With the growing environmental pollution and energy crisis, it is urgent to develop sustainable energy resources. Hydrogen is considered to be one of the most promising energy resources that can replace fossil energy [1]. Photocatalytic water splitting for hydrogen production is an environmentally friendly approach that involves transforming solar energy to hydrogen energy. The n-type semiconductor material graphitic carbon nitride C3N4(g-C3N4) is widely used as a light-absorbing material for photocatalytic hydrogen production. However, the photogenerated electron-hole pairs of pure g-C3N4 recombine rapidly and the photocatalytic activity is very low. The loading of cocatalysts is an efficient way to improve the rate of photocatalytic hydrogen production by effectively inhibiting the recombination of the photogenerated carriers. Over the years, various cocatalysts have been developed, including noble metals (Pt [2–4], Pd [5, 6]), transition metals (Fe [7], Co [8], Ni [9–11], Cu [12, 13], ) and their compounds (Co2P [14–17], NixP [18–21], NiS [22–27], MoSe2 [28], MoS2 [29, 30], Ni(OH)2 [31–35]).
The task of a cocatalyst is to transfer electrons and holes to the corresponding reactants. So, in addition to the matching energy levels between the cocatalyst and the light-absorbing material, the location of the cocatalyst is also important. The cocatalysts prepared by conventional methods are distributed randomly. However, it has been proved by Li and Yu's Group that photochemical methods can deposit cocatalysts at the outlet points of electrons and holes [36–39], which is beneficial for the next reaction. Using a photodeposition method, an oxidizing cocatalyst can be prepared by photochemical oxidation, and a reductive cocatalyst can be prepared by photochemical reduction. The photochemical method is commonly used to reduce noble metal cocatalysts, but it is still a novel method for preparing non-noble metals [40]. Since the photochemical method is lacking in the regulation of components, we tried to develop a relatively accurate and controllable photodeposition method. From previous research, it is known that H2PO2– can play an important role in the process of photodeposition, forming a variety of different composite photocatalysts which can reduce hydrogen production, such as Ni/g-C3N4 [9], NixP/g-C3N4 [18], and Ni(OH)x/g-C3N4/WO3 [31]. However, the particles formed in the presence of H2PO2– are very large and highly crystalline, which may inhibit the activity of photocatalysts because the more perfect the crystal is, the fewer defects and active sites are present on the surface of the cocatalyst. The synthesis of position-matched transition metal cocatalysts with weak crystallization is still challenging. Therefore, we aim to synthesize some weakly crystalline and small-size cocatalysts to improve the hydrogen production activity.
We carefully analyzed the role of sodium hypophosphite, and concluded that it mainly acts as a reducing agent and a sacrificial agent during photodeposition. The reducing property of H2PO2– is so strong that the reduction rate in photodeposition is very fast, while the high-valence phosphate has a weaker reducing ability, which is beneficial for slowing down the rate of reduction to form weakly crystalline cocatalysts. In this work, we designed a photochemical synthesis route using H2PO3– as the sacrificial agent, NiCl2 as the nickel source and g-C3N4 as the light-absorbing material. The results confirm that our supposition is reasonable, because the photocatalytic hydrogen evolution reaction (HER) activity of the composite photocatalyst prepared by H2PO3– is very prominent. Through various characterizations, it was confirmed that the product is Ni(OH)2/g-C3N4. The Ni(OH)2 content can be adjusted by changing the irradiation time.
All reagents, including thiourea (CH4N2S, 99%), nickel chloride hexahydrate (NiCl2·6H2O, 98%), triethanolamine (TEOA, C6H15NO3, 78%), phosphorous acid (H3PO3, 99%), sodium hypophosphite monohydrate (NaH2PO2·H2O, 98%~103%), sodium phosphate dibasic anhydrous (Na2HPO4, 99%), chloroplatinic acid hexahydrate (H2PtCl6·6H2O, 37% Pt basis) and sodium hydroxide (NaOH, 96%), were purchased from Sinopharm Chemical Reagent Co. Ltd. and used without further purification.
The g-C3N4 nanosheet was prepared by a modified method using thiourea as the starting material [9, 41]. Thiourea (20 g) was heated to 550 ℃ with a ramp rate of 2 ℃ min−1 and maintained at this temperature for 2 h in air. After cooling to room temperature, the yellow bulk was ground into a fine powder and heated at 500 ℃ for another 2 h in an open porcelain crucible. A white powder of g-C3N4 nanosheet was finally obtained with a yield of about 1%.
The Ni(OH)2/g-C3N4 composite was prepared by a photochemical deposition method. First, 20 mg of g-C3N4, 1 mL of NiCl2 (0.1 M), 1 mL of H3PO3 (0.7 M), and 8 mL of water were mixed. Before use, the pH of H3PO3 was adjusted to neutral by adding 10 mL of H3PO3 (2 M) and 11.5 mL of NaOH (3 M) into 7.1 mL of H2O. After sonication for 10 min, pure nitrogen was used to purge the mixed system from air for 40 min. Subsequently, the mixture was illuminated under UV-vis light (300 W Xe lamp). Then the precipitate was collected by centrifugation, washed with distilled water and ethyl alcohol, and dried under flowing N2. The obtained products were named as Ni(OH)2-T/g-C3N4, where T represents the illumination time (min).
The reaction was performed in a closed flask using a 300 W Xenon lamp with an AM 1.5G filter as the light source. The lamp was 8 cm away from the reaction solution. The prepared Ni(OH)2/g-C3N4 composite photocatalyst (5 mg) was dispersed in 10 mL of aqueous solution (including 20 vol% TEOA) with sonication for 10 min. The mixed system was purged with pure nitrogen gas for 1 h to remove air. The produced hydrogen was measured by gas chromatography (GC-7920, argon as carrier gas).
The quantum yields (Q.Y.) was measured using a 300 W Xe lamp with a 400 nm (± 5 nm) band-pass filter, and calculated by the following equation:
The optical power density, measured by a radiometer (CEL–NP2000), was 44.86 mw·cm−2 and the irradiation area was 6.28 cm2, corresponding to the number of incident photons (5.67 × 1017 photons per s).
Photocurrent tests were carried out to explore the role of Ni(OH)2 in enhancing the separation efficiency of photogenerated charges in a three-electrode system. It was performed on an electrochemical workstation with Pt foil (1 × 1 cm) as the counter electrode and Ag/AgCl (0.222 V vs. a normal hydrogen electrode (NHE)) as the reference electrode. The working electrode was prepared as follows [9]: 10 mg of the Ni(OH)2/g-C3N4 sample was dispersed in 100 mL of isopropanol with sonication for 3 h. Then, 10 mg of Mg(NO3)2·6H2O was added into the above suspension to form a homogeneous solution with sonication for another 1 h. Subsequently, fluorine-doped tin oxide (FTO) glass as the cathode and a Pt wire as the anode were immersed in the solution at 1 cm below the liquid level. Electrophoretic deposition was performed at 160 V for 1 min, and then the working electrode was dried in a oven at 40 ℃ for 1 h. For comparison, pure g-C3N4 was treated by the same method to prepare the working electrode.
X-ray diffraction (XRD) patterns obtained with a D2 PHASER X-ray diffractometer (Bruker AXS, German) were used to explore the composition and phases of the samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were collected using a JEM-2100 transmission electron microscope (JEOL, Japan) to analyze the size and lattice fringe. The selected area electron diffraction (SAED) patterns, TEM-energy-dispersive X-ray spectroscopy (EDX) and EDX mapping results were collected by field emission TEM on Tecnai G2 F30. X-ray photoelectron spectroscopy (XPS) analysis was conducted using an ESCALAB 250 Xi X-ray photoelectron spectrometer (Thermo, USA), with Al Kα as the excitation source (hv = 1484.6 eV). The content of Ni(OH)2 was measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, PerkinElmer 8300). UV-vis absorption spectra were recorded using a UV-3600 Plus spectrophotometer (Shimadzu, Japan). Fourier transform infrared (FT-IR) spectra were collected on a Nicolet 6700 infrared spectrometer (Thermal, USA) with a DLaTGS detector. A confocal microscopic Raman spectrometer (Renishaw InVia, England) was used to measure Raman spectra. Room temperature photoluminescence (PL) spectra with an excitation wavelength of 375 nm were measured on Cary Eclipse (Varian, USA). The time-resolved photoluminescence (TR-PL) was measured using a fluorescence spectrophotometer (Edinburgh Instruments, FLS980) with an excitation wavelength of 375 nm to measure the electronic lifetimes of the samples. The surface photovoltage (SPV) was measured using self-made equipment to study the charge features. Electrochemistry measurements were performed using a CHI660E electrochemical workstation (Chenhua Instruments Co., China).
The control experiments were performed to study the influence of synthesis conditions on the formation of Ni(OH)2/g-C3N4. Table 1 shows the different synthesis conditions. All of the products except F were collected to test their photocatalytic hydrogen production activity. As shown in Fig. 1, sample A has a photocatalytic hydrogen production rate of 13707.86 μmol·g−1·h−1, which is much higher than those of the other samples. This result indicates that only when g-C3N4, NiCl2, H3PO3 and irradiation are present at the same time, the Ni(OH)2/g-C3N4 composite photocatalyst with the highest hydrogen production rate can be obtained. We also compared the performance of catalysts synthesized with different phosphates and found that the catalyst prepared using H3PO3 has the best catalytic activity (Table S1 and Fig. S1).
The proposed photochemical synthesis route of Ni(OH)2/g-C3N4 is shown in Scheme 1, where g-C3N4 is used as the light-absorbing material. Under irradiation, g-C3N4 is excited to generate electrons and holes (Eq. (1)). Then, H2O can react with the photogenerated electron. The produced H2 has been observed by gas chromatography. Subsequently, Ni2+ in the solution reacts with the generated OH– to form Ni(OH)2 (Eq. (2)). At the same time, H2PO3– (+ Ⅲ) is oxidized to H2PO4– (+ Ⅴ) by photogenerated holes (Eq. (3)).
TEM, HRTEM and SAED were used to characterize the morphology and structure of the samples. Fig. 2(a) and Fig. 2(b) are the TEM images of pure g-C3N4 and Ni(OH)2/g-C3N4. Pure g-C3N4 appears to be a thin-layer, two-dimensional structure. Some Ni(OH)2 particles with an average size of about 10 nm are dispersed on the surface of g-C3N4 in Ni(OH)2/g-C3N4. In the HRTEM image of Ni(OH)2/g-C3N4 (Fig. 2(c)), no lattice fringe is observed, indicating an amorphous structure of Ni(OH)2. As a comparison, the HRTEM images of samples synthesized with other phosphates were collected. They reveal that the sample prepared with H2PO2– has a size of 100~200 nm (Fig. S2(a)), and the size of sample synthesized by HPO42– (Fig. S2(b)) is about 50 nm. Fig. 2(d) shows the SAED pattern of Ni(OH)2/g-C3N4, in which a broad and diffused halo ring can be observed. This result corresponds to an amorphous morphology [32], which further supports the conclusion from HRTEM. EDX and TEM–EDX mappings were collected to investigate the elemental and spatial distribution of the atoms. The EDX mapping images (Fig. 2(e)) show that C and N are uniformly distributed in the g-C3N4 nanosheet, while O and Ni are uniformly present in the small nanoparticles, indicating that the Ni(OH)2 nanoparticles exist in the g-C3N4 nanosheet. Meanwhile, all of the elements (C, N, O and Ni) were observed by TEM-EDX (Fig. S3). This result indicates that the high photocatalytic hydrogen production activity of Ni(OH)2/g-C3N4 may be related to its small size and amorphous structure.
XRD patterns (Fig. S4) were collected to investigate the crystalline structure of pure g-C3N4 and Ni(OH)2/g-C3N4. The two samples both show a weak peak at 13.1° and a sharp and strong peak at 28.0°, which belong to the feature diffraction peak of g-C3N4 [42, 43], indicating that the loading of Ni(OH)2 has not changed the structural features of g-C3N4. There is no obvious diffraction peak of Ni(OH)2 in Ni(OH)2/g-C3N4, proving the amorphous structure of Ni(OH)2 indicated by the result of HRTEM.
The functional groups of samples can be confirmed by FT-IR (Fig. S5) and Raman (Fig. S6) spectroscopy. No obvious change is observed after the deposition of Ni(OH)2, indicating that the construction of g-C3N4 is not changed.
XPS was performed to investigate the composition and chemical status of Ni(OH)2/g-C3N4. Fig. 3(a) shows the survey XPS spectra of Ni(OH)2/g–C3N4 and the high-resolution XPS spectra of C 1s, N 1s and Ni 2p. The high-resolution C 1s spectrum (Fig. 3(b)) has two peaks at 288.37 and 285.02 eV, which can be attributed to the N–C=N and C–C/C=C groups, respectively. The N1s spectrum in Fig. 3(c) has three obvious peaks at 400.72, 399.20 and 398.63 eV. The main peak at 398.63 eV is a typical signal for the N–C=N group of triazine rings [33], while the peak at 399.20 eV is assigned to N(C)3 groups. The last weak peak with a binding energy of 400.72 eV is attributed to amino functional groups. The binding energies of C 1s and N 1s are coincident with that of g-C3N4. In Fig. 3(d), the peaks of Ni 2p appear at 880.34, 874.61, 862.14 and 856.78 eV. The peaks at 874.61 and 856.78 eV are the typical signals of Ni 2p1/2 and Ni 2p3/2 in the Ni(OH)2 phase [33]. The XPS spectra results indicate that the obtained product is Ni(OH)2/g–C3N4.
The photocatalytic hydrogen production activities of the samples were examined. As shown in Fig. 4(a) and Fig. 4(b), pure g-C3N4 has a very low photocatalytic H2 evolution rate of only 18.66 μmol·g−1·h−1 due to the fast recombination of photogenerated carriers. With the loading of Ni(OH)2, the photocatalytic H2 evolution rate is clearly increased. After loading a small amount of Ni(OH)2, the photocatalytic H2 evolution rate is 3325.30 μmol·g−1·h−1 for Ni(OH)2-20/g-C3N4. Ni(OH)2-40/g-C3N4 with an optimal weight ratio of 4.36 wt% (measured by ICP-AES) shows the highest photocatalytic activity of 13707.86 μmol·g−1·h−1. It is even higher than the activity of Pt-4.36 wt%/g-C3N4 (11210.93 μmol·g−1·h−1, Fig. 4(b)). These results demonstrate that Ni(OH)2 is an effective cocatalyst for g-C3N4. However, when the irradiation time is prolonged further, the photocatalytic activity reveals a decrease, because excess Ni(OH)2 impedes light absorption and covers the active sites of g-C3N4. The H2 evolution quantum yield of Ni(OH)2-40/g-C3N4 at 400 nm was determined to be about 0.78%. Furthermore, we explored the effect of the molar ratio of H3PO3 and NiCl2 during photodeposition (Fig. S7). It is clear that when the molar ratio of H3PO3 and NiCl2 is 7, the photocatalytic HER activity reaches the highest value. At the same time, the cocatalyst with small particles and low content has higher atomic utilization when the molar ratio is 7, because the cocatalyst content increases as the ratio increases. Besides, different hole sacrificial agents were used in photocatalytic hydrogen production (Fig. S8). Among them, TEOA is the optimal sacrificial agent for Ni(OH)2/g-C3N4.
The stability of Ni(OH)2/g-C3N4 was tested in Fig. 4(c). During four cycles of photocatalytic hydrogen production, no obvious decrease of photocatalytic activity was observed, indicating its reasonable photocatalytic stability for 20 h. Subsequently, the XRD pattern (Fig. S9), FT-IR spectrum (Fig. S10), and Raman spectrum (Fig. S11) were collected to investigate the composition and structure of Ni(OH)2/g-C3N4 before and after photocatalysis. There is no obvious difference between the two samples. Additionally, TEM (Fig. S12(a)) and HR-TEM (Fig. S12(b)) of the sample after photocatalysis show a size of about 10 nm and an amorphous structure. These results indicate that the structure and morphology of the sample did not change during the reaction. The slight decrease of photocatalytic hydrogen production may be due to the loss of catalyst during washing.
UV-vis diffuse reflectance spectroscopy was carried out to investigate the light-absorption performance of the samples. According to Fig. 5(a), Ni(OH)2/g-C3N4 has a better light-absorption ability compared to pure g-C3N4, but the absorption edges of the pure g-C3N4 and Ni(OH)2/g-C3N4 are both about 450 nm. The band gap of pure g-C3N4 is measured to be 2.8 eV (Fig. S13) [9, 44–46], and that of Ni(OH)2-30/g-C3N4 is also 2.8 eV (Fig. 5(b)). This result indicates that the loading of Ni(OH)2 does not change the structure of g-C3N4. As shown in Fig. 5(c), the valence band of the sample is 1.8 eV [46], and the conduction band was evaluated to be –1.0 eV. Fig. 5(d) exhibits the H2 evolution rate of Ni(OH)2/g-C3N4 at different light wavelengths. The photocatalytic hydrogen production activity agrees well with the UV-vis diffuse reflectance spectrum of pure g-C3N4, indicating the cocatalyst role of Ni(OH)2.
To investigate the separation and transfer efficiency of photogenerated charge carriers, PL spectra, TR–PL decay spectra, SPV spectra and the transient photocurrent density were investigated. As shown in Fig. 6(a), the PL spectra were collected to demonstrate the transfer of photogenerated electron-hole pairs with an excitation wavelength of 375 nm. Pure g-C3N4 shows a strong peak in the PL spectrum, while the PL peak of Ni(OH)2/g-C3N4 composites is much weaker than that of pure g-C3N4, suggesting that the recombination of electrons and holes in g-C3N4 is significantly inhibited by Ni(OH)2. The TR-PL decay spectra (Fig. 6(b)) show that the average lifetimes of pure g-C3N4 and Ni(OH)2/g-C3N4 are 5.45 and 5.06 ns, respectively. The lifetime of electrons in Ni(OH)2/g-C3N4 is reduced compared to pure g-C3N4, indicating the accelerated separation and transfer efficiency of photogenerated charge carriers by the loading of Ni(OH)2.
In addition, the SPV spectra (Fig. 6(c)) indicate that the photovoltage intensity of Ni(OH)2/g-C3N4 is much higher than that of pure g-C3N4, confirming that the separation efficiency of photogenerated carriers from g-C3N4 is effectively enhanced by Ni(OH)2. Moreover, both pure g-C3N4 and Ni(OH)2/g-C3N4 have obvious positive photovoltage responses from 300 to 450 nm, further supporting the UV-vis result. The transient photocurrent response (Fig. 6(d)) shows that the photocurrent density of Ni(OH)2/g-C3N4 is much higher than that of pure g-C3N4, indicating higher separation and transfer efficiency, in agreement with the above results.
Scheme 2 shows the proposed photocatalytic hydrogen production mechanism of Ni(OH)2/g-C3N4. Under irradiation, g-C3N4 is excited to generate electrons and holes. Subsequently, the photogenerated electrons in the conduction band of g-C3N4 can be easily transferred to the cocatalyst Ni(OH)2. These trapped electrons can further reduce H+ and lead to H2 production. Additionally, the sacrificial agent TEOA is oxidized by the photogenerated holes left on the valence band of g-C3N4.
In conclusion, by precisely regulating the type of inorganic phosphates used, we successfully synthesized a weakly crystalline and small-size Ni(OH)2 cocatalyst with an amorphous structure and an average size of about 10 nm. The photocatalytic hydrogen production activity of the prepared Ni(OH)2/g-C3N4 was greatly improved compared to g-C3N4. Based on various characterizations, we can attribute this ability to the loading of Ni(OH)2, which accelerates the separation and transfer efficiencies of the photogenerated charge carriers. This article provides a unique reference for the precise photodeposition of transition metal cocatalysts, and is helpful to understand the function of inorganic sacrificial agents.
2020,
Vol. 41 Issue (5): 889-897 DOI: S1872-2067(19)63499-6 
PDF
2020,
Vol. 41 Issue (5): 889-897 DOI: S1872-2067(19)63499-6 PDF