Carbon-Doped ZnO Hybridized Homogeneously with Graphitic

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Carbon-Doped ZnO Hybridized Homogeneously with Graphitic Carbon Nitride Nanocomposites for Photocatalysis Yun-Pei Zhu,† Min Li,† Ya-Lu Liu,† Tie-Zhen Ren,‡ and Zhong-Yong Yuan*,† †

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), College of Chemistry, Nankai University, Tianjin 300071, China ‡ School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China

J. Phys. Chem. C 2014.118:10963-10971. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/12/18. For personal use only.

S Supporting Information *

ABSTRACT: The nanocomposite photocatalysts of carbon-doped zinc oxide (ZnO) hybridized with graphitic carbon nitride (g-C3N4) were prepared through simple one-step calcination of evaporation-dried mixture of dicyandiamide and zinc nitrate. Compared with pure ZnO and g-C3N4, the absorption of the prepared g-C3N4/ZnO nanocomposites shifted toward lower energy region, and the broader and stronger absorbance in the visible light region was observed, which was related to the content of g-C3N4 in the nanocomposites. The photocatalytic activities of the resultant g-C3N4/ZnO nanocomposites for the degradation of methylene blue (MB) dye under visible light irradiation were enhanced remarkably and much higher than that of g-C3N4. The optimal content of g-C3N4 in the prepared nanocomposites was found at a weight percent of 50.7%, which corresponded to the homogeneous hybridization between ZnO and gC3N4. The improved photocatalytic performance of the g-C3N4/ZnO nanocomposites was ascribed to the elevation of the separation efficiency of photoinduced electron−hole pairs, resulting from the heterojunction established between the interfaces of g-C3N4 and ZnO.

1. INTRODUCTION Semiconductor-mediated photocatalytic degradation of organic pollutants in industrial and household wastewater has become an area of great research interest over the past decades due to its destruction ability of contaminants, broad applicability, and environmental friendly.1−4 In spite of the great development and progress in this field, the main goal has been focused on promoting the efficiency of catalysts to cater for the demand of large-scale industrial applications. So far, a large variety of inorganic semiconductor materials, especially metal oxides and sulfides, have been explored as photocatalysts for environmental purification under UV or visible light illumination.5−10 As one of the most widely and deeply studied oxide semiconductors, ZnO shows the superiority of valuable optical and electronic properties, low cost, and nontoxicity over other kinds of semiconductor materials. However, ZnO can only respond to the UV irradiation that takes up only ∼4% of solar radiation, and the high recombination ratio of photoinduced electron−hole pairs could further restrict its applications to a great extent. And thus much effort is being made to extend the absorbance to visible light region and reduce the recombination of photogenerated carriers. Doping with nonmetallic elements (e.g., C, S, and N) was considered to reduce the band gap for wide-band-gap metal oxides.11 The incorporation of the dopants into the ZnO crystalline lattice could produce an intermediate energy level, reducing the absorption energy.12 Cdoped ZnO have exhibited enhanced optical acitivities.13,14 Another efficient approach is coupling with other materials so as to build a heterojunction structure at the interface to © 2014 American Chemical Society

enhance the separation efficiency of photogenerated electron− hole pairs during the photocatalytic process.15,16 The composites of noble metals and UV-sensitive ZnO have received extensive attention.17 In comparison with unmodified ZnO, silver-modified ZnO showed much higher photocatalytic activity to Rhodamine 6G due to the electron scavenging ability of silver.18 Meanwhile, the substitution of noble metals with cheap materials in the hetero-nanostructured photocatalysts with high photocatalytic activity, good stability, and low cost is also expected.19,20 However, achieving the doping and the heterojunction formation simultaneously through a facile strategy was scarcely reported and thus urgently required from the scientific research and industrial production point of view. Graphitic carbon nitride (g-C3N4), as an analogue of graphite, also possesses stacked two-dimensional structure, with tris-triazine building units connected with planar amino groups in each layer and weak van der Waals force between layers, owning relatively narrow band gap of ∼2.7 eV.21 Besides the simple fabrication from the thermal condensation of nitrogen-rich precursors, such as urea and dicyandiamide, the low-cost and high stability under light irradiation in solution with pH = 0−14 make g-C3N4 especially attractive in photoelectrochemical area,22 exhibiting potential for the removal of organic contaminants under visible light illuminaReceived: March 18, 2014 Revised: April 24, 2014 Published: April 29, 2014 10963

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tion,23 splitting of water into hydrogen using solar energy,24 and photoelectrochemical conversion.25 Nonetheless, bare gC3N4 suffers the predicament of limited photocatalytic activity due to low quantum yield and high recombination rate of photogenerated charges. Recently, the combination of g-C3N4 and ZnO into a heterostructure presents a feasible and inspiring route to attain an improved charge separation in the electron transfer process. A C3N4-hybridized ZnO photocatalyst was fabricated by a twostep chemisorption method, demonstrating ameliorative photocatalytic activity for the methyl blue degradation.26 Liu et al. reported the synthesis of ZnO/C3N4 photocatalysts through a ball milling method, which resulted in higher photoactivity than that of single-phase C3N4.27 Nevertheless, the general synthesis of composites of ZnO and g-C3N4 involves unfavorable multistep and insufficient combination between g-C3N4 and ZnO, which lead to inadequate photoactivity and low stability of the resultant photocatalysts. In this work, the nanocomposites of ZnO and g-C3N4 were prepared by means of a facile calcination approach with the use of commercially available dicyandiamide and zinc nitrate as precursors, where carbon-doping in ZnO was observed. A series of composites with different weight percentages of g-C3N4 were prepared by simply adjusting the ratio of the precursors, exhibiting enhanced optical property and photocatalytic activity. Methylene blue (MB) dye was chosen as probe molecule to evaluate the photocatalytic activity under visible light irradiation, and the possible photocatalytic mechanism was investigated. The facile preparation process, low cost, and the uniform hybridization make the synthesized nanocomposites show great potential in the fields of sustainable energy and environment.28

spectrophotometer, with BaSO4 acting as a reference. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Kratos Axis Ultra DLD (delay line detector) spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV). All XPS spectra were recorded by using an aperture slot of 300 × 700 μm; survey spectra were recorded with a pass energy of 160 eV and high-resolution spectra with a pass energy of 40 eV. A photoelectrochemical test was carried out on a Zennium (Zahner, German) workstation. To investigate the transition of photogenerated electrons of the synthesized photocatalysts, the corresponding electrodes were prepared as follows: 100 mg of the as-prepared photocatalyst, acetylacetone (0.15 mL), poly(ethylene glycol)800 (0.1 g), OP-10 emulsifier (0.2 g), and trace of distilled water and ethanol were blended in a mortar to get a homogeneous mixture, which was then dip-coated onto a 1.5 cm × 2 cm fluorine-doped tin oxide (FTO) glass electrode. Electrodes were calcined in air at 400 °C for 1 h to eliminate the organic additives completely. 2.3. Photocatalytic Degradation of MB. The MB was selected as the probe molecule to evaluate the photocatalytic activities of the synthesized g-C3N4/ZnO composites. In a typical photoreaction experiment, 0.1 g of photocatalyst was dispersed in the MB aqueous solution (100 mL, 10 mg L−1). The photocatalytic activities of the synthesized photocatalysts were tested at ambient condition under visible light irradiation, which was obtained from a 300 W xenon lamp with a 400 nm cutoff filter. The distance between the light source and the reactor containing the reaction mixture was fixed at 10 cm. Before the photolysis of MB, the mixture was stirred in dark for 1 h to ensure the establishment of adsorption−desorption equilibrium on the catalysts. The mixture, sampled at given intervals, was centrifuged for 5 min to discard any sediment. The absorbance of reaction solutions was measured by a Shimadzu UV-2450 UV−vis spectrophotometer at λmax = 663 nm. To estimate the photostability of the photocatalysts, the sample after one trial was collected through centrifugation, washed by water and ethanol alternatively, and dried for the subsequent cycle test.

2. EXPERIMENTAL SECTION 2.1. Photocatalyst Preparation. Typically, dicyandiamide (0.12 mol) dissolved in deionized water (25 mL) was heated and stirred at 100 °C, and different amounts of zinc nitrate were added. After continuous stirring at 100 °C to remove the water, the resultant white solids were placed in a incubator at 100 °C for 6 h, followed by calcination in a muffle oven to 500 °C for 2 h at a heating rate of 4 °C min−1. The obtained gC3N4/ZnO composites with different contents of g-C3N4 were labeled as g-C3N4/ZnO-x for simplicity, where x represented the g-C3N4 weight percent in the composites. For comparison purpose, the pure g-C3N4 and ZnO were prepared via the same route. 2.2. Characterization. Thermogravimetry analysis (TGA) was performed on a TA SDT Q600 instrument at a heating rate of 10 °C min−1 in the air atmosphere with α-Al2O3 as the reference. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out on a Jeol JSF-7500L at 5 keV and a Jeol JEM-2100F at 200 kV, respectively. All samples subjected to TEM measurements were ultrasonically dispersed in ethanol and drop-cast onto copper grids covered with a carbon film. N2 adsorption−desorption isotherms were measured on a Quantachrome NOVA 2000e sorption analyzer at liquid nitrogen temperature (77 K). The samples were degassed at 200 °C overnight prior to the measurements, and the specific surface area was obtained by the Brunauer−Emmett−Teller (BET) method. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Focus diffractometer with Cu Kα radiation (λ = 1.5418 Å) operated at 40 kV and 40 mA. Diffuse reflectance UV−vis absorption spectroscopy was performed with a Shimadzu UV-2450

3. RESULTS AND DISCUSSION The g-C3N4/ZnO-x nanocomposites were synthesized by dissolving dicyandiamide and zinc nitride in aqueous solutions under mild stirring to ensure the homogeneous mixing of the precursors, followed by evaporating the solutions to dryness and calcining the precipitates at 500 °C in air. In order to investigate the thermal stability of the obtained composites and confirm the g-C3N4 contents in the final composites, TGA was carried out from ambient temperature to 900 °C (Figure 1), exhibiting two weight loss stages. The little weight loss in the low-temperature range can be attributed to the desorption of physically adsorbed and intercalated water molecules. The main weight loss from about 500 to 820 °C can be assigned to the rapid combustion of g-C3N4 phase. Correspondingly, the weight percents of g-C3N4 in the composites can be obtained from the second weight loss stage. The g-C3N4 contents in the composite photocatalysts were calculated to be 88.2, 81.6, 65.4, 50.7, 27.5, and 10.7 wt % for g-C3N4/ZnO-88.2%, g-C3N4/ ZnO-81.6%, g-C3N4/ZnO-65.4%, g-C3N4/ZnO-50.7%, gC3N4/ZnO-27.5%, and g-C3N4/ZnO-10.7%, respectively. Figure 2a shows the typical XRD patterns of the composites. g-C3N4 exhibits two characteristic diffractions at 13.1° and 27.5° assigned to the (100) and (002) peaks of graphitic carbon 10964

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Figure 3. SEM images of g-C3N4 (a), ZnO (b), and g-C3N4/ZnO50.7% (c, d). Figure 1. TGA curves of pure g-C3N4, ZnO, and the g-C3N4/ZnO-x nanocomposites.

4). Figure 3a shows the SEM image of g-C3N4, presenting a typical multilayer stacking structure of the graphitic nature of

nitride,29 corresponding to the in-plane structural packing motif of tris-triazine units24 and the interlayer stacking of aromatic system with the inlayer distance of 0.371 nm,25 respectively, while pure ZnO presents the sharp diffractions of typical wurtzite hexagonal zinc oxide phase.30 With respect to the gC3N4/ZnO-x composites, the diffraction intensities of g-C3N4 weakened with the decrease of the weight ratio of dicyandiamide in the precursor mixtures and even disappeared when the content of g-C3N4 in the composites decreased to 50.7%, while the peaks of ZnO appeared. The intensity of ZnO peaks enhanced with the further decrease of the content of gC3N4 and the increase of the ratio of ZnO. From the partially enlarged XRD patterns (Figure 2b), the typical diffractions corresponding to ZnO for g-C3N4/ZnO-50.7% shifts to lower angles as compared with pure ZnO. Indeed, the ionic radius of C4− (0.260 nm) is much larger than that of O2− (0.140 nm).31 Thus, the substitution of O with C during the high-temperature calcination necessarily expands the lattice of ZnO, causing a peak shift toward lower angles for the ZnO component of gC3N4/ZnO-50.7%, which suggests the possible C-doping of ZnO in the resultant g-C3N4/ZnO nanocomposites. The morphology and microstructure of the synthesized samples were examined by SEM (Figure 3) and TEM (Figure

Figure 4. TEM image of g-C3N4 (a); low-magnification (b) and highresolution (c, d) TEM images of g-C3N4/ZnO-50.7%. The yellow dashed lines in (d) depict the skeleton of ZnO nanoparticles.

Figure 2. (a) XRD patterns of g-C3N4, ZnO, and the synthesized composites. (b) Partially enlarged XRD patterns of ZnO and g-C3N4/ZnO-50.7%. 10965

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carbon nitride. The pure ZnO sample is revealed to be mainly composed of irregularly aggregated spherical particles with the size of 50−120 nm (Figure 3b), whereas the g-C3N4/ZnO50.7% composite exhibits spongelike porous structure that is composed of numerous nanoparticles (Figure 3c,d), distinct from the morphology of pure g-C3N4 and ZnO. This may be due to the influence of the gases released from the precursor decomposition and condensation during high-temperature calcination in the air. This well-structured porosity may be beneficial for the photocatalysis due to the improvement of mass transport through the materials.32 Energy-dispersive X-ray spectrum (EDS) mapping analyses of the g-C3N4/ZnO-50.7% composite show the uniform distribution of the four elements (C, N, O, and Zn) throughout the whole material (Figure S1 in the Supporting Information). The TEM image of g-C3N4 (Figure 4a) shows obvious layered structure, coinciding with the SEM observation, which indicates that pure g-C3N4 is consisted of stacked graphitic planes constructed from the triazine building units. The characteristic layered structure of gC3N4 can be well retained in the ultimate nanocomposites even after hybridization with ZnO (Figure 4c). The HRTEM images of g-C3N4/ZnO-50.7% composite demonstrate ZnO nanoparticles of 4−6 nm in size homogeneously hybridized with the layered graphitic carbon nitride as well as an interplanar distance of 0.282 nm that is attributed to the (100) plane of the ZnO hexagonal wurzite phase. The N2 adsorption−desorption isotherms of the g-C3N4/ ZnO-x composites are of type III with type H3 hysteresis loops (Figure S2). Their textual properties are listed in Table 1. The

Figure 5. FT-IR spectra of g-C3N4, ZnO, and g-C3N4/ZnO-50.7%.

vibration modes for the −NH and hydroxyl of the adsorbed H2O. The FT-IR spectrum of g-C3N4/ZnO-50.7% resembles gC3N4 in the characteristic bands, revealing the typical graphitic structure of carbon nitride was well reserved after homogeneous hybridization with ZnO, though the characteristic bands of graphitic C3N4 red-shifted with the band strengths weakened, which indicates that the conjugated structures of g-C3N4 are stretched and a more widely conjugated systems containing g-C3N4 and ZnO have been generated.26 XPS analysis of g-C3N4/ZnO-50.7% was conducted (Figure 6), taken as being representative, to investigate the stoichiometric chemistry of the prepared composite catalysts. The Zn 2p spectrum contains a doublet at the binding energy of 1021.8 and 1044.6 eV, assigned to Zn 2p3/2 and 2p1/2 lines, respectively. The binding energy distance between these two lines is 22.8 eV, which is within the standard reference value of ZnO.38 The binding energies and the binding energy difference indicate that the Zn ions in the composites are of +2 states. In the O 1s XPS spectrum, the main peak centered at 531.6 eV is assigned to O2− ions in the Zn−O bonding of the wurtzite ZnO structure,39 and the shoulder peak located at 533.4 eV is related to OH group absorbed onto the surface of the composite.40 The N 1s XPS spectrum can also be fitted into one main peak at 398.7 eV attributed to the aromatic N bonded to two carbon atoms (CN−C), and a weak shoulder at 400.6 eV corresponded to the sp2-hybridized N bonded to three atoms (C−N(−C) −C or C−N(H)−C).41,42 The C 1s spectrum can be deconvoluted into four components at about 282.9, 284.5, 285.9, and 288.1 eV, respectively. The peak centered at 284.5 eV is assigned to pure graphitic sites in the carbon nitride matrix, whereas the energy contribution at 285.9 eV is attributed to the sp2-hybridized carbon atoms bonded to N in an aromatic ring.43 The peak situating at 288.1 eV can be assigned to the sp2-hybridized carbon in the aromatic ring attached to the NH2 group, resulting from the incomplete condensation of dicyandiamide during the high-temperature heat treatment. The weak peak at 282.9 eV suggests the presence of carbon atoms in carbide form,44,45 which signifies the substitution of carbon for oxygen and the formation of Zn− C bonds in the ZnO components of the composites. The formation of Zn−C in C-doped ZnO nanostructures impelled the adsorption edge red-shift to visible light region that were red-shifted relative to the UV exciton absorption of pure ZnO nanostructures, which could make better use of solar energy and avail the photocatalytic process.46

Table 1. Textual Properties and Photocatalytic Rate Constant of the Synthesized Photocatalysts sample

SBETa (m2 g−1)

Vporeb (cm3 g−1)

k (min−1)

g-C3N4 g-C3N4/ZnO-88.2% g-C3N4/ZnO-81.6% g-C3N4/ZnO-65.4% g-C3N4/ZnO-50.7% g-C3N4/ZnO-27.5% g-C3N4/ZnO-10.7% ZnO g-C3N4/ZnO-refc

8 14 16 23 27 22 18 28 24

0.023 0.041 0.039 0.064 0.063 0.062 0.065 0.053 0.053

0.00183 0.00363 0.00491 0.00746 0.0134 0.0101 0.00256 0.000875 0.00853

a

BET surface area calculated from the linear part of the multipoint BET plot. bSingle-point total pore volume of pores at P/P0 = 0.97. c Prepared by calcination of the mechanically blended precursors.

specific surface area calculated from the linear part of the multipoint plot of pure g-C3N4 and ZnO is 8 and 28 m2 g−1, respectively. The surface area of the composite photocatalyst increases with the reduction of the g-C3N4 contents in the final composites and reach the maximum of 27 m2 g−1 for the gC3N4/ZnO-50.7%. Further decreasing the g-C3N4 content leads to a lower surface area of 18 m2 g−1 for g-C3N4/ZnO-10.7%. FT-IR spectra of the synthesized composites (Figure 5) present skeletal vibrations of the heptazine heterocyclic ring. For g-C3N4, the sharp band at 808 cm−1 corresponds to the out-of-plane bending vibration of characteristics of triazine rings.33 The absorption bands in the region of 1200−1650 cm−1 are assigned to the typical stretching modes of CN heterocycles.34,35 The peaks at 1241, 1318, and 1425 cm−1 are attributed to the aromatic C−N stretching.36,37 The broad peaks at around 3100−3400 cm−1 are assigned to stretching 10966

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Figure 6. High-resolution XPS Zn 2p, O 1s, N 1s, and C 1s spectra of g-C3N4/ZnO-50.7%.

composite shows broad and strengthened absorption in the visible light range. These results may be attributed to the consequence of C-doped effect and the synergistic effect between g-C3N4 and ZnO in the composite samples.46,47 The enhanced light adsorption of the composites can make the utmost of visible light and thus result in the production of more electron−hole pairs under visible light illumination, which subsequently leads to a higher photocatalytic activity. Hitherto, all the results clearly reveal that the prepared gC3N4/ZnO-x materials are not simple physical mixtures of two separate g-C3N4 and ZnO but fine composites after calcination of the evaporation-dried precursor mixtures. There might exist a covalent bond between g-C3N4 and ZnO.48 Furthermore, to the best of our knowledge, this is the first successful synthesis of homogeneous C-doped ZnO hybridized g-C3N4 nanocomposites. In this work, zinc nitrate and dicyandiamide were used to synthesize ZnO and g-C3N4 nanocomposites. During the hightemperature treatment process in air, dicyandiamide decomposed and condensed to form graphitic carbon nitride, and the zinc nitrate thermally decomposed to wurtzite ZnO species. Meanwhile, dicyandiamide could attach on the surface of formed ZnO nanoparticles, and combustion of dicyandiamide and rearrangement of atoms (Zn, O, and C) resulted in the substitution of a fraction of oxygen atoms in the ZnO lattice with carbon atoms. The composite photocatalysts of C-doped ZnO and g-C3N4 could be therefore obtained. Figure 8 depicts the photocatalytic activities for the decomposition of MB on g-C3N4, ZnO, and the g-C3N4/ ZnO-x composite photocatalysts under visible light illumination. To have a better understanding of the reaction kinetics of the MB degradation catalyzed by the photocatalysts, the photocatalytic degradation process was fitted to a pseudo-first-

The photoabsorption behaviors of the prepared composite photocatalysts were investigated by using UV−vis absorbance spectroscopy (Figure 7). The pure wurtzite ZnO has strong

Figure 7. UV−vis absorbance spectra of g-C3N4, ZnO, and the composite photocatalysts.

absorption in the UV region wavelength threshold of 386 nm, corresponding to the band gap of 3.21 eV. Distinct from the ZnO absorption behavior, the absorption offset of g-C3N4 occurs at 458 nm, giving the band gap of 2.71 eV, which indicates its visible-light-induced photocatalytic activity. Noticeably, the absorption edges of the g-C3N4/ZnO-x composites shift significantly to the longer wavelength region in comparison with that of pure ZnO and g-C3N4, obviously demonstrating that the absorption edges of the composites moves to lower energy region. The g-C3N4/ZnO-50.7% 10967

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Figure 8. Photocatalytic activity for the degradation of MB under visible light irradiation on the synthesized photocatalysts.

order, and the value of the rate constant k is equal to the corresponding slope of the fitting line: ln(C0/C) = kt, where k represents the rate constant (min−1) and C0 and C are the initial concentration and at time t of the MB molecules, respectively. No photolysis of MB can be observed after 2 h of illumination without any photocatalysts, which indicates that MB is stable under visible light irradiation in the absence of catalysts. The pure ZnO played limited role in degrading MB due to the large band gap energy, giving photocatalytic efficiency of 9.27%. g-C3N4 has a better performance in photodegradation than ZnO, showing degradation efficiency of 17.4% after 2 h of irradiation, due to the absorption in the visible light region. The photocatalytic activities of the g-C3N4/ ZnO-x composites increased with the increase of the ZnO proportion in the composites first and then decreased. g-C3N4/ ZnO-50.7% exhibits the highest photocatalytic activity with the photodegradation efficiency of 78.6% and reaction rate constant k = 0.0134 min−1, which is comparable to some other composite photocatalysts such as g-C3N4/BiVO4 and WO3/ NiWO4.49,50 Nevertheless, further decreasing g-C3N4 content to 10.7% leads to the decrease of degradation rate, but still higher than pure g-C3N4 and ZnO. It is believed that excess ZnO bulks might act as electron−hole recombination sites and suppress the photoinduced charge transfer. For comparison, the ZnO and g-C3N4 (53.1 wt %) composite was prepared by calcination of the mechanically blended dicyandiamide and zinc nitrate, which was denoted as g-C3N4/ZnO-ref. The sample gC3N4/ZnO-ref can degrade 61.4% MB after 2 h visible light illumination (Figure 8), exhibiting photocatalytic activity (k = 0.008 53 min−1) lower than the nanocomposite g-C3N4/ZnO50.7%, though much higher than pure g-C3N4 (k = 0.001 83 min−1). This reveals the advantage of the present of one-step preparation process of g-C3N4/ZnO nanocomposite photocatalysts. To evaluate the stability of the g-C3N4/ZnO-50.7% and gC3N4/ZnO-ref photocatalysts prepared by different routes, multiple photodegradation tests of MB under visible light illumination were carried out (Figure 9). After three times recycling experiments, an obvious decrease of photoactivity of g-C 3 N 4 /ZnO-ref can be seen in Figure 9, and the corresponding photocatalytic efficiency decreases to 42.5%. In contrast, g-C3N4/ZnO-50.7% displays better stability with a less decrease of photocatalytic efficiency from 78.6 to 75.5% after three times recycles. Noticeably, the surface area and g-C3N4 weight ratio of g-C3N4/ZnO-ref are close to that of g-C3N4/ ZnO-50.7% (Figures S2 and S3). However, g-C3N4/ZnO-ref prepared by calcination of the mechanically mixed precursors demonstrates larger crystalline ZnO particle size (∼10 nm),

Figure 9. Cycling experiments of g-C3N4/ZnO-50.7% and g-C3N4/ ZnO-ref for MB degradation under visible light irradiation.

and the ZnO nanoparticles are mainly distributed on the surface of g-C3N4 (Figures S4 and S5). The abscission and photocorrosion of exposed ZnO might happen in the g-C3N4/ ZnO-ref catalyst during the photocatalytic process, which would result in the deterioration of photocatalytic activity during the recycling tests.26 Accordingly, the homogeneous hybridization between ZnO and g-C3N4 could prohibit the consumption of ZnO and enhance the activity and stability of the g-C3N4/ZnO-x composite photocatalysts thereby to meet the need of practical applications. It is well-known that the photocatalytic redox reactions are intimately relevant to the separation efficiencies of photoinduced electron−hole pairs arisen from the excited semiconductor materials. To qualitatively investigate the separation efficiency of photoinduced charges during the photoreactions, the photocurrent response was carried out for the ZnO, gC3N4, and g-C3N4/ZnO-x nanocomposites under visible light irradiation (Figure 10). It can be obviously seen that fast and stable photocurrent responses were observed in all electrodes, and the photoresponsive phenomenon was entirely reversible. Under visible light illumination, pure ZnO electrode shows the weakest response because of its large band gap. On the contrary, the photocurrent of the g-C3N4/ZnO-50.7% electrode is about 5.5 times higher than that of g-C3N4 electrode. The remarkable enhancement of the photocurrent of the composite photocatalysts reveals an enhanced separation efficiency of the photogenerated electrons and holes, which can be ascribed to the heterojunction built between g-C3N4 and ZnO.26,51 As discussed above, the significant improvement of photocatalytic activities for the composite semiconductors was mainly due to the adsorption band of the composites extending to the 10968

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high oxidative capacity, producing visible light photocatalytic activity.

4. CONCLUSIONS A series of composite photocatalysts of g-C3N4 and C-doped ZnO were successfully synthesized via a facile one-step calcination approach, in which dicyandiamide not only could thermally condense into g-C3N4 but also provide carbon source to supersede lattice oxygen of ZnO. The g-C3N4/ZnO-x composite materials with a certain g-C3N4 content showed broader and stronger adsorption in the visible light range after the uniform hybridization of g-C3N4 with ZnO than that of pure g-C3N4. On the basis of the photodegradation results, the enhanced photocatalytic activities of the composites under visible light irradiation were achieved. Such an enhanced photoactivity could be assigned to the well-established heterojunction between g-C3N4 and ZnO, which was favorable for the effective separation of the photoinduced electrons and holes. Considering the excellent properties of the composites and the easy preparation procedure, the synthesized g-C3N4 and ZnO nanocomposites presents potential in a wide range of applications including environmental remediation and solar utilization.

Figure 10. Photocurrent response curves of g-C3N4/ZnO-50.7% (a), g-C3N4/ZnO-27.5% (b), g-C3N4/ZnO-65.4% (c), g-C3N4/ZnO-81.6% (d), g-C3N4/ZnO-88.2% (e), g-C3N4/ZnO-10.7% (f), g-C3N4 (g), and ZnO (h) under visible light irradiation.

visible region and high efficiency of charge separation induced by the heterojunction structure built between ZnO and graphitic carbon nitride. A mechanism of the photoreaction process is illustrated in Figure 11. g-C3N4 could adsorb the

■

ASSOCIATED CONTENT

S Supporting Information *

N2 sorption isotherms, EDS mapping, more TG curves, SEM and TEM images (Figures S1−S5). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Fax +86 22 23502604; Tel +86 22 23509610; e-mail zyyuan@ nankai.edu.cn (Z.-Y.Y.). Notes

The authors declare no competing financial interest.

■ Figure 11. Schematic drawing illustrating the electron−hole separation, transport, and photocatalytic progress of the composite photocatalyst under visible light irradiation.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21076056 and 21073099), the Specialized Research Fund for the Doctoral Program of Higher Education (20110031110016), the Program for Innovative Research Team in University (IRT1059, IRT13022), the 111 project (B12015), and the Key Laboratory of Advanced Catalytic Materials in Zhejiang Normal University (ZJHX201301).

visible light to be excited, transporting the excited-state electrons from the valence band (VB) to the conduction band (CB). Since the CB edge potential of g-C3N4 is more negative than that of ZnO, the excited electrons in g-C3N4 transferred to the CB of ZnO.13,52,53 Reversely, the holes from the VB of ZnO were injected to that of g-C3N4. Accordingly, an internal electrostatic potential was formed in the space charge region, which was propitious to the separation of the photogenerated charge carriers. The charges would subsequently transform to the surface of the composite semiconductor to react with water and dissolved oxygen to generate superoxide and hydroxyl radicals or interact with MB directly. The radicals were able to oxidize the organics owing to their

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