Preparation and characterization of regenerated cellulose/TiO2/ZnOnanocomposites and its photocatalytic activity

Chaorong Li n, Qiuyan Liu, Shunxin Shu, Yong Xie, Yongqiang Zhao, Benyong Chen,Wenjun DongDepartment of Physics and Key Laboratory of ATMMT Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, PR China

a r t i c l e i n f o

Article history:Received 13 September 2013Accepted 2 December 2013Available online 10 December 2013

Keywords:PhotocatalyticNanocompositesNanofibersCelluloseHydrothermal

a b s t r a c t

Regenerated cellulose (RC)/TiO2/ZnO nanocomposites have been prepared via hydrothermally grown ZnOon the RC/TiO2 nanofibers. The RC/TiO2 nanofibers were prepared by electrospinning technologyfollowed via sol–gel process. Results show that a plenty of amorphous TiO2 sol particles is dispersedon the surfaces of the RC nanofibers, and ZnO (wurtzite structure) nanosheets could be fixed on thesurface of RC/TiO2 nanofibers. The photocatalytic activities indicate that the RC/TiO2/ZnO nanocompo-sites perform a higher photocatalytic property than RC/TiO2 nanocomposites nanofibers or pure ZnOnanosheets.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Cellulose has attracted much attention because of its outstand-ing properties, such as high flexibility, environmental protectingcapability and biodegradability [1]. Particularly, electrospun cellu-lose nanofibers have a number of fascinating characteristics, whichmade them be a competing candidate for many important appli-cations, such as tissue scaffolds, protective clothing and opticalelectronics [2–4]. Consequently, creating nanomaterials withpeculiar properties on the cellulose nanofibers substrates becomesthe frontiers of cellulose science [5]. Zhang et al. [6] obtained theporous renewable cellulose nanofiber by wet spinning method.The nanofibers were used as the template for the synthesis ofTiO2/cellulose nanofibers, which displayed better catalytic abilityto the degration of methyl orange. Furthermore, the cellulosenanofibers are considered as excellent host materials for nano-particles because they can improve the stability, retain the specialmorphology, and control the growth of nanoparticles, what'smore, they can combine the properties of nanoparticles withcellulose nanofibers [7–9].

Among various semiconductor oxides, TiO2 and ZnO are twokinds of important materials due to their excellent electronic,chemical and optical properties, hence they have importantapplications in a variety of fields such as solar energy conversion,photocatalysis for self-cleaning surface, air and water purificationetc [10,11]. Especially, TiO2 has been proved to be the most

excellent photocatalyst for the oxidative degradation of organiccompounds under ultraviolet (UV) irradiation. TiO2 or ZnO photo-catalysts individually coated on cellulose nanofibers have beenreported [6,12,13], however, the cellulose was modified with theircombinations has rarely been investigated.

In this paper, we have synthesized the binary oxide catalystTiO2/ZnO on the regenerated nanofibers by a facile method. Thephotocatalytic performance of RC/TiO2/ZnO also was investigated.The obtained composites may provide a new and green photo-catalytic material to solve the problem of organic pollution.

2. Experimental

Preparation of heterostructure RC/TiO2/ZnO nanocomposites:All chemical reagents are analytical grade and directly used with-out further purification. In our previous research [12], the RC/TiO2

nanocomposites were prepared on the basis of RC electrospunnanofibers. In a typical experiment, 0.075 mmol Na3C6H5O7

and 0.75 mmol hexamethylenetetramine (HMT) were added into0.75 mmol Zn(Ac)2 �H2O solution. The mixture solution wasdissolved in 25 ml H2O under magnetic stirring for 30 min atroom temperature, finally forming a homogeneous and clarifyingsolution. Afterward the right amount of RC/TiO2 nanocompositeswere transferred into a 30 ml autoclave, then sealed in an electricoven under 100 1C for 8 h. After the reactor was cooled down toroom temperature, the as-obtained nanofibers were washed withanhydrous ethanol and deionized water twice respectively, thenthe heterostructure RC/TiO2/ZnO nanocomposites were obtained.

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Materials Letters

0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.matlet.2013.12.009

n Corresponding author. Tel./fax: þ86 571 8684 3575.E-mail addresses: crli@zstu.edu.cn, chaorong_li@hotmail.com (C. Li).

Materials Letters 117 (2014) 234–236

Characterization: The morphology of the RC/TiO2/ZnO nano-composites nanofibers was characterized by a field emissionscanning electron microscope (FESEM, Hitachi S-4800) and trans-mission electron microscope (TEM, JEM-2100, 200 kV). The phasestructures of the RC/TiO2/ZnO nanocomposites were studiedby powder X-ray diffraction (XRD, Bruker AXS D8-discover). Thephotocatalytic activity towards the degradation of RhB underultraviolet irradiation (PHILIPS, 365 nm) was investigated byUV–vis absorption (Hitachi U-3900). In this experiment, appro-priate RC/TiO2/ZnO nanocomposites were added into 2�10�5 MRhB solutions for photocatalytic examination under magneticstirring. The UV–vis spectra of the RhB solution were collectedevery 40 min.

3. Results and discussions

Fig. 1a and b shows that some small particles were grown onthe surface of cellulose acetate fiber(CA) and RC. In addition, wecould easily found that large number of ZnO nanosheets (inset ofFig. 1c) is formed on the RC/TiO2. Thus it is reasonable to speculatethat the hydroxyl group on the surface of fibers is very importantto the formation of ZnO nanosheets. Furthermore, from Fig. 1c wecould observe clearly that the surface morphology of RC/TiO2/ZnOwas very smooth and uniform. Plenty of ZnO nanosheets weresuccessfully grown on the RC/TiO2 fibers, that can be verified fromthe strong peak of Zn in the EDS spectrum in Fig. 1d.

X-ray power diffraction (XRD) analysis was applied to examinethe crystal phase structure of the RC/TiO2/ZnO nanofibers. Thecurve a in Fig. 2 shows that broad diffraction peaks of theregenerated cellulose nanofibers are at 151 to 251, which can beascribed to the amorphous cellulose structure. There are noobvious TiO2 characteristic diffraction peaks in curve a, whichindicated that TiO2 is amorphous. Many sharp and strong peakscan be observed in curve b, which can be indexed to the diffractionpeaks of ZnO with wurtzit structure (JCPDS 43-1002).

The RC/TiO2/ZnO nanofibers microstructure were inspected bythe transmission electron microscopy (TEM). The low magnifica-tion TEM image of the RC/TiO2/ZnO nanofibers are displayed inFig. 3a. One can see that the nanofiber is composed of a largequantity of ZnO nanosheets. Meanwhile, a high-resolution image(HRTEM) of the RC/TiO2/ZnO nanofibers is shown in Fig. 3b. Theinterplanar distances of nanosheets are 0.278 nm, correspondingto (1 0 0) planes of hexagonal wurtzite structure of ZnO, whichclearly demonstrate the crystal lattices of ZnO nanosheets in theRC/TiO2/ZnO nanofibers.

Photocatalytic activity: The degradation of RhB has beenemployed as a model reaction to evaluate the photocatalyticactivity of the material. For comparison, the self-degradation ofRhB under UV irradiation without catalyst was measured (inset ofFig. 4a). The absorption spectra of RhB solution have a tinydecrease, which indicate that the self-degradation of RhB couldbe neglected. However, In Fig. 3b, the major absorption peak

Fig. 1. SEM images of (a) CA/ZnO nanofibers, (b) regenerated cellulose/ZnO nanofibers, (c) the RC/TiO2/ZnO nanocomposite fibers. The inset is the zoom of a nanocompositefiber, (d) EDS spectrum of RC/TiO2/ZnO nanocomposites fibers.

Fig. 2. XRD patterns of (a) RC/TiO2 nanocomposites fibers, (b) RC/TiO2/ZnOnanocomposites fibers.

C. Li et al. / Materials Letters 117 (2014) 234–236 235

intensity of RhB around 550 nm diminished gradually under UVirradiation in the presence of the RC/TiO2/ZnO nanofibers andnearly disappeared after irradiation for 200 min. This indicated thatcomposite nanofibers possessed high photodegradation efficiency.

The degradation efficiency of the samples is defined as (A0�A)/A0,where A0 and A are the value of UV maximum absorption peakof initial RhB and RhB after catalyzing, respectively. After UVirradiation for 200 min, only 10% and 70% of RhB molecular weredecomposed by RC/TiO2 nanofibers and pure ZnO nanosheetsrespectively, while almost 97% was decomposed by RC/TiO2/ZnOcomposites nanofibers in Fig. 4b. The RC/TiO2/ZnO compositesnanofibers show the highest photocatalytic activity comparedwith RC/TiO2 nanofibers and pure ZnO nanosheets. The higherphotocatalytic activity of RC/TiO2/ZnO composites nanofibers canbe mainly attributed to intrinsic properties of the material [14].(1) The TiO2/ZnO heterostructure drives the photoelectrons on theZnO to the TiO2 immediately upon light absorption, hence forciblyseparating the pairs of electrons/holes on the surface, and redu-cing their recombination. (2) The combination of crystalline ZnOand amorphous RC/TiO2 generates more active catalytic centers,which facilitates the photocatalytic performance.

4. Conclusions

The RC/TiO2/ZnO composites nanofibers were successfully fabri-cated using electrospinning, sol–gel process and hydrothermalmethod. Heterostructure of the RC/ZnO/TiO2 composited nanofibersconsists of amorphous TiO2 and hexagonal (wurtzite) structure ofZnO nanosheets is formed. The obtained RC/TiO2/ZnO compositesnanofibers show excellent photocatalytic efficiency. The RC/TiO2/ZnO composite nanofibers embody excellent integration of highlyenhanced physical and chemical properties for improved mass

transfer, and photogenerated electrons and holes separation com-pared to the RC/TiO2 nanofibers and pure ZnO nanosheets. Thisfacile method may be extended to synthesize other binary orternary complex oxide nanostructures based on cellulose. It isexpected that RC/TiO2/ZnO composited nanofibers will havepromising applications in the treatment of organic-polluted waterand air purification.

Acknowledgements

This work was supported by NSF of China (Nos. 51172209 and91122022) and the Program for Changjiang Scholars and Innova-tive Research Team in University (PCSIRT).

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Fig. 3. TEM and HRTEM images of regenerated cellulose/TiO2/ZnO nanocomposites fibers.

Fig. 4. (a) Absorption spectra of RhB solution under UV irradiation at different periods of the time in the presence of regenerated cellulose/TiO2/ZnO nanocomposite fibers.The inset was the absorption spectrum of the RhB solution under UV light without any other catalyst, (b) curves of photocatalytic degradation on RhB with different catalysts.

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