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ZnO hybrid composite with excellent photocatalytic activity
Accepted Manuscript Title: Facile preparation of well-combined lignin-based carbon/ZnO hybrid composite with excellent photocatalytic activity Authors: Huan Wang, Xueqing Qiu, Weifeng Liu, Dongjie Yang PII: DOI: Reference:
S0169-4332(17)32103-7 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.112 APSUSC 36646
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APSUSC
Received date: Revised date: Accepted date:
15-6-2017 13-7-2017 14-7-2017
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Facile preparation of well-combined lignin-based carbon/ZnO hybrid composite with excellent photocatalytic activity
Huan Wanga, Xueqing Qiua,b,*, Weifeng Liua, Dongjie Yanga,*
a
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou, 510640, China. b
State Key Lab of Pulp and Paper Engineering, South China University of
Technology, Guangzhou, 510640, China.
*Corresponding Author E-mail: [email protected]. (Prof X.Q. Qiu), E-mail: [email protected]. (Prof D.J Yang). Tel: +86-20-8711-4722. Fax: +86-20-8711-4721.
Graphical abstract
Highlights
1 The LC/ZnO hybrid was synthesized using industrial lignin by a simple carbonization method.
2 The LC/ZnO hybrid is composed of ZnO nanoparticles and carbon nanosheets.
3 The LC/ZnO hybrid exhibited excellent surface contact and photocatalytic performance.
4 The LC/ZnO hybrid showed photocatalytic mechanism for the different charged dyes.
ABSTRACT In this work, a novel lignin-based carbon/ZnO (LC/ZnO) hybrid composite with excellent photocatalytic performance was prepared through a convenient and environment friendly method using alkali lignin (AL) as carbon source. The morphological, microstructure and optical properties of the as-prepared LC/ZnO hybrid composite was characterized with scanning electron microscope (SEM), X-ray diffraction (XRD), Raman and UV-Vis. The resulting LC/ZnO hybrid is composed of highly dispersed ZnO nanoparticles embedded on a lignin-based carbon nanosheet, showing excellent photogenerated electrons and holes separation and migration efficiency. The photocatalytic activity of LC/ZnO was much higher than the pure ZnO. The LC/ZnO hybrid composite showed different photocatalytic mechanism for degradation of negative methyl orange (MO) and positive Rhodamine B (RhB). It showed that h+ was the main photocatalytic active group during the degradation of MO, ·O2- and ·OH were the photocatalytic active groups during degradation of RhB. This reported photocatalyst with selective degradation of positive and negative organic dyes may have a great application prospect for photoelectric conversion and catalytic materials. Results of this work were of practical importance for high-valued utilization of lignin for carbon materials.
Keywords: lignin-based carbon; ZnO nanoparticle; hybrid composite; photocatalysis
1. Introduction Due to the continuous development of industry, environmental pollution and resources crisis have been the problems which restrict the development and existence of human. Especially the environmental pollution caused by a large amount of industrial waste is very serious, such as huge amounts of paper-making black liquor from papermaking industry and a large number of organic waste liquor from printing and dyeing industry [1-5]. Accordingly, it is an urgent to find suitable and effective methods to solve the above problems.
The extensive development and utilization of abundant renewable lignocellulosic biomass derived from plant sources may be an effective solution, and it has received wide attention and research in recent years [6-10]. The main components of Lignocellulosic biomass are lignin (15~30%), cellulose (35~50%) and hemicellulose (25~30%), respectively [11]. Cellulose and hemicellulose have been widely utilized as the important chemical raw materials in many fields, such as papermaking, functional carbon, specialty fiber, and so on [12, 13]. However, lignin has not got widely and fully utilized. Especially, a large amount of industrial lignin produced from and paper-making industry and biological engineering as industrial by-product, only less than 2% is recycled as chemical materials for application, which not only waste the resources, but also cause environmental problems [14-16]. Therefore, the research of high-valued utilization of industrial lignin is very urgent and it is great significance for both the development of environmental protection and resources utilization. Photocatalysts based on carbon/semiconductor composites with great potential application to the treatment of organic wastewater and have received wide research in recent years. Various carbon/semiconductor nanostructures with excellent photocatalytic properties have been reported [17-19]. Among the many semiconductor photocatalyst materials (e.g., ZnO, TiO2, SnO2, etc.), ZnO has received more and more attention due to its exceptional electrical, optical and environmental friendliness [20-22]. However, the low quantum efficiency and photocorrosion of ZnO significantly limits its applications in photocatalysis, which could be solved through helping the separation and migration of the photo-generated electrons and holes by doping or combining with carbon materials [23-26]. Among various carbon materials, graphene (GR) is a novel promising ones because of excellent optical properties, mechanical flexibility and good chemical stability. Numerous studies have demonstrated that a GR-based semiconductor (such as ZnO, TiO2) can not only adsorb the organic dyes, but also enhance the optical properties, which is conducive to improving the photodegradation performance of the organic dyes [27-32]. Generally, the preparation of GR/ZnO composite consists mainly of two steps: dispersed oxide graphene was firstly prepared from graphite or GR by chemical oxidation using strong
oxidizing chemicals, and then the ZnO nanoparticles were doped onto the dispersed oxide graphene, after that the oxide graphene was reduced by high temperature treatment or reducing agent. However, the exceedingly complicated preparation process and the exorbitant price limit the widely application of GR/ZnO composites. On the other hand, the exorbitant price of the commercial graphene and graphite limit its wide application. In the industrial applications, it would be more reasonable to take full consideration of economic efficiency and environmental pollution. Owing to its abundance, renewability and low-cost, industrial lignin has been considered as a promising carbon material precursor. Industrial lignin is obtained mainly as waste liquor from the pulping and papermaking industry or biorefinery process [33-35]. Industrial alkali lignin is mainly derived from alkaline pulping, and the lignin content in the waste liquor is close to 90% [36-38]. The high-valued utilization of industrial lignin could effectively alleviate the resource and energy crisis [39]. In recent years, lignin-based materials have attracted wide academic and industrial interest, and showing high value of practical application in many fields such as adsorbent, supercapacitor electrode materials and lithium ion battery [40, 41]. In our previous study, lignin/silica composite was prepared using industrial lignin and sodium silicate with a simple method [42]. Jeon et al. reported a serious of lignin-derived nanoporous carbons using different lignin resources. The prepared porous carbon was a good candidate for supercapacitor electrode materials [43]. Wang et al. reported a porous ZnO synthesized using the lignin-amine as a template, which showed excellent photocatalytic performance [44]. However, the lignin-based carbon photocatalyst has not been reported, to our knowledge. According to the findings, lignin has been described as a three-dimensional network-structured polymer comprised of phenol hydroxyl, methoxyl and carboxyl active functional groups, which are very beneficial to form neat uniform composite structures with ZnO nanoparticles. However, the unmodified lignin lacks positively charged functional groups. Thus, the unmodified lignin could not combine well with the negatively charged ZnO. Therefore, the quaternized alkali lignin (QAL), which
contains a positively charged functional group, was synthesized from alkali lignin by a simple quaternization process. This work present a novel LC/ZnO hybrid composite synthesized by a low-cost and environmental process using industrial lignin for the first time. The prepared LC/ZnO hybrid composite showed excellent photogenerated electrons and holes separation efficiency. The photocatalytic performance of the prepared LC/ZnO hybrid composite was investigated through photodegradation of negative methyl orange (MO) and positive Rhodamine B (RhB) in water under simulated solar light. The obtained LC/ZnO hybrid composite exhibited significant enhancement photocatalytic performance compared with pure ZnO nanoparticles. The photocatalytic activity of LC/ZnO is much higher than many other reports about based on GR/ZnO which obtained from graphite or graphene oxide. Furthermore, the LC/ZnO hybrid composite showed different photodegradation mechanism of negative MO and positive RhB. The reported photocatalyst with selective degradation of positive and negative organic dyes may have other application prospect in the field of photoelectric conversion and catalytic materials. This convenient and scalable method opens a green and valuable route for preparation of photoelectric conversion materials. 2. Experimental 2.1 Chemicals and reagents Alkali lignin (AL) was purified from papermaking black liquor supplied by Shuntai Technology Development Co., Ltd. (Hunan, China). Pentahydrate sodium oxalate (Na2C2O4·5H2O, >98%) were purchased from Sigma. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), methyl orange (MO) and rhodamine B (Rh B) were purchased from Alfa Aesar. Tert-butyl alcohol (TBA, (CH3)3COH), benzoquinone (BQ, C6H4O2) and triethanolamine (TEOA, C6H15NO3) were all obtained from Aladdin. 2.2 Preparation of quaternized alkali lignin (QAL) Firstly, 100 mL of AL sodium hydroxide aqueous solution (20 wt %) was prepared, then 15.05 g of the intermediate 3-Chloro-2-hydroxypropyltrimethyl
ammonium chloride was dissolved in 50 mL of
distill water to add into the above
AL solution by a peristaltic pump at a speed of ~100 rpm under stirring. After about 15 min, 8.01 ml of a sodium hydroxide aqueous solution (20 wt %) was added into the above AL solution and stirring for another 6 h at 80 ℃ to give the QAL solution. Finally, the obtained QAL solution was purified by dialysis and freeze dried to produce QAL powder. 2.3 Preparation of LC/ZnO composite Firstly, 200 mL of Na2C2O4 (6.7 g) solution and 200 mL of Zn(NO3)2·6H2O (14.87 g) solution was prepared, respectively. Then the Zn(NO3)2 solution was dropped into the Na2C2O4 solution with constant stirring to obtain the dispersed ZnC2O4 mixture. Subsequently, 1.0 g of QAL powers was added into the above ZnC2O4 mixture and stirred for 30 min. Then the mixed liquor was filtered to give ZnC2O4/QAL precursor, and then it was calcined at 550 ℃ for 2 h under the nitrogen atmosphere in a high temperature carbonization furnace (OTF-1200X). The lignin-based carbon/zinc oxide (LC/ZnO) hybrid composite was prepared. The preparation flowchart of the synthetic procedure for the LC/ZnO hybrid composite is showed in Figure 1. 2.4 Photoelectrode Preparation The FTO/ZnO and FTO/LC/ZnO photoelectrodes were prepared by a low-spinning process [32]. A piece of FTO glass (15 mm × 10 mm) was first ultrasonically washed with ethanol and deionized water for 30 min, respectively. 10 mg of the prepared sample (ZnO and LC/ZnO respectively) powders was mixed with 0.2 ml of deionized water using an agate mortar, and then the mixture was careful ground for 15 min to make homogeneous suspension. And then it was distributed onto the cleaned FTO glass by a low-spinning process. Finally, the FTO glass loaded with the prepared samples was dried at a temperature of 60 ℃ for 6 h, the ZnO and LC/ZnO photoelectrodes were prepared. 2.5 Characterization methods The micromorphology and microstructure of the prepared samples were
examined by scanning electron microscope (SEM, Merlin, Zeiss, Germany) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). The ZnO and LC/ZnO were dispersed in ethanol and dropped on the copper grid, dried in the air atmosphere, and then performed with TEM. The prepared LC/ZnO hybrid composite were tightly packed into the sample holder. X-ray diffraction (XRD) patterns of the pure ZnO and LC/ZnO hybrid composites were collected by a powder X-ray diffractometer (D8 Advance, Bruker, Germany) with Cu Kα wavelength of 0.1541 nm irradiation at 40 mA and40 kV. Raman spectra of the prepared samples were recorded on a (LabR AMA ramis, France) micro-Raman spectrometer excited at 633 nm. X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd., England) was used to study the bonding information of the samples. Thermal-gravimetric analysis (TGA) of the prepared LC/ZnO hybrid composite was carried out by a simultaneous thermal analyzer (STA449 F3, Netzsch, Germany) in an air atmosphere at the temperature of 25 ℃ to 800 ℃. The specific surface area and pore size of the samples were studied using Brunauer-Emmett-Teller (BET, Tristar Ⅱ 3020, Micromeritics Corp., USA ) method. The optical properties of the ZnO and LC/ZnO hybrid composites were studied by UV-Vis diffuse reflectance spectra (UV-2600, Shimadzu, Japan) and photoluminescence (PL) spectra. of ZnO and LC/ZnO hybrid composites were recorded using a UV-Vis spectrophotometer (UV-2600, Shimadzu, Japan). Electron spin resonance (ESR) spectra of the ZnO and LC/ZnO in different environment were measured using a JEPL mode JES-FA200 spectrometer. The photocurrent time (I-T) current were recorded at 0.5 mV bias potential during 4-cycle light on and off. The electrochemical impedance spectroscopy (EIS) curves of the devices at the frequency range between 105 and 10-1 Hz were recorded under dark condition, under a bias voltage of 5 mV. All of the photo-electrochemistry measurements were performed in Na2SO4 (0.1 M) solution at ambient temperature. 2.6 Photocatalytic activity The photodegradation of MO was used as a model reaction for the evaluation of the photoccatalytic activity of the LC/ZnO. The experiment was carried out as follows:
A mixture of MO (15 mg L-1, 60 mL) and the prepared samples (~30 mg) was illuminated with simulated solar light (500 W Xe lamp) under stirring in a photochemical reactions instrument (GG-GHX-V, Shanghai Guigo Industrial Co., Ltd, Shanghai, China). Before the light illumination, the mixture was stirred for 30 min in the dark environment to reach the adsorption-desorption equilibrium. The temperature of the MO solution was controlled at 15 ℃ using circulating water. During the illumination, 10 mL of the MO mixture together with the catalyst was removed at each time point and filtered immediately to dislodge the catalyst. The concentration of the removed MO solution was measured by the absorbance at 464 nm using a UV-Vis spectrophotometry. The degradation percentage of the MO was reported as C/C0 (calculated by A/A0). Here, C is the concentration of the MO according to the irradiated time interval and C0 is the initial concentration of MO solution, respectively. A is the absorbance at 464 nm of the MO solution according to the irradiated time interval and A0 is the initial concentration of MO solution, respectively. The photodegradation of Rh B experiment was carried out in accordance with the same procedure of MO. 3. Results and discussion 3.1 Micromorphology and microstructure The prepared samples of ZnO and LC/ZnO hybrid composite were first characterization by Raman spectra, as the results shown in Figure 2. In the spectrum of ZnO, the bands at ~300, ~400 are assigned to the ZnO Raman feature. In the spectrum of LC/ZnO hybrid composite, the absorption bands belonging to lignin-based carbon and ZnO appeared obviously, which proves that LC/ZnO hybrid composites have been successfully prepared. Additionally, the ID/IG ratio (the D and G bands intensity ratio) is a measure of the relative concentration of the local defects for the carbon domains [45, 46]. The ID/IG intensity ratios of the obtained LC/ZnO samples was approximately ~1.01 similar to the reported carbon/ZnO using graphene, graphite and other carbon materials by other researchers [20-25]. For the pure carbon materials, the ID/IG ratio can reflect its degree of graphitization [45]. In our current
research, the Raman spectrum of the pure lignin-based carbon (LC, obtained by washing away the ZnO) was supported in Figure S1. Compared with the LC/ZnO, ID/IG ratio of pure LC is reduced, and 2D, 2G bonds show enhancement. The results indicate that part of the QAL has been graphitized after carbonization. The XPS spectra were used to investigate the elemental components and bonding state of the as-prepared pure ZnO and LC/ZnO hybrid composite, as the results shown in Figure 3. From the full scan spectrum (Figure 3a), peaks belonging to Zn, C and O elements are observed, suggesting that the as-prepared LC/ZnO mainly contains Zn, O and C elements. Particularly, the peak belonging to C in the pure ZnO was due to the carbon in the equipment. This phenomenon was also reported in other studies [7, 56]. Figure 3b shows the high-resolution C 1s spectrum of the LC/ZnO hybrid composite. The peak at 284.5 eV is corresponding to the sp2 carbon atom, while the peak at 285.6eV is attributed to the C from C=O and C-O groups [47], and the peak at ~287.8 eV should belong to the O=C-OH species [48]. The C 1s spectrum was similar to the prepared carbon/ZnO using graphene, graphite and other carbon materials by other researchers [20-25]. The prepared samples of LC/ZnO and its precursor of QAL/ZnC2O4 were characterization by FT-IR analysis, as shown in Figure S2. Compared with the precursor of QAL/ZnC2O4, a significant loss of oxygen-containing carbonaceous peaks was obviously appeared in the FT-IR spectrum of LC/ZnO, suggesting the successfully preparation of LC/ZnO. Figure 4 depicts the XRD patterns of the samples of pure ZnO and LC/ZnO hybrid composite. The pure ZnO and LC/ZnO hybrid composite exhibit similar XRD patterns, they both can be readily indexed to the hexagonal wurtzite ZnO (JCPDS data card 36-1451). It shows that the introduction of QAL in the precursor solution didn’t obstruct the formation of crystal structured ZnO nanoparticles. The crystallite size was evaluated using the (101) peak using Scherrer formula [49]:
D
cos
where 𝜆 is the wavelength of X-ray, 𝜃 is Bragg’s angle, κ is the shape factor, and 𝛽 is the full width and half maxima. Here, κ and 𝜆 are taken as 0.9 and 0.15405 nm, respectively. The calculation result was listed in Table 1. The crystallite size of the ZnO nanocrystalline in ZnO and LC/ZnO were 37.5 and 32.6 nm, respectively. It shows that the crystallite size of the ZnO was decreased due to the addition of QAL in precursor. Figure 5(a & b) shows the SEM and TEM images of the as-prepared pure ZnO nanoparticles, showing that the ZnO has a typical nanoparticle structure, and the diameter ranged approximately from 50-180 nm. Additionally, the pure ZnO nanoparticles show a serious aggregation behavior. The SEM images of the prepared LC/ZnO hybrid composite were shown in Figure 5(c & d). It shows that the LC/ZnO micron-sized cube structure is composed of LC nanosheets and ZnO nanoparticles. The Nano-sized ZnO particles were well dispersed and tightly bonded to the LC nanosheets. The particle size of ZnO in hybrid composite was much smaller than the pure ZnO. In addition, it could be observed that the LC nanosheets were well combined with the ZnO nanoparticles as TEM images shown in Figure 5(e & f). The lattice fringes of ZnO (corresponds to 0.25 nm and 019 nm) and LC (corresponds to 0.33 nm) were labeled in the Figure 5f. It shows that there is a tight interfacial contact between the LC nanosheets and ZnO nanoparticles. The photo-generated electrons and holes transfer process is intimately related to the surface contact between the carbon and semiconductor, such an intimate interfacial contact in the prepared LC/ZnO hybrid composite could be expected to improve the photo-generated electrons and holes transfer process, thus leading to the improvement and excellent photocatalytic performance. In the formation of LC/ZnO hybrid composite, the preparation of QAL/ZnC2O4 precursor is the key factor. Firstly, the CO2 and H2O gas from the decomposition of the ZnC2O4 precursor could cause the carbon nanosheet to expand and form a loose structure, which favors of the loading of ZnO in LC. Next, lignin with three-dimensional-structure as the precursor of the carbon could help disperse the
ZnO nanoparticles. The resulting LC can also modify the surface of ZnO, which could make a large number of oxygen vacancies in ZnO as shown in Figure 5f. The oxygen vacancies become positive centers, forming many holes. On the other hand, the graphic stripe similar to graphene fingerprints was observed in Figure 5f, suggesting that the resulting ZnO could accelerate the carbonization and graphitization of lignin at a relatively low temperature of 550 ℃, which was much lower than 700~900 ℃ in other reports [39-42].The BET surface areas of the QAL, ZnO and LC/ZnO were investigated by nitrogen (N2) adsorption-desorption experiments, as the results show in Figure S5. The N2 adsorption-desorption isotherms of the samples are typed assigned to the Brunauer-Emmett-Teller classification [50], which indicates the presence of macropores. Especially, the LC/ZnO hybrid composite exhibits high adsorption at relative pressure (P/P0) close to 1.0, indicating that there is a lot of mesopores and macropores. On the other hand, there is a rapid growth in gas adsorption capacity in the low relative pressure area, which is due to the micropores [51]. The measured surface areas of the LC/ZnO, ZnO and QAL were 139.53 m2·g-1, 45.38 m2·g-1 and 26.49 m2·g-1, respectively. The LC/ZnO hybrid composite shows much larger surface area than ZnO probably due to the nanosheet-shaped lignin-based carbon. The large specific surface area of the LC/ZnO hybrid composite could enhance the adsorption ability for organic dye molecules, which may contribute to enhance the photocatalytic performances. 3.2 Optical and photoelectrical properties Figure 6a shows the UV-Vis diffuse reflectance spectra of the as-prepared ZnO and LC/ZnO. Both the ZnO and LC/ZnO all show the typical very high absorption intensity in the UV region. The LC/ZnO hybrid composite shows higher UV absorption than pure ZnO, due to the benzene ring structure and C=C and C=O double bonds [52]. Furthermore, the LC/ZnO hybrid composite shows enhanced absorption intensity in the visible-light compared with the pure ZnO, which was cause by the background absorption of LC. This is in consistent with the black color of the LC/ZnO [28].
However, the absorption band-edge of the LC/ZnO hybrid composite has red-shifted. The band gap energy of the semiconductors has been calculated by the Kubelka-Munk method [53]. The relationship between the photon energy (hν) and the absorption coefficient (α) for direct band gap semiconductor could be determined by the following equation: α = Βd(hν - Eg)1/2/hν where the Βd is the absorption constants, α can be determined from the reflectance spectra according to the Kubelka-Munk theory. Plots of he (αhν)2 versus hν are shown in Figure 6b. The direct band gap energies of the pure ZnO and LC/ZnO are approximately 3.25 and 3.03 eV calculated from the intercept of the tangents to the plots, respectively. The lower band gap of LC/ZnO is possibly due to the synergistic interfacial interaction between the LC and ZnO [28]. The absorption edge of LC/ZnO hybrid composite shows a red shift compared to the pure ZnO nanoparticles, which is beneficial to enhance The photoluminescence (PL) spectra of the as-prepared pure ZnO and LC/ZnO are presented in Figure 7. Pure ZnO shows a significantly high and wide PL peak around 500 nm, indicating that there were many photogenerated electrons and holes in the pure ZnO. However, the photogenerated electrons and holes show high recombination probability. On the other hand, the PL intensity of the LC/ZnO is much lower than the PL intensities of pure ZnO. The remarkable decrease of PL intensity indicates that the recombination probability of photogenerated electrons and holes greatly decreases and their separation and migration efficiency obviously increases [17, 27]. The photo-generated electrons were efficiency transferred to the lignin-based carbon, which will be a great contribution to the photoactivity enhancement. The photoelectronchemical properties of the semiconductor are performed to investigate the charge separation and transfer process [31]. Figure 8 shows the EIS and I-T curves of the ZnO and LC/ZnO devices. Figure 8a shows that the arc radius in the Nyquist plot of the LC/ZnO is smaller than that of the pure ZnO, revealing that the interface charge transfer efficiency of the LC/ZnO is improved. Figure 8b shows the
IT curves of the ZnO and LC/ZnO devices with several photoelectric response cycles with intermittent light irradiation. It shows that the current intensities of the ZnO and LC/ZnO devices increase sharply upon light irradiation. Noticeably, the photocurrent of the LC/ZnO is much higher than the photocurrent of the pure ZnO, which reveals that the photogenerated charge carrier efficiency of the LC/ZnO is higher than the photo-generated charge carrier efficiency of the pure ZnO. The photoelectrochemical properties were consistent with the results of PL spectra. The higher efficiency of charge generation and transfer speed could be attributed from the excellent combination of LC and ZnO. And the excellent photoelectrochemical properties of the LC/ZnO hybrid composite would be in accordance with a high photocatalytic activity. 3.3 Evaluation of photocatalytic performance The photocatalytic performance of the prepared LC/ZnO hybrid composite was evaluated by aqueous phase photodegradation of organic dye pollutants under simulated solar light illumination, the negative MO and positive Rh B were chose as the photo-degraded objects. Prior to photocatalytic experiments, the adsorption-desorption equilibrium curves for the catalysts and dyes were measured, as the results show in Figure S6. It shows that the dyes adsorption is balanced within 30 min. Figure 9(a & b) shows the photodegradation of MO and Rh B over the pure ZnO and LC/ZnO hybrid composite, respectively. Figure 9a shows that the concentration reduction of MO based on LC/ZnO is obvious higher than that of pure ZnO, and the LC/ZnO exhibited higher photocatalytic activities than the ZnO. Furthermore, MO was almost fully degraded by the catalyst of LC/ZnO after light irradiation 30 min, but only 75.3% of MO was degrade by the pure ZnO after light illumination 50 min. Figure 9c shows the pseudo-first-order kinetic of the photodegradation of MO based on ZnO and LC/ZnO. The reaction rates of ZnO and LC/ZnO are 0.0366 and 0.18454, respectively. The LC/ZnO exhibits a 5-fold enhancement in photodegradation of MO than the pure ZnO. Figure 9b shows that the photodegradation rate of Rh B based on LC/ZnO is much higher that of pure ZnO. It means that the Rh B is easily adsorbed by the
LC/ZnO. The photodegradation efficiency and rate for Rh B based on LC/ZnO is much higher than that based on pure ZnO. Figure 9d shows that the reaction rates of the ZnO and LC/ZnO are 0.0072 and 0.019, respectively. The photodegradation rate of Rh B based on LC/ZnO increased 2.6 times compared with pure ZnO. The detailed comparison of the photocatalytic data for LC/ZnO and ZnO is shown in the Table 2. Especially, the enhanced speed and efficiency in photodegradation of MO is higher than that of in photodegradation Rh B over the LC/ZnO. It may mean that the mechanism of photocatalytic degradation of MO and Rh B based on LC/ZnO is different. On the other hand, the photodegradation of MO based on the LC/ZnO hybrid composites with varied pH has been studied, as the results shown in Figure S7. It shows that that the LC/ZnO hybrid composite has a good catalytic degradation effect in the pH of 7 to 11. But the LC/ZnO shows poor catalytic performance under acidic conditions. Furthermore, the photo stability of LC/ZnO for degradation of MO based on LC/ZnO has been investigated, as shown in Figure S8. It shows that the photocorrosion of ZnO nanoparticles can be efficiently inhibited by mixing LC into the composite. The photoactive groups in photocatalytic degradation of organic dyes are mainly superoxide radicals (·O2-), hydroxyl radicals (·OH) and holes (h+) [54]. In order investigate the photoactive radical species of photocatalyst, the electron spin resonance (ESR) spectra were measured, as the results shown in Figure 10. Figure 10a shows the ESR spectra of the pure ZnO and LC/ZnO in the pure aqueous dispersion without DMPO. The ZnO showed a very weak peak in dark, indicating it contained very little h+. However, the LC/ZnO showed a clear peak in dark, indicating it contained many h+. In addition, the peaks intensity of ZnO and LC/ZnO both significant enhanced when they were irradiated with light, indicating many h+ were generated. Figure 10b shows the ESR spectra of pure ZnO and LC/ZnO in aqueous solution doped with DMPO. Figure 10c shows the ESR spectra of pure ZnO and LC/ZnO in methanol doped with DMPO. The characteristic peaks corresponds to
DMPO-·OH and DMPO-·O2- were appeared both in ZnO and LC/ZnO under light irradiation [28]. On the other hand, the peak intensity of the DMPO-·OH and DMPO-·O2- based on ZnO and LC/ZnO under light illumination was much stronger than that in dark. The content of ·OH and ·O2- based on LC/ZnO was much more than that based on pure ZnO. This is the reason why LC/ZnO showed the higher photocatalytic activity than pure ZnO. The photocatalytic mechanism exploration was carried out by investigating the main photoactive species during the photodegradation process via dissolving various trapping agents in solution [55]. The added trapping agent TEOA, BQ and TBA were used to eliminate the h+, ·O2- and ·OH, respectively. Figure 11a shows the results of photodegradation of MO over LC/ZnO with various trapping agents. The photodegradation rate and efficient decreased obviously when added the trapping agents compared with no scavenger, indicating that the photo-generated h+, ·O2- and ·OH were the photocatalytic active groups in photodegradation MO. Especially, MO could hardly be photo-degraded when TEOA was added, suggesting that the h+ was main photocatalytic active groups. Figure 11b shows the result of photodegradation of RhB based on LC/ZnO with different trapping agents. The photodegradation rate and efficient decreased obviously after adding trapping agents of BQ and TBA. However, there was almost no effect on the photodegradation rate and efficient when adding TEOA during photocatalytic, suggesting that the ·O2- and ·OH radicals were the main photocatalytic active groups, h+ almost had no effect in photodegradation of RhB. According to experiment results of photoactive species from ESR and trapping agents. The photodegradation mechanism of based on LC/ZnO composite for organic dyes in this study could be explained as follows (Figure 12) [17, 48, 49, 56]: LC/ZnO + hν → ZnO(h+) + LC(e-), e- + O2 → ·O2-, h+ + H2O/OH- → ·OH ·OH or ·O2- or h+ + MO → CO2 and H2O mainly ·OH or ·O2- + RhB → CO2 and H2O mainly
Under illumination of simulated solar light, the electrons (e-) in ZnO will be excited from valence band (VB) to conduction band (CB), producing photogenerated electrons and holes. The photogenerated electrons and holes in ZnO can transfer to the lignin-based carbon nanosheets through the intimate interfacial contact, which could help to separate the photogenerated electrons and holes effectively. The transferred holes in the lignin-based carbon nanosheet could oxidize the absorbed H2O or OH- to produce ·OH radicals. Additional, the photogenerated electrons can reduce the molecular oxygen absorbed on the lignin-based carbon nanosheet to produce superoxide ions (·O2-). Both ·OH and ·O2- can degrade MO and RhB due to the strong oxidative abilities. On the other hand, the MO absorbed on the surface of ZnO could be directly degrade by the holes, but the RhB was photo-degraded only by the ·OH and ·O2-. Furthermore, in order to study the why the photoactive species were difference between the photodegradation MO and RhB over LC/ZnO. The Zeta potentials of the LC/ZnO and LC dispersed in aqueous solution were measured, as shown in Table S2. The Zeta potentials of the LC/ZnO and LC were -11.85 and -15.5 mV, respectively. As is known, MO molecules in the water solution are negatively charged, RhB molecules in the water solution are positively charged. The positive RhB is more likely to be adsorbed to the positively charged LC/ZnO than the negative MO due to the electrostatic adsorption, which was confirmed by the results of the adsorption experiments, as shown in Figure 9(a & b) and Figure S6. However, the RhB was firmly adsorbed on the surface of LC due to the electrostatic adsorption between the positive and negative charges. The positive RhB could not be exclusion to the surface of ZnO to be degraded by the large number of holes. On the other hand, the negative MO could be excreted to the surface of ZnO being directly degraded by the holes. This may be the main reason why the enhancement rate of LC/ZnO degradation of negative MO is higher than that of degradation of positive RhB. It means that the prepared LC/ZnO could selectively degrade organic dyes with different polarities. This prepared catalyst may have other potential application value in other fields.
Conclusions In summary, a novel LC/ZnO hybrid composite was successfully prepared through a facile yet efficient one-pot carbonization method by calcination the QAL/ZnC2O4 precursor at 550 ℃ for the first time. The small-sized ZnO nanoparticles were closely associated to lignin-based carbon nanosheets. The prepared LC/ZnO showed excellent photoelectron transfer properties due to its excellent surface contact. The LC/ZnO hybrid composite exhibited an excellent photocatalytic performance: the LC/ZnO showed significant enhancement of photocatalytic activity over the pure ZnO, and it showed different photocatalytic mechanism for the degradation of negative MO and positive RhB: h+ was the main photoactive specie during the degradation of negative MO, ·O2- and ·OH were the main photoactive specie during the degradation of positive RhB. This work reported a photocatalyst with selective degradation of positive and negative organic dyes. In addition, the prepared lignin-based carbon has great potential as a replacement for expensive graphene in the field of photocatalysis. It may have other valued application prospect in the field of photoelectric conversion and catalytic materials. Acknowledgments This work was funded by the Guangdong Province science and technology plan project (2014B050505006) and the National Natural Science Foundation of China (NSFC) (No. 21436004, 21576106).
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Figure 1. Schematic illustration for the preparation of LC/ZnO hybrid composite.
Figure 2. Raman spectra of the prepared ZnO and LC/ZnO.
Figure 3. XPS spectra of the pure ZnO and LC/ZnO hybrid composite, (a) is overview and (d) is the high-resolution spectra of C 1s.
Figure 4. XRD patterns of the as-prepared samples of ZnO and LC/ZnO hybrid composite.
Figure 5 SEM images of the prepared ZnO (a) and LC/ZnO (c & d), TEM images of the prepared ZnO (b) and LC/ZnO (e & f).
the photocatalytic performance.
Figure 6. (a) UV-Vis absorption spectra of pure ZnO and LC/ZnO, (b) the plots of calculated by Kubelka-Munk function to the energy of light.
Figure 7. PL spectra of the prepared pure ZnO and LC/ZnO hybrid composite excited with wavelength of 365 nm.
Figure 8. EIS (a) and I-T (b) curves of the ITO/ZnO and ITO/LC/ZnO devices with a bias voltage at 0.1 V.
Figure 9. Photocatalytic degradation MO (a) and Rh B (b) over the pure ZnO nanoparticles and LC/ZnO hybrid composite, pseudo-first-order kinethic plots photodegradation MO (c) and Rh B (d).
Figure 10. ESR spectra of pure ZnO and LC/ZnO with or without light irradiation in different environment, (a) pure aqueous dispersion without DMPO, (b) in aqueous dispersion doped with DMPO, (c) in methanol dispersion doped with DMPO.
Figure 11. Photodegradation of MO (a) and RhB (b) with different scavenger, TBA, BQ and TEOA used as ·OH, O2·- and h+ scavenger, respectively.
Figure 12. Photocatalytic mechanism for the degradation of MO and RhB over the LC/ZnO.
Table 1. Structural information calculated from the XRD patterns in Figure 4. (hkl)
2θ(°)
ZnO
101
36.4607
0.263
37.5
LC/ZnO
101
36.3052
0.302
32.6
Sample Name
FWHM(°)
Crystallite size (nm)
Table 2. Comparison of the photodegradation performance of ZnO and LC/ZnO. Adsorbance (%)
Efficiency (%)
Time (min)
k
MO
RhB
MO
RhB
MO
RhB
MO
RhB
LC/ZnO
10.5
37.5
99.9
79.2
30
50
0.1845
0.019
ZnO
5.2
4.5
69.8
31.1
50
50
0.0366
0.0072
Photocatalyst
S0169-4332(17)32103-7 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.112 APSUSC 36646
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Facile preparation of well-combined lignin-based carbon/ZnO hybrid composite with excellent photocatalytic activity
Huan Wanga, Xueqing Qiua,b,*, Weifeng Liua, Dongjie Yanga,*
a
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou, 510640, China. b
State Key Lab of Pulp and Paper Engineering, South China University of
Technology, Guangzhou, 510640, China.
*Corresponding Author E-mail: [email protected]. (Prof X.Q. Qiu), E-mail: [email protected]. (Prof D.J Yang). Tel: +86-20-8711-4722. Fax: +86-20-8711-4721.
Graphical abstract
Highlights
1 The LC/ZnO hybrid was synthesized using industrial lignin by a simple carbonization method.
2 The LC/ZnO hybrid is composed of ZnO nanoparticles and carbon nanosheets.
3 The LC/ZnO hybrid exhibited excellent surface contact and photocatalytic performance.
4 The LC/ZnO hybrid showed photocatalytic mechanism for the different charged dyes.
ABSTRACT In this work, a novel lignin-based carbon/ZnO (LC/ZnO) hybrid composite with excellent photocatalytic performance was prepared through a convenient and environment friendly method using alkali lignin (AL) as carbon source. The morphological, microstructure and optical properties of the as-prepared LC/ZnO hybrid composite was characterized with scanning electron microscope (SEM), X-ray diffraction (XRD), Raman and UV-Vis. The resulting LC/ZnO hybrid is composed of highly dispersed ZnO nanoparticles embedded on a lignin-based carbon nanosheet, showing excellent photogenerated electrons and holes separation and migration efficiency. The photocatalytic activity of LC/ZnO was much higher than the pure ZnO. The LC/ZnO hybrid composite showed different photocatalytic mechanism for degradation of negative methyl orange (MO) and positive Rhodamine B (RhB). It showed that h+ was the main photocatalytic active group during the degradation of MO, ·O2- and ·OH were the photocatalytic active groups during degradation of RhB. This reported photocatalyst with selective degradation of positive and negative organic dyes may have a great application prospect for photoelectric conversion and catalytic materials. Results of this work were of practical importance for high-valued utilization of lignin for carbon materials.
Keywords: lignin-based carbon; ZnO nanoparticle; hybrid composite; photocatalysis
1. Introduction Due to the continuous development of industry, environmental pollution and resources crisis have been the problems which restrict the development and existence of human. Especially the environmental pollution caused by a large amount of industrial waste is very serious, such as huge amounts of paper-making black liquor from papermaking industry and a large number of organic waste liquor from printing and dyeing industry [1-5]. Accordingly, it is an urgent to find suitable and effective methods to solve the above problems.
The extensive development and utilization of abundant renewable lignocellulosic biomass derived from plant sources may be an effective solution, and it has received wide attention and research in recent years [6-10]. The main components of Lignocellulosic biomass are lignin (15~30%), cellulose (35~50%) and hemicellulose (25~30%), respectively [11]. Cellulose and hemicellulose have been widely utilized as the important chemical raw materials in many fields, such as papermaking, functional carbon, specialty fiber, and so on [12, 13]. However, lignin has not got widely and fully utilized. Especially, a large amount of industrial lignin produced from and paper-making industry and biological engineering as industrial by-product, only less than 2% is recycled as chemical materials for application, which not only waste the resources, but also cause environmental problems [14-16]. Therefore, the research of high-valued utilization of industrial lignin is very urgent and it is great significance for both the development of environmental protection and resources utilization. Photocatalysts based on carbon/semiconductor composites with great potential application to the treatment of organic wastewater and have received wide research in recent years. Various carbon/semiconductor nanostructures with excellent photocatalytic properties have been reported [17-19]. Among the many semiconductor photocatalyst materials (e.g., ZnO, TiO2, SnO2, etc.), ZnO has received more and more attention due to its exceptional electrical, optical and environmental friendliness [20-22]. However, the low quantum efficiency and photocorrosion of ZnO significantly limits its applications in photocatalysis, which could be solved through helping the separation and migration of the photo-generated electrons and holes by doping or combining with carbon materials [23-26]. Among various carbon materials, graphene (GR) is a novel promising ones because of excellent optical properties, mechanical flexibility and good chemical stability. Numerous studies have demonstrated that a GR-based semiconductor (such as ZnO, TiO2) can not only adsorb the organic dyes, but also enhance the optical properties, which is conducive to improving the photodegradation performance of the organic dyes [27-32]. Generally, the preparation of GR/ZnO composite consists mainly of two steps: dispersed oxide graphene was firstly prepared from graphite or GR by chemical oxidation using strong
oxidizing chemicals, and then the ZnO nanoparticles were doped onto the dispersed oxide graphene, after that the oxide graphene was reduced by high temperature treatment or reducing agent. However, the exceedingly complicated preparation process and the exorbitant price limit the widely application of GR/ZnO composites. On the other hand, the exorbitant price of the commercial graphene and graphite limit its wide application. In the industrial applications, it would be more reasonable to take full consideration of economic efficiency and environmental pollution. Owing to its abundance, renewability and low-cost, industrial lignin has been considered as a promising carbon material precursor. Industrial lignin is obtained mainly as waste liquor from the pulping and papermaking industry or biorefinery process [33-35]. Industrial alkali lignin is mainly derived from alkaline pulping, and the lignin content in the waste liquor is close to 90% [36-38]. The high-valued utilization of industrial lignin could effectively alleviate the resource and energy crisis [39]. In recent years, lignin-based materials have attracted wide academic and industrial interest, and showing high value of practical application in many fields such as adsorbent, supercapacitor electrode materials and lithium ion battery [40, 41]. In our previous study, lignin/silica composite was prepared using industrial lignin and sodium silicate with a simple method [42]. Jeon et al. reported a serious of lignin-derived nanoporous carbons using different lignin resources. The prepared porous carbon was a good candidate for supercapacitor electrode materials [43]. Wang et al. reported a porous ZnO synthesized using the lignin-amine as a template, which showed excellent photocatalytic performance [44]. However, the lignin-based carbon photocatalyst has not been reported, to our knowledge. According to the findings, lignin has been described as a three-dimensional network-structured polymer comprised of phenol hydroxyl, methoxyl and carboxyl active functional groups, which are very beneficial to form neat uniform composite structures with ZnO nanoparticles. However, the unmodified lignin lacks positively charged functional groups. Thus, the unmodified lignin could not combine well with the negatively charged ZnO. Therefore, the quaternized alkali lignin (QAL), which
contains a positively charged functional group, was synthesized from alkali lignin by a simple quaternization process. This work present a novel LC/ZnO hybrid composite synthesized by a low-cost and environmental process using industrial lignin for the first time. The prepared LC/ZnO hybrid composite showed excellent photogenerated electrons and holes separation efficiency. The photocatalytic performance of the prepared LC/ZnO hybrid composite was investigated through photodegradation of negative methyl orange (MO) and positive Rhodamine B (RhB) in water under simulated solar light. The obtained LC/ZnO hybrid composite exhibited significant enhancement photocatalytic performance compared with pure ZnO nanoparticles. The photocatalytic activity of LC/ZnO is much higher than many other reports about based on GR/ZnO which obtained from graphite or graphene oxide. Furthermore, the LC/ZnO hybrid composite showed different photodegradation mechanism of negative MO and positive RhB. The reported photocatalyst with selective degradation of positive and negative organic dyes may have other application prospect in the field of photoelectric conversion and catalytic materials. This convenient and scalable method opens a green and valuable route for preparation of photoelectric conversion materials. 2. Experimental 2.1 Chemicals and reagents Alkali lignin (AL) was purified from papermaking black liquor supplied by Shuntai Technology Development Co., Ltd. (Hunan, China). Pentahydrate sodium oxalate (Na2C2O4·5H2O, >98%) were purchased from Sigma. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), methyl orange (MO) and rhodamine B (Rh B) were purchased from Alfa Aesar. Tert-butyl alcohol (TBA, (CH3)3COH), benzoquinone (BQ, C6H4O2) and triethanolamine (TEOA, C6H15NO3) were all obtained from Aladdin. 2.2 Preparation of quaternized alkali lignin (QAL) Firstly, 100 mL of AL sodium hydroxide aqueous solution (20 wt %) was prepared, then 15.05 g of the intermediate 3-Chloro-2-hydroxypropyltrimethyl
ammonium chloride was dissolved in 50 mL of
distill water to add into the above
AL solution by a peristaltic pump at a speed of ~100 rpm under stirring. After about 15 min, 8.01 ml of a sodium hydroxide aqueous solution (20 wt %) was added into the above AL solution and stirring for another 6 h at 80 ℃ to give the QAL solution. Finally, the obtained QAL solution was purified by dialysis and freeze dried to produce QAL powder. 2.3 Preparation of LC/ZnO composite Firstly, 200 mL of Na2C2O4 (6.7 g) solution and 200 mL of Zn(NO3)2·6H2O (14.87 g) solution was prepared, respectively. Then the Zn(NO3)2 solution was dropped into the Na2C2O4 solution with constant stirring to obtain the dispersed ZnC2O4 mixture. Subsequently, 1.0 g of QAL powers was added into the above ZnC2O4 mixture and stirred for 30 min. Then the mixed liquor was filtered to give ZnC2O4/QAL precursor, and then it was calcined at 550 ℃ for 2 h under the nitrogen atmosphere in a high temperature carbonization furnace (OTF-1200X). The lignin-based carbon/zinc oxide (LC/ZnO) hybrid composite was prepared. The preparation flowchart of the synthetic procedure for the LC/ZnO hybrid composite is showed in Figure 1. 2.4 Photoelectrode Preparation The FTO/ZnO and FTO/LC/ZnO photoelectrodes were prepared by a low-spinning process [32]. A piece of FTO glass (15 mm × 10 mm) was first ultrasonically washed with ethanol and deionized water for 30 min, respectively. 10 mg of the prepared sample (ZnO and LC/ZnO respectively) powders was mixed with 0.2 ml of deionized water using an agate mortar, and then the mixture was careful ground for 15 min to make homogeneous suspension. And then it was distributed onto the cleaned FTO glass by a low-spinning process. Finally, the FTO glass loaded with the prepared samples was dried at a temperature of 60 ℃ for 6 h, the ZnO and LC/ZnO photoelectrodes were prepared. 2.5 Characterization methods The micromorphology and microstructure of the prepared samples were
examined by scanning electron microscope (SEM, Merlin, Zeiss, Germany) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). The ZnO and LC/ZnO were dispersed in ethanol and dropped on the copper grid, dried in the air atmosphere, and then performed with TEM. The prepared LC/ZnO hybrid composite were tightly packed into the sample holder. X-ray diffraction (XRD) patterns of the pure ZnO and LC/ZnO hybrid composites were collected by a powder X-ray diffractometer (D8 Advance, Bruker, Germany) with Cu Kα wavelength of 0.1541 nm irradiation at 40 mA and40 kV. Raman spectra of the prepared samples were recorded on a (LabR AMA ramis, France) micro-Raman spectrometer excited at 633 nm. X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd., England) was used to study the bonding information of the samples. Thermal-gravimetric analysis (TGA) of the prepared LC/ZnO hybrid composite was carried out by a simultaneous thermal analyzer (STA449 F3, Netzsch, Germany) in an air atmosphere at the temperature of 25 ℃ to 800 ℃. The specific surface area and pore size of the samples were studied using Brunauer-Emmett-Teller (BET, Tristar Ⅱ 3020, Micromeritics Corp., USA ) method. The optical properties of the ZnO and LC/ZnO hybrid composites were studied by UV-Vis diffuse reflectance spectra (UV-2600, Shimadzu, Japan) and photoluminescence (PL) spectra. of ZnO and LC/ZnO hybrid composites were recorded using a UV-Vis spectrophotometer (UV-2600, Shimadzu, Japan). Electron spin resonance (ESR) spectra of the ZnO and LC/ZnO in different environment were measured using a JEPL mode JES-FA200 spectrometer. The photocurrent time (I-T) current were recorded at 0.5 mV bias potential during 4-cycle light on and off. The electrochemical impedance spectroscopy (EIS) curves of the devices at the frequency range between 105 and 10-1 Hz were recorded under dark condition, under a bias voltage of 5 mV. All of the photo-electrochemistry measurements were performed in Na2SO4 (0.1 M) solution at ambient temperature. 2.6 Photocatalytic activity The photodegradation of MO was used as a model reaction for the evaluation of the photoccatalytic activity of the LC/ZnO. The experiment was carried out as follows:
A mixture of MO (15 mg L-1, 60 mL) and the prepared samples (~30 mg) was illuminated with simulated solar light (500 W Xe lamp) under stirring in a photochemical reactions instrument (GG-GHX-V, Shanghai Guigo Industrial Co., Ltd, Shanghai, China). Before the light illumination, the mixture was stirred for 30 min in the dark environment to reach the adsorption-desorption equilibrium. The temperature of the MO solution was controlled at 15 ℃ using circulating water. During the illumination, 10 mL of the MO mixture together with the catalyst was removed at each time point and filtered immediately to dislodge the catalyst. The concentration of the removed MO solution was measured by the absorbance at 464 nm using a UV-Vis spectrophotometry. The degradation percentage of the MO was reported as C/C0 (calculated by A/A0). Here, C is the concentration of the MO according to the irradiated time interval and C0 is the initial concentration of MO solution, respectively. A is the absorbance at 464 nm of the MO solution according to the irradiated time interval and A0 is the initial concentration of MO solution, respectively. The photodegradation of Rh B experiment was carried out in accordance with the same procedure of MO. 3. Results and discussion 3.1 Micromorphology and microstructure The prepared samples of ZnO and LC/ZnO hybrid composite were first characterization by Raman spectra, as the results shown in Figure 2. In the spectrum of ZnO, the bands at ~300, ~400 are assigned to the ZnO Raman feature. In the spectrum of LC/ZnO hybrid composite, the absorption bands belonging to lignin-based carbon and ZnO appeared obviously, which proves that LC/ZnO hybrid composites have been successfully prepared. Additionally, the ID/IG ratio (the D and G bands intensity ratio) is a measure of the relative concentration of the local defects for the carbon domains [45, 46]. The ID/IG intensity ratios of the obtained LC/ZnO samples was approximately ~1.01 similar to the reported carbon/ZnO using graphene, graphite and other carbon materials by other researchers [20-25]. For the pure carbon materials, the ID/IG ratio can reflect its degree of graphitization [45]. In our current
research, the Raman spectrum of the pure lignin-based carbon (LC, obtained by washing away the ZnO) was supported in Figure S1. Compared with the LC/ZnO, ID/IG ratio of pure LC is reduced, and 2D, 2G bonds show enhancement. The results indicate that part of the QAL has been graphitized after carbonization. The XPS spectra were used to investigate the elemental components and bonding state of the as-prepared pure ZnO and LC/ZnO hybrid composite, as the results shown in Figure 3. From the full scan spectrum (Figure 3a), peaks belonging to Zn, C and O elements are observed, suggesting that the as-prepared LC/ZnO mainly contains Zn, O and C elements. Particularly, the peak belonging to C in the pure ZnO was due to the carbon in the equipment. This phenomenon was also reported in other studies [7, 56]. Figure 3b shows the high-resolution C 1s spectrum of the LC/ZnO hybrid composite. The peak at 284.5 eV is corresponding to the sp2 carbon atom, while the peak at 285.6eV is attributed to the C from C=O and C-O groups [47], and the peak at ~287.8 eV should belong to the O=C-OH species [48]. The C 1s spectrum was similar to the prepared carbon/ZnO using graphene, graphite and other carbon materials by other researchers [20-25]. The prepared samples of LC/ZnO and its precursor of QAL/ZnC2O4 were characterization by FT-IR analysis, as shown in Figure S2. Compared with the precursor of QAL/ZnC2O4, a significant loss of oxygen-containing carbonaceous peaks was obviously appeared in the FT-IR spectrum of LC/ZnO, suggesting the successfully preparation of LC/ZnO. Figure 4 depicts the XRD patterns of the samples of pure ZnO and LC/ZnO hybrid composite. The pure ZnO and LC/ZnO hybrid composite exhibit similar XRD patterns, they both can be readily indexed to the hexagonal wurtzite ZnO (JCPDS data card 36-1451). It shows that the introduction of QAL in the precursor solution didn’t obstruct the formation of crystal structured ZnO nanoparticles. The crystallite size was evaluated using the (101) peak using Scherrer formula [49]:
D
cos
where 𝜆 is the wavelength of X-ray, 𝜃 is Bragg’s angle, κ is the shape factor, and 𝛽 is the full width and half maxima. Here, κ and 𝜆 are taken as 0.9 and 0.15405 nm, respectively. The calculation result was listed in Table 1. The crystallite size of the ZnO nanocrystalline in ZnO and LC/ZnO were 37.5 and 32.6 nm, respectively. It shows that the crystallite size of the ZnO was decreased due to the addition of QAL in precursor. Figure 5(a & b) shows the SEM and TEM images of the as-prepared pure ZnO nanoparticles, showing that the ZnO has a typical nanoparticle structure, and the diameter ranged approximately from 50-180 nm. Additionally, the pure ZnO nanoparticles show a serious aggregation behavior. The SEM images of the prepared LC/ZnO hybrid composite were shown in Figure 5(c & d). It shows that the LC/ZnO micron-sized cube structure is composed of LC nanosheets and ZnO nanoparticles. The Nano-sized ZnO particles were well dispersed and tightly bonded to the LC nanosheets. The particle size of ZnO in hybrid composite was much smaller than the pure ZnO. In addition, it could be observed that the LC nanosheets were well combined with the ZnO nanoparticles as TEM images shown in Figure 5(e & f). The lattice fringes of ZnO (corresponds to 0.25 nm and 019 nm) and LC (corresponds to 0.33 nm) were labeled in the Figure 5f. It shows that there is a tight interfacial contact between the LC nanosheets and ZnO nanoparticles. The photo-generated electrons and holes transfer process is intimately related to the surface contact between the carbon and semiconductor, such an intimate interfacial contact in the prepared LC/ZnO hybrid composite could be expected to improve the photo-generated electrons and holes transfer process, thus leading to the improvement and excellent photocatalytic performance. In the formation of LC/ZnO hybrid composite, the preparation of QAL/ZnC2O4 precursor is the key factor. Firstly, the CO2 and H2O gas from the decomposition of the ZnC2O4 precursor could cause the carbon nanosheet to expand and form a loose structure, which favors of the loading of ZnO in LC. Next, lignin with three-dimensional-structure as the precursor of the carbon could help disperse the
ZnO nanoparticles. The resulting LC can also modify the surface of ZnO, which could make a large number of oxygen vacancies in ZnO as shown in Figure 5f. The oxygen vacancies become positive centers, forming many holes. On the other hand, the graphic stripe similar to graphene fingerprints was observed in Figure 5f, suggesting that the resulting ZnO could accelerate the carbonization and graphitization of lignin at a relatively low temperature of 550 ℃, which was much lower than 700~900 ℃ in other reports [39-42].The BET surface areas of the QAL, ZnO and LC/ZnO were investigated by nitrogen (N2) adsorption-desorption experiments, as the results show in Figure S5. The N2 adsorption-desorption isotherms of the samples are typed assigned to the Brunauer-Emmett-Teller classification [50], which indicates the presence of macropores. Especially, the LC/ZnO hybrid composite exhibits high adsorption at relative pressure (P/P0) close to 1.0, indicating that there is a lot of mesopores and macropores. On the other hand, there is a rapid growth in gas adsorption capacity in the low relative pressure area, which is due to the micropores [51]. The measured surface areas of the LC/ZnO, ZnO and QAL were 139.53 m2·g-1, 45.38 m2·g-1 and 26.49 m2·g-1, respectively. The LC/ZnO hybrid composite shows much larger surface area than ZnO probably due to the nanosheet-shaped lignin-based carbon. The large specific surface area of the LC/ZnO hybrid composite could enhance the adsorption ability for organic dye molecules, which may contribute to enhance the photocatalytic performances. 3.2 Optical and photoelectrical properties Figure 6a shows the UV-Vis diffuse reflectance spectra of the as-prepared ZnO and LC/ZnO. Both the ZnO and LC/ZnO all show the typical very high absorption intensity in the UV region. The LC/ZnO hybrid composite shows higher UV absorption than pure ZnO, due to the benzene ring structure and C=C and C=O double bonds [52]. Furthermore, the LC/ZnO hybrid composite shows enhanced absorption intensity in the visible-light compared with the pure ZnO, which was cause by the background absorption of LC. This is in consistent with the black color of the LC/ZnO [28].
However, the absorption band-edge of the LC/ZnO hybrid composite has red-shifted. The band gap energy of the semiconductors has been calculated by the Kubelka-Munk method [53]. The relationship between the photon energy (hν) and the absorption coefficient (α) for direct band gap semiconductor could be determined by the following equation: α = Βd(hν - Eg)1/2/hν where the Βd is the absorption constants, α can be determined from the reflectance spectra according to the Kubelka-Munk theory. Plots of he (αhν)2 versus hν are shown in Figure 6b. The direct band gap energies of the pure ZnO and LC/ZnO are approximately 3.25 and 3.03 eV calculated from the intercept of the tangents to the plots, respectively. The lower band gap of LC/ZnO is possibly due to the synergistic interfacial interaction between the LC and ZnO [28]. The absorption edge of LC/ZnO hybrid composite shows a red shift compared to the pure ZnO nanoparticles, which is beneficial to enhance The photoluminescence (PL) spectra of the as-prepared pure ZnO and LC/ZnO are presented in Figure 7. Pure ZnO shows a significantly high and wide PL peak around 500 nm, indicating that there were many photogenerated electrons and holes in the pure ZnO. However, the photogenerated electrons and holes show high recombination probability. On the other hand, the PL intensity of the LC/ZnO is much lower than the PL intensities of pure ZnO. The remarkable decrease of PL intensity indicates that the recombination probability of photogenerated electrons and holes greatly decreases and their separation and migration efficiency obviously increases [17, 27]. The photo-generated electrons were efficiency transferred to the lignin-based carbon, which will be a great contribution to the photoactivity enhancement. The photoelectronchemical properties of the semiconductor are performed to investigate the charge separation and transfer process [31]. Figure 8 shows the EIS and I-T curves of the ZnO and LC/ZnO devices. Figure 8a shows that the arc radius in the Nyquist plot of the LC/ZnO is smaller than that of the pure ZnO, revealing that the interface charge transfer efficiency of the LC/ZnO is improved. Figure 8b shows the
IT curves of the ZnO and LC/ZnO devices with several photoelectric response cycles with intermittent light irradiation. It shows that the current intensities of the ZnO and LC/ZnO devices increase sharply upon light irradiation. Noticeably, the photocurrent of the LC/ZnO is much higher than the photocurrent of the pure ZnO, which reveals that the photogenerated charge carrier efficiency of the LC/ZnO is higher than the photo-generated charge carrier efficiency of the pure ZnO. The photoelectrochemical properties were consistent with the results of PL spectra. The higher efficiency of charge generation and transfer speed could be attributed from the excellent combination of LC and ZnO. And the excellent photoelectrochemical properties of the LC/ZnO hybrid composite would be in accordance with a high photocatalytic activity. 3.3 Evaluation of photocatalytic performance The photocatalytic performance of the prepared LC/ZnO hybrid composite was evaluated by aqueous phase photodegradation of organic dye pollutants under simulated solar light illumination, the negative MO and positive Rh B were chose as the photo-degraded objects. Prior to photocatalytic experiments, the adsorption-desorption equilibrium curves for the catalysts and dyes were measured, as the results show in Figure S6. It shows that the dyes adsorption is balanced within 30 min. Figure 9(a & b) shows the photodegradation of MO and Rh B over the pure ZnO and LC/ZnO hybrid composite, respectively. Figure 9a shows that the concentration reduction of MO based on LC/ZnO is obvious higher than that of pure ZnO, and the LC/ZnO exhibited higher photocatalytic activities than the ZnO. Furthermore, MO was almost fully degraded by the catalyst of LC/ZnO after light irradiation 30 min, but only 75.3% of MO was degrade by the pure ZnO after light illumination 50 min. Figure 9c shows the pseudo-first-order kinetic of the photodegradation of MO based on ZnO and LC/ZnO. The reaction rates of ZnO and LC/ZnO are 0.0366 and 0.18454, respectively. The LC/ZnO exhibits a 5-fold enhancement in photodegradation of MO than the pure ZnO. Figure 9b shows that the photodegradation rate of Rh B based on LC/ZnO is much higher that of pure ZnO. It means that the Rh B is easily adsorbed by the
LC/ZnO. The photodegradation efficiency and rate for Rh B based on LC/ZnO is much higher than that based on pure ZnO. Figure 9d shows that the reaction rates of the ZnO and LC/ZnO are 0.0072 and 0.019, respectively. The photodegradation rate of Rh B based on LC/ZnO increased 2.6 times compared with pure ZnO. The detailed comparison of the photocatalytic data for LC/ZnO and ZnO is shown in the Table 2. Especially, the enhanced speed and efficiency in photodegradation of MO is higher than that of in photodegradation Rh B over the LC/ZnO. It may mean that the mechanism of photocatalytic degradation of MO and Rh B based on LC/ZnO is different. On the other hand, the photodegradation of MO based on the LC/ZnO hybrid composites with varied pH has been studied, as the results shown in Figure S7. It shows that that the LC/ZnO hybrid composite has a good catalytic degradation effect in the pH of 7 to 11. But the LC/ZnO shows poor catalytic performance under acidic conditions. Furthermore, the photo stability of LC/ZnO for degradation of MO based on LC/ZnO has been investigated, as shown in Figure S8. It shows that the photocorrosion of ZnO nanoparticles can be efficiently inhibited by mixing LC into the composite. The photoactive groups in photocatalytic degradation of organic dyes are mainly superoxide radicals (·O2-), hydroxyl radicals (·OH) and holes (h+) [54]. In order investigate the photoactive radical species of photocatalyst, the electron spin resonance (ESR) spectra were measured, as the results shown in Figure 10. Figure 10a shows the ESR spectra of the pure ZnO and LC/ZnO in the pure aqueous dispersion without DMPO. The ZnO showed a very weak peak in dark, indicating it contained very little h+. However, the LC/ZnO showed a clear peak in dark, indicating it contained many h+. In addition, the peaks intensity of ZnO and LC/ZnO both significant enhanced when they were irradiated with light, indicating many h+ were generated. Figure 10b shows the ESR spectra of pure ZnO and LC/ZnO in aqueous solution doped with DMPO. Figure 10c shows the ESR spectra of pure ZnO and LC/ZnO in methanol doped with DMPO. The characteristic peaks corresponds to
DMPO-·OH and DMPO-·O2- were appeared both in ZnO and LC/ZnO under light irradiation [28]. On the other hand, the peak intensity of the DMPO-·OH and DMPO-·O2- based on ZnO and LC/ZnO under light illumination was much stronger than that in dark. The content of ·OH and ·O2- based on LC/ZnO was much more than that based on pure ZnO. This is the reason why LC/ZnO showed the higher photocatalytic activity than pure ZnO. The photocatalytic mechanism exploration was carried out by investigating the main photoactive species during the photodegradation process via dissolving various trapping agents in solution [55]. The added trapping agent TEOA, BQ and TBA were used to eliminate the h+, ·O2- and ·OH, respectively. Figure 11a shows the results of photodegradation of MO over LC/ZnO with various trapping agents. The photodegradation rate and efficient decreased obviously when added the trapping agents compared with no scavenger, indicating that the photo-generated h+, ·O2- and ·OH were the photocatalytic active groups in photodegradation MO. Especially, MO could hardly be photo-degraded when TEOA was added, suggesting that the h+ was main photocatalytic active groups. Figure 11b shows the result of photodegradation of RhB based on LC/ZnO with different trapping agents. The photodegradation rate and efficient decreased obviously after adding trapping agents of BQ and TBA. However, there was almost no effect on the photodegradation rate and efficient when adding TEOA during photocatalytic, suggesting that the ·O2- and ·OH radicals were the main photocatalytic active groups, h+ almost had no effect in photodegradation of RhB. According to experiment results of photoactive species from ESR and trapping agents. The photodegradation mechanism of based on LC/ZnO composite for organic dyes in this study could be explained as follows (Figure 12) [17, 48, 49, 56]: LC/ZnO + hν → ZnO(h+) + LC(e-), e- + O2 → ·O2-, h+ + H2O/OH- → ·OH ·OH or ·O2- or h+ + MO → CO2 and H2O mainly ·OH or ·O2- + RhB → CO2 and H2O mainly
Under illumination of simulated solar light, the electrons (e-) in ZnO will be excited from valence band (VB) to conduction band (CB), producing photogenerated electrons and holes. The photogenerated electrons and holes in ZnO can transfer to the lignin-based carbon nanosheets through the intimate interfacial contact, which could help to separate the photogenerated electrons and holes effectively. The transferred holes in the lignin-based carbon nanosheet could oxidize the absorbed H2O or OH- to produce ·OH radicals. Additional, the photogenerated electrons can reduce the molecular oxygen absorbed on the lignin-based carbon nanosheet to produce superoxide ions (·O2-). Both ·OH and ·O2- can degrade MO and RhB due to the strong oxidative abilities. On the other hand, the MO absorbed on the surface of ZnO could be directly degrade by the holes, but the RhB was photo-degraded only by the ·OH and ·O2-. Furthermore, in order to study the why the photoactive species were difference between the photodegradation MO and RhB over LC/ZnO. The Zeta potentials of the LC/ZnO and LC dispersed in aqueous solution were measured, as shown in Table S2. The Zeta potentials of the LC/ZnO and LC were -11.85 and -15.5 mV, respectively. As is known, MO molecules in the water solution are negatively charged, RhB molecules in the water solution are positively charged. The positive RhB is more likely to be adsorbed to the positively charged LC/ZnO than the negative MO due to the electrostatic adsorption, which was confirmed by the results of the adsorption experiments, as shown in Figure 9(a & b) and Figure S6. However, the RhB was firmly adsorbed on the surface of LC due to the electrostatic adsorption between the positive and negative charges. The positive RhB could not be exclusion to the surface of ZnO to be degraded by the large number of holes. On the other hand, the negative MO could be excreted to the surface of ZnO being directly degraded by the holes. This may be the main reason why the enhancement rate of LC/ZnO degradation of negative MO is higher than that of degradation of positive RhB. It means that the prepared LC/ZnO could selectively degrade organic dyes with different polarities. This prepared catalyst may have other potential application value in other fields.
Conclusions In summary, a novel LC/ZnO hybrid composite was successfully prepared through a facile yet efficient one-pot carbonization method by calcination the QAL/ZnC2O4 precursor at 550 ℃ for the first time. The small-sized ZnO nanoparticles were closely associated to lignin-based carbon nanosheets. The prepared LC/ZnO showed excellent photoelectron transfer properties due to its excellent surface contact. The LC/ZnO hybrid composite exhibited an excellent photocatalytic performance: the LC/ZnO showed significant enhancement of photocatalytic activity over the pure ZnO, and it showed different photocatalytic mechanism for the degradation of negative MO and positive RhB: h+ was the main photoactive specie during the degradation of negative MO, ·O2- and ·OH were the main photoactive specie during the degradation of positive RhB. This work reported a photocatalyst with selective degradation of positive and negative organic dyes. In addition, the prepared lignin-based carbon has great potential as a replacement for expensive graphene in the field of photocatalysis. It may have other valued application prospect in the field of photoelectric conversion and catalytic materials. Acknowledgments This work was funded by the Guangdong Province science and technology plan project (2014B050505006) and the National Natural Science Foundation of China (NSFC) (No. 21436004, 21576106).
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Figure 1. Schematic illustration for the preparation of LC/ZnO hybrid composite.
Figure 2. Raman spectra of the prepared ZnO and LC/ZnO.
Figure 3. XPS spectra of the pure ZnO and LC/ZnO hybrid composite, (a) is overview and (d) is the high-resolution spectra of C 1s.
Figure 4. XRD patterns of the as-prepared samples of ZnO and LC/ZnO hybrid composite.
Figure 5 SEM images of the prepared ZnO (a) and LC/ZnO (c & d), TEM images of the prepared ZnO (b) and LC/ZnO (e & f).
the photocatalytic performance.
Figure 6. (a) UV-Vis absorption spectra of pure ZnO and LC/ZnO, (b) the plots of calculated by Kubelka-Munk function to the energy of light.
Figure 7. PL spectra of the prepared pure ZnO and LC/ZnO hybrid composite excited with wavelength of 365 nm.
Figure 8. EIS (a) and I-T (b) curves of the ITO/ZnO and ITO/LC/ZnO devices with a bias voltage at 0.1 V.
Figure 9. Photocatalytic degradation MO (a) and Rh B (b) over the pure ZnO nanoparticles and LC/ZnO hybrid composite, pseudo-first-order kinethic plots photodegradation MO (c) and Rh B (d).
Figure 10. ESR spectra of pure ZnO and LC/ZnO with or without light irradiation in different environment, (a) pure aqueous dispersion without DMPO, (b) in aqueous dispersion doped with DMPO, (c) in methanol dispersion doped with DMPO.
Figure 11. Photodegradation of MO (a) and RhB (b) with different scavenger, TBA, BQ and TEOA used as ·OH, O2·- and h+ scavenger, respectively.
Figure 12. Photocatalytic mechanism for the degradation of MO and RhB over the LC/ZnO.
Table 1. Structural information calculated from the XRD patterns in Figure 4. (hkl)
2θ(°)
ZnO
101
36.4607
0.263
37.5
LC/ZnO
101
36.3052
0.302
32.6
Sample Name
FWHM(°)
Crystallite size (nm)
Table 2. Comparison of the photodegradation performance of ZnO and LC/ZnO. Adsorbance (%)
Efficiency (%)
Time (min)
k
MO
RhB
MO
RhB
MO
RhB
MO
RhB
LC/ZnO
10.5
37.5
99.9
79.2
30
50
0.1845
0.019
ZnO
5.2
4.5
69.8
31.1
50
50
0.0366
0.0072
Photocatalyst