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ZnIn2S4 heterostructures for enhanced photocatalytic hydrogen evolution
Chemical Engineering Journal 353 (2018) 15–24
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Fabrication of two-dimensional Ni2P/ZnIn2S4 heterostructures for enhanced photocatalytic hydrogen evolution ⁎
Xu-li Lia, Xiao-jing Wanga,b, , Jia-yu Zhua, Yu-pei Lia, Jun Zhaoa, Fa-tang Lia, a b
T
⁎
College of Science, Hebei University of Science and Technology, Shijiazhuang 050018, China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Lower overpotential of 2D Ni P na• nosheets in photocatalytic H evolu2
2
tion are reported.
2D/2D Ni P/ZnIn S photocatalyst is • fabricated for enhanced HER perfor2
2 4
mance.
quantum efficiency of 7.7% • Apparent at 420 ± 20 nm is achieved. structure exhibits lower surface • 2D energy barrier and shorter carriers distance.
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalytic hydrogen evolution 2D-2D Overpotential Interfacial charge transfer Ni2P ZnIn2S4
Promoting electron-hole separation and migration and lowering the overpotential of hydrogen evolution reactions are two effective solutions for improving photocatalytic hydrogen performance. Suitable co-catalyst and appropriate interfacial contacts can effectively lower overpotential and can also construct an electric field at the interface to increase the separation efficiency of the carriers. In this work, we design and fabricate a 2D-2D type of Ni2P co-catalyst modified with ZnIn2S4 for boosting the performance of photocatalytic hydrogen evolution. As a co-catalyst, the 2D Ni2P nanosheets exhibit a lower overpotential and smaller charge transfer resistance in hydrogen evolution reactions, and is much improved in both respects compared to Ni2P nanoparticles. Based on this, 2D/2D Ni2P/ZnIn2S4 nanohybrids with large contact regions and shorter transmission distances of the charges were fabricated, which effectively improve the separation of photo induced carriers and the interfacial charge transfer. By taking advantage of the above features, the fabricated 2D-2D Ni2P/ZnIn2S4hybrid exhibits a superior hydrogen evolution rate of 2066 μmol·h−1·g−1 under visible light irradiation, and the apparent quantum yield was 7.7% at 420 ± 20 nm. This activity far exceeds performance of the 0D/2D Ni2P/ZnIn2S4 hybrid, and is ascribed to better charge separation and accelerated surface reactions of the Ni2P nanosheets.
1. Introduction The overuse of fossil fuels and related environment issues have stimulated more researchers to exploit renewable and carbon-free energy alternatives. Hydrogen fuel is the cleanest proposed energy source
⁎
and has zero emissions, and photocatalysis technology is considered to be a sustainable and environmentally friendly strategy for solving the increasing environmental and energy crisis. Hitherto, unremitting efforts have been devoted to developing technologies for the photocatalytic evolution of hydrogen using various semiconductors [1,2].
Corresponding authors at: College of Science, Hebei University of Science and Technology, Shijiazhuang 050018, China (X.J. Wang). E-mail addresses: [email protected] (X.-j. Wang), [email protected] (F.-t. Li).
https://doi.org/10.1016/j.cej.2018.07.107 Received 1 May 2018; Received in revised form 21 June 2018; Accepted 14 July 2018 Available online 17 July 2018 1385-8947/ © 2018 Published by Elsevier B.V.
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performance above was attributed to better charge separation and more active sites provided by MoS2 nanosheets. However, little research has focused on the fabrication of two dimensional TMP nanosheet co-catalysts and the substantial effect of those TMP materials with different dimensionality on physicochemical properties during the photocatalytic hydrogen production process. Zhang et al. [38] demonstrated a kind of CoP nanosheets with outstanding electrochemical hydrogen evolution performance: a lower overpotential and smaller Tafel slope which is far better than that of CoP nanoparticles. Smaller Tafel slope of the electrocatalyst often means faster reaction kinetics, which is beneficial to accelerate the surface reaction of hydrogen evolution. Inspired by this report and to fully maximize the merits of 2D TMP materials, we prepare 2D–2D Ni2P/ZnIn2S4 photocatalyst for the first time using a post-synthesis method. This work presents an efficient route for maximizing the function of transition metal phosphides as co-catalysts.
TiO2 is the most widely studied photocatalyst owing to its high photocatalytic activity, stability, nontoxicity and low cost. However, TiO2 has a wide band gap, which affects its ability to harvest of solar energy [3]. Compared with metal oxides, metal sulfides possess relatively lower conduction band (CB) positions and narrower band gaps, which are beneficial for expanding the response to light in the visible light range [4,5]. ZnIn2S4, as one of the important ternary chalcogenide semiconductors (AB2S4), is a good photocatalyst candidate due to its low toxicity, suitable band-gap (∼2.4 eV), and relatively high chemical stability [6–8]. Nonetheless, as with most of the semiconductor-based photocatalysts, bare ZnIn2S4 usually has low activity during the process of photocatalytic hydrogen production because it has a large hydrogen evolution overpotential, and the photocatalytic efficiency is strictly restricted by fast electron-hole recombination [9]. Generally, the photocatalytic hydrogen evolution performance of semiconductor photocatalysts can be improved in several: (i) enhancing the utilization of solar energy, (ii) improving charge separation and migration to the surface, (iii) lowering the surface overpotential of redox reactions and (iv) increasing the number of reactive sites [10]. It is well known that suitable co-catalysts are indispensable in artificial photocatalytic hydrogen evolution systems, which can decrease the overpotential for hydrogen production, improve host reaction active sites, create an electric field at the interface, and thereby increase the separation efficiency of electrons and electron holes. Noble metals have usually been employed as co-catalyst candidates in the majority of previous studies [11]. However, noble metals suffer from high costs and low reserves. In consideration of large-scale applications, it is essential to find low cost and efficient noble metal-free co-catalysts. Up until now, various noble-metal-free co-catalysts have been available, such as nickel-based alloys [12], sulfide-based materials [13,14], carbides [15], nitrides [16] and multicomponent co-catalysts [17,18]. Recently, transition metal phosphides (TMPs), typical representatives of burgeoning non-noble metal co-catalysts (including CoP, Ni2P, and MoP), have been intensively studied for both electrocatalytic [19] and photocatalytic [20–25] hydrogen evolution due to their high stability and low overpotential during hydrogen evolution. Meanwhile, metal phosphides can promote the release of hydrogen from active sites owing to structural arrangements [26]. As a low-cost TMP, it has been reported that Ni2P is destined to act as an excellent hydrogen evolution catalyst because of weak binding of hydrogen on the Ni2P surface [27]. Research has achieved superior photocatalytic hydrogen activity employing Ni2P as co-catalyst for CdS [28,29]. Chen et al. [30] reported on the general applicability of nanocrystalline Ni2P as a noble-metal-free co-catalyst to boost photocatalytic hydrogen generation. Other researches anchored zero-dimensional nanocrystalline Ni2P onto g-C3N4, which exhibited a conspicuous enhancement in hydrogen evolution activity under visible light radiation [31,32]. In our previous work, we also identified that the analogous metallic character of Ni2P nanoparticles can accelerate the transfer and consumption of photo-generated electrons [33]. However, how to maximize the function of suitable transition metal phosphides as co-catalysts still remains a formidable challenge. Two-dimensional (2D) ultrathin materials have been extensively studied as ideal materials because of their large specific surface area, short electron/carrier transfer distance and richness in active sites. For instance, graphene is employed as an effective electrocatalyst [34] or co-catalyst for photocatalysis [35] in hydrogen evolution processes due to its superior electrical transport properties. Xia et al. [36]constructed different carbon materials by modifying ZIS nanosheets to enhance the photoactivity on hydrogen evolution and found that the 2D RGO exhibited unique advantages among the studied carbon materials. Li et al. [37] assembled ZnIn2S4 nanosheets on several layers of MoS2 nanosheets to produce ultra-thin and intimate-contacting 2D hybrid photocatalysts, and Yuan et al. [10] reported a 2D Cu2+-doped ZnIn2S4 nanosheets modified with 2D MoS2 presents highly photocatalytic hydrogen evolution performance. The enhancement of photocatalytic
2. Experimental 2.1. Chemicals All raw materials were used without further purification. Red phosphorus, lactic acid, ammonia, sodium carboxymethyl cellulose (CMC300), thioacetamide, NaH2PO4·2H2O, H2PtCl6·6H2O, Zn (NO3)2·6H2O, NiCl2·6H2O, Ni(AC)2·4H2O and In(NO3)3·4.5H2O were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Nafion (Sigma-Aldrich) was used for electrode preparation. 2.2. Preparation of ZnIn2S4 (ZIS) nanosheets In a typical experiment, 1.439 mmol Zn(NO3)2, 2.439 mmol In (NO3)3·xH2O and 10 mmol thioacetamide were dissolved in 35 mL water with vigorous stirring for 20 min at room temperature. Next, the mixture was transferred into a 50 mL Teflon-lined autoclave and maintained at 160 °C for 12 h. After cooling to room temperature naturally, the precipitate was washed with distilled water and combined with ultrasound irradiation several times and was finally placed in 10 mL ethanol for the subsequent synthesis. 2.3. Preparation of Ni2P nanosheets (SNPs) To prepare Ni(OH)2 nanosheets, 0.5 g Ni(AC)2·4H2O was dissolved in a mixture of 10 mL deionized water and 20 mL 1 g/L CMC300 solution while stirring, followed by the addition of 2 mL of ammonia aqueous solution (25 wt%). After stirring for another 3 h at room temperature, the suspension was transferred into a 50 mL Teflon-lined autoclave and heated at 160 °C for 12 h. The precipitate was collected by centrifugation and sonication, washed with hot deionized water several times, and then dried via vacuum freeze-drying [39]. To prepare SNPs. Ni2P nanosheets were obtained by putting Ni (OH)2 (100 mg) and NaH2PO2·2H2O (700 mg) into a steamer like porcelain boat, which was separated by a ceramic filter. NaH2PO2·2H2O was put on bottom and Ni(OH)2 was placed on the ceramic filter. Subsequently, the samples were heated at 300 °C for 180 min in a fluent Ar2 atmosphere at a warming rate of 5 °C min−1. During the calcining process, the Ni(OH)2 nanosheets captured PH3, which was generated from the thermal decomposition of NaH2PO4. After cooling to room temperature under the Ar2 environment, the products were collected for further measurements. In comparison, Ni2P nanoparticles (PNPs) were prepared using a hydrothermal method, which was performed according to preciously described methods [40]. 2.4. Preparation of 2D-2D Ni2P/ZnIn2S4(SNP/ZIS) composites The process for preparing SNP/ZIS composites is illustrated in 16
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Fig. 1. Schematic representation of the synthesis procedures of 2D-2D SNP/ZIS composite photocatalysts.
Na2SO4 solution, and fluorine-doped tin oxide (FTO) combined with sample was used as the working electrode. The working electrode was prepared as follows: 40 mg of catalyst was dispersed in 100 μL ethanol and then mixed with 10 μL 5% Nafion solution by sonicating for 20 min. Finally, 10 μL of the abovementioned suspension was deposited on the FTO and dried at room temperature.
Fig. 1. In detail, a certain amount of SNPs was dispersed into 10 mL of ethanol by sonication for 30 min and then 30 mL turbid liquid containing of 0.2 g ZIS was added while stirring magnetically. After being stirred for 12 h, the suspension was dried at 60 °C. In contrast, 0D/2D Ni2P/ZnIn2S4 (PNP/ZIS) composite was synthesized using the same strategy, except that the Ni2P nanosheets were replaced by Ni2P nanoparticles. The obtained samples were denoted as xSNP/ZIS or xPNP/ ZIS, where x is the wt% of NP in the composite calculated from the initial addition of SNPs or PNPs.
2.7. Photocatalytic hydrogen production activity Photocatalytic hydrogen evolution was detected in a gas-closed circulation system. Photocatalyst powders (50 mg) were dispersed in a 100 mL aqueous solution containing 90 mL of deionized water and 10 mL of lactic acid (LA). A 300 W Xe lampwas used as the visible-light source, with light that was passed through a cut-off filter (λ > 400 nm for visible light illumination). Before irradiation by light, the reactor was degassed with Ar2 for 30 min to remove O2 from the system. 1 mL of gas was intermittently sampled and analyzed by a gas chromatography (GC7900, Ar2 carrier, thermal conductive detector). The apparent quantum efficiency(QE) of the hydrogen evolution at 420 ± 20 nm was estimated using the following equation.
2.5. Characterization The sample nanostructures were analyzed by X-ray diffraction (XRD) using CuKα radiation on a RigakuD/MAX 2500 X-ray diffractometer. A scanning electron microscope (SEM) (HITACHI S-4800) was used to characterize the morphology of the powder samples. Transmission electron microscopy (TEM), high resolution transmission electron microscopy(HRTEM) and elemental mapping images were acquired on an FEI Tecnai G2 F20 S-TWIN field-emission scanning electron microscope. The X-ray photoelectron spectroscopy was used to analyze surface using a PHI 1600 ESCA XPS system. Ultraviolet–visible diffuse reflectance spectra (UV–Vis DRS) was recorded with a Thermo Fisher Evolution220 transmission spectrophotometer. The separation characteristics of the photogenerated charge carriers were tested by surface photovoltage spectroscopy (SPS), which consists of a source of monochromatic light with a light chopper and a lock-in amplifier. The work function of ZIS and NP was measured using an SKP5050 Scanning Kelvin Probe.
AQE =
2×number of evolved hydrogen molecules × 100% number of incident photons
3. Results and discussion 3.1. Structures and morphologies of as-prepared samples The XRD patterns of pure ZIS, SNP and xSNP/ZIS samples are shown in Fig. 2. All the diffraction peaks of the ZIS can be assigned to the hexagonal ZnIn2S4phase (JCPDS card No. PDF#065-2023), and all of the main diffraction peaks of ZIS are apparent in the SNP/ZIS composites [40]. A typical XRD pattern for SNP is presented, which is indexed to the hexagonal Ni2P phase JCPDS card PDF#03-0953, and exhibits all of the main peaks with 2θ values of 40.8°, 44.6°,47.3°,54.2°,55°,66.3°,72.7°, and 74.9°, which correspond to the (1 1 1), (2 0 1), (2 1 0), (3 0 0), (2 1 1), (3 1 0), (3 1 1) and (2 1 2) planes. A feeble diffraction peak at approximately 40.8° was found in the composites when the SNP loading exceeded 10%. No diffraction peaks corresponding to Ni2P were observed in the XRD patterns at lower loading amounts because of the small amount and relatively low diffraction intensity of Ni2P. The microstructures and morphologies of samples were investigated by SEM and TEM. As illustrated in Fig. 3a and b, bare ZIS (Fig. 3a, d) and SNP (Fig. 3b, e) samples are composed of stacked nanosheets, whose thicknesses are approximately 5–10 nm. The morphology of the
2.6. Electrochemical measurements The polarization curves, Nyquist plots and photocurrent measurements were all recorded by a CHI660E electrochemical analyzer (ChenHua Instrument Co. Ltd.) using a standard three-electrode system, where an Ag/AgCl and a graphite rod electrode were used as the reference and counter electrodes, respectively. The polarization curves were studied in a 0.5 M H2SO4 solution, the samples were loaded at 0.28 mg·cm−2 on a glassy carbon (GC) electrode as the working electrode. The polarization data curves were acquired at a scan rate of 5 mV·s−1. Moreover, in order to achieve a steady-state condition, we collected the final data after cycle voltammetry (CV) tests at scan rate 0.03 V·s−1 for more than 20 cycles. The electrochemical impedance spectroscopy (EIS) was tested by the abovementioned working electrodes and measured over a frequency range of 0.1–105 Hz, using a 0.5 M H2SO4 solution as the electrolyte. Transient photocurrent measurements were conducted in a 0.5 M 17
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10SNP/ZIS sample is shown in Fig. 3c and f, and other SEM images of the SNP/ZIS samples are displayed in Fig. S1. It is evident that uniform nanosheets were obtained in all SNP/ZIS samples and seem to be smaller than bare ZIS and SNP. Smaller nanosheets can lead to more exposed surfaces and active edges, which may contribute to the enhancement in HER performance [34]. HRTEM images of 10SNP/SZIS samples are depicted in Fig. 3g. Obviously, the lattice fringes correspond to the interplanar distance of 0.22 nm and 0.32 nm, respectively, which can be attributed to the (1 1 1) plane of the Ni2P phase and the (1 0 1) plane of the ZnIn2S4 phase. The intimate contact could contribute to the efficient transfer of the charges between the components and therefore improve the photocatalytic activity. To further testify the distribution of the two substances, a TEM image and the corresponding energy-dispersive X-ray (EDX) spectrum mapping is shown in Fig. 3h. It is obvious that there are uniform distributions of Zn, In, S, Ni and P elements in the sample of 10SNP/ZIS, indicating that the SNP/ZIS hybrid structure was successfully constructed. The BET surface areas of the prepared samples and the effect of SNPs were investigated using nitrogen adsorption-desorption isotherms. As shown in Fig. 4, all of the SNPs, pure ZIS and 10SNP/ZIS
Fig. 2. XRD patterns of ZIS, SNP and SNP/ZIS composites.
Fig. 3. SEM images of (a) ZIS (b) SNP and (c) 10 SNP/ZIS; TEM images of (d) ZIS (e) SNP and (f) 10SNP/ZIS; (g) HRTEM:10SNP/ZIS (h) STEM and EDX elemental mapping images of the 10SNP/ZIS sample. 18
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Fig. 4. Nitrogen adsorption-desorption isotherms of ZIS, SNP and10SNP/ZIS.
samples. XPS survey spectra of ZIS, SNPs and 10SNP/ZIS composites are shown in Fig. 6a, confirming that the composite is mainly composed of Zn, In, S, Ni and P. Fig. 6b–d display Zn 2p, In 3d and S 2p XPS spectra, respectively. The two peaks located at 1022.4 eV and 1045.4 eV can be attributed to the Zn 2p3/2 and Zn 2p1/2, respectively [44]. The binding energies of In 3d3/2 and In 3d5/2 are located at 452.9 eV and 445.5 eV, respectively, which are indexed to the In–S band [45]. The S 2p spectrum (Fig. 6d) shows two peaks at 161.7 eV and 163.0 eV. It is notable that compared with pure ZIS, the coupled 10SNP/ZIS composite possesses slightly higher binding energy for all elements, indicating strong chemical bonding between SNPs and ZIS. Fig. 6e and f show the Ni 2p and P 2p XPS spectra, respectively. The peaks at 853.4 and 129.4 eV correspond to the elements Ni and P with oxidation states of nearly zero, which are assigned to combined Ni2P. Meanwhile, the peaks at 856.4 and 134.4 eV are ascribed to Ni2+ and nickel phosphate species from surface oxidation. Combining with reported results, the Ni–P bond is of covalent nature with charge transfer characteristics, which is beneficial for the migration of fast electrons, additionally, facilitating the electron transfer of the ZIS sample and achieving the enhancement of photocatalytic hydrogen evolution [28]. Compared with SNPs, the Ni2p and P2p spectra appear very weak due to the low concentration of SNPs in the composites. Furthermore, by means of fitting analysis, it was found that a slight shift of Ni2p to a lower binding energy and P2p to a higher binding energy in the 10SNP/ZIS composite implies a possible chemical bonding actions between SNPs and ZIS rather than a simple physical mixture [44].
composite samples exhibit a type-Ⅳisotherm, and the isotherms of ZIS and 10SNP/ZIS present an evident H2 type-shape in the hysteresis loop data, indicating the presence of an ink bottle-type mesoporous nature. The BET surface area of the samples is listed in Table S1. Interestingly, a larger surface area is observed for the SNP/ZIS samples. As mentioned in Section 2.2, the assembly process for SNP with ZIS happened before the ZIS drying process, therefore, by evaluating the SEM images, we infer that SNP could inhibit the agglomeration of ZIS nanosheets during the drying process. As is common knowledge, a large surface area contributes more adsorption/reaction sites during the photocatalytic reaction process, which is beneficial for the improvement of photocatalytic activity [41]. 3.2. Optical absorptive performance Fig. 5a shows the UV–vis absorption spectra of pure ZIS, SNPs and different ratios of SNP/ZIS samples. It is apparently that the SNP spectrum shows a high and wide adsorption coefficient throughout the tested range of the ultraviolet-visible spectrum, and ZIS shows an absorption edge at approximately 514 nm, corresponding to a band gap of 2.41 eV, which is estimated by the Kubelka-Munkfunction and shown in Fig. 5b [42]. Adding a different amount of Ni2P would increase the absorption between 460 ∼ 800 nm. The valence band of ZIS is measured by XPS valence spectra and shown in Fig. 5c. It can be observed that the valence band edge of ZIS is located at approximately 1.42 eV, which is consistent with previous studies [43]. Combined with the band-gap energy, the conduction band edge of ZIS is estimated to be approximately −0.99 eV.
3.4. Photocatalytic properties The photocatalytic hydrogen production activity of the as-prepared samples were evaluated in a lactic acid solution (10%) containing 50 mg of photocatalysts. Fig. 7a shows the hydrogen evolution activity of SNP/ZIS photocatalysts with different loading amounts of SNPs in
3.3. Interaction between ZIS and SNP To further study the interaction of ZIS and SNPs, XPS measurements were employed to elucidate the chemical compositions of as-prepared
Fig. 5. (a) UV–Vis DRS of ZIS, SNP and SNP/ZIS composites, (b) the plots of (ahν)2 versus hν of ZIS, (c) XPS valence band spectra of ZIS. 19
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Fig. 6. XPS spectra of (a) survey (b) Zn 2p (c) In 3d (d) S 2p of (e) Ni 2p (f) P 2p ZIS, SNP or SNP/ZIS composites.
1%Pt/ZIS(AQY = 3.9%). However, if we continue to increase SNP loading, the hydrogen rate starts to decrease, which is caused by excess SNPs on the surface of ZIS blocking the absorption light by ZIS. It is worth mentioning that the initial photocatalytic hydrogen rate of SNP/ ZIS is relatively low, probably owing to the existence of phosphate species bonded to the photocatalyst [29]. The comparison of photocatalytic hydrogen evolution activity for some co-catalyst modified ZIS photocatalysts are listed in Table S2. It can be seen that the SNP/ZIS photocatalyst presented here exhibits relatively high photocatalytic hydrogen evolution activity. A long-time photocatalytic hydrogen test was conducted to test the stability of 10SNP/ZIS and is shown in Fig. 7c.
the visible light range. Pure ZIS shows relatively low photocatalytic hydrogen production activity because of the rapid recombination of photogenerated electron-hole pairs. No substantial hydrogen production is observed in the control experiment with SNPs, which reveals that NP is not an active photocatalyst for hydrogen evolution. Nevertheless, with SNP loading, the SNP/ZIS hybrid photocatalysts exhibit a significantly enhanced hydrogen evolution rate. In contrast, a 0D/2D Ni2P/ZnIn2S4 (PNP/ZIS) composite was synthesized by using the same strategy. As shown in Fig. 7b, the highest hydrogen evolution reached 2066 μmol·g−1·h−1 with 10SNP/ZIS, and the AQY was 7.7% at 420 ± 20 nm, which is higher than both 10PNP/ZIS(AQY = 4.6%) and 20
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Fig. 7. Photocatalytic activity for hydrogen production of (a) ZIS、SNP and SNP/ZIS composites (b) different co-catalysts for ZIS under visible light irradiation (c) cycling runs of photocatalytic hydrogen evolution over 10SNP/ZIS sample. Reaction condition: 50 mg of photocatalyst, 100 mL of aqueous solution containing 10% lactic acid, 300 W Xe lamp.
reason for decreased photovoltage value may come from the intimated connections between SNPs and ZIS, and the SNP nanosheets could act as an electron acceptor, therefore, more photogenerated electrons are transferred to SNPs instead of to the top electrode. Abundant photogenerated electrons are accumulated on the surface of the SNP sample, which is beneficial for photocatalytic reduction [52].
After 5 cycles, the activity of the material remained essentially unchanged, indicating good stability. The crystal structure of the 10SNP/ ZIS composite shows no obvious change after a 20 h test as presented in Fig. S2, which indicates sufficient photostability of the sample for photocatalytic hydrogen evolution. 3.5. Photogenerated carriers transfer behavior
3.6. Function of 2D Ni2P nanosheets To further study the photogenerated charge carrier transfer properties, transient photocurrent responses were measured by several onoff intermittent visible light irradiation cycles, and the results are shown in Fig. 8a. Notably, compared with pristine ZIS, the photocurrent composites have higher values. The 10SNP/ZIS photoelectrode exhibits the highest photocurrent density, which is nearly 4 times that of the pristine ZIS, indicating the efficient separation of photogenerated charge carriers in the composite [46]. In addition, the rapid transient response of current during the switch between light-on and light-off statuses implies the well-formed junction contact between SNPs and ZIS. It is worth mentioning that SNP/ZIS composites not only possess a higher photocurrent density than PNP/ZIS but also display better stability, which is attributed to the excellent interfacial contacts between ZIS and SNPs [47,48]. The metallicity and flaky morphology of SNPs is analogous with graphene, which can act as an electron accepter, accelerating the photogenerated electron transfer from ZIS to the SNP cocatalyst [49]. To further study the effect of SNPs on photoexcited charges, SPV spectra and phase spectrum were conducted to investigate the behavior of photogenerated carrier transfer. Fig. 8b and c shows the surface photovoltage signal and phase spectrum of ZIS, 10PNP/ZIS, and 10SNP/ZIS, respectively. For all of the samples, the phase values are in the second or fourth quadrant in the range of photon-absorption, indicating that the photoinduced electrons mostly transfer to the top electrode [50,51]. Nevertheless, the photovoltage value of 10SNP/ZIS displays the weakest signal compared with the two other samples. Through combination with the SPV phase spectrum, we infer that the
To further investigate the function of flaky SNPs in photocatalytic hydrogen evolution, the electrocatalytic performance of the Ni2P with two different kinds of morphologies was systematically evaluated by the electrochemical method. The linear sweep voltammetry (LSV) was measured with Pt/C (20%) as a reference. As shown in Fig. 9a, the SNP nanosheets show a smaller onset overpotential of approximately 130 mV for HER than that for PNP particles (180 mV). The decrease of the overpotential makes it easier for electrons to transfer to protons. At a current density of 10 mA cm−2(η-10), the potential of the SNP sample is only 259 mV, which is much better than that of the PNP sample (406 mV), indicating a stronger reductive ability [53]. To further analyze the kinetics of charge transfer, the Tafel slope method was used to investigate the transportation of electrons. As shown in Fig. 9b, the resulting Tafel slopes are 64 mV·dec−1 and 92 mV·dec−1 for SNPs and PNPs, simultaneously implying that the SNPs correspond to faster electron transfer and reactionkinetics [54]. Electrochemical impedance spectroscopy (Nyquist plots) indicates the phenomena of charge transfer resistance and hydrogen evolution from the surface. Fig. 9c shows Nyquist plots of the SNPs and PNPs; SNPs display the smaller semicircle, presaging a lower charge transfer resistance and higher rate of hydrogen evolution [55]. The above findings suggest that Ni2P can lower the overpotential of hydrogen evolution and improve the separation efficiency of photoinduced carriers, especially SNPs. Nevertheless, the direction of the photoinduced electrons on photocatalysts is also crucial for the
Fig. 8. (a) Transient photocurrent responses, (b) SPV spectra and (c) SPV phase spectra of ZIS, 10PNP/ZIS and 10SNP/ZIS composites. 21
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Fig. 9. (a) LSV curves (b) Tafel plots and (c) Nyquist plot of electrochemical impedance spectra of the Ni2P nanoparticles and nanosheets.
Fig. 10. (a) The working function of ZIS and SNP (b) Proposed photocatalytic hydrogen evolution reaction mechanism of SNP/ZIS.
visible light irradiation, the photogenerated charges are easy to recombine, and the free electrons are difficult to be used for hydrogen evolution because of the large hydrogen overpotential. Improving separation efficiency of photogenerated charges and lowering surface hydrogen overpotential can reduce the reaction energy barriers for hydrogen evolution and increase photocatalysis activity. After loading SNPs with a lower hydrogen overpotential, the photoexcited electrons tend to transfer to SNPs due to the different Fermi energy levels and the interfacial electric field; thus, more free electrons are allowed to react with protons with a lower energy barrier. As shown in Fig. 11, compared with PNP particles, SNPs possess larger contact areas, shorter distances, smaller impedances and lower Tafel slopes, which lead to the increase of reactive sites, improvement of the separation efficiency and migration rate of the charge carriers, lower energy barriers and faster reaction kinetics, resulting in a significant enhancement in photocatalytic hydrogen evolution.
photocatalytic reduction process. The surface functional test will be a great help for us to better understand the direction of interfacial electric fields and motion directions of the photoinduced electrons on hybrid photocatalysts. Fig. 10a shows the work function(WF) of ZIS and SNPs, which were detected by the Kelvin Probe that offers CPDs between the samples and Au (5.1 eV). The WF of ZIS and SNPs is 4.75 eV and 5.02 eV, respectively (Fig. 10b), calculated by the equation: WF (eV) = WF(Au) + ΔCPD/1000. It has been universally acknowledged that the interfacial electric field is the primary driving force for charge separation. When SNPs were loaded on the surface of ZIS and in contact, the free electrons will transfer between the semiconductors due to the different Fermi energy levels until an equilibrium state is formed. The data for WF measurements also reveal that the interfacial electric field is approximately 0.26 V, oriented from ZIS towards the SNPs [56,57]. In other words, negative charges can accumulate on the SNP side, whereas positive charges accumulate on the ZIS side. In the irradiation of visible light, the electrons of ZIS transfer to the conduction band, and then the photogenerated electrons would prefer transfer to the contact side on the ZIS, which has positive charge attraction, meanwhile the holes transfer to another side and are consumed by the sacrificial agent. Simultaneously, due to higher electronic conductivity, faster electrons transfer kinetics and shorter transport routes of SNP nanosheets, the accumulated negative charges on SNP side would more easily migrate to its surface. Furthermore, due to the lower overpotential of SNP nanosheets, the electrons can participate in the reduction of hydrogen evolution more easily. For these reasons, the photogenerated charges are efficiently separated and the photocatalytic hydrogen evolution is enhanced significantly [58–60]. Based on the above investigations and discussions, the photocatalytic mechanism is proposed and embodied in Figs. 10b and 11. For pristine ZIS, belonging to a typical n-type of semiconductor, the surface is bent upward and responds to the positive photovoltage signal. Under
4. Conclusions In conclusion, a 2D/2D noble-metal-free Ni2P/ZnIn2S4 photocatalyst was successfully constructed, and shows highly enhanced visible photocatalytic activity for hydrogen evolution. The photocatalytic hydrogen evolution rate of the SNP/ZIS composite could achieve 2066 μmol·g−1·h−1 under visible light and its apparent quantum yield reached up to 7.7% at 420 ± 20 nm, which is higher than both PNPs and Pt modified ZnIn2S4. The superior photocatalytic hydrogen production activity of the SNP/ZIS photocatalyst could be attributed to the positive synergies of stable junctions, the larger contact area of 2D/2D modificatory, faster electron transport and shorter transport distance of SNPs, which improved the efficiency of photo induced carrier separation and the transition of interfacial charge transfer. 22
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Fig. 11. The Schematic illustrations of the possible charge separation and transportation over SNP/ZIS and PNP/ZIS.
Acknowledgements
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Fabrication of two-dimensional Ni2P/ZnIn2S4 heterostructures for enhanced photocatalytic hydrogen evolution ⁎
Xu-li Lia, Xiao-jing Wanga,b, , Jia-yu Zhua, Yu-pei Lia, Jun Zhaoa, Fa-tang Lia, a b
T
⁎
College of Science, Hebei University of Science and Technology, Shijiazhuang 050018, China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Lower overpotential of 2D Ni P na• nosheets in photocatalytic H evolu2
2
tion are reported.
2D/2D Ni P/ZnIn S photocatalyst is • fabricated for enhanced HER perfor2
2 4
mance.
quantum efficiency of 7.7% • Apparent at 420 ± 20 nm is achieved. structure exhibits lower surface • 2D energy barrier and shorter carriers distance.
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalytic hydrogen evolution 2D-2D Overpotential Interfacial charge transfer Ni2P ZnIn2S4
Promoting electron-hole separation and migration and lowering the overpotential of hydrogen evolution reactions are two effective solutions for improving photocatalytic hydrogen performance. Suitable co-catalyst and appropriate interfacial contacts can effectively lower overpotential and can also construct an electric field at the interface to increase the separation efficiency of the carriers. In this work, we design and fabricate a 2D-2D type of Ni2P co-catalyst modified with ZnIn2S4 for boosting the performance of photocatalytic hydrogen evolution. As a co-catalyst, the 2D Ni2P nanosheets exhibit a lower overpotential and smaller charge transfer resistance in hydrogen evolution reactions, and is much improved in both respects compared to Ni2P nanoparticles. Based on this, 2D/2D Ni2P/ZnIn2S4 nanohybrids with large contact regions and shorter transmission distances of the charges were fabricated, which effectively improve the separation of photo induced carriers and the interfacial charge transfer. By taking advantage of the above features, the fabricated 2D-2D Ni2P/ZnIn2S4hybrid exhibits a superior hydrogen evolution rate of 2066 μmol·h−1·g−1 under visible light irradiation, and the apparent quantum yield was 7.7% at 420 ± 20 nm. This activity far exceeds performance of the 0D/2D Ni2P/ZnIn2S4 hybrid, and is ascribed to better charge separation and accelerated surface reactions of the Ni2P nanosheets.
1. Introduction The overuse of fossil fuels and related environment issues have stimulated more researchers to exploit renewable and carbon-free energy alternatives. Hydrogen fuel is the cleanest proposed energy source
⁎
and has zero emissions, and photocatalysis technology is considered to be a sustainable and environmentally friendly strategy for solving the increasing environmental and energy crisis. Hitherto, unremitting efforts have been devoted to developing technologies for the photocatalytic evolution of hydrogen using various semiconductors [1,2].
Corresponding authors at: College of Science, Hebei University of Science and Technology, Shijiazhuang 050018, China (X.J. Wang). E-mail addresses: [email protected] (X.-j. Wang), [email protected] (F.-t. Li).
https://doi.org/10.1016/j.cej.2018.07.107 Received 1 May 2018; Received in revised form 21 June 2018; Accepted 14 July 2018 Available online 17 July 2018 1385-8947/ © 2018 Published by Elsevier B.V.
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performance above was attributed to better charge separation and more active sites provided by MoS2 nanosheets. However, little research has focused on the fabrication of two dimensional TMP nanosheet co-catalysts and the substantial effect of those TMP materials with different dimensionality on physicochemical properties during the photocatalytic hydrogen production process. Zhang et al. [38] demonstrated a kind of CoP nanosheets with outstanding electrochemical hydrogen evolution performance: a lower overpotential and smaller Tafel slope which is far better than that of CoP nanoparticles. Smaller Tafel slope of the electrocatalyst often means faster reaction kinetics, which is beneficial to accelerate the surface reaction of hydrogen evolution. Inspired by this report and to fully maximize the merits of 2D TMP materials, we prepare 2D–2D Ni2P/ZnIn2S4 photocatalyst for the first time using a post-synthesis method. This work presents an efficient route for maximizing the function of transition metal phosphides as co-catalysts.
TiO2 is the most widely studied photocatalyst owing to its high photocatalytic activity, stability, nontoxicity and low cost. However, TiO2 has a wide band gap, which affects its ability to harvest of solar energy [3]. Compared with metal oxides, metal sulfides possess relatively lower conduction band (CB) positions and narrower band gaps, which are beneficial for expanding the response to light in the visible light range [4,5]. ZnIn2S4, as one of the important ternary chalcogenide semiconductors (AB2S4), is a good photocatalyst candidate due to its low toxicity, suitable band-gap (∼2.4 eV), and relatively high chemical stability [6–8]. Nonetheless, as with most of the semiconductor-based photocatalysts, bare ZnIn2S4 usually has low activity during the process of photocatalytic hydrogen production because it has a large hydrogen evolution overpotential, and the photocatalytic efficiency is strictly restricted by fast electron-hole recombination [9]. Generally, the photocatalytic hydrogen evolution performance of semiconductor photocatalysts can be improved in several: (i) enhancing the utilization of solar energy, (ii) improving charge separation and migration to the surface, (iii) lowering the surface overpotential of redox reactions and (iv) increasing the number of reactive sites [10]. It is well known that suitable co-catalysts are indispensable in artificial photocatalytic hydrogen evolution systems, which can decrease the overpotential for hydrogen production, improve host reaction active sites, create an electric field at the interface, and thereby increase the separation efficiency of electrons and electron holes. Noble metals have usually been employed as co-catalyst candidates in the majority of previous studies [11]. However, noble metals suffer from high costs and low reserves. In consideration of large-scale applications, it is essential to find low cost and efficient noble metal-free co-catalysts. Up until now, various noble-metal-free co-catalysts have been available, such as nickel-based alloys [12], sulfide-based materials [13,14], carbides [15], nitrides [16] and multicomponent co-catalysts [17,18]. Recently, transition metal phosphides (TMPs), typical representatives of burgeoning non-noble metal co-catalysts (including CoP, Ni2P, and MoP), have been intensively studied for both electrocatalytic [19] and photocatalytic [20–25] hydrogen evolution due to their high stability and low overpotential during hydrogen evolution. Meanwhile, metal phosphides can promote the release of hydrogen from active sites owing to structural arrangements [26]. As a low-cost TMP, it has been reported that Ni2P is destined to act as an excellent hydrogen evolution catalyst because of weak binding of hydrogen on the Ni2P surface [27]. Research has achieved superior photocatalytic hydrogen activity employing Ni2P as co-catalyst for CdS [28,29]. Chen et al. [30] reported on the general applicability of nanocrystalline Ni2P as a noble-metal-free co-catalyst to boost photocatalytic hydrogen generation. Other researches anchored zero-dimensional nanocrystalline Ni2P onto g-C3N4, which exhibited a conspicuous enhancement in hydrogen evolution activity under visible light radiation [31,32]. In our previous work, we also identified that the analogous metallic character of Ni2P nanoparticles can accelerate the transfer and consumption of photo-generated electrons [33]. However, how to maximize the function of suitable transition metal phosphides as co-catalysts still remains a formidable challenge. Two-dimensional (2D) ultrathin materials have been extensively studied as ideal materials because of their large specific surface area, short electron/carrier transfer distance and richness in active sites. For instance, graphene is employed as an effective electrocatalyst [34] or co-catalyst for photocatalysis [35] in hydrogen evolution processes due to its superior electrical transport properties. Xia et al. [36]constructed different carbon materials by modifying ZIS nanosheets to enhance the photoactivity on hydrogen evolution and found that the 2D RGO exhibited unique advantages among the studied carbon materials. Li et al. [37] assembled ZnIn2S4 nanosheets on several layers of MoS2 nanosheets to produce ultra-thin and intimate-contacting 2D hybrid photocatalysts, and Yuan et al. [10] reported a 2D Cu2+-doped ZnIn2S4 nanosheets modified with 2D MoS2 presents highly photocatalytic hydrogen evolution performance. The enhancement of photocatalytic
2. Experimental 2.1. Chemicals All raw materials were used without further purification. Red phosphorus, lactic acid, ammonia, sodium carboxymethyl cellulose (CMC300), thioacetamide, NaH2PO4·2H2O, H2PtCl6·6H2O, Zn (NO3)2·6H2O, NiCl2·6H2O, Ni(AC)2·4H2O and In(NO3)3·4.5H2O were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Nafion (Sigma-Aldrich) was used for electrode preparation. 2.2. Preparation of ZnIn2S4 (ZIS) nanosheets In a typical experiment, 1.439 mmol Zn(NO3)2, 2.439 mmol In (NO3)3·xH2O and 10 mmol thioacetamide were dissolved in 35 mL water with vigorous stirring for 20 min at room temperature. Next, the mixture was transferred into a 50 mL Teflon-lined autoclave and maintained at 160 °C for 12 h. After cooling to room temperature naturally, the precipitate was washed with distilled water and combined with ultrasound irradiation several times and was finally placed in 10 mL ethanol for the subsequent synthesis. 2.3. Preparation of Ni2P nanosheets (SNPs) To prepare Ni(OH)2 nanosheets, 0.5 g Ni(AC)2·4H2O was dissolved in a mixture of 10 mL deionized water and 20 mL 1 g/L CMC300 solution while stirring, followed by the addition of 2 mL of ammonia aqueous solution (25 wt%). After stirring for another 3 h at room temperature, the suspension was transferred into a 50 mL Teflon-lined autoclave and heated at 160 °C for 12 h. The precipitate was collected by centrifugation and sonication, washed with hot deionized water several times, and then dried via vacuum freeze-drying [39]. To prepare SNPs. Ni2P nanosheets were obtained by putting Ni (OH)2 (100 mg) and NaH2PO2·2H2O (700 mg) into a steamer like porcelain boat, which was separated by a ceramic filter. NaH2PO2·2H2O was put on bottom and Ni(OH)2 was placed on the ceramic filter. Subsequently, the samples were heated at 300 °C for 180 min in a fluent Ar2 atmosphere at a warming rate of 5 °C min−1. During the calcining process, the Ni(OH)2 nanosheets captured PH3, which was generated from the thermal decomposition of NaH2PO4. After cooling to room temperature under the Ar2 environment, the products were collected for further measurements. In comparison, Ni2P nanoparticles (PNPs) were prepared using a hydrothermal method, which was performed according to preciously described methods [40]. 2.4. Preparation of 2D-2D Ni2P/ZnIn2S4(SNP/ZIS) composites The process for preparing SNP/ZIS composites is illustrated in 16
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Fig. 1. Schematic representation of the synthesis procedures of 2D-2D SNP/ZIS composite photocatalysts.
Na2SO4 solution, and fluorine-doped tin oxide (FTO) combined with sample was used as the working electrode. The working electrode was prepared as follows: 40 mg of catalyst was dispersed in 100 μL ethanol and then mixed with 10 μL 5% Nafion solution by sonicating for 20 min. Finally, 10 μL of the abovementioned suspension was deposited on the FTO and dried at room temperature.
Fig. 1. In detail, a certain amount of SNPs was dispersed into 10 mL of ethanol by sonication for 30 min and then 30 mL turbid liquid containing of 0.2 g ZIS was added while stirring magnetically. After being stirred for 12 h, the suspension was dried at 60 °C. In contrast, 0D/2D Ni2P/ZnIn2S4 (PNP/ZIS) composite was synthesized using the same strategy, except that the Ni2P nanosheets were replaced by Ni2P nanoparticles. The obtained samples were denoted as xSNP/ZIS or xPNP/ ZIS, where x is the wt% of NP in the composite calculated from the initial addition of SNPs or PNPs.
2.7. Photocatalytic hydrogen production activity Photocatalytic hydrogen evolution was detected in a gas-closed circulation system. Photocatalyst powders (50 mg) were dispersed in a 100 mL aqueous solution containing 90 mL of deionized water and 10 mL of lactic acid (LA). A 300 W Xe lampwas used as the visible-light source, with light that was passed through a cut-off filter (λ > 400 nm for visible light illumination). Before irradiation by light, the reactor was degassed with Ar2 for 30 min to remove O2 from the system. 1 mL of gas was intermittently sampled and analyzed by a gas chromatography (GC7900, Ar2 carrier, thermal conductive detector). The apparent quantum efficiency(QE) of the hydrogen evolution at 420 ± 20 nm was estimated using the following equation.
2.5. Characterization The sample nanostructures were analyzed by X-ray diffraction (XRD) using CuKα radiation on a RigakuD/MAX 2500 X-ray diffractometer. A scanning electron microscope (SEM) (HITACHI S-4800) was used to characterize the morphology of the powder samples. Transmission electron microscopy (TEM), high resolution transmission electron microscopy(HRTEM) and elemental mapping images were acquired on an FEI Tecnai G2 F20 S-TWIN field-emission scanning electron microscope. The X-ray photoelectron spectroscopy was used to analyze surface using a PHI 1600 ESCA XPS system. Ultraviolet–visible diffuse reflectance spectra (UV–Vis DRS) was recorded with a Thermo Fisher Evolution220 transmission spectrophotometer. The separation characteristics of the photogenerated charge carriers were tested by surface photovoltage spectroscopy (SPS), which consists of a source of monochromatic light with a light chopper and a lock-in amplifier. The work function of ZIS and NP was measured using an SKP5050 Scanning Kelvin Probe.
AQE =
2×number of evolved hydrogen molecules × 100% number of incident photons
3. Results and discussion 3.1. Structures and morphologies of as-prepared samples The XRD patterns of pure ZIS, SNP and xSNP/ZIS samples are shown in Fig. 2. All the diffraction peaks of the ZIS can be assigned to the hexagonal ZnIn2S4phase (JCPDS card No. PDF#065-2023), and all of the main diffraction peaks of ZIS are apparent in the SNP/ZIS composites [40]. A typical XRD pattern for SNP is presented, which is indexed to the hexagonal Ni2P phase JCPDS card PDF#03-0953, and exhibits all of the main peaks with 2θ values of 40.8°, 44.6°,47.3°,54.2°,55°,66.3°,72.7°, and 74.9°, which correspond to the (1 1 1), (2 0 1), (2 1 0), (3 0 0), (2 1 1), (3 1 0), (3 1 1) and (2 1 2) planes. A feeble diffraction peak at approximately 40.8° was found in the composites when the SNP loading exceeded 10%. No diffraction peaks corresponding to Ni2P were observed in the XRD patterns at lower loading amounts because of the small amount and relatively low diffraction intensity of Ni2P. The microstructures and morphologies of samples were investigated by SEM and TEM. As illustrated in Fig. 3a and b, bare ZIS (Fig. 3a, d) and SNP (Fig. 3b, e) samples are composed of stacked nanosheets, whose thicknesses are approximately 5–10 nm. The morphology of the
2.6. Electrochemical measurements The polarization curves, Nyquist plots and photocurrent measurements were all recorded by a CHI660E electrochemical analyzer (ChenHua Instrument Co. Ltd.) using a standard three-electrode system, where an Ag/AgCl and a graphite rod electrode were used as the reference and counter electrodes, respectively. The polarization curves were studied in a 0.5 M H2SO4 solution, the samples were loaded at 0.28 mg·cm−2 on a glassy carbon (GC) electrode as the working electrode. The polarization data curves were acquired at a scan rate of 5 mV·s−1. Moreover, in order to achieve a steady-state condition, we collected the final data after cycle voltammetry (CV) tests at scan rate 0.03 V·s−1 for more than 20 cycles. The electrochemical impedance spectroscopy (EIS) was tested by the abovementioned working electrodes and measured over a frequency range of 0.1–105 Hz, using a 0.5 M H2SO4 solution as the electrolyte. Transient photocurrent measurements were conducted in a 0.5 M 17
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10SNP/ZIS sample is shown in Fig. 3c and f, and other SEM images of the SNP/ZIS samples are displayed in Fig. S1. It is evident that uniform nanosheets were obtained in all SNP/ZIS samples and seem to be smaller than bare ZIS and SNP. Smaller nanosheets can lead to more exposed surfaces and active edges, which may contribute to the enhancement in HER performance [34]. HRTEM images of 10SNP/SZIS samples are depicted in Fig. 3g. Obviously, the lattice fringes correspond to the interplanar distance of 0.22 nm and 0.32 nm, respectively, which can be attributed to the (1 1 1) plane of the Ni2P phase and the (1 0 1) plane of the ZnIn2S4 phase. The intimate contact could contribute to the efficient transfer of the charges between the components and therefore improve the photocatalytic activity. To further testify the distribution of the two substances, a TEM image and the corresponding energy-dispersive X-ray (EDX) spectrum mapping is shown in Fig. 3h. It is obvious that there are uniform distributions of Zn, In, S, Ni and P elements in the sample of 10SNP/ZIS, indicating that the SNP/ZIS hybrid structure was successfully constructed. The BET surface areas of the prepared samples and the effect of SNPs were investigated using nitrogen adsorption-desorption isotherms. As shown in Fig. 4, all of the SNPs, pure ZIS and 10SNP/ZIS
Fig. 2. XRD patterns of ZIS, SNP and SNP/ZIS composites.
Fig. 3. SEM images of (a) ZIS (b) SNP and (c) 10 SNP/ZIS; TEM images of (d) ZIS (e) SNP and (f) 10SNP/ZIS; (g) HRTEM:10SNP/ZIS (h) STEM and EDX elemental mapping images of the 10SNP/ZIS sample. 18
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Fig. 4. Nitrogen adsorption-desorption isotherms of ZIS, SNP and10SNP/ZIS.
samples. XPS survey spectra of ZIS, SNPs and 10SNP/ZIS composites are shown in Fig. 6a, confirming that the composite is mainly composed of Zn, In, S, Ni and P. Fig. 6b–d display Zn 2p, In 3d and S 2p XPS spectra, respectively. The two peaks located at 1022.4 eV and 1045.4 eV can be attributed to the Zn 2p3/2 and Zn 2p1/2, respectively [44]. The binding energies of In 3d3/2 and In 3d5/2 are located at 452.9 eV and 445.5 eV, respectively, which are indexed to the In–S band [45]. The S 2p spectrum (Fig. 6d) shows two peaks at 161.7 eV and 163.0 eV. It is notable that compared with pure ZIS, the coupled 10SNP/ZIS composite possesses slightly higher binding energy for all elements, indicating strong chemical bonding between SNPs and ZIS. Fig. 6e and f show the Ni 2p and P 2p XPS spectra, respectively. The peaks at 853.4 and 129.4 eV correspond to the elements Ni and P with oxidation states of nearly zero, which are assigned to combined Ni2P. Meanwhile, the peaks at 856.4 and 134.4 eV are ascribed to Ni2+ and nickel phosphate species from surface oxidation. Combining with reported results, the Ni–P bond is of covalent nature with charge transfer characteristics, which is beneficial for the migration of fast electrons, additionally, facilitating the electron transfer of the ZIS sample and achieving the enhancement of photocatalytic hydrogen evolution [28]. Compared with SNPs, the Ni2p and P2p spectra appear very weak due to the low concentration of SNPs in the composites. Furthermore, by means of fitting analysis, it was found that a slight shift of Ni2p to a lower binding energy and P2p to a higher binding energy in the 10SNP/ZIS composite implies a possible chemical bonding actions between SNPs and ZIS rather than a simple physical mixture [44].
composite samples exhibit a type-Ⅳisotherm, and the isotherms of ZIS and 10SNP/ZIS present an evident H2 type-shape in the hysteresis loop data, indicating the presence of an ink bottle-type mesoporous nature. The BET surface area of the samples is listed in Table S1. Interestingly, a larger surface area is observed for the SNP/ZIS samples. As mentioned in Section 2.2, the assembly process for SNP with ZIS happened before the ZIS drying process, therefore, by evaluating the SEM images, we infer that SNP could inhibit the agglomeration of ZIS nanosheets during the drying process. As is common knowledge, a large surface area contributes more adsorption/reaction sites during the photocatalytic reaction process, which is beneficial for the improvement of photocatalytic activity [41]. 3.2. Optical absorptive performance Fig. 5a shows the UV–vis absorption spectra of pure ZIS, SNPs and different ratios of SNP/ZIS samples. It is apparently that the SNP spectrum shows a high and wide adsorption coefficient throughout the tested range of the ultraviolet-visible spectrum, and ZIS shows an absorption edge at approximately 514 nm, corresponding to a band gap of 2.41 eV, which is estimated by the Kubelka-Munkfunction and shown in Fig. 5b [42]. Adding a different amount of Ni2P would increase the absorption between 460 ∼ 800 nm. The valence band of ZIS is measured by XPS valence spectra and shown in Fig. 5c. It can be observed that the valence band edge of ZIS is located at approximately 1.42 eV, which is consistent with previous studies [43]. Combined with the band-gap energy, the conduction band edge of ZIS is estimated to be approximately −0.99 eV.
3.4. Photocatalytic properties The photocatalytic hydrogen production activity of the as-prepared samples were evaluated in a lactic acid solution (10%) containing 50 mg of photocatalysts. Fig. 7a shows the hydrogen evolution activity of SNP/ZIS photocatalysts with different loading amounts of SNPs in
3.3. Interaction between ZIS and SNP To further study the interaction of ZIS and SNPs, XPS measurements were employed to elucidate the chemical compositions of as-prepared
Fig. 5. (a) UV–Vis DRS of ZIS, SNP and SNP/ZIS composites, (b) the plots of (ahν)2 versus hν of ZIS, (c) XPS valence band spectra of ZIS. 19
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Fig. 6. XPS spectra of (a) survey (b) Zn 2p (c) In 3d (d) S 2p of (e) Ni 2p (f) P 2p ZIS, SNP or SNP/ZIS composites.
1%Pt/ZIS(AQY = 3.9%). However, if we continue to increase SNP loading, the hydrogen rate starts to decrease, which is caused by excess SNPs on the surface of ZIS blocking the absorption light by ZIS. It is worth mentioning that the initial photocatalytic hydrogen rate of SNP/ ZIS is relatively low, probably owing to the existence of phosphate species bonded to the photocatalyst [29]. The comparison of photocatalytic hydrogen evolution activity for some co-catalyst modified ZIS photocatalysts are listed in Table S2. It can be seen that the SNP/ZIS photocatalyst presented here exhibits relatively high photocatalytic hydrogen evolution activity. A long-time photocatalytic hydrogen test was conducted to test the stability of 10SNP/ZIS and is shown in Fig. 7c.
the visible light range. Pure ZIS shows relatively low photocatalytic hydrogen production activity because of the rapid recombination of photogenerated electron-hole pairs. No substantial hydrogen production is observed in the control experiment with SNPs, which reveals that NP is not an active photocatalyst for hydrogen evolution. Nevertheless, with SNP loading, the SNP/ZIS hybrid photocatalysts exhibit a significantly enhanced hydrogen evolution rate. In contrast, a 0D/2D Ni2P/ZnIn2S4 (PNP/ZIS) composite was synthesized by using the same strategy. As shown in Fig. 7b, the highest hydrogen evolution reached 2066 μmol·g−1·h−1 with 10SNP/ZIS, and the AQY was 7.7% at 420 ± 20 nm, which is higher than both 10PNP/ZIS(AQY = 4.6%) and 20
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Fig. 7. Photocatalytic activity for hydrogen production of (a) ZIS、SNP and SNP/ZIS composites (b) different co-catalysts for ZIS under visible light irradiation (c) cycling runs of photocatalytic hydrogen evolution over 10SNP/ZIS sample. Reaction condition: 50 mg of photocatalyst, 100 mL of aqueous solution containing 10% lactic acid, 300 W Xe lamp.
reason for decreased photovoltage value may come from the intimated connections between SNPs and ZIS, and the SNP nanosheets could act as an electron acceptor, therefore, more photogenerated electrons are transferred to SNPs instead of to the top electrode. Abundant photogenerated electrons are accumulated on the surface of the SNP sample, which is beneficial for photocatalytic reduction [52].
After 5 cycles, the activity of the material remained essentially unchanged, indicating good stability. The crystal structure of the 10SNP/ ZIS composite shows no obvious change after a 20 h test as presented in Fig. S2, which indicates sufficient photostability of the sample for photocatalytic hydrogen evolution. 3.5. Photogenerated carriers transfer behavior
3.6. Function of 2D Ni2P nanosheets To further study the photogenerated charge carrier transfer properties, transient photocurrent responses were measured by several onoff intermittent visible light irradiation cycles, and the results are shown in Fig. 8a. Notably, compared with pristine ZIS, the photocurrent composites have higher values. The 10SNP/ZIS photoelectrode exhibits the highest photocurrent density, which is nearly 4 times that of the pristine ZIS, indicating the efficient separation of photogenerated charge carriers in the composite [46]. In addition, the rapid transient response of current during the switch between light-on and light-off statuses implies the well-formed junction contact between SNPs and ZIS. It is worth mentioning that SNP/ZIS composites not only possess a higher photocurrent density than PNP/ZIS but also display better stability, which is attributed to the excellent interfacial contacts between ZIS and SNPs [47,48]. The metallicity and flaky morphology of SNPs is analogous with graphene, which can act as an electron accepter, accelerating the photogenerated electron transfer from ZIS to the SNP cocatalyst [49]. To further study the effect of SNPs on photoexcited charges, SPV spectra and phase spectrum were conducted to investigate the behavior of photogenerated carrier transfer. Fig. 8b and c shows the surface photovoltage signal and phase spectrum of ZIS, 10PNP/ZIS, and 10SNP/ZIS, respectively. For all of the samples, the phase values are in the second or fourth quadrant in the range of photon-absorption, indicating that the photoinduced electrons mostly transfer to the top electrode [50,51]. Nevertheless, the photovoltage value of 10SNP/ZIS displays the weakest signal compared with the two other samples. Through combination with the SPV phase spectrum, we infer that the
To further investigate the function of flaky SNPs in photocatalytic hydrogen evolution, the electrocatalytic performance of the Ni2P with two different kinds of morphologies was systematically evaluated by the electrochemical method. The linear sweep voltammetry (LSV) was measured with Pt/C (20%) as a reference. As shown in Fig. 9a, the SNP nanosheets show a smaller onset overpotential of approximately 130 mV for HER than that for PNP particles (180 mV). The decrease of the overpotential makes it easier for electrons to transfer to protons. At a current density of 10 mA cm−2(η-10), the potential of the SNP sample is only 259 mV, which is much better than that of the PNP sample (406 mV), indicating a stronger reductive ability [53]. To further analyze the kinetics of charge transfer, the Tafel slope method was used to investigate the transportation of electrons. As shown in Fig. 9b, the resulting Tafel slopes are 64 mV·dec−1 and 92 mV·dec−1 for SNPs and PNPs, simultaneously implying that the SNPs correspond to faster electron transfer and reactionkinetics [54]. Electrochemical impedance spectroscopy (Nyquist plots) indicates the phenomena of charge transfer resistance and hydrogen evolution from the surface. Fig. 9c shows Nyquist plots of the SNPs and PNPs; SNPs display the smaller semicircle, presaging a lower charge transfer resistance and higher rate of hydrogen evolution [55]. The above findings suggest that Ni2P can lower the overpotential of hydrogen evolution and improve the separation efficiency of photoinduced carriers, especially SNPs. Nevertheless, the direction of the photoinduced electrons on photocatalysts is also crucial for the
Fig. 8. (a) Transient photocurrent responses, (b) SPV spectra and (c) SPV phase spectra of ZIS, 10PNP/ZIS and 10SNP/ZIS composites. 21
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Fig. 9. (a) LSV curves (b) Tafel plots and (c) Nyquist plot of electrochemical impedance spectra of the Ni2P nanoparticles and nanosheets.
Fig. 10. (a) The working function of ZIS and SNP (b) Proposed photocatalytic hydrogen evolution reaction mechanism of SNP/ZIS.
visible light irradiation, the photogenerated charges are easy to recombine, and the free electrons are difficult to be used for hydrogen evolution because of the large hydrogen overpotential. Improving separation efficiency of photogenerated charges and lowering surface hydrogen overpotential can reduce the reaction energy barriers for hydrogen evolution and increase photocatalysis activity. After loading SNPs with a lower hydrogen overpotential, the photoexcited electrons tend to transfer to SNPs due to the different Fermi energy levels and the interfacial electric field; thus, more free electrons are allowed to react with protons with a lower energy barrier. As shown in Fig. 11, compared with PNP particles, SNPs possess larger contact areas, shorter distances, smaller impedances and lower Tafel slopes, which lead to the increase of reactive sites, improvement of the separation efficiency and migration rate of the charge carriers, lower energy barriers and faster reaction kinetics, resulting in a significant enhancement in photocatalytic hydrogen evolution.
photocatalytic reduction process. The surface functional test will be a great help for us to better understand the direction of interfacial electric fields and motion directions of the photoinduced electrons on hybrid photocatalysts. Fig. 10a shows the work function(WF) of ZIS and SNPs, which were detected by the Kelvin Probe that offers CPDs between the samples and Au (5.1 eV). The WF of ZIS and SNPs is 4.75 eV and 5.02 eV, respectively (Fig. 10b), calculated by the equation: WF (eV) = WF(Au) + ΔCPD/1000. It has been universally acknowledged that the interfacial electric field is the primary driving force for charge separation. When SNPs were loaded on the surface of ZIS and in contact, the free electrons will transfer between the semiconductors due to the different Fermi energy levels until an equilibrium state is formed. The data for WF measurements also reveal that the interfacial electric field is approximately 0.26 V, oriented from ZIS towards the SNPs [56,57]. In other words, negative charges can accumulate on the SNP side, whereas positive charges accumulate on the ZIS side. In the irradiation of visible light, the electrons of ZIS transfer to the conduction band, and then the photogenerated electrons would prefer transfer to the contact side on the ZIS, which has positive charge attraction, meanwhile the holes transfer to another side and are consumed by the sacrificial agent. Simultaneously, due to higher electronic conductivity, faster electrons transfer kinetics and shorter transport routes of SNP nanosheets, the accumulated negative charges on SNP side would more easily migrate to its surface. Furthermore, due to the lower overpotential of SNP nanosheets, the electrons can participate in the reduction of hydrogen evolution more easily. For these reasons, the photogenerated charges are efficiently separated and the photocatalytic hydrogen evolution is enhanced significantly [58–60]. Based on the above investigations and discussions, the photocatalytic mechanism is proposed and embodied in Figs. 10b and 11. For pristine ZIS, belonging to a typical n-type of semiconductor, the surface is bent upward and responds to the positive photovoltage signal. Under
4. Conclusions In conclusion, a 2D/2D noble-metal-free Ni2P/ZnIn2S4 photocatalyst was successfully constructed, and shows highly enhanced visible photocatalytic activity for hydrogen evolution. The photocatalytic hydrogen evolution rate of the SNP/ZIS composite could achieve 2066 μmol·g−1·h−1 under visible light and its apparent quantum yield reached up to 7.7% at 420 ± 20 nm, which is higher than both PNPs and Pt modified ZnIn2S4. The superior photocatalytic hydrogen production activity of the SNP/ZIS photocatalyst could be attributed to the positive synergies of stable junctions, the larger contact area of 2D/2D modificatory, faster electron transport and shorter transport distance of SNPs, which improved the efficiency of photo induced carrier separation and the transition of interfacial charge transfer. 22
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Fig. 11. The Schematic illustrations of the possible charge separation and transportation over SNP/ZIS and PNP/ZIS.
Acknowledgements
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