Dalton Transactions View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
PAPER
Cite this: Dalton Trans., 2014, 43, 16441
View Journal | View Issue
CO2-assisted synthesis of mesoporous carbon/C-doped ZnO composites for enhanced photocatalytic performance under visible light Fangxiao Wang,a Lin Liang,b Lei Shi,a Mengshuai Liua and Jianmin Sun*a,c Visible-light-responsive mesoporous carbon/C-doped ZnO (mC/C-ZnO) composites were fabricated using a facile, fast, one-step process in CO2-expanded ethanol solution. It is a green and sustainable process that does not need tedious pretreatment, surfactants or precipitants. CO2 played triple roles in the synthesis of mC/C-ZnO composites; the first was to provide a simple physical expansion to evenly dope the carbon in the ZnO; the second was to offer some chemical groups such as CO32− and HCO3−, facilitating the uniform and complete deposition through the coordination of a metallic cation with these anions; and the third was to offer CO32− acting as a template for the formation of mesoporosity in the carbon. When used as a photocatalyst for the photodegradation of RhB and the organic pollutant phenol, the mC/C-ZnO composites with glucose content at 22 wt% (mC/C-ZnO-CE-2) synthesized in CO2expanded ethanol exhibited better recycling stability and photodegradation rate than the corresponding sample synthesized in pure ethanol. Such improved photocatalytic performance was attributed to
Received 10th July 2014, Accepted 6th September 2014
the well-mixing of the mesoporous carbon and the small sized C-doped ZnO particles in the mC/ C-ZnO-CE-2 composites. The facile and fast synthesis method could be extended to other mesoporous
DOI: 10.1039/c4dt02098g
carbon/C-doped metal oxide composites, which are expected to be good photocatalyst candidates, or in
www.rsc.org/dalton
other application fields.
1.
Introduction
Zinc oxide (ZnO) has been widely used as a photocatalyst for solar energy conversion and a treatment for organic or dye pollutants due to its low toxicity, high abundance, photostability and photocatalytic efficiency.1–3 However, the application of pure ZnO in visible light is limited due to its low efficiency because of the wide band gap (3.27 eV). One of the solutions to this problem is to modify ZnO to utilize the total solar energy, such as through dye sensitization,4,5 semiconductor coupling6,7 or metal or non-metal doping.8–10 Doping of ZnO with carbon was found to be very efficient in visible-light induced photocatalysis not only due to forming an intermediate energy band gap but it also promoted separation efficiency in photoelectrons and holes by channelizing the photo-excited electrons to the doped carbon.11–15 However, the relatively
a The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150080, China b School of Life Science and Technology, Harbin Institute of Technology, Harbin 150080, China c State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150080, China. E-mail: [email protected]; Tel: +86 451 86403715
This journal is © The Royal Society of Chemistry 2014
small surface areas of carbon still limits the adsorption and transfer of photoelectrons, thus resulting in unsatisfactory catalytic degradation efficiency. Mesoporous carbons, which possess large pore sizes and high surface areas, provide a good opportunity for synthesizing mesoporous carbon/C-doped ZnO composites. The coexistence of mesoporous carbon and C-doped ZnO may be a promising efficient photocatalyst. However, to the best of our knowledge, there are few reports on materials composed of mesoporous carbon and C-doped ZnO for the degradation of contaminants. In the pioneering work, soft or hard templates were usually used for the fabrication of mesoporous carbon in advance,16–18 then the mesoporous C-based composites were prepared by post-processing. However, from the viewpoint of green chemistry, the utilization of high cost organic templates as well as the complexity of the synthetic procedures for mesoporous C might restrict their industrial applications. Recently, Zhou et al. prepared a uniform hamburger-like mesoporous carbon-incorporated ZnO nanoarchitecture by one-pot solvothermal method, but the long synthesis time of up to 24 h was not satisfactory.19 Therefore, the challenges associated with facile, fast and one-step synthesis of mesoporous carbon/ C-doped ZnO composites with large surface areas continue to be of the utmost importance to their application.
Dalton Trans., 2014, 43, 16441–16449 | 16441
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Paper
As a green and sustainable technology, supercritical or compressed CO2 has recently drawn great interest in different fields due to its environmentally friendly and unique features, such as tunable physical properties, low viscosity, high diffusivity and absence of surface tension.20–25 Simultaneously, CO2-expanded ethanol (CE) technique has shown superior advantages in the material’s synthesis because it could reduce the solvent strength caused by hydroxyl groups originating from ethanol, and thus greatly lowers the aggregation of the final particles.26–30 In addition, CO2-expanded ethanol solvents can largely mitigate the mass-transfer limitation to facilitate the infiltration of precursors in the complex and thus realize uniform doping. Sun et al. demonstrated that CO2-expanded ethanol could disperse hydrous metal nitrates inside and outside of carbon nanotubes (CNTs) and form an even oxide coating on tubes via in situ decomposition of precursors.26 More recently, Zhao’s group successfully prepared a series of monodispersed metal oxides and their composites with carbon colloids in CO2-expanded ethanol media, wherein the carbon colloids were prepared beforehand.27,30,31 Herein, we present a simple, fast and one-step method for synthesis of mC/C-doped ZnO composites directly from C and Zn precursors of glucose and Zn(NO3)2·6H2O in CO2-expanded ethanol solvent. Then, the roles of CO2 in the synthesis of mC/C-ZnO composites were investigated thoroughly. Furthermore, the photocatalytic activities of the composites were evaluated by degrading RhB dye and the organic pollutant phenol under visible light. The mC/C-ZnO composites with glucose content at 22 wt% (mC/C-ZnO-CE-2) synthesized in CO2-expanded ethanol manifested excellent recycling stability and rate performance, compared with the reference samples synthesized in pure ethanol and various glucose contents.
2. Experimental 2.1
Sample preparation
The reagents with AR purity used in this work were ethanol, glucose and zinc nitrate hexahydrate [Zn(NO3)2·6H2O]. In a typical run for the synthesis of mC/C-ZnO in CO2expanded ethanol, 0.60 g of Zn(NO3)2·6H2O and 0.168 g of glucose were added into 20 mL of absolute ethanol solution under vigorous stirring for 10 min at ambient temperature. Subsequently, the mixture was transferred into a 50 mL stainless autoclave, then the autoclave was heated to 150 °C for 8 h with pressurized CO2 at 10 MPa. After it was cooled to ambient temperature, the autoclave was slowly depressurized to atmosphere. Finally, the solid products were separated by filtration, dried at 60 °C for 6 h then calcined at 450 °C for 3 h under N2 atmosphere. The sample was designated as mC/C-ZnO-CE-2 composites. Similarly, mC/C-ZnO-CE-1 and mC/C-ZnO-CE-3 were prepared with glucose contents at 0.084 g and 0.336 g, respectively. Additionally, designated mC/C-ZnO-E-2 composites were also prepared in pure ethanol at 150 °C for 12 h with the same compositions as mC/C-ZnO-CE-2 only without CO2 for comparison.
16442 | Dalton Trans., 2014, 43, 16441–16449
Dalton Transactions
2.2
Characterization
Powder X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance X-ray powder diffractometer with Cu Kα radiation (40 kV, 40 mA). Nitrogen adsorption/desorption isotherms were measured at −196 °C on an ASAP 2020 volumetric analyzer. Before analysis, the sample was outgassed at 200 °C for 12 h under vacuum. The surface area was calculated using the Brunauer–Emmett–Teller (BET) method and total pore volume was determined from the amount of nitrogen adsorbed at P/P0 ca. 0.99. Transmission electron microscopy (TEM) experiments were performed on a Tecnai G2 Spririt electron microscope with an acceleration voltage of 120 kV. UV-vis spectroscopy was recorded on a Perkin Elmer Lambda 750 in the range of 200–800 nm. X-ray photoelectron spectroscopy (XPS) measurement was carried out on a Phi Quantera spectrometer using Al Ka X-ray as the excitation source. The photoluminescence spectra (PL) were obtained by a Varian Cary Eclipse spectrometer with an excitation wavelength of 325 nm. 2.3
Photocatalytic reaction
In order to evaluate the photocatalytic activity of the samples, photocatalytic degradation of RhB and phenol under visible light were chosen as model reactions. A 300 W Xe lamp with a 420 nm cutoff filter was used as the light source to provide visible light irradiation. In a typical experiment, 100 mg of photocatalyst was added to 100 mL of 5 mg L−1 RhB solution under magnetic stirring. Before the light irradiation, the dispersion was kept in the dark for 60 min under magnetic stirring to reach the adsorption–desorption equilibrium. During the illumination, solutions were collected every 30 min and centrifuged to remove the catalyst then analyzed on a UV-vis spectrometer at 552 nm. For comparison, the reactions were carried out in the presence of commercial ZnO powder and the samples synthesized in pure ethanol. Moreover, the photocatalytic degradation of the organic pollutant phenol was similar to that of RhB and determined on a UV-vis spectrometer at 270 nm. The percentage of degradation was calculated by C/C0, wherein C is the concentration of remaining pollutant solution at each irradiated time, and C0 is the initial concentration.
3. Results and discussion The powder XRD patterns of the mC/C-ZnO composites are shown in Fig. 1. The diffraction peaks at 2θ of 31.7°, 34.4°, 36.2°, 47.5°, 56.6° and 62.8° were respectively indexed to (100), (002), (101), (102), (110) and (103) diffraction planes of hexagonal wurtzite structure ZnO (JCPDS no. 36-1451).32 Apart from ZnO peaks, no obvious diffraction peaks of carbon were observed in the mC/C-ZnO composites, which might be due to its lower content or higher dispersity of carbon.33 No diffraction peaks from other impurities were observed, indicating that mC/C-ZnO composites were successfully synthesized. More importantly, the synthesis time was shortened markedly to 8 h at 10 MPa CO2 compared with 24 h in the absence of
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Dalton Transactions
Fig. 1 XRD patterns of (a) mC/C-ZnO-E-2, (b) mC/C-ZnO-CE-1, (c) mC/C-ZnO-CE-2 and (d) mC/C-ZnO-CE-3.
CO2 in the earlier report,19 implying that the anions CO32− and HCO3− formed in situ in water favored the fast fabrication process of carbon-based intermediates for the further formation of mC/C-ZnO composites. Whereas, the relatively prolonged synthesis time for 12 h in the case of mC/C-ZnO-E-2 composites came from the weaker coordination ability of NO3− to metal ions into intermediates than CO32− and HCO3−. A similar phenomenon of enhancement in crystallization rates was reported in the related literature.34 In order to examine the chemical states and surface compositions of the mC/C-ZnO composites, XPS study was performed in Fig. 2. The overall survey spectra (Fig. 2A) indicated that all
Paper
the mC/C-ZnO composites contained only C, Zn and O elements.35 The Zn 2p (Fig. 2B) spectra showed two distinct characteristic peaks of Zn 2p3/2 at 1022.7 eV and Zn 2p1/2 at 1045 eV, respectively, which confirmed the presence of a ZnO moiety in the composites.11,35 C 1s spectra (Fig. 2C) divided into three peaks at 284.6 eV, 286.1 eV and 288.8 eV, corresponding to C–C, C–O and CvO bonds, respectively.11,12 The binding energy at 286.1 eV suggested that the carbon was doped into the interstitial positions of ZnO lattice and formed the band of C–O–Zn,11,19 thus leading to the generation of a new intermediate band above the VB of ZnO,11 and this new intermediate band enabled mC/C-ZnO to absorb the visible light. O 1s spectra (Fig. 2D) were divided into two peaks at 530.3 eV and 532 eV, which corresponded to the lattice oxygen of ZnO and chemisorbed oxygen caused by the surface hydroxyl, respectively.35,36 Besides, the different surface compositions of all the mC/C-ZnO composites are given in Table 1. The sequence of carbon contents in the various mC/C-ZnO samples was mC-ZnO-CE-3 > mC-ZnO-E-2 ≈ mC-ZnO-CE-2 > mC-ZnO-CE-1, consistent with the amounts of glucose added. The composed and doped carbons in the composites were favourable for adsorption to pollutants and absorption to visible lights, which facilitated the photocatalytic reaction process under visible light. N2 sorption measurement was conducted to investigate the BET surface area and porous structure of the different samples in Fig. 3A. All the mC/C-ZnO composites had broad hysteresis loops in the relative pressures between 0.5 and 1.0, exhibiting
Fig. 2 XPS of (A) survey spectra, (B) Zn 2p, (C) C 1s and (D) O 1s for (a) mC/C-ZnO-E-2, (b) mC/C-ZnO-CE-1, (c) mC/C-ZnO-CE-2 and (d) mC/ C-ZnO-CE-3.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 16441–16449 | 16443
View Article Online
Paper Table 1
Dalton Transactions Textural parameters of various samples
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Surface compositionsd
Sample
SBETa (m2 g−1)
Vmesob (cm3 g−1)
Dmesoc (nm)
C (at%)
O (at%)
Zn (at%)
Pure ZnO mC/C-ZnO-E-2 mC/C-ZnO-CE-1 mC/C-ZnO-CE-2 mC/C-ZnO-CE-3
2.4 52.5 48.1 54.2 58.5
— 0.135 0.166 0.116 0.114
— 7–20 7.0 5.6 5.4
— 37.2 29.2 35.7 41.0
—e 30.5 35.3 32.2 30.1
—e 32.3 35.5 32.1 28.9
a BET surface area. b Mesoporous volume. c Mesopore size distribution calculated from desorption branch by BJH method. d Determined by XPS. e Not measured.
type IV isotherms according to the IUPAC classification. The obvious type IV isotherms indicated the abundant mesopores existing in the mC/C-ZnO composites. In contrast, there were no obvious hysteresis loops in the commercial ZnO. Through the XPS spectra, carbons in the composites were in the form of mesoporous carbon particles and doped C. Therefore, the present photocatalysts were a mixture of carbon-doped ZnO particles and mesoporous carbon. The formation of mesoporosity in carbon in mC/C-ZnO composites was related to the presence of anions such as NO3− and CO32−, which acted as the templates for the formation of mesoporous carbon.34 After thermal treatment in N2 atmosphere, NO3− and CO32− were removed and thus mesopores were introduced into the carbon. Correspondingly, the average pore diameters of all samples are displayed in Fig. 3B, further indicating the narrow mesoporous distributions in the mC/C-ZnO-CE composites. Additionally, the BET surface areas and pore volumes of various samples are summarized in Table 1. The mC/C-ZnO-CE composites possessed larger specific surface areas at 48–58 m2 g−1, which was almost 20 times higher than that of the commercial ZnO (2.4 m2 g−1) and the reported C-doped ZnO (about 7 m2 g−1) sample.12 The high specific surface areas and the presence of mesoporous C were both favorable for improving the adsorption capability, which occurred prior to the photodegradation
reaction. In addition, the mesoporous channels also could provide paths to separate the photoelectrons and holes during the photocatalytic reaction, improving the separation efficiency and thus enhancing the photodegradation efficiency. The morphologies of mC/C-ZnO-E-2 and mC/C-ZnO-CE-2 composites were further elucidated by TEM results. Fig. 4A and B exhibited the TEM image and EDX analysis inside the red rectangle area of mC/C-ZnO-E-2 composites. From the EDX pattern, only Zn and O elements were present, implying a bad doping and mixing of ZnO with carbon. From Fig. 4C, the disordered worm-like mesopores were clearly observed, and the EDX pattern indicated that the mesoporous C and C-doped ZnO nanoparticles were well-mixed. Moreover, the sizes of C-doped ZnO particles in mC/C-ZnO-CE-2 composites were obviously smaller than those in mC/C-ZnO-E-2; small ZnO particles were advantageous to the photocatalytic activity. The HRTEM image of mC/C-ZnO-CE-2 composites in Fig. 4E shows apparent mesopores in carbon and the lattice spacing of 0.26 nm attributed to the distance between the (002) planes of ZnO crystal lattice,12 presenting an intuitive contact of mesoporous C and C-doped ZnO nanoparticles in mC/C-ZnO-CE-2 composites. The presence of CO2 could facilitate the infiltration of C and Zn precursors in the composites, and realized the well-mixing of carbon and ZnO. The well-mixed mesoporous C in mC/C-ZnO-CE-2 composites accelerated the adsorption to pollutants and separation efficiency of the photoelectrons and holes, thus favoring the enhancement of photocatalytic activity. According to the results and discussion above, a possible formation procedure for the fabrication of mC/C-ZnO-CE composites was illustrated in Scheme 1. A primary solution of precursors Zn(NO3)2·6H2O and glucose (Scheme 1a) was firstly expanded by CO2 to form a homogeneous fluid (Scheme 1b) wherein the glucose molecules and zinc nitrate had enough space for thorough dispersion and uniform reaction in the solution, thereby resulting in the preservation of good dispersity of the composites even after precipitation. Subsequently with increasing temperature, the soluble Zn precursors and glucose converted into Zn-salt compounds [Znx(OH)y(NO3)z(CO3)m·n(H2O)]31
Fig. 3 (A) Nitrogen isotherms and (B) pore size distribution of (a) commercial ZnO, (b) mC/C-ZnO-E-2, (c) mC/C-ZnO-CE-1, (d) mC/C-ZnO-CE-2 and (e) mC/C-ZnO-CE-3. The isotherms for (c), (d) and (e) were offset by 50, 100 and 150 cm3 g−1 based on (b) along the vertical axis for clarity, respectively.
16444 | Dalton Trans., 2014, 43, 16441–16449
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Dalton Transactions
Fig. 4 (A) TEM image and (B) EDX analysis for mC/C-ZnO-E-2 composites. (C) TEM image, (D) EDX analysis and (E) HRTEM image for mC/ C-ZnO-CE-2 composites.
and carbon-based intermediates (CO32−@C-intermediates),34 respectively. The specific physicochemical properties of CO2expanded ethanol facilitated the formation and mixing of Znsalt compounds and carbon-based intermediates, therefore leading to the perfect mixing and doping of C and ZnO (Scheme 1c and d). After thermal treatment in N2 atmosphere, a large number of pores were left in the carbon and the Zn-salt
This journal is © The Royal Society of Chemistry 2014
compounds were converted into small sized C-doped ZnO avoiding the aggregation (Scheme 1e). Therefore, CO2 played triple roles in the synthesis of mC/C-ZnO composites; the first was to provide a simple physical expansion to make the mesoporous carbon and C-doped ZnO mix evenly; the second was to offer some chemical groups such as CO32− and HCO3−, facilitating the homogenous and complete deposition through the coordi-
Dalton Trans., 2014, 43, 16441–16449 | 16445
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Paper
Dalton Transactions
Scheme 1 Possible formation procedure for mC/C-ZnO-CE composites. (a) A primary solution of precursors within an autoclave, (b) the primary solution was expanded by CO2 to form a homogeneous fluid, (c) the precursors were converted to intermediates, (d) enlarged for the intermediate, (e) mC/C-ZnO-CE composites were obtained after thermal treatment in N2 atmosphere.
Fig. 5 (A) UV-visible diffuse reflectance spectra and (B) the corresponding plots of (αhν)1/2 vs. hν of (a) commercial ZnO, (b) mC/C-ZnO-E-2, (c) mC/C-ZnO-CE-1, (d) mC/C-ZnO-CE-2 and (e) mC/C-ZnO-CE-3.
nation of a metallic cation with these anions; and the third was to offer CO32− acting as templates for the formation of mesopores in carbon. The optical properties of the mC/C-ZnO composites were probed by UV-visible diffuse reflectance spectroscopy in Fig. 5A. The absorption spectrum of the commercial ZnO only displayed absorption in the ultraviolet region, and its band gap absorption edge was at 385 nm. However, the absorption spectra of mC/C-ZnO composites were extended to the visible regions at around 430, 450, 465 and 500 nm for mC/ C-ZnO-E-2, mC/C-ZnO-CE-1, mC/C-ZnO-CE-2 and mC/ C-ZnO-CE-3, respectively, which suggested that mC/C-ZnO composites could absorb more visible light energy due to the presence of the new energy level. The distinct differences in absorption characteristics proved that ZnO was effectively carbon-doped.11 Moreover, the band gap energies of the mC/ C-ZnO composites were determined from the plots of (αhν)1/2 vs. hν, as presented in Fig. 5B. By extrapolating the straight line to the x-axis in this plot, the band gap energies of the commercial ZnO, mC/C-ZnO-E-2, mC/C-ZnO-CE-1, mC/ C-ZnO-CE-2 and mC/C-ZnO-CE-3 were estimated at about 3.22, 2.75, 2.72, 2.45, and 2.35 eV, respectively, which further proved that the existence of doped carbon narrowed the band gaps of ZnO.11,37 In addition, the absorption intensity of the mC/
16446 | Dalton Trans., 2014, 43, 16441–16449
C-ZnO composites was improved significantly with the carbon content increasing, which was consistent with the previous report.12 Hence, the optical results suggested that the mC/ C-ZnO composites were more strongly responsive to the visible light than the commercial ZnO, thus favoring visible light catalytic activity. The PL spectra of nanostructured semiconductor materials are relative to the transfer behavior of the photoinduced electrons and holes, in other words, the intensity of the PL peak reflects the separation or recombination rates of the photogenerated carriers.38,39 It is generally believed that a weaker PL intensity means a lower recombination probability of the photogenerated charge carrier. Fig. 6 shows the PL spectra of commercial ZnO, mC/C-ZnO-E-2 and mC/C-ZnO-CE-2 composites with an excitation wavelength of 325 nm. Commercial ZnO showed the highest intensity and mC/C-ZnO-E-2 showed higher intensity than the mC/C-ZnO-CE-2 composites. Therefore, the presence of well-mixed and doped mesoporous carbon in mC/C-ZnO-CE-2 composites improved the separation rates of photogenerated electrons and holes, which were crucial to the improved photodegradation activity of mC/ C-ZnO-CE composites. In order to explore the photocatalytic ability of mC/C-ZnO composites, photodegradations of RhB and phenol by visible
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Dalton Transactions
Fig. 6 Photoluminescence spectra of (a) commercial ZnO, (b) mC/ C-ZnO-E-2 and (c) mC/C-ZnO-CE-2 composites.
Paper
light irradiation were chosen as the model reactions. Fig. 7A shows the adsorption equilibriums within 60 min for all the photocatalysts in dark, and the adsorbed amounts of RhB were 1.5% for commercial ZnO, 11.9% for mC/C-ZnO-E-2, 6.6% for mC/C-ZnO-CE-1, 13.4% for mC/C-ZnO-CE-2 and 22.5% for mC/ C-ZnO-CE-3. Noticeably, the mC/C-ZnO remarkably improved the adsorption capabilities for organic dye, which could enrich more dye molecules on the photoactive C-doped ZnO nanoparticles, thus leading to the enhanced rates of photocatalytic reactions. Fig. 7B displays the photocatalytic activities over various samples. When the commercial ZnO catalyst was used, the RhB concentration gradually decreased and the degradation efficiency was about 28% after 120 min. The degradation efficiencies of mC/C-ZnO-E-2 and mC/C-ZnO-CE-2 composites were improved to 63% and 99%, respectively, under the same conditions. Obviously, mC/C-ZnO-CE-2 exhibi-
Fig. 7 (A) The adsorption curves of RhB, (B) photocatalytic activities to RhB, (C) first-order kinetic plots, (D) the rate constants for photodegradation of RhB; (E) the adsorption curves of phenol, (F) photocatalytic activities to phenol, (G) first-order kinetic plots and (H) the rate constants for photodegradation of phenol over (a) commercial ZnO, (b) mC/C-ZnO-E-2, (c) mC/C-ZnO-CE-1, (d) mC/C-ZnO-CE-2 and (e) mC/C-ZnO-CE-3.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 16441–16449 | 16447
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Paper
Fig. 8
Dalton Transactions
(A) Recyclability and (B) XRD patterns of (a) before and (b) after three recycles over mC/C-ZnO-CE-2.
ted higher degradation activity than mC/C-ZnO-E-2, which implied that the presence of CO2 during the synthesis process had a significant effect on the activity of the photocatalyst. The existence of CO2 favored the mesoporous even mixing of C and small sized C-doped ZnO nanoparticles due to the high dispersity of the CO2-expanded ethanol system. The intimate contact between the well-mixed mesoporous C and the small-sized C-doped ZnO in the mC/C-ZnO-CE-2 composites assisted the enhanced adsorption to dye and strengthened absorption to visible light, together with the narrowed band gap and improved separation rates of photoelectrons and holes, thus leading to the acceleration in the photocatalytic rates. Additionally, the amount of carbon also had an influence on the photocatalytic activity. The degradation activity improved with increased carbon content, that is, the activity of mC/ C-ZnO-CE-2 was higher than mC/C-ZnO-CE-1. However, a decrease in the activity was caused with further increase of carbon in mC/C-ZnO-CE-3, suggesting that the carbon content was influential to the optimal photocatalytic activity. The mC/ C-ZnO-CE-2 composites exhibited the highest photocatalytic activity, almost 99% RhB was photodegraded within 120 min. In the case of mC/C-ZnO-CE-3 composites, although the carbon amount-to-composites fraction was higher and the band gap was narrower than mC/C-ZnO-CE-2 composites, that is, more light and dyes could be absorbed, less can be used for photocatalysis due to a low amount of ZnO in the composites.14 A linear relationships between ln(C0/C) and the irradiation time is shown in Fig. 7C. The linear relationships suggest that the photocatalytic degradation curves in all cases fit well with pseudo-first-order kinetics. Correspondingly, the degradation rate constants of various mC/C-ZnO composites are listed in Fig. 7D. Especially, mC/C-ZnO-CE-2 exhibited the highest rate constant at 0.03798 min−1, which was approximately 15.5 times larger than 0.00245 min−1 on commercial ZnO. Additionally, the adsorption and degradation phenomena for the organic pollutant phenol (Fig. 7E and F) were extremely similar to RhB dye. The mC/C-ZnO-CE-3 composites showed the highest adsorption capacity due to the highest surface area and carbon content, but mC/C-ZnO-CE-2 exhibited the highest degradation activity up to 38% within 3 h due to the appropriate proportion of mesoporous carbon to ZnO. The linear relationships (Fig. 7G) also suggested that the
16448 | Dalton Trans., 2014, 43, 16441–16449
photocatalytic degradation curves in all cases fit well with pseudo-first-order kinetics. Besides, the rate constant of mC/ C-ZnO-CE-2 for degradation to phenol reached 0.00258 min−1 (Fig. 7H), which was approximately 31.6 times larger than commercial ZnO under the same conditions. The stability of a photocatalyst is important for practical applications so the catalytic test on RhB was repeated using mC/C-ZnO-CE-2 to check the recyclability and stability under the same conditions. After every run of photodegradation, the separated photocatalysts were washed with deionized water and ethanol, dried at 60 °C for 12 h then reused for the next run. After three cycles, it was found that there was no significant reduction in the catalytic activity of mC/C-ZnO-CE-2 (Fig. 8A), which confirmed that mC/C-ZnO-CE-2 composites possessed good stability and were not photocorroded during the photodegradation process. In addition, the catalyst structure was further measured after three photodegradation runs by XRD (Fig. 8B). The peaks did not change obviously before and after three recycles, which indicates that the catalyst structure was stable.
4.
Conclusions
Visible-light-responsive mesoporous carbon/C-doped ZnO composites were fabricated using a facile, fast, one-step process directly from C and Zn precursors in CO2-expanded ethanol solution. The utilization of CO2 in the synthesis has the following advantages: (1) providing a simple physical expansion to make the carbon precursor mix and dope with the Zn precursor effectively. The good mixing and contact between carbon and C-doped ZnO are very crucial to the improved activity of the mC/C-ZnO-CE photocatalyst. (2) The use of CO2 can offer some chemical groups, such as CO32− and HCO3−, which facilitate the deposition of small sized C-ZnO completely and uniformly through the coordination to metallic cations. (3) The chemical group CO32− anions act as mesoporous templates for the formation of mesoporous carbon. In summary, it is a green or sustainable process and could be extended to the synthesis of other mesoporous carbon mixed and/or doped metal oxide composites. The photocatalytic activities of mC/C-ZnO-CE composites were evaluated for
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Dalton Transactions
degrading RhB dye and phenol under visible light. The mC-ZnO-CE composites synthesized in CO2-expanded ethanol manifested excellent catalytic performance compared to the reference sample mC/C-ZnO-E synthesized in pure ethanol and commercial ZnO. The improved photocatalytic activity of mC/C-ZnO in visible light came from the synergistic effects from the enhanced absorption to visible light and enriched adsorption to pollutants, together with the improved separation efficiency in photogenerated electrons and holes and narrowed band gap. Moreover, the small sized C-doped ZnO nanoparticles in composites also helped to enhance the photocatalytic activity. The mesoporous carbon/C-doped ZnO composites were easy and fast to prepare and displayed attractive photocatalytic performance, which favors their wide practical applications.
Acknowledgements We sincerely acknowledge the financial supports from National Natural Science Foundation of China (21373069), Science Foundation of Harbin City (NJ20140037), State Key Lab of Urban Water Resource and Environment of Harbin Institute of Technology (HIT2013TS01) and the Fundamental Research Funds for the Central Universities (HIT. IBRSEM. 201327).
References 1 E. S. Jang, J. H. Won, S. J. Hwang and J. H. Choy, Adv. Mater., 2006, 18, 3309. 2 N. Kislov, J. Lahiri, H. Verma, D. Y. Goswami, E. Stefanakos and M. Batzill, Langmuir, 2009, 25, 3310. 3 S. Chakrabarti and B. K. Dutta, J. Hazard. Mater., 2004, 112, 269. 4 Z. Dong, X. Lai, J. E. Halpert, N. Yang, L. Yi, J. Zhai, D. Wang, Z. Tang and L. Jiang, Adv. Mater., 2012, 26, 1046. 5 J. Jiang, X. Zhang, P. Sun and L. Zhang, J. Phys. Chem. C, 2011, 115, 20555. 6 V. Jeena and R. S. Robinson, Chem. Commun., 2012, 48, 299. 7 S. Balachandran and M. Swaminathan, Dalton Trans., 2013, 42, 5338. 8 S. Anandan and M. Miyauchi, Phys. Chem. Chem. Phys., 2011, 13, 14937. 9 D. E. Zhang, J. Y. Gong, J. J. Ma, G. Q. Han and Z. W. Tong, Dalton Trans., 2013, 42, 16556. 10 S. Liu, C. Li, J. Yu and Q. Xiang, CrystEngComm, 2011, 13, 2533. 11 S. T. Kochuveedu, Y. H. Jang, Y. J. Jang and D. H. Kim, J. Mater. Chem. A, 2013, 1, 898. 12 S. Cho, J. W. Jang, J. S. Lee and K. H. Lee, CrystEngComm, 2010, 12, 3929.
This journal is © The Royal Society of Chemistry 2014
Paper
13 H. F. Yu and H. Y. Chou, Powder Technol., 2013, 233, 201. 14 S. Y. Lian, H. Huang, J. M. Zhang, Z. H. Kang and Y. Liu, Solid State Commun., 2013, 155, 53. 15 P. Zhang, C. Shao, Z. Zhang, M. Zhang, J. Mu, Z. Guo and Y. Liu, Nanoscale, 2011, 3, 2943. 16 Y. Y. Zhou, S. Ko, C. W. Lee, S. G. Pyo, S. K. Kim and S. Yoon, J. Power Sources, 2013, 244, 777. 17 Y. F. Zhao, L. Zhao, K. X. Yao, Y. Yang, Q. Zhang and Y. Han, J. Mater. Chem., 2012, 22, 19726. 18 G. Q. Ma, Z. Y. Wen, J. Jin, Y. Lu, K. Rui, X. W. Wu, M. F. Wu and J. C. Zhang, J. Power Sources, 2014, 254, 353. 19 M. J. Zhou, X. H. Gao, Y. Hu, J. F. Chen and X. Hu, Appl. Catal., B, 2013, 138–139, 1. 20 S. C. Li, Z. Wang, X. W. Yu, J. X. Wang and S. C. Wang, Adv. Mater., 2012, 24, 3196. 21 R. Mello, A. Olmos, J. P. Carbonell, M. G. Nunez and G. Asensio, Green Chem., 2009, 11, 994. 22 J. M. Sun, C. L. Wang, L. Wang, L. Liang, X. F. Liu, L. Chen and F.-S. Xiao, Catal. Today, 2010, 158, 273. 23 J. L. Zhang, B. X. Han, C. X. Zhang, W. Li and X. Y. Feng, Angew. Chem., Int. Ed., 2008, 47, 3012. 24 M. Poliakoff, J. M. Fitzpatrick, T. R. Farren and P. T. Anastas, Science, 2002, 297, 807. 25 F. X. Wang, L. Liang, J. Ma and J. M. Sun, Mater. Lett., 2013, 111, 201. 26 Z. Y. Sun, X. R. Zhang, B. X. Han, Y. Y. Wu, G. M. An, Z. M. Liu, S. D. Miao and Z. J. Miao, Carbon, 2007, 45, 2589. 27 L. H. Zhuo, Y. Q. Wu, L. Y. Wang, J. Ming, Y. C. Yu, X. B. Zhang and F. Y. Zhao, J. Mater. Chem. A, 2013, 1, 3954. 28 Z. M. Liu and B. X. Han, Adv. Mater., 2009, 13, 825. 29 N. Farhangi, R. R. Chowdhury, Y. M. Gonzalez, M. B. Ray and P. A. Charpentier, Appl. Catal., B, 2011, 110, 25. 30 L. Y. Wang, L. H. Zhuo, C. Chao and F. Y. Zhao, Chem. – Eur. J., 2014, 20, 4308. 31 J. Ming, C. Y. Wu, H. Y. Cheng, Y. C. Yu and F. Y. Zhao, J. Supercrit. Fluids, 2011, 57, 137. 32 J. C. Sina, S. M. Lama, I. Satoshi, K. T. Leea and A. R. Mohamed, Appl. Catal., B, 2014, 148–149, 258. 33 L. Shi, L. Liang, J. Ma, F. X. Wang and J. M. Sun, Dalton Trans., 2014, 43, 7236. 34 J. Ming, Y. Q. Wu, G. F. Liang, J. B. Park, F. Y. Zhao and Y. K. Sun, Green Chem., 2013, 15, 2722. 35 M. Ahmad, E. Ahmed, Z. L. Hong, N. R. Khalid, W. Ahmed and A. Elhissi, J. Alloys Compd., 2013, 577, 717. 36 N. S. Ramgir, I. S. Mulla and V. K. Pillai, J. Phys. Chem. B, 2006, 110, 3995. 37 J. Cao, B. Luo, H. Lin and S. Chen, J. Mol. Catal. A: Chem., 2011, 344, 138. 38 L. Shi, L. Liang, J. Ma, Y. N. Meng, S. F. Zhong, F. X. Wang and J. M. Sun, Ceram. Int., 2014, 40, 3495. 39 Z. F. Zhang, Y. Li, K. F. Li, K. Chen, Y. Z. Yang, X. G. Liu, H. S. Jia and B. S. Xu, J. Mater. Sci., 2014, 49, 2347.
Dalton Trans., 2014, 43, 16441–16449 | 16449
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
PAPER
Cite this: Dalton Trans., 2014, 43, 16441
View Journal | View Issue
CO2-assisted synthesis of mesoporous carbon/C-doped ZnO composites for enhanced photocatalytic performance under visible light Fangxiao Wang,a Lin Liang,b Lei Shi,a Mengshuai Liua and Jianmin Sun*a,c Visible-light-responsive mesoporous carbon/C-doped ZnO (mC/C-ZnO) composites were fabricated using a facile, fast, one-step process in CO2-expanded ethanol solution. It is a green and sustainable process that does not need tedious pretreatment, surfactants or precipitants. CO2 played triple roles in the synthesis of mC/C-ZnO composites; the first was to provide a simple physical expansion to evenly dope the carbon in the ZnO; the second was to offer some chemical groups such as CO32− and HCO3−, facilitating the uniform and complete deposition through the coordination of a metallic cation with these anions; and the third was to offer CO32− acting as a template for the formation of mesoporosity in the carbon. When used as a photocatalyst for the photodegradation of RhB and the organic pollutant phenol, the mC/C-ZnO composites with glucose content at 22 wt% (mC/C-ZnO-CE-2) synthesized in CO2expanded ethanol exhibited better recycling stability and photodegradation rate than the corresponding sample synthesized in pure ethanol. Such improved photocatalytic performance was attributed to
Received 10th July 2014, Accepted 6th September 2014
the well-mixing of the mesoporous carbon and the small sized C-doped ZnO particles in the mC/ C-ZnO-CE-2 composites. The facile and fast synthesis method could be extended to other mesoporous
DOI: 10.1039/c4dt02098g
carbon/C-doped metal oxide composites, which are expected to be good photocatalyst candidates, or in
www.rsc.org/dalton
other application fields.
1.
Introduction
Zinc oxide (ZnO) has been widely used as a photocatalyst for solar energy conversion and a treatment for organic or dye pollutants due to its low toxicity, high abundance, photostability and photocatalytic efficiency.1–3 However, the application of pure ZnO in visible light is limited due to its low efficiency because of the wide band gap (3.27 eV). One of the solutions to this problem is to modify ZnO to utilize the total solar energy, such as through dye sensitization,4,5 semiconductor coupling6,7 or metal or non-metal doping.8–10 Doping of ZnO with carbon was found to be very efficient in visible-light induced photocatalysis not only due to forming an intermediate energy band gap but it also promoted separation efficiency in photoelectrons and holes by channelizing the photo-excited electrons to the doped carbon.11–15 However, the relatively
a The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150080, China b School of Life Science and Technology, Harbin Institute of Technology, Harbin 150080, China c State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150080, China. E-mail: [email protected]; Tel: +86 451 86403715
This journal is © The Royal Society of Chemistry 2014
small surface areas of carbon still limits the adsorption and transfer of photoelectrons, thus resulting in unsatisfactory catalytic degradation efficiency. Mesoporous carbons, which possess large pore sizes and high surface areas, provide a good opportunity for synthesizing mesoporous carbon/C-doped ZnO composites. The coexistence of mesoporous carbon and C-doped ZnO may be a promising efficient photocatalyst. However, to the best of our knowledge, there are few reports on materials composed of mesoporous carbon and C-doped ZnO for the degradation of contaminants. In the pioneering work, soft or hard templates were usually used for the fabrication of mesoporous carbon in advance,16–18 then the mesoporous C-based composites were prepared by post-processing. However, from the viewpoint of green chemistry, the utilization of high cost organic templates as well as the complexity of the synthetic procedures for mesoporous C might restrict their industrial applications. Recently, Zhou et al. prepared a uniform hamburger-like mesoporous carbon-incorporated ZnO nanoarchitecture by one-pot solvothermal method, but the long synthesis time of up to 24 h was not satisfactory.19 Therefore, the challenges associated with facile, fast and one-step synthesis of mesoporous carbon/ C-doped ZnO composites with large surface areas continue to be of the utmost importance to their application.
Dalton Trans., 2014, 43, 16441–16449 | 16441
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Paper
As a green and sustainable technology, supercritical or compressed CO2 has recently drawn great interest in different fields due to its environmentally friendly and unique features, such as tunable physical properties, low viscosity, high diffusivity and absence of surface tension.20–25 Simultaneously, CO2-expanded ethanol (CE) technique has shown superior advantages in the material’s synthesis because it could reduce the solvent strength caused by hydroxyl groups originating from ethanol, and thus greatly lowers the aggregation of the final particles.26–30 In addition, CO2-expanded ethanol solvents can largely mitigate the mass-transfer limitation to facilitate the infiltration of precursors in the complex and thus realize uniform doping. Sun et al. demonstrated that CO2-expanded ethanol could disperse hydrous metal nitrates inside and outside of carbon nanotubes (CNTs) and form an even oxide coating on tubes via in situ decomposition of precursors.26 More recently, Zhao’s group successfully prepared a series of monodispersed metal oxides and their composites with carbon colloids in CO2-expanded ethanol media, wherein the carbon colloids were prepared beforehand.27,30,31 Herein, we present a simple, fast and one-step method for synthesis of mC/C-doped ZnO composites directly from C and Zn precursors of glucose and Zn(NO3)2·6H2O in CO2-expanded ethanol solvent. Then, the roles of CO2 in the synthesis of mC/C-ZnO composites were investigated thoroughly. Furthermore, the photocatalytic activities of the composites were evaluated by degrading RhB dye and the organic pollutant phenol under visible light. The mC/C-ZnO composites with glucose content at 22 wt% (mC/C-ZnO-CE-2) synthesized in CO2-expanded ethanol manifested excellent recycling stability and rate performance, compared with the reference samples synthesized in pure ethanol and various glucose contents.
2. Experimental 2.1
Sample preparation
The reagents with AR purity used in this work were ethanol, glucose and zinc nitrate hexahydrate [Zn(NO3)2·6H2O]. In a typical run for the synthesis of mC/C-ZnO in CO2expanded ethanol, 0.60 g of Zn(NO3)2·6H2O and 0.168 g of glucose were added into 20 mL of absolute ethanol solution under vigorous stirring for 10 min at ambient temperature. Subsequently, the mixture was transferred into a 50 mL stainless autoclave, then the autoclave was heated to 150 °C for 8 h with pressurized CO2 at 10 MPa. After it was cooled to ambient temperature, the autoclave was slowly depressurized to atmosphere. Finally, the solid products were separated by filtration, dried at 60 °C for 6 h then calcined at 450 °C for 3 h under N2 atmosphere. The sample was designated as mC/C-ZnO-CE-2 composites. Similarly, mC/C-ZnO-CE-1 and mC/C-ZnO-CE-3 were prepared with glucose contents at 0.084 g and 0.336 g, respectively. Additionally, designated mC/C-ZnO-E-2 composites were also prepared in pure ethanol at 150 °C for 12 h with the same compositions as mC/C-ZnO-CE-2 only without CO2 for comparison.
16442 | Dalton Trans., 2014, 43, 16441–16449
Dalton Transactions
2.2
Characterization
Powder X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance X-ray powder diffractometer with Cu Kα radiation (40 kV, 40 mA). Nitrogen adsorption/desorption isotherms were measured at −196 °C on an ASAP 2020 volumetric analyzer. Before analysis, the sample was outgassed at 200 °C for 12 h under vacuum. The surface area was calculated using the Brunauer–Emmett–Teller (BET) method and total pore volume was determined from the amount of nitrogen adsorbed at P/P0 ca. 0.99. Transmission electron microscopy (TEM) experiments were performed on a Tecnai G2 Spririt electron microscope with an acceleration voltage of 120 kV. UV-vis spectroscopy was recorded on a Perkin Elmer Lambda 750 in the range of 200–800 nm. X-ray photoelectron spectroscopy (XPS) measurement was carried out on a Phi Quantera spectrometer using Al Ka X-ray as the excitation source. The photoluminescence spectra (PL) were obtained by a Varian Cary Eclipse spectrometer with an excitation wavelength of 325 nm. 2.3
Photocatalytic reaction
In order to evaluate the photocatalytic activity of the samples, photocatalytic degradation of RhB and phenol under visible light were chosen as model reactions. A 300 W Xe lamp with a 420 nm cutoff filter was used as the light source to provide visible light irradiation. In a typical experiment, 100 mg of photocatalyst was added to 100 mL of 5 mg L−1 RhB solution under magnetic stirring. Before the light irradiation, the dispersion was kept in the dark for 60 min under magnetic stirring to reach the adsorption–desorption equilibrium. During the illumination, solutions were collected every 30 min and centrifuged to remove the catalyst then analyzed on a UV-vis spectrometer at 552 nm. For comparison, the reactions were carried out in the presence of commercial ZnO powder and the samples synthesized in pure ethanol. Moreover, the photocatalytic degradation of the organic pollutant phenol was similar to that of RhB and determined on a UV-vis spectrometer at 270 nm. The percentage of degradation was calculated by C/C0, wherein C is the concentration of remaining pollutant solution at each irradiated time, and C0 is the initial concentration.
3. Results and discussion The powder XRD patterns of the mC/C-ZnO composites are shown in Fig. 1. The diffraction peaks at 2θ of 31.7°, 34.4°, 36.2°, 47.5°, 56.6° and 62.8° were respectively indexed to (100), (002), (101), (102), (110) and (103) diffraction planes of hexagonal wurtzite structure ZnO (JCPDS no. 36-1451).32 Apart from ZnO peaks, no obvious diffraction peaks of carbon were observed in the mC/C-ZnO composites, which might be due to its lower content or higher dispersity of carbon.33 No diffraction peaks from other impurities were observed, indicating that mC/C-ZnO composites were successfully synthesized. More importantly, the synthesis time was shortened markedly to 8 h at 10 MPa CO2 compared with 24 h in the absence of
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Dalton Transactions
Fig. 1 XRD patterns of (a) mC/C-ZnO-E-2, (b) mC/C-ZnO-CE-1, (c) mC/C-ZnO-CE-2 and (d) mC/C-ZnO-CE-3.
CO2 in the earlier report,19 implying that the anions CO32− and HCO3− formed in situ in water favored the fast fabrication process of carbon-based intermediates for the further formation of mC/C-ZnO composites. Whereas, the relatively prolonged synthesis time for 12 h in the case of mC/C-ZnO-E-2 composites came from the weaker coordination ability of NO3− to metal ions into intermediates than CO32− and HCO3−. A similar phenomenon of enhancement in crystallization rates was reported in the related literature.34 In order to examine the chemical states and surface compositions of the mC/C-ZnO composites, XPS study was performed in Fig. 2. The overall survey spectra (Fig. 2A) indicated that all
Paper
the mC/C-ZnO composites contained only C, Zn and O elements.35 The Zn 2p (Fig. 2B) spectra showed two distinct characteristic peaks of Zn 2p3/2 at 1022.7 eV and Zn 2p1/2 at 1045 eV, respectively, which confirmed the presence of a ZnO moiety in the composites.11,35 C 1s spectra (Fig. 2C) divided into three peaks at 284.6 eV, 286.1 eV and 288.8 eV, corresponding to C–C, C–O and CvO bonds, respectively.11,12 The binding energy at 286.1 eV suggested that the carbon was doped into the interstitial positions of ZnO lattice and formed the band of C–O–Zn,11,19 thus leading to the generation of a new intermediate band above the VB of ZnO,11 and this new intermediate band enabled mC/C-ZnO to absorb the visible light. O 1s spectra (Fig. 2D) were divided into two peaks at 530.3 eV and 532 eV, which corresponded to the lattice oxygen of ZnO and chemisorbed oxygen caused by the surface hydroxyl, respectively.35,36 Besides, the different surface compositions of all the mC/C-ZnO composites are given in Table 1. The sequence of carbon contents in the various mC/C-ZnO samples was mC-ZnO-CE-3 > mC-ZnO-E-2 ≈ mC-ZnO-CE-2 > mC-ZnO-CE-1, consistent with the amounts of glucose added. The composed and doped carbons in the composites were favourable for adsorption to pollutants and absorption to visible lights, which facilitated the photocatalytic reaction process under visible light. N2 sorption measurement was conducted to investigate the BET surface area and porous structure of the different samples in Fig. 3A. All the mC/C-ZnO composites had broad hysteresis loops in the relative pressures between 0.5 and 1.0, exhibiting
Fig. 2 XPS of (A) survey spectra, (B) Zn 2p, (C) C 1s and (D) O 1s for (a) mC/C-ZnO-E-2, (b) mC/C-ZnO-CE-1, (c) mC/C-ZnO-CE-2 and (d) mC/ C-ZnO-CE-3.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 16441–16449 | 16443
View Article Online
Paper Table 1
Dalton Transactions Textural parameters of various samples
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Surface compositionsd
Sample
SBETa (m2 g−1)
Vmesob (cm3 g−1)
Dmesoc (nm)
C (at%)
O (at%)
Zn (at%)
Pure ZnO mC/C-ZnO-E-2 mC/C-ZnO-CE-1 mC/C-ZnO-CE-2 mC/C-ZnO-CE-3
2.4 52.5 48.1 54.2 58.5
— 0.135 0.166 0.116 0.114
— 7–20 7.0 5.6 5.4
— 37.2 29.2 35.7 41.0
—e 30.5 35.3 32.2 30.1
—e 32.3 35.5 32.1 28.9
a BET surface area. b Mesoporous volume. c Mesopore size distribution calculated from desorption branch by BJH method. d Determined by XPS. e Not measured.
type IV isotherms according to the IUPAC classification. The obvious type IV isotherms indicated the abundant mesopores existing in the mC/C-ZnO composites. In contrast, there were no obvious hysteresis loops in the commercial ZnO. Through the XPS spectra, carbons in the composites were in the form of mesoporous carbon particles and doped C. Therefore, the present photocatalysts were a mixture of carbon-doped ZnO particles and mesoporous carbon. The formation of mesoporosity in carbon in mC/C-ZnO composites was related to the presence of anions such as NO3− and CO32−, which acted as the templates for the formation of mesoporous carbon.34 After thermal treatment in N2 atmosphere, NO3− and CO32− were removed and thus mesopores were introduced into the carbon. Correspondingly, the average pore diameters of all samples are displayed in Fig. 3B, further indicating the narrow mesoporous distributions in the mC/C-ZnO-CE composites. Additionally, the BET surface areas and pore volumes of various samples are summarized in Table 1. The mC/C-ZnO-CE composites possessed larger specific surface areas at 48–58 m2 g−1, which was almost 20 times higher than that of the commercial ZnO (2.4 m2 g−1) and the reported C-doped ZnO (about 7 m2 g−1) sample.12 The high specific surface areas and the presence of mesoporous C were both favorable for improving the adsorption capability, which occurred prior to the photodegradation
reaction. In addition, the mesoporous channels also could provide paths to separate the photoelectrons and holes during the photocatalytic reaction, improving the separation efficiency and thus enhancing the photodegradation efficiency. The morphologies of mC/C-ZnO-E-2 and mC/C-ZnO-CE-2 composites were further elucidated by TEM results. Fig. 4A and B exhibited the TEM image and EDX analysis inside the red rectangle area of mC/C-ZnO-E-2 composites. From the EDX pattern, only Zn and O elements were present, implying a bad doping and mixing of ZnO with carbon. From Fig. 4C, the disordered worm-like mesopores were clearly observed, and the EDX pattern indicated that the mesoporous C and C-doped ZnO nanoparticles were well-mixed. Moreover, the sizes of C-doped ZnO particles in mC/C-ZnO-CE-2 composites were obviously smaller than those in mC/C-ZnO-E-2; small ZnO particles were advantageous to the photocatalytic activity. The HRTEM image of mC/C-ZnO-CE-2 composites in Fig. 4E shows apparent mesopores in carbon and the lattice spacing of 0.26 nm attributed to the distance between the (002) planes of ZnO crystal lattice,12 presenting an intuitive contact of mesoporous C and C-doped ZnO nanoparticles in mC/C-ZnO-CE-2 composites. The presence of CO2 could facilitate the infiltration of C and Zn precursors in the composites, and realized the well-mixing of carbon and ZnO. The well-mixed mesoporous C in mC/C-ZnO-CE-2 composites accelerated the adsorption to pollutants and separation efficiency of the photoelectrons and holes, thus favoring the enhancement of photocatalytic activity. According to the results and discussion above, a possible formation procedure for the fabrication of mC/C-ZnO-CE composites was illustrated in Scheme 1. A primary solution of precursors Zn(NO3)2·6H2O and glucose (Scheme 1a) was firstly expanded by CO2 to form a homogeneous fluid (Scheme 1b) wherein the glucose molecules and zinc nitrate had enough space for thorough dispersion and uniform reaction in the solution, thereby resulting in the preservation of good dispersity of the composites even after precipitation. Subsequently with increasing temperature, the soluble Zn precursors and glucose converted into Zn-salt compounds [Znx(OH)y(NO3)z(CO3)m·n(H2O)]31
Fig. 3 (A) Nitrogen isotherms and (B) pore size distribution of (a) commercial ZnO, (b) mC/C-ZnO-E-2, (c) mC/C-ZnO-CE-1, (d) mC/C-ZnO-CE-2 and (e) mC/C-ZnO-CE-3. The isotherms for (c), (d) and (e) were offset by 50, 100 and 150 cm3 g−1 based on (b) along the vertical axis for clarity, respectively.
16444 | Dalton Trans., 2014, 43, 16441–16449
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Dalton Transactions
Fig. 4 (A) TEM image and (B) EDX analysis for mC/C-ZnO-E-2 composites. (C) TEM image, (D) EDX analysis and (E) HRTEM image for mC/ C-ZnO-CE-2 composites.
and carbon-based intermediates (CO32−@C-intermediates),34 respectively. The specific physicochemical properties of CO2expanded ethanol facilitated the formation and mixing of Znsalt compounds and carbon-based intermediates, therefore leading to the perfect mixing and doping of C and ZnO (Scheme 1c and d). After thermal treatment in N2 atmosphere, a large number of pores were left in the carbon and the Zn-salt
This journal is © The Royal Society of Chemistry 2014
compounds were converted into small sized C-doped ZnO avoiding the aggregation (Scheme 1e). Therefore, CO2 played triple roles in the synthesis of mC/C-ZnO composites; the first was to provide a simple physical expansion to make the mesoporous carbon and C-doped ZnO mix evenly; the second was to offer some chemical groups such as CO32− and HCO3−, facilitating the homogenous and complete deposition through the coordi-
Dalton Trans., 2014, 43, 16441–16449 | 16445
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Paper
Dalton Transactions
Scheme 1 Possible formation procedure for mC/C-ZnO-CE composites. (a) A primary solution of precursors within an autoclave, (b) the primary solution was expanded by CO2 to form a homogeneous fluid, (c) the precursors were converted to intermediates, (d) enlarged for the intermediate, (e) mC/C-ZnO-CE composites were obtained after thermal treatment in N2 atmosphere.
Fig. 5 (A) UV-visible diffuse reflectance spectra and (B) the corresponding plots of (αhν)1/2 vs. hν of (a) commercial ZnO, (b) mC/C-ZnO-E-2, (c) mC/C-ZnO-CE-1, (d) mC/C-ZnO-CE-2 and (e) mC/C-ZnO-CE-3.
nation of a metallic cation with these anions; and the third was to offer CO32− acting as templates for the formation of mesopores in carbon. The optical properties of the mC/C-ZnO composites were probed by UV-visible diffuse reflectance spectroscopy in Fig. 5A. The absorption spectrum of the commercial ZnO only displayed absorption in the ultraviolet region, and its band gap absorption edge was at 385 nm. However, the absorption spectra of mC/C-ZnO composites were extended to the visible regions at around 430, 450, 465 and 500 nm for mC/ C-ZnO-E-2, mC/C-ZnO-CE-1, mC/C-ZnO-CE-2 and mC/ C-ZnO-CE-3, respectively, which suggested that mC/C-ZnO composites could absorb more visible light energy due to the presence of the new energy level. The distinct differences in absorption characteristics proved that ZnO was effectively carbon-doped.11 Moreover, the band gap energies of the mC/ C-ZnO composites were determined from the plots of (αhν)1/2 vs. hν, as presented in Fig. 5B. By extrapolating the straight line to the x-axis in this plot, the band gap energies of the commercial ZnO, mC/C-ZnO-E-2, mC/C-ZnO-CE-1, mC/ C-ZnO-CE-2 and mC/C-ZnO-CE-3 were estimated at about 3.22, 2.75, 2.72, 2.45, and 2.35 eV, respectively, which further proved that the existence of doped carbon narrowed the band gaps of ZnO.11,37 In addition, the absorption intensity of the mC/
16446 | Dalton Trans., 2014, 43, 16441–16449
C-ZnO composites was improved significantly with the carbon content increasing, which was consistent with the previous report.12 Hence, the optical results suggested that the mC/ C-ZnO composites were more strongly responsive to the visible light than the commercial ZnO, thus favoring visible light catalytic activity. The PL spectra of nanostructured semiconductor materials are relative to the transfer behavior of the photoinduced electrons and holes, in other words, the intensity of the PL peak reflects the separation or recombination rates of the photogenerated carriers.38,39 It is generally believed that a weaker PL intensity means a lower recombination probability of the photogenerated charge carrier. Fig. 6 shows the PL spectra of commercial ZnO, mC/C-ZnO-E-2 and mC/C-ZnO-CE-2 composites with an excitation wavelength of 325 nm. Commercial ZnO showed the highest intensity and mC/C-ZnO-E-2 showed higher intensity than the mC/C-ZnO-CE-2 composites. Therefore, the presence of well-mixed and doped mesoporous carbon in mC/C-ZnO-CE-2 composites improved the separation rates of photogenerated electrons and holes, which were crucial to the improved photodegradation activity of mC/ C-ZnO-CE composites. In order to explore the photocatalytic ability of mC/C-ZnO composites, photodegradations of RhB and phenol by visible
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Dalton Transactions
Fig. 6 Photoluminescence spectra of (a) commercial ZnO, (b) mC/ C-ZnO-E-2 and (c) mC/C-ZnO-CE-2 composites.
Paper
light irradiation were chosen as the model reactions. Fig. 7A shows the adsorption equilibriums within 60 min for all the photocatalysts in dark, and the adsorbed amounts of RhB were 1.5% for commercial ZnO, 11.9% for mC/C-ZnO-E-2, 6.6% for mC/C-ZnO-CE-1, 13.4% for mC/C-ZnO-CE-2 and 22.5% for mC/ C-ZnO-CE-3. Noticeably, the mC/C-ZnO remarkably improved the adsorption capabilities for organic dye, which could enrich more dye molecules on the photoactive C-doped ZnO nanoparticles, thus leading to the enhanced rates of photocatalytic reactions. Fig. 7B displays the photocatalytic activities over various samples. When the commercial ZnO catalyst was used, the RhB concentration gradually decreased and the degradation efficiency was about 28% after 120 min. The degradation efficiencies of mC/C-ZnO-E-2 and mC/C-ZnO-CE-2 composites were improved to 63% and 99%, respectively, under the same conditions. Obviously, mC/C-ZnO-CE-2 exhibi-
Fig. 7 (A) The adsorption curves of RhB, (B) photocatalytic activities to RhB, (C) first-order kinetic plots, (D) the rate constants for photodegradation of RhB; (E) the adsorption curves of phenol, (F) photocatalytic activities to phenol, (G) first-order kinetic plots and (H) the rate constants for photodegradation of phenol over (a) commercial ZnO, (b) mC/C-ZnO-E-2, (c) mC/C-ZnO-CE-1, (d) mC/C-ZnO-CE-2 and (e) mC/C-ZnO-CE-3.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 16441–16449 | 16447
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Paper
Fig. 8
Dalton Transactions
(A) Recyclability and (B) XRD patterns of (a) before and (b) after three recycles over mC/C-ZnO-CE-2.
ted higher degradation activity than mC/C-ZnO-E-2, which implied that the presence of CO2 during the synthesis process had a significant effect on the activity of the photocatalyst. The existence of CO2 favored the mesoporous even mixing of C and small sized C-doped ZnO nanoparticles due to the high dispersity of the CO2-expanded ethanol system. The intimate contact between the well-mixed mesoporous C and the small-sized C-doped ZnO in the mC/C-ZnO-CE-2 composites assisted the enhanced adsorption to dye and strengthened absorption to visible light, together with the narrowed band gap and improved separation rates of photoelectrons and holes, thus leading to the acceleration in the photocatalytic rates. Additionally, the amount of carbon also had an influence on the photocatalytic activity. The degradation activity improved with increased carbon content, that is, the activity of mC/ C-ZnO-CE-2 was higher than mC/C-ZnO-CE-1. However, a decrease in the activity was caused with further increase of carbon in mC/C-ZnO-CE-3, suggesting that the carbon content was influential to the optimal photocatalytic activity. The mC/ C-ZnO-CE-2 composites exhibited the highest photocatalytic activity, almost 99% RhB was photodegraded within 120 min. In the case of mC/C-ZnO-CE-3 composites, although the carbon amount-to-composites fraction was higher and the band gap was narrower than mC/C-ZnO-CE-2 composites, that is, more light and dyes could be absorbed, less can be used for photocatalysis due to a low amount of ZnO in the composites.14 A linear relationships between ln(C0/C) and the irradiation time is shown in Fig. 7C. The linear relationships suggest that the photocatalytic degradation curves in all cases fit well with pseudo-first-order kinetics. Correspondingly, the degradation rate constants of various mC/C-ZnO composites are listed in Fig. 7D. Especially, mC/C-ZnO-CE-2 exhibited the highest rate constant at 0.03798 min−1, which was approximately 15.5 times larger than 0.00245 min−1 on commercial ZnO. Additionally, the adsorption and degradation phenomena for the organic pollutant phenol (Fig. 7E and F) were extremely similar to RhB dye. The mC/C-ZnO-CE-3 composites showed the highest adsorption capacity due to the highest surface area and carbon content, but mC/C-ZnO-CE-2 exhibited the highest degradation activity up to 38% within 3 h due to the appropriate proportion of mesoporous carbon to ZnO. The linear relationships (Fig. 7G) also suggested that the
16448 | Dalton Trans., 2014, 43, 16441–16449
photocatalytic degradation curves in all cases fit well with pseudo-first-order kinetics. Besides, the rate constant of mC/ C-ZnO-CE-2 for degradation to phenol reached 0.00258 min−1 (Fig. 7H), which was approximately 31.6 times larger than commercial ZnO under the same conditions. The stability of a photocatalyst is important for practical applications so the catalytic test on RhB was repeated using mC/C-ZnO-CE-2 to check the recyclability and stability under the same conditions. After every run of photodegradation, the separated photocatalysts were washed with deionized water and ethanol, dried at 60 °C for 12 h then reused for the next run. After three cycles, it was found that there was no significant reduction in the catalytic activity of mC/C-ZnO-CE-2 (Fig. 8A), which confirmed that mC/C-ZnO-CE-2 composites possessed good stability and were not photocorroded during the photodegradation process. In addition, the catalyst structure was further measured after three photodegradation runs by XRD (Fig. 8B). The peaks did not change obviously before and after three recycles, which indicates that the catalyst structure was stable.
4.
Conclusions
Visible-light-responsive mesoporous carbon/C-doped ZnO composites were fabricated using a facile, fast, one-step process directly from C and Zn precursors in CO2-expanded ethanol solution. The utilization of CO2 in the synthesis has the following advantages: (1) providing a simple physical expansion to make the carbon precursor mix and dope with the Zn precursor effectively. The good mixing and contact between carbon and C-doped ZnO are very crucial to the improved activity of the mC/C-ZnO-CE photocatalyst. (2) The use of CO2 can offer some chemical groups, such as CO32− and HCO3−, which facilitate the deposition of small sized C-ZnO completely and uniformly through the coordination to metallic cations. (3) The chemical group CO32− anions act as mesoporous templates for the formation of mesoporous carbon. In summary, it is a green or sustainable process and could be extended to the synthesis of other mesoporous carbon mixed and/or doped metal oxide composites. The photocatalytic activities of mC/C-ZnO-CE composites were evaluated for
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 08 September 2014. Downloaded by University of Illinois at Chicago on 23/10/2014 17:57:17.
Dalton Transactions
degrading RhB dye and phenol under visible light. The mC-ZnO-CE composites synthesized in CO2-expanded ethanol manifested excellent catalytic performance compared to the reference sample mC/C-ZnO-E synthesized in pure ethanol and commercial ZnO. The improved photocatalytic activity of mC/C-ZnO in visible light came from the synergistic effects from the enhanced absorption to visible light and enriched adsorption to pollutants, together with the improved separation efficiency in photogenerated electrons and holes and narrowed band gap. Moreover, the small sized C-doped ZnO nanoparticles in composites also helped to enhance the photocatalytic activity. The mesoporous carbon/C-doped ZnO composites were easy and fast to prepare and displayed attractive photocatalytic performance, which favors their wide practical applications.
Acknowledgements We sincerely acknowledge the financial supports from National Natural Science Foundation of China (21373069), Science Foundation of Harbin City (NJ20140037), State Key Lab of Urban Water Resource and Environment of Harbin Institute of Technology (HIT2013TS01) and the Fundamental Research Funds for the Central Universities (HIT. IBRSEM. 201327).
References 1 E. S. Jang, J. H. Won, S. J. Hwang and J. H. Choy, Adv. Mater., 2006, 18, 3309. 2 N. Kislov, J. Lahiri, H. Verma, D. Y. Goswami, E. Stefanakos and M. Batzill, Langmuir, 2009, 25, 3310. 3 S. Chakrabarti and B. K. Dutta, J. Hazard. Mater., 2004, 112, 269. 4 Z. Dong, X. Lai, J. E. Halpert, N. Yang, L. Yi, J. Zhai, D. Wang, Z. Tang and L. Jiang, Adv. Mater., 2012, 26, 1046. 5 J. Jiang, X. Zhang, P. Sun and L. Zhang, J. Phys. Chem. C, 2011, 115, 20555. 6 V. Jeena and R. S. Robinson, Chem. Commun., 2012, 48, 299. 7 S. Balachandran and M. Swaminathan, Dalton Trans., 2013, 42, 5338. 8 S. Anandan and M. Miyauchi, Phys. Chem. Chem. Phys., 2011, 13, 14937. 9 D. E. Zhang, J. Y. Gong, J. J. Ma, G. Q. Han and Z. W. Tong, Dalton Trans., 2013, 42, 16556. 10 S. Liu, C. Li, J. Yu and Q. Xiang, CrystEngComm, 2011, 13, 2533. 11 S. T. Kochuveedu, Y. H. Jang, Y. J. Jang and D. H. Kim, J. Mater. Chem. A, 2013, 1, 898. 12 S. Cho, J. W. Jang, J. S. Lee and K. H. Lee, CrystEngComm, 2010, 12, 3929.
This journal is © The Royal Society of Chemistry 2014
Paper
13 H. F. Yu and H. Y. Chou, Powder Technol., 2013, 233, 201. 14 S. Y. Lian, H. Huang, J. M. Zhang, Z. H. Kang and Y. Liu, Solid State Commun., 2013, 155, 53. 15 P. Zhang, C. Shao, Z. Zhang, M. Zhang, J. Mu, Z. Guo and Y. Liu, Nanoscale, 2011, 3, 2943. 16 Y. Y. Zhou, S. Ko, C. W. Lee, S. G. Pyo, S. K. Kim and S. Yoon, J. Power Sources, 2013, 244, 777. 17 Y. F. Zhao, L. Zhao, K. X. Yao, Y. Yang, Q. Zhang and Y. Han, J. Mater. Chem., 2012, 22, 19726. 18 G. Q. Ma, Z. Y. Wen, J. Jin, Y. Lu, K. Rui, X. W. Wu, M. F. Wu and J. C. Zhang, J. Power Sources, 2014, 254, 353. 19 M. J. Zhou, X. H. Gao, Y. Hu, J. F. Chen and X. Hu, Appl. Catal., B, 2013, 138–139, 1. 20 S. C. Li, Z. Wang, X. W. Yu, J. X. Wang and S. C. Wang, Adv. Mater., 2012, 24, 3196. 21 R. Mello, A. Olmos, J. P. Carbonell, M. G. Nunez and G. Asensio, Green Chem., 2009, 11, 994. 22 J. M. Sun, C. L. Wang, L. Wang, L. Liang, X. F. Liu, L. Chen and F.-S. Xiao, Catal. Today, 2010, 158, 273. 23 J. L. Zhang, B. X. Han, C. X. Zhang, W. Li and X. Y. Feng, Angew. Chem., Int. Ed., 2008, 47, 3012. 24 M. Poliakoff, J. M. Fitzpatrick, T. R. Farren and P. T. Anastas, Science, 2002, 297, 807. 25 F. X. Wang, L. Liang, J. Ma and J. M. Sun, Mater. Lett., 2013, 111, 201. 26 Z. Y. Sun, X. R. Zhang, B. X. Han, Y. Y. Wu, G. M. An, Z. M. Liu, S. D. Miao and Z. J. Miao, Carbon, 2007, 45, 2589. 27 L. H. Zhuo, Y. Q. Wu, L. Y. Wang, J. Ming, Y. C. Yu, X. B. Zhang and F. Y. Zhao, J. Mater. Chem. A, 2013, 1, 3954. 28 Z. M. Liu and B. X. Han, Adv. Mater., 2009, 13, 825. 29 N. Farhangi, R. R. Chowdhury, Y. M. Gonzalez, M. B. Ray and P. A. Charpentier, Appl. Catal., B, 2011, 110, 25. 30 L. Y. Wang, L. H. Zhuo, C. Chao and F. Y. Zhao, Chem. – Eur. J., 2014, 20, 4308. 31 J. Ming, C. Y. Wu, H. Y. Cheng, Y. C. Yu and F. Y. Zhao, J. Supercrit. Fluids, 2011, 57, 137. 32 J. C. Sina, S. M. Lama, I. Satoshi, K. T. Leea and A. R. Mohamed, Appl. Catal., B, 2014, 148–149, 258. 33 L. Shi, L. Liang, J. Ma, F. X. Wang and J. M. Sun, Dalton Trans., 2014, 43, 7236. 34 J. Ming, Y. Q. Wu, G. F. Liang, J. B. Park, F. Y. Zhao and Y. K. Sun, Green Chem., 2013, 15, 2722. 35 M. Ahmad, E. Ahmed, Z. L. Hong, N. R. Khalid, W. Ahmed and A. Elhissi, J. Alloys Compd., 2013, 577, 717. 36 N. S. Ramgir, I. S. Mulla and V. K. Pillai, J. Phys. Chem. B, 2006, 110, 3995. 37 J. Cao, B. Luo, H. Lin and S. Chen, J. Mol. Catal. A: Chem., 2011, 344, 138. 38 L. Shi, L. Liang, J. Ma, Y. N. Meng, S. F. Zhong, F. X. Wang and J. M. Sun, Ceram. Int., 2014, 40, 3495. 39 Z. F. Zhang, Y. Li, K. F. Li, K. Chen, Y. Z. Yang, X. G. Liu, H. S. Jia and B. S. Xu, J. Mater. Sci., 2014, 49, 2347.
Dalton Trans., 2014, 43, 16441–16449 | 16449
