Transition metal phosphide of nickel and cobalt-modified Zn_0.5Cd_0.5S for efficient photocatalytic hydrogen evolution with visible light irradiation

Abstract

Transition metal phosphides (TMPs), as an environmentally friendly non-noble metal cocatalyst, play a key role in promoting photocatalytic hydrogen evolution. In this work, three metal phosphating compounds (NiCoP, Ni₂P, and Co₂P) were synthesized by solvothermal method using red phosphorus as a phosphorus source. The three kinds of co-catalysts were individually loaded onto Zn_0.5Cd_0.5S by physical agitation method to form binary nanocomposites (ZnCdS/TMPs). The results showed that the three composite photocatalysts all showed excellent HER performance in the Na₂S/Na₂SO₃ sacrifice solution. Among them, the hydrogen production rate of ZnCdS/NiCoP-5% reached 64 μmol at 5 h, which was significantly higher than that of pure Zn_0.5Cd_0.5S, ZnCdS/Ni₂P-5%, and ZnCdS/Co₂P-5%. According to the results of various characterization techniques, NiCoP nanoparticles provide more active sites for Zn_0.5Cd_0.5S and the composite photocatalytic materials exhibit excellent hydrogen evolution performance. Moreover, the Schottky barrier between Zn_0.5Cd_0.5S and NiCoP nanoparticles effectively enhances the charge separation rate, thus promoting the improvement of photocatalytic hydrogen evolution. Therefore, low-cost transition metal phosphide as a cocatalyst has broad prospects in the field of photocatalysis.

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1. Introduction

As a clean energy source, hydrogen energy has the advantages of being environmentally friendly and having high energy density, which has attracted extensive attention and research of scientists.^1–3 Among them, semiconductor photocatalytic hydrogen production technology is the most commonly used method to solve environmental and energy problems.^4,5 At present, semiconductors that have been discovered include metal sulfides (ZnIn₂S₄^6,7) and metal oxides (CaTiO₃,⁸ ZnO^9,10). Therefore, the exploration of a low cost and environmental protection semiconductor photocatalyst can effectively improve the photocatalytic hydrogen evolution rate.

Metal sulfide semiconductors have attracted much attention due to their suitable band gap and excellent photocatalytic hydrogen evolution performance.^11,12 As a kind of metal sulfide, Zn_0.5Cd_0.5S has outstanding hydrogen production performance.^13,14 Zn_0.5Cd_0.5S easily causes photocorrosion. This is because the valence band of Zn_0.5Cd_0.5S is less electronegative, which leads to the autooxidation of the holes generated by light excitation.^15,16 In addition, in the study of ZnCdS, the problem of a photoexcited electron–hole pair rapid recombination needs to be further improved, so as to effectively improve the charge separation efficiency and hydrogen evolution activity. Therefore, the supported cocatalyst can effectively inhibit the recombination of electron holes in the Zn_0.5Cd_0.5S solid solution, thus promoting the improvement of its hydrogen evolution performance. Therefore, the exploration of a cocatalyst without precious metals has become the main method to solve this problem.¹⁷

However, the most effective cocatalysts currently rely on noble metals, but the high cost of precious metals limits their large-scale application. Therefore, it has been reported that transition metal phosphide is widely used in electrocatalysis due to its excellent electrical conductivity and stability, such as Ni₂P,¹⁸ Co₂P¹⁹ and FeP.²⁰ Compared with transition metal oxides, transition metal phosphide has the characteristics of low cost, easy access and high catalytic performance. According to previous studies, Ni₂P as a co-catalyst of one-dimensional semiconductor nanorods can effectively improve the photocatalytic hydrogen evolution efficiency of composite photocatalysts.²¹ Yu et al. reported the performance comparison of Ni_0.1Co_0.9P with Ni₂P and CoP, and the results showed that Ni_0.1Co_0.9P exhibited the most significant catalytic performance in neutral water.²² In addition, MoP was used as a co-catalyst to support the light-absorbent CdS, and an efficient system of photocatalytic hydrogen evolution without noble metals was constructed, which was similar to the rate of hydrogen evolution when Pt was used as a co-catalyst.²³

In this work, a series of ZnCdS/TMPs composite photocatalysts were successfully prepared by solvothermal method and physical agitation method. The effects of Ni₂P, Co₂P and NiCoP as co-catalysts on the hydrogen evolution performance were studied. Under simulated solar irradiation, the ZCS/NCP-5% composite photocatalyst with Na₂S/Na₂SO₃ as a sacrificial agent had the highest hydrogen production rate, and its 5 h hydrogen production activity reached 64 μmol, which was significantly higher than that of pure Zn_0.5Cd_0.5S, ZCS/NP-5% and ZCS/CP-5%. A series of characterization studies showed that TMPs nanoparticles, as a cocatalyst, not only effectively improved the active hydrogen evolution site of Zn_0.5Cd_0.5S, but also improved the charge separation efficiency of Zn_0.5Cd_0.5S, thus effectively enhancing the hydrogen evolution performance of ZnCdS/TMPs composite photocatalyst.

2. Experimental section

2.1 Materials

The chemicals used in this experiment are nickel chloride hydrate (NiCl₂·6H₂O), sodium dodecyl benzene sulfonate (SDBS), cobaltous nitrate hexahydrate (Co(NO₃)₂·6H₂O), red phosphorus, hexadecyl trimethyl ammonium bromide (CTAB), thiourea (CH₄N₂S), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), sodium sulfide nonahydrate (Na₂S·9H₂O), cadmium nitrate tetrahydrate (Cd(NO₃)₂·4H₂O), and sodium sulfite (Na₂SO₃).

2.2 Sample preparation

2.2.1 The preparation of Zn_0.5Cd_0.5S. Zn_0.5Cd_0.5S was prepared by hydrothermal method. 0.5950 g Zn(NO₃)₂·6H₂O, 0.6169 g Cd(NO₃)₂·4H₂O and 0.3806 g thiourea were dissolved in 60 mL distilled water. After stirring for 0.5 h, the mixed solution was placed in a 100 mL Teflon lined reactor at 180 °C for 15 h. The solution was cooled and filtered to obtain Zn_0.5Cd_0.5S.

2.2.2 Preparation of TMPs (NiCoP, Ni₂P and Co₂P) nanoparticles. 1.5 g red phosphorus was added to a 100 mL Teflon lined reactor. A volume of 60 mL deionized water was added and the mixture was stirred for 30 min. Then, the reactor was placed in an oven at 200 °C for 12 h. The solution was cooled and filtered to obtain pure RP.

1.1188 g NiCl₂·6H₂O, 0.4365 g Co(NO₃)₂·6H₂O, 0.7743 g pure RP, 0.1 g CTAB and 0.1 g SDBS were dissolved in a 50 mL mixture of deionized water and ethanol (volume ratio is 1 : 1). After stirring for 1 h, the mixed solution was placed in a 100 mL Teflon lined reactor in 200 °C for 24 h. The solution was cooled and filtered to obtain the NiCoP nanoparticles. Ni₂P and Co₂P nanoparticles were synthesized by the same method, except for hydrothermal temperature and time. Ni₂P can be synthesized by reacting for 12 h at 140 °C, while Co₂P can be synthesized by reacting for 10 h at 200 °C.

2.2.3 The preparation of ZCS/TMP-X. 0.1 g of the above prepared Zn_0.5Cd_0.5S was taken and NiCoP nanoparticles with percentages of 1%, 5%, 10%, 15% and 20% were separately added. The mixed solution was ultrasonicated for 5 min and stirred for 3 h. Finally, the composite catalysts ZCS/NCP-X were obtained after washing and filtration with deionized water, and they were named as ZCS/NCP-1%, ZCS/NCP-5%, ZCS/NCP-10%, and ZCS/NCP-15%. Similarly, ZCS/NP-5% and ZCS/CP-5% of Zn_0.5Cd_0.5S with 5% Ni₂P and 5% Co₂P were also prepared.

2.3 Characterization

The crystal diffraction (XRD) data of the photocatalyst were determined by an X-ray diffractometer (Rigaku SmartLab). The morphology, structure and elemental composition of the samples were analyzed by scanning electron microscopy (SEM, Zeiss EVO 18), transmission electron microscopy (TEM, JEM-2100) and energy dispersive X-ray spectroscopy (EDX), respectively. X-Ray photoelectron spectroscopy (XPS) measurements were obtained with a Thermo Escalab 250XI instrument. The UV-Vis spectra (UV-Vis) of the sample was recorded by an UV-Vis (UV2550, Shimadu) apparatus. Nitrogen adsorption–desorption tests were performed on the samples using a Micromeritics ASAP 2020 instrument. Stable state photoluminescence (PL) and time-resolved fluorescence (TRF) of the catalyst were measured using a Fluoromax-4 instrument.

2.4 Photocatalytic H₂ evolution experiments

The experiment of photocatalytic hydrogen production was conducted in a multichannel illuminated photocatalytic reaction system with a 5 W LED light source. A mixture of 10 mg catalyst powder and 30 mL Na₂S·9H₂O and Na₂SO₃ was placed in a quartz glass reactor. After agitation and ultrasound for 10 min, the mixed solution was degassed with nitrogen for 5 min. The amount of H₂ produced was measured with a gas chromatograph (Ruili SP-2100) every hour after irradiation. The gas chromatograph was a 13 X Zeolite packed column, and N₂ was used as the carrier gas. Under the same condition, the cycle stability experiment was also carried out in order to compare the hydrogen production effect between ZnCdS/TMPs and the composite photocatalysis supported by the Pt cocatalyst. H₂PtCl₆ solution and Na₂S/Na₂SO₄ as electron sacrificial agents were used to test the hydrogen production performance of ZnCdS/Pt.

The apparent quantum yield (AQY) of the catalyst was measured under a 300 W xenon lamp. The experiment was conducted in a closed quartz glass reaction flask, in which 0.05 g catalyst was added into a quartz reaction flask containing 100 mL Na₂S/Na₂SO₄ solution for ultrasonic agitation. The reaction flask was degasified with nitrogen for 10 min and the light source was a 300 W xenon lamp, with a fixed wavelength monochromatic light every 1 h for the hydrogen production test. A total wavelength range of 450–550 nm was used for the hydrogen production rate with the AQY calculation formula below:³⁷

2.5 Photoelectrochemical experiments

The electrochemical properties of different catalysts were measured in 0.2 M sodium sulfate electrolyte solution using a three-electrode electrochemical workstation (CHI660E). A saturated silver chloride electrode (Ag/AgCl) and platinum wire were used as the counter electrode and reference electrode, respectively. The catalyst powder was dispersed in a mixture of 5% Nafion solution and ethanol, and then the suspension was coated onto ITO glass with an effective area of 1.0 cm² as the working electrode. The photocurrent response curves (I–T) of different catalysts were measured during the switching cycle of a 300 W xenon lamp. In addition, the electrochemical impedance (EIS) and scanning linear voltammetry (LSV) curves of different catalysts were measured.

3. Results and discussion

3.1 Crystal structure analysis (XRD)

In Fig. 1(a), the crystal structures of the NiCoP, Zn_0.5Cd_0.5S and ZCS/NCP-5% photocatalysts were studied by XRD patterns. The diffraction peaks of NiCoP at 40.99°, 44.90°, 47.58° and 54.44° correspond to the (111), (201), (210) and (300) crystal faces of the NiCoP standard card (PDF#71-2336), respectively,²⁴ indicating the successful synthesis of NiCoP. Similarly, in Fig. 1(b), the diffraction peaks of Ni₂P and Co₂P correspond to the standard cards (PDF#74-1385) and (PDF#32-0306), respectively.²⁵ Meanwhile, the Zn_0.5Cd_0.5S in the figure corresponds to the characteristic peak of hexagonal phase CdS. Among them, 24.81°, 26.51°, 28.18°, 43.68° and 51.82° correspond to the crystal faces of (100), (002), (101), (110) and (112) of the CdS standard card (PDF#41-1049), respectively. Since the radius of Cd²⁺ was larger than that of Zn²⁺, the introduction of Zn²⁺ into the lattice of CdS reduces the lattice fringe distance of CdS, resulting in a large angular displacement of the peak position of Zn_0.5Cd_0.5S. These characteristics indicate the formation of the Zn_XCd_1-XS solid solution. The result conforms to Vegard's law.^26,27 Because the content of NiCoP in the ZCS/NCP-5% composite catalyst was very small, the (111) crystal plane belonging to NiCoP shows relatively weak characteristic peaks. In addition, no diffraction peaks with other characteristics were observed in the composite photocatalysts ZCS/NCP-5%, ZCS/NP-5% and ZCS/CP-5%, indicating that no other impurities were generated.

Fig. 1 The XRD patterns of (a) NiCoP, Zn_0.5Cd_0.5S, ZCS/NCP-5% and (b) Ni₂P, Co₂P, ZCS/NP-5%, ZCS/CP-5%.

3.2 SEM and TEM analysis

In Fig. 2, SEM images were used to analyze the microscopic morphology of the sample. As shown in Fig. 2(a), Zn_0.5Cd_0.5S is a leaf-like structure with a length of 4–6 μm.²⁷ It can be observed from Fig. 2(b–d) that the Ni₂P, Co₂P and NiCoP nanoparticles have similar structures with an average diameter of about 0.1–1 μm. In Fig. 2(e), it is obvious that the Ni₂P nanoparticles are distributed on the Zn_0.5Cd_0.5S surface. In particular, it can be found from Fig. 2(f and g) that both Co₂P and NiCoP are distributed on Zn_0.5Cd_0.5S. The microstructure of ZCS/NCP-5 was analyzed by TEM and HRTEM, as shown in Fig. 3. In Fig. 3(a and b), Zn_0.5Cd_0.5S has good interface contact with the NiCoP nanoparticles. In particular, lattice fringes with lattice spacing of 0.34 nm and 0.21 nm correspond to the (002) and (110) crystal faces of Zn_0.5Cd_0.5S, respectively. The lattice fringes with lattice spacing of 0.22 nm belong to the (110) plane of NiCoP. The results are consistent with the XRD diffraction peaks of Zn_0.5Cd_0.5S and NiCoP. According to the EDX diagram in Fig. 3(c), it is proved that Zn, Cd, S, P, Ni and Co elements are present in the composite catalyst. In addition, the distribution of each element is observed from the elemental mapping in Fig. 3(d–j). Due to the small content of NiCoP in the composite catalyst, the strength of elements Ni, Co and P is lower than that of elements Zn, Cd and S.

Fig. 2 SEM image of (a) Zn_0.5Cd_0.5S, (b) Ni₂P, (c) Co₂P, (d) NiCoP, (e) ZCS/NP-5%, (f) ZCS/CP-5%, and (g) ZCS/NCP-5%.

Fig. 3 TEM images of (a) ZCS/NCP-5%, (b) HRTEM image of ZCS/NCP-5%, (c) EDX of ZCS/NCP-5%, (d) HAADF of ZCS/NCP-5%. (e–j) Elemental-mapping of Zn, Cd, S, Ni, Co and P elements.

3.3 XPS analysis

In order to further analyze the elemental composition and valence state of the sample, we carried out XPS tests on the sample. Fig. 4(a) shows the XPS spectrum calibrated by C 1s (282.8 eV). The full spectrum chart shows that the samples are mainly composed of Zn, Cd, S, Ni, P and Co elements. In Fig. 4(b), the spin–orbit splitting peaks of 1044.3 eV and 1021.4 eV corresponded to Zn 2p_1/2 and Zn 2p_3/2, respectively. This effectively indicates the presence of Zn²⁺.^17,28Fig. 4(c) shows the map of Cd 3d, which was similar to that of Zn 2p. There were two strong peaks at 411.2 eV and 403.9 eV, corresponding to Cd 3d_3/2 and Cd 3d_5/2, respectively. This proves the presence of Cd²⁺.²⁹Fig. 4(d) shows the fine spectrum of S 2p for ZCS/NCP-5%. The spin–orbit splitting peaks at 161.4 eV and 160.1 eV correspond to S 2p_1/2 and S 2p_3/2, respectively, proving the existence of S²⁻. The smaller twin peaks of 166.9 eV and 167.8 eV represent S₂²⁻.³⁰ Compared with Zn_0.5Cd_0.5S, the XPS peaks of Zn 2p, Cd 3d and S 2p spectra move towards the direction of lower binding energy. These results indicate that Zn_0.5Cd_0.5S has strong electron interaction with NiCoP nanoparticles, which greatly improves the charge transfer rate and hydrogen evolution performance of the ZCS/NCP-5% composite photocatalyst.^27,34

Fig. 4 (a) XPS survey spectrum of ZCS/NCP-5%; (b–d) XPS spectra of Zn 2p, Cd 3d, and S 2p for different photocatalysts.

In the diagram of Ni in Fig. 5(a), Ni 2P_3/2 has two pairs of spin splitting peaks of 852.5 eV and 854.9 eV. The spin splitting peaks at 879.1 eV and 872.6 eV correspond to Ni 2p_1/2. Among them, the subpeaks at 852.5 eV and 879.1 eV effectively prove the existence of Ni²⁺, while the peaks at 854.9 eV and 872.6 eV effectively prove the existence of Ni³⁺.³¹ In Fig. 5(b), the Co element has an optional split slit similar to that of the Ni element. The spin splitting peaks at 777.9 eV and 780.7 eV correspond to Co 2p_3/2. The spin-splitting peaks at 792.9 eV and 796.6 eV correspond to Co 2p_1/2. Among them, the subpeaks of 777.9 eV and 792.9 eV represent the existence of Co³⁺.³² In Fig. 5(c), the peaks at 128.8 eV, 129.2 eV and 131.8 eV correspond to P 2p_3/2, P 2p_1/2 and P oxide on the sample surface, respectively.³³

Fig. 5 (a–c) XPS spectra of Ni 2p, Co 2p and P 2p for ZCS/NCP-5%.

3.4 BET analysis

As shown in Fig. 6, the N₂ adsorption–desorption isothermal curve of the sample provides a basis for studying the specific surface area of the photocatalyst. In Fig. 6(a–c), all catalysts exhibit typical type IV isotherms. In addition, they all have H3 type hysteresis loops, indicating that Zn_0.5Cd_0.5S, NiCoP and the composite photocatalyst ZCS/NCP-5% are mesoporous structures. As shown in Table 1, the specific surface areas of Zn_0.5Cd_0.5S, NiCoP and the composite photocatalyst ZCS/NCP-5% were 11.902 m² g⁻¹, 41.088 m² g⁻¹ and 16.416 m² g⁻¹, respectively. After loading NiCoP on Zn_0.5Cd_0.5S, the specific surface area of Zn_0.5Cd_0.5S was effectively increased. Similarly, the average aperture of ZCS/NCP-5% was also improved compared to Zn_0.5Cd_0.5S. Therefore, the addition of NiCoP can effectively improve the specific surface area of Zn_0.5Cd_0.5S, thus contributing to the improvement of the hydrogen evolution activity of the ZCS/NCP-5% composite photocatalyst.

Fig. 6 (a–c) N₂ adsorption–desorption isotherms and BJH adsorption mean pore sizes of Zn_0.5Cd_0.5S, NiCoP and ZCS/NCP-5% samples.

Table 1 The S_BET, pore volume and average pore size of different samples

Samples	S BET (m^2 g^−1)	Pore volume^b (cm^3 g^−1)	Average pore size^c (nm)
a BET specific surface area. b BJH adsorption cumulative volume of pores. c BJH adsorption average pore diameter.
Zn0.5Cd0.5S	11.902	0.030	10.020
NiCoP	41.088	0.152	14.786
ZCS/NCP-5%	16.416	0.059	14.315

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3.5 UV-Vis DRS analysis

UV-Vis spectra provide a basis for studying the light absorption properties of different photocatalysts. Generally speaking, the stronger the light absorption intensity of photocatalyst, the more easily electrons are excited by light, which is favorable for the photoexcited carrier transfer and hydrogen evolution of the photocatalyst.³⁴ As shown in Fig. 7(a), NiCoP, Ni₂P and Co₂P show strong light absorption in the whole band because they were black powders. The absorbance edge of pure Zn_0.5Cd_0.5S was about 600 nm. After loading TMPs nanoparticles on the photocatalyst Zn_0.5Cd_0.5S, the absorption of visible light by Zn_0.5Cd_0.5S was significantly improved.^25,35 The band gaps of different samples were obtained by converting the UV-Vis DRS data into Tauc plots using eqn (1).³⁶Fig. 7(b) clearly shows that the band gaps of Zn_0.5Cd_0.5S is 2.21 eV.

ahν = A(hν – E_g)ⁿ (1)

where a is the absorption coefficient; A is a constant; hν is the photon energy; and E_g is the band-gap energy.

Fig. 7 (a) UV-Vis DRS of different catalysts. (b) Tauc plots of different catalysts.

3.6 Photocatalytic hydrogen evolution

The hydrogen evolution performance of different photocatalysts was measured with Na₂S/Na₂SO₃ as a sacrificial agent. As shown in Fig. 8(a), Zn_0.5Cd_0.5S showed weak photocatalytic hydrogen evolution performance, and its hydrogen evolution rate reached 3.54 μmol h⁻¹. This corresponds to its weak light capture ability in the ultraviolet spectrum and low electron–hole pair separation ability in the fluorescence spectrum. The hydrogen evolution activity of the ZCS/NP-5%, ZCS/CP-5% and ZCS/NCP-5% composite photocatalysts was significantly increased by loading Ni₂P, Co₂P and NiCoP nanoparticles onto the Zn_0.5Cd_0.5S surface, respectively. The hydrogen evolution rate in ZCS/NCP-5% was significantly higher than that in ZCS/NP-5% and ZCS/CP-5%, reaching 12.8 μmol h⁻¹. In Fig. 8(b), the hydrogen evolution performance of the composite photocatalyst ZCS/TMPs was further optimized by adjusting the molar ratio of Zn_0.5Cd_0.5S to NiCoP. It is obvious in the figure that when NiCoP load reaches 5%, the hydrogen yield of the ZCS/NCP-5% composite photocatalyst reaches the highest. As the NiCoP loading continued to increase, the H₂ evolution rate of the composite catalyst decreased significantly. This is because the excess NiCoP load can obscure the active site of Zn_0.5Cd_0.5S. It is clear from Fig. 8(c) that the stability of the ZCS/NCP-5% photocatalytic hydrogen evolution performance has not significantly decreased in the five cycle experiments, proving that the composite catalyst has good stability. In particular, Fig. 8(d) shows the XRD patterns of ZCS/NCP-5% before and after hydrogen production, which were compared to further prove that ZCS/NCP-5% has good stability.

Fig. 8 (a and b) H₂ evolution of the different photocatalyst; (c) cycle test of the ZCS/TMPs-5%; (d) XRD patterns of the ZCS/NCP-5% before and after H₂ production.

As shown in Fig. 9, we measured the apparent quantum efficiency (AQY) at 450 nm, 475 nm, 500 nm, 520 nm and 550 nm, among which the AQY at 500 nm was the highest, reaching 1.32%. It can also be seen from the figure that the variation trend of the AQY value is consistent with the light absorption characteristics of the photocatalyst.^37–39

Fig. 9 AQY of ZCS/NCP-5%.

3.7 PL analysis

The charge separation ability of the photocatalyst is an important factor to determine its photocatalytic performance. The PL spectra results show the charge separation ability of the composite photocatalyst. As the sample is irradiated by light of a certain energy, the ground state electrons of the sample will transition from the low energy level to high energy level, resulting in electron hole pairs.⁴⁰ However, the electrons in the excited state will go back to the ground state because of their instability, and that will produce a fluorescence spectrum. Therefore, the lower PL intensity indicates that the excited electrons return to the ground state with fewer electrons, indicating that the charge separation efficiency of the photocatalyst is higher.⁴¹ As shown in Fig. 10(a), the composite photocatalyst ZCS/NP-5%, ZCS/CP-5% and ZCS/NCP-5% generated after loading Ni₂P, Co₂P and NiCoP on Zn_0.5Cd_0.5S were all lower than that of pure Zn_0.5Cd_0.5S. This indicates that the introduction of TMPs nanoparticles in Zn_0.5Cd_0.5S effectively promotes the charge separation. Among them, the peak intensity of ZCS/NCP-5% was the lowest, which further proves that NiCoP can accelerate the separation efficiency of photoexcited carriers of the ZCS/NCP-5% composite photocatalyst, thus promoting the improvement of the hydrogen evolution performance of the composite photocatalyst.

Fig. 10 (a) PL spectra of the Zn_0.5Cd_0.5S, ZCS/NP-5%, ZCS/CP-5% and ZCS/NCP-5%; (b) TRPL spectra of the Zn_0.5Cd_0.5S, ZCS/NP-5%, ZCS/CP-5% and ZCS/NCP-5%.

The lifetime of the photocatalyst was further determined by the time-resolved transient decay curve of the sample. According to the formula, the decay curve of the sample can be well fitted by using the three-exponential equation, as shown in Fig. 10(b):²⁷

τ_i: Decay life; A_i: pre-exponential factor; 〈τ〉: average life.

The decay curve of TRPL shows that the average life of ZCS/NP-5%, ZCS/CP-5% and ZCS/NCP-5% decrease successively, which were 3.10 ns, 3.06 ns and 2.99 ns, respectively. The main reason for the decrease in the mean lifetime of ZCS/NCP-5% was the effective electron transfer between Zn_0.5Cd_0.5S and NiCoP, which promotes the separation of the electron hole pairs.^40,42,43 Therefore, the ZCS/NCP-5% photocarrier separation rate was the fastest, which was conducive to its hydrogen evolution performance.

3.8 Photoelectrochemical test analysis

The charge separation ability of the photocatalyst was further studied by photochemical experiment. As shown in Fig. 11(a), intermittent irradiation was carried out with a 300 W Xenon lamp light source, and the I–T curves of Zn_0.5Cd_0.5S, ZCS/CP-5%, ZCS/NP-5% and ZCS/NCP-5% were measured. All photocatalysts show different photocurrent responses under light, but with the continuous accumulation of holes, the photocurrent response intensity decreases slightly.⁴⁴ In Fig. 11(a), the photocurrent response intensity of the composite photocatalyst ZCSNCP-5% was the highest, indicating that its electron transfer rate was the fastest, which can promote hydrogen evolution performance.

Fig. 11 (a) I–T curves of Zn_0.5Cd_0.5S, ZCS/CP-5%, ZCS/NP-5%, and ZCS/NCP-5%; (b) Corresponding EIS curves, (c) LSV plots of Zn_0.5Cd_0.5S, ZCS/CP-5%, ZCS/NP-5%, and ZCS/NCP-5%.

In Fig. 11(b), the EIS of different photocatalysts reflect the resistance of the photocatalysts during charge transfer. The smaller the arc radius of the photocatalyst, the smaller the charge transfer resistance in the photocatalyst.⁴⁵ The composite photocatalyst supported by 5% NiCoP nanoparticles has the smallest radius of arc, indicating that the conductivity of the composite photocatalyst ZCS/NCP-5% was greater than that of ZCS/NP-5% and ZCS/CP-5%. The introduction of NiCoP, a cocatalyst, provides more catalytic active sites for Zn_0.5Cd_0.5S by rapidly capturing photoelectrons. Therefore, Zn_0.5Cd_0.5S and NiCoP have excellent electrical conductivity and low resistance, which effectively promote the photocatalytic hydrogen evolution performance.

Fig. 11(c) shows that ZCS/NCP-5% has the highest photocurrent density, indicating that the overpotential of ZCS/NCP-5% was lower than that of Zn_0.5Cd_0.5S, ZCS/CP-5% and ZCS/NP-5%. The decrease of the overpotential of the hydrogen production was mainly caused by the rapid transfer of electrons between the Zn_0.5Cd_0.5S and NiCoP interface.^44,46 This further indicates that the ZCS/NCP-5% composite photocatalyst has strong activity of hydrogen evolution. In conclusion, the ZCS/NCP-5% composite photocatalyst had a faster charge separation efficiency and a lower recombination efficiency, thus effectively improving the activity of the hydrogen evolution of the photocatalyst.

Fig. 12 shows the Mott–Schottky results for Zn_0.5Cd_0.5S, NiCoP, Co₂P, and Ni₂P, and all line plots have positive slopes, indicating that they are typical n-type semiconductors.⁴⁷ It is obvious from the figure that the flat band potentials of the Zn_0.5Cd_0.5S, NiCoP, Co₂P, and Ni₂P are −1.03 eV, −0.64 eV, −0.62 eV and −0.60 eV (relative to SCE), respectively. In general, the conduction band potential (E_CB) of the n-type semiconductors is 0.1 V or 0.2 V more negative than the flat band potential.^36,48,49 Therefore, the E_CB of the Zn_0.5Cd_0.5S, NiCoP, Co₂P, and Ni₂P are estimated to be −1.13 eV, −0.74 eV, −0.72 eV and −0.70 eV (relative to SCE), respectively. According to formula,⁵⁰ the E_CB of the Zn_0.5Cd_0.5S, NiCoP, Co₂P, and Ni₂P are −0.89 eV, −0.50 eV, −0.48 eV and −0.46 eV (relative to NHE), respectively. According to the band gap value of Zn_0.5Cd_0.5S at 2.21 V, the valence band potential (E_VB) of Zn_0.5Cd_0.5S is 1.32 eV (relative to NHE).

Fig. 12 (a and b) Mott–Schottky plots of Zn_0.5Cd_0.5S, Ni₂P, Co₂P, and NiCoP.

3.10 Photocatalytic mechanism

Combined with the photoelectric chemistry experiment and fluorescence spectrum, a possible interface charge transfer direction between Zn_0.5Cd_0.5S and TMPs nanoparticles was proposed, as shown in Fig. 13. In particular, it had been reported that the work function results of Zn_0.5Cd_0.5S have been characterized by the Kelvin probe method,⁵¹ which provides an important basis for studying the direction of the charge transfer. Under visible light irradiation, the photoexcited electrons in the Zn_0.5Cd_0.5S transfer from the valence band (VB) to the conduction band (CB), leaving holes in the valence band. However, due to the rapid recombination between electrons and holes, the photocatalytic hydrogen evolution performance of Zn_0.5Cd_0.5S has not been well developed. Compared with Zn_0.5Cd_0.5S, the XPS peaks of ZCS/NCP-5% Zn 2p, Cd 3d and S 2p move towards the direction of lower binding energy. It is possible that the work function of the TMPs nanoparticles is smaller than that of ZnCdS. Thus, the free electrons are easily transferred from the TMPs nanoparticles to Zn_0.5Cd_0.5S until their Fermi energy levels reach equilibrium,⁵² thus forming a Schottky barrier. After simulated visible light irradiation, photoexcited electrons are transferred from Zn_0.5Cd_0.5S to TMPs nanoparticles, and the electrons on the surface of TMPs reduce H⁺ to H₂, while the VB holes of Zn_0.5Cd_0.5S are consumed by the sacrifice reagent Na₂S/Na₂SO₃.^31,53 On the other hand, TMPs nanoparticles, as a cocatalyst, not only effectively improve the visible light absorption capacity of Zn_0.5Cd_0.5S, but also increase the active site of the hydrogen evolution and charge transfer rate of Zn_0.5Cd_0.5S, thus effectively enhancing its photocatalytic hydrogen evolution performance. Compared with Ni₂P, Co₂P and NiCoP, the composite photocatalytic hydrogen evolution with 5% NiCoP was the best. This is because in most TMPs, the active sites for hydrogen production are in the metal center and phosphating center. When 5% NiCoP nanoparticles were loaded on Zn_0.5Cd_0.5S, the hydrogen evolution rate of ZCS/NCP-5% was the highest, indicating that NiCoP provided more H⁺ receptor centers. This also means that NiCoP has more active sites.^54,55 Therefore, TMPs nanoparticles loaded on Zn_0.5Cd_0.5S effectively improve the charge separation efficiency and increase more active sites for photocatalytic hydrogen evolution, thus improving the activity of hydrogen evolution.

Fig. 13 Schematic diagram of the electron transfer of the ZnCdS/TMPs composite photocatalyst.

4. Conclusion

In this work, a series of ZnCdS/TMPs composite photocatalysts were successfully prepared by solvothermal method and physical agitation method. The effects of Ni₂P, Co₂P and NiCoP as co-catalysts on the hydrogen evolution performance were studied. After loading TMPs nanoparticles on the Zn_0.5Cd_0.5S nanomaterial, all photocatalysts show good photocatalytic H₂ produce performance. Under visible light irradiation, the hydrogen evolution rate of ZCS/NCP-5% reached 12.80 μmol h⁻¹, which was significantly higher than that of pure Zn_0.5Cd_0.5S (3.54 μmol h⁻¹), ZCS/NP-5% (12.10 μmol h⁻¹) and ZCS/CP-5% (9.16 μmol h⁻¹). According to the results of fluorescence and photoelectrochemical tests, TMPs nanoparticles, as a co-catalyst, not only effectively improve the visible light absorption capacity of Zn_0.5Cd_0.5S, but also increase the active site of hydrogen evolution and charge transfer rate of Zn_0.5Cd_0.5S, thus effectively enhancing its photocatalytic hydrogen evolution performance. In particular, compared with Ni₂P and Co₂P, NiCoP has more active sites, which effectively promotes the charge transfer rate of ZCS/NCP-5%. It provides a new idea for the study of high efficiency and low cost photocatalyst.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Natural Science Foundation of Ningxia Province (2021AAC03180) and the Central Universities of North University for Nationalities (2021KJCX02).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1nj04251c

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