Graphdiyne Heterojunction for Enhanced

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In Situ Synthesis of CdS/Graphdiyne Heterojunction for Enhanced Photocatalytic Activity of Hydrogen Production Jia-Xin Lv, Zhi-Ming Zhang, Juan Wang, Xiu-Li Lu, Wen Zhang,* and Tong-Bu Lu* Institute for New Energy Materials and Low Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China S Supporting Information *

ABSTRACT: Hydrogen production through artificial photosynthesis has been regarded as a promising strategy for dealing with energy shortage and environmental problems. In this work, graphdiyne (GD) was first introduced to the visible-light catalytic system for hydrogen production, in which a CdS/GD heterojunction was prepared through a simple in situ growth process by adding Cd(AcO)2 into a dimethyl sulfoxide (DMSO) solution containing GD substrate. The as-prepared CdS/GD heterojunction exhibits much higher performance for photocatalytic hydrogen evolution compared to that of pristine GD and CdS nanoparticles. The photocatalytic performance of CdS/GD heterostructure containing 2.5 wt % of GD (GD2.5) is 2.6 times higher than that of the pristine CdS nanoparticles. The enhanced catalytic performance can be ascribed to the formation of CdS/GD heterojunction, in which the presence of GD can not only stabilize CdS nanoparticles by preventing the agglomeration of CdS nanoparticles but also act as a photogenerated hole transfer material for efficiently separating photogenerated electron−hole pairs in CdS. Accordingly, this work provides the potential of GD-derived materials for solar energy conversion and storage. KEYWORDS: graphdiyne, CdS, heterojunction, photocatalysis, hydrogen evolution

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heterojunction,18 CdS/g-C3N4 heterojunction,19 or CdS/WS2/ graphene composite20 to enhance the separation and migration of photogenerated carriers; integration of noble and non-noble metals as electron-transportors;21−24 and incorporation with other semiconductors to afford heterogeneous materials.25,26 For example, the tripodal S-donor capping agents have been used to stabilize the CdSe nanocrystals to achieve a remarkable catalytic performance for hydrogen evolution;14 the mesoporous material and layered hydrotalcites were both used to confine CdS nanoparticles for constructing efficient hydrogen evolution photocatalysts;27,28 and the combination of CdS nanoparticles with 2D graphene serving as a electron transporter results in a high efficient composite photocatalyst for hydrogen evolution.29 Accordingly, it will be an effective way to enhance the photocatalytic hydrogen production activity of CdS nanoparticles by proper selection of the supporting substrates, which could effectively suppress the aggregation of CdS nanoparticles as well as promote the separation of photogenerated carriers in the ultrafine CdS nanoparticles.

INTRODUCTION Hydrogen energy has been widely considered as a potential alternative of fossil fuels because of its cleanness and high energy density.1 Photocatalytic water-splitting into hydrogen represents one of the most attractive strategies to address energy and environmental issues.2−4 In the past decades, various semiconductors were explored and used as effective catalysts for photocatalytic water splitting into hydrogen. It represents a promising way for the simulation of natural photosynthetic system to transform sunlight into fuels.5−10 Among them, metal chalcogenides, especially CdS nanoparticles, have attracted much attention because of their relatively negative conduction band (CB) position and suitable band gaps for the utilization of solar light in energy conversion.11−13 However, for isolated CdS nanoparticles, its application is still limited by several problems. For instance, CdS nanoparticles are prone to aggregate during the synthetic and catalytic processes, which will greatly reduce their surface area and photocatalytic activity. In addition, the severe recombination of photoexcited charge carriers of CdS nanoparticles could decrease their photocatalytic activity. To address these issues, many efficient strategies have been explored to improve the photocatalytic performance of CdS nanoparticles, such as the synthesis of ultrafine quantum dot with surface ligand modification to increase its surface area;14−16 loading both hole and electron cocatalysts on the CdS surface to improve its activity and stability;17 constructing CdS/ZnO © XXXX American Chemical Society

Special Issue: Graphdiyne Materials: Preparation, Structure, and Function Received: February 26, 2018 Accepted: April 23, 2018

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DOI: 10.1021/acsami.8b03326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Preparation of CdS/GD Composite and Its Photocatalytic Process

nanoparticles decorated on GD substrate were synthesized by the solvothermal reaction. Generally, GD powder and cadmium acetate were dispersed in DMSO (as solvent and sulfide source), and the mixture was heated to 180 °C for 12 h in an autoclave. The 0, 0.5, 1, 2.5, and 5% mass ratios of GD to cadmium acetate were used in the synthetic process, with the resulting products denoted as CdS, GD0.5, GD1, GD2.5, and GD5, respectively. Detailed synthetic process of these composites is given in the Supporting Information. Characterization. PXRD (Powder X-ray diffraction) was performed on a X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å) (Smart Lab 9 KW, Rigaku, Japan). UV−vis diffused reflectance spectra were carried out on a Lambda 750 UV/vis/NIR spectrophotometer. Raman spectrum was recorded on a highresolution laser confocal fiber Raman spectrometer (HORIBA EVOLVTION, HORIBA Jobinyvon, France). XPS (X-ray photoelectron spectroscopy) was detected with Al Kα as the excitation source on an ESCALAB 250 Xi spectrometer (Thermo Scientific, America). The acquisition parameters for VB spectra: pass energy 40.00 eV, energy step size 0.200 eV, total acquisition time 225.9 s, number of scans 20, number of energy steps 226. The analysis area in the selected lens mode was determined to be 500 μm in diameter. TEM (Transmission electron microscope) and high-resolution TEM (HRTEM) images were performed on Talos F200X, FEI, America using 200 kV acceleration voltage. The SEM (scanning electron microscopy) images were acquired on an Environmental SEM with FEG (Quanta FEG 250, FEI, America). EDS mapping were taken on Talos F200X, FEI, America. Photoluminescence (PL) spectra were detected by a fluorescence spectrophotomer (F-7000, Hitachi, Tokyo, Japan). Electrochemical experiments were performed using a CHI 760E electrochemical workstation. Mott−Schottky plots were determined by impedance-potential technique using a three-electrode system, FTO (10 Ω sq−1) with a geometrical area of 1.0 × 2.5 cm2, Ag/AgCl (in 3 M KCl) and platinum plate (1.0 × 1.0 cm2) as the working electrode, reference electrode and counter electrode, respectively. For the capacitance of the semiconductor-electrolyte interface, it was determined with 10 mV AC voltage amplitude at 1 kHz, in pH 7, 0.5 M Na2SO4 aqueous solution. In a typical process, 5 mg of each sample was dispersed into 0.5 mL of DI water. Then, 40 μL of this dispersion was dispersed to FTO surface, which was further dried at 60 °C for 30 min. Photocatalytic Reaction. The photocatalytic hydrogen reactions were performed by adding 2 mg of photocatalyst CdS/GDx in 5 mL of aqueous solution (0.3 M triethanolamine (TEOA)) in a quartz bottle

Graphdiyne (GD), as a new member of carbon materials, is composed of both sp- and sp2-hybridized carbon atom to form extended two-dimensional layers.30 Benefiting from its special electronic structure and geometric framework, the GD and its derivatives have attracted much attention in diverse fields, such as lithium-ion batteries,31,32 hydrogen storage,33 deposition of ultrafine noble clusters,34 photodegradation,35−37 electrocatalytic oxygen reduction,38 etc. According to the theoretical calculations, the hole mobility of GD can reach to 1 × 104 cm2 V−1 S1−,39 thus, the GD could be introduced into perovskite solar cells as a hole-transporting layer or employed as photocathodes for hydrogen production.40−42 However, the GD has never been used as a hole-transfer material for catalytic hydrogen evolution driven by visible light. Herein, the CdS/GD heterojunction was facilely prepared by directly adding Cd(AcO)2 into a GD-containing DMSO solution. In this process, 2D GD serves as a suitable supporting substrate for the growth of the CdS nanoparticles into the CdS/GD heterojunction. As the high hole mobility of GD, the photogenerated holes in CdS could transfer to GD through the in situ formed CdS/GD heterojunction, which can efficiently suppress the recombination of photogenerated carriers. Moreover, a proper content of GD can efficiently extend the visible light absorption of ultrafine CdS nanoparticles. The preparation of CdS/GD composites and the photogenerated carriers interfacial migration process were shown in Scheme 1. For CdS, the photoinduced holes could be efficiently injected into the valence band (VB) of the GD under the irradiation at 450 nm due to the formation of the CdS/GD heterostructure. Consequently, the photogenerated electrons in CdS have longer time to participate in the proton reduction. The results demonstrate that GD can act as a promising substrate to support semiconductor nanoparticles and a hole-transfer material for photocatalytic H2 production simultaneously.

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EXPERIMENTAL SECTION

Synthesis of CdS/GD Composites. As shown in Scheme 1, the GD was synthesized via in situ growth strategy on a copper foil using hexaethynylbenzene (HEB) as the monomer.30 Then the CdS B

DOI: 10.1021/acsami.8b03326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. SEM images of (a) isolated CdS nanoparticles and (b) GD2.5; (c) TEM and (d) HRTEM images of GD2.5 (inset: the SAED pattern of GD2.5).

Figure 2. (a) HADDF image for GD2.5; (b−e) elemental mapping images of GD2.5; and (f) XPS pattern of GD 2.5.

solvothermal reaction at 180 °C for 12 h, then the CdS/GD heterojunction with different amount of GD could be obtained. To confirm the formation of CdS/GD composites, we performed XRD, SEM, and TEM analysis on these CdS/GD composites. The XRD results (Figure S1) show that CdS/GD composites with different contents of GD exhibit similar diffraction peaks at 52.1°, 44.0° and 26.5°, corresponding to the diffractions of (311), (220), and (111) lattice planes of isolated cubic phase CdS (JCPDS 80−0019), respectively. SEM images reveal that, in the absence of GD substrate, CdS tends to form aggregated spheres with an average diameter of around 200 nm (Figure 1a). In the presence of GD, much smaller CdS nanoparticles with 7−15 nm diameters are uniformly distributed and tightly packed on GD surface (Figure 1b). As

sealed with a rubber tube. Before the irradiation, the solution was bubbled with argon for 20 min to eliminate residual oxygen. Further, a blue LED light (Zolix, MLED4, λ = 450 nm) was used to irradiate the mixture. The generated gas products were detected by a Shimadzu GC-2014 gas chromatography (thermal conductivity detector, TDX01 packed column) with argon as a carrier gas.

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RESULTS AND DISCUSSION Synthesis and Characterization of CdS/GD Composites. CdS/GD heterostructure was synthesized by a one-step solvothermal method. In short, as-prepared GD was used as the substrate, cadmium acetate as the cadmium source, and DMSO as both the sulfide source and solvent. After a sufficient dispersion of these precursors, the mixture was subjected to C

DOI: 10.1021/acsami.8b03326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. XPS spectra of C 1s in GD and GD 2.5.

corresponding to C−S at 286.5 eV and O−CO at 288.7 eV appeared, which may result from the combination of CdS with the GD and residual acetate on CdS surface, respectively. These results suggest that some of the acetylenic groups in the GD react with CdS precursors to form stable CdS/GD heterojunction during solvothermal reaction. The XPS of pure CdS nanoparticles only displays three peaks belonging to the residual acetate on CdS surface (Figure S11), and no peaks belonging to GD were observed. Photocatalytic Hydrogen Production. To access the photocatalytic performance of CdS/GDx, we performed the photocatalytic hydrogen evolution experiments in aqueous solution with TEOA as a sacrificial reagent. The results show that the contents of GD have a significant impact on the photocatalytic activity of CdS (Figure 4a). The mass activity of

shown in Figures S2 and S3, the isolated GD exhibits a smoother surface than the CdS/GD composite, which indicates that CdS nanoparticles were successfully anchored on GD surface. Further, the HADDF image of GD2.5 and corresponding elemental mapping confirm that the Cd, S and C elements uniformly distributed on the composites (Figure 2). These findings suggest that GD can serve as a supporting matrix to efficiently anchor CdS nanoparticles on its surface and prevent the aggregation during the synthetic process. To further investigate the subtle structures of the CdS/GD composites, TEM was operated on GD2.5 sample as the example. As shown in Figure 1c, there are numerous CdS nanoparticles covering the GD skeleton, which matches well with the results of SEM (Figure 1b). As shown in Figure 1d, HRTEM image displays the defined lattice spacing of 0.175, 0.208, and 0.333 nm on the crystalline nanoparticles, which correspond to the (311), (220), and (111) crystalline planes of cubic CdS, respectively. The amorphous region is attributed to the GD substrate. Such close connections between CdS and GD demonstrate the formation of the CdS/GD heterojunction. The selected area electron diffraction (SAED) image shows that these CdS nanoparticles on GD are polycrystalline (Figure 1d). Three inside diffraction rings are in line with (111), (220), and (311) crystalline planes of cubic CdS, respectively, consistent with the XRD patterns (Figure S1). All the above images demonstrate the successful preparation of CdS/GD heterojunction with CdS nanoparticles homogeneously anchored on the GD scaffold. The formation of the heterojunction might promote the injection of photoinduced holes from CdS to GD substrate. The chemical compositions and bonds in CdS/GD composites were investigated by EDX and XPS, respectively. Different from the isolated GD substrate mostly composed of carbon (Figures S4−S7), the XPS pattern of CdS/GD composite obviously shows the binding energies of Cd and S (Figure 2f and Figures S8 and S9), also consistent with the EDX results (Figure S10). The peaks of C 1s of GD and CdS/ GD composite were analyzed in detail. As shown in Figure 3a, b, both the GD and the CdS/GD show the binding energy of C 1s at 284.7 eV. For pure GD, four subpeaks were observed in the spectrum of C 1s (Figure 3a). These peaks correspond to bind energies of sp2 (CC, 284.4 eV), sp (CC, 285.2 eV), C−O (286.8 eV), and CO (288.5 eV), respectively. Integration of the sp and sp2 peaks gives the calculated sp/ sp2 ratio as 2, consistent with that of the C sp/C sp2 in the HEB monomer. By contrast, the ratio of sp/sp2 decreases to 1.4 in the XPS spectrum of GD2.5 composite (Figure 3b), as the diacetylene units in the GD skeleton are usually unstable in solvothermal treatment. Meanwhile, two additional peaks

Figure 4. (a) Photocatalytic H2 production of CdS, GD and GDx (x = 0.5, 1, 2.5, and 5). (b) Photocatalytic evolution of H2 catalyzed by GD2.5 (red), CdS (black), and the mixture of CdS and GD (blue) (97.5/2.5, w/w), respectively. Light source: 450 nm LED, 200 mW cm−2, illumination area, 0.8 cm2) in the presence of 2 mg of catalyst and TEOA (0.3 M) in 5 mL ofaqueous solution at 25 °C. Each photocatalytic reaction was repeated at least three times.

the catalysts increased along with the increase of GD content, and the highest H2 evolution activity is achieved by using GD2.5 (4.1 mmol g−1), 2.6 times of that of pure CdS (1.6 mmol g−1). When the content of GD further increased, the hydrogen evolution activity of CdS/GD composite (GD5.0) significantly decreased, which can be attributed to the higher opacity and stronger light scattering with the higher contents of D

DOI: 10.1021/acsami.8b03326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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morphology changes was detected for GD2.5 (Figure S13). In contrast, the morphology of pure CdS shows an obvious aggregation to form larger particles (200−800 nm) (Figure S14). These comparative results indicate that the CdS/GD heterostructure was relatively stable compared to pure CdS during photocatalytic hydrogen evolution. Therefore, the GD could improve the stability and the photocatalytic performance of CdS-based catalysts. When the photocatalytic system was enlarged from 5 mL (containing 2 mg catalyst) to 25 mL (containing 10 mg catalyst), the amounts of hydrogen production are nearly 5-fold increased (Table S1 and Figure S15), demonstrating the data shown in Figure 4 are believable, and the difference of photocatalytic activity caused by the experimental error can be excluded. To better understand the photocatalytic process with the CdS/GD heterostructure, we evaluated the electronic band structures of GD and CdS by UV−vis absorption spectra, Mott−Schottky plots, XPS spectra and PL spectra. First, the direct band gap of CdS was determined by UV−vis spectra with an estimated value of 2.30 eV (Figure 6a). The variations in energy bands for GD and CdS were validated by detecting their flat-band potential. From Figure 6b, it can be found that the flat-band potentials (corresponding to the CB) for GD and CdS obtained from the Mott−Schottky plots are −1.22 and −0.88 V versus Ag/AgCl, respectively. Moreover, the VB spectra of XPS indicate that the VB of GD is 0.58 eV higher than that of CdS (Figure 6c). Accordingly, we can determine the relative energy-band position between GD and CdS, which can confirm the transfer direction of photogenerated carriers. Figure 6d shows that the photogenerated holes of CdS can transfer to the VB of the GD, thus the probability of the recombination of photogenerated carriers can be greatly suppressed. Thus, the photogenerated electrons in CdS have more opportunity to attend the proton reduction. Steady-state PL spectrum was also conducted to confirm the efficient separation of photogenerated carriers in CdS/GD composite (Figure S16). The emission of GD2.5 was quenched by 45%,

GD. Therefore, the introduction of an appropriate amount of GD into the heterojunction plays an important role in improving the photocatalytic performance of CdS/GD.43 For pure GD, no hydrogen could be obtained (Figure 4a), suggesting that the GD is inactive for visible-light-driven hydrogen evolution in this setup. Further, the photocatalytic experiment was also performed with the physical mixture of GD and CdS. As shown in Figure 4b, compared with pure CdS, there is no observable improvement of hydrogen evolution in this case, indicating the formation of CdS/GD heterojunction is critical to improve the catalytic performance of CdS. In addition, the stability of CdS/GD photocatalyst was confirmed, as no obvious deactivation of hydrogen evolution activity was observed after four consecutive cycles of the photocatalytic experiments with the as-prepared GD2.5 (Figure 5). For

Figure 5. Recycling experiments for H2 production of GD2.5 (red) and CdS (black), light source: 450 nm LED, 200 mW cm−2, illumination area, 0.8 cm2) in the presence of 2 mg of catalyst and TEOA (0.3 M) in 5 mL of aqueous solution at 25 °C.

comparison, the recycling photocatalytic experiments of pure CdS were operated, where the photocatalytic performance of CdS decreased by 40% after four cycles (Figure 5, black line). Moreover, the SEM images of GD2.5 and CdS after photocatalytic reaction also demonstrate that no significant

Figure 6. (a) UV−vis spectrum of CdS, and (inset) the corresponding Tauc plots ((αhv)2) vs photon energy (hν). α, h, and ν represent the absorption coefficient, Planck’s constant, and light frequency, respectively; (b) Mott−Schottky plots; (c) XPS valence spectra of GD and CdS; and (d) band structures of GD and CdS. E

DOI: 10.1021/acsami.8b03326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was financially supported by NSFC (21790052, 21702146, and 21331007).

indicating the formation of CdS/GD heterojunction with GD as a hole-transfer layer can greatly suppress the recombination of photogenerated carriers in CdS, which could dramatically improve the photocatalytic performance of CdS for hydrogen production. In addition, the UV−vis absorption spectra of CdS and GDx (x = 0.5, 1.0, 2.5, and 5.0) show that the light absorption (>450 nm) of CdS/GD composite was much enhanced with the increase of the GD contents (see Figure 7), and the color of the

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composites changed from pale yellow to light green simultaneously. These results indicate the formation of CdS/ GD heterostructure could efficiently improve its light-adsorbing ability, so as to enhance its photocatalytic performance for H2 production.

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CONCLUSIONS In summary, GD was first utilized as an efficient hole-transfer material in the photocatalytic hydrogen evolution system. Through a facile in situ growth process, the ultrafine CdS nanoparticles were successfully decorated on the GD substrate. The construction of heterojunction between CdS and GD can efficiently accelerate the injection of photoinduced holes from CdS to GD, and suppress the electron−hole recombination in CdS/GD composites. As a result, the as-synthesized CdS/GD heterojunction possesses much higher activity and stability in photocatalytic hydrogen evolution than those of isolated GD and CdS nanoparticles, respectively. The results indicate that the GD, with unique properties can act as a fine substrate and a good hole-transfer material for the preparation of stable semiconductor materials. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03326. Preparation of GD and CdS/GD composites, XRD measurements, SEM, TEM, Raman spectra, EDX spectra, UV−vis absorption spectrum, PL spectra (PDF)

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Figure 7. UV−vis absorption spectra of the heterostructures and CdS.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhi-Ming Zhang: 0000-0003-3116-756X Tong-Bu Lu: 0000-0002-6087-4880 Notes

The authors declare no competing financial interest. F

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DOI: 10.1021/acsami.8b03326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX