Constructing Co₃O₄/g-C₃N₄ Ultra-Thin Nanosheets with Z-Scheme Charge Transfer Pathway for Efficient Photocatalytic Water Splitting

Abstract

Photocatalytic water splitting for hydrogen generation is a significant pathway for sustainable energy conversion and production. The photocatalysts with a Z-scheme water splitting charge transfer pathway is superior due to the good separation and migration ability of photoexcited charge carriers. Herein, Co₃O₄/g-C₃N₄ photocatalysts with Z-scheme charge transfer pathway were successfully constructed by an electrostatic interaction-annealing method. The as-prepared Co₃O₄/g-C₃N₄ ultra-thin nanosheets were tested and analyzed by XRD, EA, ICP, SEM, TEM, AFM, XPS, UV-Vis DRS, PL and photoelectrochemical measurements. Moreover, the influences of fabrication parameters on performance of Co₃O₄/g-C₃N₄ catalysts were investigated, and 0.5% Co₃O₄/g-C₃N₄ exhibited the optimal activity. Based on the characterization and catalytic performance, the Z-scheme charge transfer pathway of Co₃O₄/g-C₃N₄ was established and put forward. To further improve the catalytic performance of Co₃O₄/g-C₃N₄, 0.5% Pt was added as a co-catalyst. The obtained Pt/0.5% Co₃O₄/g-C₃N₄ was recyclable and remained the original catalytic water splitting performance within 20 h. The modification of Co₃O₄ and Pt improved the separation and migration of e⁻ and h⁺, and induced the increased hydrogen evolution rate of g-C₃N₄.

1. Introduction

Photocatalytic hydrogen generation is a high efficiency, environmentally friendly and economically practical technology for utilizing solar energy [1,2,3]. It promises a sustainable alternative via semiconductors to address environmental issues and energy shortages all around the world [4,5,6]. The strong optical absorption ability, high separation and migration efficiency and stability are major factors for the photocatalysts’ activity for hydrogen evolution [7,8]. Now efforts have been devoted to fabricating more and more effective semiconductors for solar-energy utilization and conversion, majorly categorized as metal oxides, amorphous (oxy)-hydroxides, (oxy)nitrides, (oxy)sulphides and polymeric catalysts [9,10].

Graphitic carbon nitride (g-C₃N₄) is a robust and nontoxicity polymeric catalyst with good properties [11]. It was constructed by the polymerization method as a photocatalytic water splitting catalyst in 2009 [12]. Up to now, g-C₃N₄ has been widely developed as an emerging and prospective catalyst in different areas, for example photocatalytic degradation of pollution [13], hydrogen evolution through photocatalytic water splitting [14] and CO₂ reduction [15]. Nowadays g-C₃N₄ with various morphologies have emerged, such as quantum dots [16,17], nanotubes [18,19], bulk g-C₃N₄ and g-C₃N₄ nanosheets [20]. Two-dimensional g-C₃N₄ nanosheets have been intensively explored because of their outstanding photocatalytic performance in the past few years [20]. For example, the hydrogen generation ability of two-dimensional g-C₃N₄ nanosheets was improved by Lu and co-workers [21]. It is reported that g-C₃N₄ nanosheets have an improved catalytic performance compared with bulk g-C₃N₄, benefiting from its enlarged redox potentials [22], and prolonged charge carrier lifetime. As has been reported, a short transfer path can be obtained in the g-C₃N₄ nanosheets [22]. Therefore, efficient performance of g-C₃N₄ with 2D ultra-thin structure can be anticipated. In 2015, Liu and co-workers reported that the as-synthesized g-C₃N₄ nanosheets possessed an efficient water splitting ability by nature-inspired environment “phosphorylation” [23]. Unfortunately, g-C₃N₄ has a lot of defects, such as an inner self-combination of electrons and holes, limited light response ability, and so on [24]. To overcome the above shortcomings and greatly enhance its photocatalytic H₂ production activity, semiconductor composite [25], dye sensitization, and co-catalyst modification [26] have been tried.

Among the semiconductor composites, the Z-scheme charge transfer pathway is a practical strategy. Its prominent advantages are the efficient separation of e⁻ and h⁺ at conduction band edges (E_CB) of one semiconductor and valence band edges (E_VB) of other semiconductors, respectively. Therefore, it can inhibit the inner self-combination of electrons and holes. The transfer process schematic diagram has the same shape as the letter Z, called the Z charge transfer pathway. What is more, it is extensively utilized by researchers to boost the effective separation and migration of charges, facilitate hydrogen generation of water splitting and perform a superior photocatalytic performance. For instance, Xie and co-workers synthesized Ag-AgI/BiOI-Bi₂O₃ photocatalyst with Z-scheme multi-charge transfer pathway, which exhibited excellent photocatalytic performance [27]. Qu et al. constructed Ag₂MoO₄/Ag/AgBr Z-scheme composites. The obtained composites exhibited an excellent performance for RhB photocatalytic degradation under various reaction conditions [28]. The photocatalysts with Z-scheme photocatalytic charge transfer pathway have different band structures. The p-type transition metal oxide semiconductor, Co₃O₄, is a narrow band gap photocatalyst, and has been widely employed as an excellent catalyst [29]. Co₃O₄ and g-C₃N₄ can meet the requirements of Z-scheme charge transfer pathway due to their suitable valence band edges and conduction band edges [30]. Through the modification of Co₃O₄ and construction of Z-scheme charge transfer pathway, an electric field between Co₃O₄ and g-C₃N₄ photocatalysts is constructed, which is of benefit to the charge separation and migration. Therefore, improved photocatalytic performance can be obtained on Co₃O₄/g-C₃N₄.

Co₃O₄/g-C₃N₄ catalysts have been applied to degrade tetracycline [30], several dye pollutants [31] and photocatalytic water oxidation [32]. They all exhibited outstanding photocatalytic performances. For instance, Jin et al. synthesized Co₃O₄/g-C₃N₄ as peroxymonsulfate-mediated photocatalytic catalysts, which showed good activity for tetracycline degradation [30]. Qu et al. synthesized Co₃O₄/g-C₃N₄ composites with outstanding photocatalytic peroxymonosulfate activation performance for dyes’ degradation [31].

As has been reported, introducing co-catalysts can greatly improve the hydrogen evolution reaction performance of semiconductors. Researchers have found that modification with noble metal Pt can boost the photocatalysts’ water splitting activity [33,34]. As an electron mediator, Pt has a low Fermi level, which has an important impact on the H₂ evolution performance.

Based on former reports, g-C₃N₄ nanosheets with uniform thickness were synthesized by a polymerization method in the paper. Co₃O₄ was then introduced via an electrostatic interaction–calcination method to form Z-scheme charge transfer pathway in order to obtain efficient separation of charge carriers and high photocatalytic water splitting performance. With further Pt modification, better activity was obtained. The Z-scheme photocatalytic charge transfer pathway of Co₃O₄/g-C₃N₄ was put forward based on the experimental data.

2. Materials and Methods

2.1. Materials

All the reagents that were used were analytical pure. Triethanolamine (TEOA) was purchased from Aladdin Industry Corporation, Shanghai, China. Melamine was purchased from Tianjin Jiangtian Chemical Technology Co. Ltd., Tianjin, China. Ammonia chloride was purchased from Wind Ship Chemical Reagent Technology Co. Ltd., Tianjin, China. Anhydrous ethanol and ammonium bicarbonate were purchased from Guangfu Technology Development Co. Ltd., Tianjin, China. Cobalt chloride hexahydrate and chloroplatinic acid were purchased from Masco Chemical Co. Ltd., Tianjin, China.

2.2. Fabrication of g-C₃N₄

The large-scale production of few-layer and ultra-thin g-C₃N₄ nanosheets with uniform thickness was performed by the facile method of polymerization. 5 g melamine and 10 g ammonia chloride were fully grinded into powder and then calcinated at 550 °C for 4 h in the air atmosphere. This product was collected and named as g-C₃N₄.

2.3. Synthesis of the Ultra-Thin Co₃O₄/g-C₃N₄ Catalysts

The series Co₃O₄/g-C₃N₄ nanosheets were prepared via electrostatic interaction and calcination methods. 0.72 g g-C₃N₄ was sonicated for 0.5 h to disperse in 100 mL anhydrous ethanol. Subsequently, different masses of CoCl₂·6H₂O were added into the above system, followed with a stoichiometric ratio of NH₄HCO₃, and n(CoCl₂·6H₂O): n(NH₄HCO₃) = 1:3 [31]. After continuous stirring for 6 h, the sample was centrifugated, washed and dried at 85 ℃. The as-synthesized materials were calcinated at 350 °C for 2 h. The series Co₃O₄/g-C₃N₄ catalysts with various mass ratios were collected, and are denoted as 0.3, 0.5, 1, 3, 6, 12 wt.% Co₃O₄/g-C₃N₄, respectively.

For comparison, the pure Co₃O₄ was fabricated by calcination of CoCl₂·6H₂O at 600 °C for 4 h.

2.4. Characterization

The X-ray powder diffraction (XRD) was characterized by a diffractometer from Rigaku Corporation. Scanning electron microscopy image was observed by a SU3500 (Hitachi, Tokyo, Japan) microscope. Elemental analysis was explored by Elemental Analyzer (Vario EL cube, Langenselbold, Germany). The ICP analysis was performed by Inductive Coupled Plasma Emission Spectrometer (SpectroBlue, Kleve, Germany). Transmission electron microscopy (TEM) was observed by a FEI TalosF200X instrument. EDS analyses were characterized by an energy dispersive X-ray spectroscope (Bruker, Billerica, MA, USA). Atomic force microscopy (AFM) images were performed by the Bruker icon AFM instrument. The X-ray photoelectron spectroscopy (XPS, Thermo escalab 250Xi) analysis was characterized on a photoemission spectroscopy, using a mono AlKα radiation source. The UV–Vis diffuse reflection spectra (UV–Vis DRS) was explored by a UV–Vis spectrophotometer (UV3600-Plus, Shimadzu, Kyoto, Japan). Photoluminescence spectra (PL) was conducted by a spectrophotometer (PTI, New York, NY, USA). Electrochemical experiments were explored with a workstation (Zahner Zenium, Kronach, Germany).

2.5. Photocatalytic Activity

The H₂ production tests were implemented with the catalyst (50 mg), which was dispersed in triethanolamine solution (10% vol TEOA) in the Labsolar-6A online system. 0.5 wt.% Pt, which was used as co-catalyst, was in-situ reduced by light on the catalysts by adding H₂PtCl₆ aqueous solution in the above suspension of the reaction. After vacuuming, the Argon gas was added. A 300 W Xenon lamp was served to achieve the system irradiation during tests. The solution temperature was controlled at 5 °C to avoid the thermal effect produced by light. Additionally, the tests were carried out using a gas chromatograph and the carrier gas is Argon (GC D7900 II, Tianmei, Shanghai, China).

3. Results and Discussion

3.1. XRD Analysis

The XRD measurement was characterized to confirm the phase of the catalysts, as shown in Figure 1. The peaks at 2θ = 13. 0 and 27.7° were indexed to the characteristic peaks of g-C₃N₄ [5,35]. The diffraction peaks at 2θ = 19.0°, 31.3°, 36.9°, 38.5°, 44.8°, 55.7°, 59.4° and 65.2° were indexed as (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) plane of cubic Co₃O₄ (PDF#42-1467), respectively. It was evident that the peaks of Co₃O₄ and g-C₃N₄ were both existed in the 6% and 12% Co₃O₄/g-C₃N₄ photocatalysts’ XRD patterns. No obvious Co₃O₄ peaks were observed in Co₃O₄/g-C₃N₄ samples with 0.3%, 0.5%, 1% and 3% Co₃O₄. That might be owing to the relatively low content and high distribution of Co₃O₄ [36]. What is more, as clearly demonstrated, the peaks of the photocatalysts were sharp and strong, illustrating that the samples possessed an excellent crystallization. There were no peaks of other phases in Figure 1, which indicated the good purity of the synthesized photocatalysts.

3.2. EA and ICP

For the g-C₃N₄ sample, the atomic ratio of C element and N element was investigated by EA. The value exhibited a result that the atomic ratio of C/N = 3.00: 4.27, which was very close to the value of g-C₃N₄. Based on XRD analysis results, the as-synthesized products were pure g-C₃N₄.

The amounts of Co element in the as-prepared catalysts were monitored by the ICP. The actual contents of 0.3%, 0.5%, 1%, 3%, 6% and 12% Co₃O₄/g-C₃N₄ is 0.3540, 0.5354, 1.146, 3.038, 6.253, 12.44 wt.%, respectively. The corresponding contents of Co₃O₄ in the series Co₃O₄/g-C₃N₄ photocatalysts are close to theoretical values. Due to the negative zeta potentials of g-C₃N₄, there was strong electrostatic interaction between Co²⁺ and g-C₃N₄ nanosheets [37]. As a result, the added Co²⁺ should have no considerable loss. Actually, the actual weight ratio of Co₃O₄ to g-C₃N₄ in the prepared Co₃O₄/g-C₃N₄ catalysts was a little higher than original CoCl₂·6H₂O to g-C₃N₄. It was assignable to the slight weight loss after the second calcination process of g-C₃N₄.

3.3. Morphology Analysis

The microstructure of as-fabricated catalysts was presented in Figure 2. From Figure 2a, Co₃O₄ had polyhedral structure, and the 0.5% Co₃O₄/g-C₃N₄ showed an irregular and crinkly structure. Pt/0.5% Co₃O₄/g-C₃N₄ exhibited 2D morphology with a crinkly nanosheet structure, which seem to be a loose and soft product with a diameter of several micrometers. As observed in the images, the microstructures of 0.5% Co₃O₄/g-C₃N₄ and Pt/0.5% Co₃O₄/g-C₃N₄ were nanosheets. The composition and elements distribution of 0.5% Co₃O₄/g-C₃N₄ and Pt/0.5% Co₃O₄/g-C₃N₄ were further investigated by EDS maps, as displayed in Figure 3 and Figure 4. All of the elements were uniformly dispersed in the photcatalysts, and no signals of other elements were observed. Furthermore, the existence of C, N, Co, O and Pt were highly coincided with the area of g-C₃N₄. It seems that Co₃O₄ and Pt could be uniformly deposited on the g-C₃N₄.

3.4. AFM Analysis

To further investigate the structure and height profiles of the photocatalysts, AFM was performed. The images and corresponding thickness of samples were presented in Figure 5. The architectures of the g-C₃N₄ were 2D ultra-thin nanosheets, with 1–2 nm step height. As illustrated in Figure 5c,d, there is no obvious step height change for 0.5% Co₃O₄/g-C₃N₄ ultra-thin nanosheets. Additionally, g-C₃N₄ and 0.5% Co₃O₄/g-C₃N₄ nanosheets both exhibited few-layer ultra-thin nanosheets with uniform thickness. Ultrathin nanosheets had a short bulk diffusion length [22,38]. Compared with bulk g-C₃N₄, ultrathin g-C₃N₄ nanosheets and 0.5% Co₃O₄/g-C₃N₄ photocatalysts had a shorter electron transfer path, which can increase the lifetime of photoexcited electrons. The unique structure of few-layer g-C₃N₄ photocatalyst generated numerous charge transfer nanochannels and provided a short transfer path for electron transfer, which would promote the efficient separation and migration of e⁻ and h⁺ [22].

3.5. XPS Analysis

To confirm the element compositions and chemical states in the as-synthesized 0.5% Co₃O₄/g-C₃N₄, the XPS analysis was conducted. From XPS survey spectrum (Figure 6a), C 1s, N 1s and O 1s peaks were evident. What is more, no signals assigned to other elements were displayed in this survey spectrum, suggesting that the purity of the samples is very high. As displayed in Figure 6b, the peak at 287.7 eV is due to sp² C atoms, which can bond to N atoms in catalysts [39]. Additionally, the peak at 293.1 eV is owing to π-excitation [40]. Evidently, N 1s has three different peaks at 398.1 eV, 400.0 eV and 404.0 eV in Figure 5c assigning to sp² N atoms of triazine rings, bridging nitrogen atoms in (N–(C)₃) and the charging effects of the hetero-cycles or positive charge localization [41], respectively. The Co 2p at the peaks of 780.4 and 796.1 eV are ascribed to Co 2p_3/2 and Co 2p_1/2, suggesting that Co₃O₄ exists in the photocatalysts [42,43]. Compared with g-C₃N₄, there was negative energy shift of C 1s and N 1s in 0.5% Co₃O₄/g-C₃N₄ photocatalysts, indicating that the e⁻ are transferred from Co₃O₄ to g-C₃N₄ [31]. As discussed in the Z-scheme charge transfer system, e⁻ on CB of Co₃O₄ can migrate to VB of the other semiconductor. Therefore, Z-scheme charge transfer pathway of Co₃O₄/g-C₃N₄ system agreed with the results of negative binding energy shift in 0.5% Co₃O₄/g-C₃N₄ photocatalysts. These results take firm evidence that the Co₃O₄ and g-C₃N₄ were co-existed in the samples and Z-scheme charge transfer pathway was constructed in photocatalysts.

3.6. UV–Vis DRS Analysis

The UV–Vis DRS spectrophotometer analysis was performed to measure optical absorption properties of the catalysts. 0.5% Co₃O₄/g-C₃N₄ possessed a strong absorption and its edge located at 443 nm in Figure 6a. Additionally, although the absorption edges and bandgap energies of g-C₃N₄ and 0.5% Co₃O₄/g-C₃N₄ had no apparent distinction, optical absorption intensity of 0.5% Co₃O₄/g-C₃N₄ was apparently increased, which made it easier to harvest light. The spectra (Figure 7a) of the catalysts suggested that the samples displayed a strong absorption all between 200–440 nm, which covered ultraviolet and visible regions. These photocatalysts can respond to the visible light and their responsiveness was enhanced, which has probably a more important advantage to photocatalytic water splitting. Bandgaps of Co₃O_4, g-C₃N₄ and 0.5% Co₃O₄/g-C₃N₄ photocatalysts were achieved by the Tauc plots, and results are presented in Figure 7b. The bandgap (E_g) of catalysts can be received by the Tauc Equation [44]. Because the Co₃O₄ [45] and g-C₃N₄ [46] are typical direct semiconductors, n in the Tauc equation should be all 1/2. The E_g of as-prepared g-C₃N₄ and Co₃O₄ are 2.8 eV and 2.0 eV, respectively. The E_CB values of Co₃O₄ and g-C₃N₄ are obtained via this equation.

$${\mathrm{E}}_{\mathrm{CB}}{=\chi -\mathrm{E}}_{\mathrm{e}}-0{.5\mathrm{E}}_{\mathrm{g}}$$

(1)

χ for g-C₃N₄ and Co₃O₄ are 4.64 [47] and 5.90 [48], respectively. E_CB of g-C₃N₄ and Co₃O₄ are −1.26 eV and 0.4 eV, respectively. So E_VB values of g-C₃N₄ and Co₃O₄ are 1.54 eV and 2.4 eV, respectively. Because of the appropriate band structure of two semiconductors, the e⁻ would like to transfer from CB of Co₃O₄ to VB of g-C₃N₄. Finally, e⁻ and h⁺ accumulate at CB of g-C₃N₄ and VB of Co₃O₄, respectively. Charge carriers can be effectively separated. These data of g-C₃N₄ and Co₃O₄ can be known based on the formulas and experiments.

3.7. PL

As displayed in Figure 8, the recombination ability of e⁻ and h⁺ was speculated from the photoluminescence intensity of the samples. All the photocatalysts were evaluated at the excitation wavelength, which was 320 nm. Generally, the higher intensity of the PL emission spectra means a worse separation ability of carriers [49]. The spectrum of 0.5% Co₃O₄/g-C₃N₄ catalyst showed lower emission intensity than the intensity of g-C₃N₄, leading to a lower recombining frequency of e⁻ and h⁺ in 0.5% Co₃O₄/g-C₃N₄. The lower the peak of photoluminescence spectra presented, the more excellent catalytic performance can be anticipated. It seems that 0.5% Co₃O₄/g-C₃N₄ catalyst may have a better photocatalytic performance than pure g-C₃N₄.

3.8. Photoelectrochemical Analysis

The photocurrent measurement was produced under the irradiation of 429 nm wavelength light, as illustrated in Figure 9. The photocurrent immediately increased to a value and remained when the light irradiated on the catalysts. The photocurrent decreased as soon as the light turned off. It manifested that 0.5% Co₃O₄/g-C₃N₄ has a photocurrent response and the photocurrent value is around 10 nA at 0.1 V. However, no photocurrent existed in g-C₃N₄. It indicated that 0.5% Co₃O₄/g-C₃N₄ is visible to light-responsive photocatalysts. After Co₃O₄ was introduced, the g-C₃N₄ catalyst can respond to visible light, and a photocurrent was exhibited. That should be attributed to the formation of Z-scheme charge transfer pathway, which resulted in improved separation and migration of the photo-induced charges. As a result, a photocurrent with higher intensity can be obtained.

3.9. Photocatalytic Activity and Cycling Tests

As illustrated in Figure 10a, g-C₃N₄ exhibits no activity. However, the series Co₃O₄/g-C₃N₄ catalysts all present good photocatalytic performance, and 0.5% Co₃O₄/g-C₃N₄ has the fine H₂ evolution rate of 8.774 μmol∙g⁻¹∙h⁻¹. Obviously, the modification of Co₃O₄ can greatly promote the water splitting reaction rate of g-C₃N₄. The results of photocatalytic activity tests can be proved by photoelectrochemical analysis. As indicated in the photoelectrochemical analysis, the g-C₃N₄ can respond to 429 nm only after Co₃O₄ was modified. Therefore, the quantum yield of g-C₃N₄ is low. Catalytic activity of g-C₃N₄ is also low. The photoluminescence spectra analysis showed that e⁻ and h⁺ in the g-C₃N₄ have higher recombination rates. It suggested that g-C₃N₄ has a lower photocatalytic hydrogen production.

According to the band structure of Co₃O₄ and g-C₃N₄ as we noted above, the reasonable photocatalytic charge transfer pathway was preliminarily put forward to explain the reason for catalytic improved-performance of 0.5% Co₃O₄/g-C₃N₄ in Figure 11. The electrons of Co₃O₄ and g-C₃N₄ are photo-generated from VB to CB in response to irradiation. After calcination, interaction between Co₃O₄ and g-C₃N₄ was enhanced. The e⁻ on CB of Co₃O₄ would quickly migrate to VB of g-C₃N₄ and combine with h⁺, which replaced inner self-combination. Some holes were reacted with TEOA, which worked as an oxidation sacrificial agent. Therefore, the electric field between Co₃O₄ and g-C₃N₄ was formed, with electrons and holes accelerated at different parts. The electric field can promote the directional transfer of charges and introduction of Co₃O₄ can boost the electric field of Co₃O₄/g-C₃N₄. Hence, the existence of Co₃O₄ effectively promoted the interfacial charges to separate and transfer and the hydrogen generation, leading to superior photocatalytic performance. What is more, the XPS analysis indicated that e⁻ are transferred from Co₃O₄ to g-C₃N₄ nanosheets. It suggested that the Z-scheme charge transfer pathway was in agreement with experiment results.

The probable reactions process of photocatalytic hydrogen generation is as follows:

$${\mathrm{Co}}_3{\mathrm{O}}_4{/\mathrm{g}\text{-}\mathrm{C}}_3{\mathrm{N}}_4+\mathrm{hv}\to {\mathrm{Co}}_3{\mathrm{O}}_4{(\mathrm{e}}^{-}{+\mathrm{h}}^{+}{)/\mathrm{g}\text{-}\mathrm{C}}_3{\mathrm{N}}_4{(\mathrm{e}}^{-}{+\mathrm{h}}^{+})$$

(2)

$${\mathrm{h}}_{\mathrm{VB}}^{+}{(\mathrm{Co}}_3{\mathrm{O}}_4)+\mathrm{TEOA}\to {\mathrm{oxidation}\mathrm{product}\mathrm{of}\mathrm{sacrificial}\mathrm{reagent}}_{}$$

(3)

$${\mathrm{e}}_{\mathrm{CB}}^{-}{(\mathrm{g}\text{-}\mathrm{C}}_3{\mathrm{N}}_4{)+2\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{+2\mathrm{OH}}^{-}$$

(4)

Thence, the proposed Z-scheme charge transfer pathway of photocatalysts showed good separation and migration of e⁻ and h⁺. The obvious enhancement of the hydrogen generation ability of the 0.5% Co₃O₄/g-C₃N₄ ultra-thin nanosheets may attribute to the significant and ideal Z-scheme photocatalytic charge transfer pathway.

After Pt was added, further efforts were made to improve the performance of Co₃O₄/g-C₃N₄. As presented in Figure 10a, Pt/0.5% Co₃O₄/g-C₃N₄ exhibited a predominant rate and the average rate was up to 1620 μmol∙g⁻¹∙h⁻¹. This is 2.1 times the photocatalytic performance of Pt/g-C₃N₄.

To explore the influence of Co₃O₄ contents on the performance of the Pt/Co₃O₄/g-C₃N₄, results of Pt/Co₃O₄/g-C₃N₄ with different Co₃O₄ contents were shown in Figure 10b. From this figure, it can be observed that all photocatalysts manifested improved hydrogen generation activity compared with Pt/g-C₃N₄, except Pt/12% Co₃O₄/g-C₃N₄. Owning to the relatively low content of Co₃O₄ (0.5 % Co₃O₄), Pt was mainly deposited on g-C₃N₄ nanosheets. When the catalyst was illuminated, the electrons migrated from Co₃O₄ to g-C₃N₄, and were finally transferred to Pt. The H⁺ in the solution obtained the electrons from the Pt particles to form H₂. When the Co₃O₄ content was relatively low (0.3%), Z-scheme charge transfer pathway is still the main pathway in this photocatalytic system. However, the amount of electrons and holes in Co₃O₄ was relatively low than that in the 0.5% one. Therefore relatively large amounts of holes in g-C₃N₄ were reacted with TEOA instead of combining with the electrons in Co₃O₄. As a result, e⁻ and h⁺ in the Pt/0.3% Co₃O₄/g-C₃N₄ have higher recombination rates than in Pt/0.5% Co₃O₄/g-C₃N₄, and the Pt/0.3% Co₃O₄/g-C₃N₄ exhibited lower photoactivity. When Co₃O₄ contents increased, Co₃O₄ will accumulate. More and more Pt/Co₃O₄ interfaces formed, and the electrons migrated from Co₃O₄ to Pt, instead of combining with the holes in g-C₃N₄. Consequently, the Z-scheme charge transfer pathway was blocked, and the activity of Co₃O₄/g-C₃N₄ decreased with the increasing Co₃O₄ contents.

Cycling stability tests of the optimal Pt/0.5% Co₃O₄/g-C₃N₄ were evaluated by a recycle experiment, which was presented in Figure 12. It could suggest that after four successive cycles, no noticeable diminution was observed within 20 h with TEOA as sacrificing agents, suggesting that the Pt/0.5% Co₃O₄/g-C₃N₄ photocatalyst had an excellent stability and it is reusable for practical applications.

4. Conclusions

In this paper, g-C₃N₄ ultra-thin nanosheets were fabricated via the polymerization method. The Co₃O₄/g-C₃N₄ series were fabricated by an electrostatic interaction-calcination treatment. The various characterization results indicated that the Co₃O₄/g-C₃N₄ catalysts exhibit Z-scheme charge transfer pathway, and possessed excellent photocatalytic hydrogen evolution performance with high stability and fine charge transfer ability. After Pt is added, the Co₃O₄/g-C₃N₄ exhibited better activity, and Pt/0.5% Co₃O₄/g-C₃N₄ exhibited the best activity, with a hydrogen generation rate of 1620 μmol∙g⁻¹∙h⁻¹. Based on the characterization and environment data, the catalytic reaction charge transfer pathway of Pt/Co₃O₄/g-C₃N₄ was proposed. Therefore, the work provided a promising method to design efficient and reusable photocatalytic systems with Z-scheme charge transfer pathways for solar-energy conversion. Further practical applications of this Z-scheme charge transfer pathway were anticipated.