BiOCl heterojunction with enhanced photocatalytic activity for environmental pollutant elimination

Accepted Manuscript Full Length Article Structure of Z-scheme CdS/CQDs/BiOCl heterojunction with enhanced photocatalytic activity for environmental pollutant elimination Jinbo Pan, Jianjun Liu, Shengli Zuo, Usman Ali Khan, Yingchun Yu, Baoshan Li PII: DOI: Reference:

S0169-4332(18)30203-4 https://doi.org/10.1016/j.apsusc.2018.01.189 APSUSC 38328

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

22 October 2017 10 January 2018 22 January 2018

Please cite this article as: J. Pan, J. Liu, S. Zuo, U. Ali Khan, Y. Yu, B. Li, Structure of Z-scheme CdS/CQDs/BiOCl heterojunction with enhanced photocatalytic activity for environmental pollutant elimination, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.01.189

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Structure of Z-scheme CdS/CQDs/BiOCl heterojunction with enhanced photocatalytic activity for environmental pollutant elimination Jinbo Pan, Jianjun Liu *, Shengli Zuo, Usman Ali Khan, Yingchun Yu, Baoshan Li State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Abstract Z-scheme

CdS/CQDs/BiOCl

heterojunction

was

synthesized

by

a

facile

region-selective deposition process. Owing to the electronegativity of the groups on the surface of Carbon Quantum Dots (CQDs), CQDs can be sandwiched between CdS and BiOCl, based on the stepwise region-selective deposition process. The samples were systematically characterized by X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse

reflectance

spectroscopy

(UV–vis

DRS),

photoelectrochemical

measurements and Photoluminescence (PL). The results indicate that CQDs with size of 2-5 nm and CdS nanoparticles with size of 5-10 nm dispersed uniformly on the surface of cuboid BiOCl nanosheets. The photocatalytic performance tests reveal that the CdS/CQDs/BiOCl heterojunction exhibits much higher photocatalytic activity than that of BiOCl, CdS/BiOCl and CQDs/BiOCl for Rhodamine B (RhB) and phenol photodegradation under visible and UV light illumination, respectively. The enhanced photocatalytic performance should be attributed to the Z-scheme structure of CdS/CQDs/BiOCl, which not only improves visible light absorption and the migration efficiency of the photogenerated electron-holes but also keeps high redox

ability of CdS/CQDs/BiOCl composite. Keywords: Z-scheme; CdS/CQDs/BiOCl; Region-selective deposition; Photocatalytic activity; 1. Introduction BiOCl as a kind of bismuth oxyhalides with underlying strong oxidizing ability has attracted much attention and been used as photocatalyst for industrial wastewater treatment [1, 2]. The layered structure of BiOCl consists of [Bi2 O2]2+ layers embedded in the middle of two Cl ions, which is beneficial for reducing the recombination rate of the photogenerated electron−hole pairs due to the presence of internal static electric fields [3, 4]. However, the large band gap of 3.5 eV brings not only strong oxidizing property but also a vital drawback, which is that BiOCl can only utilize the photons in ultraviolet region [5, 6]. Therefore, it is hindered for the practical application of BiOCl as a kind of wide-spectrum photocatalyst. In order to improve the photocatalytic activity of semiconductor photocatalysts, all-solid-state Z-scheme photocatalytic systems have been widely used in the water splitting, degradation of pollutants and CO2 conversion [7-9]. Compared to two-component (2C) heterojunction systems, the Z-scheme 3C heterojunction demonstrated much higher photocatalytic activity, owing to the excellent redox ability [10, 11]. All kinds of Z-scheme 3C heterojunction were researched to improve the photocatalytic activity of semiconductor photocatalysts. For instance, H Tata [12] structured

TiO2-Au-CdS

3C

heterojunction

by

a

photochemical

deposition-precipitation method, which shows much improved photocatalytic activity than that of 2C Au-TiO2 and TiO2-CdS systems. Whereafter, AgBr-Ag-AgI [13],

CdS-Ag-TiO2 [14], BiOCl-Au-CdS [15] and g-C3N4/RGO/Bi2 WO6 [16] were structured to improve the light absorption performance and the transfer efficiency of photogenerated electron-hole pairs.

However,

most

of the

Z-scheme 3C

heterojunction used noble metal nanoparticles as the charge transport material, which hinders the practical application for environmental pollutant treatment. Carbon materials, such as graphene[8, 16, 17] and CQDs [18-20] are used extensively as photoelectric materials, owing to the excellent electrical conductivity and the ability to store/release electrons, which facilitates the rate of electrontransfer in catalytic reaction. Recent years, CQDs has been using as cocatalyst to improve the visible-light absorption and the transfer efficiency of photogenerated electron-hole pairs generated by semiconductor photocatalyst. For example, the construction of heterojunction such as CQDs/TiO2[21], CQDs/Bi2 WO6[22], CQDs/Cu2O[23], CQDs/Fe2O3[24] and CQDs/Ag3PO4[25] improves the charge separation and stabilisation, which enhanced the photocatalytic activity for the photodegradation of dyes and H2 generation through water splitting. Therefore, CQDs can be used as charge transport material in Z-scheme 3C heterojunction, such as the Z-scheme photocatalytic systems of BiVO4/CQDs/CdS [26]. Besides, CdS as a kind of visible-light photocatalyst has been broadly used for the contaminants elimination and hydrogen generation due to its narrow band gap of 2.4 eV [27, 28]. Therefore, CdS was coupled with various semiconductors (TiO2[29], ZnO[30], BiOCl[31], etc.) to improve the optical absorption properties and promote the separation of photogenerated charge carriers. To the best of our

knowledge, the preparation of Z-scheme CdS/CQDs/BiOCl heterojunction and their photocatalytic activities for environmental pollution treatment have never been reported so far. In this paper, Z-scheme CdS/CQDs/BiOCl heterojunction was constructed by a facile region-selective deposition method. Cuboid BiOCl nanosheets exposing (001) facet with the size of 300-400 nm coupled with CQDs were synthesized by improved hydrothermal method, in which CQDs with the size of 2-5 nm distribute uniformly on the surface of cuboid BiOCl nanosheets. After region-selective deposition process, CdS nanoparticles with the size of 5–10 nm were anchored onto the surface of CQDs, owing to the electrostatic interaction. The photodegradation of RhB and phenol were carried out to study the photocatalytic activities of the CdS/CQDs/BiOCl nanocomposites under visible and UV light, respectively. The possible photocatalytic and region-selective deposition mechanism of CdS/CQDs/BiOCl nanocomposites were discussed.

2. Experimental 2.1 Synthesis of CQDs CQDs were synthesized by a facile hydrothermal method using glucose as precursor, the preparation process is presented as follows: 6g glucose was added into 54 mL deionized water, followed by a constant stirring for 30 min at room temperature. Then, the aqueous solution of glucose was transferred into a 100 mL teflon-lined stainless steel autoclave, treated at 160°C for 3 h. After cooling down to room temperature, the obtained CQDs were purified by dialysis against deionized

water for 12 h. 2.2 Synthesis of CdS Quantum Dots (CdS QDs) CdS QDs were prepared as following [28]: 0.5 mL TGA was added into a 200 mL 17.5 mmol CdCl2·2.5H2O solution, the pH of above solution was adjusted to 10 by dropwise addition of 1 M NaOH solution. The Na2S solution obtained by dissolving 0.84 g Na2S·9H2O into 10 mL water was added dropwise to above solution, and then the mixture was stirred at 60◦C for 1 h. After that, the solution was centrifuged and washed with deionized water. The obtained sample was redispersed in distilled water to obtain the pure CdS QDs aqueous solution. 2.3 Preparation of CQDs/BiOCl and BiOCl composites CQDs/BiOCl was prepared as follows: 3.9 mmol Bi(NO3)3·5H2O and 3.9 mmol NaCl were dissolved into 30 mL glycol and deionized water, respectively, followed by a constant stirring for 30 min at ambient temperature. Then, the NaCl solution was dropwise added into the Bi(NO3)3·5H2O solution, which was kept stirring for 30 min at room temperature. After that, 2 mL CQDs was added into above mixture. After being stirred for 30 min, the mixture solution was transferred into a 100 mL teflon-lined stainless steel autoclave, treated at 120°C for 12 h and then naturally cooled to room temperature. The samples were separated by filtration, washed with distilled water and ethanol three times and dried at 60 °C for 12h. Finally, the photocatalyst was grinded as powder. The preparation process of pure BiOCl was the same as the above mentioned process for CQDs/BiOCl synthesis without adding CQDs solution.

2.4 Preparation of CdS/CQDs/BiOCl and CdS/BiOCl composites The preparation process of CdS/CQDs/BiOCl composites was carried out as follows: 0.5 g CQDs/BiOCl powder was dispersed into 30 mL distilled water, followed by a constant stirring for 30 min. CdS QDs solution was dispersed into 20mL distilled water and treated by ultrasonic processing for 15 min. After that, CdS QDs solution was gradually added into the as-prepared CQDs/BiOCl mixture. After being stirred for 12 h, the product was collected by centrifugation and washed with distilled water for three times, followed by drying in an oven at 80 ◦C for 12h. Finally, the samples were heated at 250

◦

C in a muffle furnace for 1 h. Thus,

CdS/CQDs/BiOCl composites were prepared. The preparation process of CdS/BiOCl was the same as the above mentioned process for CdS/CQDs/BiOCl synthesis, replacing CQDs/BiOCl with BiOCl. 2.5. Characterization XRD was used to measure the crystal structure of the samples by a Bruker D8FOCUS powder diffractometer with Cu Kα irradiation (λ = 0.15406 nm). TEM and HRTEM images were taken with Hitachi 7700 microscope and JEOL-3010 microscope, respectively. SEM was carried out by Hitachi S7800. XPS was measured by an ESCALAB-250 with monochromated Al Kα radiation. UV–vis DRS absorption spectra were measured using a Shimadzu UV3600 spectrophotometer. PL spectra were performed on a Hitachi F7000 fluorescence spectrophotometer with photomultiplier tube voltage of 400 V and scanning speed of 1200 nm min−1. The photoelectrochemical measurements were measured using an electrochemical

workstation with three-electrode system (CHI660E, Chenhua Instruments Co. Shanghai, China). Pt wire and saturated calomel electrode were used as the counter electrode and the reference electrode, respectively. The working electrode was made by an indium-tin oxide (ITO) sheet glass with the active area of 1cm2 containing 10 mg photocatalyst. Transient photocurrent responses and EIS of different samples were carried out in 0.5 M Na2SO4 aqueous solution with bias potential of 50 mV under a 300 W Xe lamp with AM 1.5 cut-off filter. 2.6. Photocatalytic evaluation The photocatalytic activity of the samples was evaluated by degradation of rhodamine RhB and phenol under visible and UV light irradiation, respectively. Both of 500 W Xenon lamp with 400 nm cut off filter and 500W mercury lamp were chosen to offer visible light and UV light source, respectively. The distance between the light source and the surface of the reaction solution was 15 cm. 0.1 g photocatalyst was suspended in 100 mL of 10 ppm 20 ppm RhB or 40 ppm phenol aqueous solution, and then stirred in the dark for 30min to reach the adsorption–desorption equilibrium. During the light irradiation, 5 mL solution containing the sample was taken from the reaction suspensions every 15 min, and then centrifuged to remove the photocatalyst particles. Subsequently, the solutions were measured with the UV–vis spectrophotometer at wavelength of 554 nm and 270 nm for RhB and phenol, respectively. The concentration changes were described by C/C0, where C0 is the initial concentration of RhB and C is the remained concentration of RhB. The RhB and phenol degradation percentage (Dp) was expressed as Dp = [1 − (C/C0)] × 100%.

3. Results and discussion 3.1 Characterization and the synthetic mechanism of CdS/CQDs/BiOCl composites XRD patterns of CdS, BiOCl, CdS/BiOCl, CQDs/BiOCl and CdS/CQDs/BiOCl composites are shown in Fig. 1. All the diffraction peaks of pure CdS match to the cubic structure of CdS according to JCPDS Card No. 89-0440 [28, 32]. The diffraction peaks of pure BiOCl are in good accordance with the tetragonal structures of BiOCl (JCPDS Card No. 73-2049) without appearing any impurity peaks [33, 34]. The sharp diffraction peaks at 2θ=11.90, 25.85, 32.52 and 33.66° corresponds to the (001), (011), (110) and (012) crystal planes of BiOCl nanosheet indicating the high crystallinity at the above planes. However, no obvious diffraction peaks for CdS could be found in CdS/BiOCl nanocomposites, which can be attributed to the tiny particle size, high dispersion, and low loading amount of CdS. Similar phenomenon was observed in CdS QDs/BiOI [35]. After depositing CQDs previous to hydrothermal reaction, all the diffraction peaks, particularly those of (001) crystal planes of BiOCl nanosheet were weakened, indicating some CQDs embed into the crystal lattice of BiOCl, which is beneficial to the charge transfer. Similarly, no obvious diffraction peaks for CQDs could be found in CQDs/BiOCl and CdS/CQDs/BiOCl, such as the characterization of CQDs/BiOCl [20] composites. The SEM images of the pure BiOCl, CQDs/BiOCl, CdS/BiOCl and CdS /CQDs/BiOCl heterojunction are shown in Fig 2. It can be clearly seen from Fig 2(a),

pure BiOCl are is composed of cuboid BiOCl nanosheets with smooth surface, which are in the size of 300-400 nm. After deposition of CQDs, the surface of CQDs/BiOCl nanosheet is not smooth any more as shown in Fig 2 (b), which indicates that CQDs nanoparticles dispersed on the surface of BiOCl nanosheets. Similar phenomenon can be seen after depositing CdS on the surface of BiOCl nanosheets as shown in Fig 2 (c). Owing to the similar size and low content of CdS and CQDs, the superficial morphology of CdS/CQDs/BiOCl resembles that of CQDs/BiOCl and CdS/BiOCl. BiOCl, CQDs/BiOCl, CdS/BiOCl and CdS/CQDs/BiOCl were further characterized by TEM and HRTEM, as shown in Fig. 3. The pure BiOCl exhibits cuboid sheet-shaped morphology and the surfaces of pure BiOCl are clean and smooth shown in Fig. 3 (a), in keeping with the characterization of SEM. After coupled with CQDs, many nanoparticles with the size less than 5 nm loaded uniformly on the surface of BiOCl marked by white circles, as shown in Fig. 3 (b). Meanwhile, it can be seen that the CdS nanoparticles with an average diameter about 10 nm were deposited homogeneously on the surface of CdS/BiOCl nanosheets, which shows much smaller size and better dispersion of CdS than that of the previous work[15, 31], owing to the region-selective deposition process. Compared to CQDs/BiOCl and CdS/BiOCl heterojunctions, CdS/CQDs/BiOCl shows similar morphology, but the CdS and CQDs nanoparticles interlap together. CdS and CQDs nanoparticles interlap together in CdS/CQDs/BiOCl. Besides, CdS nanoparticles with the size of 15 nm in CdS/CQDs/BiOCl are bigger than that of CdS/BiOCl, which can be ascribed to the hydrophilic functional groups of CQDs,

shown in scheme 1, attracting the CdS QDs together. Therefore, after heated CdS nanoparticles will be a little stacked. Meanwhile, CQDs display bigger size in CdS/CQDs/BiOCl than that of CQDs/BiOCl, owing to the formation of heterojunction

between

CdS

nanoparticles

and

CQDs.

HRTEM

of

CdS/CQDs/BiOCl is presented in Figure 3 (e), the clear lattice fringes with spacing of 0.275 nm correspond to the (110) planes of BiOCl [36, 37]. The interplanar spacing is deduced to be 0.34 nm which agrees well with (111) planes reflection of the cubic lattice of CdS [32, 38]. The enlarged HRTEM image displays the size of CQDs nanoparticles with 2-5 nm and the lattice spacing was determined to be 0.32 nm, which corresponding to the (002) crystal plane of CQDs [19, 39]. In summary, the formation of sandwich structure of CdS/CQDs/BiOCl heterojunction indicates that CQDs were caught between BiOCl nanosheets and CdS nanoparticles constituting all-solid-state Z-scheme photocatalytic systems. The synthetic mechanism of Z-scheme CdS/CQDs/BiOCl heterojunction based on region-selective deposition are is illustrated in scheme 1. Owing to the hydrophilic functional groups, such as –OH and –COOH, attached on the surface of the as-synthesized CQDs [40, 41], CQDs not only can keep smaller size in the solution based on the charge rejection, but also are liable to attach the Bi ions in [Bi2O2]2+ layers during the hydrothermal treatment. Therefore, CQDs and hydrophilic functional groups can be deposited on the surface of BiOCl nanosheets. After the CQDs/BiOCl nanosheets were mixed with CdS QDs in distilled water, CdS QDs can be selectively anchored on the surface of hydrophilic functional groups of CQDs.

Therefore, the hydrophilic functional groups on the surface of CQDs can be used as the bridge, linking CdS and BiOCl. After heated in 250 ◦C, the hydrophilic functional groups in the middle of CdS, CQDs and BiOCl will be was decomposed. As a result, the all-solid-state Z-scheme CdS/CQDs/BiOCl was successfully structured. X-ray photoelectron spectroscopy The XPS of the CdS/CQDs/BiOCl heterojunction is exhibited in Fig. 4. The XPS survey spectra indicates that the sample CdS/CQDs/BiOCl is primarily composed of Bi, O, Cl, Cd, S and C elements, as shown in Fig. 4(a). In Fig. 4 (b), the peaks of Bi 4f XPS spectra at 159.0 eV and 164.2 eV belong to Bi 4f7/2 and Bi 4f5/2, respectively, indicating the Bi3+ in the BiOCl nanosheets [42, 43]. The O 1s peak at 530.5 eV can be attributed to the O2- in BiOCl shown in Fig. 4 (c) [20, 33]. The peak at 198.3 eV is ascribed to Cl 2p, which indicates the Cl- in BiOCl [44]. The two peaks located at 412.4 eV and 405.6 eV can be assigned to Cd2+, meanwhile, the peak at 164.3eV and 158.9 eV corresponds to the S 2p3/2 and S 2p1/2 in CdS [45]. The peak at 284.9 eV should be attributed to the C−C bond with sp2 orbital, which corresponds with the CQDs[39, 46]. In conclusion, it is further demonstrated that the as-prepared CdS/CQDs/BiOCl sample is composed of CdS, CQDs and BiOCl. UV–vis DRS were was used to characterize the optical absorption of pure CdS, BiOCl, CQDs/BiOCl, CdS/BiOCl and CdS/CQDs/BiOCl composites, as shown in Fig. 6. The pure CdS have an absorption edge at 540 nm, corresponding to the results reported in the related literature [28, 32].The pure BiOCl can only absorb the UV light below 370 nm, which can be assigned to the intrinsic band gap of BiOCl for 3.5

eV [5, 47]. After coupled with CdS, the CdS/BiOCl composite shows much improved optical absorption than pure BiOCl in visible light region, owing to the narrow bandgap of CdS and the bonding interaction between CdS and BiOCl. Based on excellent optical absorption and electrical conductivity properties of CQDs, the visible light absorption ability of CQDs/BiOCl and CdS/CQDs/BiOCl heterojunctions were improved further. Photoluminescence PL spectrum was used to study the recombination of photogenerated electrons and holes of the as-prepared samples. PL spectra of the pure BiOCl, CdS/BiOCl, CQDs/BiOCl and CdS/CQDs/BiOCl heterojunction in the range of 500–600 nm excited with wavelength of 360 nm are shown in Fig. 6. The PL spectrum of the pure BiOCl shows strong emission peak presenting at the wavelength of 518 nm, which indicates that the recombination efficiency of photogenerated electrons and holes of pure BiOCl is high. CdS/BiOCl composite shows lower fluorescence intensity than that of pure BiOCl, indicating the higher separation efficiency of photogenerated electron−hole pairs owing to matched band gap and interaction between CdS and BiOCl. Based on the excellent electrical conductivity properties of CQDs, the emission peaks of CQDs/BiOCl and CdS/CQDs/BiOCl are much lower than that of pure BiOCl and CdS/BiOCl, which can be interpreted that CQDs promotes the charge separation and migration efficiently. Transient photocurrents and electrochemical impedance spectra (EIS) are usually carried out to investigate the separation and transfer efficiency of photogenerated electrons and holes of semiconductor photocatalysts [15, 48]. As

shown in Fig. 7 (a), the modified samples show much higher photocurrent than that of pure BiOCl, indicating the much higher transfer efficiency of the photogenerated

electrons

and

holes

of

CdS/BiOCl,

CQDs/BiOCl

and

CdS/CQDs/BiOCl. Especially, CdS/CQDs/BiOCl shows the highest transient photocurrents among the modified samples. The mechanism of enhancing the photocurrents can be elucidated as follows: CQDs as the electron transfer medium sandwiched between CdS and BiOCl can transmit the electrons photogenerated in the conduction band of CdS and the holes photogenerated in the valence band of BiOCl effectively. Similar electron transfer medium, such as Ag [13, 14], Au [15], RGO [16], constructed the Z-scheme structure for enhanced photocatalyst which can not only improve the separation efficiency of the photogenerated electrons and holes but also keep the higher redox potentials. Beyond that, the EIS furtherly confirms the much better electronic conductivity and the charge transfer efficiency of CdS/CQDs/BiOCl, owing to smaller semicircle diameter shown in Fig. 7. (B), implying that CQDs play a crucial part in the charge transfer efficiency of Z-scheme CdS/CQDs/BiOCl photocatalyst. 3.2 Photocatalytic activity RhB was used as simulated organic pollutants to evaluate the photodegradation activities of as-prepared samples under visible light. As shown in Fig.8, the adsorption-desorption equilibrium was reached of all samples by stirring in the dark for 10 min. CdS/CQDs/BiOCl heterojunction shows much improved adsorption than other samples, which can be attributed to the specific adsorption

owing to specific layered structure of BiOCl [3, 49] and the probable porous structure piled up by CdS and CQDs. Compared to pure BiOCl, CdS/BiOCl and CQDs/BiOCl

composites

show

higher

photocatalytic

activity

for

the

photodegradation of RhB, occupying the photodegradation kinetic fitting curve with 0.0159 min−1 and 0.0252 min−1, which derives from the matched band gap of CdS/BiOCl and excellent light absorption and charge transfer properties of CQDs in CQDs/BiOCl. CdS/CQDs/BiOCl heterojunction exhibited the highest photocatalytic performance with the degradation rate constant for 0.056 min−1, which can be ascribed to the Z-scheme photocatalytic systems, keeping high charge-separation efficiency and strong redox ability of CdS/CQDs/BiOCl. The photocatalytic activity of the as-prepared samples were also evaluated by the photodegradation of phenol under UV light. As shown in Fig. 9 (a), the photodegradation ratio of phenol is about 42% after 2 h of UV light irradiation with pure BiOCl. The CdS/BiOCl and CQDs/BiOCl composites show higher photocatalytic activity than that of pure BiOCl, which is similar to the photocatalytic degradation regularity of RhB. CdS/CQDs/BiOCl heterojunction shows the best photocatalytic activity, possessing the photodegradation ratio of phenol for 99.5% within 105 min under UV light irradiation. The kinetics of phenol degradation under UV light is exhibited in Fig. 9 (b). The apparent reaction rate constants of P25, BiOCl, CdS/BiOCl, CQDs/BiOCl and CdS/CQDs/BiOCl are 0.0088 min−1, 0.0040 min−1, 0.0172 min−1, 0.0246 min−1, and 0.0385 min−1, respectively, which indicates that the Z-scheme

photocatalytic

systems

dramatically

improved

the

photocatalytic

performance of BiOCl for the degradation of phenol. In order to characterize the stability of photocatalytic activity, cyclic degradation experiments were carried out by degradation of phenol using CdS/CQDs/BiOCl as the photocatalyst under UV light, as shown in Fig.10. In the cycle evaluations, the photocatalytic activity of the CdS/CQDs/BiOCl shows little decrease after three cycle evaluations, indicating the high stability of as-prepared sample under UV light irradiation. 3.3. Mechanism of photocatalytic activity of CdS/CQDs/BiOCl composite During the photodegradation processes, a series of reactive species take part in the degradation reactions of RhB and phenol [15, 42]. In order to explore the reactive species involved in the degradation of organic pollutants over CdS/BiOCl and Z-scheme CdS/CQDs/BiOCl heterojunction, isopropyl alcohol (IPA), Ethylene Diamine Tetraacetic Acid (EDTA-2Na), and p-benzoquinone (BQ) were used as effective •OH, holes and •O2− scavengers for photocatalytic reaction. As shown in Fig 11, CdS/BiOCl and CdS/CQDs/BiOCl heterojunction were used as photocatalyst for degradation of phenol under UV light, in the presence of EDTA-2Na, BQ and IPA, which indicates that the dominant active species in the photodegradation processes are h+ and •O2−, however, the function of •OH is secondary. Besides, the photocatalytic performance of CdS/CQDs/BiOCl heterojunction for degradation of phenol in the presence of EDTA and BQ is much better than that of CdS/BiOCl, which can be attributed to the excellent redox ability in the Z-scheme photocatalytic system of CdS/CQDs/BiOCl heterojunction. Based on the above results, the photogenerated charge transfer in the CdS

/CQDs/BiOCl heterojunction is illustrated in Scheme 2. Under simulated solar light irradiation, electron-hole pairs will be generated in CdS and BiOCl. The electrons transfer from the VB of CdS and BiOCl to the CB. CQDs as the excellent electron transporting material caught in the middle of CdS and BiOCl. Therefore, the electrons in CB of BiOCl and the holes in VB of CdS are easily transferred to CQDs and recombined quickly [26]. So the photogenerated electron-hole pairs of CdS and BiOCl separate effectively. Besides, the holes with stronger oxidation ability on VB of BiOCl and the electrons with stronger reduction ability on CB of CdS are reserved, which is much better than that of the combination of CdS and BiOCl. Since the CB potential of CdS (-0.97 eV vs NHE) is more negative than E0 (O2/•O2-)(-0.046 eV), the electrons on the CB of BiOCl can react with O 2 to form •O2 - [50], meanwhile, •O2is the dominant active species in the photodegradation processes. Therefore, the structure of Z-scheme photocatalytic system of CdS/CQDs/BiOCl heterojunction promote the charge separation efficiency and reserve the excellent redox ability. Conclusion A facile region-selective deposition process was used to prepare Z-scheme CdS/CQDs/BiOCl heterojunction. CQDs and CdS with the size of 2-5 nm and 5-10 nm, respectively, distributed uniformly on the surface of BiOCl nanosheets. CQDs as the excellent photoelectric transmission material transfer the photogenerated electrons in the CB of BiOCl to the VB of CdS, which improved the separation efficiency of photogenerated electrons-holes and kept the high redox ability of CdS/CQDs/BiOCl heterojunction. Besides, the optical absorption performance of BiOCl was much

improved in visible light region after coupling CQDs and CdS. The CdS/CQDs/BiOCl composite shows enhanced photocatalytic activities for the degradation of RhB and phenol under visible and UV light irradiation, respectively. This work may be helpful for the construction of non-noble metal Z-scheme photocatalytic systems for environmental remediation and water purification.

Acknowledgements This work was financially supported by the Natural Science Foundation of China under Grant no. 10972025.

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Figure captions: Fig.1. XRD patterns of the CdS (a), BiOCl (b) , CdS/BiOCl (c), CQDs/BiOCl (d) and CdS/CQDs/BiOCl (e). Fig.2. SEM images of the (a) pure BiOCl, (b) CQDs/BiOCl, (c) CdS/BiOCl and (d) CdS /CQDs/BiOCl composites. Fig.3. TEM images of the (a)BiOCl, (b) CQDs/BiOCl, (c) CdS/BiOCl, (d) CdS/CQDs/BiOCl and HRTEM image of (e) CdS/CQDs/BiOCl. Scheme 1. Schematic illustration of the synthetic mechanism of Z-scheme CdS/CQDs/BiOCl based on region-selective deposition. Fig.4. XPS spectra of CdS/CQDs/BiOCl composite. (a) full survey spectra, (b) Bi 4f, (c) O 1s , (d) Cl 2p, (e) Cd 3d, (f) S 2p and (g) C 1s. Fig.5. UV–vis DRS of the CdS ,BiOCl, CdS/BiOCl, CQDs/BiOCl and CdS/CQDs/BiOCl composites. Fig.6. PL spectra of the pure BiOCl , CdS /BiOCl, CQDs/BiOCl and CdS/CQDs/BiOCl composites (λ ex=360nm). Fig. 7. (A) Transient photocurrents and (B) Nyquist plots of the electrochemical impedance spectra of BiOCl, CdS/BiOCl, CQDs/BiOCl and CdS/CQDs/BiOCl under simulated solar light irradiation (300 W Xe lamp with an AM 1.5 filter). Fig.8. (a)Photocatalytic degradation of RhB and (b) kinetic fit for the degradation of RhB by the P25 (a), BiOCl (b), CdS/BiOCl (c), CQDs/BiOCl (d) and CdS/CQDs/BiOCl (e) composites under

visible light. Fig.9. (a)Photocatalytic degradation of phenol and (b) kinetic fit for the degradation of phenol by the P25 (a), BiOCl (b), CdS/BiOCl (c), CQDs/BiOCl (d) and CdS/CQDs/BiOCl (e) composites under UV light. Fig.10. Cyclic degradation experiments of photocatalytic degradation of phenol over the CdS/CQDs/BiOCl composite under UV light irradiation. Fig. 11. Effect of different scavengers on the phenol degradation under UV light in the presence of (a) CdS/BiOCl and (b) CdS/CQDs/BiOCl. Scheme. 2. Photocatalytic mechanism scheme of CdS/CQDs/BiOCl composite.

Fig. 1. XRD patterns of the CdS (a), BiOCl (b) , CdS/BiOCl (c), CQDs/BiOCl (d) and CdS/CQDs/BiOCl (e).

Fig. 2. SEM images of the (a) pure BiOCl, (b) CQDs/BiOCl, (c) CdS/BiOCl and (d) CdS /CQDs/BiOCl composites.

Fig. 3. TEM images of the (a)BiOCl, (b) CQDs/BiOCl, (c) CdS/BiOCl, (d) CdS/CQDs/BiOCl and HRTEM image of (e) CdS/CQDs/BiOCl.

Scheme. 1. Schematic illustration of the synthetic mechanism of Z-scheme CdS/CQDs/BiOCl based on region-selective deposition.

Fig. 4. XPS spectra of CdS/CQDs/BiOCl composite. (a) full survey spectra, (b) Bi 4f, (c) O 1s , (d) Cl 2p, (e) Cd 3d, (f) S 2p and (g) C 1s.

Fig. 5. UV–vis DRS of the CdS ,BiOCl, CdS/BiOCl, CQDs/BiOCl and CdS/CQDs/BiOCl composites.

Fig. 6. PL spectra of the pure BiOCl , CdS/BiOCl, CQDs/BiOCl and CdS/CQDs/BiOCl composites (λ ex=360nm).

Fig. 7. (A) Transient photocurrents and (B) Nyquist plots of the electrochemical impedance spectra of BiOCl, CdS/BiOCl, CQDs/BiOCl and CdS/CQDs/BiOCl under simulated solar light irradiation (300 W Xe lamp with an AM 1.5 filter).

Fig. 8. (a)Photocatalytic degradation of RhB and (b) kinetic fit for the degradation of RhB by the P25 (a), BiOCl (b), CdS/BiOCl (c), CQDs/BiOCl (d) and CdS/CQDs/BiOCl (e) composites under visible light.

Fig. 9. (a)Photocatalytic degradation of phenol and (b) kinetic fit for the degradation of phenol by the P25 (a), BiOCl (b), CdS/BiOCl (c), CQDs/BiOCl (d) and CdS/CQDs/BiOCl (e) composites under UV light.

Fig. 10. Cyclic degradation experiments of photocatalytic degradation of phenol over the CdS/CQDs/BiOCl composite under UV light irradiation.

Fig. 11. Effect of different scavengers on the phenol degradation under UV light in the presence of (a) CdS/BiOCl and (b) CdS/CQDs/BiOCl.

Scheme 2. Photocatalytic mechanism scheme of CdS/CQDs/BiOCl composite.

Graphical abstract

Z-scheme CdS/CQDs/BiOCl heterojunction was synthesized by a stepwise facile region-selective deposition process.

Highlights 1. Z-scheme CdS/CQDs/BiOCl heterojunction was synthesized by facile region-selective deposition process. 2. CQDs with size of 2-5 nm were sandwiched between BiOCl nanosheets and CdS nanoparticles with size of 5-10 nm. 3. The sandwich-like Z-scheme CdS/CQDs/BiOCl heterojunction keeps not only excellent redox ability but also efficient charge transfer and improved visible light absorption. 4. The photocatalytic degradation activities of CdS/CQDs/BiOCl heterojunction are excellent for RhB dyes and phenol contaminant under visible and UV light, respectively.