Construction of Z-Scheme System for Enhanced Photocatalytic H2

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Article 2

Construction of Z-scheme System for Enhanced Photocatalytic H Evolution Based on CdS Quantum Dots/CeO Nanorods Heterojunction 2

Yongjin Ma, Yuan Bian, Yi Liu, Anquan Zhu, Hong Wu, Hao Cui, Dewei Chu, and Jun Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04049 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Sustainable Chemistry & Engineering

Construction of Z-scheme System for Enhanced Photocatalytic H2 Evolution Based on CdS Quantum Dots/CeO2 Nanorods Heterojunction

Yongjin Maa, Yuan Biana, Yi Liua, Anquan Zhua, Hong Wua, Hao Cuib,*, Dewei Chuc, Jun Pana,*

a State Key Laboratory for Powder Metallurgy, Central South University, Changsha Lushan South Road 932, P. R. China b Sino-Platinum Metals Co. Ltd., Kunming Institute of Precious Metals, Kunming, People West Road121, P. R. China c School of Materials Science & Engneering, University of New South Wales, Sydney, NSW 2052, Australia. *To whom correspondence should be addressed: [email protected]

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ABSTRACT Photocatalytic hydrogen evolution from water splitting is a promising approach in energy conversion and storage. Here, the 0D/1D CdS Quantum Dots (QDs)/CeO2 nanorods heterojunction was designed and fabricated by a facile two-step method. The optimum photocatalytic H2 evolution activity for /CeO2-based composites with 3 at. % CdS QDs (101.12 µ mol h−1 g−1) was 45 times as high as that of pure CeO2 nanorods (2.25 µ mol h−1 g−1) under light irradiation. Meanwhile, the photocurrent response intensity increased 17.75 times higher than pure CeO2 nanorods. Furthermore, the 0D/1D CdS QDs/CeO2 heterojunctions exhibited enhanced photocatalytic stability for long lifetime (60 h). The reasons that dramatically enhanced photocatalytic performance could be the improved light harvesting, enhanced photoresponse and stronger electronic conductivity while the CdS QDs was loaded in CeO2 nanorods to form the 0D/1D heterojunctions CdS QDs/CeO2 nanocomposites. What’s more, the remarkably increased photocatalytic performance of CdS QDs/CeO2 composites was mainly attributed to the Z-scheme between CdS QDs and CeO2 nanorods, which was confirmed by the PL (photoluminescence) method. Therefore, the proposed system is highly promising for large scale photocatalytic hydrogen evolution.

KEYWORDS: 0D/1D heterojunctions, CdS Quantum Dots, CeO2 nanorods photocatalytic H2 evolution, Z-scheme photocatalytic system


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INTRODUCTION For decades, semiconductor photocatalysts are pivotal function materials for

environmental protection, H2 evolution, photodetectors and photovoltaic devices.1-5 With the advantages of zero-carbon emission, recyclability and great efficiency, photocatalytic water splitting into hydrogen is a promising approach in energy conversion and storage.6-8 However, photocatalytic activity and stability are important factors in large scale H2 evolution. The potential ways for improving photocatalytic activity and stability are to develop the efficient semiconductor photocatalysts that can utilize the photon energy across the whole solar spectrum and concurrently enhance the transfer and separation of the photogenerated carriers on the suitable energy levels for implementing the redox reaction in photocatalytic H2 evolution.9-11 Hence, numerous attempts have been performed toward developing the efficient and recyclable photocatalysts.12-20 Interestingly, extending light-harvesting range of the semiconductor photocatalysts is leading to improve its redox ability.21,22 A socalled direct Z-scheme photocatalytic system has put forward in order to solve this problems, which not only extends light-harvesting range, but also enhances the photogenerated carriers transfer and separation and implements the semiconductor photocatalysts redox reaction.23-36 Xiong’s group has fabricated the Janus-like g-MnS/ Cu7S4 structures displaying dramatically enhanced photocatalytic hydrogen production rates by construction of Z-scheme system.37 More and more Z-scheme semiconductor photocatalysts are



explored to facilitate the charge transfer between semiconductors for promoting H2 evolution. As is known, 0-dimensional (0D) and 1-dimensional (1D) nanomaterials have been explored in many kinds of photocatalysts. However, several drawbacks largely impede their practical applications, for 0D nanomaterials, including vulnerable self-aggregation, abundant surface defects and heavy recombination of photoinducted charges compared with their bulk counterparts.38,39 Besides, the rigorous synthesis condition and low productivity limit its practical applications. Meanwhile, 1D materials get a lot of attention due to their unique advantages over bulk materials such as short path lengths for ion insertion/extraction and electronic transport, a very large surface to volume ratio in the field of electronic applications, its homogenous dispersion and stability in photocatalytic reaction. Cerium dioxide (CeO2) is one of the most potential photocatalysts.40,41 As one of metal oxides, it has some inherent advantages including stability, abundant in natural, nontoxic and low-cost, which has been widely researched in multitudinous semiconductor photocatalysis materials.42-45 In addition, it has recently proven that the photocatalytic activities of cerium-based materials are determined not only by low dimensional nanostructure, non-photo corrosive but also by the catalyst kinetics distributions of oxygen vacancies and catalytic active sites.46 However, pure CeO2 has a relatively low photocatalytic activity caused by weak light absorption and poor charge separation. Accordingly, various strategies were attempted to solve those problems, such as ion doping, noble-metal loading and the construction of a heterojunction structure. CdS QDs have lots of advantages such as unique small size


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(< 10 nm), suitable band gap, short effective charge-transfer length and extensive light-harvested, which make it a highly promising research in semiconductor photocatalytic field.47-50 T.Q. Lian and his co-workers have reported that excitons in QDs could dissociate by either electron (e-) or hole (h+) transfer, although the control factors not well understood.51 However, CdS also suffered from photocorrosion by its photoinducted h+ resulting to the decrease of photocatalytic activity.52-56 Thus it is critical to separate or transfer photoinducted h+. What is worth mentioning is that the construction of heterojunction structure contribute to improve separation and restrain the recombination of the photoinducted charges. As a result, it’s possbile to obtain enhanced photocatalytic activity due to high efficiency of the interfacial charge transfer. Herein, the 0D/1D CdS QDs/CeO2 nanorods heterojunction were rationally designed and fabricated by a facile two-step method. As a result, the photocatalytic H2 evolution activity was conspicuously improved and the photocurrent response intensity was remarkably increased under light irradiation. The reasons that the enhanced photocatalytic activity of CdS QDs/CeO2 composites were also explored by DRS, transient photocurrent, EIS and PL. Furthermore, the possible Z-scheme photocatalytic mechanism was in-depth discussed.

EXPERIMENTAL SECTION Synthesis of the Photocatalysts. All chemicals are analytical reagents without

further purification. Cerium nitrate hexahydrate, Cadmium chlorlde, Sodium sulfide nonahydrate, Sodium sulfite anhydrate and Thioglycolic acid are purchased in



Aladdin. Sodium hydroxide is purchased in Xilong Chemical Co. In this work, the synthesized CdS QDs are refered to a paper by Feng et al.57 Specifically, 0.05 L of 10 mM Cadmium chlorlde aqueous solution is poured into 100 mL beaker at first; Following, 0.25 mL Thioglycolic acid is added to above solution; Next, adjusting the pH reached to 11 by adding 1 M Sodium hydroxide aqueous solution. Then, 5 mL of 0.1 M Sodium sulfide nonahydrate aqueous solution was dropwise injected into above solution and stirred for 4 h. The reaction was always under N2 atmosphere. Finally, CdS QDs were synthesized. In a novel solvothermal approach, CeO2 nanorods were synthesized as following: 1mmol Cerium nitrate hexahydrate was added in 30 mL 4M Sodium hydroxide aqueous solution for 120 °С hydrothermal reaction keeping 12 h. CdS QDs/CeO2 composites are prepared with tunable molar ratios of CdS QDs and Ce(NO3)3 in a range of 1:100, 2:100, 3:100 and 4:100 regarded as CdS QDs/CeO2-1, CdS QDs/CeO2-2, CdS QDs/CeO2-3 and CdS QDs/CeO2-4, respectively. The samples were washed three times with deionized water by centrifugation and then dried in vacuum. Characterization. XRD (X-ray diffraction) researches were performed on a D/max 2550, Rigaku Corporation. The range is from 10° to 80°. The scanning electron microscope (Nova Nano SEM 230, FEI Electron Optics B.V) and field emission transmission electron microscope (JEM-2100F, Japanese electronics co. LTD) were used to observe shape, size and lattice structure of the prepared samples. Surface chemical compositions and chemical status were analyzed by X-ray photoelectron spectroscopy (K-Alpha 1063, Thermo Fisher). Photoluminescence spectra were


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measured on the fluorescence spectrophotometer (F-4600, Japan's Hitachi LTD) at an excited wavelength of 328 nm. The UV-Vis spectra were obtained by the UV-Vis spectrometer (Evolution 220, Thermo Fisher Scientific) to characterize the light response range of the above samples and BaSO4 was utilized as the reflectance standard material. The surface area of the as-prepared samples was investigated with a micromeritics ASAP 2010 system at 77.3 K. Transient PL was detected with 328 nm emission wavelength by FLS980 series of fluorescence spectrometers. Electron spin resonance (ESR) testing was in JES FA200. Photocatalytic Measurement. Photocatalytic H2 evolution reaction was done under A 300 W Xe lamp (Perfect Light, Microsolar300, λ> 300 nm). Typically, 100 mg sample was added in 80 mL aqueous solution containing 20 mmol sodium sulfite anhydrate and 25 mmol sodium sulfide nonahydrate as the sacrificial electron donor and kept it for vigorous stirring. The hydrogen amount was periodically detected by an on-line gas chromatograph (GC, 4000) with a thermal conductivity detector (TCD) using N2 as a carrier gas. The incident light intensity was measured by the hand-held optical photometer (Thorlabs, S302C) and Power Energy Meter w. Graphics Display (Thorlabs, PM100D). Photoelectrochemical Measurements. The photoelectrochemical measurements were performed on a standard three electrode cell with a 1 cm2 platinum sheet as the counter electrode and an Ag/AgCl electrode in 3 M KCl as the reference electrode. The prepared samples deposited on In-doped SnO2-coated glass (ITO coated glass, Zhuhai Kaivo Optoelectronic Technology Co., Ltd) were employed as the working



electrode with an area of 1 cm2, Sodium sulfite anhydrate aqueous solution (0.2 M, pH = 10.2) was used as the electrolyte. The photocatalyst was dispersed in Ethylene glycol (Sinopharm Chemical Reagent Co., Ltd.), and the suspension was added dropwise onto the ITO glass under 393 K to speed drying. The film was annealed at 513 K for 0.5 h under a flow of N2 gas, and the typical surface density of the photocatalyst was about 1 mg cm-2. The light source was the same as the Photocatalytic measurement. The transient current-time (I-t) curves with light on/off cycles measurements were performed in an electrochemical workstation (CHI 660E, Shanghai Chenhua Instrument Co. Ltd. China). The Mott–Schottky measurement and electrochemical impedance spectroscopy (EIS) were performed under 0.2 V (vs NHE).

RESULTS AND DISCUSSION X-ray Diffraction. The XRD patterns of the obtained samples indicated the

formation of CeO2 cubic phase showed in Fig. 1. All of the diffraction peaks matched well with JCPDS card No. 34-0394 of CeO2 and there was no impurity in CeO2 sample. Besides, the characteristic peaks of CeO2 cubic phase were observed at 28.54°, 47.50° and 56.15° corresponding to (111), (220) and (311) crystal planes, respectively. Interestingly, a peak at 42.5° was found in CdS QDs/CeO2 nanorods with the amount of CdS QDs increased.58 This peak was assigned to the (110) plane of the CdS, which confirmed that the CdS QDs/CeO2 composites were successfully synthesized. SEM, TEM and HRTEM Analysis. The morphologies of prepared samples were


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explored by SEM as shown in the Fig. 2. Obviously, pure CeO2 and CdS QDs/CeO2 composites samples had no other morphology but uniform nanorods with an average length of around 400 nm and a width of about 15 nm. The constituent of CeO2 was constituted by Ce and O elements as shown in Fig. 2 (f). The morphology and structure of the synthesized samples were farther elucidated by TEM as shown in Fig. 3. Fig. 3 (a), (c) and (e) showed the TEM images of the CdS QDs, CeO2 nanorods and CdS QDs/CeO2-3 composites, respectively. From the Fig.3 (a), the CdS QDs had good dispersity and the sphere diameter was about 5 nm. The HRTEM of the CdS QDs as shown in Fig. 3 (b), the crystallographic planes spacing of was determined to be 0.32 nm, which agrees well with the (002) CdS.59 This result indicated CdS QDs was successfully synthesized. The pure CeO2 nanorods and CdS QDs/CeO2-3 composites had no obvious difference in morphology as shown in Fig. 3 (c) and (e). Fig. 3 (d) mainly displayed the lattice spacing about 0.33 nm, which corresponding to the (111) plane of CeO2.The (002) plane of CdS and the (111) plane of CeO2 were observed in Fig.3 (f), which farther confirmed that the CdS QDs/CeO2 composites were successfully synthesized. Furthermore, the mapping images showed that O, Ce, S and Cd were distributed uniformly over the entire architecture as shown in Fig. 4, which confirmed that CdS QDs uniformly dispersed in the surface of CeO2 nanorods. The TEM images clearly manifested that the 0D/1D CdS QDs/CeO2 nanorods composites were synthesized. XPS Analysis. To farther confirm the elemental composition and reveal the chemical status of the obtained samples, the XPS patterns of the as-prepared samples was performed



as shown in Fig. 5. Fig. 5 (a) displayed the survey XPS spectrum of pure CeO2 and QDs CdS/CeO2-3, which showed QDs CdS/CeO2-3 had the elements of Ce, O, Cd and S, respectively, pure CeO2 had the peaks of Ce and O. The detected Cd and S peaks indicated the existence of CdS in QDs CdS/CeO2-3 sample. The present of the Ce and O peaks indicated the existent of CeO2. Fig. 5 (b) displayed the XPS spectrum of O 1s for the pure CeO2 and QDs CdS/CeO2-3. The peak at 529.4 eV was corresponded to O2- in the normal crystal structure and the higher binding energy (BE) peak was possibly related to surface adsorbed O2- or OH-.60,61 Fig. 5 (c) showed the XPS spectrum of Ce 3d for the pure CeO2 and QDs CdS/CeO2-3. The peaks at BEs 882.50, 888.45, 898.51, 900.67, 907.23, and 916.66 eV were corresponded to Ce4+ and Ce3+, respectively. The observed peaks could be assigned to CeO2 (Ce4+) as 898.51, 888.45, 882.50 and 916.66, 907.23, 900.67eV for Ce 3d5/2 and Ce 3d3/2, respectively. 62-64 the peaks at 888.45 and 882.50 were assigned to Ce3+ 3d5/2 and the peaks at 907.23 and 900.67 eV were assigned to Ce3+ 3d3/2, respectively. Besides, the peaks at 405.30 and 412.04 eV were assigned to Cd 3d as shown in Fig. 5 (d), which matched well with the Cd 3d5/2 and Cd 3d3/2 of Cd2+ in the CdS QDs. Fig. 5 (e) showed the main S 2p peaks centered at 161.71 eV and 162.74 eV, which match well with the S ions in CdS QDs.65 Further, the XPS spectra farther confirmed that the 0D CdS Quantum Dots loaded in 1D CeO2 nanorods and the formation 0D/1D CdS QDs/CeO2 heterojunctions had enhanced stability and catalytic activity. UV-vis Dffuse Reflection Spectra Analysis. Fig. 6 (a) showed the UV-vis diﬀuse reflection spectra of the pure CeO2 and CdS QDs/CeO2 samples. A significant


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phenomenon was observed that the photoresponse and absorption intensity were gradually enhanced with the amount of CdS QDs increased. What’s more, the CdS QDs/CeO2-3 had better photoresponse and absorption intensity in CdS QDs/CeO2 composites, obviously. Adsorption intensity was in the order of pure CeO2 < CdS QDs/CeO2-1 < CdS QDs/CeO2-4 < CdS QDs/CeO2-2 < CdS QDs/CeO2-3 according to the UV–vis diffuse reflection spectra. Besides, the absorption edges were same for CdS QDs/CeO2 composites. To estimate the bandgap energy (Eg) of the as-obtained samples that the typical Tauc equation was ulteriorly employed as equation a*Ephoton = K (Ephoton - Eg) 2. And the (aEphoton)2 versus Ephoton curves of samples were shown in Fig. 6 (b). The Eg of CeO2 was 2.95 eV and the Eg of CeO2 was 2.94-3 eV for CdS QDs/CeO2 composites and the Eg of CdS was 2.30-2.37 eV. This result was consistent with reported literatures.66-69 Photocatalytic Hydrogen Evolution Analysis. To compare the photocatalytic activity between pure CeO2 nanorods and 0D/1D CdS QDs/CeO2, photocatalytic hydrogen evolution experiments were carried out as shown in Fig. 7 (a). The results indicated 0D/1D heterojunctions of CdS Quantum Dots/ CeO2 nanorods had better photocatalytic hydrogen evolution under light irradiation. The average rates were 2.25, 54.13, 87.07, 101.12 and 59.72 µmol h-1g-1 for pure CeO2 nanorods, CdS QDs/CeO2-1, CdS QDs/CeO2-2, CdS QDs/CeO2-3 and CdS QDs/CeO2-4, respectively, showed in Fig. 7 (b). Besides, the composites possessed excellent stability in 60 h compared to pure CeO2 nanorods. What’s more, the optimal Moore ratio was 3 % of CdS QDs to CeO2.

The reason can be ascribed to that the CdS QDs improved the light



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absorption of CeO2 and decreased charge transfer resistance and promoted the charge separation for photocatalytic H2 evolution. This could be well-evidenced by fluorescence analysis, transient photocurrent evolution and EIS, which were effective and sensitive measures to confirm the transfer and recombination of photoinduced charges. BET and PL Analysis. As is known, the photocatalytic activity was tightly associated with the activity sites of the photocatalyst surface. In other words, the number of activity sites had a close-kint connection with the BET surface areas, so the BET was tested showed in table 1. The BET surface areas were 107.62, 108.85, 106.08, 104.19 and 104.90 m2 g-1 for CeO2 nanorods, CdS QDs/CeO2-1, CdS QDs/CeO2-2, CdS QDs/CeO2-3 and CdS QDs/CeO2-4, respectively.

The surface

areas of the obtained samples were roughly equal from Table 1. Thus the BET surface areas had a little influence for photocatalytic activity. The recombination rate of photogenerated carries in CdS QDs/CeO2 nanocomposites were tested, which was clearly decreased relative to pure CeO2 nanorods by photoluminescence spectra at 328 nm excitation wavelength in Fig. 8 (a). The significant depression of the CdS QDs/CeO2 composition emission suggested that effective charges separation occurred at the CdS QDs and CeO2 interfaces. Furthermore, the transient PL plot of the CeO2 and CdS QDs/CeO2-3 composites were performed showed in Fig. 8 (b). The average lifetimes of charge carriers in pure the CeO2 and CdS QDs/CeO2-3 composites were estimated by fitting the decay curve with three exponential terms to yield 2.14 and 4.05 ns, respectively. The enhanced lifetime for CdS QDs/CeO2-3 composites


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indicated fast separation and transfer of the charge carriers. It further confirmed that CdS QDs/CeO2 composites possessed higher photocatalytic activity.

Table 1 the BET of the prepared samples.

Sample

BET(m2/g)

Pure CeO2

107.62

CdS QDs/

CdS QDs/

CdS QDs/

CdS QDs/

CeO2-1

CeO2-2

CeO2-3

CeO2-4

108.85

106.08

104.19

104.90

Transient Photocurrent and EIS Analysis. To farther study the role of CdS QDs for the improvement photogenerated charge separation and transfer properties in CdS QDs/CeO2 composite, transient photocurrent evolution and electrochemical impedance spectroscopy (EIS) were presented in Fig. 8 (c) and (d). As expected every photocatalyst showed obvious photocurrents with good reproducibility when they were illuminated by light. Furthermore, the photocurrents on CdS QDs/CeO2 composites were higher than the pure CeO2 nanorods, which indicated the introduction of CdS QDs formed heterostructure between 0D CdS QDs and 1D CeO2 nanorods significantly suppressed the recombination of photoexcited electrons and holes. Besides, CdS QDs/CeO2-3 nanocomposites showed the highest photocurrent in CdS QDs/CeO2 composites, which farther confirmed the CdS QDs/CeO2-3 had the optimal photocatalytic H2 evolution performance. Moreover, the Nyquist impedance plots for pure CeO2 nanorods and CdS QDs/CeO2-3 nanocomposites samples without light illumination were shown in Fig. 8 (d). Obviously, the arc radius of CdS QDs/CeO2-3 electrode was smaller than the pure CeO2 electrode, which suggested that the CdS QDs/CeO2-3 composites had a stronger electronic conductivity in the nonphotoexcitated state and this structure would be very beneficial for photoexcitated charges separation.70 It determined that the CdS QDs/CeO2 composites had lower charge transfer resistance. This would increase the light utilization and decrease the carrier recombination. This could be farther confirmed the CdS QDs/CeO2-3



composites

possessed enhanced

photocatalytic

H2 evolution

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and

transient

photocurrent. Crystal Structure Stability Analysis. To confirm the sample stability of crystal structure after 60 h photocatalytic H2 evolution, the before and after reaction XRD of the CdS QDs/CeO2-3 sample was showed in Fig. 9. From the results, we are confident that the crystalline structure was no obvious change after 60 h photocatalytic H2 evolution. It confirmed the CdS QDs/CeO2-3 sample had enhanced photostability. Possible Photocatalytic Mechanism Exploration. Possible photocatalytic mechanism for the photocatalytic hydrogen evolution over the CdS/CeO2 heterostructures was explored. The band gap of the CdS and CeO2 were 2.35 and 2.95 eV, respectively, as shown in Fig. 6 (b). The flat band of the CdS and CeO2 were -0.79 and -0.33 eV, respectively, as shown in Fig. 10. The conduction ban was negative about 0.1 eV compared to the flat band. Thus the conduction band (CB) of the CdS and CeO2 were -0.89 and -0.43 eV, respectively. Accordingly, the valence band (VB) of the CdS and CeO2 were 1.46 and 2.52 eV, respectively. The possible photocatalytic mechanism, the heterojunction-type and Z-scheme, was explored over the CdS QDs/CeO2 heterostructures as shown in Fig. 11. In order to explore the possible photocatalytic mechanism, hydroxyl radicals (•OH) (active species) was detected by the PL method showed in Fig. 13 (a).71 Terephthalic acid (TA) was generally utilized as a probe molecule because it can react with •OH to generate the fluorescent agent 2-hydroxyterephthalic acid (TAOH).72 Thus the fluorescence intensity of TAOH was proportional to the generated amount of •OH. The detected fluorescence spectra of TAOH at a wavelength of 425 nm were showed in Fig. 13 (a). No fluorescence was detected before irradiation, which indicated that there was no •OH in the original solution. The fluorescence intensity increased gradually with extended irradiation time, indicating that more and more •OH radicals were generated under light irradiation. However, the VB position of CdS (+1.46 eV) was more negative than E (•OH/OH-) (+2.38 V vs NHE) and E (•OH/ H2O) (+2.27 V vs NHE), the holes in the VB of CdS could’t react with adsorbed water molecules (or surface hydroxyls) to form •OH radicals. In contrast, the VB position of CeO2 (+2.52 eV) was more positive ACS Paragon Plus Environment

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than E (•OH/OH-) and E (•OH/H2O), thus holes in the VB of CeO2 could react with these groups to produce •OH radicals. The generation of a large number of •OH radicals, confirmed by the fluorescence experiment, suggested that the holes must come from the VB of CeO2. To investigate the reactive oxygen species generated during the photocatalytic process and confirm the reaction mechanism, the DMPO spin-trapping ESR was employed on CdS QDs/CeO2-3 composites as shown in Fig. 12 (a) and (b). No signals were detected for CdS QDs/CeO2-3 composites when the system was in the dark, while six characteristic signals of DMPO-•O2- and four characteristic signals of DMPO-•OH could be clearly observed under light irradiation. The DMPO-•O2- DMPO-•OH signals intensities were strengthened with light irradiation time enhanced. Therefore, a dual mechanism in the photocatalytic process is predicted, involving oxidation of both •OH radicals and •O2- radicals. Therefore, it could certainly be concluded that the photogenerated holes tend to remain in the VB of CeO2. A direct Z-scheme photocatalytic system were proposed as shown in Fig. 13 (b). 3D-simulated charge distribution at the CdS QDs/CeO2 interface as shown in Fig. 13 (c).

CONCLUSION In summary, we have succeeded in preparing the 0D/1D heterojunctions CdS QDs/ CeO2 nanorods. The composites had been tuned by varying the mass content of the CdS QDs from 0 to 4 %. The optimum mass content of the CdS QDs was 3 at %, where its photoactivity in H2 evolution was up to 45 times as high as that of pure CeO2 nanorods under light irradiation. Furthermore, the remarkably increased photocatalytic performance of CdS QDs/CeO2 composites was mainly attributed to the Z-scheme between CdS QDs and CeO2 nanorods, which was confirmed by the PL (photoluminescence) method. The Z-scheme between CdS QDs and CeO2 nanorods could consume the h+ generated by CdS QDs employing the e- generated by CeO2



nanorods, which improved the photocatalytic activity and stability. Therefore, our proposed system is highly promising for large scale photocatalytic hydrogen evolution.

ACKNOWLEDGEMENTS We great acknowledge financial support by Science Fund for Distinguished Young Scholars of Hunan Province (2015JJ1026), Program for Shenghua Overseas Talent (90600-903030005; 90600-996010162), the project of Innovation–driven plan in Central South University (2015CXS004, 2016CX003) and the National Natural Science Foundation of China (11674398).


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FOR TABLE OF CONTENTS USE ONLY

3D-simulated charge distribution at the CdS QDs/CeO2 nanorods 0D/1D heterostructures.



Fig.1 The XRD patterns of pure CeO2, CdS QDs/CeO2-1, CdS QDs/CeO2-2, CdS QDs/CeO2-3 and CdS QDs/CeO2-4. 47x37mm (300 x 300 DPI)


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Fig.2 The SEM images for (a) pure CeO2; (b) CdS QDs/CeO2-1; (c) CdS QDs/CeO2-2; (d) CdS QDs/CeO2-3; (e) CdS QDs/CeO2-4 and (f) the EDX of pure CeO2. 81x45mm (300 x 300 DPI)



Fig.3 The TEM and the HRTEM images for (a, b) CdS QDs; (c, d) pure CeO2; (e, f) QDs CdS/CeO2-3. 144x144mm (300 x 300 DPI)


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Fig.4 The STEM images for (a) CdS QDs/CeO2; (b) the mapping of all elements; (c) O; (d) Ce; (e) S; (f) Cd. 146x128mm (300 x 300 DPI)



Fig.5 The XPS spectras for (a) the survey spectra of the pure CeO2 and QDs CdS/CeO2-3; (b) O1s; (c) Ce 3d; (d) S 2p; (e) Cd 3d. 119x140mm (300 x 300 DPI)


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Fig.6 (a) UV–vis diffuse reflection spectra and (b) plot of (αhv)2 versus band gap (hv) of as-obtained samples. 49x20mm (300 x 300 DPI)



Fig. 7 (a) Photocatalytic hydrogen evolution in 60 h and (b) photocatalytic hydrogen evolution rates over the prepared samples. 109x46mm (300 x 300 DPI)


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Fig. 8 (a) the PL plot; (b) the transient PL plot; (c) the transient photocurrent evolution plot of the asobtained samples; (d) the EIS plot of the the CeO2 and CdS QDs/CeO2-3 composition. 119x99mm (300 x 300 DPI)



Fig. 9 the before and after reaction XRD of the CdS QDs/CeO2-3 sample. 49x41mm (300 x 300 DPI)


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Fig. 10 the Ms plot of CdS QDs/CeO2-3 composites. 60x50mm (300 x 300 DPI)



Fig. 11 The possible photocatalytic mechanism exploration of the heterojunction-type and Z-scheme over the CdS QDs/CeO2 nanorods 0D/1D heterostructures. 70x44mm (300 x 300 DPI)


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Fig. 12 the DMPO spin-trapping ESR spectra of CdS QDs/CeO2-3 composites (a) DMPO-•O2−and (b) DMPO•OH under dark and light irradiation. 49x22mm (300 x 300 DPI)



Fig.13 (a) Fluorescence spectra of CdS QDs/CeO2-3 in a TA solution irradiated at different irradiation times; (b) Proposed mechanism for the photocatalytic hydrogen evolution over the CdS QDs/CeO2 heterostructures; (c) 3D-simulated charge distribution at the CdS QDs/CeO2 nanorods 0D/1D heterostructures. 98x96mm (300 x 300 DPI)


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Construction of Z-Scheme System for Enhanced Photocatalytic H2

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