Introduction

Over the past few decades, environmental and energy problems have attracted worldwide attention, especially, wastewater has become a threat of ecological crisis (Mehrjouei et al. 2015; Xiao et al. 2015; Teh et al. 2017). Traditional treatment of wastewater usually involves adsorption and coagulation (Nair et al. 2014; Teh et al. 2016) however, these methods only transfer or condense these contaminants but which are decomposed and required additional treatment to avoid secondary pollution (Liu et al. 2016b). Photocatalysis, a clean and efficient technology, has been widely used for wastewater treatment (Yao et al. 2016; Wang et al. 2016), gas degradation (Sano et al. 2013; Zhang et al. 2010) and is also potentially a route to the production of chemicals and fuels. It requires photocatalyst that must be cheap, non-toxic, easy to handle and above all efficient (Lv et al. 2016; Wang et al. 2017).

Zinc oxide (ZnO) is among the most appealing ones, being an n-type semiconductor with a wide band gap and is commonly used as a photocatalyst for contaminants degradation because of its unique electronic structure (Yu et al. 2016; Jo and Selvam 2015). As a photocatalyst, specific surface area plays an important role in the photocatalytic performance since by determining the number of active sites and the distance between the sites of photon absorption and electron/hole redox in order to enhance photocatalytic efficiency (Saikia et al. 2017; Ge et al. 2017). The facets exposed are also of great importance since not all of them present the same active species and their redox-potential. Thus, much attention has been paid to the synthesis of ZnO with morphology-controlled methods (Boury and Plumejeau 2015; Xia et al. 2016), such as nanotubes (Samadipakchin et al. 2017), nanorods (Liu 2016), nanowires (Chang 2014), nanospheres (Song et al. 2016) etc. Until now, different methods, such as hydrothermal process, sol–gel (Khodadadi et al. 2016), magnetron sputtering (Perez-Gonzalez et al. 2017) and chemical vapor deposition (Wang et al. 2012) have been used to prepare ZnO photocatalysts. Templated-directed mineralization is a promising nanofabrication technique because both physical and chemical cues from the template could control over the size and structure of the resulting materials (Ceylan et al. 2013). Recently, researchers have prepared some novel ZnO doping with metal Fe2+ (Ambika et al. 2016), AgNPs (Basuny et al. 2015) in order to enhance the photocatalytic activity. Additionally, biomass template not only fabricates ZnO advanced materials, but provides carbon sources doping ZnO (Zhao et al. 2017; Wang et al. 2014).

Cellulose is one of the most utilized and cheapest polysaccharides which is used as both chelating and stabilizing agents because of its abundant hydroxyl groups which support affinities and active sites to the precursors and growth of inorganic nanoparticles (Liu et al. 2011; Lidor-Shalev et al. 2017). Additionally, cellulose is a potentially reducing agent in the formation of metal nanoparticles such as Ag, Au and Pt (Boury and Plumejeau 2015). In the past, it has been already used as a template for the preparation of ZnO and the formation of ZnO/cellulose was reported (Lefatshe et al. 2017; Yuan et al. 2017). Cellulose is also known as being selectively oxidized by 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) agent that reacts with the primary hydroxyl groups (C-6) and oxidizes it into carboxyl groups (Saito et al. 2007). Two additional points must be considered: first, in solution, it is easier to form carboxylate than hydroxylate and secondly, it is known that carboxylate can better coordinate metal cations than do hydroxyl function. Therefore, TEMPO-oxidized cellulose should have stronger adsorption capacity toward coordination of Zn2+. We thought attractive to see if such enhanced reactivity can speed up, limit or orient the growth of growth of nanocrystals (Foresti et al. 2017; Liu et al. 2016a) in order to finally obtain ZnO nanomaterials with highly active crystal phase in optimal conditions. We check the effect of the carboxylate content by controlling the TEMPO oxidation. Interestingly, we found that this carboxylate content has also an impact on the C-content of the ZnO after calcination at 400 °C. It thus appears that we came across a simple process to access to carbon/ZnO nanocomposites. The effect of such treatment of the corresponding material was checked by looking to their photocatalytic activity estimated using methyl orange solution as target degradation molecule, the latter being commonly used in the photodegradation tests and allowing comparison (Khan et al. 2015; Danwittayakul et al. 2013).

In this paper, carbon/ZnO nanocomposites were successfully synthesized by using cellulose as a template but previously oxidized by TEMPO in agreement with well known method (Isogai et al. 2011). The chemical composition, crystallinity and morphology of carbon/ZnO nanocomposites were investigated by XRD-ray, BET, EDX, SEM and TEM etc. Effects of type of TEMPO-oxidized cellulose on photocatalytic degradation of methyl orange were also investigated.

Materials and methods

Materials

Zinc nitrate hexahydrate, urea, sodium hypochlorite, sodium bromide and sodium hydroxide, hydrochloric acid, absolute ethanol were of analytical grade and utilized without purification. TEMPO agent (98%) and methyl orange (96%) were obtained from Aladdin Industrial Corporation. Bamboo dissolving pulp (α-cellulose ≥ 95%) was obtained from Fujian Qingshan Paper Industry Co., Ltd. The bamboo cellulose fiber, a common fast-growing non-wood in southern China, was used to prepare TEMPO-oxidized cellulose without pretreatment before experiment.

Preparation of TEMPO-oxidized cellulose

Bamboo pulp (5.00 g) was dispersed in deionized water (500 mL) at room temperature (20 °C) using mechanical stirring (500 r/min) for 2 h. Then 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO solution in water, 0.1 mmol/g) and sodium bromide (NaBr in water, 1.0 mmol/g) were added into the suspension. Then sodium hypochlorite (NaClO in water, x mmol/g) was added drop by drop and pH was maintained at about 10 using sodium hydroxide solution (NaOH in water, 0.1 mmol/L). The obtained suspension was stirred (300 r/min) for 6 h at 20 °C and terminated by neutralization of hydrochloric acid in order to obtain pH 7. The wet TEMPO-oxidized cellulose fiber was then centrifuged at 10,000 r/min for 20 min and collected. Finally, the sample was freezing-dried at − 50 °C for 24 h. The final products were named for Blank, T3, T6 and T12, respectively according to the amount of NaClO (0, 3, 6, 12 mmol/g). Carboxyl contents in the TEMPO-oxidized celluloses were determined by the electric conductivity titration method (Saito and Isogai 2004).

Preparation of carbon/ZnO nanocomposites with TEMPO-oxidized cellulose

TEMPO-oxidized cellulose (1.89 g, as 1 wt.% water suspension) was dispersed in a 250 mL round bottomed flask with water (150 mL) and Zinc nitrate (Zn(NO3)2·6H2O, 2.97 g, 10 mmol) was added and magnetically stirred (400 r/min) for 2 h at 30 °C. Then urea (CH4ON2, 0.36 g, 6 mmol) was dissolved in the mixture. The latter was warmed in an oil bath at 90 °C for 4 h. After being cooled down, the suspension was centrifugated (8000 r/min) in order to obtain crude product, and washed with deionized water and absolute ethanol (two times, successively) and then vacuum dried at 50 °C for 24 h. Finally, the dried products were calcined at 400 °C in air atmosphere for 2 h, leading respectively to samples ZnO-FT, ZnO-T3, ZnO-T6 and ZnO-T12. The synthesis routes of carbon/ZnO nanocomposites were shown in Fig. 1.

Fig. 1
figure 1

The schematic illustration of the fabrication of carbon/ZnO nanocomposites templated by TEMPO-oxidized cellulose

Full size image

Characterizations

Optical morphologies of TEMPO-oxidized cellulose were determined by fiber analyzer (Techpap,MorFi Compact,France). X-ray diffraction (XRD) analysis was carried out by X-ray diffractometer (Rigaku, Ultima IV, Japan) using Cu Kα radiation, tube voltage 40 kV and tube current 45 mA. A field emission scanning electron microscope (Hitachi, SU8010, Japan) was used to observe the surface morphology and microstructure of carbon/ZnO samples. The detailed microstructures of all the samples were investigated by using a (JEOL, JEM-2100, Japan) transmission electron microscopy (TEM) with a field emission gun operating at 200 kV. The specific surface area and pore characteristics of carbon/ZnO samples were measured by N2 adsorption–desorption specific surface area analyzer (Micromeritics, ASAP 2020, USA). The chemical compositions of carbon/ZnO samples were studied by energy dispersive spectroscopy (HORIBA, XMX 1011, Japan). Diffuse reflectance UV–Vis (DR UV–Vis) spectra were recorded on Cary 500, Varian Corporation.

Evaluation of photocatalytic activity

The photocatalytic source was launched out by ultraviolet lamp with power about 36 W. The distance between the light source and the reaction tube was 10 cm. Many dyes are visible in water at concentrations as low as 1 mg/L, which is enough to present an aesthetic problem (Low et al. 2012; Subramonian and Wu 2014). In this study, a mixture containing of carbon/ZnO (25 mg, 0.309 mmol) was added to a solution of methyl orange (100 mL of MO in aqueous solution, 5 mg/L), magnetically stirred (300 r/min) at 30 °C for 30 min in the dark and irradiated range from 0 to 120 min without feeding oxygen. Then, irradiation was started and samples of 4 mL were withdrawn and filtered through a 0.22 μm PES membrane (Jin Teng Experimental Equipment Co., Ltd., 13 × 0.22 μm, Tianjin, China) at 15, 30, 60, 90 and 120 min, respectively. During the illumination, no additional oxygenation was introduced. The absorbance of the filtrate was measured by UV–Vis spectrophotometer (Shimadzu, UVmini-1240, Japan).

Results and discussion

Characteristics of different TEMPO-oxidized cellulose

Treatment of cellulose by TEMPO/NaClO was performed with different amount of NaClO (Fig. 2) with the aim to show its effect on the morphologies of the cellulose and carboxylic content. As expected, increasing of the amount of NaClO, result in an increase of the carboxyl groups content from 0.105 to 1.478 mmol/g as shown in Table 1. These new carboxyl groups are favorable for the reaction with zinc precursor as reported (Qian et al. 2012).

Fig. 2
figure 2

Photographs of TEMPO-oxidized cellulose with different amounts of NaClO (a 0 mmol/g, b 3 mmol/g, c 6 mmol/g, d 12 mmol/g)

Full size image
Table 1 Characteristics of different TEMPO-oxidized cellulose
Full size table

However, the oxidation process has another effect that is to shorten the fibers, a process that is ascribed to hydrolysis/oxidation of some less thick part of the fiber. Indeed, increasing the NaClO lead to an average decrease of the average fibers’ length from 1.559 to 0.361 mm with no important change of the diameter. Meanwhile, the ratio of length to width dropped from 66 to 15.

Characteristics of carbon/ZnO nanocrystals templated by TEMPO-oxidized cellulose

The crystalline phases of carbon/ZnO samples templated by TEMPO-oxidized cellulose are characterized by XRD measurements. For comparison, pure ZnO sample (ZnO-FT), which was without the use of TEMPO-oxidized cellulose is also tested. As shown in Fig. 3, after calcination, both ZnO-FT and ZnO-T exhibit the uniform wurtzite structure with characteristic 2θ values at (31.96°, 34.60°, 36.46°, 47.74°, 56.78°, 63.06°, 66.52°, 68.16°, 69.26°). The diffraction peaks originated from TEMPO-oxidized cellulose template are hardly detected in carbon/ZnO samples. Moreover, there is no remarkable shift of all diffraction peaks. To further investigate the crystallization characteristics of carbon/ZnO samples templated by TEMPO-oxidized cellulose, crystalline size from three main crystal planes were shown in (supporting information Table 1). Crystalline size of carbon/ZnO became smaller after template synthesis process due to oriental induction of TEMPO-oxidized cellulose rich in carboxyl and hydroxyl groups (Goncalves et al. 2009; Costa et al. 2013). Thus, these carbon/ZnO samples had potential advantage of the photodegradation of wastewater (Kumaresan et al. 2017).

Fig. 3
figure 3

XRD patterns of carbon/ZnO nanocrystals templated by different TEMPO-oxidized cellulose

Full size image

Figure 4 shows that the SEM images of the carbon/ZnO samples after calcination. It can be observed that they all present a similar morphology resulting from aggregated lamella. However, the carbon/ZnO sample prepared with the highest carboxylate-content present a remarkable difference, the lamella being less packed and aggregated. ZnO samples are composed of multidimensional structures. After calcinations process, the ZnO prepared by template method gradually changed from nanosheets to nanorods with the increase of carboxyl of TEMPO-oxidized cellulose. In our work, uniformly distributed carboxyl groups of TEMPO-oxidized cellulose can coordinate with Zn2+ to form stable and dispersive ZnO nanograins, and the polymer network prevent ZnO nanoparticles from agglomeration efficiently at the same time. So, the ZnO nanorods had been formed by TEMPO-oxidized cellulose as templates. However, the original cellulose has not to prevent the agglomeration of ZnO nanoparticles because of weak bonding between hydroxyl groups and Zn2+. Moreover, ZnO nanoparticles have not grown in orient direction along the original cellulose because of its larger size (Li et al. 2009; Zhao et al. 2017). Herein, ZnO nanosheets had been formed by original cellulose.

Fig. 4
figure 4

SEM images of ZnO-FT (a), ZnO-T3 (b), ZnO-T6 (c) and ZnO-T12 (d), TEM images of ZnO-FT (e) and ZnO-T12 (f), SAED images of ZnO-FT (g) and ZnO-T12 (h)

Full size image

The carbon/ZnO samples are further confirmed by Brunauer–Emmett–Teller (BET) measurements and the N2 adsorption–desorption isotherms are shown in Fig. 5. The samples displayed a typical isotherm curve with an obvious hysteresis loop at high relative pressure (P/P0 = 0.985), indicating the presence of mesoporous materials that probably results from intergranular void between lamella particles. The pore size distribution is determined by Barrett–Joyner–Halenda (BJH) analysis of the desorption branch of the isotherm from Fig. 4. It can be seen from Table 2 that the BET surface area does not increases with the content of carboxyl groups, except for the sample prepared with the highest carboxylate content; this fact being in agreement with the SEM observations. The large surface area and porosity will contribute to the photocatalytic activity (Chen et al. 2013a; Subramonian et al. 2017).

Fig. 5
figure 5

N2 adsorption–desorption isotherms of carbon/ZnO samples (a) and pore size distribution derived from desorption isotherm (b)

Full size image
Table 2 Characteristics of the carbon/ZnO templated by different TEMPO-oxidized cellulose
Full size table

From Energy-dispersive X-ray (EDX) characterization, all samples of carbon/ZnO templated by TEMPO-oxidized cellulose were investigated. It can be seen from Fig. 6 that ZnO samples were composed of zinc, oxygen and carbon. The presence of the latter because the calcination temperature was only 400 °C and with the increase of carboxyl content, the more residual carbon element was embedded into the ZnO samples. Zinc ions, the precursor of ZnO samples, could be adsorbed preferentially to the TEMPO-oxidized cellulose with more carboxyl groups. Thus, TEMPO-oxidized cellulose can combine the ZnO samples more closely. After calcinations, residual carbon from template was compounded into the ZnO samples, playing an important role to improve the photocatalytic activity.

Fig. 6
figure 6

EDX spectra of ZnO-FT (a), ZnO-T3 (b), ZnO-T6 (c) and ZnO-T12 (d)

Full size image

Photocatalytic activity of carbon/ZnO nanocrystals for methyl orange degradation

The UV–vis DR spectra of the as-prepared mesoporous carbon/ZnO samples are shown in Fig. 7a, b. By increasing the content of carboxyl groups of TEMPO-oxidized cellulose, the UV–vis DR spectra of carbon/ZnO by template are slightly shifted to visible light as compared with ZnO free of template (ZnO-FT). The absorbance edge of the ZnO by template is shifted from 417 nm to 432 nm (shown in supporting information Table 2) when the content of carboxyl groups is increased from 0.105 to 1.478 mmol/g, due to the fact that the carbon compounded into the ZnO crystals can slightly increase the light absorption ability. The band gap energy of the ZnO samples were calculated from the UV–vis DR spectra as following the formula: E g  = 1240/λ g , where E g (eV) is the band gap energy, λ g (nm) is absorbance edge. From Fig. 6b, along with the increase of the content of carboxyl groups from 0.105 to 1.478 mmol/g, the band gap energy of carbon/ZnO samples were decreased from 2.97 to 2.87 eV.

Fig. 7
figure 7

UV-Vis DR spectra for carbon/ZnO samples (a), UV–vis relationship between band gap energy and [Ahγ]1/2 of all the samples (b), photocatalytic activity of carbon/ZnO samples (c), reusability of ZnO-FT and ZnO-T12 on photocatalytic activity (d), photocatalytic activity of ZnO-T12 (e) and ZnO-FT (f) with different reaction time

Full size image

Photodegradation experiments of MO were studied under UV to investigate photocatalytic activity of ZnO templated by TEMPO-oxidized cellulose. Figure 7c displays the testing results for ZnO templated by TEMPO-oxidized cellulose with different carboxylate contents. It was observed that MO removal rate significantly increased by the carboxylate amount of cellulose template. Activity are similar whatever are the carboxylate content used for the preparation of the sample, as long as it reach the highest carboxylate contents of 1.478 mmol/g that clearly leads to a much better photocatalytic activity increased by a factor ≈ 2. After dark reaction, methyl orange only decreased by 1–2%. While the methyl orange photodegradation by ZnO-FT, ZnO-T3, ZnO-T6 and ZnO-T12 decreased by 65.92, 71.19, 72.76 and 96.11%, respectively. This is because ZnO-T12 with high surface area and narrow band was templated from TEMPO-oxidized cellulose with high content of carboxyl groups, making it easier to contact with pollutants and inspire more electron hole pairs to increase the photodegradation activity. The photocatalytic performance for degradation of methyl orange in the present work is superior to the previous reported (Yan et al. 2015; Chen et al. 2013b). Figure 7d shows the reusability of ZnO samples for photocatalytic degradation of methyl orange. It can be seen that ZnO-FT and ZnO-T12 samples have good photocatalytic stability. Methyl orange had been removed 38.68 and 79.78% initially by ZnO-FT and ZnO-T12, respectively. After 5 times for photodegradation, methyl orange can still hold 30.50 and 70.53% removal by ZnO-FT and ZnO-T12, respectively.

Figure 7e, f show full wavelength curves for photocatalytic degradation of methyl orange at different times of ZnO-T12 and ZnO-FT, respectively. It can be seen that the degradation rate of ZnO-T12 for methyl orange was significantly faster than ZnO-FT. With the extension of time, on the one hand, the maximum absorption peak of 463 nm became smaller indicating that the methyl orange concentration decreased; on the other hand, the absorption peak at 463 nm shifted to shorter wavelengths, implying that methyl orange degradation into other molecules (Kaur and Singhal 2014).

Conclusions

The novel carbon/ZnO photocatalyst had been successfully prepared via precipitation approach using TEMPO-oxidized cellulose as soft template. On the morphological structure, the ZnO samples were significantly affected by the addition of TEMPO-oxidized cellulose. The crystalline size of ZnO-T was smaller than that of ZnO-FT. And the surface area and pore volume of ZnO-T was increased at the same time. On the chemical characteristics, more carbon from TEMPO-oxidized cellulose was compounded into the ZnO crystals so that the band gap energy of ZnO had been narrowed resulting enhanced photocatalytic activity. Therefore, T12 was considered as the best bio-template to exhibit appropriate microstructure and chemical properties. ZnO-T12 displayed the highest photocatalytic activity in the degradation of methyl orange under UV irradiation with 96.11% degradation of methyl orange and good stability for five cycles.