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Facile synthesis of water-soluble Schiff-base polymers for photocatalytic H2 production from water
* Corresponding author: E-mail address: liuxing1127@sina.com (X. Liu)
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Received: ,
Accepted: ,
Abstract
Solar photocatalysis for splitting water to produce hydrogen has gained recognition as a promising technology for H2 generation. The linchpin of realizing this technology lies in developing effective, cost-effective, and practical photocatalysts. Polymeric photocatalysts have gained prominence compared to semiconductor ones, owing to their high structural versatility and tunable band gaps. In our current research, we synthesized two straightforward polymers (UP-1 and UP-2) using Schiff-base chemistry through a one-step hydrothermal process with urea (UA) and p-phthalaldehyde (PPA) as monomers. Specifically, UP-1 (UA:PPA = 1:1) features an imine (–C=N–) structure, whereas UP-2 (UA:PPA = 2:1) predominantly comprises an aminal (N–C–N) structure. The photocatalytic hydrogen production capabilities of UP-1 and UP-2 were assessed at room temperature, utilizing Pt as a cocatalyst and triethanolamine (TEOA) as an electron donor. Notably, UP-1 exhibits a H2 generation rate of 7.60 mL·h−1·g−1, significantly outperforming that of UP-2 (4.89 mL·h−1·g−1). It was postulated that UP-1’s imine structure possesses a superior conjugation, facilitating the generation of more photoelectron (e−)-hole (h+) pairs upon light irradiation and enabling smoother carrier migration compared to UP-2’s aminal structure. The work anticipates contributing to advancements in the generation of “gren H2” through solar photocatalysis using synthetically accessible polymers.
Keywords
Hydrogen production
Photocatalysis
Schiff base
Water-soluble polymers
1. Introduction
Photocatalytic hydrogen production is broadly acknowledged as an environmentally friendly and sustainable method for converting solar energy into chemical energy and obtaining “gren hydrogen”, with photocatalysts serving as pivotal components in this process [1-4]. In contrast to conventional semiconductor-based photocatalytic systems, polymeric photocatalysts represented by graphitic carbon nitride architectures have gained prominence owing to their exceptional structural tunability and modifiable electronic band structures [5,6]. Recent advances have highlighted novel conjugated polymer systems and crystalline covalent organic frameworks, featuring precisely engineered linear or cross-linked configurations, as next-generation photocatalysts for solar-driven hydrogen evolution. These materials demonstrate superior photocatalytic water splitting performance primarily attributed to their extended π-conjugation networks, customizable chemical functionalities, and intrinsic nanoporous architectures that facilitate charge carrier transport and surface redox processes [7,8].
It should be noted that most organically conjugated polymers experience a few problems, such as short-haul exciton diffusion and low charge mobility, which lead to fast hole-electron recombination and a poor photocatalytic performance [9]. Therefore, it is necessary to exploit organic photocatalysts that exhibit excellent dispersibility or solubility in water, as this can minimize the distance that photo-driven carriers must travel to reach the solid-liquid interface. This effectively reduces the likelihood of exciton recombination within the polymers [10]. To improve water dispersibility of conjugated polymers, some strategies including soluble support substrates [11], surface functionalization [10], conjugated polyelectrolytes [9], et al can be employed.
Schiff-base polymers (SBPs), known for their cost-effectiveness and the abundance of their building blocks, have garnered significant interest since their initial synthesis by Schwab et al. in 2009 [12]. SBPs have been found diverse applications in areas such as: (i) gas storage and separation [13,14] because they are lightweight, highly thermally stable and exhibit permanent porosity; (ii) catalysis because of their suitable energy gap, porosity, crystalline nature and structural regularity [15,16]; (iii) sensing [17] due to fine tuning of different characteristics of the SBPs such as the pore size, the ionization potential, the electron affinity; and (v) energy storage due to their high-power density, exceptional cycle life, and low maintenance cost [18]. A notable characteristic of SBPs is their ability to readily form π-interactions and hydrogen bonds with aromatic heterocycle-containing compounds, demonstrating a strong affinity for such substances [15]. While numerous photocatalysts derived from highly porous materials have been utilized as photosensitizers and supports for active nanoparticle loading, thereby boosting catalytic activity [19], many of these materials suffer from limited photocatalytic efficiency due to issues with dispersibility and/or stability.
The Schiff base reaction mechanism, extensively studied and characterized, involves multiple reversible steps that form a well-documented pathway [20-23]. Notably, the dynamic covalent chemistry inherent in imine bond formation has enabled the assembly of complex architectures such as macrocyclic ligands and mechanically interlocked molecular systems [24]. Additionally, under optimized reaction conditions, the C=N double bond in imines becomes susceptible to nucleophilic attack by primary amines, ultimately yielding aminal derivatives through this cascade process.
Most SBPs were synthesized through solvothermal reactions, a process that necessitates severe conditions, including reaction in a sealed tube under an inert environment, as well as extended reaction durations. Herein, simple linear polymers based on Schiff-base chemistry were prepared through a facile hydrothermal process without any additives, evolving p-phthalaldehyde (PPA) and urea (UA) as precursors. The comparisons between the used UP-1/UP-2 and other SBP photocatalysts were presented in Table S1 (Supporting Information). The polymer demonstrates the ability to photocatalyze H2 production with triethanolamine (TEOA) as an electronic donor and Pt as a co-catalyst. The effects of different structures of polymers on photocatalytic H2 evolution were discussed. It is thought that the imine (–C=N–) structure of UP-1 has a good conjugated system, which could generate more photoinduced electron-hole pairs under light excitation, and the charge migration is also more facile, compared with the aminal structure (–N–C–N–) of UP-2. It is anticipated that this linear polymer photocatalyst will play a role in advancing and implementing photocatalytic technology.
2. Materials and Methods
2.1. Polymers synthesis
SBPs were produced using a hydrothermal process, utilizing UA and PPA as starting materials. Typically, 6.0 mmol of UA was dissolved in 50.0 mL of distilled water in an 80°C water bath. With continuous stirring, 6.0 mmol or 3.0 mmol of PPA was added to the solution. The UA and PPA were of analytical grade (purity >99%) and used without further purification. Subsequently, the reaction mixtures were transferred to a 100 mL Teflon-lined autoclave and heated in an oven at 180°C for 12 h. Once cooled, the mixtures were filtered, and the resulting filtrates were allowed to stand at room temperature (RT) for several days. After the water was volatilized, the pale-yellow solid products were obtained, which were recorded as UP-1 and UP-2, respectively. Here, the replicated number of independent synthesis experiments is four. The synthesis reactions have been presented in Scheme 1.
- Hydrothermal synthesis of UP-1 and UP-2.
2.2. H2 production reaction
The experiment for photocatalytic hydrogen production was conducted in a Pyrex cell maintained at RT. The cell was loaded with 100 mg of polymers, 78.0 mL of a solution containing TEOA and H2PtCl6. The mixture was then dispersed by sonication for 5 min to create a uniform solution. Prior to irradiation, nitrogen gas was bubbled through the cell for 25 min to purge the air. A 300W xenon lamp served as the light source. The produced H2 was quantified using a gas chromatograph equipped with a thermal conductivity detector and a molecular sieve column. The replicated number of hydrogen production experiments is three, and the relative error is ∼6%.
3. Results and Discussion
3.1. Characterization of the polymers
In the polymer’s synthesis, the stoichiometric ratios of UA:PPA = 1:1, and 2:1 were used to obtain UP-1 (imine structure) and UP-2 (aminal structure). When the ratio is higher than 2:1 (such as 3:1), the same aminal structural product UP-2 is obtained. When the ratio is lower than 1:1 (such as 0.5:1), the same imine structural product UP-1 is obtained, but with obviously reduced yields. The concentration of UA is selected as 0.12 mol/L, because a higher concentration of UA means required more PPA but the water solubility of PPA is limited. A lower concentration of UA means a reduced yield of SBP. The optimized reaction temperature and time is 180°C and 12 h, respectively. When the reaction temperature is lower than 180°C (such as 160°C) or the reaction time is shorter than 12 h, only a few products are obtained. However, if the reaction temperature is higher than 180°C (such as 200°C) or the reaction time is longer than 12 h, the obtained products have the same structures and similar photocatalytic H2 production properties compared with UP-1 and UP-2.
The Fourier-transform infrared (FTIR) spectra of two monomers (PPA and UA), UP-1, and UP-2, have been shown in Figure 1(a). The raw PPA showed a stretching peak of the C=O from the -CHO group at 1694 cm−1, and stretching peaks of the C-H in the -CHO group at 2757, 2807, and 2866 cm−1. Pure UA showed a stretching of the amide C=O (amide I peak) at 1682 cm−1, a scissoring vibration peak of the amide -NH2 (amide II peak) at 1624 cm−1, and a stretching vibration peak of the amide C-N at 1457 cm−1. The wide peaks of 3211, 3352, and 3487 cm−1 could be attributed to the stretching peak of the amino (-NH2) group. In the products UP-1 and UP-2, the -CHO group’s C-H peak from PPA disappears between 2750 and 2870 cm−1, and the -NH2 peak from UA also disappears between 3200 and 3500 cm−1, indicating that these groups have participated in the Schiff base reaction. The UP-1 and UP-2 still showed a peak of the C=O, indicating that this structure was still retained in the products (from UA). The peak of 1610 cm−1 in UP-1 was ascribed to the imine (-C=N-) group [25], indicating that a Schiff base structure may have been formed.
- (a) FTIR spectra of UP-1, UP-2, UA and PPA, and (b) PXRD patterns of UP-1, and UP-2.
Nevertheless, UP-2 exhibited no such peak, suggesting that the formed -C=N- may have undergone further reaction with UA to produce an aminal (-C-(NH-)2) structure. However, identifying the characteristic C-N peak of this structure is challenging due to the interference from the overlapping C-N signals originating from UA.
As shown in Figure 1(b), UP-1 and UP-2 exhibit X-ray diffraction (XRD) peaks at 2θ=10.26°, 18.27°, 20.18°, 24.59°, 27.02°, and 28.93°, the peak positions of the UP-1 and UP-2 are roughly identical. The powder-XRD (PXRD) result confirms that both materials exhibit a degree of crystallinity.
The ultraviolet-visible (UV) absorption spectra of UP-1, UP-2, UA, and PPA are shown in Figure 2. It can be found that UA has no absorption peak in the whole UV-visible light region, while PPA has an absorption band at 260 nm in the UV region. This absorption band belongs to the B band, which is produced by the vibration of the benzene ring and the π→π* transition. The reaction products (UP-1, UP-2) have an absorption peak at 257 nm, and UP-1 has a higher absorbance. Compared with PPA, the UV absorption peaks of UP-1 and UP-2 have a 3 nm blue shift, which is attributed to the variation in the electronic circumstance around the benzene ring after forming the polymer.
- UV absorption spectra of UP-1, UP-2, UA, and PPA in aqueous solution (1.0 g/L).
The surface chemical composition and bonding configurations of UP-1 and UP-2 were characterized through X-ray Photoelectron Spectroscopy (XPS). As evidenced by the survey spectra in Figure 3(a), both materials demonstrate characteristic photoelectron peaks corresponding to carbon, nitrogen, and oxygen elements, confirming their shared elemental constituents at the surface level. Furthermore, the C 1s spectra depicted in Figure 3(b) can be deconvoluted into three peaks centered at 284.7 eV, 285.5 eV, and 288.4 eV, corresponding to C atoms originating from benzene rings (–C–C, –C=C), imine (–C=N–) or aminal (N–C–N) structures, and carbonyl groups (–C=O), respectively, as reported by Lin et al. [26] and Yang et al. [27]. The N 1s (Figure 3c) peak can be fitted into one peak at 399.2 eV for UP-1 and 399.4 eV for UP-2. The binding energy of N 1s peak in UP-2 is higher than that of UP-1, which may be because of their different linking styles, which are C–N–C and =C=N-, respectively [28].
- XPS spectra (a) total survey, (b) C1s, and (c) N1s of UP-1 and UP-2.
Thermogravimetric (TG) analysis was conducted to assess the thermal stability of UP-1 and UP-2. Figure 4 displays the TG-DTG curves for both materials. The weight loss observed can be categorized into three distinct stages: the first occurring from RT to 138°C, attributed to the desorption of absorbed water molecules; the second spanning from 140 to 295°C, corresponding to the removal of residual UA and PPA; and the third, ranging from 300 to 480°C, representing the decomposition of UP-1 and UP-2. The DTG peak of the decomposition of UP-1 is 386°C, which is higher than that of UP-2 (369°C), indicating that the imine structure is more stable than the aminal structure.
- TG and DTG curves of (a) UP-1 and (b) UP-2.
The nuclear magnetic resonance 13C (NMR) spectra of UP-1 and UP-2 (DMSO as a solvent) have been given in Figure 5. In UP-1, the typical signal at 150 ppm indicates the formation of imine bonds (–C=N–) via the condensation of UA and PPA [29]. The resonances at 126-135, 167, and 193 ppm may be assigned to the C atoms in the benzene ring, and amide (-(O=C)-NH2) and aldehyde groups (-CHO), respectively. In UP-2, except of the resonances at 126-135, 167, and 193 ppm, the resonance observed at 173 ppm can be attributed to the C atoms present in the imide group (-(O=C)-NH-), while the resonance approximately at 63 ppm may correspond to the C atoms in the aminal structure (N–C–N) [30]. The 13C NMR spectra further proved that UP-1 mainly consists of imine (–C=N–) structure, whereas UP-2 mainly consists of the aminal structure (N–C–N).
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13C NMR spectra of UP-1 and UP-2.
3.2. Hydrogen evolution properties
The photocatalytic hydrogen production performances of UP-1 and UP-2 were investigated under RT, utilizing Pt as a cocatalyst and TEOA as an electronic donor. As illustrated in Figure 6(a), both UP-1 and UP-2 individually exhibited very limited hydrogen evolution activity in the absence of co-catalysts. The photocatalytic performance demonstrates significant enhancement when employing in situ photoreduction-generated platinum as a cocatalyst (denoted as UP-1@Pt and UP-2@Pt systems). Experimental measurements revealed hydrogen evolution rates of 7.60 and 4.89 mL·h⁻1·g⁻1 for UP-1@Pt and UP-2@Pt, respectively. The hydrogen evolution activity of UP-1 and UP-2 can be comparable to some reported SBPs (see Table S1 in Supporting Information), indicating their immense potential for solar water splitting in future applications. This activity disparity primarily arises from structural differences between the frameworks: The imine linkage in UP-1 facilitates an extended conjugated system, enabling both enhanced e⁻-h+ pair generation under illumination and superior charge transfer kinetics compared to the aminal configuration in UP-2. Crucially, control experiments confirmed the photochemical nature of this process, with no detectable hydrogen production observed under dark conditions. No appreciable H2 was detected in the system without sacrificial donors, which suggested that a sacrificial donor is indispensable for photocatalytic H2 evolution. As revealed in Figure 6(b), the study examined the effects of different electronic donors, namely TEOA, trimethylamine (TMA), triethylamine (TEA), and methanol (MeOH), on the UP-1 hydrogen production activity. The results indicated that the hydrogen production activity decreases in the following order: TEOA, TMA, TEA, and MeOH. Consequently, TEOA, which is a commonly utilized electronic donor, was chosen for this work. The dependence of hydrogen production activity on TEOA content is presented in Figure S1(a) (Supporting Information). As TEOA content increases from 1 v% to 10 v%, the activity of UP-1 improves from 4.23 to 7.60 mL·h−1·g−1. As the concentration of the electronic donor increases, the clearing rate of photo-driven holes accelerates, resulting in prohibited hole/electron recombination and enhanced photocatalytic activity. When TEOA content increases to 20 v%, the activity of UP-1 is almost unchanged, so 10 v% TEOA was selected in this work. Figure 6(c) illustrates the effect of the initial concentration of H2PtCl6 on hydrogen production activity. Initially, the activity rises with increasing H2PtCl6 concentration, but subsequently decreases, with peak activity observed at 0.5 × 10-4 M. As the concentration of H2PtCl6 increases from 0 to 0.5 × 10-4 M, there is a corresponding increase in the number of Pt0 active sites available for the hydrogen production reaction, leading to an improved rate of hydrogen production. However, an excessive amount of Pt0 particles can obstruct incoming photons, thereby decreasing photoactivity. The impact of catalyst concentrations on hydrogen production activity has been illustrated in Figure S1(b) of the Supporting Information. Notably, no hydrogen was detected in the absence of UP-1. When the concentration of UP-1 is raised from 0.625 to 1.25 g·L−1, the hydrogen evolution rate increases from 5.0 to 7.60 mL·h−1·g−1. Nevertheless, further increasing the UP-1 concentration to 2.50 g·L−1 results in only a minor change in activity, prompting the selection of 1.25 g·L−1 as the optimal UP-1 concentration. The time-dependent profiles of photocatalytic hydrogen evolution for UP-1@Pt and UP-2@Pt are depicted in Figure 6(d). As the irradiation time elongates, the amount of hydrogen evolved increases; the amount of hydrogen evolution during the 2nd cycle is only ∼ half of that of the initial cycle (Figure S2a in Supporting Information). To understand the reasons for reduced photoactivity, the XRD pattern of UP-1 after photocatalytic reaction was recorded (Figure S2b). XRD results indicate that MP-1 lost its crystallinity after photocatalytic reaction; similar phenomena have been found by B.V. Lotsch [20] and our previous work [29].
- (a) Photocatalytic H2 evolution rates of various systems, (b) Dependences of H2 evolution activity of UP-1 on hole scavenger, (c) Influence of initial H2PtCl6 concentration on H2 evolution activity of UP-1, and (d) Time courses of H2 production from UP-1@Pt and UP-2@Pt.
In order to understand the photoinduced H2 production phenomena of UP-1/UP-2 and their differences in H2-evolution activities. Electrochemical cyclic voltammetry (CV), steady-state photoluminescence (PL), and time-resolved photoluminescence (TRPL) decay spectra were conducted. The energy band structures of UP-1 and UP-2 were measured by the CV method. As shown in Figure 7, the reduction potential of UP-1 and UP-2 is −0.64 and −0.61 V (vs. NHE), respectively, since the potential of H+/H2 is −0.53 V at pH = 9 (the pH value of the photocatalytic system, including TEOA, is about 9). Thus, in thermodynamics, the UP-1 and UP-2 can reduce H+ into H2 [31]. Furthermore, UP-1 exhibits a more negative reduction potential compared to UP-2, suggesting that the thermodynamic driving force for the H2-evolution reaction is greater for UP-1 than for UP-2, thereby resulting in UP-1. Possessing a higher activity than UP-2. Besides, it was thought that the conjugated structures in UP-1 are favorable for the separation and diffusion of photoexcited charge-carriers during photocatalysis, which is similar to the case of conjugated polymer (PMTPA) with melamine and PPA precursors [32], thereby resulting in UP-1 possessing a higher activity than UP-2. This inference was supported by PL and TRPL spectra. As illustrated in Figure S3 (Supporting Information), under 327 nm excitation, UP-1 and UP-2 polymers exhibit distinct fluorescence emission peaks centered at 466 nm and 475 nm, respectively. This emission behavior is widely attributed to radiative recombination of photogenerated e−-h+ pairs in conjugated polymer systems [33]. Notably, UP-1 demonstrates significantly stronger fluorescence intensity compared to UP-2, suggesting enhanced generation of photo-induced charge carriers (e− and h+) in UP-1 during light excitation. The TRPL decay spectra (Figure S4) showed that the average lifetime of UP-1 is 1.53 ns, longer than that of UP-2 (1.39 ns), indicating the improved separation efficiency of photoexcited electron-hole pairs, possibly due to its conjugated imine structure. This improved charge carrier generation and prolonged photoelectron lifetime directly correlate with UP-1’s superior photocatalytic hydrogen evolution performance, as the increased population of free charges facilitates more efficient redox reactions at catalytic sites.
- CV curves of UP-1 and UP-2.
4. Conclusions
In conclusion, simple linear Schiff base polymers UP-1 and UP-2 were prepared by a one-step hydrothermal method with UA and PPA as starting materials. The synthesized polymers were characterized by FTIR, XRD, XPS, 13C-NMR, UV, TG analysis, CV, PL, and TRPL. The photo-induced hydrogen production performances of UP-1 and UP-2 were evaluated using Pt as a cocatalyst and TEOA as an electronic donor. Notably, UP-1 exhibited a hydrogen production activity of 7.60 mL·h−1·g−1, significantly exceeding that of UP-2, which was 4.89 mL·h−1·g−1. This is attributed to the favorable conjugated system of the imine structure in UP-1, which promotes the generation of more e−-h+ pairs upon light excitation and facilitates the carrier transfer compared to UP-2 with an aminal structure.
Acknowledgment
This work was supported by the Scientific Research Fund of Hunan Provincial Education Department (No. 21B0632) and the Foundation of Key Laboratory of Functional Metal-Organic Compounds of Hunan Province of Hengyang Normal University (No.2023HSKFJJ008).
CRediT authorship contribution statement
Xing Liu: Methodology, Data curation, Writing-Original draft preparation, Writing-Reviewing and Editing, Funding acquisition; Bingyan Liu and Yunyan Wu: Investigation, Validation, Writing- Original draft preparation; Longxin Hu: Formal analysis, Visualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data will be made available on request.
Declaration of Generative AI and AI-assisted technologies in the writing process
The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.
Supplementary data
Supplementary material to this article can be found online at https://dx.doi.org/10.25259/AJC_136_2025
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