Direct Z-Scheme Heterojunction Catalysts Constructed by Graphitic-C₃N₄ and Photosensitive Metal-Organic Cages for Efficient Photocatalytic Hydrogen Evolution

Abstract

The demand for improving the activity, durability, and recyclability of metal-organic cages (MOCs) that work as photocatalytic molecular devices in a homogeneous system has promoted research to combine them with other solid materials. An M₂L₄ type photosensitive metal-organic cage MOC-Q2 with light-harvesting ligands and catalytic Pd²⁺ centers has been synthesized and further heterogenized with graphitic carbon nitride to prepare a robust direct Z-scheme heterojunction photocatalyst for visible-light-driven hydrogen generation. The optimized g-C₃N₄/MOC-Q2 (0.7 wt%) sample exhibits a high H₂ evolution activity of 6423 μmol g⁻¹ h⁻¹ in 5 h, and a total turnover number of 39,695 after 10 h, significantly superior to the bare MOC-Q2 used in the homogeneous solution and the comparison sample Pd/g-C₃N₄/L-4. The enhanced performances of g-C₃N₄/MOC-Q2 can be ascribed to its direct Z-scheme heterostructure, which effectively improves the charge separation and transfer efficiency. This work presents a rational approach of designing a binary photocatalytic system through combing micromolecular MOCs with heterogeneous semiconductors for water splitting.

1. Introduction

It is meaningful but challenging to convert solar energy into chemical energies and to produce useful fuels such as H₂ and CH₄ [1,2,3]. Inspired by enzymatic photosynthesis in nature, scientists have attempted to develop photochemical molecular devices (PMDs) that can conduct catalysis reactions under appropriate conditions, and have adjustable reactivity and properties as well as highly researchable structure—activity relationships [4,5,6]. Metal-organic cages (MOCs) are a kind of discrete cage compound assembled by organic ligands and metal ions, which are named metal-organic polyhedrons (MOPs), macrocycles, containers, cages, and so on [7,8]. In particular, photosensitive MOCs that involve light-harvesting ligands and redox-active metal cation centers can transfer exciting electrons from ligands to catalytic centers fast, and thus work as PMDs, attracting extensive interest from researchers in the field of visible-light driven H₂ production [9,10,11,12].

However, most photocatalytic MOC systems suffer from easy deactivation, are difficult to recycle, and have poor stability in water [13]. To overcome these drawbacks, efforts have been paid to developing composite catalysts formed by MOCs and some other solid materials for improving the photocatalytic performance and durability of MOCs. For example, Su and co-workers successfully heterogenized an as-synthesized MOC (MOC-16) by embedding it into a metal-organic framework (MOF) named ZIF-8, obtaining a heterogeneous PMD, MOC-16@CZIF [14]. MOC-16@CZIF inherited the efficiently directional electron transfer property of MOC-16, and the MOF matrix provided a platform for proton transfer. Therefore, the H₂ production activity and durability of the MOC-16@CZIF was much higher than homogeneous MOC-16.

Since Wang and co-workers first used graphite phase carbon nitride (g-C₃N₄) for photocatalytic H₂ production [15], g-C₃N₄ has become a widely investigated photocatalyst [16,17,18,19,20,21,22,23,24], because it is a non-toxic, low-cost, easily-prepared, and stable metal-free polymer with an appropriate energy band structure to absorb visible light and is used for many redox reactions. Nevertheless, the bare g-C₃N₄ has some disadvantages of a poor visible light response, low charge transfer rate, and low electron-hole separation efficiency, limiting its application in photocatalysis. Therefore, various strategies, such as morphology regulation, metal/non-metal element doping, dye sensitization, co-catalyst loading, and the formation of type II or Z-scheme heterojunction [25], have been investigated. Among these heterojunctions, type II heterojunction systems are widely used, but the photogenerated electrons in the type II heterojunction tend to accumulate on the more positive conduction band (CB), while the holes are prone to gather on the more negative valence band (VB), thus the redox activity is natively weakened thermodynamically. In contrast, the Z-scheme system [26], which simulates the electron transfer process of between photosystem I and photosystem II in nature, reserves both the higher excited energy level and the lower oxidized energy level, and thus shows a stronger redox ability than the traditional type II heterojunction [27,28].

Our group designed a non-photosensitive MOC-PC6 and a photosensitive MOC-Q1, and combined them with semiconductor matrixes including TiO₂ [29] and g-C₃N₄ [30,31] for enhancing their performances. In 2021, we synthesized a M₂L₄ type photosensitive metal-organic cage MOC-Q2 with light-harvesting ligands and catalytic Pd²⁺ centers (Scheme 1). MOC-Q2 has been successfully heterogenized through combining it with TiO₂ semiconductor through a simple sol−gel method. The as-prepared MOC-Q2-TiO₂ photocatalyst showed a dramatically higher activity than the bare MOC-Q2 in a homogeneous solution [32]. To further enhance the performances of the MOC-Q2-based PMD, we herein designed a new binary Z-Scheme heterojunction system constructed with g-C₃N₄ and MOC-Q2. The optimal g-C₃N₄/MOC-Q2 (0.7 wt%) catalyst achieved a remarkable H₂ evolution efficiency, giving a production rate of 6423 µmol g⁻¹ h⁻¹ after 5 h and a turnover number (TON) value of 39,694 based on MOC-Q2 after 10 h, representing the highest of all of the reported MOC-based PMD under visible light irradiation.

2. Materials and Methods

2.1. Materials

The solvent acetonitrile utilized in the cyclic voltammetry test was an HPLC grade reagent, and dimethyl sulfoxide was dried with A4 molecular sieves. All the other chemicals and reagents were purchased from commercial channels. Unless otherwise specified, further purification was unnecessary before use.

2.2. Characterizations

Powder X-ray diffraction (PXRD) curves were tested under Cu-Kα1 radiation (λ = 1.54056 Å) in a Rigaku Smart Lab diffractometer with Bragg−Brentano geometry (Rigaku corporation, Tokyo, Japan). Transmission electron microscopy (TEM) and element mapping were recorded by a JEM-ARM200P spherical aberration corrected transmission electron microscope (JEOL Ltd., Tokyo, Japan). Scanning electron microscope (SEM) images came from Hitachi ultra-high-resolution FE-SEM SU8010 (Hitachi High-Technologies Corporation, Tokyo, Japan). The UV−VIS absorption spectra were obtained on a Shimadzu UV-3600 spectrometer (Shimadzu Corporation, Kyoto, Japan). The infrared spectra were measured by a Frontier Fourier transform infrared (FT-IR) spectrometer with an average of 64 scans in a spectra range of 4000–450 cm⁻¹ (PerkinElmer, Waltham, MA, United States). The steady-state and time-resolved photoluminescence (PL) spectra were tested with an Edinburgh FLSP980 fluorescence spectrometer (Edinburgh Instruments Ltd., Livingston, UK). The cyclic voltammetry (CV) curves were tested using a CH Instruments CHI760E electrochemical work station (CH Instruments Inc., Austin, TX, USA). The nitrogen adsorption−desorption isotherms were measured with a Quantachrome Auosorb-iQ2-MP analyzer (Quantachrome Instruments Inc., Boynton Beach, FL, USA). X-ray photoelectron spectroscopy (XPS) were performed in an ultrahigh vacuum chamber (ESCALAB 250Xi) using an XR6 monochromated AlKα X-ray source (hν = 1486.6 eV) with a 900 mm spot size (Thermo Fisher Scientific Inc., Waltham, MA, USA). The H₂ yields were tested with a Fuli GC9790 II gas chromatography (Fuli Instruments Inc., Zhejiang, China). The apparent quantum yields (AQYs) were achieved using a monochromatic LED light source (Zolix, MLED4-1/M425, λ = 425 nm), and the photon flux was ascertained to be 1083 μmol h⁻¹ (Zolix Instruments CO., LTD, Beijing, China). The Pd contents in all of the samples were obtained by inductively coupled plasma-atomic emission spectrometry (ICP-AES; SPECTRO CIROS VISION, spectra range: 120–800 nm, and holographic grating: 2924 line/mm) (SPECTRO Analytical Instruments, Kleve, Germany).

2.3. Synthesis and Preparation

2.3.1. Synthesis of MOC-Q2

The solution of Pd(BF₄)₂(MeCN)₄ (44 mg, 0.1 mmol, 1 eq) in DMF (1 mL) was added into a stirring solution of L-2 (144 mg, 0.2 mmol, 2 eq) in DMF (2 mL), and the resulting solution was stirred at room temperature for 10 min and filtered. Then, the product was precipitated by adding diethyl ether, and it was washed with ethyl acetate and dried in a vacuum to give 118 mg MOC-Q2 as a brown solid (yield: 63%), which was in accordance with our previous work [32].

2.3.2. Preparation of g-C₃N₄

First, 10 g of urea was put in a 50 mL crucible and calcinated in a muffle furnace at 550 °C for 2 h at a heating rate of 10 °C/min. Then, the obtained solid was ground into powder and treated at 500 °C for another 2 h [30].

2.3.3. Preparation of g-C₃N₄/MOC-Q2

The composite material g-C₃N₄/MOC-Q2 was prepared using a solution dipping method. First, 2 mg MOC-Q2 was dissolved in 100 μL DMSO. Then, 50 mg g-C₃N₄ was ultrasonically dispersed in 10 mL acetonitrile for 1 h. Thereafter, different volumes of MOC-Q2 solution were added dropwise into the above g-C₃N₄ suspension, i.e., 7.5, 17.5, 25.0, and 50.0 µL. For keeping the same total volume of the mixtures, 42.5, 32.5, and 25.0 µL of DMSO was subsequently added to the above mixtures, respectively. Afterward, the mixture was vigorously agitated for 12 h at room temperature, followed by evaporation and vacuum drying. Finally, resulting samples with MOC-Q2 mass fractions of 0.3, 0.7, 1.0, and 2.0 wt% were obtained.

2.3.4. Preparation of g-C₃N₄/L-2 (0.7 wt%)

The preparation method was the same as that of g-C₃N₄/MOC-Q2 (0.7 wt%), except for replacing MOC-Q2 with L-2.

2.3.5. Preparation of Pd/g-C₃N₄/L-2 (0.7 wt%)

Pd nanoparticles were loaded onto g-C₃N₄/L-2 (0.7 wt%) using a photo-deposition method. The calculated amounts of H₂PdCl₄ corresponding to 0.7 wt% were added into a g-C₃N₄/L-2 (0.7 wt%) suspension and then stirred for 2 h under 300 W Xenon lamp irradiation. Then, the mixture was centrifuged to obtain the grey powder, which was washed with deionized water and dried under a vacuum to gain product.

2.4. Photocatalytic H₂ Generation

The photocatalytic H₂ generation experiments were performed in a closed quartz reactor. In detail, 5 mg of catalyst powder was added into a quartz reactor containing 2 mL triethanolamine (TEOA) and 18 mL water. The mixture was ultrasonic-dispersed and bubbled with N₂ for 30 min to eliminate O₂. Then, the reactor was irradiated with a Xenon lamp (300 W, λ ≥ 420 nm), and the amounts of produced H₂ were detected by gas chromatography (GC). Especially, for 5 mg MOC-Q2, considering its poor solubility in the aqueous phase, it was dissolved in a mixed solution containing 16 mL DMSO, 2 mL water, and 2 mL TEOA.

2.5. Apparent Quantum Yield (AQY) Measurements for H₂ Evolution

The AQY measurements were performed in a closed quartz reactor. In detail, 5 mg of catalyst powder was added into a quartz reactor containing 4.5 mL water and 0.5 mL TEOA. The mixture was ultrasonic-dispersed and bubbled with N₂ for 30 min to eliminate O₂. Then, the reactor was irradiated with a 425 nm monochromatic LED, and the amounts of produced H₂ were detected by GC. The AQYs were measured with the following formula:

$$\mathsf{\Phi }=\frac{\mathrm{number} \mathrm{of} \mathrm{transfered} \mathrm{electrons}}{\mathrm{number} \mathrm{of} \mathrm{incident} \mathrm{photons}}\times 100\%=\frac{2\times \mathrm{n}({\mathrm{H}}_2)}{\mathrm{n}\left(\mathrm{photons}\right)}\times 100\%$$

2.6. Hydroxyl Radical Trapping Experiment

First, 20 mg of g-C₃N₄, MOC-Q2 or g-C₃N₄/MOC-Q2 (0.7 wt%) was dispersed in a sealed 40 mL quartz glass bottle containing 10 mL aqueous solution containing 5 × 10⁻⁴ M terephthalic acid (TA) and 2 × 10⁻³ M NaOH. Then, the reactor was irradiated with a Xenon lamp (300 W, λ ≥ 420 nm) for 1 h. After the reaction, the mixture was filtered through an aqueous phase membrane filter, and the filtrate was used for the fluorescence measurement.

3. Results and Discussion

3.1. Synthesis of MOC-Q2 and g-C₃N₄/MOC-Q2

The synthesis of MOC-Q2 and the preparation of hybrid materials g-C₃N₄/MOC-Q2 and Pd/g-C₃N₄/L-2 are given in the Experimental Section. The successful formation and the geometric configuration of MOC-Q2 was confirmed in our previous study [32]. The actual MOC-Q2 contents of the prepared samples g-C₃N₄/MOC-Q2 confirmed by ICP-AES are listed in Table S1.

3.2. Characterization of g-C₃N₄/MOC-Q2

The PXRD patterns of g-C₃N₄, MOC-Q2, and the hybrid samples g-C₃N₄/MOC-Q2 are depicted in Figure 1. The pristine g-C₃N₄ and g-C₃N₄/MOC-Q2 materials displayed similar characteristic peak of 13.1° and 27.7°, demonstrating the (100) and (002) crystal plane of g-C₃N₄. Moreover, when MOC-Q2 contents gradually increased in the hybrids, the (002) peak of the g-C₃N₄/MOC-Q2 (0.3/0.7/1.0/2.0 wt%) samples slightly shifted to a lower angle. According to the Bragg formula, the decrease in the angle of the (002) peak indicated the interlayer spacing of g-C₃N₄/MOC-Q2 became larger, which can be attributed to the successful incorporation of MOC-Q2 into g-C₃N₄.

The SEM and TEM images displayed the stacked nanosheet structure of the g-C₃N₄/MOC-Q2 (Figure 2a–d). Owing to the tiny size of MOC-Q2, scarce signals related to MOC-Q2 were detected in the spectrogram. However, from the element mapping results of g-C₃N₄/MOC-Q2 (Figure 2e,f), it could be seen that the S and Pd elements, the exclusive elements of MOC-Q2, were detective all over the sample, thus confirming the homogeneously scattering of MOC-Q2 in the composite material.

The nitrogen adsorption−desorption tests of g-C₃N₄ and g-C₃N₄/MOC-Q2 (0.7 wt%) samples were carried out at 77 K. The isotherms are shown in Figure S1, and the BET surface areas and pore volumes are presented in Table S2. As shown in Figure S1a, g-C₃N₄ and g-C₃N₄/MOC-Q2 (0.7 wt%) exhibited H3 type hysteresis loops, which agreed with the layered structure of g-C₃N₄ materials. Furthermore, the pore size distribution diagram in Figure S1b showed that there were many mesopores, and a few micropores and macropores in g-C₃N₄. As for the g-C₃N₄/MOC-Q2 (0.7 wt%), there were a large number of mesopores and a small amount of macropores, but the micropores belonging to g-C₃N₄ disappeared. The specific surface area and pore volume in g-C₃N₄/MOC-Q2 (0.7 wt%) was 54.5 cm² g⁻¹ and 0.33 cm³ g⁻¹, respectively, both smaller than those of g-C₃N₄, indicating that some pores in g-C₃N₄ were blocked by MOC-Q2.

The UV−VIS diffuse reflection spectra of g-C₃N₄, MOC-Q2, and a series of g-C₃N₄/MOC-Q2 samples are illustrated in Figure 3. g-C₃N₄ showed a strong UV absorption peak with an edge of ca. 457 nm, related to its 2.71 eV optical band gap according to the Tauc plot (Figure S2). MOC-Q2 had a wide absorption region from 300 to 600 nm, owing to its effective π−π* transition. As for the g-C₃N₄/MOC-Q2 composites, superimposed absorption bands of g-C₃N₄ and MOC-Q2 were observed. As the MOC-Q2 contents increased, the absorption intensity at a range of 400–600 nm was gradually enhanced.

To explore the interaction between MOC-Q2 and g-C₃N₄ in the composite material, the FT-IR spectra were measured as shown in Figure 4. The absorption peaks of MOC-Q2 were found in the region of 700 to 1600 cm⁻¹. The peaks at 1637, 1573, 1460, 1407, 1316, and 1240 cm⁻¹ in g-C₃N₄ represented the stretching vibrations of the triazine rings, while the peak at 812 cm⁻¹ was a characteristic absorption that came from the deformation vibration of triazine rings. The g-C₃N₄/MOC-Q2 (2 wt%) showed some typical peaks of g-C₃N₄ as well, but the deformation vibration peak of the triazine ring shifted to 814 cm⁻¹, which might be due to the π−π stacking effect that existed between MOC-Q2 and g-C₃N₄. In addition, the broadened and weakened peaks in the range from 3000 to 3500 cm⁻¹ in the g-C₃N₄/MOC-Q2, which were ascribed to the amine stretching vibrations of g-C₃N₄, demonstrated that hydrogen bonds formed between the uncoordinated pyridine moieties in MOC-Q2 and the amine groups on g-C₃N₄.

For testifying the electron transfer feasibility of this system, the energy levels of the constituents were measured. In the CV curve of MOC-Q2 in Figure S3a,b, the onset oxidation potential (E_ox) of MOC-Q2 appearing at 0.97 V vs. NHE, which corresponded to the Highest occupied molecular orbital (HOMO) energy level, was more positive than the CB edge potential (−1.03 V vs. NHE) of g-C₃N₄ [31], allowing for electron transfer from the CB edge of g-C₃N₄ to the oxidized MOC-Q2 so as to regenerate the neutral MOC-Q2. As seen from the normalized absorption and emission spectra of MOC-Q2 in Figure S3c, its E_0-0 transition energy was determined to be 2.58 eV from the intersection (Table S3). Therefore, the excited state potential, which corresponded to its Lowest unoccupied molecular orbital (LUMO) level, was calculated to be −1.61 V vs. NHE judging by its E_ox and E_0-0 value. The reduction potential of H⁺ was more positive than the LUMO value of MOC-Q2, so that MOC-Q2 could conduct a proton reduction reaction via the Pd²⁺ catalytic sites. On the other hand, the VB edge potential (1.68 V vs. NHE) of g-C₃N₄ [31] was more positive than that of the sacrificial agent TEOA (0.84 ± 0.12 V vs. NHE) [33] and therefore it is thermodynamically feasible for the efficient supply of electrons from TEOA to the positive-charged VB of g-C₃N₄ during the reaction.

3.3. Photocatalytic H₂ Evolution

The photocatalytic H₂ evolution experiments were performed under visible light (λ > 420 nm) irradiation in an aqueous solution with triethanolamine (TEOA) as the sacrificial agent. As shown in Figure 5 and Table S4, when the content of MOC-Q2 on the g-C₃N₄ increased from 0.3 to 0.7 wt%, the photocatalytic activity enhanced from 2693 to 6423 µmol g⁻¹ h⁻¹ in 5 h. For g-C₃N₄/MOC-Q2 (0.7 wt%), the initial turnover number (TON) reached 20,955 based on the MOC-Q2 amounts, or 10,478 based on the number of Pd atoms. However, further increasing the incorporation amounts of MOC-Q2 conversely lowered the activity. For the g-C₃N₄/MOC-Q2 (1.0 and 2.0 wt%) samples, H₂ yielded rates of 3153 and 2953 µmol g⁻¹ h⁻¹, respectively. We speculated that excessive amounts of MOC-Q2 would cause the accumulation of MOC-Q2 on the g-C₃N₄ surface, which resulted in intermolecular quenching, hindered the electron transfer efficiency, and thus decreased the catalyst activity. In addition, all the g-C₃N₄/MOC-Q2 samples exhibited a much higher H₂ generation activity than the bare MOC-Q2 used in the homogeneous solution (855 µmol g⁻¹ h⁻¹), implying that the heterostructure of g-C₃N₄/MOC-Q2 could improve the effective charge transfer. As controls, Pd/g-C₃N₄/L-2 (0.7 wt%) samples with the same element composition were prepared and tested. The hydrogen production rate of Pd/g-C₃N₄/L-2 (0.7 wt%) was determined to be 156 µ mol g⁻¹ h⁻¹, far lower than that of g-C₃N₄/MOC-Q2 (0.7 wt%), indicating that MOC-Q2 played an essential role in the photocatalytic process, in which the ligand-to-metal charge transfer transition between L-2 and Pd²⁺ occurred.

To explore the durability of the MOC-Q2/g-C₃N₄ system, two reaction cycles were performed (Figure 6). The H₂ yield in the second cycle decreased slightly, which might be due to the desorption or dissociation of a small part of MOC-Q2 during the photocatalytic reaction. A total H₂ yield of 60.82 mmol g⁻¹ was obtained, corresponding to a total TON_[MOC-Q2] of 39,695 and TON_[Pd] of 19,847, which was the highest of all of the reported MOC-based PMD under visible light irradiation (

Table 1. Performances of photocatalytic systems based on MOC for visible-light-driven hydrogen evolution in recent years.

No.	Reference	Photocatalyst	H2 Yield	TON	Time
1	Appl. Catal. B-Environ. 2021, 285, 119782. [34]	RhB-Zr-Bpydc-PtCl2 MOCs	230 μmol/g	378[Pt]	22 h
2	ACS Appl. Mater. Interfaces 2021, 13, 25960. (our work) [31]	g-C3N4/MOC-Q1	83,692 μmol/g	19,268[MOC-Q1]	25 h
3	ACS Appl. Mater. Interfaces 2021, 13, 57230. (our work) [29]	FL@MOC-PC6-TiO2	29.8 mmol/g	4356[MOC-PC-6]	40 h
4	Chem Asian J. 2021, 16, 2055.(our work) [32]	TiO2-MOC-Q2	309.8 mmol/g	11,739[MOC-Q2]	35 h
5	ACS Appl. Energy Mater. 2020, 3, 12108. [35]	PdNPs@C4R/g-C3N4	5487 μmol/g h	–	20 h
6	Chem. Eur. J. 2019, 25,2824. [36]	Pt/ZrT-1-NH2	1060 μmol/g h	–	4 h
7	Angew. Chem. Int. Ed. 2019, 59, 2639. [9]	MOC-16/TTF	3173 μmol/μM	1202[Pd]	47 h
8	J. Am. Chem. Soc. 2019, 141, 13057. [14]	MOC-16@CZIF	97.0 mmol/g	35,000[Pd]	24 h
9	Chem. Commun. 2019, 55, 13156. [37]	[Ni6L8]∞	14 μmol/μM	2824[[Ni6L8]∞	69 h
10	Chinese J. Catal. 2019, 40, 1198.(our work) [30]	MOC-16/g-C3N4	10.0 mmol/g	517[Pd]	15 h
11	Chem. Commun. 2019, 55, 8524. [38]	Co-NAS/Ru(bpy)32+	10 mL/mM	360[Co-NAS]	9 h
12	Chem. Eur. J. 2018, 24, 16395. [39]	1·Fe4(Zn-L)6	–	0.4[1·Fe4(Zn-L)6]	2 h
13	Angew. Chem. Int. Ed. 2017, 56, 15284. [40]	Co-TPC/QHQ	0.4 mL/μM	–	12 h
14	Sci. Rep. 2017, 7, 14347. [41]	Co-ZPB/Fl	50 mL/mM	400[Co-ZPB]	4 h
15	RSC Adv. 2017, 7, 48989. [42]	Cu-OBP/Fl	0.1 mL/μM	1200[Cu-OBP]	20 h
16	Angew. Chem. Int. Ed. 2017, 56, 11759. [43]	Ni-TFT/Fl	0.3 mL/mM	25,000[Ni-TFT]	20 h
17	Inorg. Chem. 2017, 56, 13286. [44]	Co6L8/Ru(bpy)3	12 μmol/h	43[Co6L8]	2 h
18	Inorg. Chem. Front. 2016, 3, 1256. [45]	Ni-SSC/Fl	1 mL/μM	1250[Ni-SSC]	8 h
19	Chem. Eur. J. 2016, 22,5253. [46]	Ni-YL/Ru(dcbpy)3	0.2 mL/μM	1600[Ni-YL]	5 h
20	Chem. Eur. J. 2016, 22,18107. [47]	Cage2/Fl	0.1 mL/μM	700[Cage2]	15 h
21	Nat. Commun. 2016, 7, 13169. [10]	MOC-16	50 μmol/μM	635[Pd]	48 h
22	J. Am. Chem. Soc. 2015, 137, 3967. [48]	Co-TFT/Fl	1.5 mL/μM	11,000[Co-TFT]	15 h

).

3.4. Mechanism Study

XPS measurements were performed to monitor the chemical states of Pd before and after the photocatalytic reaction (Figure S4a,b). Considering that the Pd contents in g-C₃N₄/MOC-Q2 (0.7 wt%) were too low to be detected, g-C₃N₄/MOC-Q2 (2 wt%) with high Pd contents was used for the test. For the fresh sample, two peaks located at 337.7 and 342.8 eV with an area ratio of 3:2 could be found, which could be attributed to 3d_5/2 and 3d_3/2 orbits of Pd²⁺, respectively. After reaction for 10 h, the 3d_5/2 and 3d_3/2 peaks of Pd²⁺ shifted to 336.7 and 341.9 eV. In addition, two new peaks appeared at 334.8 and 340.1 eV with an area ratio of 3:2, representing the 3d_5/2 and 3d_3/2 orbits of Pd⁰. These changes indicated that the break of Pd-N took place during the photocatalytic process, resulting in the formation of simple substance Pd⁰ (Pd black), which was in accordance with our previous report [31]. In addition, the XRD patterns of the spent sample showed a negligible change compared with that of the fresh sample, testifying that the crystal form of the sample did not change during the reaction (Figure S4c). In addition, the isolated feature of Pd on the spent g-C₃N₄/MOC-Q2 (2 wt%) sample after the photocatalytic reaction was observed by high-resolution TEM images (Figure S5).

To verify the electron transfer properties in g-C₃N₄/MOC-Q2, the steady-state and time-resolved PL spectra of g-C₃N₄, MOC-Q2, and g-C₃N₄/MOC-Q2 (0.7 wt%) were tested. As shown in Figure S6a, the bare g-C₃N₄ exhibited strong fluorescence centered at 465 nm under 360 nm excitation, and MOC-Q2 showed trace fluorescence. Whereas for g-C₃N₄/MOC-Q2 (0.7 wt%), a fluorescence peak at 465 nm, but with a significantly decreased intensity compared to g-C₃N₄, was observed. This result suggested that some photogenerated electrons in g-C₃N₄ transferred to MOC-Q2, and thus led to the reduced fluorescence intensity of g-C₃N₄/MOC-Q2. The fitted fluorescence lifetime from the time-resolved PL spectra (Figure S6b) were 10.1 ns for g-C₃N₄, 9.5 ns for g-C₃N₄/MOC-Q2 (0.7 wt%), and 1.2 ns for MOC-Q2, consistent with the steady-state PL results.

It is very important to confirm the Z-scheme mechanism for the heterojunction of g-C₃N₄/g-C₃N₄. In the presence of H⁺, the O₂ in the reaction mixture can be reduced by the electron on the CB of g-C₃N₄ (Figure 7a), and the specific processes are listed as Equations (1)–(5) [49].

g-C₃N₄ + hv → h_VB⁺ + e_CB⁻

(1)

e_CB⁻ + O₂ → O₂•⁻

(2)

O₂•⁻ + H⁺ → HO₂•

(3)

e_CB⁻ + HO₂• + H⁺ → H₂O₂

(4)

H₂O₂ + e_CB⁻ → •OH + OH⁻

(5)

Herein, we carried out the experiments of capturing •OH free radicals by terephthalic acid (TA), because TA could quickly combine with •OH to form a strongly fluorescent compound TA-OH, which has been widely used to testify the Z-scheme mechanism. As seen from Figure 8, the reaction mixture of the g-C₃N₄ sample with TA presented strong fluorescence at 430 nm, using 315 nm as the excitation wavelength, while that of the MOC-Q2 sample with TA had no fluorescence. As for the reaction mixture of the g-C₃N₄/MOC-Q2 (0.7 wt%) sample with TA, fluorescence with a medium intensity was observed. Considering that the produced •OH in the reaction mixture could only be generated by the electrons in the CB of g-C₃N₄, the attenuation of the fluorescence intensity came from the incorporation of MOC-Q2, which accepted some electrons in the CB of g-C₃N₄ and then reduced the yields of •OH (Figure 7b). Thus, it was verified that the as-prepared g-C₃N₄/MOC-Q2 system worked under a Z-scheme electron transfer route in the photocatalytic reaction. Therefore, we reasonably speculated the working mechanism of this system as follows: under visible light irradiation, g-C₃N₄ and MOC-Q2 synchronously entered an excited state and formed photogenerated electron−hole pairs. Then, the photo excited electrons on the CB of g-C₃N₄ combined with the holes on the HOMO of MOC-Q2, while the electrons on the LUMO of MOC-Q2 and the holes on the VB of g-C₃N₄ were retained to participate in the external reduction/oxidation reactions, respectively (Figure 9).

4. Conclusions

In summary, we designed and synthesized a photosensitive tridentate ligand L-2, and used four L-2 ligands with two catalytic Pd²⁺ centers to construct a M₂L₄-type molecular cage MOC-Q2. The as-prepared MOC-Q2 featured visible-light-harvesting and photo-induced LMCT transition properties, which allowed it to serve as a PMD for visible-light-driven water splitting. To overcome the instability of the homogeneous MOC catalyst and further improve the performances, we combined MOC-Q2 with g-C₃N₄ to prepare a direct Z-Scheme composite material g-C₃N₄/MOC-Q2. The optimized g-C₃N₄/MOC-Q2 (0.7 wt%) catalyst achieved a remarkable H₂ generation activity of 6423 µmol g⁻¹ h⁻¹ and a TON value of 39,694 based on MOC amounts under visible light, far outperforming the homogenous MOC-Q2. The increased performance of the g-C₃N₄/MOC-Q2 catalyst could be owing to the superior charge-separation property of the Z-Scheme heterojunction, as well as the effective stabilizing effect through heterogenizing MOC-Q2 PMD with the polymeric g-C₃N₄ matrix. This research provides a promising idea to develop heterogeneous MOC-based catalysts for efficient and persistent photocatalytic water splitting.