Abstract
Well-crystallized and hexagonal wurtzite ZnO was synthesized with nanocellulose using a facile hydrothermal method. Many highly active (001) facets were retained in the obtained ZnO nanocrystals, presumably due to interaction between the polar facet of ZnO and the nanocellulose. Given its effective surface area, the synthesized ZnO exhibited good photocatalytic activity of degrading methylene blue. Its degradation efficiency reached 94.4% within 30 min (UV irradiation power of 6 W), which was 34% higher than that of Degussa TiO2 P25. The ZnO photocatalyst also exhibited excellent reusability, confirmed by no obvious abatement after its being re-used for 8 cycles. These ZnO nanomaterials were synthesized using renewable nanocellulose derived from cotton. This environmentally friendly and cost-effective approach is anticipated to be applied in the future synthesis of small-sized ZnO photocatalysts.
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Synthesis of ZnO with Enhanced Photocatalytic Activity: A Novel Approach Using Nanocellulose
Guangyu Wei,a Hong-Fen Zuo,b Yuan-Ru Guo,b,* and Qing-Jiang Pan a,*
Well-crystallized and hexagonal wurtzite ZnO was synthesized with nanocellulose using a facile hydrothermal method. Many highly active (001) facets were retained in the obtained ZnO nanocrystals, presumably due to interaction between the polar facet of ZnO and the nanocellulose. Given its effective surface area, the synthesized ZnO exhibited good photocatalytic activity of degrading methylene blue. Its degradation efficiency reached 94.4% within 30 min (UV irradiation power of 6 W), which was 34% higher than that of Degussa TiO2 P25. The ZnO photocatalyst also exhibited excellent reusability, confirmed by no obvious abatement after its being re-used for 8 cycles. These ZnO nanomaterials were synthesized using renewable nanocellulose derived from cotton. This environmentally friendly and cost-effective approach is anticipated to be applied in the future synthesis of small-sized ZnO photocatalysts.
Keywords: Nanocrystalline ZnO; Exposed (001) active facets; Nanocellulose; Photocatalytic activity; Hydrothermal method
Contact information: a: Key Laboratory of Functional Inorganic Material Chemistry (Ministry of Education), School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China; b: Key Laboratory of Bio-based Material Science & Technology (Ministry of Education), Department of Materials Science and Engineering, Northeast Forestry University, Harbin 150040, China;
* Corresponding authors: panqjitc@163.com; guoyrnefu@163.com
INTRODUCTION
Replacing conventional chemicals with renewable biomass is an important and effective way to reduce environmental contamination. Cellulose is an abundant organic compound present in biomass (Hokkanen et al. 2013). Because it is a cheap renewable resource (Albertsson 2012; Gericke et al. 2013), many countries have devoted themselves to developing and applying various forms of cellulose. Recently, nanoscale cellulose has become popular (Moser et al. 2015).
Due to serious environment contamination problems, many methods, such as degradation or adsorption, have been applied to eliminate the pollution (Nouri et al. 2014; Aziz and Anise 2016). As an n-type semiconductor (Maryam and Aziz 2016), ZnO has excellent optoelectrical behavior due to its natured wide band gap and large exciton binding energy. Nanoscale ZnO is a next-generation semiconductor material with potential applications in the catalytic and luminescent fields (Mishra et al. 2015). These outstanding properties, which are to some degree attributable to small-size effects, allow wide applications of nano-ZnO in wastewater treatment, sterilization, and environmental protection (Memarian et al. 2011). The method used to prepare nano-ZnO affects both its morphology and performance. Great progress has been made in various synthetic methods such as vapor deposition sol-gel, homogeneous precipitation, and hydrothermal methods (Yang et al. 2015). However, a facile, low-cost, and environmentally friendly preparation approach for ZnO with specific performance is still highly demanded.
ZnO crystals have a basal positive polar plane Zn (001) and a negative polar plane O (001) (Memarian et al. 2011). Due to thermodynamic instability, these polar planes grow fast to reduce surface energy. Consequently, the (001) plane vanishes in the most situations, for instance, forming sharp-end (needle-like) ZnO. It is well known that the polar (001) plane of the ZnO crystallite can serve as optimally active sites to enhance photocatalysis. Therefore, it is vital to maintain a large number of active and exposed (001) facets during nano-ZnO preparation to improve the photocatalytic performance of the product.
Many studies reported preparation of ZnO/cellulose composite (Ghule et al. 2006; Perelshtein et al. 2009). Previously, microcrystalline cellulose (MCC) was applied to synthesize ZnO, yielding a product with good photocatalytic performance (Zuo et al. 2014). In this work, nanocrystalline ZnO with highly active (001) facets has been successfully fabricated using a facile, environmentally friendly, and cost-effective approach. In addition to assistance in preparing ZnO nanomaterials, the nanocellulose (NCs) surfactant protects energetically unstable (001) facets in the synthetic process, allowing more high-energy planes to be retained. Thus, ZnO exhibits excellent photocatalytic activity, which can be superior to Degussa TiO2 P25. Because of its high reusability, the newly synthesized ZnO is anticipated to be a promising photocatalyst.
EXPERIMENTAL
NCs Synthesis
NCs was derived from a renewable biomass resource (cotton) and prepared as previously described (Li et al. 2013). Anhydrous phosphoric acid (74% P2O5) was prepared by mixing polyphosphoric acid with 85% phosphoric acid for 1.5 h at 48 °C, followed by cooling to 1 to 3 °C. Degreased cotton was immersed with stirring into the anhydrous phosphoric acid system at a mass ratio of 1:5 cotton to solution. After 2 h, the mixture was neutralized to pH 7.5 with aqueous sodium hydroxide (20% w/w) in an ice bath. NCs was obtained after washing with deionized water and centrifugation. The prepared cellulose has a rod-like shape with diameter of 10 nm.
Nano-ZnO Synthesis
The ZnO nanocrystallites were fabricated via a facile solution-processing hydrothermal technique. With stirring at room temperature, 15 mL of zinc acetate dihydrate solution (Zn(CHCOO)2·2H2O) (2 mol·L-1) and 15 mL of sodium hydroxide solution (0.5 mol·L-1) were added dropwise to a 7 mL NCs suspension (0.5% w/w). After half an hour, the mixed solution was moved to the hydrothermal reactor. The reactor was put into a blast oven at 100 °C for 16 h. White products were harvested by centrifugation and washed with deionized water. The nano-ZnO was obtained after being calcined for 1 h at 550 °C.
For comparison, ZnO was also prepared at the same condition with glucose, MCC and without NCs.
X-Ray Diffraction (XRD) Analysis
X-ray powder diffraction was performed on a Rigaku D/Max-RC apparatus (Tokyo, Japan) using a CuKα radiation, and scans were performed from (2θ) 5° to 80° at a rate of 4°·min-1.
Electron Microscopy
Scanning electron microscopy (SEM) images were taken with a Sirion field emission microscope (FEI, Hillsboro, OR, USA). Samples were put on conductive tapes for the test. Transmission electron microscopy (TEM) images were performed on a JEM-2100 electron microscope (JEOL, Tokyo, Japan) with an acceleration voltage of 200 kV. Carbon-coated copper grids were used as the sample holders. The samples were dispersed on sample holders by absolute ethyl. Pore size and lattice spacing were calculated according to the scale of the apparatus.
Photoluminescence Spectra and IR Spectra
Photoluminescence performance was examined using a PerkinElmer Fluorescence Spectrophotometer (U.K.) with Xe lamp at room temperature and an excitation wavelength is 325 nm.
FT-IR spectra were obtained with a FT-IR spectrometer (PerkinElmer Frontier).
Photocatalytic Analysis
The reaction suspension was formed by mixing 40 mL of methylene blue (MB) with 0.1 g ZnO, and the suspension was stirred in the dark for 0.5 h to reach the adsorption-desorption equilibrium. The suspension was then exposed to UV irradiation (6 W, wavelength = 365 nm, WFH-203) under ambient conditions. The distance between the UV light source and the photoreaction vessel was 10 cm. At a definite time interval, 4 mL of suspension was collected for analysis. Photodegradation was investigated by measuring its absorbance in a UV-vis spectrophotometer (UV762, Shanghai, China) at the maximum absorption (λmax) of 664 nm. A good linear relationship was observed between the concentration and absorbance at low concentration according to Lambert-Beer’s law (Eq. 1),
η = (C0–Ct) ·100%/C0 = (A0–At) ·100%/A0 (1)
where C0 is the initial concentration of MB solution, A0 the initial absorbance, Ct the concentration at the t moment, and At the absorbance at the t moment.
RESULTS AND DISCUSSION
XRD Analysis
The XRD results showed two peaks at 20° and 22° (2θ), which indicated that the NCs consisted of cellulose II (Fig. 1). The crystallinity and purity of the prepared ZnO was also analyzed by XRD. As shown in Fig. 1, sharp peaks matched well with the standard hexagonal wurtzite structure of ZnO (JCPDS no. 36-1451; P63 mc; cell parameters: a = b = 3.250 Å and c = 5.207 Å). No extra diffraction peaks corresponding to impurities such as Zn(OH)2 or NCs were detected, indicating that pure hexagonal wurtzite ZnO had been synthesized. The average crystallite size was estimated to be 31.7 nm by computing the (100), (002), and (101) diffraction peaks with the Scherrer formula (Eq. 2),
L(hkl) = 0.9λ / Δ(hkl) cosθ (2)
where L is the average crystallite size in nm, Δ is the FWHM of the peak, λ is the wavelength of the incident X-ray, and θ refers to the Bragg angle subtended at maximum intensity. The letters “hkl” denote the Miller indices corresponding to the lattice planes.
Fig. 1. X-ray powder diffraction patterns
The intense diffraction peaks demonstrated the high crystallinity of the ZnO. It has been suggested that the relative intensity of the (100) vs. (002) peaks in the XRD patterns can quantitatively estimate the ratio of exposed nonpolar and polar facets of ZnO (Chen et al. 2013). The (100)/(002) intensity ratio was 1.30 for the bulk ZnO (JCPDF:36-1451); for the synthesized product, this value was 1.07. The smaller ratio indicates more polar facets exposed in the synthesized ZnO.
Electron Microscopy Analysis
The morphology of the ZnO was determined by scanning electron microscopy (SEM; Fig. 2a~e) and transmission electron microscopy (TEM; Fig. 2f).
Fig. 2. SEM of ZnO prepared a) without NCs, (b) with NCs, (c) with MCC, and (d) with glucose; TEM of ZnO prepared by NCs with the inset of HRTEM (e), and EDS (f).
As can be seen in Fig. 2a, the ZnO prepared without NCs was characterized as flakes with 1 μm length. These random microflakes tended to aggregate to each other. However, ZnO fabricated by NCs showed the same morphology as flakes with the approximately identical size (Fig. 2b). Those flakes were well-dispersed, indicating that NCs can reduce the ZnO coacervate. Nano-flower-like ZnO was obtained using MCC (Fig. 2c). The prepared ZnO flowers were composed by the flakes, which resembled the ZnO flakes prepared by NCs. Utilization of the glucose to prepare ZnO led to formation of nanoparticle clusters (Fig. 2d). These clusters were constituted by particles having a uniform size of 100 nm. Notably, the ZnO prepared with NCs showed the most dispersive character according to SEM images.
The TEM method was used for further analysis of the ZnO structure prepared with NCs. The image in Fig. 2f shows many homogeneous pores, consistent with a large effective surface area. The average pore diameter was approximately 9.1 nm. High resolution transmission electron microscopy (HRTEM) was used to ascertain the growth orientation of the ZnO crystallites (upper left corner of Fig. 2f). The lattice spacing of the ZnO nanocrystallites was calculated to be 0.52 nm. This is attributed to the (001) planar spacing, corresponding to the most active facet in crystalline ZnO (Mclaren et al. 2009; Tian et al. 2012). Both large effective contact area and highly-active exposed polar (001) planes would benefit the photocatalytic activity of the synthesized nano-ZnO. In this case, only Zn and O elements were found, which was evidenced by the energy dispersive spectroscopy (EDS) measurement of the components of ZnO. Therefore, the ZnO prepared by this approach was judged to be pure.
IR Spectra
Infrared (IR) spectra of the NCs and the ZnO prepared by NCs were determined and displayed in Fig. 3a. One can see that the NCs showed intense absorption bands at 3430 and 1630 cm−1. These features are attributed to OH-stretching and -bending vibrational frequencies, respectively. The peak at 860 cm−1 is related to the C-H rocking vibration of cellulose. Bands at 1090 and 1061 cm−1 correspond to the OH vibration of intramolecular hydrogen bond and the C-O stretching of cellulose, respectively. Regarding the ZnO materials, only strong peaks at 3430 and 1630 cm−1 were observed, and they were assigned to vibrations of OH from water absorbed on the ZnO surface. Additionally, the vibration of Zn-O, being much weaker, was absent in the IR spectra. Comparison of IR spectra between NCs and ZnO revealed that the ZnO materials were pure, i.e. the NCs had been totally removed.
Fig. 3. (a) IR spectra of the NCs and the ZnO prepared by NCs, and (b) photoluminescence spectra of ZnO prepared without and with NCs
Optical Properties of ZnO
The optical properties of the ZnO prepared with and without NCs were compared by determining their photolumiscence spectra at room temperature. As can be seen in Fig. 3b, the ZnO displayed two prototypical bands at 400 and 400-700 nm, regardless of whether NCs were included in the preparation. The first intense peak at 400 nm belongs to the exciton emission, which originates from the direct recombination of the conduction band electrons and the valence band holes. The broad emission at 400-700 nm is visible emission, which arises from transition of an excited electron from the conduction band of the nanomaterials to their defects. Notably, the ZnO prepared with NCs showed much more intense visible emission than the one without NCs. This indicates that the ZnO made with the NCs had more defect states while being prepared with the NCs. As a result, the utilization of NCs would improve catalytic property of materials.
Photocatalytic Performance Analysis
The photocatalytic performance of ZnO was evaluated by degrading methylene blue (MB) under both ultraviolet (UV) and visible light irradiation at room temperature. The degradation efficiency reached 94.4% after 30 min of UV irradiation (6 W, wavelength = 365 nm) using the ZnO photocatalyst (Fig. 4a). Comparatively, the Degussa TiO2 P25 catalyst and ZnO prepared without using NCs were tested under the same experimental conditions, resulting in 60.5% and 23.0% degradation efficiency, respectively. These results reveal that the ZnO exhibited 34% and 72% higher photocatalytic efficiency than TiO2 P25 and ZnO prepared without NCs, respectively.
When performing degradation experiments with visible light irradiation (250 W high-pressure mercury lamp, 420 nm-cut off filter), the ZnO achieved approximately 10% better performance than TiO2 P25 (Fig. 4b).
Fig. 4. Photodegradation curves of MB: irradiated under (a) UV light and (b) the visible light, and ZnO prepared by different additives under UV light (c), together with the ZnO degradation efficiency within 30 min (d).
To more fully understand the NCs’ effect on the catalytic property of ZnO, catalytic experiments of the ZnO prepared by MCC and glucose were carried out under the same conditions. As can be seen in Fig. 4c, ZnO prepared with NCs had the highest degradation efficiency, compared with ZnO prepared with either MCC or glucose. After 2 h, the degradation efficiency of ZnO prepared by NCs reached 98.2%. More importantly, it also exhibited a dramatically fast rate of degradation of the MB, i.e., reaching over 90% in 30 min. Comparatively, ZnO materials with MCC and glucose showed low degradation rates within 30 min, although degradation efficiency retained 89.8 and 82.4% in 2 h. Fig. 4d shows the degradation efficiency of ZnO prepared by different additives and TiO2 P25 within 30 min, which indicates that ZnO prepared by NCs had better catalytic activity and achieved a faster degradation rate than TiO2P25 and ZnO prepared with other additives or none.
Eight recycling tests were carried out to evaluate the reuse efficiency of ZnO prepared by NCs (Fig. 5). At the end of each cycle, wet ZnO was separated by centrifugation and directly reused without any further treatment such as washing or drying. For the first six-cycle tests, the UV irradiation time was 30 min, and the concentration of MB was 5 mg·L-1. The measured degradation efficiency was higher than 92%, even after six cycles (Fig. 5a). Moreover, there was not any obvious sign of abatement. To further assess the photocatalytic activity and the reuse efficiency, the MB concentration was increased to 10 mg·L-1 in the 7th cycle and 15 mg·L-1 in the 8th cycle (Fig. 5b). The degradation percentage reached 98.2% in the 7th cycle and 97.9% in the 8th cycle after 90 min of UV illumination; thus, ZnO prepared by NCs is a promising photocatalyst due to its high photocatalytic ability and outstanding recycled performance.
a
b
Fig. 5. (a) Reusable capability of ZnO for MB photodegradation, where 5 mg·L-1 MB and 30 min degradation time were used in the first six cycles, and 10 and 15 mg·L-1 MB with 90 min used in the 7th and 8th cycles, respectively. (b) Photodegradation curves at specific MB concentration.
The Possible Growth Mechanism
Surface areas and active planes are key factors to influence photocatalytic activity of ZnO. A synergetic effect of them was found to enhance photocatalytic activity of the materials evaluated in this work. A not-high specific surface area of 21 cm2g-1 was measured for these materials, but many active and exposed (001) planes were retained, which would play an important role in improving photocatalytic behaviors. To strengthen this effect, the NCs used in the experiment were prepared from degreased cotton cellulose.
Cellulose does not naturally appear as an isolated individual molecule, but as assemblies of individual cellulose chain-forming fibers (Fig. 6). When degreased cotton was immersed in phosphoric acid, crystalline NCs were isolated from the fibers. The assembled nanocrystals carry a negative charge on the surface and produce the electric double layer in the solution.
Gedanken and Ghule and their coworkers reported that the ZnO and cellulose can adhere to each other by hydrogen bonds (Ghule et al. 2006; Perelshtein et al. 2009). Since the nanocellulose carries a negative charge and the ZnO crystal has a polar plane, we propose that the formation mechanism of ZnO is associated with the behaviors of ZnO crystal growth. During the growth of crystallite ZnO, the positively-charged Zn (001) planes are absorbed by negatively-charged NCs surface via electrostatic attraction. Thus, Zn (001) planes are covered by NCs. As a result, this greatly reduces the surface energy of (001) planes, suppresses the growth along the [001] direction, and eventually produces the flake ZnO morphology, consistent with Figs. 2b and 2f. Subsequent calcination removes the NCs that protect high-energy surface and the active (001) planes are exposed. This is evidenced by HRTEM in Fig. 2f.
Fig. 6. Formation of NCs from cotton and their influence on formation of ZnO
CONCLUSIONS
- Nano-ZnO with exposed (001) facets was fabricated via a facile hydrothermal method with NCs.
- The NCs was prepared from the easily available degreased cotton that is a renewable biomass resource, making the synthetic approach low-cost and environmentally friendly.
- Nano-ZnO prepared by CNs showed outstanding photocatalytic activity, which was much better than TiO2 P25, as well as by ZnO prepared with MCC and glucose. Further association with high reuse efficiency enhances the potential application of ZnO prepared by CNs as a photocatalytic material.
ACKNOWLEDGMENTS
This work was supported by the Fundamental Research Funds for the Central Universities (DL12EB05-02), and the Natural Science Foundations of China (30901136, 21273063) and of Chinese Heilongjiang Province (B201318).
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Article submitted: February 19, 2016; Peer review completed: May 1, 2016; Revised version received: May 18, 2016; Accepted: May 25, 2016; Published: June 2, 2016.
DOI: 10.15376/biores.11.3.6244-6253