Facile preparation and photocatalytic performance of anatase TiO2/nanocellulose composite

Anatase TiO2/nanocellulose composite was prepared for the first time via a onestep method at a relatively low temperature by using cellulose nanofibers as carrier and tetrabutyl titanate as titanium precursor. The morphology, structure and element composition of the composite were characterized by SEM, EDS, TEM, XRD and XPS. The specific surface area and thermal stability of the composite were investigated by N2 adsorption-desorption and thermogravimetric analysis, respectively. In addition, the prepared composite was used for the photocatalytic degradation of methyl orange (aqueous solution, 40 mg·L). It was found that the composite had a good morphology and anatase crystal structure, and Ti-O-C bond was formed between TiO2 and nanocellulose. The specific surface area of composite was increased and the thermal stability was decreased compared with the cellulose nanofiber. Moreover, the degradation rate of methyl orange was achieved as 99.72% within 30 min, and no


Introduction
Organic dyes are widely used in textile, paper and dye industries. However, wastewater produced by this process is toxic and difficult to degrade, which is harmful to humans' health and the environment. Therefore, it is highly urgent to explore effective methods for the degradation of organic dye wastewater. At present, the methods for treating organic dye wastewater mainly include physical adsorption, membrane filtration, ion exchange, chemical oxidation, biochemistry, etc. [1][2][3][4][5][6], but these methods have some disadvantages (such as incomplete degradation of pollutants and secondary pollution). In this regard, photocatalytic degradation is considered as one of the most effective methods for the treatment of organic dye wastewater due to its advantages such as simplicity, high efficiency, and environmentally-friendly characteristic [7][8][9][10], and the organic dyes can be degraded into non-toxic and harmless molecules (such as CO2 and H2O) under light conditions by using photocatalysts [10][11][12].
As a semiconductor, nano TiO2 has been widely used in the field of photocatalytic degradation owing to the advantages (such as good stability and non-toxicity) [13][14].
As for TiO2, there are mainly three crystal structures: rutile, anatase and brookite.
Among them, anatase has better photocatalytic activity and is most widely used in practical applications due to its wider band gap [15]. However, TiO2 nanoparticle tends 3 to agglomerate inevitably in practical photocatalytic applications and it is also difficult to recycle. Therefore, in recent years, researchers have reported the strategy by supporting nanoparticles on various carriers (such as glass, stainless steel [16][17][18], cellulose [19][20][21][22][23][24][25]) to overcome the above problem, meanwhile, the construction of TiO2based composite would be beneficial to achieve better photocatalytic performance.
As the most abundant natural biomass resource, cellulose has the advantages of non-toxic, environmentally friendly, and renewable. Therefore, the preparation of nano TiO2 composite with cellulose as a carrier has become a research hotspot in this field.
According to the reports, there are mainly four preparation methods: 1) TiO2 gel film/cellulose composite material was prepared via a sol-gel method and then it was calcined to obtain anatase TiO2/nanocellulose composite [19][20]; 2) TiO2 gel film/cellulose composite was first prepared by sol-gel method with multilayer deposition, and then followed by hydrothermal treatment to get the anatase TiO2/cellulose composite [21]. 3) Nanocellulose aerogel was prepared before tetrabutyl titanate adsorbed, and followed by hydrothermal treatment to get the TiO2/nanocellulose composite [22]. 4) Nano TiO2 was first prepared, and then it was mixed with nanocellulose solution to prepare the TiO2/nanocellulose composite [23][24]. However, the above methods included multi-step synthesis and the preparation process was relatively complicated, and the conversion of amorphous or rutile TiO2 into anatase TiO2 usually required high temperature. In this regard, this work reported the one-step method to synthesize anatase TiO2/cellulose composites at a lower temperature by using flexible cellulose nanofibers as a carrier. Tetrabutyl titanate was used as titanium precursor, ethanol and water was used as solvent, and the mixture was stirred for 5 h at 55 °C, and followed by aging, washing and freeze-drying to get the anatase TiO2/nanocellulose composite, and it was found that the prepared composite exhibited excellent performance for the degradation of methyl orange.

SEM and EDS analysis
The surface morphology and element composition of the sample was investigated by scanning electron microscope equipped with an energy spectrum detector, and the results were shown in Figure 1. It could be seen from Figure 1a that three-dimensional network structure of cellulose interwoven by fine nanofibrils still remained, and numerous nanopores on the surface were also observed. In addition, it was found that a large number of TiO2 nanoparticles were supported uniformly on nanocellulose.

TEM analysis
The microstructure of the sample was further investigated by transmission electron microscope (TEM). It could be found from Figure 2a that the diameter of the nano TiO2 nanoparticles was about 30 nm with relatively uniform size, and the particles agglomerated to form an irregular nanoporous structure. In Figure 2b, a lattice fringe with an interplanar spacing of 0.353 nm was obviously observed, which was assigned to (101) plane of anatase TiO2 [25]. In addition, this result also indicated that the prepared TiO2 had good crystallinity.   Figure 4c were ascribed to Ti 2p3/2 and Ti 2p1/2, respectively, indicating that the existence of Ti 4+ among the composite [26]. Compared to that of the bare TiO2, the binding energy of Ti 2p of the composite shifted to higher position, indicating that the electronic environment of Ti had changed due to the existence of Ti-O-C bond [27]. Such a shift of binding energy showed that TiO2 was loaded on nanocellulose by chemical bonds with a strong chemical effect. Figure

N2 adsorption-desorption analysis
The specific surface area, pore specific surface area, pore volume and pore diameter of CNF and TiO2/CNF were measured by N2 adsorption-desorption, and the results were shown in Table 1. It can be seen that the specific surface area and pore volume of the TiO2-CNF composite were 54.2 m 2 ·g -1 and 0.0253 cm 3 ·g -1 , respectively, which were significantly increased compared to those of CNF. The pore specific surface area was 50.099 m 2 ·g -1 , which accounted for up to 92.46% of the total specific surface area. That was due to the fact that tetrabutyl titanate had penetrated into the voids of the nanocellulose during the preparation process and chemically interacted with its hydroxyl groups, which destroyed the cross-linked structure of CNF and promoted the formation of mesopores. On the other hand, the formation of nano TiO2 promoted the formation of three-dimensional structures inside and on the surface of the composite. However, the pore size of TiO2/CNF did not change significantly compared to that of CNF, and the pore specific surface area ratio was very high, indicating that the generated TiO2 particles might have a large number of small nanopores.

Thermogravimetric analysis
Thermogravimetric analysis was used to characterize the thermal stability of CNF and TiO2/CNF. It was seen from Figure 5 that the thermal decomposition process of CNF and TiO2/CNF was divided into three stages: initial thermal decomposition, rapid thermal decomposition and slow thermal decomposition. The initial decomposition

Photocatalytic activity test
The photocatalytic degradation performance of bare TiO2, CNF and TiO2/CNF on methyl orange was investigated, and the results were shown in Figure 6. It was found that the photocatalytic degradation rates of bare TiO2 and TiO2/CNF to methyl orange increased with the extension of UV irradiation time. Compared to that of bare TiO2, the photocatalytic degradation rate of TiO2/CNF was much higher, and the bare CNF  degradation rate was tested, and the results were shown in Figure 7. It was found that as the number of cycles increased, the degradation rate decreased slightly. After five cycles, the degradation rate of the composite to methyl orange was still as high as 98.39%, no obvious loss in catalytic activity was observed, displaying excellent photocatalytic stability. The result showed that the TiO2 was supported on nanocellulose strongly to form the TiO2/CNF composite, and because of the good loading performance, CNF still provided a transfer environment for the photogenerated electrons of TiO2, further reduced the photo-corrosion of TiO2 and ensured its good photocatalytic stability.  Degradation rate/% cycle-index

Conclusion
(1) The results of SEM, EDS, TEM, XRD and XPS showed that the anatase TiO2/nanocellulose composite with good crystallinity was successfully prepared by one-step method at low temperature. The degradation rate of methyl orange solution was as high as 99.72%, displaying fast and efficient photocatalytic performance.
(2) BET and TG results indicated that the combination of nanocellulose significantly increased the specific surface area and pore volume of TiO2/CNF composite, but the thermal stability was reduced, and the loading mass content of TiO2 was about 17.53%.
(3) TiO2/nanocellulose composite showed good recycling performance. After five cycles, the degradation rate of methyl orange was only slightly reduced, still as high as 98.39%. The prepared TiO2/CNF composite was a promising candidate for the degradation of organic dye wastewater.

Photocatalytic activity test
In order to study the photocatalytic performance of anatase TiO2/nanocellulose composite, a photocatalytic experiment was carried out with methyl orange as the Bare TiO2 was also prepared via the above method for the purpose of comparison.
In order to investigate the photocatalytic degradation stability, the sample after the photocatalytic reaction was centrifuged and the supernatant was removed. 20 mL of methyl orange solution (40 mg/L) was added again, and then the photocatalytic reaction was carried out for 30 min under the same light conditions. The degradation rate was determined, and it was repeated five times to investigate the change of its photocatalytic degradation performance.