- Nano Express
- Open Access
Renewable Lignosulfonate-Assisted Synthesis of Hierarchical Nanoflake-Array-Flower ZnO Nanomaterials in Mixed Solvents and Their Photocatalytic Performance
© Li et al. 2016
- Received: 10 March 2016
- Accepted: 9 May 2016
- Published: 21 May 2016
With the assistance of sodium lignosulfonate, hierarchical nanoflake-array-flower nanostructure of ZnO has been fabricated by a facile precipitation method in mixed solvents. The sodium lignosulfonate amount used in our synthetic route is able to fine-tune ZnO morphology and an abundance of pores have been observed in the nanoflake-array-flower ZnO, which result in specific surface area reaching as high as 82.9 m2 · g−1. The synthesized ZnO exhibits superior photocatalytic activity even under low-power UV illumination (6 W). It is conjectured that both nanoflake-array structure and plenty of pores embedded in ZnO flakes may provide scaffold microenvironments to enhance photocatalytic activity. Additionally, this catalyst can be used repeatedly without a significant loss in photocatalytic activity. The low-cost, simple synthetic approach as well as high photocatalytic and recycling efficiency of our ZnO nanomaterials allows for application to treat wastewater containing organic pollutants in an effective way.
- ZnO nanomaterials
- Mixed solvents
- Sodium lignosulfonate
- Catalytic properties
- Hierarchical structure
With strengthening consciousness of environmental protection and effective utilization of resources, much more attention has been paid to using renewable materials to synthesize highly efficient catalyst for sewage treatment . As a low-cost renewable biomass resource, lignin is the most abundant organic material in nature . It is derived from black liquor generated in the pulping process. Every year, tens of million tons of lignin are produced all over the world  Unfortunately, most of them are discarded as waste, greatly contaminating our environment, while only a small part is inferiorly used. Therefore, exploration on high-effective utilization of lignin not only makes good use of natural resource but also protects the environment from pollution of black liquor [4, 5].
Sodium lignosulfonate (SL for short) is one of the most important derivatives of lignin. Lignosulfonate can be obtained by the sulfonated modification of lignin. Moreover, SL itself is a by-product of sulfite pulp and paper industry. It is a biomacromolecule polyelectrolyte and can act as an anionic surfactant , because of its highly cross-linked polymers formed by various hydrophobic phenylpropanoid units and diverse hydrophilic groups. Various active functional groups ensure its favorable water solubility and effective surface charges. In different solvents, SL can form associations and the degree of associations changes according to the environment of the solution (such as SL concentration, the type of the solvents, and ion strength). The aggregation state of SL will change as a result of the existence of electrostatic repulsion [7–9]. Spheroidal shape and irregularly disk-like shape of SL in different conditions were corroborated by previous works . The layer-by-layer self-assembly of SL can also easily be produced in saline solution because of its capability of binding transition metal ions such as Cu2+ and Zn2+. . All of these outstanding performances have enabled SL to be applied in the synthesis of nanoparticles of transition metal oxide.
ZnO, as a typical transition metal oxide, has recently attracted increasing attention in view of its outstanding properties [12–15]. All these provide ZnO with diverse applications in dye-sensitized solar cell, nanoimprint lithography, drug delivery, and photocatalysis. Particularly, ZnO has excited much interest of researchers because of photocatalytically degrading organic pollutants and is becoming more promising alternative to TiO2.
It has well established that properties and performance of ZnO strongly depend on its structure and morphology . A variety of ZnOs have been fabricated to meet practical applications [17–19]. More fascinating merits have been found for hierarchical three-dimensionally (3D) porous micro-/nano-architecture ZnO, which is self-assembled by low-dimensional nano-sized building blocks. The high porosity of hierarchical 3D ZnO greatly facilitates gas diffusion and mass transport in sensor and surface chemical reaction materials . However, most of the synthetic methods for these complex structures are restricted because of requiring rigorous conditions.
Apart from morphology discussed above, specific surface area is also one of the important factors to affect ZnO properties. So far, the specific surface area of commercial ZnO is very low, ranging 4–5 m2 · g−1. Using highly ordered mesoporous carbon templates, Polarz and co-workers synthesized ZnO of 195 m2 · g−1 . Larger surface area of 305 m2 · g−1 was achieved by Goswami et al., in which the use for drug delivery was emphasized and synthetic details were not clearly described . Although with relatively high surface area, the above ZnO does not show the hierarchical architecture. Currently, lower than 50 m2 · g−1 specific surface area has been reported for ZnO having the hierarchical structure [23–25]. Therefore, it is highly demanded to synthesize ZnO materials having both 3D hierarchical architecture and high specific surface area, especially in the case of using a simple and economical approach with a low-cost surfactant.
In our previous work, we have developed a preparation method of ZnO using the SL [26, 27]. The prepared ZnO shows good catalytic performance with specific surface area about 20–30 m2 · g−1. On the basis of previous work, the mixed solvent approach xhas been developed for the synthesis of ZnO nanomaterials. Hierarchically porous flake-array-flower structure with high specific surface area has been successfully fabricated. Its photocatalytic performance was tested, and the corresponding growth process and possible mechanism have been proposed. Herein, we have used the cost-effective SL surfactant, derived from industrially discarded lignin. So, this study allows for not only effective utilization of resources and protection of environment but also a good way of massively producing hierarchical porous nanomaterials of ZnO.
In this work, ZnO architectures were synthesized by a surfactant-assistant solution-based method. Zinc acetate (Zn(OAc)2 · 2H2O) (Fuchen chemical factory, Tianjin) was taken as Zn2+ source and sodium hydroxide (NaOH) (Tianda Chemical Corp., Tianjin) was used as the precipitating agent. SL (Tumen Qianjinfuli chemical Co. Ltd.) was used as surfactant material. According to manufacturer’s specifications, the parameters of the SL is that pH = 4.5–6, water content ≤8.5 %, insoluble matter ≤1.0 %, sugar content ≤12.0 %, Ca and Mg content ≤1.5 %, and inorganic salt (Na2SO4) ≤5.0 %.
In a typical experiment, 7 g of SL was added to 50 mL of deionized water to form the surfactant solution. And 3.5 g of Zinc acetate (0.53 M) was dissolved to 30 mL deionized water forming the Zn2+ solution. These two solutions were mixed under stirring and then 100-mL anhydrous ethanol was added to this mixed solution. We added 20 mL of NaOH (2.5 M) solution into the above mixture dropwise. The final solution was kept stirring for 30 min to form a homogeneous precursor solution. Finally, the precursor solution was kept stirring in 80 °C water bath for 5 h. Precipitate was harvested by centrifugation and washed and dried at 50 °C. Then, the as-obtained powders were calcined in air atmosphere at 500 °C for 2 h to obtain the final ZnO product.
X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-RC diffractometer using Cu Kα radiation, and scans were performed from (2θ) 5° to 80° by rate 4°/min. Scanning electron microscopy (SEM) images were taken with a field emission microscope FEI Sirion. The transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images of the samples were performed on a JEM-2100 electron microscope (JEOL, Japan) with an acceleration voltage of 200 kV. Carbon-coated copper grids were used as the sample holders. Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption experiments were carried out on the automated surface area and pore-size analyzer (ST-2000).
The photocatalytic activity of the prepared hierarchical ZnO to decompose the methylene blue (MB) was investigated. In the experiment, 0.1 g ZnO was dispersed in 40 mL of MB (10 mg · L−1) solution. And the stirring suspensions were exposed to the UV irradiation (6 W, 365 nm, WFH-203) under ambient conditions. After being kept in the dark for 30 min, the solution reaches the absorption-desorption equilibrium. The distance between UV light and the photoreaction vessel is 12 cm. In order to evaluate the efficiency of the degradation processes, the suspension was analyzed at a definite time interval, by recording variations at the maximum absorption around λ = 664 nm using a UV–vis spectrophotometer (Shanghaijingmi instrument Co., Ltd. UV762).
Synthetic Condition and SEM
Effect of SL on ZnO Morphology
Calcination and Cosurfactant of Ethanol
The effect of ethanol on the formation of the ZnO hierarchical flower-like structure has been examined as seen in Fig. 3a, c. Also, both optimal 7 g SL surfactant and calcination were used. Addition of ethanol to the reaction solution results in much more regular ZnO slices in Fig. 3a than those in Fig. 3c. It is rationalized that ethanol raised surface activity of SL and enhanced its coordination to Zn2+ ions. At the same time, once nuclei of ZnO were formed in the solution, the presence of ethanol decreases solubility of ZnO, which makes it easy to form small size nanostructure and eventually produces a hierarchical structure.
Structural Properties of Optimal Sample
Further detailed structural analysis of individual porous lamina was carried out using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The TEM image of the edge part of single flower-like ZnO shows that each lamina is constructed from agglomerative nanoflakes, which is featured with a wide size distribution ranging from 20 to 80 nm (Fig. 4c). It is found that sizes measured from TEM are larger than those calculated by Scherrer’s equation according to XRD. This discrepancy is attributed to the aspect ratio for non-spherically shaped crystallites, as well as the FWHM by the microstrain in the crystallite . These cross-linked nanoflakes show abundant porous structure, in accordance with the SEM results. Meanwhile, some mesopores with several nanometers in size can be clearly observed, which are scattering in the nanoflake array. Well-resolved two-dimensional lattice fringes observed in the HRTEM image (Fig. 4d) suggest good crystallinity of our synthesized nanoparticles. The interplanar distances between adjacent lattice planes were calculated to be 0.26 and 0.28 nm. They are attributed to d-spacing (002) and (100) planes of ZnO in the wurtzite phase, respectively.
Abundant pores in our synthesized ZnO materials are also evidenced by nitrogen adsorption-desorption isotherms and Barrett–Joyner–Halenda (BJH) pore-size distribution (Fig. 5b). The measurements find two kinds of pores in the nanoflake-array-flower ZnO. First is small mesopores, whose diameters range from 3 to 4 nm. They are formed by interconnected individual ZnO nanocrystallites. Meso and macropores are the second kind. Their diameters are relatively large, about 80 nm. These pores are constructed by nanoflakes’ interlacing, as observed in SEM images Fig. 4a, b).
It has been well established that SL is polyphenolic material, made from the copolymerization of three phenylpropanoid monomers such as coniferyl, sinapyl, and p-coumaryl alcohol. Due to containing both rich hydrophilic and hydrophobic sites, the macromolecular SL would form a structure of layer-by-layer self-assembly  in controlled experimental condition.
The association degree and the aggregation shape of SL are significantly affected by the SL concentration . For example, low concentration SL, using 1 ~ 3 g amount, shows relatively weak surface activity ; as a result, irregular slices of ZnO are formed . Too much amount of SL (for instance 11 g) added into the reaction system would destroy the layer-by-layer structure of SL, yielding the broccoli-like ZnO . In contrast, suitable SL concentration (7 g SL in 200 mL solvents) would retain surface activity as well as facilitate its lay-by-lay self-assembly. And hierarchical nanoflake-array-flower ZnO materials with high surface area are fabricated.
In brief, the formation of final ZnO architecture is attributed to a synergistic effect, i.e., the SL aggregation shape and the electrostatic attraction between SL and polar ZnO crystals. The absorption of ZnO enhances the layer-by-layer shape of SL, which in turn induces the ZnO to form flake-based structural unit and then self-assembly superstructures.
A SL-assisted self-assembly of certain morphology of ZnO architecture has been achieved via a simple precipitation synthetic approach. Various morphologies of ZnO have been fabricated by fine-tuning amount of SL. It is revealed that a suitable amount plays an important role in self-assembly forming the hierarchical nanoflake-array-flower morphology of ZnO. The interesting architecture is featured with assembled lamina, while each lamina is constructed from agglomerative sub-level nanoflake array. A wide size distribution ranging from 20 to 80 nm was measured for these nanoflakes. It is observed in the flower-like ZnO that nanoflakes are standing perpendicularly to face of the lamina. All these structural arrangement has eventually resulted in a great increase of specific surface area of ZnO, reaching as high as 82.9 m2 · g−1. This surface area is much higher than those of reported ZnO materials with 3D hierarchical nanostructure. The formation mechanism of this nanostructure has been proposed as a synergistic effect of the SL aggregation shape and polar ZnO crystals.
The hierarchical nanoflake-array-flower ZnO has been examined to show a superior photocatalytic performance of degrading methylene blue, even under a very low-power UV illumination. This is attributed to both featured hierarchical nanostructure and high surface area of our ZnO. Its abundant pores and nanoflake array, simultaneously, may serve as scaffold microenvironments, which would enhance photocatalytic activity. It is found that photocatalytic efficiency of our ZnO is comparable to that of Degussa P25 TiO2. Moreover, a perfect durability in the photodegradation of MB has been observed for our hierarchical ZnO.
In the synthesis of hierarchical nanostructure ZnO, the cost-effective SL surfactant has successfully applied, allowing for a large-scale production. Moreover, SL is derived from renewable biomass lignin that is main component of black liquor from the pulping industry and is usually discarded as waste. So the use of SL would not only recycle industrial waste and protect environment but also extend application of renewable lignin in adding its value. In brief, our facile and low-cost synthetic approach is expected to be promising for the preparation of ZnO-based photocatalyst materials.
This work was supported by the Fundamental Research Funds for the Central Universities (2572014DB02). The Natural Science Foundations of China (21273063, 30901136) and Heilongjiang Province (B201318) and the Program for New Century Excellent Talents in University (NCET-11-0958) are also greatly acknowledged.
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