Research Article pubs.acs.org/journal/ascecg
Noble-Metal-Free Iron Phosphide Cocatalyst Loaded Graphitic Carbon Nitride as an Efficient and Robust Photocatalyst for Hydrogen Evolution under Visible Light Irradiation Hui Zhao,*,† Junwei Wang,† Yuming Dong,‡ and Pingping Jiang‡ †
Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Jiangsu Engineering Technology Research Center of Environmental Cleaning Materials, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology (NUIST), 219 Ningliu Road, Nanjing 210044, China ‡ Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Avenue, Wuxi 214122, China S Supporting Information *
ABSTRACT: Photocatalytic hydrogen (H2) evolution from water is a promising and sustainable approach for solar-to chemical energy conversion. However, the development of an efficient, robust, and low-cost visible-light-driven photocatalyst for H2 evolution is still one of the great challenges. Graphitic carbon nitride (g-C3N4) is an attractive candidate, but the activity of pristine g-C3N4 is largely limited. Herein, for the first time, we report the noble-metal-free iron phosphide cocatalyst decorated graphitic carbon nitride (g-C3N4/FexP) as a photocatalyst for the highly efficient and stable H2 evolution from water splitting irradiated by visible light. The peak H2 evolution rate of g-C3N4/ FexP is ca. 277 times higher than that of pristine g-C3N4 and is almost comparable with the g-C3N4 modified by noble metal Pt cocatalyst. Additionally, g-C3N4/FexP demonstrates almost negligible photocatalytic degradation capability after five repeated cycles. Based on the detail analyses of photoluminescence and surface photovoltage spectra, we find the presence of FexP cocatalyst significantly accelerates the separation and transfer of photogenerated electrons of g-C3N4, hence resulting in the high photocatalytic efficiency of g-C3N4/FexP for H2 production. In addition, the adjacent Fe and P atoms in FexP act as dual proton adsorption sites to synergistically facilitate the fast H2 generation from water. KEYWORDS: graphitic carbon nitride, iron phosphide, hybrid photocatalyst, hydrogen evolution
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tion, and easy availability.4−6 Together with the appropriate band gap and position, g-C3N4 has been an attractive low-cost photocatalyst for H2 evolution under the irradiation of visible light.7−12 However, the H2 evolution activity of pristine g-C3N4 is relatively low due to the fast recombination of the photogenerated electron−holes pairs. In order to accelerate the charge separation and transfer of g-C3N4, coupling with a cocatalyst has been identified as a feasible and effective strategy.13,14 Noble metal Pt is widely used and considered as the best cocatalyst for g-C3N4 in H2 evolution reaction.15−17 However, the large-scale application of such a system is largely limited by the high cost and extreme rarity of Pt. As a result, the development of an effective, low-cost, noble-metal-free cocatalyst has attracted lots of researchers’ interest. In recent years, several earth-abundant metal compounds have emerged
INTRODUCTION Nowadays the energy crisis and environmental pollution caused by fossil fuel combustion have been highly valued in the world. To help solve the shortage problem of fossil fuel and mitigate the environmental issue, the development of green renewable energy has received a great deal of attention. Hydrogen (H2) is currently pursued as a clean energy due to the high heat and product water alone during combustion. Since solar energy is essentially infinite and the harvesting process is inherently environmentally benign, the generation of H2 from water via photocatalysis is widely regarded as a valuable and promising approach.1−3 In view of effectively utilizing solar energy (ca.43% for visible light) and large-scale application, the construction of a low-cost visible-light-driven photocatalyst with high efficiency and good durability is of great importance in the H2 evolution reaction. As a metal-free polymer semiconductor photocatalyst, graphitic carbon nitride (g-C3N4) combines the advantages of nontoxicity, high stability, earth-abundant elemental composi© 2017 American Chemical Society
Received: May 26, 2017 Revised: July 11, 2017 Published: August 14, 2017 8053
DOI: 10.1021/acssuschemeng.7b01665 ACS Sustainable Chem. Eng. 2017, 5, 8053−8060
Research Article
ACS Sustainable Chemistry & Engineering
The heating rate is controlled at 2 °C min−1. The resultant yellowcolored product is washed with H2O and then dried at 120 °C. Finally, pristine g-C3N4 production is obtained. Preparation of the g-C3N4/FeOOH composite: The g-C3N4/ FeOOH composite is prepared using a hydrothermal method. In a typical synthesis, g-C3N4 (200 mg) is added into water (22 mL). The mixture is sonicated for 180 min. FeSO4·7H2O with different masses (3.5, 6.9, 13.0, and 25.5 mg) and hydrogen peroxide (30 wt %, 6 mL) are then added into the mixture. After stirring for 30 min, the suspension is transferred to a Teflon-lined autoclave (50 mL) that maintains at 150 °C for 6 h. The resultant product is separated, washed with water and ethanol, and dried at 80 °C. The g-C3N4/ FeOOH composite is finally obtained. Pristine FeOOH is prepared using the same method without adding g-C3N4. The XRD patterns and SEM image of pristine FeOOH are provided in Figure S1. Preparation of g-C3N4/FexP composite: The low-temperature phosphidation method reported in our previous work is used to prepare the g-C3N4/FexP composite.30,31 In a typical synthesis, the asobtained g-C3N4/FeOOH (100 mg) and monohydrate sodium hypophosphite (50 mg) are mixed together and ground to a fine powder. The mixture is then calcinated at 300 °C for 2 h at a heating rate of 2 °C min−1 in an argon gas atmosphere. The obtained product is washed with water and ethanol and dried at 80 °C. The resultant product is denoted as g-C3N4/FexP−Y (Y = 0.04, 0.08, 0.15, and 0.22 wt %), where Y represents the measured weight percent of Fe. The same procedure is used to obtain phosphidation-treated g-C3N4 in the condition of pristine g-C3N4 (100 mg) and monohydrate sodium hypophosphite (50 mg). Pristine FexP is prepared by the same method using FeOOH (30 mg) and monohydrate sodium hypophosphite (150 mg). The XRD patterns and TEM image of pristine FexP are provided in Figure S2. Characterization. The XRD patterns are recorded on a XRD-6000 X-ray diffractometer (Shimadzu, Japan) using Cu Kα radiation at the wavelength of λ = 1.541 Å. The TEM image, EDX spectrum, and elemental mapping are collected using EDX analysis on a Tecnai G2 F20 scanning transmission electron microscopy instrument (FEI, U.S.A.). The SEM images are collected on a JSM-6340F scanning electron microscope (JEOL, Japan). The FTIR spectra are recorded using PerkinElmer Fourier Transform Infrared Spectrometer GX (PerkinElmer, U.S.A.). X-ray photoelectron spectroscopy (XPS) analysis is conducted using an ESCALAB 250 Xi X-ray photoelectron spectrometer (Thermo, U.S.A.) with Al Ka line as the excitation source (hν = 1484.8 eV), and adventitious carbon (284.8 eV for binding energy) is used as a reference to correct the binding energy of the sample. The inductively coupled plasma optical emission spectrometer (ICP-OES) with the model of Agilent 725 (Agilent, U.S.A.) is used to measure the content of Fe element. UV−vis diffuse reflectance spectra are measured on a Lambda 750 UV/vis/NIR spectrophotometer (PerkinElmer, U.S.A.). Photoluminescence (PL) spectra are recorded on a RF5301 spectrofuorophotometer (Shimadzu, Japan) with an excitation wavelength of 325 nm. The surface photovoltage (SPV) spectrum is determined based on a self-made instrument.33 Photocatalytic H2 Evolution. The photocatalytic experiments are performed in a 37 mL flask at ambient temperature using a 300 W Xe lamp equipped with a UV cut off filter (λ > 420 nm). The intensity of the light source is estimated to be 180 mW cm−2. A total of 10 mg of photocatalyst is added to a solution mixture of 9 mL of water and 1 mL of triethanolamine (TEOA). Before each experiment, the suspension is purged with N2 gas for 30 min to remove oxygen. The amount of generated H2 at intervals is measured by gas chromatography (Agilent 7890A) with thermal conductivity detector.
as good candidate cocatalysts for g-C3N4, including MoS2,18,19 NiSx (x = 1 and 2),20−22 CoS,20,23 and Ni(OH)2.24,25 However, these cocatalysts are reported to have the drawback of instability during the photocatalytic reaction.26 Transition metal phosphides, such as Ni2P, CoP, MoP, FeP, and Fe2P, are an important class of compounds with metallic characteristics and good electrical conductivity.27 They have been found to have a high electrochemical catalysis activity and good stability for the H2 evolution reaction in acid or alkali solutions.27 Based on the high electron-transfer activity and good stability, at present typical Ni2P and CoP have been used as efficient cocatalysts for g-C3N4 to accelerate the charge transfer and separation, resulting in the enhanced H2 evoltuion activity. For example, Yan et al. prepared the noble-metal-free CoP/g-C3N4 hybrid photocatalyst that showed much higher H2 evolution activity than pristine g-C3N4 due to the good absorption ability of visible light, highly effective separation, and low charge recombination rate in the presence of CoP cocatalyst.28 Chen and Xu et al. reported the significantly enhanced H2 evolution activity of g-C3N4 after loading CoP cocatalyst and revealed that the unique P(δ−)−Co(δ+)−N(δ−) surface bonding states led to the accelerated charge transfer and separation, resulting in the high photocatalytic activity of CoP/ g-C3N4.29 In our recent studies, Ni2P and CoP loaded g-C3N4 composites were found to exhibit efficient and stable performance in the photocatalytic H2 evolution reaction.30,31 Fe is the most abundant transition metal, and its price is typically at least 2 orders of magnitude less than that of other highly abundant and catalytically relevant metals, including Ni and Co.26,32 These advantages make Fe based phosphides attractive alternative cost-effective cocatalysts. For example, Du et al. reported the enhanced photocatalytic H2 production activity of CdS in water under visible light using noble-metalfree Fe2P as an active cocatalyst.33 Fu et al. constructed FeP/ CdS composite photocatalyst with much higher visible-lightdriven H2 evolution activity than pristine CdS.26 Lewis et al. confirmed that FeP as cocatalyst deposited on the TiO2 surface showed exceptionally active for sustained H2 evolution under UV light irradiation.32 However, according to our knowledge, there is no relevant study available in the literature about Fe based phosphides as cocatalysts for g-C3N4 to obtain improved H2 generation performance by photocatalysis. As a result, in this work the g-C3N4 modified by iron phosphide cocatalyst (g-C3N4/FexP) is obtained for the first time. They are synthesized by two-step hydrothermal and phosphidation method. The loading mass of FexP can be adjusted by systematically controlling the amount of FeOOH precursor generated from the initial hydrothermal process. The structural characteristics of g-C3N4/FexP are well characterized and recognized. The photocatalytic activity of g-C3N4/FexP is discussed in the H2 generation reaction. The stability of gC3N4/FexP is evaluated in cycling experiments. The H2 evolution activity of g-C3N4/FexP is compared with that of noble metal Pt loaded g-C3N4. The interfacial charge transfer is investigated between g-C3N4 and FexP, and a possible photocatalytic mechanism is proposed for g-C3N4/FexP in the H2 evolution reaction.
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RESULTS AND DISCUSSION Photocatalyst Structure. Figure 1 shows the XRD patterns of pristine g-C3N4 and g-C3N4/FexP composites with different FexP loading amounts. Pristine g-C3N4 and g-C3N4/ FexP composites all have two distinct diffraction peaks at 27.6° and 13.0° that can be indexed to the (022) and (110) planes of g-C3N4 with graphitic structure (JCPDS no. 87-1526). This
EXPERIMENTAL SECTION
Preparation. Preparation of pristine g-C3N4: A simple heating treatment method is used to prepare pure g-C3N4, which has been reported in our previous work.30,31 In short, urea (40 g) is added in a crucible with a cover and heated at 550 °C for 4 h in a muffle stove. 8054
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results from the Cu grid used to obtain the TEM image. When FexP (section 3) is characterized by HRTEM technique, two interplanar spacings of ca. 0.20 and 0.27 nm are found (Figure 2d). They are in good accordance with the (201) lattice plane of Fe2P and (011) lattice plane of FeP, respectively. The same results are obtained for g-C3N4/FexP-2.58, as shown in Figure S3. The XPS spectra of typical g-C3N4/FexP-0.08 are recorded in Figure 3. In the high resolution XPS spectrum of C 1s (Figure 3a), there are three deconvolution peaks at 284.8, 286.1, and 288.4 eV, respectively. They correspond to graphite carbon atoms, carbon atoms in C-NH2 species, and the sp2-hybridized carbon in the aromatic ring (N−CN).28 As displayed in the high resolution N 1s spectrum, three deconvolution peaks at 398.9, 400.0, and 401.1 eV observed are assigned to the sp2 N atoms in the triazine units, bridging N in the N-(C)3 of N−H, and N in the heterocycles and cyano groups, respectively.34 These results are well consistent with that for g-C3N4 alone. There is a weak peak appearing at 404.4 eV that can be attributed to terminal nitrate groups, charging effects, or π excitations.17 Due to the low loading amount and self-resistance to the XPS signal of FexP, there is a very weak XPS signal observed for FexP,35,36 as shown in Figure S4. However, a peak is found to appear at 133.3 eV in the high resolution P 2p spectrum (Figure S4a). It is typical for P−N coordination indicating that some C in g-C3N4 is probably replaced by P in the phosphidation process.37,38 However, in the FTIR spectrum of g-C3N4/FexP-0.08 (Figure S5), no obvious vibration evidence of P-doing (at around 950 cm−139,40) is observed. This result indicates a very small amount of P-doping exists. In order to clearly identify the XPS signals of loaded FexP on the g-C3N4 surface, the g-C3N4/FexP-2.58 is selected to be investigated. As shown in Figure 3c, besides the peak at
Figure 1. XRD patterns of pristine g-C3N4 and g-C3N4/FexP composites. The inset is the XRD patterns of g-C3N4/FexP-2.58.
result indicates that the introduction of FexP does not incur obvious change on the structure of g-C3N4. According to the XRD patterns of pristine FexP (Figure S2), we find the asobtained pristine FexP has a mixed crystal structure of FeP (JCPDS no. 65-2595) and Fe2P (JCPDS no. 51-0943). As to the as-prepared g-C3N4/FexP composites, no diffraction peak assigned to FexP is observed. This is due to the low loading mass of FexP on the g-C3N4 surface. When the loading content of FexP is significantly increased (g-C3N4/FexP-2.58), as presented in the inset of Figure 1, the FexP (x = 1 and 2) diffraction peaks are found to be more apparent. Figure 2a presents the TEM image of g-C3N4/FexP-0.08. The FexP are found to be deposited on the g-C3N4 surface. The loaded FexP has a nanorod morphology that is consistent with pristine FexP (Figure S2b). In order to determine the morphology of g-C3N4, section 1 is chosen to be magnified. A layered structure is clearly observed for g-C3N4 (Figure 2b). On the basis of EDX spectrum of FexP (section 2), elemental Fe and P are clearly found (Figure 2c). The weak signal of C and N arises from the g-C3N4 matrix. The emerging Cu signal
Figure 2. TEM image of g-C3N4/FexP-0.08 (a). TEM image of g-C3N4 in g-C3N4/FexP-0.08 (b). EDX spectrum (c) and HRTEM image (d) of loaded FexP in g-C3N4/FexP-0.08. 8055
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Figure 3. High resolution spectra of C 1s (a) and N 1s (b) in g-C3N4/FexP-0.08 and P 2p (c) and Fe 2p (d) in g-C3N4/FexP-2.58.
Photocatalytic Activity. Figure 5a shows the photocatalytic H2 generation activity of different photocatalysts. During a reaction time of 2 h, the amount of H2 evolved over gC3N4/FexP-0.08 composite catalyst reaches up to 166.4 μmol g−1. In comparison, the H2 evolution amounts over pristine gC3N4 and the corresponding g-C3N4/FeOOH precursor are 0.6 and 1.2 μmol g−1, respectively. The H2 evolution rate over gC3N4/FexP-0.08 is about 277 and 139 times that over pristine g-C3N4 and g-C3N4/FeOOH precursor, respectively. Besides, in order to exclude the effect of g-C3N4 with the same phosphidation treatment (phosphidation-treated g-C3N4), we investigate its H2 evolution activity, but the corresponding H2 evolution amount is only 5.2 μmol g−1 in 2 h. This result demonstrates that phosphidation-treated g-C3N4 nearly has a negligible effect on the H2 generation over g-C3N4/FexP-0.08. In our recent work,41 with the assistance of the XPS technique, we discover the P-doping as a main action mode between P and g-C3N4 is existed in this phosphidation-treated g-C3N4 almost with the same doping amount as that in g-C3N4/FexP-0.08 due to the nearly unchanged XPS peak area (Figure S3a). As a result, the much lower H2 evolution amount of phosphidationtreated g-C3N4 also indicates the inappreciable effect of P-doing in g-C3N4/FexP-0.08. In addition, the mixture of g-C3N4 and FexP (0.12 wt %) presents low H2 evolution amount of 7.6 μmol g−1 under the same condition. Based on these results, we could thus make a conclusion that the distinct enhancement of H2 generation activity over g-C3N4/FexP-0.08 results from the effective interaction between g-C3N4 and FexP species. The photocatalytic H2 evolution activity of g-C3N4/FexP composites largely depends on the loading amount of FexP. As given in Figure 5b, when the weight percent of FexP increases to 0.12 wt %, the highest H2 evolution amount could be obtained during the reaction time of 2 h. However, the H2 evolution activity gradually decreases with further increment in FexP loading which, on the one hand, originates from the excess of FexP species seriously blocking the absorption of the incident light by g-C3N4. This is supported by the UV−vis diffuse reflection spectra of g-C3N4/FexP composites (Figure 4) and
133.3 eV, a new peak appears at 129.4 eV. It is attributed to the P in FexP.35,36 Figure 3d gives the high resolution XPS spectrum of Fe 2p. The peak at 707.3 eV corresponds to Fe 2p3/2 that is in good accordance with that in FexP.35,36 The UV−vis diffuse reflection spectra of pristine FexP and gC3N4 and g-C3N4/FexP composites with different FexP loading amounts are given in Figure 4. Evidently, pristine FexP has a
Figure 4. UV−vis diffuse reflection spectra of pristine FexP and gC3N4 and g-C3N4/FexP composites with different FexP loading amounts.
high adsorption in the range from 350 to 800 nm. On the other hand, pristine g-C3N4 has an absorption edge at about 450 nm, which corresponds to a band gap value of 2.71 eV for g-C3N4. When FexP is introduced, the g-C3N4/FexP shows nearly the similar absorption edge as compared to pristine g-C3N4, indicating that there is hardly obvious structural change of gC3N4 observed in the loading process of FexP. However, in comparison with pristine g-C3N4, a broader absorption in the wavelength range from 450 to 800 nm is founded for g-C3N4/ FexP composite photocatalysts. This is due to the introduction of FexP with high absorption ability of visible light. In addition, the absorption intensity for g-C3N4/FexP in the visible region gradually enhances with the increasing content of FexP loading. Based on these characterization techniques above, we can conclude that the FexP component is successfully deposited on the g-C3N4 surface. 8056
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Figure 5. (a) H2 evolution activities over different photocatalysts. (b) H2 evolution activities over g-C3N4/FexP composites with different FexP loading amounts. (c) Comparison of photocatalytic H2 evolution activity between g-C3N4/FexP-0.08 and g-C3N4/Pt-0.12. (d) H2 evolution stability over g-C3N4/FexP-0.08 in the reused experiments. Notably, the involving content of FexP in the g-C3N4/FexP composite is calculated by reference to FeP on the basis of the measured Fe content.
well consistent with previous work.19,21 On the other hand, the surplus FexP species reduce the oxidation reaction sites on the g-C3N4 surface, thus leading to a decreased H2 evolution activity of g-C3N4/FexP.42 The photocatalytic H2 evolution activity of g-C3N4/FexP0.08 is compared with that of noble metal Pt cocatalyst modified g-C3N4. In order to maintain an unchanged loading amount, the g-C3N4/Pt heterostructure with Pt weight percent of 0.12 wt % (labeled as g-C3N4/Pt-0.12) is prepared via an in situ photodeposition method.43 Under the same reaction conditions, the g-C3N4/FexP-0.08 nearly exhibits the equivalent H2 evolution amount with g-C3N4/Pt-0.12, as shown in Figure 5c. This result indicates that FexP possibly becomes an alternative cocatalyst for the replacement of expensive and scarce Pt due to its high efficiency and much lower cost. Stability is a decisive factor when considering the practical application of a photocatalyst. In order to evaluate the stability of g-C 3 N 4 /Fe xP-0.08, we performed the time-circle H 2 evolution experiment, and the results are presented in Figure 5d. After five runs of photocatalysis, negligible degradation in terms of H2 production is observed. This result demonstrates that g-C3N4/FexP-0.08 photocatalyst possesses stable activity for H2 generation. This robust photocatalytic stability possibly results from highly efficient electron transfer between FexP and g-C3N4, and high stability of FexP for H2 evolution in aqueous media.35,36 Such heterostructure may be expected to be a promising noble-metal-free candidate photocatalyst for H2 evolution with high efficiency. Photocatalytic Mechanism. In the process of H 2 evolution reaction over g-C3N4/FexP-0.08 photocatalyst, the g-C3N4 in g-C3N4/FexP-0.08 is excited under the irradiation of visible light and then produces electron−hole pairs. In contrast to pristine g-C3N4 facing the problem of rapid recombination of electron−hole pairs, the photogenerated electrons in the conduction band (CB) of g-C3N4 could be easily transferred to surface FexP cocatalyst for H2O reduction to H2 in the gC3N4/FexP-0.08 composite due to the intimate contact with the
FexP cocatalyst. The holes in the valence band (VB) of g-C3N4 are quickly quenched by a TEOA sacrificial agent. The presence of FexP cocatalyst accelerates the separation and transfer of photogenerated electrons of g-C3N4 thus resulting in the enhanced photocatalytic activity in the H2 evolution reaction. As shown in Figure 6, the schematic diagram of H2 evolution over g-C3N4/FexP-0.08 photocatalyst is provided.
Figure 6. Schematic diagram of photocatalytic H2 evolution over gC3N4/FexP-0.08 photocatalyst under visible light irradiation.
To verify the accelerated transfer and separation of photoexcited charges in g-C3N4/FexP-0.08, the PL and SPV experiments are performed, and the results are given in Figure 7. According to the PL spectra (Figure 7a), it is found that the g-C3N4/FexP-0.08 has a lower peak intensity at about 450 nm in comparison with pristine g-C3N4. This result demonstrates that the charge recombination of g-C3N4 can be efficiently suppressed with FexP modification.30,31 In the SPV spectra (Figure 7b), both pristine g-C3N4 and g-C3N4/FexP-0.08 present obvious positive photovoltage responses when irradiated by light ranging from 300 to 450 nm, and the higher signal intensity is determined for g-C3N4/FexP-0.08 than pristine g-C3N4. These results indicate a stronger photoelectric 8057
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Figure 7. Comparisons of PL spectra (a) and SPV spectra (b) between pristine g-C3N4 and g-C3N4/FexP-0.08 composite photocatalyst.
C3N4/FexP-0.08, which is obviously higher than pristine gC3N4, and is almost comparable with noble metal Pt decorated g-C3N4. Additionally, g-C3N4/FexP-0.08 shows robust stability for H2 generation. The introduction of FexP cocatalyst significantly accelerates the separation and transfer of photogenerated charges thus leading to the highly efficient H2 evolution activity of g-C3N4. Moreover, the adjacent Fe and P atoms in FexP act as dual proton adsorption sites to produce a synergistic effect, facilitating the fast generation of H2 from water. It is believed that this work is important for developing high-performance and low-cost photocatalytic materials in H2 evolution reaction.
intensity and higher separation efficiency of photogenerated charge for g-C3N4 in the presence of FexP coccatalyst.33 On the basis of XPS results of elemental Fe and P in the gC3N4/FexP composite (Figure 3c,d), we find the peak of Fe 2p is positively shifted in contrast to metallic Fe (706.8 eV44) while the P 2p peak is negatively shifted with respect to elemental P (130.2 eV45), implying a transfer of electron density from Fe to P. In this case, Fe and P atoms have a partial positive (δ+) and negative (δ−), respectively. Combined with previous research results,26,35 we believe the FexP component in g-C3N4/FexP composite features pendant basic P(δ−) in close proximity to Fe(δ+), and that Fe(δ+) and P(δ−) can act as the hydride-acceptor and proton-acceptor centers, respectively. First, due to the presence of long pair electrons in 3d and 3p orbitals for Fe(δ+) and P(δ−), respectively, one proton from a H2O molecule can be adsorbed on their surfaces forming a transition state of two molecules adsorption (step 1). The electrons of the H−O bond in a H2O molecule transfer to the O atom to produce a dual protonation transition state along with the releasing of OH− ions (step 2). The photogenerated electrons in CB of g-C3N4 transfer to the unoccupied d orbital of the Fe(δ+) atom leading to the formation of the hydride at the Fe(δ+) center which then combines with the proton at the adjacent P(δ−) center to form a H2 molecule (step 3). Evidently, the adjacent Fe and P atoms in FexP act as dual proton adsorption sites to produce a synergistic effect, contributing to the fast H2 generation from H2O. This result is in good accordance with that reported in recent studies.29,30 The schematic diagram of charge transfer and reaction mechanism for FexP on the g-C3N4 surface is shown in Figure 8.
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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/acssuschemeng.7b01665. Figure S1: XRD patterns (a) and SEM image (b) of pristine FeOOH. Figure S2: XRD patterns (a) and TEM image (b) of pristine FexP. Figure S3: TEM image (a) and STEM-EDX mapping (b) of g-C3N4/FexP-2.58 and TEM (c) and HRTEM (d) images of loaded FexP in gC3N4/FexP-2.58. Figure S4: High resolution spectra of P 2p (a) and Fe 2p (b) in g-C3N4/FexP-0.08. Figure S5: FTIR spectrum of g-C3N4/FexP-0.08. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected].
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ORCID
CONCLUSIONS In conclusion, a novel g-C3N4/FexP hybrid photocatalyst consisting of earth-abundant elements has been presented. The highest H2 production activity is determined for typical g-
Hui Zhao: 0000-0003-3063-5720 Yuming Dong: 0000-0002-2999-1325 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the support from the Startup Foundation for Introducing Talent of NUIST (No. 2243141601061). The authors are very grateful to Prof. Zhichuan J. Xu and Dr. Shengnan Sun in Nanyang Technological University, Singapore for their help in preparation, characterization, and performance of g-C3N4/FexP.
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Figure 8. Schematic diagram of charge transfer and reaction mechanism for FexP cocatalyst on the g-C3N4 surface to accelerate H2 evolution.
REFERENCES
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ACS Sustainable Chemistry & Engineering
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DOI: 10.1021/acssuschemeng.7b01665 ACS Sustainable Chem. Eng. 2017, 5, 8053−8060
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DOI: 10.1021/acssuschemeng.7b01665 ACS Sustainable Chem. Eng. 2017, 5, 8053−8060
