In Situ Hydrothermal Construction of Direct Solid-State Nano-Z

Jun 16, 2017 - Moreover, very few works have focused on the geometry architectures of g-C3N4-based Z-scheme systems, such as size, spatial distributio...
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In Situ Hydrothermal Construction of Direct Solid-State Nano-ZScheme BiVO4/Pyridine-Doped g‑C3N4 Photocatalyst with Efficient Visible-Light-Induced Photocatalytic Degradation of Phenol and Dyes Qingguo Meng,† Haiqin Lv,† Mingzhe Yuan,† Zhen Chen,‡ Zhihong Chen,*,†,§ and Xin Wang*,‡ †

Shenyang Institute of Automation, Guangzhou, Chinese Academy of Science, No. 1121, Haibin Road, Nansha District, Guangzhou City, Guangdong Province 511458, P.R. China ‡ Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, No. 378, Outer Ring Road of Guangzhou University City, Panyu District, Guangzhou City, Guangdong Province 511400, P.R. China § Guangzhou Science and Technology Innovation Service Center, No. 25, South Ring Road, Nansha District, Guangzhou City, Guangdong Province 510640, P.R. China S Supporting Information *

ABSTRACT: In the current study, a mediator-free solid-state BiVO4/ pyridine-doped g-C3N4 nano-Z-scheme photocatalytic system (BDCN) with superior visible-light absorption and optimized photocatalytic activity was constructed via an in situ hydrothermal method for the first time. The pyridine-doped g-C3N4 (DCN) nanosheets show strong absorbance in the visible-light region by pyridine doping, and the BiVO4 (∼10 nm) nanoparticles are successfully in situ grown on the surface of DCN nanosheets by the controlled hydrothermal method. Under the irradiation of visible light (λ > 420 nm), the BiVO4/DCN nanocomposite photocatalysts efficiently degrade phenol and methyl orange (MO) and display much higher photocatalytic activity than the individual DCN, bulk BiVO4, or the simple physical mixture of DCN and BiVO4. The greatly improved photocatalytic ability is attributed to the construction of the direct Z-scheme system in the BiVO4/DCN nanocomposite free from any mediator, which leads to enhanced separation of photogenerated electron−hole pairs, as confirmed by the photocurrent analysis. The possible Z-scheme mechanism of the BiVO4/DCN nanocomposite photocatalyst was investigated by transient time-resolved luminescence decay spectrum, active species trapping experiments, electron paramagnetic resonance (EPR) technology, and hydrogen evolution test.

1. INTRODUCTION

the photogenerated electron. However, the photocatalytic activity of g-C3N4 is restricted by its insufficient visible-light absorbance, slow charge-carrier transport, and high recombination of photogenerated charge carriers. To overcome these inherent drawbacks of g-C3N4, various methods, including structure regulations,6,7 molecular doping,8−19 and heterojunction,20−28 have been developed to improve the photocatalytic activity and selectivity of g-C3N4. Inspired by the thermal polymerization process of the precursors during the fabrication of C3N4, Wang et al.8 exploited a molecular doping strategy in which small organic molecules were incorporated into the g-C3N4 network by a copolymerization process, which can not only enhance the light absorbance of g-C3N4 but also create surface dyadic heterostructures to promote the

Semiconductor-based photocatalysis has attracted increasing attentions as it has the potential to eliminate the environmental contamination because of its low cost and eco-friendly nature.1,2 Photocatalysts with efficient solar energy utilization capability and quantum efficiency are necessary for applying the photocatalysis technology into practical applications. Unfortunately, the limitation on practical applications of the most applied semiconductor photocatalysts is that they can only absorb ultraviolet (UV) light.3 As a result, exploiting visiblelight-driven photocatalysts with high efficiency and good durability has been a hot spot in the field of photocatalysis. Recently, graphitic carbon nitride (g-C3N4) has been regarded as a promising visible-light-driven photocatalyst because of its appropriate band gap and unique optical properties with promising performance in the degradation of organic pollutants.4,5 The relatively very negative conduction band (CB) (−1.13 eV) enables a strong reduction potential of © 2017 American Chemical Society

Received: March 21, 2017 Accepted: June 5, 2017 Published: June 16, 2017 2728

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Figure 1. (A) Solid-state 13C NMR spectra of CN, BCN, DCN, and BDCN4 and (B) solid-state 13C NMR spectra of BDCN4 before and after light irradiation.

catalytic property has not been reported yet. Moreover, very few works have focused on the geometry architectures of gC3N4-based Z-scheme systems, such as size, spatial distribution, and morphologies, which have critical influences on the lightharvesting activity, charge-carrier separation, and photocatalytic activity of the g-C3N4-based Z-scheme system. In our current work, a novel and mediator-free solid-state BiVO4/pyridine-doped g-C3N4 (BDCN) nano-Z-scheme photocatalytic system with superior visible-light absorbance ability and photocatalytic activity was demonstrated for the first time, which was obtained by the successful in situ growth of BiVO4 nanoparticles (∼10 nm) on the surface of pyridine-doped gC3N4 (denoted as DCN) nanosheets via a controlled hydrothermal method. The obtained BDCN nano-Z-scheme photocatalytic system was characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), and diffuse reflectance spectrum (DRS). Degradation of phenol and methyl orange (MO) was performed to evaluate the photocatalytic activity of the samples under visible-light irradiation (λ > 420 nm). Furthermore, the possible photocatalytic Z-scheme mechanism was investigated and confirmed via active species trapping experiments, electron paramagnetic resonance (EPR) measurement, and hydrogen evolution test, and the stability and recyclability of the BDCN photocatalytic system were also examined.

separation of the generated charge carriers on the surface of the molecular-doped g-C3N4, thus resulting in a higher photocatalytic activity. Latterly, various types of monomer precursors with cyano and/or amino groups have been developed for integrating organic molecules with different functional groups into the CN frameworks.9−19 However, after modification by the organic molecular doping method, the valence band (VB) position of g-C3N4 was sharply shifted toward the negative potential position, which was detrimental to its applications in the organic pollutant degradation by dramatically decreasing the oxidizability of the organic-molecule-doped g-C3N4.19,29 Therefore, it is highly desired to develop a novel strategy to maintain or enhance the reducibility and oxidizability of the organic-molecule-doped g-C3N4. More recently, the construction of the g-C3N4-based Zscheme system has been developed as another feasible and efficient method to promote the photocatalytic performance of g-C3N4 because of the fact that the Z-scheme charge-carriertransfer channel can facilitate the separation and restrain the recombination of photoinduced charge carriers.30−39 In the Zscheme charge-carrier-transfer channel, the photogenerated electrons in the CB of oxidation semiconductor will recombine with the photogenerated holes in the VB of reduction semiconductor through the Z-scheme charge-carrier-transfer channel rather than the vastly reported heterojunction channel in the composites. Thus, the reducibility of photogenerated electron in the CB of reduction semiconductor and the oxidizability of photogenerated holes in the CB and VB of oxidation semiconductor in the Z-scheme system were retain and enhanced. Therefore, it enlightened us to enhance the reducibility and oxidizability of the organic-molecule-doped gC3N4 by the construction of a mediator-free solid-state g-C3N4based Z-scheme system. The band gap of BiVO4 is 2.40 eV, which is visible-light-responsive semiconductor and thus shows a great potential in visible-light-driven Z-scheme photocatalysts.40 The relatively low VB position makes it a good candidate in the construction of the Z-scheme photocatalytic system with g-C3N4. It is highly anticipated that combining the enhanced light absorbance ability by the organic molecular doping strategy and the preserved or enhanced reducibility and oxidizability by the Z-scheme system strategy, our proposed BiVO4/organic-molecule-doped g-C3N4 Z-scheme photocatalytic system will be quite promising as an excellent photocatalytic system. To the best of our knowledge, the combination of the organic-molecule-doped g-C3N4 by copolymerization and mediator-free solid-state Z-scheme designation to synergistically achieve the enhanced photo-

2. RESULTS AND DISCUSSION 2.1. Structural and Optical Analysis. To verify the successful doping of the pyridine ring into the carbon nitride network in the DCN and BDCN samples, the solid-state 13C NMR spectrum measurement was recorded, and the obtained results are shown in Figure 1A. Compared with the pure gC3N4 (CN) and the BiVO4/undoped g-C3N4 nanocomposite (denoted as BCN, prepared by the same hydrothermal method, and the mass percentage of g-C3N4 was optimized in terms of the results of photocatalytic analysis), the DCN and BDCN4 (40% mass percentage of DCN in BDCN) samples show an additional broad peak centered at 115.8 ppm in the 13C NMR spectra, suggesting that the pyridine ring was doped successfully into the carbon nitride network in the DCN and BDCN4 samples. Moreover, the BDCN4 sample shows the identical solid state 13C NMR spectrum before and after photocatalytic reaction (Figure 1B), suggesting that the pyridine ring in the carbon nitride network is stable under light irradiation. The XRD patterns was taken to detect the crystalline and chemical structures of the samples, and the outputs are shown 2729

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in Figure 2. Two pronounced peaks in the DCN sample at 13.04° and 27.47° are observed, corresponding to the (100)

Figure 3. Ultraviolet−visible (UV−vis) DRS of the samples and the absorption edge of BiVO4 and DCN (inset).

broader absorption edge at 576 nm, relating to a band gap energy of 2.16 eV. After combined with BiVO4, the obtained BDCN composites present a gradual shift to the red edge of the absorption band as the DCN contents increase, indicating that DCN has a significant light-harvesting ability in the BDCN composite photocatalysts. From Figure S3, it can be found that both DCN and BDCN samples show much wider light absorption range than the pure CN and BCN samples, suggesting that the pyridine doping can efficiently extend the light absorbance of the doped carbon nitride and BDCN samples. The morphology of DCN, BiVO4, and BDCN was characterized by scanning electron microscopy (SEM), as shown in Figure 4. DCN shows the crumpled layered structure

Figure 2. XRD patterns of DCN, bulk BiVO4, and BDCN composites.

and (002) diffractions of DCN, respectively. The (100) and (002) diffractions are attributed to the in-plane structural packing mode and the interlayer stacking of aromatic networks, respectively.4,5 The diffraction pattern of the bulk BiVO4 sample shows a series of peaks that can be indexed to the monoclinic shelties phase of BiVO4 (JCPDS card #14-0688). In the case of the BDCN nanocomposites, the XRD pattern of BDCN shows both the characteristic diffractions of DCN and the monoclinic shelties of BiVO4, implying that the hydrothermal process leads to the successful in situ formation of a BiVO4 structure and has no dramatic influence on the structure of DCN. In addition, the intensities of DCN diffraction peaks gradually increase from BDCN1 to BDCN5, corresponding to the variation of DCN contents from 10 to 50%. The FT-IR spectra of DCN, BiVO4, and BDCN1−5 samples are shown in Figure S1. The pure BiVO4 sample shows the peaks at 839 and 745 cm−1, which correspond to ν1 symmetric stretching and ν3 asymmetric stretching vibrations of VO4, respectively. The DCN sample shows typical peaks at 900− 1500 and 809 cm−1, representing the typical stretching vibration of CN heterocyclic and plane breathing and vibration of triazine units, respectively. Moreover, the broad peak at 2800−3450 cm−1 is ascribed to the adsorbed O−H vibration in water molecules and the N−H vibration because of the surface uncondensed amine groups. For the BDCN composites, all BDCN samples show the characteristic peaks of both DCN and BiVO4, further confirming the composite structure between DCN and BiVO4. It also indicates that the hydrothermal treatment did not change the C−N heterocycles and the chemical bond dramatically in the carbon nitride structure. The actual mass percent of DCN in the composite photocatalyst was determined by thermogravimetric analysis (TGA). DCN was decomposed at 1000 K, whereas BiVO4 remained stable above 1000 K (Figure S2). Thus, the mass percent of DCN in the composite photocatalysts was estimated by the mass loss between 300 and 1000 K, which was determined as 10.8, 19.9, 30.4, 39.9, and 49.7% for the samples BDCN1−BDCN5, respectively. This is in good agreement with the theoretical mass percentages. The optical properties of the DCN, BiVO4, and BDCN composites were monitored by DRS, as shown in Figure 3. Pure bulk BiVO4 shows a steep absorption edge at around 524 nm, which reveals a band gap energy of 2.37 eV. DCN has a

Figure 4. SEM images of DCN (A), bulk BiVO4 (B), BDCN4 (C), and their corresponding EDS patterns (D).

containing several stacking layers, indicating the planar graphite-like structure of carbon nitride. The blank BiVO4 exhibits a decagonal shape with a size of 150−300 nm. Compared with the morphology of DCN and BiVO4, the smallsized BiVO4 particles with a size of about 10 nm are found to disperse on the crumpled DCN surface, which will be further confirmed by the TEM images. Additionally, the energydispersive spectrometry (EDS) was also adopted to analyze the 2730

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is clearly evidenced in Figure 5A, as evidenced by the result of SEM. In the absence of DCN, the BiVO4 particles obtained by the hydrothermal route show the agglomerations with diameters ranging from 150 to 300 nm, and no nanosized BiVO4 particles are obtained (Figure 5B). However, it is very interesting to note from Figure 5C that the BiVO4 nanoparticles with the average size of approximately 10 nm were successfully in situ synthesized and deposited on the surface of DCN uniformly in the presence of DCN during the hydrothermal process after precise control over the hydrothermal synthesis. Compared with the bulk BiVO4, the size of in situ synthesized BiVO4 decreases obviously with the introduction of DCN. Bi3+ ions can be bound on the surface of DCN by chemisorption between the Bi3+ ion and the heptazine rings of DCN, which successfully prevents the overgrowth and agglomeration of the BiVO4 particles. This demonstrates that the in situ hydrothermal method is a feasible route to construct the uniform BiVO4/DCN nanocomposite photocatalyst. The HRTEM image in Figure 5D shows that the lattice spacings of 0.292 and 0.308 nm correspond to the (040) and (121) planes of monoclinic shelties of BiVO4, respectively, and the lattice spacing of 0.324 nm corresponds to the (020) plane of DCN. Figure 5D also reveals the intimate interface between DCN nanosheets and BiVO4 nanoparticles, indicating that the BiVO4 nanoparticles are tightly attached to the surface of DCN, which is beneficial to the transport of photoinduced charge carriers. X-ray photoelectron spectroscopy (XPS) technology was applied to further study the surface chemical composition and the oxidation state of the elements in the BDCN4, as shown in Figure S4. The C 1s, N 1s, Bi 4f, V 2p, and O 1s are all found in the survey XPS spectrum (Figure S4A). The peak of C 1s at 284.6 eV can be attributed to the adventitious carbon on the surface of g-C3N4, and the two peaks at 286 and 288.2 eV both belong to the sp2-hybridized C [C−(N)3] (Figure S4B). The

elements in the BDCN4 nanocomposite, and the result is given in Figure 4D. It can be clearly found that C, N, Bi, V, and O elements exist in the BDCN4 samples, which further confirms the compositional structure of the BDCN samples. Moreover, the morphology of DCN, BiVO4, and contact interface between DCN and BiVO4 in the BDCN sample was analyzed by TEM and high-resolution TEM (HRTEM), as shown in Figure 5. The typical graphite-like structure of DCN

Figure 5. TEM images of DCN (A), bulk BiVO4 (B), BDCN4 (C), and HRTEM image of BDCN4 (D).

Figure 6. Photocatalytic activities of the samples on the photodegradation and corresponding rate constants for phenol (A,B) and MO (C,D) under visible-light irradiation (λ > 420 nm). 2731

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photodecomposition in water over the photocatalysts. As illustrated in Figure 7, the TOC in the phenol and MO solution

main features of N 1s include a broad and wide peak from 398 to 403.5 eV (Figure S4C), in which the existence of the sp2bonded DCN in the BDCN4 nanocomposites was verified by the sp2-hybridized nitrogen (CN−C) of N 1s peak at 398.8 eV. The peak at 400.1 eV corresponds to tertiary nitrogen N− (C)3 groups, and the peak at 401.3 eV relates to the effects of charging reaction.17−19 The binding energies of Bi 4f7/2 and Bi 4f5/2 are 164.3 and 159.2 eV, respectively (Figure S4D), and the two peaks at 516.6 and 526.1 eV can be attributed to V 2p (Figure S4E). In the O 1s region (Figure S4F), one peak at 529.4 eV belongs to the Bi−O bonds of (Bi2O2)2+ units and one minor peak at 530.1 eV is attributed to the −OH groups on the surface. The XPS results further demonstrate the composition of DCN and BiVO4 in the prepared BDCN nanocomposites. 2.2. Photocatalysis Analysis. The photocatalytic activities of the as-prepared DCN, BiVO4, BCN, and BDCN samples were examined by the photodegradation of phenol (Figure 6A,B) and MO (Figure 6C,D) under visible-light irradiation (λ > 420 nm). Because of the poor oxidation ability of DCN associated with the relatively high level of VB position, the photodegradation of phenol is not quite efficient. The DCN sample presents only 21.2% removal efficiency within 150 min (Figure 6A) on phenol. The bulk BiVO4 shows better oxidation ability because of its low level of VB position, achieving 49.5% removal efficiency in the degradation of phenol in 150 min. Interestingly, the BDCN nanocomposites demonstrate much higher activities than pristine DCN or bulk BiVO4, up to 69.8, 77.3, 87.4, 97.1, and 92.0% removal efficiency by BDCN1, BDCN2, BDCN3, BDCN4, and BDCN5 nanocomposite photocatalysts, respectively, which increase with the doping concentration of DCN from 10 to 40% but levels off at 50%. This decreased photocatalytic activity was due to the decrease in the light absorbance of BiVO4 and/or the enhanced recombination of photoinduced charge carriers when the mass percent of DCN is over 50%. In the presence of the BDCN4 nanocomposite, which shows the best photocatalytic activity over all BDCN nanocomposite photocatalysts, nearly 92% of phenol and 97% of MO molecules (Figure 6C) are photodecomposed within 150 min under visible-light irradiation. Moreover, the apparent rate constant for phenol photodecomposition obtained from the BDCN4 sample is 0.02202 min−1, which is about 13.7 times higher than that of the DCN sample (0.0016 min−1) and 5.0 times higher than that from the BiVO4 sample (0.0044 min−1) (Figure 6B). The apparent rate constant for the MO decomposition obtained from the BDCN4 sample is 0.0215 min−1, which is about 6.3 times higher than that from the DCN sample (0.0034 min−1) and 4.2 times higher than that from the BiVO4 sample (0.0051 min−1) (Figure 6D). Moreover, the BDCN4 sample shows a more effective photocatalytic activity than the pyridine-free undoped BiVO4/g-C3N4-4 (the BCN nanocomposite with the mass percentage of g-C3N4 at 40%), a simple physical mixture of DCN and BiVO4 (40% of DCN by mass), BiVO4/CN samples made by a mixing-calcination method, and BiVO4/CN made by ultrasound-assisted41 and calcination methods,42 implying there occurred more efficient charge-carrier separation and transfer in the BDCN nanocomposite than those in the simple physical mixture, mixing-calcination-prepared sample, and much more efficient visible-light-harvesting capability than the BCN sample (Figure S5). The total organic carbon (TOC) analysis was also applied to evaluate the mineralization ratio of organic contaminant

Figure 7. TOC removal of phenol (A) and MO (B) over the BDCN4 sample under visible-light irradiation.

during photodegradation decreases with the irradiation time and down to 38.6 and 26.3% after 150 min irradiation, respectively, indicating that about 73.6 and 61.4% organic carbon in phenol and MO are photodegraded into inorganic carbon (CO2). The TOC results indicate that phenol and MO are extensively degraded by the photocatalytic process over the BDCN nanocomposites, rather than just being decolorized to some organic intermediates, which may be still hazardous to the environment. However, the TOC in the phenol and MO solution did not decrease to zero, suggesting that there must be some intermediate products in the final solution. The main intermediate products and their distribution in the photodegradation of phenol and MO in the presence of BDCN4 were detected by liquid chromatography−mass spectrometry (LC−MS), and the mass spectrum intensity variation of the intermediate products is displayed in Figure 8. The m/z of phenol, MO, and the corresponding intermediate products in the degradation process are presented in Figure S6. Under visible-light irradiation, phenol (m/z = 94) is directly oxidized into catechol (m/z = 110) and hydroquinone (m/z = 110) first. The peak intensities of catechol and hydroquinone reach a maximum with the irradiation time, which then gradually transfer to maleic anhydride (m/z = 98) through the ring cleavage process and can be ultimately mineralized into carbon dioxide and water. For the MO degradation, after visible-light irradiation, the peak intensity of MO (m/z = 304) was oxidized to some fragment ions including [M − H − SO2]− (m/z = 241), [M − H − C6H4SO3]− (m/z = 147), and [M − H − C6H4(CH3) SO2]− (m/z = 150), which increase and reach the maximum rapidly and then decrease by decomposing them to a smaller molecule fragment, such as [M − H − C6H5SO2]− (m/ z = 162) and [M − H − N2C6H4(CH3)2]− (m/z = 156).43 The interfacial charge-carrier-transfer dynamics between the DCN, BiVO4, and BDCN photocatalysts were assessed by photocurrent−time measurement, and transient photocurrent responses and the results are shown in Figure 9A. Obviously, all BDCN nanocomposites exhibit a higher photocurrent than DCN and BiVO4, indicating that the formation of the BDCN nanocomposite by combining DCN with BiVO4 can facilitate the separation of photogenerated electron−hole pairs. However, BDCN5 shows a decreased photocurrent that is attributed to the excess DCN that decreases the light absorbance of BiVO4 and enhances the recombination of photogenerated electron−hole pairs. In addition, a comparison of the 2732

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Figure 8. Intensity of the intermediate products of (A) phenol and (B) MO photodecomposition in the mass spectrum.

Figure 9. (A) Transient photocurrent responses (0.5 V vs Ag/AgCl) of DCN (a), BiVO4 (b), BDCN1 (c), BDCN2 (d), BDCN3 (e), BDCN4 (f), and BDCN5 (g) and (B) transient photocurrent responses (0.5 V vs Ag/AgCl) of CN (a), BCN (b), DCN (c), and BDCN4 (d).

DCN samples after the addition of AO, IPA, and BQ was also investigated, as shown in Figure 10B,C, from which a conclusion can be drawn that •O2− plays the most crucial role in the oxidation process over DCN, whereas for the pure bulk BiVO4, •OH was the main active species during the photodegradation process. Therefore, in general, the active species of the BDCN nanocomposite in the degradation process is determined to be •O2− and •OH species. For the sake of explaining the enhanced photocatalytic activity and exploring the possible mechanism, the band edge positions of the VB and CB of BiVO4 and DCN were determined by VB XPS and the empirical equation (EVB = ECB + Eg), which is shown in Figure 10D. The band gap energy of DCN was assumed to be 2.16 eV, as discussed above in the results of DRS. According to the VB XPS and empirical eq 2, the VB and CB positions of DCN were calculated as 1.81 and −0.35 eV versus the normal hydrogen electrode (NHE), respectively. For BiVO4, the positions of the VB and CB were calculated by the following empirical eqs 1 and 2.

photocurrent among CN, DCN, BCN, and BDCN4 samples was made, and the results are shown in Figure 9B. The DCN and BDCN4 samples show much higher photocurrent than CN and BCN because of the incorporation of pyridine into the CN network, which can delocalize the aromatic system. This enhances the separation of photogenerated electron−hole pairs. 2.3. Mechanism of Photocatalytic Degradation. A series of active oxygen species, such as hole (h+), hydroxyl radical (•OH), and superoxide radical (•O2−), are hypothesized to be involved in the current photocatalytic degradation of phenol or dye.44 To investigate the photocatalytic mechanism, ammonium oxalate (AO), isopropanol (IPA), and benzoquinone (BQ) were introduced into the photocatalytic oxidation process separately over the BDCN4 photocatalyst and acted as the scavengers for h+, •OH, and •O2−, respectively. The photocatalytic analysis results (Figure 10A) indicate that the addition of IPA and AO slightly hinders the MO degradation rate but the addition of BQ strongly suppresses the MO degradation rate, indicating that •O2− is the major oxygen active species, whereas •OH is the minor oxygen active species in photodegradation. The addition of IPA and AO shows a similar inhibition effect, which suggests that •OH is generated by the reaction between h+ and H2O. In the meanwhile, the photocatalytic experiments over the BDCN4 photocatalyst were also performed under different atmospheres (O2 and N2 gases) to investigate the active oxy-radical, and the results are given in Figure 10B. It is found that the degradation rate is suppressed under an N2 atmosphere but slightly improved by bubbling O2, revealing that O2 dissolved in the suspension acts as the electron trapper, thus resulting in the generation of •O2−. For comparison, the photocatalytic activity of the BiVO4 and

ECB = χ − E e − 1/2Eg

(1)

E VB = ECB + Eg

(2)

where E VB corresponds to the VB edge potential; E CB corresponds to the CB edge potential; χ corresponds to the electronegativity of the semiconductor, which is the geometric average of the absolute electronegativity of the constituent atoms (the χ value of BiVO4 is 6.15 eV); Ee corresponds to the free-electron energy on the hydrogen scale (Ee ≈ 4.5 eV); and Eg corresponds to the band gap energy of the semiconductor. The band gap energy of BiVO4 was calculated as 2.36 eV from 2733

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Figure 10. Photocatalytic activity of BDCN4 (A), BiVO4 (B), and DCN (C) photocatalysts in the process of the MO photodegradation with different scavengers existing under visible-light irradiation (λ > 420 nm) and VB XPS of DCN (D).

Figure 11. EPR spectra (A) of DMPO−•O2− and (B) TEMPO−•OH adducts over the BDCN4, DCN, and BiVO4 in the MO solution before and after the photodegradation process.

the result of DRS. Thus, the CB position of the BiVO4 was calculated to be 0.47 eV versus NHE, and the corresponding VBs was calculated to be 2.83 eV versus NHE. To further investigate the photocatalytic mechanism of BDCN, an EPR spin-trap technique with 5,5-dimethyl-1pyrroline-N-oxide (DMPO) was applied to further confirm the production of the active species •O2− and •OH. Typically, DMPO acts as a radical scavenger by the formation of the stable DMPO−•O2− or the DMPO−•OH adduct species. No obvious EPR signals of DMPO−•O2− and DMPO−•OH adducts in all MO solution were observed in the dark. Under the light irradiation, the four characteristic peaks of DMPO−•O2− adducts (Figure 11A) are found in the solutions of BDCN and DCN, and the six characteristic peaks of the DMPO−•OH adducts (Figure 11B) are found in the solutions of BDCN and BiVO4, suggesting that •O2− was produced as the active species only in DCN and •OH was produced as the active species only in BiVO4, whereas both the •OH and •O2−

radicals were produced as the active species in BDCN photoreaction systems. If the transfer of photogenerated electron−hole pairs of BDCN follows a heterojunction pathway (shown in scheme 1A), because the CB and VB edge potentials of DCN are more negative than the CB and VB edge potentials of BiVO4, the photogenerated electrons in the CB of DCN will migrate to the CB of BiVO4, and the holes in the VB of BiVO4 will transfer to the VB of DCN. However, the electrons in the CB of BiVO4 cannot react with the dissolved oxygen molecule to produce • O2− as the active species because the CB position of BiVO4 is 0.47 V versus NHE, which is more positive than that of O2/•O2− (−0.33 V vs NHE). Similarly, the holes in the VB of DCN cannot react with H2O to produce •OH because the position of VB of DCN is 1.81 V versus NHE, which is more negative than that of H2O/•OH (2.72 V vs NHE). Thus, •OH and •O2− cannot be detected as the active species in the photocatalytic reaction systems if the transfer pathway of the 2734

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photogenerated electron−hole pairs of BDCN follows a heterojunction pathway. To further confirm this hypothesis, the transient timeresolved luminescence decay and photoluminescence (PL) measurement experiment was conducted, and the results are shown in Figure 12. These decay curves are well-fitted to a double-exponential PL decay model, as shown in Figure 12A− C. The fast PL lifetime component (τ1) and the slow PL lifetime component (τ2) are ascribed to the surface-related nonradiative recombination processes and the recombination of free exciton,45 respectively. Compared with BiVO4 (4.78 ns) and DCN (4.95 ns), the BDCN4 sample displays a longer PL decay lifetime of 10.22 ns. Both the τ1 and τ2 and PL life times (τ) of BDCN are longer than those of BiVO4 and DCN, suggesting that the separation of the photogenerated charge carrier of BDCN is higher than those of BiVO4 and DCN.46,47 Moreover, the PL spectra show that the PL intensities of BDCN are higher than those of DCN and BiVO4 (Figure 12D), suggesting a higher recombination probability of the photogenerated charge carrier of the composites than that of DCN and BiVO4. If the charge-carrier transfer of BDCN4 is heterojunction, a faster PL decay kinetics and a lower PL intensity should be observed. However, BDCN4 shows a much slower PL decay process and weaker PL intensity, and as shown in the result of photocurrent−time measurement above, the charge-carrier transfer of BDCN is more efficient than those of other photocatalysts, which are contradictory to traditional heterojunction, suggesting that the photogenerated electron− hole pairs between BiVO4 and DCN follow another transfer channel than heterojunction. Furthermore, the photocatalytic hydrogen generation experiment was carried out, and the result is shown in Figure S7. The average hydrogen generation rate of BDCN is 26.3 μmol/h, which is about two times higher than that of DCN (14.4 μmol/h), whereas BiVO4 cannot generate

Scheme 1. Proposed Photocatalytic Charge-Carrier-Transfer Mechanisms in the BDCN Nanocomposite System under Visible-Light Irradiation: (A) Heterojunction and (B) ZScheme

Figure 12. PL decay traces of (A) BiVO4, (B) DCN, and (C) BDCN4 and (D) photoluminescence of BiVO4, DCN, and BDCN4 excited by 450 nm in air at the normal atmospheric temperature. 2735

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obvious change in the crystalline structure was observed, further demonstrating the superior stability of our BDCN nanocomposite photocatalyst.

hydrogen, revealing that the CB position of BDCN is more negative, which is further against the heterojunction mechanism. Consulting the similar works reported by other groups,36−40,45−48 the Z-scheme charge-carrier-transfer channel existing in natural photosynthesis appears to be more proper for our BDCN photocatalysts, as shown in Scheme 1B. Under visible-light irradiation, both DCN and BiVO4 can be excited and electron−hole pairs are generated. The photoexcited electrons in the CB of BiVO4 are transferred to the VB of DCN quickly through the interface between DCN and BiVO4 and then combine with the holes in the VB of DCN, which leads to a slower PL decay process and a weak PL intensity of BDCN than those of DCN and BiVO4. Consequently, the most negative electrons in the CB of DCN reduce the molecular oxygen dissolved in water to yield •O2−, and the most positive holes in the VB of BiVO4 generate the active •OH radicals. The verification of •O2− ion and •OH radicals as the active species confirms the likely direct Z-scheme mechanism of our photocatalytic system. Namely, the Z-scheme charge-carriertransfer channel is favorable to preserve the strong reducing capacity of electrons in the CB of DCN and the oxidizing capacity of holes in the VB of BiVO4 for the production of •O2− and •OH reactive species, thus leading to significantly efficient separation of photogenerated electron−hole pairs and enhanced photocatalytic performance. In general, the much higher photocatalytic activity of BDCN than that of DCN, bulk BiVO4, BCN, or physical mixture can be ascribed to (1) the enhanced absorption ability by pyridine doping method, (2) the in situ growth of small nanosized BiVO4 particles, and (3) the mediator-free Z-scheme chargecarrier-transfer route between the doped DCN and BiVO4 nanoparticles, which can not only preserve the high oxidation potential of the photogenerated electrons and holes in the CB of DCN and VB of BiVO4 but also reduce the recombination rate of electron−hole pairs. The stability and reusability of the BDCN4 photocatalyst were assessed by the recycling photodegradation experiment, and the result is shown in Figure 13. Even after five times of

3. CONCLUSIONS A direct solid-state nano-Z-scheme BiVO4/DCN photocatalytic system with a superior visible-light-harvesting ability and a photocatalytic activity was synthesized via an in situ hydrothermal method in this work. The BiVO4 nanoparticles were successfully in situ deposited on the DCN nanosheets uniformly. All as-synthesized BiVO4/DCN nanocomposite photocatalysts showed photocatalytic activity superior to those of the pristine DCN, bulk BiVO4, and physical DCN/ BiVO4 mixture under visible-light irradiation (λ > 420 nm). • O2− and •OH are confirmed to be the oxygen active species by the active species trapping experiments and EPR technology. Moreover, the hydrogen evolution test, decreased PL decay process, and increase in the PL intensity further verified that the Z-scheme charge-carrier-transfer channel is more suitable to explain the mechanism than that of heterojunction for the BDCN photocatalyst. The significantly improved photocatalytic activity under visible-light irradiation of our successful design of solid-state nano-Z-scheme principled BiVO4/DCN nanocomposite can be ascribed to the enhanced visible-light absorption and largely reduced the recombination of photogenerated electron−hole pairs. Moreover, the excellent stability and the recycling ability of our BiVO4/DCN nanocomposite photocatalyst ensure its practical applications in the environmental remediation. 4. EXPERIMENTAL SECTION 4.1. Preparation of DCN, BiVO4, and BDCN. The DCN sample was synthesized via organic pyridine doping approach and thermal copolymerization treatment, as reported in our previous research.14 Briefly, 70 mg of 2,6-diaminopyridine (DPY) and 3.0 g of dicyandiamide were dispersed into 15 mL of water, and the mixed solution was maintained at 100 °C under stirring to remove water. Finally, the obtained solid precursor was put in a crucible and maintained at 550 °C for 4 h in a muffle furnace with the heating rate of 15 °C/min under the air atmosphere. The obtained solid pyridine-doped g-C3N4 was denoted as DCN. The BDCN nanocomposite photocatalysts were in situ synthesized by a controlled hydrothermal route. The Bi3+ ions were absorbed on the surface of g-C3N4 by a chemical bond49,50 because of the tri-s-triazine (heptazine) ring structure of g-C3N4, which benefited the growth and dispersion of BiVO4 nanoparticles on the surface of the DCN sheet. The controlled in situ hydrothermal synthesis procedure of BDCN is described as follows: 1.81 g of Bi(NO3)3·5H2O, 0.435 g of NH4VO3, 1.16 g of urea, and different amounts of DCN were mixed in 30 mL of deionized (DI) water. HNO3 (6 M) was used to adjust the pH of the mixture to 1, and the mixture was stirred for 1 h at 25 °C. The obtained mixture solution was sealed into a Teflon-lined stainless steel autoclave (50 mL) and kept at 180 °C for 4 h. After being cooled down to 25 °C naturally, the precipitate was filtered, collected, washed by DI water, and finally maintained at 60 °C for drying. The obtained BDCN nanocomposite photocatalysts with different mass percentages of DCN varying from 10 to 50% were denoted as BDCN1, BDCN2, BDCN3, BDCN4, and BDCN5, respectively. In the absence of DCN, only the bulk BiVO4 sample was obtained by the same hydrothermal process.

Figure 13. Cycle runs of the BDCN4 nanocomposite photocatalyst for MO degradation under visible-light irradiation (λ > 420 nm).

successive recycling photodegradation experiments, the BDCN4 photocatalyst did not show obvious decrease in the photocatalytic degradation activity under visible-light irradiation, implying that the BDCN4 photocatalyst is sufficiently stable for photocatalytic degradation. Moreover, the crystalline structure of BDCN4 photocatalysts after photocatalytic experiments was examined by XRD, as shown in Figure S8. No 2736

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electrochemical properties of the samples in a conventional three-electrode cell. The working electrodes were made by the doctor-blading method. A Pt sheet was used as a counter electrode, and an Ag/AgCl electrode was used as a reference electrode. The Na2SO4 aqueous solution (1.0 M) was used as the supporting electrolyte. The area exposed in the solution and illuminated by the lamp was 0.25 cm2. With a voltage of 0.5 V bias versus Ag/AgCl and a perturbation signal of 10 mV with a frequency at 1 kHz set, the periodic on/off photocurrent response of the modified fluorine-doped tin oxide glass (FTO) was measured.

For comparison, the pure g-C3N4 was prepared by thermal polymerization (denoted as CN). In addition, the BiVO4/ undoped g-C3N4 nanocomposite was also prepared by the same hydrothermal method (denoted as BCN, the mass percentage of g-C 3N 4 was optimized in terms of the results of photocatalytic analysis). 4.2. Characterization. Powder XRD measurement was carried out on a D/Max-IIIA instrument by using Cu Kα radiation (the scanning rate was 0.02°·s−1). The FT-IR spectrum was recorded on a Vector 33 infrared spectrometer. Thermal degradation was identified via TGA (TGA 92-18, Setaram, N2 atmosphere, 313−1173 K). DRS were recorded by a Shimadzu U-3010 spectrophotometer with BaSO4 as the reflectance standard. XPS of the samples was obtained by an ESCALAB 250Xi spectrometer equipped with a pre-reduction chamber. SEM and TEM images were obtained by S-3700 scanning TEM and JEM 2100 field emission TEM. The solidstate 13C NMR spectra were measured by a Bruker Avance III 500 spectrometer. The transient time-resolved luminescence decay spectra were recorded by FLS980 series of fluorescence spectrometers. EPR measurements with and without light irradiation were recorded using a Bruker model A300 spectrometer. Two filters, an IR cutoff (λ < 800 nm) and a UV cutoff (λ > 420 nm), installed between the 300 W Xe lamp and the samples, filtered out the light, which were used as the visible-light source. The TOC of the phenol or MO solution was tested by a high-temperature TOC/TNb analyzer (Liqui TOC II, Elementar, Germany). 4.3. Photocatalytic Activity. Degradation of the phenol and MO solution under visible-light irradiation was performed to evaluate the photocatalytic activities of the catalysts. The phenol compound is considered to be a direct threat to the health of humankind because of its highly toxic, persistent, and biorecalcitrant properties. In a typical experiment, 0.05 g of the as-prepared photocatalyst was dispersed into 150 mL of the phenol or MO aqueous solution with a concentration of 10 mg/L under stirring. Before light irradiation, the suspension was stirred for half an hour in the dark with the light turned off to get the adsorption−desorption equilibrium. After light irradiation, about 5 mL of the suspension was taken every 0.5 h and centrifuged to remove the photocatalysts and get a clear solution in the upper level, of which the absorption spectrum was obtained by a Shimadzu UV-2050 spectrophotometer. The experiments of the active species trapping were carried out with the aim to explore the active species involved during the degradation process and the possible photocatalytic mechanism. A certain amount of AO, BQ, and IPA was introduced into the reaction suspensions to capture the possible active species, such as hole (h+), superoxide radical (•O2−), and hydroxyl radical (•OH), respectively. N2 and O2 gases were also bubbled into the reaction system to further confirm the active species. To test the stability and reusability of the BDCN nanocomposite photocatalyst, the BDCN4 nanocomposite photocatalyst was centrifuged and collected after the photocatalytic test experiment, dried in 80 °C, and redispersed into the fresh phenol or MO solution. The recycling photocatalytic reaction was repeated under the same condition for successive five times, and the XRD pattern of the BDCN4 sample was recorded and compared with that of the original sample to detect any degradation of the photocatalyst after each photocatalytic run. 4.4. Electrochemical Analysis. An electrochemical analyzer (CHI-660E, Chenhua, China) was used to test the

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00338. Additional spectral and characterization data, including FT-IR, TGA, UV−vis, XPS, photocatalytic activity, intermediate products of LC−MS, hydrogen evolution of the samples from water, and XRD before and after photocatalytic degradation (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-02022912534. Fax: +86-020-22912534 (Z.C.). *E-mail: [email protected]. Phone: +86-020-39314813. Fax: 86-020-39314813 (X.W.). ORCID

Zhihong Chen: 0000-0001-7510-5112 Xin Wang: 0000-0002-4771-8453 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is financially supported by NSFC (grant no. 51602111), the National Key R&D Program of China (2016YFB0401502), the “Hundred Talents Program of Chinese Academy Sciences”, Guangdong Province Grant (no. 2015A030310196), the Pearl River S&T Nova Program of Guangzhou (no. 201506040045), and Guangzhou postdoctoral initial funding.

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REFERENCES

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