Mojiao Zhou^a, Xuehui Gao^a, Yong Hu^a,∗, Jiafu Chen^b, Xiao Hu^c

^a Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, 321004, PR China

^b Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, 230026, PR China

^c School of Materials Science & Engineering, Nanyang Technological University, Singapore, 639798, Singapore

A b s t r a c t

Uniform hamburger-like mesoporous carbon-incorporated ZnO (MC-ZnO) nanoarchitectures were facilely prepared by a simple, economical and environmentally benign solvothermal method using ethylene glycol (EG) as solvent in the presence of glucose. The as-obtained samples possessed a high surface area of 104 m2 g−1 and narrow pore size distribution around 7.2 nm. The adsorption behavior of Congo red (CR) onto the products fitted well the Langmuir isotherm and the adsorption process followed the pseudo-second-order kinetic model. The maximum adsorption capacity of CR as 162 mg g−1 was achieved by Langmuir equation, while only 18.8 mg g−1 was found for by commercial ZnO nanoparticles. Furthermore, the as-prepared nanoarchitectures exhibited enhanced photocatalytic activity for the decolorization of photosensitized dyes (CR and rhodamine-B) under visible-light illumination. Therefore, the MC-ZnO nanoarchitectures developed in this work may be a promising potential material for wastewater treatment.

1. Introduction

In recent years, the development of nanotechnology has extended from the stage of individual nanocomponent to that of three-dimensional (3D) stacking nanoarchitectures assembled from low-dimensional building blocks [1]. Up to now, a wide variety of inorganic materials have been successfully prepared with hierarchical shapes via various strategies [2–4]. Due to low cost and potential advantage for large-scale production, solution-phase chemical route was one of the most promising methods [5]. However, narrow size distribution and high dispersibility were the prerequisites for assembling [6]. Therefore, exploring rational and simple synthetic methods for controlling construction of complex 3D nanoarchitectures offunctional materials remains as a challenging but exciting topic in materials science.

Zinc Oxide (ZnO), as one kind of versatile semiconductor nanomaterials, has been extensively applied in electronics, photoelectronics, optical devices and ecology owing to its significant quantum confinement effect, strong UV absorption properties, unique UVlaser emissionand excellent photocatalytic performance [7–9]. ZnO is extremely inclined to aligned along the c-axis to become the one-dimensional six prism shapes via a hydrothermal method [10]. However, the smaller aggregation rate allows ZnO particles have enough time to rotate so that further reduces the surface energy from the kinetic point of view in higher viscosity non-aqueous medium. By this way, there is certainly a strong thermodynamic driving force for oriented attachment to become nanospheres [11].

    

Herein, 3D uniform hamburger-like mesoporous carbonincorporated ZnO (MC-ZnO) nanoarchitectures are obtained by a simple, economical and environmentally benign solvothermal method in ethylene glycol (EG) solution in the presence of glucose. In particular, this synthesis does not involve any surfactants or toxic materials. In this case, glucose not only acts as precursor of carbon incorporation, but also plays a critical role of coordinating reagent in reducing the activity of nanobuilding blocks to prevent the crystal lattice from fusing directly among ZnO nanoparticles and promote the ordered self-assembly [12]. A plausible formation mechanism has been proposed based on systematic investigation of the assembly process. With carbon incorporation, specifific surface area and surface functional groups increased, the increasing electrostatic interaction and ion exchange reaction will further enhance adsorption capability [13,14]. Meanwhile, the photoresponce of ZnO can also extends toward visible light [15], resulting  in the enhanced decolorization effificiency of dyes. Thus, the asobtained 3D uniform hamburger-like MC-ZnO nanoarchitectures exhibit very promising performance in water treatment. As an example, we show that the products exhibit enhanced adsorption capability when evaluated as the adsorbents for the removal of dye pollutant Congo red (CR) from aqueous solution. For comparison, the removal capacity of CR by commercial ZnO nanoparticles was also studied.Additionally,the equilibriumisothermandkinetic characteristics of CRadsorption on 3D uniformhamburger-likeMC ZnO nanoarchitectures were investigated thoroughly. On the other hand, the decolorization behaviors of CR and rhodamine-B (RhB) under visible-light irradiation were also investigated by using MC ZnO nanoarchitectures, the as-obtained ZnO nanoparticles without glucose and commercial ZnO, respectively.

Photodecolorization of CR and RhB

Photocatalytic activities ofthe as-prepared hamburger-like MC-ZnO were evaluated by the decolorization of CR and RhB under visible-light irradiation of a 300W Xe lamp (CEL-HXF300) with a 420 nm cut off filter. The reaction cell was placed in a sealed black box with the top opened, and the cut off fifilter was placed to provide visible-light irradiation. All experiments were conducted at room temperature in air. In a typical process, 40 mg of as-prepared photocatalysts were added into 100 ml of CR solution (concentration: 50 mg L⁻¹) and RhB solution (concentration: 5 mg L⁻¹), respectively. After being dispersed in an ultrasonic bath for 5 min, the solution was stirred for 2 h in the dark to reach adsorption equilibrium between the catalyst and the solution and then was exposed to visible-light irradiation. The samples were collected by centrifugation at given time intervals to measure the dyes degradation concentration by UV–vis spectroscopy.

4. Conclusions

In this study, we developed a facile solvothermal route to synthesize uniform hamburger-like MC-ZnO nanoarchitectures in the presence of glucose, which concurred with “green” chemistry as it was simple and environmentally friendly. As carbon incorporated mesoporous self-assembly architectures, the samples have high surface area, superior adsorption capacity and enhanced photode colorization of dyes under visible-light irradiation. The adsorption kinetics and isotherms studies demonstrated that the adsorption process obeyed the pseudo-second-order kinetics and Langmuir isotherm model, respectively. The maximum adsorption capacity was 162 mg g−1 calculated by Langmuir equation. It is believed that this facile strategy is scalable to synthesize other carbonincorporated metal oxides and this fascinating adsorbent will find great potential applications in the adsorptions and decolorization of dyes from wastewaters.