Developing efficient non-precious metal semiconductor photocatalysts is highly desirable for photocatalytically splitting water. In this work, the composite of the nanocrystal twinned Zn0.5Cd0.5S (ZCS) solid solution decorated with highly dispersed Ni2P nanoparticles was successfully formed by in situ growth method, and it exhibited remarkable photocatalytic hydrogen production activity of visible light. A high rate of hydrogen production of 30473 µmol h−1 g−1 was achieved, and the apparent quantum yield (AQY) was as high as 83.5% at 420 nm. Moreover, the sample could maintain outstanding photocatalytic hydrogenation activity after 4-cycle continuous catalytic process. The unique nano-twinned structure of ZCS and synergistic effects between the Ni2P and the twinned ZCS are responsible for the dramatically improved catalytic activities of photocatalysts composite.
Ni2P is highly dispersible on the surface of ZCS with distinctive double lattice structure, and it exhibits remarkable visible-light photocatalytic hydrogen production activity
Hydrogen energy has been widely recognized as an effective substitute to fossil fuels for its cleanliness and high-energy density.1 Photocatalytic hydrogen evolution, which uses solar power to produce hydrogen efficiently, is a promising hydrogen production strategy.2 Since the discovery of photocatalytic splitting water for hydrogen production with TiO2, the development of new semiconductor catalysts presented an accelerating trend.3 As is well known, photocatalytic activity is determined by the efficiency of transport and separation of electrons and holes generated by light, which greatly depends on the band energy levels and the microcrystal structure of the catalysts.4 Therefore, it is highly desirable to refine the microstructure and reduce the band gap energy of semiconductor photocatalysts, which is advantageous for not only the separation of photoelectrons and holes, but also for the transport of holes.5
In the past decades, various semiconductors were discovered, such as oxides, metal sulfides and oxynitrides,6,7,8,–9 which have been identified as valid catalysts for photocatalytic hydrogen production.10 Among them, cadmium sulfide (CdS) has gained extensive attention because of its relatively narrow band gap of 2.42 eV, which is more easily responsive for visible-light and more negative than the redox potential of H+/H2.11 However, the practical application of CdS is limited due to its low apparent quantum yield resulting from rapid electron recombination and severe photo-corrosion. To solve these problems, the construction of ternary metal sulfides provides a feasible strategy.4,12,13 Ternary ZnxCd1−xS is one of the widely used photocatalysts, because the band gap of ZnxCd1−xS can be controlled and its conduction band can be shifted to a more negative position to improve the photocatalytic H2 production activity. Even so, the single nanocrystal structure of ZnxCd1−xS does not have a high enough efficiency for space charge separation, which greatly shortens the life of photo-generated carriers. Recently, some works reveal that the ZnxCd1−xS with nano-twinned structure can improve the charge transport properties (due to its highly ordered structures) and offer effective spatial isolation of photo-generated electrons/holes to prevent their recombination (due to the possibility of forming electrostatic field).5 This twinning structure is formed by alternating arrangements of zinc blende/wurtzite (ZB/WZ) and arranging the twinning superlattice periodically. The homojunctions between two different phases within the same nanocrystals can improve the separation efficiency of electron-hole pairs and reduce their recombination.14,15 Supporting suitable cocatalysts onto semiconductors has been proved an effective method for further improving photocatalytic activity.16 The cocatalyst can trap photogenerated charge, restrain the rapid recombination of electron/hole pairs and decrease the activation energy or overpotential of H2 decomposition. At present, most reported cocatalysts are noble metals (such as Au and Pt),17,18 however, the high price of precious metals makes their applications in hydrogen production impractical. Consequently, it is very crucial to develop the non-precious metal catalysts with cost-effective.19 With the efforts of many groups, various promising cocatalysts including transition metal sulfides (Cu2S, Cu1.94S, NiS, Mo2S and CoS),20,21,22,23,–24 hydroxides Ni(OH)2, Co(OH)2)25,26 and transition metal phosphides (Cu3P, Co2P, CoP and Ni2P)27,28,29,–30 have been developed. Among them, Ni2P has particular optoelectronic properties and has been reported as a highly efficient and earth-abundant cocatalyst,31 which significantly improves the activity of photocatalytic hydrogen production. Taking into account the characteristics of ZnxCd1−xS and Ni2P, it will be significant to couple N2P with twinned ZnxCd1−xS, which can fully exploit the advantages of Ni2P and ZnxCd1−xS. We expected that excellent photocatalytic performance can be achieved.
Herein, we develop a facile approach to grow Ni2P on pre-synthesized twinned Zn0.5Cd0.5S (ZCS) nanocrystals coupled with hydrothermal synthesis. To our knowledge, there is seldom research on photocatalytic performance of combining Ni2P with twinned ZCS.4 The as-prepared photocatalyst (Ni2P-ZCS) exhibited excellent visible-light photocatalytic activity for hydrogen evolution with a high hydrogen production rate of 30473 µmol h−1 g−1. The apparent quantum yield (AQY) could reach as high as 83.5% under visible light (λ ≥ 420 nm), and the activity retention rate could maintain 90.5% of primitive H2-evolution rate after 4-cycle continuous catalytic process for 16 h. Twinned ZCS offers high electron-hole pairs separation rate to a large extent, and Ni2P not only restrains the rapid recombination of electron-hole pairs but also decreases the activation energy. Meanwhile, the synergistic effect between Ni2P and ZCS can dramatically elevate the photocatalytic performance as well. The results reveal great potential of Ni2P-ZCS as high-performance photocatalytic materials. Besides, the simple and scalable method employed in this work may be feasible for preparing other transition metal oxides (hydroxides)-ZCS-based composites with excellent photocatalytic performance, extending their potential applications in many fields.
Synthesis of twinned ZCS
The twinned ZCS was formed by a hydrothermal precipitation method (Figure 1).5 A certain amount of cadmium acetate (0.01 mmol) and zinc acetate (0.01 mmol) were added to the deionized water (40 mL), and then 10 mL of sodium hydroxide was added after mixing well. Next, 0.25 mmol of thioacetamide were added into suspension by stirring for 30 min and then transferred into a 100 mL Teflon-lined stainless-steel autoclave and maintained at 180 °C for 24 h. Twinned ZCS was obtained by rinsing the above solid with anhydrous ethanol and deionized water for three times, followed by vacuum drying at 60 °C for 12 h.
Synthesis of Ni2P-ZCS
Ni2P-ZCS composites were prepared by an in situ growth method (Figure 1).32 0.4 g of the as-prepared twinned ZCS was dissolved in ethylenediamine (60 mL), and then after stirring uniformly, a certain amount of Ni(NO3)2·6H2O and excess red phosphorus nanoparticles (molar ratio of Ni to P is 1:5) were added to ensure completely phosphating. After continuous socinate for half an hour, the mixed homogeneous solution was transferred to 100 mL Teflon-lined autoclave and maintained at 160 °C for 24 h. Subsequently, the product was collected and washed with deionized water and anhydrous ethanol for three times. Finally, the product was dried in a vacuum oven at 80 °C for 5 h to obtain Ni2P-ZCS. For comparison, pure Ni2P was also synthesized using the same method without adding of ZCS.
X-ray diffraction (XRD) patterns of the prepared photocatalysts were confirmed by an X-ray diffractometer using a Cu K irradiation source (=1.54056 Å), and all the samples were scanned between 10° and 90°. Scanning electron microscopy (SEM) were obtained by Hitachi S2400 to observe the morphologies of the samples. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) were acquired on a FEI Talos f200s to characterize the microstructures of the samples. X-ray photoelectron spectroscopy (XPS) measurement was enforced on a Thermo Scientific ESCALAB 250 instrument with Al Ka source to analyze the element composition. UV-visible (UV-vis) absorption spectra were acquired from a HITACHI U4100 spectrophotometer. Photoluminescence (PL) spectra were obtained using a Hitachi F-700 fluorescence spectrophotometer. Electrochemical impedance spectroscopy (EIS) and photocurrent response were measured by Chenhua CHI660E. Nitrogen adsorption-desorption isotherms were tested on the Quantachrone Autosorb iQ-MP-C.
Photocatalytic hydrogen evolution experiments were performed in a 100 mL vacuum reactor (Perfect light Labsolar-III AG photocatalytic online analysis system), which was connected to a cryostat to maintain the reaction temperature at 9 °C during the characterization. The photocatalyst powder (10 mg) was uniformly dispersed in a 100 mL aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3, and the system was vacuumed for 0.5 h to remove air. The reaction vessel was then illuminated with a 300 W xenon lamp (PLS-SXE 300C) with a cut-off filter (420 nm). Hydrogen production was tested every 0.5 h for 3 h using a gas chromatograph (FULI GC 9790) equipped with a thermal conductivity detector (TCD), and high purity Argon as the carrier gas. In the cycling test, 0.5 h dark conditions and gas draws were taken between every cycle and each cycle last for 4 h.
AQY refers to the utilization rate of photoquantum in photochemical reaction, which was gauged using visible light (420 nm) and calculated according to the following formula:33
In this formula, I = 15 A is the light intensity; t = 10800 s is the irradiation time; A is the irradiation area (about 38.5 cm2); λ is the incident wavelength of the light source (nm); h = 6.626 × 10−34 Js is the Planck constant; c is the speed of light (3.0 × 108 m s−1). All the source conditions of the above data are consistent with the hydrogen production performance test conditions.
The saturated calomel electrode (SCE) was used as the reference electrode, the platinum plate was used as the counter electrode, and the photocurrent was measured by an electrochemical analyzer (CHI660E) under a standard three-electrode structure. The working electrode was produced by three drops (30 μL per drop) of the photocatalyst suspension in 5% nafion solution/ethanol (20 mg mL−1) on the surface of fluorine-doped tin oxide (FTO) glass, and then dried at room temperature. The photocurrent density at a bias voltage of 0.1 V was measured under irradiation with a 300 W Xe-lamp (optical switching period of 50 s) equipped with a 420 nm cut-off filter. The supporting electrolyte is 0.5 M of Na2SO4 solution. EIS were recorded in potentiation mode with a 10 mV sinusoidal wave with frequencies ranging from 100 kHz to 0.05 Hz.
Results and Discussion
Characterization of samples
The crystal structure of the as-prepared samples were determined by XRD analysis (Figure 2). Compared to the standard diffraction patterns of cubic phase ZnS (JCPDS card no. 05-0566) and hexagonal phase CdS (JCPDS card no. 41-1049), it can be seen that the diffraction pattern of Zn0.5Cd0.5S exhibits multiphase characteristics.4 After a certain amount of Cd were added into the ZnS crystal (for the Zn0.5Cd0.5S sample), the diffraction peaks of Zn0.5Cd0.5S showed an evident shift to the lower angle.14 This phenomenon is attributed to the larger radius of Cd2+ than that of the Zn2+ ion. These results indicated the formation of the twinned Zn0.5Cd0.5S solid solution.17 All the peaks of Ni2P-ZCS maintain nearly at the same position, indicating that the crystal structure of Zn0.5Cd0.5S is not affected by the deposition of Ni2P. However, the apparent peaks of ZCS move to a little higher angle, suggesting that ions of Zn2+ doped into CdS lattice. Notably, the characteristic diffraction peak of Ni2P (Figure S1, Supplementary Information) is not observed in Ni2P-ZCS composite, attributing to low content of Ni2P in Ni2P-ZCS composite which exceeds the sensitive of XRD characterization and fine particles of Ni2P. Nevertheless, the presence of Ni2P in Ni2P-ZCS can be easily proved by TEM and XPS techniques, which will be discussed later.
To observe the microstructure of Ni2P-ZCS, the prepared samples were characterized by SEM, TEM and HRTEM. SEM shows that Ni2P-ZCS exists as an irregular granule with a size range of 30–100 nm Figure 3(a), which appears same as ZCS (Figure S2, Supplementary Information). As revealed in Figure 3(b), the twin nanocrystal structure of ZCS is confirmed by the lattice fringes of the zinc blende/wurtzite (ZB/WZ) arranged alternately and periodically arranged in twinning superlattice.17 In addition, the black and white strips on the surface of ZCS can be observed in the TEM images (the inset of Figure 3(b)), also suggesting the existence of twinned structure of ZCS.4 In the TEM image of Ni2P-ZCS composite in (Figure 3(c)), some fine Ni2P particles are found on the twinned ZCS surface with high dispersion compared with pure ZCS (Figure S3, Supplementary Information). In Figure 3(d), the identified lattice fringes of 0.32 nm correspond to the (111) lattice plane of twinned ZCS phase,34 and the interplanar spacing of 0.22 nm is ascribed to Ni2P (111).35 These results indicate that Ni2P and twinned ZCS components form a close contacted interface in the composite samples, which contributes to the efficient transfer of the charges between the components, thereby increasing the photocatalytic activity.16 The corresponding energy-dispersive X-ray (EDX) spectra are shown in Figure 3(e–j), further demonstrating the distribution of the two substances. Obviously, the elements of Zn, Cd, S, Ni and P are coexist and distributed uniformly in the surface of Ni2P-ZCS, further confirming that Ni2P-ZCS composite photocatalysts are successfully prepared. In addition, the EDX results show that the molar ratio of Zn to Cd is approximately 1:1, which is consistent with the theoretical value. However, the loading amount of Ni2P is lower than the theoretical value (4wt%), which is mainly because Ni2+ in the solution is not completely deposited on the surface of ZCS during the hydrothermal process. (Table 1, Table S1 and Figure S4, Supplementary Information).
The XPS was tested to analyze the surface chemistry and bonds of the photocatalysts. As shown in Figure 4(a), the survey spectrum provides the signal peaks of Zn, Cd, Ni, S and P elements, which is accorded with the results of EDX (Figure 3). In the high-resolution XPS spectrum of Zn 2p in Figure 4(b), two peaks at 1021.9 and 1044.9 eV correspond to Zn 2p3/2 and Zn 2p1/2, respectively, which indicates that the valence state of Zn is +2. Two peaks at 404.8 and 411.7 eV in Figure 4(c) are attributed to Cd 2p3/2 and Cd 2p1/2, which confirms the presence of Cd2+. Another two peaks at 161.5 and 162.6 eV in Figure 4(d) can be assigned to S 2p3/2 and S 2p1/2, respectively.36 The Ni 2p spectrum shows six fitted peaks (Figure 4(e)), which are related to the Ni 2p1/2 and Ni 2p3/2 energy levels. The peaks at 853.3 and 871.3 eV are ascribed to Niδ+ (0 < δ < 2) in Ni2P, while two peaks at 856.3 and 876.1 eV result from the surface oxidation, and the other two peaks at 861.7 and 881.2 eV are the shake-up satellite signal related with multi-electron excitation of Ni.15,37,38 For the P 2p spectrum in Figure 4(f), the binding energies at 129.5 and 132.9 eV are attributed to Pδ− of metal phosphides, following two satellite peaks at 130.6 and 133.9 eV.38,39 These results suggest that Ni2P is obtained and grows successfully on the surface of ZCS. Furthermore, it is noteworthy that the Zn 2p, Cd 3d and S 2p peaks of Ni2P-ZCS composite move a little to higher binding energies compared with those of pure ZCS in Figure 4(b–d). These shifts indicate that there exists a strong interaction between ZCS and Ni2P, which can lead to effective migration of photogenerated electrons from ZCS to Ni2P and is expected to have superior photocatalytic performance.35 This result is in good accordance with the above XRD, SEM, TEM, HRTEM and EDX analyses, and further demonstrate the existence of Ni, P, Cd, Zn and S for Ni2P-ZCS. According to the XPS characterization results, we can conclude that Ni2P was successfully deposited on the surface of Zn0.5Cd0.5S.
Photocatalytic H2-evolution activity and stability
The photocatalytic H2 evolution rates of the ZCS with different loading of Ni2P and pure Ni2P were carried out in visible light (λ ≥ 420 nm). As presented in Figure 5, Ni2P is inactive for photocatalytic H2 evolution, and pure twinned ZCS has a low H2 producing rate of 20.3 mmol h−1 g−1. With the increase in the loading of Ni2P, the hydrogen production rate of the product increased gradually. This can be attributed to the improved separation efficiency of photo-generated electrons at the interface between Ni2P and ZCS and higher surface area of Ni2P-ZCS compared to ZCS (11.03 vs 8.32 m2 g−1, Figure S6, Supplementary Information). The hydrogen production rate can reach the maximum value of 30.3 mmol h−1 g−1 when the Ni2P is 4wt%. Moreover, an excess of Ni2P may result in the marked decrease of H2 evolution rate. These results show that Ni2P is an effective cocatalyst for water splitting, and at the same time, excessive Ni2P in the hybrid photocatalyst will shield the absorption of light and reduce the number of surface-active sites.
In addition to hydrogen production rate, the AQY is also a crucial parameter used to evaluate the activity for photocatalytic hydrogen production. The AQY was measured and calculated under the same conditions. The AQY value of 4wt% Ni2P-ZCS can reach the highest value of 83.5%. Higher apparent quantum yield means higher utilization efficiency of photons in the photochemical reaction and higher photocatalytic hydrogen production rate.38 Compared with other ZCS-based composite photocatalysts reported (Table 2), 4wt%Ni2P-ZCS exhibits a relatively high level of H2 evolution rate and AQY, implying its great potential application as photocatalyst. Compared with other similar systems, the excellent hydrogen production performance of Ni2P-ZCS is mainly attributed to the following aspects: (i) ZCS has a distinctive double lattice structure (Figure 3(b)). (ii) Ni2P is highly dispersed on the surface of ZCS with special structure (Figure 3(c)). (iii) ZCS has unique nano-twinned structure and there exists synergistic effects between the Ni2P and the twinned ZCS (iv) The close contact between Ni2P and twinned ZCS interface can accelerate the charge transfer.
The stability of the photocatalysts is an important factor for their practical applications. Consequently, Ni2P-ZCS (4wt%) was used in continuous photocatalytic process to investigate its durability (Figure 6). It is noticed that the hydrogen production rate has a slight decrease after 4-cycle continuous running, and Ni2P-ZCS composite can maintain 90.5% of its original H2-evolution rate. However, the activity of the pure twinned ZCS remains only 60% after four cycles. This result illustrates that 4wt%Ni2P-ZCS composite can maintain a high stability for photocatalytic process.
Optical and electrochemical analysis
In order to explore the optical absorption characteristic of the as-prepared samples, the UV-vis absorption spectra were shown in Figure 7(a). It can be observed that twinned ZCS and Ni2P-ZCS have almost the same steep absorption edge in the wavelength range of 400–490 nm. The absorbance of twinned ZCS with different Ni2P contents at the visible region (λ > 490 nm) are significantly higher than that of pure twinned ZCS, which is attributed to the absorption of Ni2P.35 The band gaps of the prepared samples are further calculated according to the Kubelka-Munk (KM) method through the ultraviolet pattern by the equation:40
Here, α is the absorption coefficient; hν is the photon energy; Eg is the direct band gap, and A is a constant. By calculation, the band gap width of the 4wt%Ni2P-ZCS is 2.45 eV, which is smaller than ZCS (2.53 eV), leading to an enhancement in photoactivity.36 Furthermore, the band gap width decreases with the increasing of Ni2P loading amounts (Figure S5, Supplementary Information). The smaller band gap width means that electrons can be easily excited from valence band (VB) to conduction band (CB), which leads to higher intrinsic carrier concentration and higher conductivity. However, it should be noted that excessive Ni2P would hinder the absorption of twinned ZCS in visible light range, which reduces the photogenic electrons and decreases the photocatalytic activity ultimately.41
Photoluminescence (PL) spectra of pure twinned ZCS and 4wt%Ni2P-ZCS were obtained under an excitation wavelength of 345 nm as shown in Figure 7(b). The pure twinned ZCS has a relatively powerful emission peak at 565 nm, however, the emission intensity of Ni2P-ZCS is significantly reduced.24 The weak emission of Ni2P-ZCS indicates that the Ni2P can act as charge carrier trapping center to improve separation efficiency of electron-hole pairs and decrease their recombination, thus improving the activity and stability of Ni2P-ZCS catalyst.42
The transient photocurrent was used to investigate the behavior of charge separation (Figure 8(a)). Compared with response photocurrent of pure twinned ZCS, the 4wt%Ni2P-ZCS exhibits a more rapid response rate and higher photocurrent. The photocurrent density of the Ni2P-ZCS electrode is about three times of pure twinned ZCS. Moreover, the photocurrent response capability of pure twinned ZCS weakens gradually. The result is in good agreement with the results described above, which further proves that the Ni2P plays an important role in improving the separation efficiency of photoelectrons at the interface of composite materials.43
The charge transfers resistance (RCT) of the photocatalyst was measured using EIS. Under visible-light irradiation, the pure twinned ZCS exhibits a much higher interfacial charge transfer resistance with a larger diameter than that of Ni2P-ZCS composites (Figure 8(b)), indicating that after the introduction of Ni2P, a faster charge transfer occurs on their interfaces. This result is consistent with the photocurrent test.44
The mechanism of photocatalytic H2 evolution
On the basis of the above results, a possible photocatalytic mechanism for photocatalytic hydrogen production using Ni2P-ZCS is proposed as depicted in Figure 9. Under the irradiation of visible light, the twinned ZCS can effectively absorb photons, and the electrons on VB are excited onto CB and the holes stayed on the VB to form photogenerated hole-electron pairs. However, without the cocatalyst, carriers may recombine rapidly, resulting in a lower rate of photocatalytic H2 evolution of twinned ZCS. The valence band edge of ZCS is located at 2.28 eV,4 and the conduction band edge of ZCS is calculated to be −0.25 eV, according to the formula EVB = ECB + Eg. The potential of Ni2P is about −0.17 eV, which locates between the CB of ZCS and the potential of H+ reduced to hydrogen molecule (E 0H+/H2 = 0). With the presence of Ni2P catalyst, Ni2P-ZCS exhibits high electron trapping ability due to its lower Fermi level than the pure ZCS, which makes photogenerated electron energy excite rapidly from the CB of the twinned ZCS to the surface of Ni2P, thus effectively separating photoexcited hole-electron pairs.34,36 Ni2P acts as an electron collector to trap photogenerated electrons and suppress the combination of the photogenerated carriers. The electrons on Ni2P will reduce H+ into H2, and the holes on the VB of ZCS are captured and consumed by the sacrificial agent in the solution. In addition, the lower H2 adsorption energy on the surface of the Ni2P is beneficial to the reduction of H+. Ni2P serves as an active reaction center in the evolution of photocatalytic H2 and can accelerate the reaction rate.36 Besides, the subtle atomic-level intimate contact and strong interaction between ZCS and Ni2P provide a larger dynamic specific surface area and maximize the efficiency of electron-to-electron transmission between the two components, and thus leads to a highly efficient photocatalytic activity. Using Ni2P-ZCS as photocatalyst under visible light (λ ≥ 420 nm), the main reaction of photocatalytic H2-evolution in an aqueous solution containing S2−/SO32− can be expressed by the following eqns. (1)–(6):
To sum up, we successfully synthesized high-efficiency catalysts by in-situ growth of Ni2P on the twinned ZCS nanocrystals. The Ni2P were scattered uniformly on the surface of twinned ZCS nanoparticles. The highest rate hydrogen evolution of 4wt%Ni2P-ZCS can reach as high as 30473 µmol h−1 g−1 and AQY reaches 83.5% at 420 nm. Moreover, the activity retention rate could maintain 90.5% of primitive H2-evolution rate after 4-cycle continuous catalytic process for 16 h. A possible photocatalytic mechanism was proposed to explain the enhanced evolution of H2 in Ni2P modified ZCS. The unique nano-twinned structure of ZCS, the close interaction between the components, and the special properties of Ni2P can effectively isolate photogenerated electrons/holes and improve their transfer, which is instrumental to boost photocatalytic hydrogen production activity. These results indicate that Ni2P is an effectively co-catalyst for twinned ZCS and has a potential application in the process of photocatalytic hydrogen production.
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This work was supported by the National Natural Science Foundation of China (Grants 21706242, 21576247, and U1804140).
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Zhou, X., Yin, L., Dai, K. et al. Preparation of Ni2P on twinned Zn0.5Cd0.5S nanocrystals for high-efficient photocatalytic hydrogen production. J Chem Sci 132, 26 (2020). https://doi.org/10.1007/s12039-019-1727-1
- hydrogen production
- water splitting