Carbon-Doped ZnO Nanostructures: Facile Synthesis and Visible

Article pubs.acs.org/JPCC

Carbon-Doped ZnO Nanostructures: Facile Synthesis and Visible Light Photocatalytic Applications Xinyu Zhang,† Jiaqian Qin,*,‡ Ruru Hao,† Limin Wang,† Xi Shen,§ Richeng Yu,§ Sarintorn Limpanart,‡ Mingzhen Ma,† and Riping Liu*,† †

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, P. R. China Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok 10330, Thailand § Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡

S Supporting Information *

ABSTRACT: Zinc oxide (ZnO) has been widely used as a photocatalyst for solar energy conversion and treatment of organic pollutants because of its low toxicity and high photocatalytic efficiency. However, the applicability of ZnO in visible light is limited because of the wide band gap of the material, which results in low efficiency during solar photoconversion. In this paper, we report the facile one-pot, morphology-controlled, and large-scale synthesis of carbon-doped ZnO through urea-assisted thermal decomposition of zinc acetate. Nanorods and nanospheres of carbon-doped ZnO were successfully prepared by using this one-step method with various weight percent of urea. The photocatalytic activities of nanocrystals obtained with different morphologies and carbon contents were evaluated through degradation of methylene blue with visible light irradiation. Results showed that incorporation of carbon decreases the energy bandgap of ZnO, improves the separation efficiency of its electron− hole pairs, and significantly enhances the visible light photocatalytic activity.

1. INTRODUCTION In recent decades, semiconductor-mediated photocatalysis has become an area of significant research interest because it is considered a potential approach to solve current environmental and energy issues.1 ZnO has been intensively studied because of its high photosensitivity, nontoxic nature, large bandgap, and low cost.2,3 Moreover, the morphologies of ZnO photocatalysts are easily controlled, which is an important factor influencing photocatalytic performances.2,3 However, to date, the photocatalytic activities achieved thus far with ZnO have been modest because ZnO has a wide bandgap that limits its absorption of ultraviolet (UV) light. UV light accounts for only a small fraction (∼5%) of the solar radiation spectrum compared with visible light (∼45%), and the high recombination ratio of photoinduced electron−hole pairs further restricts the applications of the material to a considerable extent. Therefore, the mechanisms by which the bandgap of ZnO may be increased to extend its working spectrum to the visible light region and use sunlight more efficiently are an important area © 2015 American Chemical Society

of research in solar energy conversion and environmental cleaning.4−6 Some effort has been devoted to reduce recombination of photogenerated electron−hole pairs and improve utilization of sunlight to ZnO; doping,7,8 deposition of metals,9,10 and combination of ZnO with another semiconductor,11 for example, have been proposed. Doping with nonmetallic elements (e.g., C, S, and N) has also been considered to reduce the bandgap of wide-bandgap semiconductors.5 To address issues related to the poor performance of ZnO, we prepared carbon-doped ZnO nanorods and nanospheres for dye degradation. ZnO is a direct bandgap semiconductor with a bandgap and band edge position similar to those of TiO2. Introduction of carbon to the crystal lattice of ZnO will result in an intermediate energy level in the bandgap of the material Received: July 22, 2015 Revised: August 9, 2015 Published: August 20, 2015 20544

DOI: 10.1021/acs.jpcc.5b07116 J. Phys. Chem. C 2015, 119, 20544−20554

Article

The Journal of Physical Chemistry C

Figure 1. (a) Schematic representation of the formation process through urea-assisted technique to synthesize carbon-doped ZnO nanostructure materials. Step 1: formation of the homogeneous mixture of the zinc acetate and urea. Step 2: the obtained homogeneous mixture was transferred into an aluminum crucible. The carbon-doped ZnO nanostructures were obtained by calcination in air at 450 °C (heating rate, 10 °C min−1) for 2 h. Step 3: the sample was collected and washed.

materials (Figure 1). During the preparation process, the melted urea and generated gas can modify the morphology of ZnO crystals according to the amount of urea added. The weight percent of urea was controlled at 0, 10, 20, 30, 40, and 50% in the mixture of zinc acetate and urea. The corresponding samples were named as ZnO, 90ZA, 80ZA, 70ZA, 60ZA, and 50ZA, respectively. Carbon-doped ZnO nanorods and nanospheres with large surface areas are achieved. The facile lowcost high-throughput urea-assisted production of ZnO enables its immediate practical utilizations. The quality of the resultant carbon-doped ZnO is suitable for large-scale applications, such as in photocatalysis. We also confirm the high photocatalytic activity of the products under visible light irradiation as a result of bandgap red-shifting and the high surface area.

and reduce its absorption energy. Previous studies on carbondoped ZnO nanopowders have shown a red-shift in light absorption wavelength.12 Cho et al.12 reported the synthesis of carbon-doped ZnO using vitamin C and observed enhanced visible light photocatalytic activity; however, the photocatalytic performance obtained was still low. Other studies on carbondoped ZnO have also been reported,13,14 but these studies mainly focused on ferromagnetism, magnetotransport properties, and p-type conduction properties. The techniques presented in these works also required expensive equipment, complex process controls, and stringent reaction conditions. Solid-based methods are promising alternatives for producing nanostructured materials because of their simplicity and high yield.15 Compared with solution-based approaches, solid-based methods present no reagent concentration limitation, thereby allowing the preparation of materials in high yield. Tian et al.16 recently reported that calcination of zinc acetate (Zn(Ac)2· 2H2O) at high temperature provides a cost-effective strategy for large-scale preparation of ZnO photocatalyst. The prepared ZnO showed better photocatalytic activity than that of commercial TiO2 photocatalyst (P25) for dye degradation (methyl orange, rhodamine B, and methylene blue [MB]). In our previous study, we also demonstrated that calcination of Zn(Ac)2·2H2O at different temperatures with mechanical assistance showed different photocatalytic activities.17 Organic additives can influence the nucleation and growth of ZnO crystals, and some elements may be incorporated into the ZnO structure. Urea is a common and low-cost raw material widely available in the industry. Urea is an active molecular precursor under thermal treatment and has been used to produce graphitic carbon nitride (g-C3N4).18−20 During thermal decomposition, urea is converted into cyanuric acid with generation of gas. Thus, urea may potentially be a growth modifier in the solid-state synthesis of ZnO crystals from Zn(Ac)2·2H2O. In the present study, we develop a urea-assisted technique to synthesize carbon-doped ZnO nanostructure

2. EXPERIMENTAL SECTION 2.1. Synthesis of Carbon-Doped ZnO. Zn(Ac)2·2H2O and urea were obtained from Carlo. Methylene Blue (MB) were purchased from Alfa Aesar. All other reagents were of analytical grade and used without further purification. Carbondoped ZnO was synthesized by grinding 9.0 g of Zn(Ac)2· 2H2O and 1.0 g of urea in a mortar for 30 min and then annealing the powders in an alumina crucible at 450 °C (heating rate, 10 °C min−1) for 2 h under an air atmosphere in a muffle furnace. After completion of the reaction, the powders were washed with distilled water and dried in an oven at 80 °C for 8 h. The obtained sample was denoted as 90ZA. The sample obtained from 8.0 g of Zn(Ac)2·2H2O and 2.0 g of urea was denoted as 80ZA, the sample obtained from 7.0 g of Zn(Ac)2· 2H2O and 3.0 g of urea was denoted as 70ZA, the sample obtained from 6.0 g of Zn(Ac)2·2H2O and 4.0 g of urea was denoted as 60ZA, and the sample obtained from 5.0 g of Zn(Ac)2·2H2O and 5.0 g of urea was denoted as 50ZA. Pure ZnO was also prepared by grinding 10.0 g of Zn(Ac)2·2H2O without additives in a mortar for 30 min and then annealing at the same conditions. 20545


Article


Figure 2. XRD patterns of as-obtained ZnO and carbon-doped ZnO. (a) XRD of the powders obtained from different weight ratios of zinc acetate and urea. All of the samples were indexed as hexagonal wurtzite ZnO. (b) Enlarged area of 2θ from 31° to 37° in (a); peaks corresponding to the (100), (002), and (101) planes showed marked changes under different weight ratios of the precursors.

2.2. Characterization of Carbon-Doped ZnO. Powder X-ray diffraction (XRD) was carried out using a Phillips X’pert diffractometer with Cu Kα radiation. The instrument was equipped with an X’celerator detector and operated at 40 kV and 25 mA. Scanning was performed over the 2θ range of 20°− 80° with a step size of 0.02 and step time of 4 s. Carbon content was determined by using a sulfur and carbon analyzer. Field emission scanning electron microscopy (FESEM) images were obtained using a Hitachi S-4700, and transmission electron microscopy (TEM) images were acquired by a JEM2010 system operated at 200 kV at Yanshan University. Samples were prepared by drop-casting the catalysts dispersed in ethanol on carbon-coated Cu-grids and drying in a vacuum. Aberration-corrected scanning transmission electron microscopy (STEM) experiments were performed on a JEOL ARM200F transmission electron microscope equipped with double CS correctors for the condenser and objective lenses. High-angle annular dark-field (HAADF) and annular bright field (ABF) images were acquired at acceptance angles of 11.5− 23.0 and 90−370 mrad, respectively. Prior to the STEM measurements, samples were dispersed in alcohol and deposited on a SiN film-coated silicon grid with a diameter of 3 mm. Thermal analysis was carried out on a NETZSCH STA 409 simultaneous thermal analyzer, and optical properties were measured by UV−vis diffuse reflectance spectroscopy. The X-ray photoelectron spectroscopy (XPS) spectra of the samples were recorded on an XPS ESCA Thermo Fischer Scientific Multilab 2000 system using monochromatic Al Kα radiation. Binding energies were corrected with respect to graphitic carbon at 284.6 eV. Surface areas were determined by the Brunauer−Emmett−Teller (BET) method using ASAP 2020 HD88 surface area and porosity analyzer. The PL excitation/emission measurements were performed on a JobinYvon Nanolog-3 spectrofluorometer at room temperature. In all cases, samples were excited by a 325 nm xenon laser beam through an optical lens. Scanning was performed at excitation wavelengths from 350 to 700 nm with 5 nm steps for both samples. 2.3. Photodegradation of MB. The photocatalytic activities of carbon-doped ZnO, ZnO, P25 TiO2, and g-C3N4 were estimated by measuring the decomposition rate of MB aqueous solution. To maintain the MB solution at room

temperature, water was circulated through the exterior of the reactor. Reaction suspensions were prepared by adding 300 mg of catalysts to 300 mL of aqueous MB solution with an initial concentration of 10 mg/L. The solution mixture was stirred for 1 h in the dark to attain equilibrium adsorption. The aqueous suspension containing MB and the photocatalyst was then irradiated by a 500 W metal halide lamp with a 420 nm cutoff filter. The distance between the light source and the reactor containing the reaction mixture was fixed at 30 cm. Analytical samples were collected from the suspension at regular intervals, centrifuged, and filtered. The concentration of MB in each sample was analyzed using a UV−vis spectrophotometer (Shimadzu, 1700 UV−vis) at a wavelength of 664 nm. Photocatalytic efficiency was calculated using the expression η = (1 − C/C0) × 100, where C0 is the concentration of MB before illumination and C is the concentration of MB after irradiation.

3. RESULTS AND DISCUSSION 3.1. Characterization and Properties of CarbonDoped ZnO. The carbon-doped ZnO nanostructure materials were synthesized by grinding of Zn(Ac)2·2H2O and urea in a mortar for 30 min to ensure homogeneous mixing of the precursors and calcining of the mixture at 450 °C in air. Figure 2 shows the typical X-ray diffraction (XRD) patterns of the obtained samples. The peaks were indexed as hexagonal wurtzite. No diffraction peaks from other impurities were observed in the XRD patterns of ZnO, 90ZA, 80ZA, 70ZA, and 60ZA (Figure 2a). 50ZA was indexed as ZnO and g-C3N4 (Figure S1). To investigate the thermal stability of the obtained powders, thermogravimetric analysis (TGA) was performed from ambient temperature to 900 °C (Figure S2). TGA results revealed that ZnO, 90ZA, 80ZA, 70ZA, and 60ZA exhibit weight loss over the entire temperature range studied; the weight loss observed can be attributed to desorption of physically adsorbed and intercalated water molecules. For 50ZA, the minimal weight loss observed in the low-temperature range can be attributed to desorption of physically adsorbed and intercalated water molecules. The main weight loss of 50ZA from 500 to 700 °C can be assigned to rapid combustion of the g-C3N4 phase. This result further demonstrates that 20546


Article


Figure 3. SEM images of synthesized samples. (a) Pure ZnO, (b) 90ZA, and (c) 80ZA. As the urea concentration increased, the rod-like structures of ZnO became smooth, and the aspect ratio of the rods decreased. (d) 70ZA and (e) 60ZA. As the urea concentration increased, sphere-like ZnO structures of were obtained. (f) 50ZA. As the urea concentration increased to 50 wt %, the sponge-like structure of g-C3N4 and smaller particles of ZnO composites were obtained. EDS of (g) 80ZA and (h) 60ZA.

the SEM images of the synthesized samples. Figure 3a shows the rod-like structures of ZnO formed from direct calcination of Zn(Ac)2·2H2O without urea. When urea (10 and 20 wt %) was added to the starting materials, the obtained ZnO still showed rod-like structures, but the crystal surface became smooth and the aspect ratio of ZnO rods decreased (Figures 3b and 3c). Addition of urea to the precursor decreased ZnO crystal growth along the rod direction; such a result was also demonstrated by TEM (Figure 4). Figures 4a and 4b show the TEM images of ZnO and 90ZA, respectively. ZnO prepared from direct calcination of Zn(Ac)2·2H2O exhibited lengths of above 500 nm. By contrast, 90ZA, which was synthesized from direct calcinations of Zn(Ac)2·2H2O and 10 wt % urea, shows lengths of ∼200 nm. When 30 and 40 wt % of urea were mixed with Zn(Ac)2·2H2O, sphere-like ZnO structures were formed (Figure 3d,e). 60ZA (40 wt % urea mixed with Zn(Ac)2· 2H2O) was also examined through TEM, and sphere-like structures with grain sizes of ∼15 nm (Figure 4c,d) were observed. When higher concentrations of urea (50 wt %) were added to the precursor, sponge-like g-C3N4 and smaller particles of ZnO were obtained (Figure 3f). EDS analysis allows detection of carbon in the crystals of samples synthesized with urea assistance (Figure 3g,h). Sulfur and carbon analysis showed that the carbon contents of 90ZA,

90ZA, 80ZA, 70ZA, and 60ZA only contain the ZnO phase, whereas 50ZA contains ZnO and g-C3N4 phases. 90ZA, 80ZA, 70ZA, and 60ZA had calculated lattice constants of a = 0.3256 nm and c = 0.521 nm, a = 0.3257 nm and c = 0.5211 nm, a = 0.3258 nm and c = 0.5213, and a = 0.3259 nm and c = 0.5216 nm, respectively. These lattice constants are larger than those observed for pure ZnO (a = 0.3254 nm and c = 0.5208 nm). Such results reveal that lattice constants increase with increasing amount of urea added. The ionic radius of C4− (0.260 nm) is much larger than that of O2− (0.140 nm),21 and substitution of O with C necessarily expands the lattice. Therefore, the lattice constants of the synthesized samples were larger than those of pure ZnO. Figure 2b shows the enlarged area of 2θ from 31° to 37° in Figure 2a. The full width at half maxima of pure ZnO, 90ZA, and 80ZA are similar, but significant peak broadening may be observed in 70ZA and 60ZA. The crystallite sizes of pure ZnO, 90ZA, 80ZA, 70ZA, and 60ZA calculated from the XRD patterns obtained were 47, 52, 50, 33, and 12 nm, respectively. Addition of 10 and 20 wt % urea resulted in ZnO grain sizes nearly identical to that of pure ZnO. Further increases in the weight percentage of added urea to 30% and 40% decreased the grain size sharply. The morphologies and microstructures of the prepared samples were examined by FESEM and TEM. Figure 3 shows 20547


Article


Figure 4. TEM images of synthesized ZnO: (a) pure ZnO, (b) 90ZA, (c) 60ZA, and (d) higher resolution TEM of 60ZA.

80ZA, 70ZA, and 60ZA are 0.19, 0.23, 0.28, and 0.56 wt %, respectively. These results reveal that the carbon content of the samples increases with increasing urea concentration. The Brunauer−Emmett−Teller (BET) surface area (SBET) of the samples were investigated by using N2 adsorption− desorption isotherms (Table S1). The BET surface areas calculated from the linear part of the multipoint plot of pure ZnO and g-C3N4 are 8 and 14 m2 g−1, respectively. The BET surface areas of the carbon-doped ZnO increased with the urea concentration and reached a maximum value of 48 m2 g−1 in 60ZA. Further increases in urea concentration led to a maximum surface area of 99 m2 g−1 in 50ZA because of formation of ZnO/g-C3N4 composites. To investigate the microstructure of carbon-doped ZnO, HRTEM, SAED, and STEM were carried out on 90ZA and 60ZA. Figure 5a presents the HRTEM image of 90ZA; here, good-quality single crystalline ZnO could be obtained with low weight percentages of urea. The crystal has a lattice spacing of 0.526 nm, which corresponds to the distance between the (001) planes of the ZnO crystal lattice. From the corresponding SAED pattern (Figure 5b), the ZnO can be identified to present rod shapes in the [001] orientation, thereby implying the growth direction as [001] with side planes of [100]*. Figure 5c−f shows low- and high-resolution HAADF and ABF images of the ZnO samples. From the high-resolution HAADF and ABF images, the ZnO nanorods may be confirmed to exhibit preferential growth in the [001] direction.

These results suggest that the [001] direction is the fastest growth direction and that ZnO (001) has the highest low-index surface energy.22 Therefore, with or without urea, ZnO crystals preferentially grew along the [001] direction to form rod-like structures (Figures 3−5). STEM EDX mapping demonstrated that Zn (Figure 5h), O (Figure 5i), and C (Figure 5j) are distributed homogeneously throughout the structures. The 60ZA samples were further characterized by using HRSTEM (Figure 6). Figure 6a−d shows low- and high-resolution HAADF and ABF images of the samples. The corresponding HRTEM reveals that the crystal has a lattice spacing of 0.26 nm (Figure 6e), which corresponds to the distance between the (002) planes of the ZnO crystal lattice. The ZnO particles were single crystalline, as confirmed by HAADF, ABF, and the corresponding HRTEM, but the crystals were stacked and aggregated together, as shown in the HAADF (Figure 6b), ABF (Figure 6d), HRTEM (Figure 6e), and SAED (Figure 6f) images. EDX elemental mapping indicated that the samples were composed of only Zn (Figure 6h), O (Figure 6i), and C (Figure 6j); thus, homogeneous distribution of Zn, O, and C may be observed even in thicker areas of the samples. The EDX signal of carbon was relatively faint in thinner areas of the samples because of the absence of a carbon distribution on this area. To gain a better understanding of the electronic structures of the carbon-doped ZnO nanostructures, XPS was performed. The Zn 2p spectrum can be assigned to Zn 2p3/2 and 2p1/2 20548


Article


ZnO (80ZA and 60ZA). In pure ZnO, the large peak at 529.7 eV is assigned to O2− ions in the Zn−O bonds of wurtzite ZnO crystals, and another peak at 531.1 eV is due to chemically adsorbed surface oxygen species, such as OH− and carbonates (Figure 7a).23 The XPS spectra of carbon-doped ZnO (80ZA [Figure 7b] and 60ZA [Figure 7c]) nanostructures could be fitted to three peaks at ∼529.9, ∼530.5, and ∼531.7 eV. The main peak at ∼529.9 eV is attributed to Zn−O bonds in the ZnO crystals. The peaks at ∼531.7 and ∼530.5 eV can be assigned to the surface-adsorbed oxygen species of OH− and carbonates, such as C−O and CO bonds, which are due to Zn−Ovac and Zn−O−C bonds, respectively. The C 1s spectrum of pure ZnO can be deconvoluted into two components at 284.6 and 288.5 eV (Figure 8a), while those of 90ZA, 80ZA, 70ZA, and 60ZA can be deconvoluted into four components at ∼283.3, ∼284.6, ∼285.8, and ∼288.5 eV (Figure 8b−e and Table S2). For all obtained samples, the peak centered at 284.6 eV could be assigned to pure graphitic sites in the nanostructures. Satellite peaks for pure ZnO and carbondoped ZnO samples at ∼288.5 eV are attributed to adsorption of CO2 and structural carbonate species containing CO.14 The peak at 285.8 eV could be due to Zn−O−C bonds,23 whereas the smallest fitted peak at ∼283.3 eV could be associated with Zn−C bonds connected to oxygen vacancies (Ovac) because additional electron density would be imposed by negatively charged Ovac on the C in the Zn−C−Ovac bond. Wang et al.24 reported that Ovac may result in bandgap narrowing; therefore, the formed Zn−C and Ovac may induce visible light responses in carbon-doped ZnO. The N 1s XPS spectrum of the samples is also presented in Figure 8f, which shows that N 1s cannot be detected in 90ZA, 80ZA, 70ZA, and 60ZA. By contrast, 50ZA exhibited binding energies in the N 1s region. This peak can be ascribed to C−N−C (398.6 eV), N− (C)3 (400.1 eV), and N−H groups (401.2 eV, which are typical peaks of g-C3N4.25 These results reveal that carbon-doped ZnO was successfully synthesized in 90ZA, 80ZA, 70ZA, and 60ZA and that ZnO/g-C3N4 composites were fabricated on 50ZA. The optical properties of the carbon-doped ZnO nanostructures were studied by UV−vis diffuse reflectance spectroscopy (UV-DRS), and results are presented in Figure 9a. Compared with pure ZnO, carbon-doped ZnO displayed significant redshifting of its optical bandgap absorption edge into the visible light region as well as enhanced light absorption in the entire UV−vis band. Although visible light absorption is important for carbon-doped ZnO as a visible light photocatalyst, stronger UV absorbance would also benefit the photocatalytic applications of solar light because UV light is a major component of solar light. By applying the Kubelka−Munk rule, the precise direct bandgap energies of ZnO (∼3.19 eV), 90ZA (∼3.11 eV), 80ZA (∼3.02 eV), 70ZA (∼2.86 eV), 60ZA (∼2.72 eV), and g-C3N4 (∼2.5 eV) may be extracted from their (ahv)2 versus hv plots (Figure 9b). The UV-DRS results suggest that carbon-doped ZnO is more strongly responsive to visible light than pure ZnO and thus present better visible light photocatalytic activity. The PL spectra of nanostructured ZnO and carbon-doped ZnO semiconductor materials are related to the transfer behavior of the photoinduced electrons and holes; hence, the intensity of the PL peak reflects the separation or recombination rates of photogenerated hole−electron carriers. Weaker PL intensities generally reveal the lower recombination probability of photogenerated charge carriers. Figure 10 shows the PL spectra of the pure ZnO and carbon-doped ZnO (90ZA, 80ZA, 70ZA, and 60ZA) obtained at an excitation wavelength

Figure 5. Microstructure of 90ZA. (a) HRTEM and (b) corresponding SAED pattern of the sample. (c) HAADF and (d) corresponding high-resolution HAADF images of the sample. (e) ABF image and (f) corresponding high-resolution ABF images of the sample. (g) HAADF image of the ZnO structure. (h) Elemental mapping of Zn, (i) elemental mapping of O, and (j) elemental mapping of C.

Figure 6. Microstructure of 60ZA. (a) HAADF, (b) corresponding high-resolution HAADF, (c) ABF, and (d) corresponding highresolution ABF STEM images. (e) HRTEM image and (f) corresponding SAED pattern of the sample. (g) HAADF image of the ZnO structure, (h) elemental mapping of Zn, (i) elemental mapping of O, and (j) elemental mapping of C.

(Figure S3). The binding energy distance (Table S2) between these two lines is within the standard reference value of ZnO.23 Figure 7 presents the O 1s XPS of pure ZnO and carbon-doped 20549


Article


Figure 7. High-resolution XPS O 1s spectra of the obtained samples: (a) ZnO, (b) 80ZA, and (c) 60ZA.

Figure 8. Core-level XPS spectra. High-resolution XPS C 1s spectra of (a) ZnO, (b) 90ZA, (c) 80ZA, (d) 70ZA, and (e) 60ZA. (f) High-resolution XPS N 1s spectra of the obtained samples.

Figure 9. Optical properties of pure ZnO, carbon-doped ZnO, and g-C3N4. (a) UV−vis diffuse adsorption spectra and (b) the corresponding (ahv)2 vs hv plots. 20550


Article


nanostructures. Therefore, carbon doping of ZnO may further reduce recombination rates, which is crucial to improving photocatalytic activity. 3.2. Growth Mechanism of Carbon-Doped ZnO. Carbon-doped ZnO was synthesized through easy control of the ratio of Zn(Ac)2·2H2O to urea. During synthesis, addition of urea to the Zn(Ac)2·2H2O precursor caused a decrease in the ZnO crystal growth rate along the [001] direction.26 According to previous results,27 urea condenses to form cyanuric acid, which can be synthesized at ∼175 °C, with the release of ammonia. Cyanuric acid, which features −OH groups, preferentially adsorb on the positively charged Zn2+ (001) surface and block contact between growth units and the (001) crystal surface. ZnO crystal growth along the six symmetric directions was enhanced under the experimental conditions; thus, decreases in length and increases in the diameter of ZnO rods were observed (Figures 3 and 4). When the urea ratio was 30 wt %, the ZnO showed sphere-like structures. As the urea ratio increased to 40 wt %, sphere-like structured ZnO with fine grain sizes was synthesized. Higher urea concentrations promoted cyanuric acid formation at a high temperature of ∼175 °C. Cyanuric acid anions adsorb on ZnO crystals during thermal decomposition of Zn(Ac)2·2H2O at

Figure 10. PL spectral intensity of pure ZnO and carbon-doped ZnO (90ZA, 80ZA, 70ZA, and 60ZA) decreases with increasing carbon doping contents.

of 325 nm. Pure ZnO exhibited the highest intensity, and PL intensity decreased with increasing carbon contents in the ZnO

Figure 11. Photocatalytic activity of ZnO for MB degradation under visible-light irradiation. (a) Adsorption properties and photocatalytic activities of the synthesized photocatalysts; the properties of P25-TiO2 are also shown for comparison. (b) Kinetic plots based on the data in (a). (c) Photodegradation rate constants of the obtained photocatalysts and P25-TiO2. (d) Cycling experiments of carbon-doped ZnO 60ZA for MB degradation under visible-light irradiation. 20551


Article

The Journal of Physical Chemistry C ∼350−450 °C17 and impede contact between ZnO growth units, thereby preventing free growth of ZnO. During calcinations in air ZnO crystallization, carbon combustion and Zn, O, and C atom rearrangement occurred. Thus, carbondoped ZnO rod- and sphere-like nanostructures were successfully prepared. 3.3. Photocatalytic Activity. Figure 11a shows the adsorption properties and photocatalytic activities of the obtained pure ZnO, carbon-doped ZnO prepared with different urea concentrations (90ZA, 80ZA, 70ZA, and 60ZA), ZnO/gC3N4 composite (50ZA), and g-C3N4 for degradation of MB under visible light. MB photodegradation using commercial P25-TiO2 photocatalyst is also presented for comparison. The original UV−vis spectra with different irradiation time are presented in Figure S4. The ZnO/g-C3N4 composite (50ZA) exhibited the highest adsorption ability because it features the largest surface area among the samples studied. Carbon-doped ZnO 60ZA, g-C3N4, and commercial P25 TiO2 show the similar adsorption abilities. The amounts of MB adsorbed by the carbon-doped ZnO (90ZA, 80ZA, 70ZA, and 60ZA) increased as the carbon doping increased, consistent with the pattern of the specific surface areas of these samples. To obtain a better understanding of the reaction kinetics of MB degradation catalyzed by the photocatalysts, the degradation process was fitted to a pseudo-first-order reaction, where the rate constant k is equal to the slope of the fitting line: ln(C0/C) = kt, where k represents the rate constant (min−1) and C0 and C respectively represent the initial and time t concentrations of MB molecules. When no photocatalyst is involved with visiblelight irradiation, the organic contaminant MB slightly decreased, which indicates a very slow self-degradation process (Figures 11a,b). While pure ZnO, P25-TiO2, g-C3N4, and ZnO/g-C3N4 (50ZA) showed poor performance during MB photodegradation under visible light irradiation, the carbondoped ZnO samples exhibited high photocatalytic activity, especially as the carbon doping content increased. Among the samples studied, 60ZA showed the highest photocatalytic activity with a photodegradation efficiency of 90.3% after 2 h of irradiation. The k of this sample was 0.0207 min−1 (Figure 11c). To examine the stability of the carbon-doped ZnO, multiple photodegradation tests of MB using 60ZA under visible-light irradiation were carried out (Figure 11d). After five recycling experiments, 60ZA displayed excellent stability with limited decreases in photocatalytic efficiency from 98.2% to 96.7% after five cycles of reuse. These results reveal that carbon-doped ZnO photocatalysts feature good reusability, which meets the needs of practical applications. The urea-assisted technique developed in this work can reliably produce carbon-doped ZnO nanostructures using the inexpensive precursors Zn(Ac)2·2H2O and urea. The technique presents a universal pathway and provides new routes for synthesizing carbon-doped ZnO; it should thus be considered new member of the collection of synthetic protocols for producing carbon-doped ZnO. The carbon-doped ZnO obtained possesses a surface area (∼48 m2 g−1) larger than those of pure ZnO (∼8 m2 g−1) and g-C3N4 (∼14 m2 g−1). The photocatalytic activity of semiconductor materials depends on a number of factors, including composition, phase structures, surface hydroxyl groups, particles size, crystalline, surface defects, surface metal deposits, and adsorbates or surface bound complexes. Changes in the photocatalytic activity of carbon-doped ZnO are related to the separation efficiencies of

photoinduced electron−hole pairs arising from excitation of the semiconductor material. The high activity of the carbon doped ZnO by this facile synthesis can be ascribed to the strong visible light absorption, high surface area compared with P25-TiO2, and undoped ZnO. Photocatalyst with large surface area can offer more active adsorption sites, more photocatalytic reaction centers, and faster flow rate of the gas molecules.28 The valence band (VB) electrons of ZnO and P25-TiO2 cannot be excited by visible light because of their wide bandgap. As such, MB degradation by ZnO and P25-TiO2 is attributed to dye-sensitized photocatalysis. As carbon doping can significantly reduce the bandgap from ∼3.19 to ∼2.72 eV, carbondoped ZnO may be excited by visible light to excite valence electrons into the conduction band (CB) and leave holes in the VB. Thus, significant improvements in the photocatalytic activity of carbon-doped ZnO may be mainly due to the extension of the absorption band of the material to the visible light under excitation, during which excited-state electrons are transported from the VB to the CB. The O 1s XPS spectra reveal that Ovac may function as electron acceptors and trap photogenerated electrons temporarily to reduce surface recombination of electrons and holes. These Ovac may thus be considered trap centers that can prolong the lifetime of electron−hole pairs. The PL spectra also confirm that carbon doping can decrease the PL spectral intensity, which results in reductions in recombination rates, which is crucial to improving photocatalytic activity. According to the characterization and photocatalytic activities, we can speculate mechanism as shown in Figure 12.

Figure 12. Schematic diagram for the band structure of carbon-doped ZnO and the proposed photocatalytic mechanism.

For pure ZnO, the bandgap is ∼3.19 eV, thus the band to band transition hardly occurs since it cannot respond to visible light (process A, Figure 12). While the carbon doped ZnO, the carbon atoms incorporate into the substitutional sites and interstitial sites of ZnO lattice, which is confirmed by XPS results. The photogenerated holes injected by VB and generated at the energy level of interstitial carbon dopants can react with H2O or OH− adsorbed on the surface to produce hydroxyl radicals (•OH) that can further decompose MB (process B, Figure 12). Moreover, the oxygen vacancy is determined by XPS, and the oxygen vacancy always acts as electron trapping center, and the atoms close to oxygen vacancies usually play the role of reaction centers (active sites).29 The photogenerated electrons from CB and energy level of oxygen vacancies can be captured by the adsorbed O2 molecules. The •O2− can be formed and further transformed to • OH radicals to decompose the MB dye (process C, Figure 12). 20552



■

Thus, •OH radicals are suggested to play a crucial role in the photodecomposition of MB dye.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07116. Additional details including X-ray diffraction patterns of the obtained ZnO, 50ZA, and g-C3N4; TGA results of obtained samples; EDS elemental analysis; HAADF, ABF image, and corresponding high-resolution images of carbon-doped nanorods and nanospheres ZnO; XPS of Zn 2p; BET surface areas of the synthesized photocatalysts; original UV−vis spectra with different time under visible light irradiation; XPS fitting data of the obtained samples; binding energy of high-resolution XPS C 1s, Zn 2p, O 1s, and N 1s for the synthesized photocatalysts (PDF)

■

REFERENCES

(1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69−96. (2) Yu, J.; Yu, X. Hydrothermal Synthesis and Photocatalytic Activity of Zinc Oxide Hollow Spheres. Environ. Sci. Technol. 2008, 42, 4902− 4907. (3) Yang, J. L.; An, S. J.; Park, W. I.; Yi, G. C.; Choi, W. Photocatalysis Using ZnO Thin Films and Nanoneedles Grown by Metal−Organic Chemical Vapor Deposition. Adv. Mater. 2004, 16, 1661−1664. (4) Zheng, Y.; Zheng, L.; Zhan, Y.; Lin, X.; Zheng, Q.; Wei, K. Ag/ ZnO Heterostructure Nanocrystals: Synthesis, Characterization, and Photocatalysis. Inorg. Chem. 2007, 46, 6980−6986. (5) Ahn, K.-S.; Yan, Y.; Shet, S.; Deutsch, T.; Turner, J.; Al-Jassim, M. Enhanced Photoelectrochemical Responses of ZnO Films through Ga and N Codoping. Appl. Phys. Lett. 2007, 91, 231909. (6) Yang, X.; Wolcott, A.; Wang, G.; Sobo, A.; Fitzmorris, R. C.; Qian, F.; Zhang, J. Z.; Li, Y. Nitrogen-Doped ZnO Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2009, 9, 2331− 2336. (7) Qiu, X.; Li, L.; Zheng, J.; Liu, J.; Sun, X.; Li, G. Origin of the Enhanced Photocatalytic Activities of Semiconductors: A Case Study of ZnO Doped with Mg2+. J. Phys. Chem. C 2008, 112, 12242−12248. (8) Qiu, X.; Li, G.; Sun, X.; Li, L.; Fu, X. Doping Effects of Co2+ Ions on ZnO Nanorods and Their Photocatalytic Properties. Nanotechnology 2008, 19, 215703−215708. (9) Bandara, J.; Tennakone, K.; Jayatilaka, P. P. B. Composite Tin and Zinc Oxide Nanocrystalline Particles for Enhanced Charge Separation in Sensitized Degradation of Dyes. Chemosphere 2002, 49, 439−445. (10) Jing, L.; Wang, B.; Xin, B.; Li, S.; Shi, K.; Cai, W.; Fu, H. Investigations on the Surface Modification of ZnO Nanoparticle Photocatalyst by Depositing Pd. J. Solid State Chem. 2004, 177, 4221− 4227. (11) Zhang, M.; An, T.; Hu, X.; Wang, C.; Sheng, G.; Fu, J. Preparation and Photocatalytic Properties of a Nanometer ZnO−SnO2 Coupled Oxide. Appl. Catal., A 2004, 260, 215−222. (12) Cho, S.; Jang, J.-W.; Lee, J. S.; Lee, K.-H. Carbon-Doped ZnO Nanostructures Synthesized Using Vitamin C for Visible Light Photocatalysis. CrystEngComm 2010, 12, 3929−3935. (13) Tan, S. T.; Sun, X. W.; Yu, Z. G.; Wu, P.; Lo, G. Q.; Kwong, D. L. P-Type Conduction in Unintentional Carbon-Doped ZnO Thin Films. Appl. Phys. Lett. 2007, 91, 072101. (14) Pan, H.; Yi, J. B.; Shen, L.; Wu, R. Q.; Yang, J. H.; Lin, J. Y.; Feng, Y. P.; Ding, J.; Van, L. H.; Yin, J. H. Room-Temperature Ferromagnetism in Carbon-Doped ZnO. Phys. Rev. Lett. 2007, 99, 127201. (15) Wang, L.; Tian, C.; Wang, B.; Wang, R.; Zhou, W.; Fu, H. Controllable Synthesis of Graphitic Carbon Nanostructures from IonExchange Resin-Iron Complex Via Solid-State Pyrolysis Process. Chem. Commun. 2008, 5411−5413. (16) Tian, C.; Zhang, Q.; Wu, A.; Jiang, M.; Liang, Z.; Jiang, B.; Fu, H. Cost-Effective Large-Scale Synthesis of ZnO Photocatalyst with Excellent Performance for Dye Photodegradation. Chem. Commun. 2012, 48, 2858−2860. (17) Zhang, X.; Qin, J.; Xue, Y.; Yu, P.; Zhang, B.; Wang, L.; Liu, R. Effect of Aspect Ratio and Surface Defects on the Photocatalytic Activity of ZnO Nanorods. Sci. Rep. 2014, 4, 4596. (18) Liu, J.; Zhang, T.; Wang, Z.; Dawson, G.; Chen, W. Simple Pyrolysis of Urea into Graphitic Carbon Nitride with Recyclable Adsorption and Photocatalytic Activity. J. Mater. Chem. 2011, 21, 14398−14401. (19) Dong, F.; Wu, L.; Sun, Y.; Fu, M.; Wu, Z.; Lee, S. C. Efficient Synthesis of Polymeric g-C3N4 Layered Materials as Novel Efficient Visible Light Driven Photocatalysts. J. Mater. Chem. 2011, 21, 15171− 15174. (20) Zhang, Y.; Liu, J.; Wu, G.; Chen, W. Porous Graphitic Carbon Nitride Synthesized Via Direct Polymerization of Urea for Efficient

4. CONCLUSIONS We have developed a urea-assisted thermal decomposition approach to synthesize rod- and sphere-like nanostructured carbon-doped ZnO photocatalysts efficiently. The developed synthetic protocol is both reliable and scalable and presents a general method for synthesizing carbon-doped ZnO photocatalysts for organic pollutant degradation. Carbon-doped ZnO exhibited very high visible-light photocatalytic activity for photodegradation of MB. Such activity could be attributed to the uniform carbon doping on the ZnO crystals, which results in bandgap reduction from ∼3.19 to ∼2.72 eV. Carbon doping and the Ovac created could improve the separation efficiency of photogenerated electron−hole pairs, thereby enhancing the photocatalytic activity of ZnO. Considering the excellent photocatalytic performance and facile preparation procedure demonstrated in this work, the developed carbon-doped ZnO nanostructures present potential use in a wide range of applications, including environmental remediation and solar utilization.

■

Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.Q.). *E-mail [email protected] (R.L.). Author Contributions

X.Z. and J.Q. contributed equally to this work. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research is supported by Ratchadapisek Somphoch Endowment Fund of Chulalongkorn University. J.Q. acknowledges the support from Thailand Research Fund (TRG5780222), Ratchadapisek Somphoch Endowment Fund, Chulalongkorn University, granted to the Surface Coatings Technology for Metals and Materials Research Unit (GRU 57005-62-001), and Key Laboratory of Metastable Materials Science and Technology, Yanshan University. X.Z., M.M., and R.L. thank the support from NBRPC (grant 2013CB733000) and NSFC (grant 51171160/51434008). 20553


Article

The Journal of Physical Chemistry C Sunlight-Driven Photocatalytic Hydrogen Production. Nanoscale 2012, 4, 5300−5303. (21) Zhou, S.; Xu, Q.; Potzger, K.; Talut, G.; Grötzschel, R.; Fassbender, J.; Vinnichenko, M.; Grenzer, J.; Helm, M.; Hochmuth, H.; et al. Room Temperature Ferromagnetism in Carbon-Implanted ZnO. Appl. Phys. Lett. 2008, 93, 232507. (22) Meyer, B.; Marx, D. Density-Functional Study of the Structure and Stability of Zno Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 035403. (23) Mishra, D. K.; Mohapatra, J.; Sharma, M. K.; Chattarjee, R.; Singh, S. K.; Varma, S.; Behera, S. N.; Nayak, S. K.; Entel, P. Carbon Doped ZnO: Synthesis, Characterization and Interpretation. J. Magn. Magn. Mater. 2013, 329, 146−152. (24) Wang, J.; Wang, Z.; Huang, B.; Ma, Y.; Liu, Y.; Qin, X.; Zhang, X.; Dai, Y. Oxygen Vacancy Induced Band-Gap Narrowing and Enhanced Visible Light Photocatalytic Activity of ZnO. ACS Appl. Mater. Interfaces 2012, 4, 4024−4030. (25) Zou, X.-X.; Li, G.-D.; Wang, Y.-N.; Zhao, J.; Yan, C.; Guo, M.Y.; Li, L.; Chen, J.-S. Direct Conversion of Urea into Graphitic Carbon Nitride over Mesoporous TiO2 Spheres under Mild Condition. Chem. Commun. 2011, 47, 1066−1068. (26) Cho, S.; Jeong, H.; Park, D.-H.; Jung, S.-H.; Kim, H.-J.; Lee, K.H. The Effects of Vitamin C on ZnO Crystal Formation. CrystEngComm 2010, 12, 968−976. (27) Bauer, K.; Garbe, D.; Surbug, H. Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, Germany, 1988. (28) Dong, F.; Wang, H.; Wu, Z. One-Step “Green” Synthetic Approach for Mesoporous C-Doped Titanium Dioxide with Efficient Visible Light Photocatalytic Activity. J. Phys. Chem. C 2009, 113, 16717−16723. (29) Li, H.-F.; Zhang, N.; Chen, P.; Luo, M.-F.; Lu, J.-Q. High Surface Area Au/CeO2 Catalysts for Low Temperature Formaldehyde Oxidation. Appl. Catal., B 2011, 110, 279−285.

20554


Carbon-Doped ZnO Nanostructures: Facile Synthesis and Visible

Recommend Documents