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Article

A Heterostructure Photoelectrode Based on Two-Dimensional Covalent Organic Framework Film Decorated TiO2 Nanotube Arrays for Enhanced Photoelectrochemical Hydrogen Generation

by
Yue Zhang
,
Yujie Li
,
Jing Yu
,
Bing Sun
* and
Hong Shang
*
School of Science, China University of Geosciences (Beijing), Beijing 100083, China
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(2), 822; https://doi.org/10.3390/molecules28020822
Submission received: 28 December 2022 / Revised: 9 January 2023 / Accepted: 12 January 2023 / Published: 13 January 2023
(This article belongs to the Special Issue Chemical Functionalization of Two-Dimensional Materials)
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Abstract

:
The well-defined heterostructure of the photocathode is desirable for photoelectrochemically producing hydrogen from aqueous solutions. Herein, enhanced heterostructures were fabricated based on typical stable covalent organic framework (TpPa-1) films and TiO2 nanotube arrays (NTAs) as a proof-of-concept model to tune the photoelectrochemical (PEC) hydrogen generation by tailoring the photoelectrode microstructure and interfacial charge transport. Ultrathin TpPa-1 films were uniformly grown on the surface of TiO2 NTAs via a solvothermal condensation of building blocks by tuning the monomer concentration. The Pt1@TpPa-1/TiO2-NTAs photoelectrode with single-atom Pt1 as a co-catalyst demonstrated improved visible-light response, enhanced photoconductance, lower onset potential, and decreased Tafel slope value for hydrogen evolution. The hydrogen evolution rate of the Pt1@TpPa-1/TiO2-NTAs photoelectrode was five times that of Pt1@TpPa-1 under AM 1.5 simulated sunlight irradiation and the bias voltage of 0 V. A lower overpotential was recorded as 77 mV@10 mA cm−2 and a higher photocurrent density as 1.63 mA cm−2. The hydrogen evolution performance of Pt1@TpPa-1/TiO2-NTAs photoelectrodes may benefit from the well-matched band structures, effective charge separation, lower interfacial resistance, abundant interfacial microstructural sites, and surficial hydrophilicity. This work may raise a promising way to design an efficient PEC system for hydrogen evolution by tuning well-defined heterojunctions and interfacial microstructures.

1. Introduction

Hydrogen is regarded as a renewable, eco-friendly, and high-density energy source and has been attractive in the fields of both science and engineering to address the crises of energy shortage and environmental pollution [1,2]. Benefiting from the inexhaustible solar energy and plentiful water, solar-driven hydrogen evolution from water is regarded as a potential candidate to generate storable fuels for the future sustainable energy strategy without the carbon footprint [3]. A designable photocatalyst is the key component for converting solar energy to produce hydrogen with desirable efficiency [4,5,6,7]. Recently, two-dimensional covalent organic frameworks (2D COFs), as well as metal-organic frameworks (MOFs), have emerged as promising materials in hydrogen evolution due to their predictable structures, tunable π-conjugation, high crystallinity, and permanent pore channels [8,9,10,11,12]. The precisely tailored 2D COFs provide a platform to illustrate the structure–property relationships during the hydrogen evolution reaction. Various strategies have been developed to enhance the light-harvesting rate and charge separation efficiencies, such as band-gap engineering [13,14], donor–acceptor system fabrication [15,16,17,18], and linkage construction [19]. Their high crystallinity further facilitates rapid charge diffusion, ensuring optimal charge percolation. The abundant heteroatoms and high specific surface area of the frameworks provide substantial active interfaces for loading suitable co-catalysts and the reactant accessibility for catalyzing redox reactions [20]. The tailored coordinating microenvironments of 2D COFs benefit the immobilization of single-atom noble metals to reduce the overpotential, promoting the hydrogen evolution procedure [21]. Furthermore, the pre-designed heterostructures and nanocomposites were explored with further-improved separation efficiency of the photogenerated charge carriers by combining 2D COFs with other motifs such as inorganic semiconductors [22,23,24,25,26], metal-organic frameworks [27], 2D materials [28,29], etc. However, most of the 2D COF-based photocatalysts were suspended in reacting systems for hydrogen evolution, suffering from challenges of poor powder dispersibility, recycling inconvenience, and discontinuous production of hydrogen.
Photoelectrochemical (PEC) catalysis provides another cost-effective and promising route for converting water into sustainable hydrogen under solar light irradiation, which is apt to overcome the aforementioned drawbacks of photocatalysts [30]. Integrating 2D COF-based photocatalysts into PEC cells was primarily performed by coating a suspended COF slurry in organic solvents onto substrate electrodes [31,32,33]. On the other hand, oriented films of conjugated 2D COFs grown on transparent conducting fluorine-doped tin oxide and indium tin oxides were also reported as photocathodes for generating hydrogen [34]. Recently, solution-processed COF nanoplates were proposed to construct centimeter-scale-homogeneous COF films for fabricating a complex cascading photoelectrode for PEC hydrogen evolution [35]. Although these efforts have been made, constructing the 2D COF-based PEC system for hydrogen generation remains in the early stages of development.
Inspired by the advanced features of 2D COF-based photocatalysts and the desirable architecture of the PEC system for hydrogen evolution, we proposed the in situ fabrication of single-atom Pt (Pt1) loaded TpPa-1 films on TiO2 nanotube arrays (NTAs)/Ti foil (Pt1@TpPa-1/TiO2-NTAs photoelectrode, as shown in Figure 1) for PEC hydrogen evolution. Here, the cost-effectively anodized TiO2 NTAs were employed as substrates for growing COF films due to the stable and fantastic PEC performance for solar-driven water splitting [22,23]. The stable and ultrathin TpPa-1 COF films were chemically fabricated on the anodized TiO2 NTAs to prove the concept of conveniently integrating effective charge transfer heterostructure and interfacial catalytic sites in a single photoelectrode for PEC hydrogen evolution. By integrating COF-based heterostructures into the photoelectrode, the photoelectrode could inherit the cascading charge separation from COF-based heterostructures and address the challenges of poor powder dispersibility, recycling inconvenience, and discontinuous production of hydrogen in the application of COF-based photocatalysts. The surficial microstructures of TiO2 NTAs and the thickness of TpPa-1 films were also discussed as ways of impacting the PEC hydrogen evolution performance. The spectral investigation indicated the extended absorption, enhanced photocurrent density, and reduced interfacial resistance of TpPa-1/TiO2-NTAs heterostructures. The Pt1@TpPa-1/TiO2-NTAs photoelectrode demonstrated lower overpotential, higher current density, and enhanced PEC hydrogen generating performance. This work may shed light on the design and fabrication of heterostructure-based photoelectrode for PEC hydrogen evolution. The convenient fabrication of Pt1@TpPa-1/TiO2-NTAs photoelectrode would also provide a facile and potential way of improving their industrial applications.

2. Results and Discussion

2.1. Fabrication and Characterization of Pt1@TpPa-1/TiO2-NTAs Photoelectrode

The Pt1@TpPa-1/TiO2-NTAs photoelectrode is fabricated starting from the synthesis of TiO2 NTAs via the anodization-annealing method. The morphology and crystalline phase of the obtained TiO2-NTAs are confirmed by SEM observation and XRD analysis. The typical discrete nanotubes are obtained with an average diameter of 40.55 ± 8.46 nm after the anodization time of 0.25 h (Figure 2a). Few nanoparticles (40–50 nm) distribute on the surface of TiO2 NTAs. More nanoparticles emerge when the anodization time is 0.5 h, and tend to fuse covering on the top of nanotubes with the enlarged diameter (~69.60 nm, Figure S1a). The surficial nanoparticles grow larger at a longer anodization time forming a flat film (1 h or 1.5 h, Figure S1b,c). The underneath nanotubes collapse as shown in the edges. Only nanoparticle aggregates are found on SEM images as the anodization time is 2 h. According to the XRD patterns (Figure S2), these TiO2 nanostructures reveal a typical anatase phase matching well with that of JCPDS No. 21-1272 coupling with some diffraction peaks of Ti foil substrate (JCPDS No. 44-1294). The diffraction intensity of the TiO2 NTAs is weak at the anodization time of 0.25 h indicating the thinner oxidization layer with lower crystallinity and plenty of defects, and then gradually increases as prolonging the anodization times. UV–vis spectra demonstrate the common absorption of formed TiO2 nanostructures to ultraviolet light (Figure S3), while the absorption band and edge show a blue shift indicating the increased band gap as prolonging the anodization times. The highest photocurrent response and lowest impedance resistance are also obtained at the anodization time of 0.25 h (Figures S4 and S5). The longer the anodization time, the lower the photocurrent density and larger impedance resistance. The outstanding PEC performance of TiO2 NTAs at the anodization time of 0.25 h may be attributed to the smaller band-gap, higher absorbance to light irradiation, regular nanotube morphology for the mass delivery and thin oxidized TiO2 layers for charge transfer with Ti substrate electrode. Considering all the above results, the TiO2 NTAs obtained at the anodization time of 0.25 h are selected for further investigation.
TpPa-1 COF films are constructed on the surface of amino-modified TiO2 NTAs via the solvothermal polymerization of monomers with various concentrations. The surface of TpPa-1/TiO2-NTAs obtained with 0.2 mM Tp and 0.3 mM Pa reveals similar morphology and nanotube size compared to that of TiO2 NTAs (Figure 2b). This indicates that the TpPa-1 COF films tightly grow along the top and inner surface of TiO2 NTAs, forming extensively-spreading TpPa-1/TiO2-NTAs heterostructures. The TiO2 NTAs can be still clearly observed when the concentration of Tp is increased to 1.5 mM (2.25 mM for Pa). The open nanotube would facilitate the mass transfer combined with the high surface area and porous structures of TpPa-1 films. However, the nanotube diameter gradually shrinks (Figure S6). The TpPa-1 films grow continuously over the whole TiO2 NTAs surface, filling the nanotubes of TiO2 structures and further increasing the monomer concentrations. No typical diffraction peaks of TpPa-1 films are detected in the XRD patterns of TpPa-1/TiO2-NTAs (Figure S7) attributed to the ultrathin layer on TiO2 NTAs with the lower monomer concentration.
The chemical structures of TpPa-1 films are verified by FTIR and XPS spectra. As shown in Figure 2e, the stretching modes of N−H bonds in Pa (3100~3400 cm−1) and aldehyde C=O stretching bands of Tp at 1635 cm−1 disappear in the spectra of both TpPa-1 powders and TpPa-1/TiO2-NTAs. This result indicates the total consumption of monomers during the formation of TpPa-1 films. No characteristic hydroxyl (O−H) and imine (C=N) stretching vibration modes are recorded in TpPa-1 FTIR spectra, while a broadening shoulder band at 1610 cm−1 emerges assigned to the keto C=O in TpPa-1 films due to the extended conjugation and strong intramolecular hydrogen bonds [36]. A strong peak around 1582 cm−1 arises from the C=C stretching vibration mode of the formed keto configuration. TpPa-1 films synthesized from higher monomer concentrations demonstrate similar FTIR spectra with the same stretching modes (Figure S8). These results reveal the formation of TpPa-1 films on TiO2 NTAs, which is also confirmed by the element distribution of SEM (Figure 2d). Remarkable carbon and nitrogen elements locate around the TiO2 NTAs, profiling the nanotube apertures compared to the homogeneously distributed Ti and oxygen elements. The formation of TpPa-1 films is also verified by the XPS survey spectra (Figure 2f). The assignments of C1s, N1s, and O1s can well match the chemical structure of TpPa-1 COF (Figure 2g,h and Figure S9). The weak C1s and N1s peaks compared to Ti2p and O1s indicate the thinner TpPa-1 films formed on TiO2 NTAs.
The Pt1@TpPa-1/TiO2-NTAs photoelectrode is finally constructed by photo-depositing Pt species to a TpPa-1/TiO2-NTAs photoelectrode. The photo-deposited Pt does not change the morphology of TpPa-1/TiO2-NTAs (Figure 2c) and uniformly distributes on TpPa-1/TiO2-NTAs based on the Pt element mapping (Figure 2d). The typical Pt4f and Pt4d bands are indexed from the survey spectrum of Pt1@TpPa-1/TiO2-NTAs (Figure 2f). The high-resolution spectra of Pt4f cores are assigned to two bands at 74.91 eV for Pt(IV)4f7/2 and 78.25 eV for Pt(IV)4f5/2 (Figure 2i). The other two peaks are located at 72.83 eV and 76.24 eV attributed to the Pt4f7/2 and Pt4f5/2 of single-atom Pt (denoted as Pt1), respectively, according to the previous work [21]. No Pt clusters or nanoparticles are formed during the photo-deposition based on the above results. The N1s core can be assigned to the formed C−N band at 399.18 eV and protonated N at 401.37 eV, which slightly shifts to a higher binding energy due to the coordination of Pt1 with N and O moieties (Figure 2h). All the above results indicate the successful construction of the Pt1@TpPa-1/TiO2-NTAs photoelectrode.
The interfacial hydrophilicity of the Pt1@TpPa-1/TiO2-NTAs photoelectrode is an important factor in effectively producing hydrogen from water. The contact angle slightly increases to 35° in the case of TpPa-1/TiO2-NTAs from 27° of TiO2 NTAs and recovers to 26° of Pt1@TpPa-1/TiO2-NTAs (Figure S10). The hydrophilicity of Pt1@TpPa-1/TiO2-NTAs photoelectrodes may be attributed to the polarized keto form and protonated linkages of thinner TpPa-1 films, benefiting the higher affinity to water and protons leading to excellent hydrogen evolution efficiency [37,38]. Additionally, the TpPa-1/TiO2-NTAs photoelectrode demonstrates higher thermal stability based on the thermogravimetric analysis (Figure S11). These characteristics would promise the potential performance of the Pt1@TpPa-1/TiO2-NTAs photoelectrode.

2.2. Photoelectrochemical Performance of Pt1@TpPa-1/TiO2-NTAs Photoelectrodes

The absorption of TpPa-1/TiO2-NTAs photoelectrodes to light irradiation is evaluated by the UV–vis absorption spectra (Figure 3a). The TpPa-1/TiO2-NTAs show a broad absorption band covering ultraviolet and most visible light spectrum with an absorption edge of 580 nm, which is inherited from the excellent absorption nature of TpPa-1. The optical band gaps of TiO2 NTAs and TpPa-1 are evaluated as 3.17 eV and 2.02 eV, respectively, via the Kubelka-Munk equation based on the absorption spectra (Figure 3b). When the monomer concentrations are increased, the obtained TpPa-1/TiO2-NTAs demonstrate similar absorption characteristics (Figure S12).
The PEC performance of the Pt1@TpPa-1/TiO2-NTAs photoelectrode is further evaluated. As shown in Figure 3c, TpPa-1/TiO2-NTAs (0.2 mM for Tp) demonstrate enhanced photocurrent density (1.14 mA cm−2) compared to TiO2-NTAs (0.92 mA cm−2) under the full-range light irradiation, implying the improved photo-induced charge separation between the TpPa-1/TiO2-NTAs heterostructures. The effective charge separation may also be attributed to the enhanced photoconductance of TpPa-1/TiO2-NTAs as revealed by EIS spectra (Figure 3d and Figure S14). The resistance of TpPa-1/TiO2-NTAs is estimated as below 1 kΩ, which is smaller than that of TiO2 NTAs (>1.8 kΩ). However, the TpPa-1/TiO2-NTAs synthesized at the higher monomer concentrations reveal the remarkably decreased photocurrent density (Figure S13), which may originate from the increased interfacial resistance for charge transfer due to the inherent low conductivity of TpPa-1 (Figure S14). Loading Pt1 to TpPa-1/TiO2-NTAs further leads to enhanced photocurrent density (1.63 mA cm−2) and lower interfacial resistance (242 Ω). The photocurrent density is higher than the reported counterpart in previous works.[27,28,29,30] Furthermore, a steady photocurrent density is recorded after two light-on/off rounds for 3 h (Figure S15), indicating the highly stable PEC response of the Pt1@TpPa-1/TiO2-NTAs photoelectrode. The excellent PEC performance of the Pt1@TpPa-1/TiO2-NTAs photoelectrode would benefit its application for hydrogen evolution.

2.3. Photoelectrochemical Hydrogen Evolution Based on Pt1@TpPa-1/TiO2-NTAs Photoelectrode

The PEC hydrogen evolution performance of the Pt1@TpPa-1/TiO2-NTAs photoelectrode is investigated in a PEC cell under light irradiation. LSV measurements reveal that the TpPa-1/TiO2-NTAs photoelectrode exhibits lower potential for proton reduction compared to TiO2-NTAs under light irradiation; this is attributed to the effective charge separation in the TpPa-1/TiO2-NTAs heterostructures (Figure 4a). With the higher monomer concentrations, the TpPa-1/TiO2-NTAs photoelectrodes demonstrate the increased reduction potential and decayed current density (Figure S16). A thinner TpPa-1 film seems desirable to fabricate on TiO2 NTAs for enhanced PEC performance by descending the interfacial resistance. On the other hand, Pt1@TpPa-1/TiO2-NTAs show a lower reduction potential and higher current density, indicating the enhanced interfacial catalytic property of photoelectrodes with Pt1 loaded. The overpotentials of Pt1@TpPa-1/TiO2-NTAs photoelectrodes are evaluated as 77 mV@10 mA cm−2 and 180 mV@50 mA cm−2, which are significantly lower than that of TpPa-1/TiO2-NTAs and TiO2-NTAs (Figure 4b). The brilliant PEC catalytic performance of Pt1@TpPa-1/TiO2-NTAs photoelectrode for proton reduction is also confirmed by its smaller slope value via Tafel plotting (Figure 4c). Therefore, the extended light absorption, lower charge transfer resistance, robust interfacial catalytic ability, and hydrophilic surface of Pt1@TpPa-1/TiO2-NTAs photoelectrode promise its excellent PEC hydrogen evolution performance in aqueous solutions.
The hydrogen evolution capacity of the Pt1@TpPa-1/TiO2-NTAs photoelectrode is evaluated by quantitative analysis. As expected, the Pt1@TpPa-1/TiO2-NTAs photoelectrode shows a remarkably enhanced hydrogen-generating capacity under light irradiation compared to Pt1@TpPa-1 and Pt@TiO2-NTAs (Figure 4d). Almost no hydrogen generates in the case of Pt@TiO2-NTAs. The Pt1@TpPa-1/TiO2-NTAs obtained from 0.2 mM Tp and 0.3 mM Pa show superior capacity for hydrogen generation compared to other higher monomer concentrations (Figure 4f). These results are accordant with the PEC performance of Pt1@TpPa-1/TiO2-NTAs and TpPa-1/TiO2-NTAs photoelectrodes. The hydrogen evolution rate of Pt1@TpPa-1/TiO2-NTAs ascends when the initial amount of K2PtCl6 alters from 0.5 mg to 1 mg and keeps steady as Pt dosage increases further (Figure 4g). A dosage of 1 mg of K2PtCl6 (1 mL, 1 mg mL−1) is used for photo-depositing Pt1 onto TpPa-1/TiO2-NTAs. The effect of applied bias voltages on the hydrogen evolution of Pt1@TpPa-1/TiO2-NTAs is also investigated in the PEC cell setup. The positive bias voltage, applied via an electrochemical station, can slightly promote and stabilize the hydrogen evolution rate of Pt1@TpPa-1/TiO2-NTAs photoelectrodes, while an attenuate capacity is recorded under the inversed voltage (Figure 4h), which agrees with the transient photocurrent response (Figure S17). These results indicate that Pt1@TpPa-1/TiO2-NTAs photoelectrode can effectively produce hydrogen from aqueous solutions even at 0 V and a suitable bias voltage. Under the optimized conditions, the Pt1@TpPa-1/TiO2-NTAs photoelectrode can continuously generate hydrogen from aqueous solutions without a significantly-descended hydrogen evolution rate for four cycles. The hydrogen evolution rate of Pt1@TpPa-1/TiO2-NTAs is evaluated as 0.5 nmol h−1 cm−2, about 5 times that of Pt1@TpPa-1 (Figure 4e). The photoelectrode can be conveniently reused by just being activated in THF for 4 h, averting the collection of photocatalysts from reaction solutions. The steady hydrogen generating rate of 6 h for 4 cycles, coupled with the stable photocurrent density response, also implies the long lifetime of Pt1@TpPa-1/TiO2-NTAs photoelectrode. All these results indicate the stable and robust properties of Pt1@TpPa-1/TiO2-NTAs photoelectrode for PEC hydrogen evolution.

2.4. Photoelectrochemical Hydrogen Evolution Mechanism

To further understand the PEC hydrogen evolution process, the charge transfer in the TpPa-1/TiO2-NTAs heterostructures and interfacial reaction is illustrated combined with the PEC performance of Pt1@TpPa-1/TiO2-NTAs photoelectrode. The flat-band potentials of TiO2 NTAs and TpPa-1 are evaluated as −0.46 V and −0.92 V (vs. SCE), respectively, based on the Mott–Schottky curves (Figure S18). The band diagrams of TpPa-1/TiO2-NTAs heterostructures are demonstrated in Figure 5a, with their optical band gaps. The lowest unoccupied molecular orbital (LUMO) of TpPa-1 and conduct band of TiO2 NTAs are evaluated as −0.58 eV and −0.12 eV, respectively, which are available to generate hydrogen from water. Both TpPa-1 and TiO2 NTAs can be excited under full-range light irradiation. These band structures benefit the effective charge separation between TpPa-1/TiO2-NTAs heterostructures. The excited electrons in TpPa-1 can further transfer to TiO2 NTAs. The separated electrons further reduce protons to produce hydrogen with the Pt1 co-catalysts. Because of the difficulty in oxidizing water, a sacrificial electron donor (SA is used here) is usually added to PEC hydrogen production systems to regenerate the photosensitizer. The photogenerated holes in the TpPa-1 films and TiO2-NTAs can be captured by the sacrificial SA and electrons from the Ti substrate electrode. Ultrathin TpPa-1 films with regular pore size on TiO2-NTAs and the hydrophilicity of Pt1@TpPa-1/TiO2-NTAs photoelectrode are conducive to the penetration of protons, water, and ions to the reactive sites of the heterostructure surface. The highly specific surface area and lower crystallinity of TiO2-NTAs provide more anchoring surface vacancy sites for the adequate loading of single metal atoms with the further assistance of TpPa-1 coordinating sites, contributing to the higher atom utilization efficiency and hydrogen evolution on the surface of the Pt1@TpPa-1/TiO2-NTAs photoelectrode. Based on the hydrogen evolution performance of Pt1@TpPa-1/TiO2-NTAs photoelectrodes, the concept of integrating heterostructures into a single photoelectrode for PEC hydrogen evolution is verified. The synergistic effect of enhanced light absorption ability, improved charge separation efficiency, ultrathin mass transfer channels, abundant single-atom co-catalyst load, and good hydrophilicity facilitates the good hydrogen evolution performance of the Pt1@TpPa-1/TiO2-NTAs photoelectrode. The hydrogen evolution rate could be further enhanced by enlarging the photoelectrode area for promising industrial applications.

3. Materials and Methods

3.1. Fabrication of Pt1@TpPa-1/TiO2-NTAs Photoelectrode

The TiO2 NTAs were prepared by using a facile anodization method. All chemicals were used as received without any further purification. Typically, a Ti foil (Alfa Aesar, 99.8%, metal basis) with a thickness of 1 mm (10 mm × 35 mm) was cleaned by using the ultrasonic method in acetone, ethanol, and deionized water for 15 min in turn, and dried under nitrogen flow. The electrochemical anode oxidation was performed by employing Ti foil as the anode and a Pt foil (10 mm × 10 mm) as the counter electrode in a fresh ethylene glycol solution containing 0.1 wt% of NH4F (Sinopharm Chemical Reagent Co. Ltd., >96%) and 1 wt% H2O. A DC powder supplied the anodization at 60 V at various times. Then, the anodized Ti foil was annealed in a muffle furnace at 450 °C for 2 h to obtain the crystalline TiO2 NTAs on the Ti foil. The surface of the obtained TiO2 NTAs was further modified by dipping TiO2-NTAs/Ti foil in 15 mL toluene with 430 μL 3-aminopropyltrimethoxysilane (APTMS, Sigma-Aldrich, 97%) and refluxing at 80 °C for 6 h. The amino-modified TiO2-NTAs/Ti foil was rinsed with toluene and dried at 70 °C overnight.
The TpPa-1 films were constructed on the surface of TiO2 NTAs by using a solvothermal method, as shown in Figure 1. A precursor solution was prepared by suspending 1,3,5-triformylphloroglucinol (Tp, Innochem, 97%, 0.2 mM~2 mM) and p-phenylenediamine (Pa, Innochem, 97%, 0.3 mM~3 mM) in a vial containing 3 mL mesitylene, 3 mL dioxane, and 1 mL acetic acid solution (3 M, Acros Organics, >98%). The precursor solution was homogeneously mixed by ultrasonic treatment for 15 min and then transferred into a 25 mL Teflon autoclave container. A homemade scaffold was used to horizontally hold the TiO2-NTAs/Ti plate in the reacting solution, facing the bottom. The autoclave was sealed and heated at 120 °C for 72 h, growing TpPa-1 films on the TiO2 NTAs surface. The obtained TpPa-1/TiO2-NTAs/Ti plate was rinsed with tetrahydrofuran (THF), activated in THF for 24 h, and finally dried at 70 °C overnight under vacuum conditions. Pt loading was performed by dipping the TpPa-1/TiO2-NTAs/Ti plate in an H2PtCl6 (Sigma-Aldrich, ≥99.9%) aqueous solution (1 mg mL−1, 0.5 mL, 1 mL, 2 mL, and 4 mL) for 30 min under the irradiation of an Xe lamp (AM 1.5 simulated sunlight, 300 W, 100 mW cm−2) according to the previously reported in situ photo-deposition method [20]. The Pt1@TpPa-1/TiO2-NTAs/Ti plate was washed with deionized water and dried at 70 °C for further characterization and measurements.

3.2. Characterizations

The chemical structure of the as-prepared photoelectrode was verified by Fourier transform infrared (FTIR) spectra and X-ray photoelectron spectra (XPS). FTIR spectra were recorded in the range of 400~4000 cm−1 with an interval of 4 cm−1 on a PerkinElmer Frontier spectrophotometer in the attenuated total refraction (ATR) mode with an additional variable angle reflectance accessory under ambient conditions. XPS spectra were performed on a Thermo Fisher Scientific ESCALAB 250Xi spectroscope with Al Kα radiation (photon energy 1253.6 eV) as the exciting source at a working voltage of 12.5 kV. The crystalline features of the samples were characterized by using a Bruker D8 Advanced X-ray diffractometer with Cu Kα radiation (λ = 1.5416 Å), operated at 40 kV and 40 mA ranging from 1.5 to 80° with a speed of 2° min−1. The microstructures and element distribution of samples were observed on a JEOL JSM-7900F scanning electron microscope (SEM).

3.3. Photoelectrochemical Performance and Hydrogen Evolution

The absorption performance of the Pt1@TpPa-1/TiO2-NTAs photoelectrode was recorded on a PerkinElmer Lambda 750 UV–vis scanning spectrophotometer in diffuse reflection mode with an integrating-sphere accessary in the range of 200 to 800 nm. The photoelectrochemical measurements were carried out on a CHI-760E electrochemical station with or without light irradiation from the Xe lamp. A standard three-electrode system was used for measurement, where the Pt1@TpPa-1/TiO2-NTAs photoelectrode (electrode surface of 1 cm2) was employed as the working electrode, saturated calomel electrode (SCE) as the reference electrode, Pt foil (length of 10 mm, diameter of 1 mm) as the auxiliary electrode. The photocurrent response of photoelectrodes was performed in a phosphorus buffer solution (PBS, pH = 7) containing 10 mM sodium ascorbate (Sinopharm Chemical Reagent Co. Ltd., >96%). Linear sweep voltammetry measurements were performed from 0 V to −1.2 V (vs. SCE), with a scan rate of 5 mV s−1 in a 0.5 M H2SO4 solution. Mott–Schottky curves were recorded with the voltage range of ±1.5 V under the frequency of 1 kHz in 0.5 M Na2SO4 aqueous solution. Electrochemical impedance spectra (EIS) were carried out in a frequency range from 0.01 Hz to 100 kHz, with an amplitude of 5 mV in 0.5 M Na2SO4 aqueous solution.
The PEC hydrogen production was performed in a sealed PEC reacting cell made of stainless steel with a quartz window equipped with the standard three-electrode system, circulating water system, and PBS solution (50 mL, pH = 7, containing 0.01 M sodium ascorbate (SA)). The area of the working electrode (Pt1@TpPa-1/TiO2-NTAs) was fixed at 1 cm2. The electrolyte solution was degassed for 30 min with high-purity nitrogen floating under dark conditions. Hydrogen was generated when the reacting cell was irradiated under the AM 1.5 simulated sunlight. The hydrogen-producing rate measurement was carried out half-hourly by injecting 1 mL reacting gas into a Techcomp GC7900 gas chromatograph with a 15 Å molecular sieve packing column and a thermal conductivity detector. The circulating water system was applied to maintain the reacting cell at room temperature, avoiding the heat damage of the lasting light irradiation. A quantitative evaluation cycle lasted for 6 h. A fresh PBS solution was used for a new cycle.

4. Conclusions

In summary, a Pt1@TpPa-1/TiO2-NTAs photoelectrode was fabricated by the solvothermal growth of ultrathin TpPa-1 COF film on the amino-modified TiO2 NTAs/Ti foil and photo-deposition of Pt1. The well-defined TiO2 NTAs were easily obtained by a facile anodization method for 0.25 h and annealing at 450 °C for 2 h. Ultrathin TpPa-1 films could remarkably extend the absorption range and improve the photoconductance, interfacial resistance, and charge separation of TpPa-1/TiO2-NTAs heterostructures. With the single atom loaded, the Pt1@TpPa-1/TiO2-NTAs photoelectrode showed further enhanced PEC performance, lower overpotential (77 mV@10 mA cm−2), and reduced Tafel slope (61 mV dec−1). Under the optimized conditions, the hydrogen evolution rate of the Pt1@TpPa-1/TiO2-NTAs was 5 times that of the Pt1@TpPa-1 photoelectrode under AM 1.5 simulated sunlight irradiation and the bias voltage of 0 V. The excellent hydrogen evolution performance of Pt1@TpPa-1/TiO2-NTAs photoelectrode could be attributed to the well-matched band structures, effective charge separation, improved interfacial resistance, robust interfacial catalytic ability, and the hydrophilic surface of the photoelectrode. The convenient reusability and good stability of the Pt1@TpPa-1/TiO2-NTAs photoelectrode benefit its promising application in hydrogen evolution

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28020822/s1. The additional characterizations of synthesized TiO2 NTAs, TpPa-1/TiO2-NTAs, and Pt1@TpPa-1/TiO2-NTAs, further PEC performance of Pt1@TpPa-1/TiO2-NTAs; Figures S1–S18.

Author Contributions

Conceptualization, B.S. and H.S.; methodology, Y.Z. and Y.L.; software, Y.L.; validation, Y.Z. and J.Y.; formal analysis, Y.Z., Y.L. and B.S.; data curation, Y.Z. and H.S.; writing—original draft preparation, Y.Z.; writing—review and editing, B.S. and H.S.; visualization, B.S. and H.S.; supervision, B.S.; project administration, B.S.; funding acquisition, B.S. and H.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Fundamental Research Funds for the Central Universities (No. 2652019113) and the National Natural Science Foundation of China (Nos. 21802128 and 22001240). Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the founders.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Most of the data presented in this study are available in the Supplementary Material. Additional data presented in this study are available on request from the corresponding authors.

Acknowledgments

Experimental works were supported by the large instruments and equipment sharing platform of China University of Geosciences (Beijing).

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Samples of the compounds are not available.

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Molecules 28 00822 g001 550
Figure 1. (a) Fabricating procedure of the Pt1@TpPa-1/TiO2-NTAs photoelectrode on the surface of Ti foil. (b) Schematic diagram of the solvothermal method for growing TpPa-1 film on amino-modified TiO2 NTAs. (c) Chemical structure of TpPa-1 COF condensing from monomers Tp and Pa. (d) Possible coordination structure of the photo-deposited Pt1 to TpPa-1.
Figure 1. (a) Fabricating procedure of the Pt1@TpPa-1/TiO2-NTAs photoelectrode on the surface of Ti foil. (b) Schematic diagram of the solvothermal method for growing TpPa-1 film on amino-modified TiO2 NTAs. (c) Chemical structure of TpPa-1 COF condensing from monomers Tp and Pa. (d) Possible coordination structure of the photo-deposited Pt1 to TpPa-1.
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Figure 2. SEM images of (a) TiO2 NTAs on Ti foil, (b) TpPa-1/TiO2-NTAs, and (c) Pt1@TpPa-1/TiO2-NTAs. Insets: the amplified SEM images correspondingly. (d) Element mapping of Pt1@TpPa-1/TiO2-NTAs with SEM observation. (e) Comparison FTIR spectra of TpPa-1/TiO2-NTAs, TpPa-1 powders, and corresponding monomers (Tp and Pa). (f) XPS survey spectra of TpPa-1/TiO2-NTAs and Pt1@TpPa-1/TiO2-NTAs. High-resolution spectra of (g) C1s, (h) N1s, and (i) Pt 4f cores. The TpPa-1 COF films were obtained from 0.2 mM Tp and 0.3 mM Pa for the above characterization.
Figure 2. SEM images of (a) TiO2 NTAs on Ti foil, (b) TpPa-1/TiO2-NTAs, and (c) Pt1@TpPa-1/TiO2-NTAs. Insets: the amplified SEM images correspondingly. (d) Element mapping of Pt1@TpPa-1/TiO2-NTAs with SEM observation. (e) Comparison FTIR spectra of TpPa-1/TiO2-NTAs, TpPa-1 powders, and corresponding monomers (Tp and Pa). (f) XPS survey spectra of TpPa-1/TiO2-NTAs and Pt1@TpPa-1/TiO2-NTAs. High-resolution spectra of (g) C1s, (h) N1s, and (i) Pt 4f cores. The TpPa-1 COF films were obtained from 0.2 mM Tp and 0.3 mM Pa for the above characterization.
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Figure 3. (a) UV–vis absorption spectra of TpPa-1/TiO2-NTAs, TpPa-1 powders, and TiO2 NTAs on Ti foils. (b) Tauc’s plots for TiO2 and TpPa-1 based on the absorption spectra. (c) Transient photocurrent response of TiO2 NTAs, TpPa-1/TiO2-NTAs, and Pt1@TpPa-1/TiO2-NTAs with or without light irradiation. (d) EIS Nyquist plots of TiO2, TpPa-1/TiO2-NTAs, and Pt1@TpPa-1/TiO2-NTAs under light irradiation. The TpPa-1 COF films were obtained from 0.2 mM Tp and 0.3 mM Pa for the above characterization.
Figure 3. (a) UV–vis absorption spectra of TpPa-1/TiO2-NTAs, TpPa-1 powders, and TiO2 NTAs on Ti foils. (b) Tauc’s plots for TiO2 and TpPa-1 based on the absorption spectra. (c) Transient photocurrent response of TiO2 NTAs, TpPa-1/TiO2-NTAs, and Pt1@TpPa-1/TiO2-NTAs with or without light irradiation. (d) EIS Nyquist plots of TiO2, TpPa-1/TiO2-NTAs, and Pt1@TpPa-1/TiO2-NTAs under light irradiation. The TpPa-1 COF films were obtained from 0.2 mM Tp and 0.3 mM Pa for the above characterization.
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Figure 4. (a,b) Linear scan voltammetry curves and (c) Tafel plots of TiO2 NTAs, TpPa-1/TiO2-NTAs, and Pt1@TpPa-1/TiO2-NTAs under light irradiation. (d,e) Hydrogen evolution performance of Pt1@TpPa-1/TiO2-NTAs compared to Pt@TiO2 NTAs and Pt1@TpPa-1. Effect of (f) monomer concentration (based on Tp), (g) initial Pt amount, and (h) applied bias voltage (vs. SCE) on the hydrogen generation performance of Pt1@TpPa-1/TiO2-NTAs photoelectrode. (i) Cycle tests of hydrogen evolution.
Figure 4. (a,b) Linear scan voltammetry curves and (c) Tafel plots of TiO2 NTAs, TpPa-1/TiO2-NTAs, and Pt1@TpPa-1/TiO2-NTAs under light irradiation. (d,e) Hydrogen evolution performance of Pt1@TpPa-1/TiO2-NTAs compared to Pt@TiO2 NTAs and Pt1@TpPa-1. Effect of (f) monomer concentration (based on Tp), (g) initial Pt amount, and (h) applied bias voltage (vs. SCE) on the hydrogen generation performance of Pt1@TpPa-1/TiO2-NTAs photoelectrode. (i) Cycle tests of hydrogen evolution.
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Figure 5. (a) Band diagram and possible charge transfer under light irradiation for the Pt1@TpPa-1/TiO2-NTAs photoelectrode. (b) Interfacial illustration for the photoelectrochemical hydrogen evolution by using the Pt1@TpPa-1/TiO2-NTAs photoelectrode.
Figure 5. (a) Band diagram and possible charge transfer under light irradiation for the Pt1@TpPa-1/TiO2-NTAs photoelectrode. (b) Interfacial illustration for the photoelectrochemical hydrogen evolution by using the Pt1@TpPa-1/TiO2-NTAs photoelectrode.
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MDPI and ACS Style

Zhang, Y.; Li, Y.; Yu, J.; Sun, B.; Shang, H. A Heterostructure Photoelectrode Based on Two-Dimensional Covalent Organic Framework Film Decorated TiO2 Nanotube Arrays for Enhanced Photoelectrochemical Hydrogen Generation. Molecules 2023, 28, 822. https://doi.org/10.3390/molecules28020822

AMA Style

Zhang Y, Li Y, Yu J, Sun B, Shang H. A Heterostructure Photoelectrode Based on Two-Dimensional Covalent Organic Framework Film Decorated TiO2 Nanotube Arrays for Enhanced Photoelectrochemical Hydrogen Generation. Molecules. 2023; 28(2):822. https://doi.org/10.3390/molecules28020822

Chicago/Turabian Style

Zhang, Yue, Yujie Li, Jing Yu, Bing Sun, and Hong Shang. 2023. "A Heterostructure Photoelectrode Based on Two-Dimensional Covalent Organic Framework Film Decorated TiO2 Nanotube Arrays for Enhanced Photoelectrochemical Hydrogen Generation" Molecules 28, no. 2: 822. https://doi.org/10.3390/molecules28020822

APA Style

Zhang, Y., Li, Y., Yu, J., Sun, B., & Shang, H. (2023). A Heterostructure Photoelectrode Based on Two-Dimensional Covalent Organic Framework Film Decorated TiO2 Nanotube Arrays for Enhanced Photoelectrochemical Hydrogen Generation. Molecules, 28(2), 822. https://doi.org/10.3390/molecules28020822

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