Research Article Cite This: ACS Catal. 2017, 7, 8228-8234
pubs.acs.org/acscatalysis
g‑C3N4 Hydrogen-Bonding Viologen for Significantly Enhanced Visible-Light Photocatalytic H2 Evolution Ya-Nan Liu, Cong-Cong Shen, Nan Jiang, Zhi-Wei Zhao, Xiao Zhou, Sheng-Jie Zhao, and An-Wu Xu* Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China S Supporting Information *
ABSTRACT: Graphitic carbon nitride (g-C3N4) has recently emerged as a promising metal-free photocatalytic material for the conversion of solar energy into chemical energy under visible-light irradiation. Unfortunately, the photocatalytic activity of g-C3N4 is still unsatisfactory due to the serious recombination of photogenerated electron−hole pairs. Here, we develop a strategy to construct a type of g-C3N4-based composite photocatalyst (C3N4/ CBV2+), a g-C3N4 surface coupled with a viologen redox mediator (1,1′-bis(4-carboxylatobenzyl)-4,4′-bipyridinium dichloride, denoted as CBV2+) through hydrogen bonds, for enhanced H2 production from water under visible-light irradiation. The CBV2+ molecules not only provide sites for metal particle formation but also act as an efficient electron transfer mediator to transfer the photoinduced electrons from g-C3N4 to platinum nanoparticles (Pt NPs). The vectorial charge transfer results in an efficient spatial separation of electrons and holes in the C3N4/CBV2+ composite photocatalyst and facilitates the photogenerated charge carriers for direct photocatalytic water splitting. When 1 wt % CBV2+ is introduced, the hydrogen production rate of C3N4/ CBV2+ dramatically increases up to 41.57 μmol h−1, exceeding 85 times the rate over unmodified g-C3N4 (only 0.49 μmol h−1). It is noted that a negligible loss of photocatalytic activity was observed over continuous irradiation up to 20 h, demonstrating its good stability. The combination of the two emerging functional materials represents a simple but economical and powerful approach for highly effective photocatalytic hydrogen production under visible light irradiation. This study opens a window to rationally develop cost-acceptable materials for more efficient solar energy conversion applications. KEYWORDS: g-C3N4/viologen, hydrogen bond, visible-light photocatalysis, hydrogen evolution, charge separation efficiency nitride,4 sulfide,5 and metal oxides,6,7 as well as conjugated polymers.8 However, although much effort has been made in developing different visible-light-responsive photocatalysts, their practical application is still hindered due to the complicated preparation process and low recyclability. Thus, it is urgent to explore novel photocatalytic systems to solve the aforementioned problems and meet the requirements of energy technologies. Graphitic carbon nitride (g-C3N4), a fascinating polymer semiconductor, has recently attracted tremendous attention since its visible-light photocatalytic performance was first reported by Wang et al. in 2009.9 The excellent chemical and thermal stability, environmental harmlessness, and suitable band gap (2.7 eV) of g-C3N4 fulfill the basic requirements for water splitting.10 In particular, unlike the metal-containing photocatalysts that need expensive metal salts for preparation, g-C3N4 photocatalysts can be easily fabricated by thermal polymerization of cheap N-rich precursors, such as dicyana-
1. INTRODUCTION Currently, solar energy conversion and environmental remediation have become a realistic and urgent problem on account of the increasing awareness of the global energy crisis and environmental protection.1,2 Since Fujishima and Honda first reported the photoelectrochemical splitting of water into hydrogen with a TiO2 electrode in 1972,3 photocatalytic hydrogen (H2) production from water has attracted intense interest and has been shown to be a promising solution for alleviating worldwide energy issues. H2 is regarded as an ideal energy source for a future sustainable society because of it is clean burning, has a high energy yield (122 kJ/g), and has renewable character.1 As photocatalytic hydrogen evolution plays a significant role in hydrogen production, developing a highly efficient photocatalyst for hydrogen production has become a hot research field. The most critical factor for achieving effective solar hydrogen production is to design stable, efficient, and inexpensive photocatalysts that are capable of working in the visible-light spectrum, which occupies ca. 46% of the incoming solar light spectrum on the earth. A wealth of visible-light-responsive materials have been developed as photocatalysts for hydrogen generation from water, such as © XXXX American Chemical Society
Received: September 23, 2017 Revised: October 24, 2017
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DOI: 10.1021/acscatal.7b03266 ACS Catal. 2017, 7, 8228−8234
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ACS Catalysis mide,11 cyanamide,12 melamine,13 ammonium thiocyanate,14 and urea.15 In addition, in comparison with most traditional materials (TiO2, CdS, etc.), metal-free g-C3N4 has a 2D πconjugated electronic structure owing to the presence of sp2hybridized carbon and nitrogen.16 However, the practical application of bare g-C3N4 is still restricted due to the highenergy-waste recombination rate of photoinduced electron− hole pairs.2 Over the past few years, a great deal of effort has been made to boost the photocatalytic performance of g-C3N4 via various strategies, including doping with metal or nonmetal species,17,18 construction of p−n homojunctions,19 coupling with other semiconductors,20,21 and sensitizing with organic dyes.22 These modifications indeed enhance the photocatalytic performance of g-C3N4, but at the same time, they also bring about some unwanted negative effect on other important criteria: for instance, tedious synthetic processes and introduction of metal and toxic solvents, which will increase the cost and pollute environments. This situation motivated us to exploit a relatively facile approach to design efficient and environmentally friendly photocatalysts. In fact, g-C3N4 can be easily modified with organic molecules in solution because of its abundant pendant amine docking sites.23,24 Kuriki et al. reported that hydrogen bonds formed between −NH2 groups on g-C3N4 and −CH2PO3H2 groups on RuRu′ complexes.25 Furthermore, for g-C3N4-based copolymers, an intramolecular charge transfer that results in the effective separation of charge carriers could take place under irradiation of light.26,27 It is expected that some appropriate molecules can be introduced as a guide for the directional migration of photogenerated electrons in g-C3N4 to participate in the reduction reaction of water. Viologens (1,1′-disubstituted 4,4′-bipyridinium salts, V2+) are classical organic building blocks, and the best known feature of the viologens is their ability to exist in well-characterized oxidation states.28,29 Their low reduction potential and the ability to form a stable radical cation (V+•), which is easily reoxidized to a dication (V2+), make them applicable as electron transfer mediators in redox systems. 30−32 It is worth anticipating that coupling viologens with layered g-C3N4 sheets could boost photocatalytic activity. In this context, a viologen (1,1′-bis(4-carboxylatobenzyl)-4,4′-bipyridinium dichloride, denoted CBV2+) has been modified on the surfaces of g-C3N4 via intermolecular hydrogen bonds in solution. This composite can effectively transfer the light-generated electrons through the intermolecular H-bonds and further accelerate the reaction by promoting charge separation. On irradiation under visible light, the electrons are able to transfer from the conduction band of g-C3N4 to CBV2+ and then transfer from CBV2+ to platinum nanoparticles. The spatial separation of the electrons and holes can efficiently restrain the recombination of photogenerated charge carriers, consequently facilitating photocatalytic H2 generation. As expected, the obtained C3N4/CBV2+ hybirds show significantly enhanced visible-light photocatalytic H2 evolution performance in comparison to pristine g-C3N4 under optimized conditions. In attempting to explain the effect of CBV2+ in hydrogen production, we propose a possible mechanism for charge separation and transfer in the C3N4/ CBV2+ composite. We believe that our findings will provide a promising route for the rational design of photocatalysts for more efficient solar energy conversion.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Urea (≥99%), triethanolamine (TEOA, ≥78%), methanol (≥99%), lactic acid (≥85%), and ascorbic acid (≥99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. 1,1′-Bis(4-carboxylatobenzyl)-4,4′-bipyridinium dichloride (CBV2+, ≥97%) was purchased from Jinan Henghua Technology Co., Ltd. Chloroplatinic acid hexahydrate (H2PtCl6·6H2O, ≥37% Pt basis) was obtained from Aldrich. All chemicals were used as received without further purification. In addition, double-distilled water used in all experiments was purified through a SZ-93A auto-double-distillation apparatus (Ya Rong Corp., Shanghai, People’s Republic of China). 2.2. Preparation of Photocatalysts. First, urea (4.0 g) was placed in an alumina crucible and covered, and then the alumina crucible was calcined at 823 K for 4 h in a muffle furnace in an air atmosphere, with a heating rate of 5 K min−1. After they were cooled to room temperature naturally, the resulting yellow powders were collected for further use. 2.3. Characterization. X-ray diffraction (XRD) patterns of the powders were recorded at room temperature by using a Rigaku diffactometer (MXPAHF, Japan) with Cu Kα irradiation (λ = 1.541 Å), with an operating voltage of 40 kV and current of 200 mA. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM12010 highresolution transmission electron microscope operated at 200 kV. The diffuse reflectance UV−vis absorption spectra of the photocatalysts were recorded with a Shimadzu spectrophotometer (Model 2501 PC). The UV−vis absorption spectra of aqueous solutions were obtained using a Shimadzu UV-2510 spectrophotometer. The steady-state photoluminescence (PL) measurements were conducted on a fluorescence spectrophotometer (JY Fluorolog-3-Tau) with an excitation wavelength of 325 nm. The time-resolved photoluminescence (TRPL) measurements were measured on a LaserStrobe Time-Resolved Spectrofluorometer (Photon Technology International (Canada) Inc.) with a USHIO xenon lamp source, a GL-302 highresolution dye laser (lifetimes 100 ps to 50 ms, excited by a nitrogen laser), and a 914 photomultiplier detection system. Cyclic voltammetry (CV) tests were conducted using an electrochemical workstation (CHI 760E, Chenhua Instrument Company, Shanghai, People’s Republic of China). 2.4. Photocatalytic Hydrogen Production. Photocatalytic H2 evolution from water was carried out in an outer topirradiation gas-closed Pyrex glass system (500 mL) and high vacuum state. In a typical experiment, 50 mg of g-C3N4 powder and different amounts of CBV2+ were dispersed in an aqueous solution (100 mL) containing 10% triethanolamine as a sacrificial electron donor. Then, 1 wt % Pt, as a cocatalyst to boost H2 generation, was loaded onto the surface of the catalyst by in situ photodeposition of H2PtCl6·6H2O. The solution was evacuated for 1 h to remove air completely prior to irradiation with a 300 W xenon lamp (Perfect Light, PLS-SXE300C, Beijing, People’s Republic of China), which was equipped with a cutoff filter (λ ≥ 420 nm) to remove ultraviolet light. In addition, a Pyrex reactor with a double layer was continuously stirred and the temperature of the reaction solution was maintained at 10 °C by a flow of cooling ethylene glycol during the photocatalytic reaction. The amount of hydrogen evolution from photocatalytic splitting of water was measured by an online gas chromatograph (GC1120, Shanghai Sunny Hengping Limited, HTCD, N2 as carrier gas). After the reaction, the mixture of CBV2+ and g-C3N4 was separated from the reaction 8229
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ACS Catalysis solution for further characterization; the catalysts are denoted as C3N4/xCBV2+, where x (x = 0.2, 1, 2%) refers to the weight content of CBV2+ in C3N4/CBV2+ samples. The apparent quantum yields (AQY) were calculated at different monochromatic light irradiations by using C3N4/1% CBV2+ catalyst (irradiated by a 300 W Xe lamp using a bandpass filter of λ ± 5 nm for 420, 450, 500, 550, 600 nm), according to the equation no. of reacted electrons × 100 no. of incident photons no. of evolved H 2 molecules × 2 = × 100 no. of incident photons
nAQY (%) =
Figure 2. TEM images of (A) pure g-C3N4 and (B) C3N4/1% CBV2+ with 1 wt % Pt loading.
form a complex in solution through electrostatic interactions (Figure S2 in the Supporting Information).34 Then, the CBV2+ redox mediator extracts photogenerated electrons from carbon nitride and transfers the electrons to PtCl62−; therefore, Pt NPs are inclined to form on the surface of CBV2+.35 CBV2+ could provide sites for dispersing the metallic Pt nanoparticles and act as a stabilizer to control the particle size and prevent agglomeration.34 It can be seen that the Pt NPs show more uniform distribution with an average size of 4 nm after adding CBV2+, which is beneficial for hydrogen production. The optical absorption spectra of as-prepared g-C3N4, CBV2+, and C3N4/CBV2+ composites with different weight contents of CBV2+ were recorded using UV−vis diffuse reflectance spectroscopy (UV−vis DRS). As shown in Figure 3, the pristine g-C3N4 has an absorption edge of 460 nm, which can be assigned to a band gap of 2.70 eV.9 The CBV2+ shows a strong and broad absorption band ranging from 250 to 390 nm, which is attributed to the π → π* electronic transitions of aromatic rings.36,37 This result indicates that CBV2+ has no response to visible light. The obtained C 3 N 4 /CBV 2+ composites exhibit a slight blue shift in comparison with pure g-C3N4, further verifying the successful introduction of CBV2+ in the hybrid samples (inset in Figure 3A).38,39 At the same time, UV−vis measurements for g-C3N4, CBV2+, and C3N4/ CBV2+ composites in aqueous solution were carried out, as shown in Figure 3B. The obtained C3N4/CBV2+ aqueous solutions also exhibit a blue shift in comparison with pure gC3N4. The small blue-shift phenomenon reflects that the presence of CBV2+ has an effect on the absorption of light, and therefore introduction of a suitable amount of CBV2+ is beneficial to improve the photocatalytic performance of gC3N4. To investigate the effect of CBV2+ on photocalytic H2 evolution under visible-light illumination, a series of comparative experiments were conducted. Meanwhile, we choose triethanolamine (TEOA) as a sacrificial reagent after measuring the rate of H2 evolution on C3N4/1% CBV2+ in the presence of different sacrificial reagents (Figure S3 in the Supporting Information). The bare g-C3N4 sample exhibits a hydrogen evolution rate of 0.49 μmol h−1 (Figure 4a); g-C3N4 with 1 wt % Pt cocatalyst produces hydrogen at a rate of 7.12 μmol h−1 (Figure 4b). After CBV2+ is added, the photocatalytic H2 evolution rate for C3N4/1% CBV2+ is enhanced up to 41.57 μmol h−1 (Figure 4c), which is nearly 85 times higher than that of bare g-C3N4, thus demonstrating that the CBV2+ redox mediator indeed promotes the charge transfer and reduces the recombination of the electron−hole pairs. Meanwhile, the photocatalytic activity on C3N4/1% CBV2+ without Pt cocatalyst was also tested in the control experiment, and the H2 evolution rate is only 3.36 μmol h−1 (Figure 4d). The result
3. RESULTS AND DISCUSSION g-C3N4 was prepared from urea by calcination at 550 °C. A viologen, 1,1′-bis(4-carboxylatobenzyl)-4,4′-bipyridinium dichloride (denoted CBV2+) was used to modify g-C3N4 in solution, and g-C3N4/CBV2+ (named as C3N4/CBV2+) composites with different weight contents of CBV2+ were obtained (see the Experimental Section). The X-ray diffraction (XRD) patterns of obtained g-C 3 N 4 and C 3 N 4 /CBV 2+ composites are shown in Figure 1. For bare g-C3N4, the
Figure 1. XRD patterns of CBV2+ molecules, g-C3N4, and C3N4/ CBV2+ composite photocatalysts.
XRD pattern displays two pronounced diffraction peaks. The low-angle (100) peak at around 13.0° corresponds to tri-striazine units packing in the lattice planes parallel to the c axis. In addition, the high-angle diffraction peak (002) at around 27.7° stems from the periodic stacking of individual layers. The two diffraction peaks are in accordance with those of g-C3N4 in the literature.10,33 In comparison with g-C3N4, the C3N4/CBV2+ composites with different weight contents of CBV2+ show diffraction peaks similar to those of g-C3N4, indicating that the addition of CBV2+ does not change the crystal structure of gC3N4. It is noted that a characteristic peak of CBV2+ at about 32.9° is observed for both C3N4/1% CBV2+ and C3N4/2% CBV2+, suggesting the successful introduction of CBV2+ redox mediator in the composite samples. No diffraction peak of metallic Pt is detected in C3N4/CBV2+ composites, which can be ascribed to the relatively low content (1 wt %) and highly dispersed Pt nanoparticles (NPs) in the final samples with the addition of CBV2+ molecules (shown in Figure 2 and Figure S1 in the Supporting Information). In fact, positively charged CBV2+ can easily adsorb negatively charged PtCl62− ions to 8230
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Figure 3. (A) Diffuse reflectance UV−vis absorption spectra of g-C3N4, CBV2+, and C3N4/CBV2+ composites. The inset is an enlarged view of a blue shift. (B) UV−vis absorption spectra of g-C3N4, CBV2+, and C3N4/CBV2+ composites in aqueous solution.
To accurately investigate the migration, transfer, and recombination processes of the photoexcited charge carriers and the roles that CBV2+ plays in the photocatalytic reaction process, the steady-state photoluminescence (PL) spectra of gC3N4 and C3N4/1% CBV2+ were measured. As shown in Figure 5A, a strong PL emission peak located at about 450 nm is observed for native g-C3N4, which can be attributed to the band−band PL phenomenon of the photoinduced charge carriers for g-C3N4.9,10 In comparison to g-C3N4, the position of the emission peak of C3N4/1% CBV2+ sample is similar to that of g-C3N4, but the PL emission intensity is reduced drastically, illustrating that charge transfer efficiency in C3N4/ 1% CBV2+ is much higher than that in pure g-C3N4,40 which agrees well with an observation that the H2 evolution rate over C3N4/1% CBV2+ is much higher than that of bare g-C3N4. Moreover, the possible charge transfer dynamics for the two samples was further investigated by time-resolved photoluminescence (TRPL) decay spectra (Figure 5B). Clearly, the average lifetime of photogenerated electrons in C3N4/1% CBV2+ (6.02 ± 0.02 ns) is strikingly longer than that of g-C3N4 (3.83 ± 0.05 ns). This result suggests that the transfer of photoexcited electrons between g-C3N4 and CBV2+ occurs, which can retard the recombination probability of electron− hole pairs and therefore increase the number of photogenerated electrons to participate in the reduction reaction of water.41 As is known, a prolonged time generally implies an improved possibility that the excited electrons or holes participate in the photocatalytic reactions. In combination with the steady-state PL spectra with TRPL, it can be concluded that modification with a small amount of CBV2+ redox mediator can effectively
Figure 4. Comparison of hydrogen evolution rates over different samples under visible light illumination (λ ≥ 420 nm): (a) g-C3N4 without Pt cocatalyst; (b) g-C3N4 with 1 wt % Pt; (c) C3N4/1% CBV2+ with 1 wt % Pt cocatalyst; (d) C3N4/1% CBV2+ without Pt cocatalyst. Reaction conditions: 50 mg of photocatalyst, H2O (90 mL), TEOA (10 mL), 10 °C.
indicates that a key role of CBV2+ in hydrogen production is to work as an electron transfer mediator. It plays an important role in forming Pt NPs, provides sites for dispersing the metallic NPs, and acts as the stabilizer to control the particle size and prevent agglomeration. Consequently, this hybrid material is thought to provide spatially separated reduction (Pt) and oxidation reaction (g-C3N4) sites. It is not surprising that the C3N4/CBV2+/Pt composites display highly increased photocatalytic hydrogen generation rates.
Figure 5. (A) Steady-state photoluminescence spectra (the excitation wavelength is 400 nm) of the samples after hydrogen production. (B) Respective time-resolved fluorescence decay traces measured at room temperature (excitation at 400 nm and probe at 450 nm). 8231
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Figure 6. (A) Photocatalytic H2 evolution rates of C3N4/CBV2+ photocatalysts with different weight contents of CBV2+ under visible-light irradiation (λ ≥ 420 nm). Reaction conditions: 50 mg of photocatalyst, solvent 100 mL of H2O/TEOA (9/1 v/v), 300 W xenon lamp as light source, 1 wt % Pt cocatalyst photodeposited in situ on the surface of photocatalysts. (B) Apparent quantum yields (AQY) of hydrogen evolution by using C3N4/1% CBV2+ catalyst under different monochromatic light irradiation wavelengths.
of g-C3N4. This result suggests that the H2 evolution reaction proceeds through the light absorption of g-C3N4, and CBV2+ serves as an electron transfer mediator to suppress the electron−hole recombination rate. In addition to high performance, the recycling performance and durability of photocatalysts is also of importance from the viewpoint of widespread practical applications. To confirm the persistence of the photocatalytic activity of C3N4/1% CBV2+, cycle experiments of the photocatalytic H2 generation were carried out by using the same photocatalyst repeatedly for five times under the same conditions. As displayed in Figure 7, no
prevent the recombination of photoinduced electron−hole pairs in g-C3N4 and greatly increase the visible light conversion efficiency, which is the reason the C3N4/CBV2+ sample shows high photocatalytic H2 production activity. The photocatalytic H2-production performaces of C3N4/ CBV2+ photocatalysts with different amounts of CBV2+ were determined in an aqueous solution containing TEOA as sacrificial reagent under visible-light illumination (λ ≥ 420 nm). Figure 6A shows that the introduction of CBV2+ redox mediator indeed leads to a remarkable improvement in the photocatalytic H2 production activity of g-C3N4, and a suitable amount of CBV2+ is critical for optimizing the photocatalytic H2 evolution rate of C3N4/CBV2+ composites. In the absence of CBV2+, the H2 evolution rate of g-C3N4 is only 7.12 μmol h−1. When CBV2+ was introduced, all C3N4/CBV2+ composites exhibited higher H2 evolution activity in comparison to unmodified g-C3N4. The C3N4/CBV2+ composite displays the highest H2 evolution rate of 41.57 μmol h−1 when the weight content of CBV2+ was increased to 1 wt %, which is nearly 85 times the rate for g-C3N4 alone and is very high in comparison with other reported results in the literature (Table S1 in the Supporting Information).42−44 It noted that further increasing the CBV2+ content to 2.0 wt % in the composite results in a dramatic decline in the photocatalytic activity (22.98 μmol h−1), as is clearly shown in Figure 6A. The reason for this decline is likely due to the following aspects. (1) Only strong coupling facilitates charge transfer and promotes the separation of photogenerated electron−hole pairs; excessive CBV2+ could lead to covering the surface active sites of g-C3N4, subsequently hampering the photocatalytic activity.45 (2) Excessive CBV2+ on the surface of g-C3N4 could negatively affect the light absorption of g-C3N4, resulting in reduced photocatalytic activity for hydrogen evolution.41 Therefore, a suitable amount of CBV2+ introduction is of significance to optimize the photocatalytic activities of C3N4/CBV2+ composites. Figure 6B gives the AQY values of C3N4/1% CBV2+ sample under various monochromatic light irradiation wavelengths with band-pass filters (λ 420, 460, 500, 540, 580 (±5) nm). It can be seen that the highest AQY of 3.81% was achieved for C3N4/1% CBV2+ at 420 nm. In addition, the apparent quantum yield decreases with an increase in the incident light wavelength and shows a closely coincident relationship with the UV−vis absorption spectrum
Figure 7. Reusability tests for photocatalytic hydrogen production over C3N4/1% CBV2+ with 1 wt % Pt cocatalyst.
significant leveling off tendency was observed even with the consecutive 20 h irradiation runs. This result reveals that the C3N4/CBV2+ hybrid photocatalyst has remarkable stability for long-term photocatalytic hydrogen production. Our results have shown that CBV2+ can be employed as an excellent electron mediator for the enhancement of photocatalytic H2 activity. On the basis of the above analysis, an electron transfer mechanism of C3N4/CBV2+ composite for photocatalytic H2 production under visible-light illumination is proposed to help understand the photocatalytic process (Scheme 1). g-C3N4 was prepared by simple thermal condensation of urea; it can be easily modified with CBV2+ via direct hydrogen-bonding interactions because of the presence of rich amino groups on g-C 3 N 4 (Scheme 8232
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Scheme 1. (A) Structure of g-C3N4 and CBV2+ Molecules Coupled with Hydrogen Bonds, (B) Energy Level Diagram Adjusted in Relation to the Vacuum Level for the C3N4/CBV2+/Pt System, and (C) Schematic Diagram Illustration for Visible-LightInduced H2 Evolution over C3N4/CBV2+ Composite
1A).24,46−48 After CBV2+ was introduced on the g-C3N4 by the hydrogen bonds between carboxyl groups on CBV2+ and amino groups on the surface of the g-C3N4, the electron transfer would occur in this g-C3N4-based photocatalyst under visiblelight irradiation. According to previous reports, g-C3N4 is an n-type semiconductor with the redox potential of the conduction band (CB) and the valence band located at −3.4 and −6.1 eV versus the absolute vacuum scale, respectively.9,10,49 Combining the cyclic voltammetry (CV) (Figure S4A) and UV−vis absorption spectra (Figure S4B) in the Supporting Information, the HOMO and LUMO energy levels of CBV2+ were calculated to be −7.95 and −4.01 eV, respectively. As shown in Scheme 1B,C, the g-C3N4 in the composite photocatalyst generates enough electrons and holes under visible light irradiation. It can be seen that the LUMO of CBV2+ is more negative than the CB of g-C3N4, the photoexcited electrons can quickly transfer to CBV2+ to form a radical cation (CBV+·) (Figure S5 in the Supporting Information), and the radical cation state is reconverted to its dication state followed by electron transfer from CBV+· to Pt NPs with a Fermi level at 5.65 eV vs the absolute vacuum scale.50,51 The lifetime of electrons is significantly prolonged through these electron transfer processes; thereby the accumulated rich electrons in Pt NPs are readily captured by H+ in the reaction system to complete the reduction process. Additionally, the positive holes in the valence band of g-C3N4 would transfer to the surface of g-C3N4 and be captured by a sacrificial agent (TEOA) to suppress the backward reaction. Therefore, the material is thought to provide spatially separate reduction (Pt) and oxidation reaction (g-C3N4) sites, which restrains the recombination of charge carriers significantly and finally leads to a markedly improved photocatalytic hydrogen production activity of C3N4/CBV2+
under visible light irradiation, in comparison to each individual counterpart.
4. CONCLUSION In summary, a distinctive type of g-C3N4-based polymer composites has been prepared by decorating viologen molecules on the surface of g-C3N4, and it has been studied as an efficient photocatalyst for hydrogen production from water under visible-light irradiation. With an optimal 1 wt % of viologen, the obtained composite photocatalyst exhibits a high H2 generation rate of 41.57 μmol h−1 with an outstanding apparent quantum yield of 3.81% at ∼420 nm. In addition, the C3N4/viologen photocatalyst does not show obvious deactivation for H2 evolution under long-term visible-light irradiation, implying the good durability and recycling performance. The spatially isolated oxidation and reduction reaction sites are believed to play a significant role in the remarkably improved hydrogen evolution activity. This work demonstrates that a viologen derivative can be effectively used for exploiting efficient photocatalytic materials applied in H2 evolution and solar energy conversion because of its function as an excellent electron transfer mediator. Overall, our work is an important step forward on the way to developing a new class of highefficiency photocatalysts for the production of clean energy.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b03266. TEM images, UV−vis absorption spectrum, cyclic voltammogram, and comparison of photocatalytic hydrogen evolution rates under different sacrificial reagents (PDF) 8233
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AUTHOR INFORMATION
Corresponding Author
*E-mail for A.-W.X.:
[email protected]. ORCID
An-Wu Xu: 0000-0002-4950-0490 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the special funding support from the National Natural Science Foundation of China (51572253, 21771171) and a Scientific Research Grant of Hefei Science Center of CAS (2015SRG-HSC048). Cooperation between the NSFC and the Netherlands Organization for Scientific Research (51561135011) is acknowledged.
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DOI: 10.1021/acscatal.7b03266 ACS Catal. 2017, 7, 8228−8234
