang, C. Shao, Y. Liu, Bi4Ti3O12 nanosheets/ TiO2 submicron fibers heterostructures: in situ fabrication and high visible light photocatalytic activity, J. Mater. Chem. 21 (2011) 6922e6927. P. Zhou, J.G. Yu, M. Jaroniec, All-solid-state Z-scheme photocatalytic systems, Adv. Mater. 26 (2014) 4920e4935.