Carbon quantum dots assisted strategy to synthesize [email protected] for boosting photocatalytic hydrogen evolution performance of CdS

Journal Pre-proofs Carbon quantum dots assisted strategy to synthesize Co@NC for boosting photocatalytic hydrogen evolution performance of CdS Xiangyu Meng, Chenchen Zhang, Congzhao Dong, Wanjun Sun, Dong Ji, Yong Ding PII: DOI: Reference:

S1385-8947(20)30423-X https://doi.org/10.1016/j.cej.2020.124432 CEJ 124432

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

9 December 2019 19 January 2020 11 February 2020

Please cite this article as: X. Meng, C. Zhang, C. Dong, W. Sun, D. Ji, Y. Ding, Carbon quantum dots assisted strategy to synthesize Co@NC for boosting photocatalytic hydrogen evolution performance of CdS, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124432

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Carbon quantum dots assisted strategy to synthesize Co@NC for boosting photocatalytic hydrogen evolution performance of CdS

Xiangyu Menga,#, Chenchen Zhanga,#, Congzhao Donga, Wanjun Suna, Dong Jib, Yong Ding*,a,c

aState

Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metals

Chemistry and Resources Utilization of Gansu Province, and College of Chemistry and Chemical Engineering, Lanzhou University, 222 Tianshui South Road, Lanzhou 730000, P. R. China bCollege

of Petrochemical Technology, Lanzhou University of Technology, Langongping Road 28

7, Lanzhou 730050, P. R. China cState

Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical

Physics, Chinese Academy of Sciences, 18 Tianshui Middle Road, Lanzhou 730000, P. R.China #These

authors contributed equally to this work

* E-mail: [email protected]

1

Abstract Photocatalytic water splitting accompanied by the hydrogen production as a prospective method has been placed on solving the double issues of the energy crisis and environment pollution. Therefore, the development of stable and efficient hydrogen evolving photocatalysts is pretty important and urgent. Herein, the Co@NC (Co nanoparticles wrapped by nitrogen doped carbon layers) derived from the mixture of carbon quantum dots (CQDs) and Co2+ as the cocatalyst is loaded on the surfaces of the CdS nanorods (NRs) for photocatalytic hydrogen production. The results show a hydrogen evolution activity of 21.8 mmol g-1 h-1 for CdS with 3 wt % loading amount of Co@NC (CdS-3) and 29.8 times higher than that of CdS alone. In addition, an apparent quantum efficiency (AQE) of 41.8 % at 420 nm over the CdS-3 is obtained. The enhanced photocatalytic performance of the hybrid nanostructure is largely ascribed to the effective electron transfer (ET) between the CdS NRs and Co@NC,

which

is

confirmed

by

the

electrochemical

experiments

and

photoluminescence (PL) spectra. In all, this work supplies a novel strategy to synthesize universal photocatalytic cocatalyst. Keywords: Photocatalysis; 1D CdS; Carbon quantum dots; Hydrogen evolution reaction; Synergistic effect

2

1. Introduction Photocatalytic water splitting as a prospective method is placed on solving the issues of the energy crisis and environment pollution [1-3], where the hydrogen fuel obtained from water has the advantages of high energy density and zero emission. Therefore, it is pretty important to develop stable and efficient hydrogen-evolving photocatalysts. Among them, cadmium sulfide (CdS) is one of the most attractive catalysts for solar-driven hydrogen production owing to its excellent visible light response and suitable band gap structure [4, 5]. However, CdS catalysts are highly susceptible to the photocorrosion and the rapid recombination of photogenerated carriers during photocatalytic processes [6, 7], which seriously restrict its activity as well as application in practical large scale photolysis of water. Recent studies demonstrate that the photocatalytic hydrogen evolution reaction (HER) performance of the CdS can be enhanced by optimizing the own nanostructure and composition to overcome the inherent defects of single material [8, 9]. In many strategies, the attaching of a cocatalyst (such as metal sulfides [10-12], metal phosphides [13,14], etc) on the surface of CdS is deemed as one of the most convenient and effective methods, which supplies sufficient surface active sites to accelerate the transfer of photogenerated electrons in the bulk of CdS [15]. Whether in photocatalytic or electrocatalytic HER, electrons transport capacity of the catalysts always plays an important role. Thence, zero-valent metal nanoparticles (NPs) emerge as a promising category of electro/photo chemically active species for energy related systems because of their better electronic conductivity caused by metal-metal bonds [16]. Studies have revealed that metal particles show great potential in the field of electrocatalysts or photocatalyst for water splitting reaction [17-19]. Especially, composites of carbon-coated low-cost metals have attracted

3

widespread concern due to the unique high conductivity and adjustable surface electronic structure [20, 21]. This composite material can overcome the inherent defects of pure carbon or simple metal materials, which significantly improves the catalytic performance. Moreover, the metal wrapped by carbon makes the structure more stable and prolongs service life of the catalyst [22]. Metal NPs in these reports are usually obtained by in situ high temperature reduction of metal salts using the carbon-containing compounds. Nevertheless, the precursors used are relatively expensive [23, 24] or the sizes of the target products are too large to expose more active sites for effective catalysis [25]. Notably, the cheap carbon quantum dots (CQDs) which generally have a small size and large amount of carbon are rich in raw materials and simple to be synthesized [26]. Theirs terminal groups are coordinated with metal salts to make the mixtures evenly disperse. Then the metal NPs obtained from them by high temperature reduction are smaller, which exposes greater specific surface area and more active sites, resulting in a better catalytic activity for HER. Inspired by the above discussions, we rationally designed a hybrid nanostructure CdS/Co@NC (Co NPs wrapped by nitrogen doped carbon layers). It is prepared through Co@NC derived from the mixture of CQDs and Co2+ loading on the surface of the one-dimensional (1D) CdS nanorods (NRs) by self-assembly. The excellent electrons transport capacity of the Co@NC and synergy between Co NPs and the N doped the carbon make it an outstanding cocatalysts when coupled with CdS NRs. Additionally, the co-catalyst with relatively small particle size realizes the maximum atom efficiency and greatly enhances electron-hole separation efficiency in CdS/Co@NC, thereby generating an increased photocatalytic hydrogen production performance. The obtained CdS NRs with 3 wt % loading amount of Co@NC (CdS-3) effectively catalyzes the hydrogen evolution at a rapid rate of 21.8 mmol g-1 h-1, 29.8

4

times higher than that of pure CdS NRs. Moreover, the considerable photo-stability up to 10 h and an apparent quantum efficiency (AQE) of 41.8 % at 420 nm over the CdS3 are obtained. The enhanced photocatalytic performance of the hybrid nanostructure is largely ascribed to the effective electron transfer (ET) between the CdS NRs and Co@NC, which suppresses the rapid recombination of photogenerated carriers. This result is confirmed by photoluminescence (PL) spectra and transient photocurrent responses experiments. To the best of our knowledge, this is the first report on preparation of Co@NC assisted by the CQDs to act as a highly efficient photocatalytic hydrogen production promoter over CdS NRs, which supplies a novel strategy for expanding effective photocatalytic water splitting cocatalysts.

2. Experiment section 2.1. Materials Ultrapure water (18.25  cm-1) for the preparation of solutions in this work was purified by a Molecular Lab Water Purifier. All the chemical reagents were purchased from the commercial companies and directly employed without further purification. 2.2. Synthesis of Co@NC NPs For synthesis of the CQDs [27], the mixtures of glucose and dicyandiamide were dissolved in 10 mL water, and then transferred to a 25 mL Teflon autoclave followed by heating at 200 oC for 6 h. The resulting product was filtered by a 0.22 μm filter to remove the large particles and get the red brown CQDs solution. Subsequently, the 300 mg Co(NO3)2·6H2O was added in the above solution under stirring for 0.5 h, then dried to collect the CQDs-Co2+ complexes powder. Finally, the black powder was annealed at 600 °C for 2 h (rate: 5 oC min-1) under nitrogen atmosphere, recorded as

5

Co@NC. Similarly, nitrogen doped carbon layers (NC) was prepared in the same way as that of Co@NC without adding the cobalt source. 2.3. Synthesis of CdS NRs In a typical synthesis method [28], the CdS NRs were prepared by dissolving the 5.0 g cadmium nitrate and 3.7 g thiourea in 60 mL ethylenediamine, then the mixture was transferred to a 100 mL Teflon autoclave, maintaining at 180 oC for 24 h. After cooling down naturally, the obtained yellow product was collected by centrifugation, rinsed with water and dried at 50 oC overnight. 2.4. Preparation of CdS/Co@NC hybrid photocatalyst The hybrid photocatalysts were prepared by a classic self-assembly strategy. Briefly, an amount of CdS NRs were evenly dispersed in ethanol aqueous solution (50 Vol %), then the 1, 2, 3 and 4 wt % Co@NC (the mass load amount of CdS) were added to the above suspension. After ultrasonic dispersion uniformly, the obtained products were collected by centrifugation into a centrifuge tube, directly dried without washing and labeled as CdS-1, 2, 3, 4. 2.5. Preparation of comparative photocatalysts and derivative photocatalyst For preparation of comparative catalysts, method is the same as that of CdS/Co@NC except replacing the Co@NC by the same amount of NC and commercial cobalt powder, marked as the CdS-NC and CdS/Co-1. Additionally, the in situ photodeposition way was employed to synthesis the CdS/Co-2 and CdS/Pt composites. As for the derivative catalyst, the g-C3N4 nanosheet and equal mass fraction of Co@NC were fully ground. Among them, the g-C3N4 nanosheet[29] was synthesized from the calcination of melamine followed by the stripping treatment thorough concentrated hydrochloric acid. 6

2.6. Photocatalytic hydrogen production experiment Photocatalytic hydrogen evolution experiments were carried out under the below conditions. Briefly, 10 mL aqueous solution of Na2S (0.35 M)/Na2S2O3 (0.25 M) or 10 Vol % lactic acid containing 2.0 mg CdS-based photocatalyst was added into about 28 mL photoreactor (flat on one side, Fig S1a-b). Prior to irradiation, the reactor was sealed and subsequently flushed with argon to replace air completely. A 420 nm LED lamp (Fig S1c) with the intensity of 100 mW cm-2 was selected as light source and a calibrated Shimadzu GC-9A Gas Chromatograph equipped with a thermal conductivity detector (TCD) was used to determine the amount of released hydrogen gas. As for the C3N4 and Eosin Y (EY) as light harvesting materials, 2.0 mg sample is dispersed in 10 Vol % TEOA aqueous solution and other operations were the same as that of CdS-based photocatalyst. For the apparent quantum efficiency (AQY) calculation, the hydrogen evolution rate was selected based on 2.5 h photocatalytic experiment and AQY was obtained by the following equation: AQY =

2

*

number of evolved hydrogen moleculars × 100% number of incident photos

The turnover frequencies (TOF) of photocatalysts based on the amount of CdS were calculated using the following equation: TOF =

moles of produced hydrogen moles of CdS in photocatalyst

*

reaction time (h)

2.7. Electrochemical and photoelectrochemical measurements The electrochemical and photoelectrochemical measurements of the samples were performed on a CHI760D electrochemical analyzer, which used a standard three electrode system in 0.5 M Na2SO4 aqueous solution. Among them, a glassy carbon (0.071 cm2) or an FTO electrode (1 cm2), Pt wire and Ag/AgCl (3.5 M KCl, 0.208 V 7

vs. NHE) electrode are employed as the working, counter and reference electrodes, respectively. The working electrodes were attained by dispersing the samples (5 mg) in 1 mL DMF assisted by ultrasound. Then the resulting homogeneous ink and 0.5 Vol % nafion solutions were drop-casted onto glassy carbon electrode or FTO glass in turn, dried under the infrared light. Photocurrent tests were conducted under a 300W Xenon lamp with the same light intensity (100 mA cm-1). 2.8. Instrumental characterization Crystalline structures of the catalysts were characterized using a RigaKu D/MAX 2400 powder X-ray diffractometer. The morphologies of samples were measured by scanning electron microscopy (SEM, S4800) and transmission electron microscopy (TEM, Tecnai G2 TF20). Elemental analysis of the samples was determined on TJA ICP-atomic emission spectrometer (IRIS Advantage ER/S). Surface chemical states of catalyst were detected on ESCALAB250xi X-ray photoelectron spectra. The optical properties of the catalysts were characterized by a UV–vis diffuse reflectance spectrophotometer (UV–vis DRS, PerkinElmer Lambda 950). Photoluminescence (PL) spectra of samples were carried out by FLS920 fluorescence spectrometer with excitation at 380 nm.

Results and discussion

Scheme 1 Schematic preparation process of the samples.

8

The preparation diagram of the hybrid photocatalyst is demonstrated in Scheme. 1. As shown, CQDs as the carbon source and reducing reagent are synthesized by a facile hydrothermal method. After adding a cobalt source to the above CQDs solution, the obtained mixtures are dried and reduced by thermal treatment under N2 atmosphere to form the Co@NC, which is used as a cocatalyst supported on the CdS NRs surface by a self-assemble strategy. 3.1. Characterization of Co@NC and CdS-3

Figure 1 (a) TEM and HRTEM images of the CQDs; (b) XRD pattern of Co@NC; (c) Raman spectrum of NC; (d) XPS survey of Co@NC; (e) High resolution N 1s XPS spectrum; (f) HRTEM image of Co@NC.

The successful preparation of the CQDs was proved by the TEM (Fig. 1a), whose sizes have a diameter of about 2.4 nm (Fig. S2) and a lattice spacing of 0.19

9

nm, corresponding to the (101) planes of carbon (insert in Fig. 1a). In the IR spectrum of CQDs (Fig. S3), the strong stretching vibration bands of −OH, −NH, C=O and C=N are shown at about 3445, 3015, 1738 and 1600 cm−1, respectively. It indicates that terminal groups of CQDs are the hydroxyl, amino and carboxyl groups, which can well coordinate with Co2+ to form CQDs-Co2+ complex. Additionally, the obvious characteristic diffraction peaks of metal Co (PDF#15-0806, Fig. 1b) in Co@NC are detected except a peak at about 21o, which belongs to the graphitic carbon. ICP result exhibits that the carbon accounts for approximately 10% of the mass of the Co@NC. Additionally, the nearly equal ratio of intensity of D and G band at 1320 and 1590 cm1

in the corresponding Raman spectra (Fig. 1c) promises the better electrical

conductivity of the Co@NC when it is used as a cocatalyst [30]. Consistent with the above results, the XPS of Co@NC also confirms the coexistence of Co, C, O and N (Fig. 1d), where N in the high resolution N 1s spectrum involves the conductive pyridine and graphitic N at 398.7 and 401.1 eV (Fig. 1e). In their TEM images (Fig. 1f and S4), Co nanoparticles with a radius of about 15 nm are wrapped by some layers of nitrogen doped carbon, whose lattice spacing of 0.2 nm is clearly observed in Fig. 1f and attributed to (111) plane of the Co phase (PDF# 15-0806). What’s more, 778.3 eV of Co@NC (Fig. S5) in the high resolution Co 2p XPS spectrum is attributed to metallic Co, which is in agreement with the TEM result. The presence of Co2+ signals at 781.0 and 797.5 eV with the satellite peaks at 786.0 as well as 803.5 eV may source from the oxidized surface of material by air. The relatively small Co nanoparticles expose more active sites and achieve the maximum atom utilization efficiency [31-33], thereby greatly realizing an enhanced photocatalytic performance. The above results demonstrate the successful preparation of highly conductive and effective cocatalyst. After the Co@NC loading on the CdS

10

NRs surface by self-assembly, there is no difference in XRD patterns between CdS alone and CdS-3 due to the low loading content of the cocatalyst (Fig. 2a). Notably, the increased intensity of the Co peaks in XPS results of CdS-3 suggests the successful loading of the cocatalyst compared to that of pure CdS (Fig. 2b, S6), which is further proved by the change in color between CdS alone and CdS-3 (Fig. S7).

Figure 2 (a-b) XRD patterns and high resolution Co 2p spectra of CdS and CdS-3; (c-e) TEM and HRTEM images of CdS-3; (f) Element mapping of CdS-3.

In SEM images (Fig.S8), there are no differences of the catalysts before and after loading Co@NC, where both show CdS has morphology of typical nanorod with width of about 50 nm and length of roughly 1 um. The larger aspect ratio of CdS NRs 11

shortens the path of photogenerated carriers to the surface, which is beneficial to the improvement of photocatalytic activity [34, 35]. Therefore, the rod-shaped CdS is often employed for photocatalytic hydrogen production after further modification. Energy dispersive X-ray spectroscopy (EDX, Fig.S9) of the CdS-3 confirms the N atom is doped into Co@NC and Co@NC is loaded on the CdS successfully. In order to directly observe that the cocatalyst has been supported on the CdS, TEM was carried out. The surface of CdS is smooth, whose lattice spacing is measured to be 0.336 nm (PDF#77-2306, Fig. 2c, S10), corresponding to XRD results. As for the CdS after loading the cocatalyst, there are some objects on their surfaces (Fig. 2d). Their lattice stripes were measured as 0.2 nm (Fig. 2e), which belongs to the (111) plane of the Co phase and strongly confirms the existence of the cocatalyst in CdS-3. Element mapping of CdS-3 also supports the cocatalyst is loaded on the surface of CdS successfully (Fig. 2f). 3.2 Photocatalytic hydrogen production performance Photocatalytic performances of samples were conducted under the same condition. Firstly, the loading amounts of Co@NC on CdS were optimized, whose photocatalytic hydrogen production rates are enhanced and the tendency is in the common form of a volcano (Fig. 3a). The Co@NC loading results in the fast photogenerated carriers separation and increased active species, so the CdS-3 possesses the best photocatalytic hydrogen production activity and 29.8 times higher than that of pure CdS. In order to study the synergy effect between cobalt and carbon, hydrogen production activity of the equal amount of carbon, commercial cobalt powder and photodeposited cobalt NPs anchoring on the surface of CdS were measured (Fig. 3b).

12

Figure 3 Photocatalytic hydrogen evolution rates of (a) CdS with different amount of Co@NC and (b) comparative samples in the first 0.5 h; (c) kinetic curves of catalysts for 150 min; (d) cyclic stability experiments of CdS-3; (e) XRD patterns of fresh and used CdS-3; (f) TOFs and AQYs of CdS and CdS-3 in the first 0.5 h.

Obviously, the hydrogen production rate of CdS-3 (21.8 mmol g-1 h-1) is higher than that of CdS-NC (3.6 mmol g-1 h-1), CdS/Co-1 (3.1 mmol g-1 h-1) and CdS/Co-2 (9.2 mmol g-1 h-1). That is because that not only the cocatalyst composed of carbon and metal overcomes the inherent defects of single materials, but also conductive carbon is as an electronic transport layer, which significantly improve the photocatalytic performance and cause the above results. Additionally, this result was further confirmed by the linear sweep voltammeters (LSVs, Fig S11). The introduction of carbon enhances the electrocatalytic hydrogen evolution activity of 13

cobalt and this demonstrates a strong electronic interaction between cobalt and carbon. The zero-valence Co NPs obtained by photodeposition usually possess smaller sizes than those of commercial cobalt powders (TEM in Fig S12), resulting in a higher hydrogen evolution activity coupled with CdS NRs [36]. It is well known that Pt is the best hydrogen-producing cocatalyst while the hydrogen evolution rate of CdS with equal loading amount of Pt is only about one quarter of that of CdS-3. The poor activity of CdS/Pt may be caused by the non-optimal Pt loading in this HER system. When lactic acid as the sacrificial reagent for photocatalytic HER (Figure S13), CdS3 exhibits excellent photocatalytic hydrogen production activity, about six times higher than that of CdS alone. Besides, the Co@NC has certain universality as the cocatalyst to be coupled with other light harvesting materials, such as C3N4 nanosheets and EY. The photocatalytic activity of C3N4/Co@NC (3wt %) is 780 and 1.5 times than that of pure C3N4 and C3N4/Pt (3wt %) (Fig. S14). Moreover, with the addition of Co@NC, the amount of hydrogen production is increased 33 times higher than that of EY alone (Fig S15), which can well verify its universality as photocatalytic HER cocatalysts. The kinetics curve of CdS-3 (Fig. 3c) demonstrates it is a pretty stable hybrid photocatalyst due to the almost unchanged rate. Moreover, five cycles of total ten hours of stability experiments of CdS-3 (Fig. 3d) remain the almost constant hydrogen evolution rates, further suggesting the highly stable behavior of CdS-3 during the photocatalytic reaction. In addition, the result that XRD of the CdS-3 before and after the photocatalytic reaction does not change (Fig. 3e) suggests the photo-corrosion of CdS can be inhibited after supporting the cocatalyst due to rapid electron-hole separation. The single-variable experiments (Fig. S16) reveal the CdS-3 is inactive in the absence of light or sacrificial reagents. Notably, there is still hydrogen escaping

14

without CdS-3 (Fig. S17) maybe because Na2S is hydrolyzed to H2S, which is then decomposed into H2 and S according to the flowing chemical equation: Na2S + H2O → H2S + NaOH H2S → H2 + S Above analysis answers for the trace activity of cocatalyst Co@NC alone (Fig. 4a) The corresponding TOFs and AQYs of catalysts were calculated and shown in Fig. 3f, whose increased times is almost identical to that of the hydrogen evolution rate. Compared to other CdS based photocatalysts [9-11, 13, 15, 34, 37-42], the AQY of CdS-3 is at the upper middle level (Table 1). Table 1. AQYs of CdS with different cocatalysts under monochromatic wavelength. Concentration

Sacrificial



AQY

(mg/mL)

Reagent

(nm)

(%)

Cu7S4/g-C3N4 [9]

0.5

0.35 M Na2S/0.25 M Na2SO3

420

4.4

MoS2 [10]

1

10 Vol% lactic acid

420

41.37

NiSx [11]

0.5

30 Vol% lactic acid

420

60.4

O-Co2P [13]

0.25

15 Vol% lactic acid

420

22.17

Co [15]

0.25

Pure benzyl alcohol

420

63.2

Cu3P [34]

0.05

0.75 M Na2S/1.05 M Na2SO3

450

25

ZnS [37]

0.125

0.35 M Na2S/0.25 M Na2SO3

470

11.09

Ni@C [38]

0.125

25 Vol% lactic acid

420

31.2

Ni3C [39]

0.3125

0.25 M Na2S/0.25 M Na2SO3

420

8.72

Co3S4/Co [40]

0.0625

0.35 M Na2S/0.25 M Na2SO3

475

16.8

Ni [41]

0.83

Pure methanol

405

95

CQD@Pt [42]

0.167

0.25 M Na2S/0.35 M Na2SO3

400

29.8

Co@NC

0.2

0.35 M Na2S/0.25 M Na2SO3

420

41.8

Cocatalyst

15

3.3 Stability investigation of the hybrid photocatalyst

Fig. 4. (a-b) High-resolution Cd 3d and S 2p XPS spectra of CdS-3 before and after reaction; (c-d) high-resolution S 2p and Co 2p XPS spectra of Co@NC before and after dipping into sacrificial reagent.

The cycle experiments and XRD patterns of the hybrid catalyst before and after photo reaction only ostensibly indicate that the system is stable. Therefore, XPS of CdS-3 (before and after 5 h photocatalytic HER) as well as Co@NC (before and after dipping into sacrificial reagent for 5 h) were measured to further explore the stability separately. As shown, there is no change of Cd 3d as well as S 2p of CdS-3 in binding energy before and after reaction (Fig. 4a-4b), suggesting the CdS is indeed stable in this system. Correspondingly, the surface layer of the Co@NC after reaction has the presence of cobalt sulfide, which is drawn from the appearance of S 2p and binding energy displacement of Co 2p (Fig. 4c-4d). The new species is from the reaction

16

between metallic Co with S2- because of its low solubility product. The specific content of CoS in Co@NC and CdS-3 is changing during photocatalysis. 3 wt% CoS/CdS was prepared by a co-precipitation method (Co2+ + S2- → CoS, then coupled with CdS) and it was used for photocatalytic hydrogen production. However, there is no improvement in activity after loading CoS, which means CoS from in-situ sulfurization has no positive effect during photocatalysis (Figure S18). Additionally, the still existing characteristic peak of metallic Co implies one part of the Co is vulcanized, explaining that the metallic Co is still the main active center combined with the result of C3N4/Co@NC (Fig. S14). This confirms that the carbon layers result in little amount of CoS in situ formation so that the photocatalytic activity of CdS-3 is not reduced. 3.4 Electron transfer rate characterization

Figure 5 (a) LSVs and (b) EIS of CdS and CdS-3; (c) I-t curve of CdS-3; (d) Tafel slopes of CdS and CdS-3.

17

The boosted hydrogen evolution rates of the samples only apparently indicate an increase in the ET rate. For this phenomenon, PL spectrum, electrochemical and photoelectrochemical methods were conducted. Among them, the overpotentials at 1 mA cm-2 of the CdS (1.51 V vs Ag/AgCl) and CdS-3 (1.34 V vs Ag/AgCl) in LSVs (Fig. 5a) exhibit that the electrocatalytic activity of CdS [43] is greatly improved after the promoter supported, implying an enhanced ET rate. Analogously, electrochemical impedance spectra (EIS) are also related to the electrocatalytic kinetics, in which a smaller charge transport (Rct) radius indicates a faster reaction rate [44]. Fig. 5b reflects the Rct value of CdS-3 is smaller than that of CdS and suggests the electrical conductivity of CdS can be enhanced after coupled with Co@NC. At the same time, a current-time (IT, 10 mA cm-2) curve was measured to prove that the electrochemical stability of CdS-3. The current of five hour (Fig. 5c) has virtually no attenuation and proves that the catalyst is stable under ET process. In addition, the Tafel slope of catalysts is a good method to prove the mechanism of hydrogen production and catalytic activity (Fig. 5d), in which the individual CdS shows 167 mV dec-1. After anchoring the Co@NC, a smaller Tafel slope of catalyst is obtained (132 mV dec-1), indicating a better hydrogen evolution performance. Moreover, the Tafel slope of CdS-3 measured in neutral Na2SO4 solution represents the typical Volmer mechanism (H2O  Had + OHad) [45] and Volmer reaction is deemed as the rate-determining step of HER. The photocatalytic system is in a weakly alkaline system because of the hydrolysis of strong base weak acid salts; therefore CdS-3 also follows the above conclusion.

18

Figure 6 (a) Photo-current response experiments of CdS and CdS-3; (b-c) high-resolution Cd 3d and S 2p XPS spectra of CdS and CdS-3; (d) steady-state PL spectra of CdS and CdS-3.

The photo-current response experiments (Fig. 6a) can prove the separation efficiency of photogenerated carriers, where the both peak current and peak difference prove that the electrons are more efficiently separated in the composite catalyst [46, 47]. Compared to the high-resolution Cd 3d and S 2p XPS spectra in CdS and CdS-3 (Fig. 6b-6c), the binding energies of Cd 3d and S 2p in CdS-3 are apparently increased due to electrostatic repulsive force between the unpaired electrons of the Co@NC and the electrons of the CdS. This suggests that there is a strong mutual interaction effect between the cocatalyst and CdS [39], thereby improving photocatalytic hydrogen production activity of CdS-3. The combination of

19

photogenerated carriers of light-harvesting material produces fluorescence in steadystate PL spectra, where the weaker fluorescence indicates the higher separation efficiency of carriers and the better photocatalytic hydrogen production performance [48, 49]. After loading the Co@NC, the fluorescence intensity of CdS decreases, which confirms the above result (Fig. 6d).

Scheme 2 Transfer process of electrons and holes of CdS (above) and CdS-3 (below).

Based on the above results, the following mechanism of electron migration is shown in scheme 2. The carriers of CdS are excited by the light to divide into photogenerated electron-hole pairs, in which they migrate to the surface of the semiconductor to combine the H+ to form the hydrogen. When CdS alone is employed for photocatalytic reaction, there are no hydrogen-producing active sites on its surface, thus the photogenerated holes and electrons are enriched. This increases the difficulty of the photogenerated electron-hole pairs in the bulk moving out, which results in the fast surface and bulk recombination, thereby causing slow charge transfer and a poor

20

HER activity. When the surface of CdS is loaded with Co@NC, photogenerated electrons on the surface react with H+ to form hydrogen, and the remaining holes that are not combined with electrons only oxidize the hole sacrificial reagent. According to Le Chate's law, the electrons on the surface of CdS are insufficient; the electrons in the bulk will continue to migrate, leading to a fast charge separation and an outstanding HER performance. 3.5 Optical characterization and Mechanism research The UV–vis DRS spectra were employed to evaluate the optical properties of CdS, CdS-3 and Co@NC samples (Fig. 7a, Fig. S19). The result exhibits that Co@NC has the best optical absorption capacity. Therefore, the absorptions in the range from 280 to 480 and 520 to 900 nm of CdS are enhanced after anchoring the Co@NC. CdS and CdS-3 both present a similar absorption edge at approximately 512 nm owing to the essential band gap absorption of CdS [50]. The color of CdS-3 (Fig. S7) turns yellow green in comparison with pure CdS, revealing the absorption is extended to long wavelengths and further confirms the above phenomenon. From the lines of (αhν) 2 to hν plots in Fig. 7b and Fig. S19, the band gap (Eg) values of 2.42 and 0.93 eV for the pure CdS and Co@NC were obtained by the Kubelka-Munk equation: αhν = A(hν-Eg) 1/2 where α is the absorption coefficient and hν represents the photon energy.

21

Figure 7 (a) UV–vis DRS spectra of CdS and CdS-3; (b) Tauc plots of (αhν)2 versus hν of CdS; (c-d) Mott-Schottky plots of CdS and Co@NC; (e) photocatalytic HER mechanism of CdS; (f) time-resolved PL spectra of CdS and CdS-3.

Mott−Schottky plots are employed to measure the flat-band potential (EFB) of pure CdS and Co@NC. The positive slopes of CdS and Co@NC shown in Fig 7c-7d suggest both are n-type semiconductors and the EFB of CdS as well as Co@NC is – 1.25 and – 1.0 V versus Ag/AgCl electrode. Studies have revealed that the EFB of ntype semiconductor is more positive about 0.1 or 0.2 V than its conduction band

22

potential (ECB) [37, 51-52]. Therefore, the Ecb for CdS and Co@NC are roughly reckon up to – 1.45 and -1.2 V versus Ag/AgCl electrode, thus the valence band potentials (EVB, 0.55 V versus Ag/AgCl electrode) of the pure CdS is obtained through the following equation: EVB = ECB + Eg Through the above results, a reaction mechanism of the CdS-3 for enhanced photocatalytic hydrogen production is obtained (Fig. 7e). The CdS absorbs the photon from the visible light and generates the electron-hole pairs owing to its suitable band gap, in which electrons transfer to the CB of CdS. Given the ECB of Co@NC more negative than that of CdS and excellent conductivity of cocatalyst, the excited electrons from CdS can transfer to the Co@NC and afford efficient separation of electron-hole pairs of CdS. This results in a prolonged lifetime of photogenerated carriers (Fig. 7f) and enhanced photocatalytic HER activity. As for the holes of CdS, they react with hole sacrificial reagent according to the following chemical equation [53]: 2 h+ +S2-  S SO32- + H2O + 2h+ (VB)  SO42- + 2H+ SO32- + S  S2O32-

4. Conclusion In summary, a novel nitrogen doped carbon layers coated cobalt nanoparticles as the efficient cocatalyst for photocatalytic water splitting is rationally designed, which is obtained by a one-step pyrolysis of the mixtures containing the simple cobalt salt and readily available carbon quantum dot. The Co@NC coupled with CdS or C3N4 both exhibits excellent photocatalytic performance thanks to the better synergy effect

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between the carbon layers and the cobalt nanoparticles. In addition, the co-catalyst with relatively small particle size realizes the maximum atom efficiency and greatly enhances electron-hole separation efficiency in CdS/Co@NC, driving a HER activity of the 21.8 mmol g-1 h-1 for CdS-3 and 29.8 times higher than that of CdS alone. The reason of enhanced photocatalytic HER activity of CdS after loading the cocatalyst was verified by the electrochemical experiments and PL spectra. Overall, this work provides a new method to synthesize universal photocatalytic cocatalyst.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grants Nos. 21773096), Fundamental Research Funds for the Central Universities (lzujbky-2018-k08) and the Natural Science Foundation of Gansu (17JR5RA186).

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Highlights 

The CQDs coordinate with Co2+ to synthesize Co@NC via pyrolysis.



The hydrogen production rate of CdS-3 is 21.8 mmol g-1 h-1.



The interaction between CdS and Co@NC is crucial in enhanced HER activity.

Graphical Abstract

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Carbon quantum dots assisted strategy to synthesize Co@NC for boosting photocatalytic hydrogen evolution performance of CdS

Conflict of interest The authors declare no conflict of interest.

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