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Coordination polymer derived sulfur vacancies rich CdS composite photocatalyst with nitrogen doped carbon as matrix for H2 production Chunyu Jiang, Xinxin Xu, Mingliang Mei, and Fa-Nian Shi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03201 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 11, 2017

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ACS Sustainable Chemistry & Engineering

Coordination polymer derived sulfur vacancies rich CdS composite photocatalyst with nitrogen doped carbon as matrix for H2 production Chunyu Jiang,a Xinxin Xu*,a Mingliang Meia and Fa-nian Shi*b a

Department of Chemistry, College of Science, Northeastern University, NO. 3-11,

Wenhua Road, Heping District, Shenyang, Liaoning, 110819, People’s Republic of China b

School of Science, Shenyang University of Technology, No. 111, Shenliao West

Road, Economic & Technological Development Zone, Shenyang, Liaoning, People’s Republic of China

* To whom correspondence should be addressed. Email address: [email protected] (Prof. X. X. Xu) [email protected] (Prof. F. N. Shi)

ACS Paragon Plus Environment


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KEYWORDS: Coordination polymer, sulfur vacancy, CdS, nitrogen doped carbon matrix, photocatalytic H2 production

ABSTRACT With calcination, sulfur vacancies rich CdS based composite photocatalyst has been synthesized successfully with coordination polymer as the precursor, which is constructed by Cd(II) metal ion and 2-mercaptobenzimidazole ligand. After calcination, Cd(II) ion and thiol group converts to CdS nanoparticle with the size about 5 to 8 nm, which disperses evenly in nitrogen doped carbon matrix (NC) formed by benzimidazole. During this process, some coordinated nitrogen atoms dope in the lattice of CdS and replace sulfur atoms, which leads to the generation of sulfur vacancies. In NC, the major component is graphitic carbon with sp2 hybridized pattern. Besides carbon, a small fraction of nitrogen element also exists, including pyridinic-N, pyrolic-N and quaternary-N. Under visible light irradiation, the composite photocatalyst exhibits very excellent H2 production ability as well as perfect stability during H2 production, which does not decay after 30 h. For CdS based composite photocatalysts, temperature exhibits an obvious effect on photocatalytic property and the main reason accounting for this is the type of nitrogen species in NC matrix. Specifically, during photocatalysis, quaternary-N acts as the electron carrier, which promotes the separation of the electron-hole pair, pyridinic-N can serve as H+ acceptor as well as active sites for its reduction. Their contents possess great influence on H2 production activity.


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INTRODUCTION Recently, photocatalytic water splitting has been proved as a cheap and clean method in hydrogen energy application.[1-6] As an extensive concerned photocatalysts with narrow band gap (Eg = 2.42 eV) and suitable band structure (Ecb = -0.52 V, vs NHE), CdS exhibits excellent photocatalytic activity for visible light H2 production.[7-10] Compared with neat CdS, the prospect of composite photocatalyst constructed by CdS and carbon based materials (such as graphene,[11-16] carbon nanotubes,[17,18] carbon fiber and fullerene[19-21]) looks more promising, because carbon materials can separate photogenerated electron-hole pairs in time and improve H2 production efficiency. Up to now, several experimental skills, such as hydrothermal and solvothermal techniques, ultrasonic and microwave assisted methods have been employed to fabricate composite photocatalysts based on CdS and carbon materials.[22-28] In these methods, photocatalysts are formed through self-assemble or precipitation of CdS particle on carbon materials. But the random growth process brings some serious consequences, such as overgrowth or aggregation of CdS particle, as well as its inhomogeneous distribution on carbon materials. Anyone of these problems can reduce separation efficiency of the photogenerated electron-hole pair and lead to a fatal influence on H2 production activity. To resolve above problems, a unique strategy, which facilitates the generation of CdS nanoparticles with small size as well as the uniform distribution of these CdS nanoparticles on carbon matrix, is necessary. At this point, coordination polymer composed by Cd(II) metal ion and thiol group containing N-heterocycles ligands inspires us greatly, because a CdS based composite photocatalysis with nitrogen doped carbon as the matrix can be formed with this coordination polymer as precursor after calcination. In addition, coordination polymer precursor also possesses some



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prominent advantages. Firstly, in coordination polymer, Cd(II) metal ions and organic ligands are connected high orderly, so, after calcination, the resulted CdS particles are embedded in carbon matrix homogeneously. Secondly, during nucleation process, CdS is surrounded by organic ligands, which restricts its overgrowth and generates CdS particle with small size. Furthermore, coordinated nitrogen atoms can dope in the lattice of CdS and replace sites of sulfur atoms after calcination which produces sulfur vacancies. This is benefit for the improvement of photocatalytic activity, because sulfur vacancies not only promote the separation of photogenerated electron-hole pair, but also narrow the band gap of a photocatalyst.[29,30] So we speculate the calcination of coordination polymer composed by Cd(II) metal ion and thiol nitrogen heterocyclic ligand can obtain efficient CdS based composite photocatalyst.

Scheme 1. Schematic representation of the synthetic route for CdS@NC. Our assumption is proven to be feasible by successful fabrication of CdS@NC, a sulfur vacancies rich CdS based composite photocatalyst with nitrogen doped carbon as the matrix. This photocatalyst was obtained with a new coordination polymer [Cd(MEBMI)]n·n(H2O) (CP) (MEBMI = 2-mercaptobenzimidazole) as the precursor through calcination (Scheme 1). In CdS@NC, CdS particle with the size about 5 to 8 nm dispersed evenly in nitrogen doped carbon matrix. Photocatalytic water splitting experiment illustrates CdS@NC exhibits outstanding H2 production property under visible light irradiation. No obvious decrease in H2 production rate occurs during water splitting process for 30 h. Furthermore, we also discussed the influence of


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nitrogen species on H2 production activity. Up to now, some composite materials composed by metal oxide and metal sulfide with carbon as matrix have been obtained with coordination polymer precursor, but it has never been employed to fabricate sulfur vacancies containing composite materials, CdS@NC represents the first example.[31-33]

Figure 1. (a) Three-dimensional supramolecular network of CP; (b) TEM of CPNP. RESULTS AND DISCUSSION Structure and morphology of CP Single crystallographically X-ray diffraction analysis illustrates, in the fundamental unit of CP, there exists only one independent Cd atom which adopts distorted tetrahedron coordination mode. It connects with two nitrogen atoms and two sulfur atoms from four different MEBMI ligands. In this CdN2S2 pyramid, Cd-N bond distances are 2.202 and 2.258 Å respectively, while Cd-S bond distances range from 2.513 to 2.561 Å (Figure S1). In CP, the adjacent Cd atoms are connected by MEBMI, which lead to a one-dimensional chain at first. Then, with intensive π-π interactions (the distance between MEBMI ranges from 3.55 to 3.62 Å), adjacent one-dimensional chains are connected and generate three-dimensional supramolecular network (Figure 1a). CPNP exhibits similar PXRD pattern with CP (Figure S2). The morphology of CPNP was studied with TEM, which exhibits spherical appearance with the diameter ranging from 50 to 80 nm (Figure 1b). Furthermore, the thermal stability of CPNP was also studied, which illustrates its framework decomposes when the temperature is



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higher than about 350 ˚C (Figure S3). Morphology and structure of CdS@NC The structures of CdS@NC composite materials were studied with PXRD at first. The intensive diffractions appear at 25.0˚, 26.6˚, 28.2˚, 36.6˚, 43.8˚, 47.9˚, 52.0˚, 66.7˚, 70.9˚ and 75.4˚, which correspond to (100), (002), (101), (102), (110), (103), (112), (203), (211) and (105) planes of CdS in hexagonal phase (Figure 2a).[34] No peaks belonging to carbon material are observed in this analysis, which can be attributed to the overlap of its diffraction with CdS.[35] The existence of carbon can be confirmed by FTIR clearly (Figure 2b). CdS@NC shows an intensive absorption at 1622 cm-1, primarily ascribes to the aromatic C=C stretching vibration resulting from the aromatization of benzimidazole. Furthermore, element analysis suggests, although these composite materials were obtained under different temperatures, their elemental distribution are almost identical. The existence of sulfur vacancies can be proved by electron paramagnetic resonance spectrum (EPR). Compared with pure CdS, all these photocatalysts exhibit resonance signals at g = 2.006, which can be identified as the feature of sulfur vacancies (Figure 2c).[36,37] From CdS@NC(A) to CdS@NC(C), the intensity of resonance signal keeps in consistence and this illustrates concentration of sulfur vacancy is almost similar in these three photocatalysts.

Figure 2. (a) PXRD of CdS@NC; (b) FTIR of CdS@NC; (c) EPR of CdS@NC and pure CdS. The morphologies of CdS@NC composite materials were studied with SEM and


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TEM. SEM demonstrates CdS@NC keeps similar appearance with CPNP and their size also range from 50 to 80 nm (Figure S4 and S5). Although the composite materials were obtained under different temperatures, their morphologies kept in consistence with spherical appearance. Further structural details and distribution of CdS were studied by TEM. It can be observed clearly spherical shaped CdS particles with the size range from 5 to 8 nm distribute uniformly in NC matrix (Figure 3a to 3c). To reveal interface structure of CdS and NC in detail, high-resolution transmission microscopy (HRTEM) was employed. HRTEM images show a lattice spacing of 0.35 nm and this is in good consistence with the (110) plane of hexagonal CdS (Figure 3d to 3f). For CdS@NC, their unique structural characters originate from two aspects. During calcination, the growth of CdS particle is restricted by surrounding MEBMI ligands, which decreases its size in some extent. At the same time, in CP, the highly ordered arrangement of Cd(II) metal ions and MEBMI ligands lead to the homogeneous distribution of CdS particles in nitrogen doped carbon matrix.

Figure 3. TEM images of (a) CdS@NC(A); (b) CdS@NC(B) and (c) CdS@NC(C); HRTEM images of (d) CdS@NC(A); (e) CdS@NC(B) and (f) CdS@NC(C). The chemical composition and surface electronic state of CdS@NC catalysts were studied by X-ray photoelectron spectroscopy (XPS). The XPS wide-scan spectra exhibit obvious signals of S 2p, C 1s, N 1s and Cd 3d in CdS@NC (Figure 4a to 4c).



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Although these photocatalysts were obtained under different temperatures, their element content is about similar (Table 1). Further exploration illustrates high-resolution S 2p spectra can be divided into two peaks, originating from S 2p3/2 (161.1 eV) and S 2p1/2 (162.4 eV) respectively (Figure S6).[38] Compared with pure CdS, S 2p3/2 peak of CdS@NC shifts to lower binding energy region, which originates from sulfur vacancy (Figure S7). For Cd 3d, two peaks appear at 405.2 and 411.9 eV can be attributed to Cd 3d5/2 and Cd 3d3/2 (Figure S8).[39] All these facts illustrate CdS has been formed in these samples successfully. The high-resolution XPS spectra of C1s can be ascribed to Csp2-Csp2 (284.8 eV), Csp2-N (285.8 eV) and Csp3-N (287.4 eV) respectively (Figure S9).[40] For nitrogen, there exist three different states after deconvolution of high-resolution N 1s spectra at 397.9, 399.5 and 401.7 eV (Figure 4d to 4f). These singles correspond to pyridinic N, pyrrolic N and quaternary N respectively.[41] For CdS@NC obtained under different temperature, although their total N content is almost identical, the valence states of N element are great differences, which can further influence H2 production activity of these photocatalysts.

Figure 4. XPS survey of (a) CdS@NC(A); (b) CdS@NC(B) and (c) CdS@NC(C);


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High resolution N 1s spectra of (d) CdS@NC(A); (e) CdS@NC(B) and (f) CdS@NC(C). Table 1 The contents of different elements in CdS@NC. Cd (%) S (%) C (%) N (%) Pyridinic N (%) Pyrrolic N (%) Quaternary N (%)

CdS@NC(A) 5.73 5.82 83.2 5.25 74.2 19.4 6.4

CdS@NC(B) 5.66 5.77 82.8 5.77 75.4 9.9 14.7

CdS@NC(C) 5.89 6.01 82.5 5.6 40.3 45.5 14.2

The Raman spectra of CdS@NC show the characteristic bands of carbon based material at 1360 cm−1 and 1589 cm−1, ascribed to disordered graphitic and sp2-bonded carbons (Figure 5a to 5c).[42] These are features of graphitic-like materials, which indicate the generation of graphitic carbon during calcination. For carbon based materials, ID/IG, the relative intensity ratio of D to G band, is an ideal parameter to estimate the defect caused by the introduction of nitrogen atoms. From CdS@NC(A) to CdS@NC(C), the intensity ratio between D and G bands are 1.04, 1.01 and 1.02, which suggests regularity of nitrogen doped carbon material in these photocatalysts. CdS@NC photocatalysts also exhibit very excellent hydrophilicity, with contact angles are 7.9°, 6.6° and 5.7° respectively (Figure S10). This is benefit for the contact between photocatalysts and water, which can enhance H2 production activity.

Figure 5. Raman spectra (a) CdS@NC(A); (b) CdS@NC(B) and (c) CdS@NC(C).



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Figure 6. Nitrogen adsorption/desorption isotherms of CdS@NC, (a) CdS@NC(A); (b) CdS@NC(B), (c) CdS@NC(C); (d) The BJH mesoporous size distribution of CdS@NC. The specific surface area and the porous nature of CdS@NC photocatalysts were determined with BET method. In N2 adsorption/desorption isotherms of CdS@NC, H3 hysteresis loop exists in the range of relative pressure from 0.6 to 1.0, which can be considered as the predominant character of Type IV adsorption/desorption isotherm (Figure 6a to 6c).[43] From CdS@NC(A) to CdS@NC(C), their BET specific surface areas are 102.78, 104.93 and 106.39 m2/g. The pore-size distribution of CdS@NC catalysts was calculated from above isotherms with the BJH model, which demonstrates the size of their pores range from 5 to 15 nm (Figure 6d). Their average pore sizes are 9.30, 10.81 and 11.83 nm respectively. The generation of these pores can be explained as follows: as one-dimensional coordination polymer, after calcination, [Cd(MEBMI)]n chains convert to “carbon rod” with CdS nanoparticles


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embedding inside. After the packing of these “carbon rods”, one-dimension channels were formed between the “carbon rods”. To our knowledge, one-dimensional coordination polymer has never been employed as a precursor to fabricate mesoporous carbon based materials.

Figure 7. (a) Diffuse reflectance spectra (DRS) of CdS@NC; (b) Photoluminescence spectra of CdS@NC. Optical and electrochemical analysis Diffuse reflectance spectra (DRS) of CdS@NC composite photocatalysts were studied and their band gaps were calculated with Tauc equation. As shown in Figure 7a, from CdS@NC(A) to CdS@NC(C), their absorption edges appear at 570, 605 and 578 nm corresponding to band gap energy of 2.17, 2.05 and 2.14 eV respectively. For pure CdS, its absorption edge locates at 520 nm with band gap 2.40 eV. Red-shifts of absorption edges illustrate composite photocatalysts exhibit more intensive light adsorption ability than pure CdS. Furthermore, in the composite photocatalysts, CdS@NC(B) exhibits the narrowest band gap, which illustrates its photo adsorption region is broader than CdS@NC(A) and CdS@NC(C). This is benefit for the enhancement of H2 production activity. Photoluminescence spectrum was applied to study the separation of photogenerated electron-hole pair. For CdS@NC, with excitation at 420 nm, their main emission peaks centered at 515 nm (Figure 7b). As



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we all know, in photoluminescence spectra, the emission intensity is inversely proportional to separation efficiency of photogenerated electron-hole pair. So the weak emission intensity suggests high separation rate of the electron-hole pair. Here, CdS@NC(B) exhibits the lowest photoluminescence intensity, and this implies its electron-hole pair separation efficiency is higher than the other two composite photocatalysts.

Figure 8. (a) Photocurrent spectra of CdS@NC; (b) Nyquist plots of the electrochemical impedance spectra of CdS@NC. Electrochemical methods are powerful tools to investigate interfacial electron production and charge transportation on photocatalysts. Here, the separation of electron-hole pair was studied with photocurrent spectra at first. For composite photocatalysts, after irradiated with visible light, the photocurrent produces at once and then reaches the maximum value. If the light is shut off, the photocurrent disappears quickly. Furthermore, the photocurrent does not decay after several on-off cycles (Figure 8a). Compared with other photocatalysts, CdS@NC(B) shows more intensive photocurrent. This also suggests in CdS@NC(B), the photogenerated electron-hole pair can be separated quickly. To discuss charge transportation process in detail, EIS was used (Figure 8b). In these photocatalysts, CdS@NC(B) possesses the smallest arc radius, which suggests effective separation of the electron-hole pair


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and a quick charge transfer occurs on their interfaces. This can be attributed to their low charge transfer resistance as illustrated by the slope of the EIS curves. Base on optical and electrochemical results, we can conclude CdS@NC(B) exhibits more excellent photogenerated electron-hole pair separation property than CdS@NC(A) and CdS@NC(C). Table 2 Comparison of photocatalytic H2 production rate reported in the literature with CdS and carbon based materials composite photocatalysts and our works. Photocatalyst

Catalyst dose (g)

CDs/CdS CSs/CdS C60/CdS CNTs/CdS GR/CdS CNTs/CdS GO/CdS N-GR/CdS RGO/CdS GR/CdS GR/CdS RGO/CdS ZnO/RGO/CdS RGO/CdS Ni(OH)2/RGO/CdS PSGM/RGO/CdS Nb2O5/N-GR/CdS CdS@NC(B)

0.05 0.15 0.025 0.1 0.1 0.1 0.1 0.2 0.1 0.2 0.05 0.1 0.1 0.01 0.01 0.1 0.025 0.03

H2 Production (mmol·h-1·g-1) 0.05 3.90 1.73 1.77 0.70 0.50 3.14 1.05 4.20 0.50 0.40 1.09 5.10 0.51 4.73 1.75 4.00 5.84

Ref. 44 45 20 46 17 47 14 24 48 49 16 50 51 52 Our work

Photocatalytic H2 production H2 production properties of these photocatalysts were studied under visible light irradiation (λ ≥ 420 nm) with lactic acid as the sacrificial agent. Under this condition, CPNP is active with H2 production rate 0.192 mmol·h-1·g-1. After calcination, the H2 production rate improves in great extent. From CdS@NC(A) to CdS@NC(C), their H2 production rates are 4.04, 5.84 and 3.38 mmol·h-1·g-1, with AQY 6.25 %, 9.12 %



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and 5.21 % respectively (Figure 9a and 9b). Compared with recently reported other composite photocatalysts constructed by CdS and carbon based materials, H2 production rate of CdS@NC is improved greatly, which can be attributed to the existence of sulfur vacancies (Table 2).[44-52] It can be observed calcination temperature has great influence on H2 production rate. But with its increasing; H2 production rate did not rise monotonously. To explain this phenomenon, the analysis of its photocatalytic mechanism is necessary. For CdS@NC, its photocatalytic mechanism is similar with other photocatalysts composed by CdS and carbon based material. Under visible light, CdS is excited and generates electron, which transfers to its conductive band (CB). Simultaneously, the positive charged hole is formed on its valence band (VB). Because energy level of NC is lower than CB of CdS. So, the electron resides on CB shift to NC. This hinders recombination of the photogenerated electron-hole pair and enhances the H2 production rate. So, for CdS based photocatalyst, H+ adsorption ability and electron conductivity are two important factors which affect its H2 production rate. To our knowledge, H+ adsorption ability and electron conductivity are determined by morphology, dimension, surface area and chemical composition. But in CdS@NC, the similarity of the first three parameters suggests they are not decisive factors responsible for the difference in H2 production rate. Here, as revealed by XPS, the most obvious discrepancy in these photocatalysts is chemical composition, more concretely, is the valence states of N element.53,54 From 400 to 450 ˚C, with the increasing of quaternary N (6.4 % in CdS@NC(A) and 14.7 % in CdS@NC(B)), H2 production rate also increases obviously. As a nitrogen atom connects with three sp2 hybridized carbon atoms, the alone electron pair on quaternary N can generate delocalized conjugated system with adjacent sp2 hybridized carbon atoms. So, the increasing of


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quaternary N can enhance electron transportation property as well as H2 production rate. From 450 to 500 ˚C, the content of quaternary N keeps constant, but pyridinic N decreases in great extent. As an excellent proton acceptor, the increasing of pyridinic N is beneficial for the adsorption of H+ on photocatalyst, which in turn enhances H2 production rate. From CdS@NC(B) to CdS@NC(C), the content of pyridinic N reduces from 75.4 % to 40.3 %, which generates the negative effect on H2 production. So, in these three photocatalysts, CdS@NC(B) possesses the highest H2 production rate. Based on these facts, we can conclude, an excellent H2 production rate of CdS@NC originates from the synergistic effect of pyridinic N and quaternary N. For nitrogen element in CdS@NC, the alteration of valence states is in consistence with recent studies. During photocatalytic H2 production, the activity of the recycled catalyst is an important factor to evaluate the photocatalyst. For CdS@NC(B), its photocatalytic H2 production was studied five times and H2 production rate does not show noticeable decay in 30 h (Figure 9c). Furthermore, recycled photocatalyst also exhibits similar PXRD pattern with the original sample (Figure 9d). This indicates the structure of CdS@NC(B) is well retained.



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Figure 9. (a) Time courses of photocatalytic H2 production for CPNP and CdS@NC; (b) The average rate of H2 production over CPNP and CdS@NC, inset, the content of different nitrogen species in CdS@NC; (c) Repeated time courses of photocatalytic H2 production for CdS@NC(B); (d) PXRD of CdS@NC(B) before and after photocatalytic H2 production. CONCLUSIONS In summary, a new strategy has been explored to fabricate sulfur vacancies rich CdS based composite photocatalyst with nitrogen doped carbon as the matrix. Compared with other analogs, CdS@NC exhibits more excellent H2 production activity, which originates from its unique preparation method and the existence of sulfur vacancy. In CdS@NC, CdS nanoparticle with the size about 5 to 8 nm dispersed evenly in NC matrix. This reduces the combination of photogenerated electron-hole pair in great extent. To optimize the structure of CdS@NC, functions of pyridinic-N, pyrolic-N and quaternary-N are discussed in detail and the results illustrate synergistic effect of quaternary-N and pyridinic-N accounts for the improvement of H2 generation efficiency. More importantly, with coordination polymer as a precursor, a simple and feasible method has been set up to fabricate visible light driven, highly active and stable CdS based photocatalysts. ASSOCIATED CONTENT Supporting Information Synthesis of CP, CPNP and CdS@NC, Materials and Methods, Electrochemical measurements; Photocatalytic hydrogen production; X-Ray crystallography; PXRD and TGA of CPNP; SEM and size distribution of CdS@NC; S 2p, Cd 3d and C 1s spectra of CdS@NC; CIF file of CP (CCDC 1542143). Author Information


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Corresponding Author * E-mail: [email protected] (Prof. X. X. Xu) [email protected] (Prof. F. N. Shi). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation (21303010, 21571132) and Opening Project of Key Laboratory of Polyoxometalate Science of Ministry of Education (130028720).

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Graphic for the manuscript

A new strategy has been explored to obtain sulfur vacancies rich CdS based composite photocatalyst with nitrogen doped carbon as matrix with coordination polymer precursor.


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Coordination Polymer Derived Sulfur Vacancies ... - ACS Publications

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