Constructing a Novel n–p–n Dual Heterojunction between Anatase

Article pubs.acs.org/JPCC

Constructing a Novel n−p−n Dual Heterojunction between Anatase TiO2 Nanosheets with Coexposed {101}, {001} Facets and Porous ZnS for Enhancing Photocatalytic Activity Jun Zhang,*,† Xinming Ma,† Lili Zhang,† Zhengda Lu,† Erpan Zhang,† Hongbo Wang,‡ Zhe Kong,† Junhua Xi,† and Zhenguo Ji*,† †

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, People’s Republic of China College of Automation, Hangzhou Dianzi University, Hangzhou 310018, People’s Republic of China

‡

ABSTRACT: Compositing TiO2 with metal sulfide to construct heterojunction is an effective approach to improve the carriers’ separation efficiency and photocatalytic activity. However, TiO2 is an N-type semiconductor and most metal sulfide is also N-type semiconductor. It will form n−n heterojunction between TiO2 and metal sulfide. The photoinduced electrons in conduction band of metal sulfide hardly flow into the conduction band of TiO2, which will weaken the improvement of carriers’ separation efficiency. In this work, anatase TiO2 nanosheets with coexposed {101} and {001} facets were composited with porous ZnS to construct a novel n−p−n dual heterojunction. This TiO2/ZnS composite displays 108% improvement of photocatalytic activity compared to that of pristine TiO2 nanosheets. As comparison, P25 with mainly exposed {101} facets/porous ZnS with an n−n single heterojunction only show 2% enhancement of photocatalytic activity than that of P25. The n−p−n dual heterojunction displays an obvious advantage than common TiO2/ZnS n−n heterojunction. In the n−p−n dual heterojunction, first, photoinduced electrons at CB of {001} facets will flow into the CB of {101} facets, while photoinduced holes at VB of {101} facets will flow into the VB of {001} facets; second, photoinduced electrons at CB of ZnS will flow into the CB of {001} facets, while photoinduced holes at VB of {001} facets will flow into the VB of ZnS. In this way, it realizes carriers’ separation in the n−p−n dual heterojunction. This work improves a new strategy to employ crystal facets of photocatalysts to construct n−p− n dual heterojunction with metal sulfide for enhancing the photocatalytic activity.

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wide range bandgap and catalytic functions.3,7−9 In particular, ZnS is a well-known photocatalyst because of the rapid generation of photoinduced electrons and holes. Moreover, the conduction band (CB) and valence band (VB) of ZnS are both higher than those of TiO2. It is a well strategy to construct heterojunction between TiO2 and ZnS to improve the carriers’ separation efficiency. Some works have reported the TiO2/ZnS composites.7,8,10−23 However, in those works, the influence of TiO2 crystal facets in constructing heterojunction is ignored. Moreover, as TiO2 and ZnS are both N-type semiconductors due to the O or S vacancies caused by self-compensating, there will a single n−n type heterojunction forming between TiO2/ ZnS composites. It will hinder the electron transmission between the heterojunction. Usually, the mainly exposed crystal facets of anatase TiO2 are {101} facets for their low surface energy. Through changing the synthesis condition, other higher surface energy facets can be

INTRODUCTION As a potential solution to the global energy crisis and environmental pollution, photocatalysis has attracted great attention, and a lot of relative research has been reported.1 Photocatalysts always are semiconductors that can translate luminous energy to electrochemistry energy. This approach can directly use solar energy without any pollution. In the energy transition, the core progress is the segregation of photoinduced electrons and holes. Higher carriers’ separation efficiency means better photocatalytic activity. However, the separation efficiency usually is very low. Constructing heterojunction with other semiconductors is an effective approach to improve the separation efficiency.2−6 TiO2 is an ideal photocatalyst for its high activity, low price, stabilization, and nontoxicity. However, the carriers’ separation efficiency of TiO2 is not enough and should be improved. To solve this question, TiO2 was usually composited with other semiconductors to construct heterojunction. In this way, photoinduced electrons and holes can transfer through the heterojunction and reach longer life. Recently, metal sulfides have been found as advantages in photocatalysis due to their © 2017 American Chemical Society

Received: January 3, 2017 Revised: February 27, 2017 Published: February 27, 2017 6133

DOI: 10.1021/acs.jpcc.7b00049 J. Phys. Chem. C 2017, 121, 6133−6140

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The Journal of Physical Chemistry C controlling such as {001},24−27 {100},28,29 {111},30 and curved facets.31,32 In particular, anatase TiO2 nanosheets with exposed {001} facets are ideal substrate for compositing semiconductors. In our previous work, {001} facets of anatase TiO2 have been confirmed to be more superior to composited Bi4Ti3O12, Bi2WO6, and MoS2 than common {101} facets because the heterojunction constructed on {001} facets displayed higher enhancement of activity.9,33,34 The different composite effect on crystal facets of TiO2 has also been confirmed by other work.35 It is thought that higher surface {001} facets are in favor of forming binding between TiO2 and composite semiconductors to construct carriers’ transmission channel. Recently, it was found that the conduction band (CB) and valence band (VB) of {001} facets are some higher that those of {101} facets of anatase TiO2.36 In this way, a crystal n−p heterojunction will form between the {101} and {001} facets of TiO2. This anatase TiO2 with coexposed {101} and {001} facets can be employed to construct n−p−n dual heterojunction with ZnS to promote electron transmission between TiO2 and ZnS. In this work, anatase TiO2 nanosheets with coexposed {101} and {001} facets were composited with porous ZnS to construct n−p−n dual heterojunction. This TiO2 /ZnS composite displays 108% improvement of photocatalytic activity than that of pristine TiO2 nanosheets. As comparison, P25 with mainly exposed {101} facets/porous ZnS with an n−n single heterojunction was obtained. Under an optimal composite ratio, the P25/ZnS only shows 2% enhancement of photocatalytic activity than that of P25. The results confirm that the crystal faces of photocatalysts play a critical role in constructing semiconductor heterojunction; TiO2 with coexposed {101} and {001} facets are more suitable for loading ZnS than TiO2 with mainly exposed {101} facets because of n−p−n dual heterojunction between TiO2 with coexposed {101}, {001} facets, and ZnS will promote carrier transmission between TiO2 and ZnS. This work improves a strategy to construct n−p−n dual heterojunction using crystal facets of photocatalysts for enhancing the photocatalytic activity.

Samples obtained from different Zn/Ti mole ratios were respectively labeled as TZS1 (3.4 mg of ZnCl2 and 6 mg of thiourea, Zn/Ti = 1%), TZS2 (8.5 mg of ZnCl2 and 15 mg of thiourea, Zn/Ti = 2.5%), TZS3 (17 mg of ZnCl2 and 30 mg of thiourea, Zn/Ti = 5%), and TZS4 (25.5 mg of ZnCl2 and 45 mg of thiourea, Zn/Ti = 7.5%). Another hydrothermal route (340 mg of ZnCl2 and 600 mg of thiourea were dissolved in 60 mL deionized water) was executed to obtain ZnS (ZS). As a contrast, P25/ZnS composites were prepared by the some way except replacing the TO by P25 with the same qualities. Samples obtained from different Zn/Ti mole ratios were respectively labeled as PZS1 (Zn/Ti = 1%), PZS2 (Zn/Ti = 2.5%), PZS3 (Zn/Ti = 5%), and PZS4 (Zn/Ti = 7.5%). Characterizations. The crystal structures and phases of the samples were measured using an X-ray diffraction (XRD) (Miniflex600, Rigaku, Japan) with Cu−Kα radiation at a scan rate of 0.02° s−1. The accelerating voltage and the applied current were 40 kV and 15 mA, respectively. Morphologies and microstructures of the samples were characterized by highresolution transmission electron microscopy (HRTEM) (Tecnai G2 F30 S-Twin, FEI, Hillsboro, USA). X-ray photoelectron spectroscopy (XPS) measurements were made on an ESCLAB250Xi (Thermo Scientific, American) spectrometer with a charge neutralizer to gain information on the chemical binding energy of the photocatalysts. The C 1s peak at 284.8 eV of the adventitious carbon was referenced to rectify the binding energies. UV−vis diffuse reflectance spectroscopy (DRS) of the samples was applied using a UV−vis spectrophotometer (UV-3600, Shimadzu, Tokyo, Japan) with a multipurpose large sample compartment (MPC-3100, Shimadzu, Tokyo, Japan). BaSO4 was used as a reflectance standard in the DRS measurement. Fluorescence (FL) emission spectra of the samples were detected with a fluorescence spectrophotometer (RF-530TPC, Shimadzu, Japan) using a 300 nm line from a xenon lamp. Photocatalytic Experiments. The photocatalytic properties of the samples were examined by measuring the decomposition rates of methylene blue (MB) in the presence of a photocatalyst. In the UV−vis degradation, a 250 W highpressure mercury lamp was used as a light source in the experiment. The lamp was placed 8 cm above the liquid surface. Approximately 20 mg of photocatalyst was added into 100 mL of 4 × 10−5 M MB aqueous solution. Before photodegradation, the mixed solution was stirred incessantly in dark environment for 60 min to reach adsorption−desorption equilibrium, and then the residual concentration of dye was measured. After every 10 min during the photocatalytic process, 3 mL solutions were extracted to test the residual concentrations of methylene blue. Dye concentration was evaluated by measuring the change in maximum absorbance through UV−vis spectrometry (UV3600, Shimadzu, Tokyo, Japan). The absorbent peak at about 664 nm was selected, and the residual concentration was obtained though evaluating the intensity ratio between remaining and original MB solution.

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EXPERIMENTAL SECTION Synthesis of Samples. All chemicals used in this experiment were analytical-grade without further treatment. Anatase TiO2 nanosheets with coexposed {101} and {001} facets (TO) were synthesized via a hydrothermal route as in other work.26 In the obtained progress, 25 mL of Ti(OBu)4 was mixed with 4.5 mL of HF then put into a 100 mL Teflon-lined autoclave. After fully stirred, the mixture was kept at 180 °C for 24 h. The precipitates were then put into a 50 mL centrifuge tube and separated from the suspension by centrifugation (11000 rpm, 5 min). To wash the white powder, the products were further suspended and centrifuged in deionized water for three times, followed by drying at 102 °C for 12 h. Then the obtained sample was annealed at 550 °C for 1 h to clear surface F. TiO2/ZnS heterostructures were fabricated via a further hydrothermal route. In this process, relative ZnCl2 and thiourea were dissolved in 60 mL of deionized water. Then 200 mg of TO was mixed with the ZnCl2 and thiourea solution in a 100 mL Teflon-lined autoclave. The mixtures were kept at 180 °C for 24 h. The precipitates were separated from the suspension by centrifugation (11000 rpm, 5 min). To wash the powder, the products were further suspended and centrifuged in deionized water for three times, followed by drying at 102 °C for 12 h.

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RESULTS AND DISCUSSION Crystal Structure and Morphology. Anatase TiO2 has two mainly crystal facets: {101} and {001} facets. {101} facets are low energy but mainly exposed facets. By adding HF in the hydrolytic process, anatase TiO2 with coexposed {101} and {001} facets can be obtained. The XRD patterns of pristine TO, ZS, and composites with different ratio are shown in Figure 1. With an addition of 4.5 mL of HF, TO shows a typical 6134

DOI: 10.1021/acs.jpcc.7b00049 J. Phys. Chem. C 2017, 121, 6133−6140

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The Journal of Physical Chemistry C

Figure 1. XRD patterns of TO, TZS1, TZS2, TZS3, TZS4, and ZS.

anatase phase according to standard anatase TiO2 XRD pattern (JCPDS data file No. 21-1272). However, compared with standard pattern, the fabricated TiO2 showed obvious enhancement of (200) peaks as 48.08° and weakness of (004) peaks at 37.84°. This result indicates that the TiO2 have a nanosheet morphology. The sample obtained by hydrothermal route using ZnCl2 and thiourea as precursors could be classified as ZnS with a cubic crystal structure (JCPDS data file No. 65-1691), and some peaks according to hexagonal structure ZnS (JCPDS data file No. 12-0688). It seems ZnS with mixed phases will be obtained using thiourea as precursor. After composited, the XRD patterns of composites mainly shown anatase TiO2 phase as TO except a (111) peak of ZnS can be identified in TZS3 and TZS4. That can confirm the existence of ZnS in the composites. The crystallinity of TiO2 in composites is calculated, and the results are shown in Table 1. It seems the crystallinity of TiO2 showed a law of increase at first and then weakens (TZS1 has the highest crystallinity). The increase of peak intensities can be ascribed to the double hydrothermal process, and subsequent decline could be ascribed to the surface cover of ZnS. The grain sizes are also shown in Table 1. Due to the double hydrothermal progress, the grain size shows some increase, which is according to the crystallinity results. Pristine TO, ZS, and composited sample TZS2 were further observed by HRTEM as Figure 2. TO has nanosheet morphology with a longer of ∼40 nm and a thickness of ∼10 nm (Figure 2A). As the lattice spacing of 0.235 nm according to (004) faces of anatase TiO2 in Figure 2B, it can be confirmed that the square surface on the flat of nanosheet is {001} facet, while other trapezoid sides are {101} facets. The HRTEM images confirm the coexposed {101} and {001} facets. The

Figure 2. HRTEM images of (A,B) TO, (C,D) ZS, and (E,F) TZS2.

exposed percentage of {001} facets can be calculated from the following formulas:37 2 ⎛ d ⎞ ⎟ S001 = 2⎜l − ⎝ tan θ ⎠

S101 =

(1)

2d ⎛ d ⎞ ⎜2l − ⎟ sin θ ⎝ tan θ ⎠

(2)

Table 1. Structural and Property Information of the Samples Prepared with Different Experimental Conditions samples TO TZS1 TZS2 TZS3 TZS4 P25 PZS1 PZS2 PZS3 PZS4 ZS a

Zn:Tia (%) 1 2.5 5 7.5 1 2.5 5 7.5

mainly facets {101}, {101}, {101}, {101}, {101}, {101} {101} {101} {101} {101}

{001} {001} {001} {001} {001}

crystallinityb

grain size (nm)c

band gap (eV)

kd

1 1.44 1.40 1.39 1.30 0.81 1.17 1.13 0.97 1.06

10.2 21.1 20.7 20.3 20.1 19.6 19.6 19.9 20.5 20.2

3.22 3.21 3.20 3.20 3.20 3.19 3.16 3.14 3.12 3.08 3.15

0.01108 0.01898 0.02301 0.02135 0.01711 0.0218 0.01316 0.01466 0.02233 0.01593 0.00284

enhancement of k (%) 71 108 93 54 −40 −33 2 −27

Mole ratio. bRelative intensity of diffraction peak from the anatase {101} facets (reference = TO). cCalculated from the anatase (101) peaks. Degradation rate of MB under UV−vis light.

d

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DOI: 10.1021/acs.jpcc.7b00049 J. Phys. Chem. C 2017, 121, 6133−6140

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Figure 3. XPS spectra of (A) Ti 2p, (B) O 1s, (C) Zn 2p, and (D) S 2p.

P=

S001 S001 + S101

XPS survey spectrum to be 1.02%, which is little higher than the stoichiometric ratio in precursor of TZS2 due to the surface covered by ZnS. The influence of the composited on the light absorption and band gap of TiO2/ZnS can be characterized and calculated by UV−vis DRS.34 The UV−vis DRS spectra of TO, TZS1, TZS2, TZS3, TZS4, and ZS are displayed and compared in Figure 4A. Pristine TO shows only UV light absorption due to the wide band gap. Except UV light absorption, ZS also demonstrates some absorption in the visible light region at a range of 400− 800 nm. This visible light absorption can be ascribed to the structure defect of ZnS. All the composites have similar absorbance intensity as TO and ZS in the UV region, which indicates that ZnS composite has little influence on the absorption of UV light, while in the visible light region, along with the increase of composite ratio, the visible light absorbance intensities gradually elevate in the overall region, which can be ascribed to the adding of surface covered ZnS. Moreover, the spectrum onset of composites evidently red-shifted, and the precise band gaps could be calculated from the intercept of the tangents to the plots of [αhv]1/2 versus photon energy (hv) (Figure 4B). The results are listed in Table 1. The band gap of TO and ZnS is 3.22 and 3.15 eV, respectively. After composited, the band gap slightly reduces to 3.21, 3.20, 3.20, and 3.20 corresponding to TZS1, TZS2, TZS3 and TZS4 respectively. It seems ZnS composited has little influence on the band gap of composites. Photocatalytic Activity. The photocatalytic activity of all obtained samples was tested by degrading MB in aqueous solution. MB was chosen because it is very stable and could not be degraded under UV−vis illumination for 60 min without photocatalysts (Figure 5A). When photocatalysts were added, MB molecules would be adsorbed on the surface of photocatalysts and cause test errors. To avoid the influence of adsorption, all samples were stirred in the dark for 60 min to

(3)

where l is average length and d is average thickness of sheets, which can be measured from SEM images, S001 and S101 are the areas of all {001} and {101} facets, respectively, in a TiO2 single crystal, P is the exposed percentage of {001} facets, and 68.3° is the theoretical value for the angle between the [001] and [101] facets of anatase. According to the formulas, the exposed percentage of {001} facets can be calculated as 61%. The obtained pristine ZS displays a nanoparticles profile with a size of 20−50 nm as Figure 2C. The lattice spacing of 0.313 nm shown in Figure 2D can confirm the crystal grains of ZnS. In the TEM images of composite (Figure 2E), it can be obviously identified that porous ZnS covered on the surface of TiO2 nanosheets forms a core−shell heterojunction structure (Figure 2F). The porous structure was not found in pristine ZS because the loading ZnS on the surface of TiO2 was much smaller and thinner, resulting in forming defects more easily. In the highresolution images, ZnS nanograins load on the {001} facets of TiO2 to form bonding that can supply a good carriers’ transfer channel. XPS Spectra and UV−vis DRS. TZS2 was chosen to test XPS for its highest photocatalytic activity in latter results. In Figure 3, four elements, Ti, O, Zn, and S, can be distinguished in the XPS spectrum. In the local XPS spectrum of Ti (Figure 3A), two peaks at the binding energies of 463.7 and 457.9 eV can be assigned to Ti 2p1/2 and Ti 2p3/2, which confirm the existence of Ti4+ in TiO2 combining the O 1s peak at 529.2 eV (Figure 3B). The Zn2+ species can be ascribed to the Zn 2p peaks at 1043.9 and 1021.0 eV according to Zn 2P1/2 and Zn 2P3/2, respectively, in Figure 3C.22 S 2P1/2 and S 2P3/2 at peaks 161.8 and 160.4 eV in the result of S 2p core level spectrum (Figure 3D) can further confirm the exiting of ZnS in the composite. The atom ratios of Zn can be calculated from the 6136

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composites have higher rate than TO. The photocatalytic activity of photocatalysts can be compared from the degradation rates through calculating the apparent first-order rate constant as k, which was displayed in Figure 5B and Table 1. According to the degradation curve, all composites have higher activity than pristine TO and ZS, which can confirm the improvement of composite. In the composites, TZS2 has the highest rate of 108% enhancement compared to that of pristine TO. When the composite ratio exceeded 2.5%, the degradation rate constantly reduced due to the shielding light absorption of TiO2 by surface ZnS. It reveals an appropriate composite ratio is necessary for enhancing the photocatalytic activity. Comparison of P25. To study the influence of crystal faces on the constructing of heterojunction, P25 with mainly exposed {101} facets was composited with ZnS to test the photocatalytic activity. A comparison of XRD patterns of P25, ZS, and composites are shown in Figure 6. P25 has a typical anatase

Figure 4. (A) UV−vis diffuse reflectance spectra and (B) the plots of [αhv]1/2 vs photon energy of TO, TZS1, TZS2, TZS3, TZS4, and ZS.

Figure 6. XRD patterns of P25, PZS1, PZS2, PZS3, PZS4, and ZS.

phase according to standard anatase TiO2 XRD pattern (JCPDS data file No. 21-1272) and little rutile phase. After composited, an obvious peak according of ZnS(111) can be identified from the patterns, and the intensities gradually enhance along with the increase of composite ratio. The crystalline of TiO2 in the composites shows some improvement as Table 1. In the UV−vis DRS (Figure 7A), composites display some decline of absorption in UV range and little rise at visible light absorption. The enhancement of visible light absorption is lower than that of TO/ZnS composites. However, the red-shift of the spectrum onset and change of band gap are much larger: P25 has a bang gap of 3.09 eV and continuously reduces to 3.16, 3.14, 3.12, and 3.08 corresponding to PZS1, PZS2, PZS3, and PZS4, respectively (Figure 7B). The results reveal the crystal faces of TiO2 can influence the binding between TiO2 and ZnS. When the photocatalytic activity of there samples were investigated, it is unexpected that the composites show less enhancement compared with TO (only 2% with an optimal composite ratio) (Figure 8). Except for PZS3, the photocatalytic activities of other three composites become weaker. The composite of ZnS would worsen the photocatalysts. This contrast confirmed that anatase TiO2 with coexposed {101} and {001} facets are more suitable to construct heterostructure between TiO2 and ZnS to enhance the photocatalytic activity. The higher carriers’ separation efficiency by ZnS composite can be further confirmed by FL emission spectra shown in Figure 9.38,39 In the FL emission spectra, lower emission intensity means less carrier recombination and higher separation efficiency. The emission intensity of all peaks in the figure continually declines along with the

Figure 5. (A) Comparison of photocatalytic activities of TO, TZS1, TZS2, TZS3, TZS4, and ZS for methylene blue decomposition under UV−vis irradiation; (B) relative apparent first order rate constants k.

exclude the reduction of MB by adsorption. After MB aqueous solution was mixed with photocatalysts and illuminated under UV−vis, the absorbance intensity of MB continuously reduced, which can confirm the photocatalytic activity (Figure 5A). In the degradation curve, ZS shows the lowest rate, while all 6137

DOI: 10.1021/acs.jpcc.7b00049 J. Phys. Chem. C 2017, 121, 6133−6140

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Figure 9. Fluorescence spectra of TO, TZS1, TZS2, TZS3, and TZS4.

and TO/ZnS heterojunction are shown in Figure 10. In Figure 10A, as the CB and VB of ZnS are both higher than those of

Figure 7. (A) UV−vis diffuse reflectance spectra and (B) the plots of [αhv]1/2 vs photon energy of P25, PZS1, PZS2, PZS3, PZS4, and ZS.

Figure 10. Schematic diagrams of carrier exchange in the (A) P25/ ZnS n−n heterostructure and (B) TO/ZnS n−p−n dual heterostructure.

TiO2, photoinduced electrons at CB of ZnS will flow into the CB of TiO2, while photoinduced holes at VB of TiO2 will flow into the VB of ZnS. In this way, photoinduced carriers’ separation can be realized by the TiO2/ZnS heterojunction. However, as an II−VI compound semiconductor, ZnS has wide band gap and large difference between Zn2+ and S2− ions size, so that it will easily form S vacancies to produce a lot of electrons which cause n-type semiconductor of ZnS. In the same way, O vacancies will generate in TiO2 to cause n-type conductivity. An n−n heterojunction will form between P25/ ZnS composite, and the electrons in the CB of ZnS will hardly flow into the CB of TiO2 (as the imaginary line in Figure 10A), which will cause a weak improvement of carriers’ separation efficiency. When anatase TiO2 with exposed {101} and {001} facets was composited with ZnS, it forms an n−p−n dual heterojunction: an n−p heterojunction between {101} and {001} facets of TiO2 and a p−n heterojunction between {001} facets of TiO2 and covered ZnS (Figure 10B). First, in the

Figure 8. (A) Comparison of photocatalytic activities of P25, PZS1, PZS2, PZS3, PZS4, and ZS for methylene blue decomposition under UV−vis irradiation; (B) relative apparent first order rate constants k.

increase of ZnS composite, which can confirm the improvement of carriers’ separation efficiency. Mechanism of n−p−n Dual Heterojunction. To compare the carriers’ transfer progress between P25/ZnS and TO/ZnS composites, the energy band structures of P25/ZnS 6138

DOI: 10.1021/acs.jpcc.7b00049 J. Phys. Chem. C 2017, 121, 6133−6140

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(3) Zhang, J.; Huang, L.; Jin, H.; Sun, Y.; Ma, X.; Zhang, E.; Wang, H.; Kong, Z.; Xi, J.; Ji, Z. Constructing Two-Dimension MoS2/ Bi2WO6 Core-Shell Heterostructure as Carriers Transfer Channel for Enhancing Photocatalytic Activity. Mater. Res. Bull. 2017, 85, 140− 146. (4) Ren, X. H.; Qi, X.; Shen, Y. Z.; Xiao, S.; Xu, G. H.; Zhang, Z.; Huang, Z. Y.; Zhong, J. X. 2D Co-Catalytic MoS2 Nanosheets Embedded with 1D TiO2 Nanoparticles for Enhancing Photocatalytic Activity. J. Phys. D: Appl. Phys. 2016, 49, 315304. (5) Han, W. J.; Ren, L.; Gong, L. J.; Qi, X.; Liu, Y. D.; Yang, L. W.; Wei, X. L.; Zhong, J. X. Self-Assembled Three-Dimensional GrapheneBased Aerogel with Embedded Multifarious Functional Nanoparticles and Its Excellent Photoelectrochemical Activities. ACS Sustainable Chem. Eng. 2014, 2, 741−748. (6) Jiang, Z. F.; Jiang, D. L.; Yan, Z. X.; Liu, D.; Qian, K.; Xie, J. M. A New Visible Light Active Multifunctional Ternary Composite Based on TiO2-In2O3 Nanocrystals Heterojunction Decorated Porous Graphitic Carbon Nitride for Photocatalytic Treatment of Hazardous Pollutant and H-2 Evolution. Appl. Catal., B 2015, 170, 195−205. (7) Franco, A.; Neves, M. C.; Carrott, M.; Mendonca, M. H.; Pereira, M. I.; Monteiro, O. C. Photocatalytic Decolorization of Methylene Blue in the Presence of TiO2/ZnS Nanocomposites. J. Hazard. Mater. 2009, 161, 545−550. (8) Jin, S. Y.; Lian, T. Q. Electron Transfer Dynamics from Single CdSe/ZnS Quantum Dots to TiO2 Nanoparticles. Nano Lett. 2009, 9, 2448−2454. (9) Zhang, J.; Huang, L.; Lu, Z.; Jin, Z.; Wang, X.; Xu, G.; Zhang, R.; Wang, H.; Kong, Z.; Xi, J.; et al. Crystal Face Regulating MoS2/ TiO2(001) Heterostructure for High Photocatalytic Activity. J. Alloys Compd. 2016, 688, 840−848. (10) Jasieniak, J.; Pacifico, J.; Signorini, R.; Chiasera, A.; Ferrari, M.; Martucci, A.; Mulvaney, P. Luminescence and Amplified Stimulated Emission in CdSe-ZnS-Nanocrystal-Doped TiO2 and ZrO2 Waveguides. Adv. Funct. Mater. 2007, 17, 1654−1662. (11) Lee, H. J.; Bang, J.; Park, J.; Kim, S.; Park, S. M. Multilayered Semiconductor (CdS/CdSe/ZnS)-Sensitized TiO2 Mesoporous Solar Cells: All Prepared by Successive Ionic Layer Adsorption and Reaction Processes. Chem. Mater. 2010, 22, 5636−5643. (12) Sambur, J. B.; Parkinson, B. A. Cdse/Zns Core/Shell Quantum Dot Sensitization of Low Index TiO2 Single Crystal Surfaces. J. Am. Chem. Soc. 2010, 132, 2130−2131. (13) Antoniadou, M.; Daskalaki, V. M.; Balis, N.; Kondarides, D. I.; Kordulis, C.; Lianos, P. Photocatalysis and Photoelectrocatalysis Using (CdS-ZnS)/TiO2 Combined Photocatalysts. Appl. Catal., B 2011, 107, 188−196. (14) Cheng, S. L.; Fu, W. Y.; Hai, B. Y.; Zhang, L. N.; Ma, J. W.; Zhao, H.; Sun, M. L.; Yang, L. H. Photoelectrochemical Performance of Multiple Semiconductors (CdS/CdSe/ZnS) Cosensitized TiO2 Photoelectrodes. J. Phys. Chem. C 2012, 116, 2615−2621. (15) Zhu, Y. F.; Zhang, J.; Xu, L.; Guo, Y.; Wang, X. P.; Du, R. G.; Lin, C. J. Fabrication and Photoelectrochemical Properties of ZnS/ Au/TiO2 Nanotube Array Films. Phys. Chem. Chem. Phys. 2013, 15, 4041−4048. (16) Labiadh, H.; Ben Chaabane, T.; Balan, L.; Becheik, N.; Corbel, S.; Medjahdi, G.; Schneider, R. Preparation of Cu-Doped Zns QdS/ TiO2 Nanocomposites with High Photocatalytic Activity. Appl. Catal., B 2014, 144, 29−35. (17) Nagao, Y.; Fujiwara, H.; Sasaki, K. Analysis of Trap-State Dynamics of Single Cdse/Zns Quantum Dots on a TiO2 Substrate with Different Nb Concentrations. J. Phys. Chem. C 2014, 118, 20571− 20575. (18) Rao, S. S.; Punnoose, D.; Tulasivarma, C. V.; Kumar, C.; Gopi, C.; Kim, S. K.; Kim, H. J. A Strategy to Enhance the Efficiency of DyeSensitized Solar Cells by the Highly Efficient TiO2/Zns Photoanode. Dalton Trans. 2015, 44, 2447−2455. (19) Chen, A. Q.; Zhu, X. D.; Xi, J. H.; Qin, H. Y.; Ji, Z. G.; Zhu, K. G. Effects of Nickel Doping on the Preferred Orientation and Oxidation Potential of Ti/Sb-SnO2 Anodes Prepared by Spray Pyrolysis. J. Alloys Compd. 2016, 684, 137−142.

crystal heterojunction of TiO2, the CB and VB of {001} facets are both higher than those of {101} facets; photoinduced electrons at CB of {001} facets will flow into the CB of {101} facets, while photoinduced holes at VB of {101} facets will flow into the VB of {001} facets. Second, in the TiO2/ZnS heterojunction, photoinduced electrons at CB of ZnS will flow into the CB of {001} facets, while photoinduced holes at VB of {001} facets will flow into the VB of ZnS. In this way, it realizes carriers’ separation in the n−p−n dual heterojunction. Compared with P25/ZnS composites, in TO/ZnS, {001} facets of TiO2 embeds a p-type layer between two n-type semiconductors to reach an n−p−n dual heterojunction.

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CONCLUSIONS Overall, anatase TiO2 nanosheets with coexposed {101} and {001} facets were composited with porous ZnS to construct n− p−n dual heterojunction. This TiO2/ZnS composite displays 108% improvement of photocatalytic activity compared to that of pristine TiO2 nanosheets. As comparison, P25 with mainly exposed {101} facets/porous ZnS with an n−n single heterojunction was obtained. Under an optimal composite ratio, the P25/ZnS only shows 2% enhancement of photocatalytic activity compared to that of P25. Commonly, TiO2 and ZnS are both n-type semiconductors. The photoinduced electrons in CB of ZnS hardly flow into the CB of TiO2. However, TiO2 with coexposed {101} and {001} facets composited with ZnS will form a novel n−p−n dual heterojunction. In the crystal heterojunction of TiO2, photoinduced electrons at CB of {001} facets will flow into the CB of {101} facets, while photoinduced holes at VB of {101} facets will flow into the VB of {001} facets. In the TiO2/ZnS semiconductors heterojunction, photoinduced electrons at CB of ZnS will flow into the CB of {001} facets, while photoinduced holes at VB of {001} facets will flow into the VB of ZnS. In this way, it realizes carriers’ separation in the n− p−n dual heterojunction. This work improves a strategy to construct n−p−n dual heterojunction using crystal facets of photocatalysts for enhancing the photocatalytic activity.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 0571 8687 8609. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jun Zhang: 0000-0002-7510-2218 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Chinese National Natural Science Foundation (No. 51602086 and 61072015), Zhejiang Provincial Natural Science Foundation of China (No. LY16E020007 and LY13F040006), Zhejiang Xinmiao Talents Program (No. 2016R407034), Science and Technology Project of Zhejiang Province (No. 2015C37037).

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REFERENCES

(1) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107, 2891−2959. (2) Dahl, M.; Liu, Y. D.; Yin, Y. D. Composite Titanium Dioxide Nanomaterials. Chem. Rev. 2014, 114, 9853−9889. 6139

DOI: 10.1021/acs.jpcc.7b00049 J. Phys. Chem. C 2017, 121, 6133−6140

Article

The Journal of Physical Chemistry C (20) Lee, Y. S.; Gopi, C.; Venkata-Haritha, M.; Kim, H. J. Recombination Control in High-Performance Quantum Dot-Sensitized Solar Cells with a Novel TiO2/ZnS/CdS/ZnS Heterostructure. Dalton Trans. 2016, 45, 12914−12923. (21) Roy, E.; Patra, S.; Madhuri, R.; Sharma, P. K. A Single Solution for Arsenite and Arsenate Removal from Drinking Water Using Cysteine@ZnS:TiO2 Nanoparticle Modified Molecularly Imprinted Biofouling-Resistant Filtration Membrane. Chem. Eng. J. 2016, 304, 259−270. (22) Yan, L. L.; Li, Z. C.; Sun, M. X.; Shen, G. Q.; Li, L. Stable and Flexible CuInS2/Zns:Al-TiO2 Film for Solar-Light-Driven Photodegradation of Soil Fumigant. ACS Appl. Mater. Interfaces 2016, 8, 20048−20056. (23) Yan, L. L.; Qin, J. L.; Kong, L.; Zhi, H. B.; Sun, M. X.; Shen, G. Q.; Li, L. Optimized Synthesis of CuInS2/Zns:Al-TiO2 Nanocomposites for 1,3-Dichloropropene Photodegradation. RSC Adv. 2016, 6, 77777−77785. (24) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638−641. (25) Dai, Y.; Cobley, C. M.; Zeng, J.; Sun, Y.; Xia, Y. Synthesis of Anatase TiO2 Nanocrystals with Exposed {001} Facets. Nano Lett. 2009, 9, 2455−2459. (26) Han, X.; Kuang, Q.; Jin, M.; Xie, Z.; Zheng, L. Synthesis of Titania Nanosheets with a High Percentage of Exposed (001) Facets and Related Photocatalytic Properties. J. Am. Chem. Soc. 2009, 131, 3152−3153. (27) Yang, H. G.; Liu, G.; Qiao, S. Z.; Sun, C. H.; Jin, Y. G.; Smith, S. C.; Zou, J.; Cheng, H. M.; Lu, G. Q. Solvothermal Synthesis and Photoreactivity of Anatase TiO2 Nanosheets with Dominant {001} Facets. J. Am. Chem. Soc. 2009, 131, 4078−4083. (28) Li, J.; Xu, D. Tetragonal Faceted-Nanorods of Anatase TiO2 Single Crystals with a Large Percentage of Active {100} Facets. Chem. Commun. 2010, 46, 2301−2303. (29) Pan, J.; Liu, G.; Lu, G. Q. M.; Cheng, H. M. On the True Photoreactivity Order of {001},{010}, and {101} Facets of Anatase TiO2 Crystals. Angew. Chem., Int. Ed. 2011, 50, 2133−2137. (30) Xu, H.; Reunchan, P.; Ouyang, S.; Tong, H.; Umezawa, N.; Kako, T.; Ye, J. Anatase TiO2 Single Crystals Exposed with HighReactive {111} Facets toward Efficient H2 Evolution. Chem. Mater. 2013, 25, 405−411. (31) Yang, S.; Yang, B. X.; Wu, L.; Li, Y. H.; Liu, P. R.; Zhao, H. J.; Yu, Y. Y.; Gong, X. Q.; Yang, H. G. Titania Single Crystals with a Curved Surface. Nat. Commun. 2014, 5, 5355. (32) Zhang, J.; Wu, B.; Huang, L.; Liu, P.; Wang, X.; Lu, Z.; Xu, G.; Zhang, E.; Wang, H.; Kong, Z.; et al. Anatase Nano-TiO2 with Exposed Curved Surface for High Photocatalytic Activity. J. Alloys Compd. 2016, 661, 441−447. (33) Zhang, J.; Huang, L.; Liu, P.; Wang, Y.; Jiang, X.; Zhang, E.; Wang, H.; Kong, Z.; Xi, J.; Ji, Z. Heterostructure of Epitaxial (001) Bi4Ti3O12 Growth on (001) TiO2 for Enhancing Photocatalytic Activity. J. Alloys Compd. 2016, 654, 71−78. (34) Zhang, J.; Huang, L.; Yang, L.; Lu, Z.; Wang, X.; Xu, G.; Zhang, E.; Wang, H.; Kong, Z.; Xi, J.; et al. Controllable Synthesis of Bi2WO6(001)/TiO2(001) Heterostructure with Enhanced Photocatalytic Activity. J. Alloys Compd. 2016, 676, 37−45. (35) Huang, Z. A.; Sun, Q.; Lv, K. L.; Zhang, Z. H.; Li, M.; Li, B. Effect of Contact Interface between TiO2 and G-C3N4 on the Photoreactivity of G-C3N4/TiO2 Photocatalyst: (001) Vs (101) Facets of TiO2. Appl. Catal., B 2015, 164, 420−427. (36) Yu, J. G.; Low, J. X.; Xiao, W.; Zhou, P.; Jaroniec, M. Enhanced Photocatalytic CO2-Reduction Activity of Anatase TiO2 by Coexposed {001} and {101} Facets. J. Am. Chem. Soc. 2014, 136, 8839−8842. (37) Zhang, J.; Xi, J. H.; Ji, Z. G. Mo Plus N Codoped TiO2 Sheets with Dominant {001} Facets for Enhancing Visible-Light Photocatalytic Activity. J. Mater. Chem. 2012, 22, 17700−17708. (38) Jiang, Z. F.; Zhu, C. Z.; Wan, W. M.; Qian, K.; Xie, J. M. Constructing Graphite-Like Carbon Nitride Modified Hierarchical

Yolk-Shell TiO2 Spheres for Water Pollution Treatment and Hydrogen Production. J. Mater. Chem. A 2016, 4, 1806−1818. (39) Jiang, Z. F.; Wan, W. M.; Wei, W.; Chen, K. M.; Li, H. M.; Wong, P. K.; Xie, J. M. Gentle Way to Build Reduced Titanium Dioxide Nanodots Integrated with Graphite-Like Carbon Spheres: From Dft Calculation to Experimental Measurement. Appl. Catal., B 2017, 204, 283−295.

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DOI: 10.1021/acs.jpcc.7b00049 J. Phys. Chem. C 2017, 121, 6133−6140