1 Introduction

Nanocellulose, derived from the biopolymer cellulose, has received considerable attention owing to its many potential advantages over traditional materials such as the abundance of cellulose resources, renewability and biodegradability,exceptional mechanical properties, and nano-scaled dimensions[1]. In recent years, developing nanocellulosebased bionanomaterials with advanced functionalities for diversified applications has attracted increasing attention[2]. For instance, Henriksson et al[3] successfully achieved an eye-catching cellulose nanopaper with remarkably high toughness using wood nanofibrils. The as-prepared cellulose nanopaper exhibited a network composed of intertwined nanofibrils, with an aspect ratio exceeding 100 and a tensile index of 214 MPa[4].Sun et al[5] fabricated transparent cellulose films based on 2,2,6,6-tetramethylpiperidine-1-oxyl radical oxidized cellulose nanocrystals, which exhibited a high tensile strength of 236.5 MPa, close to that of industrial steel(250 MPa). In this regard, the feasibility and possibility of producing nanocellulose-based films with advanced functionality is of significant interest to academic and industrial communities. In particular, a nanocellulosebased film imparted with electro-conductivity may be promising for applications in a wide range of fields such as electronics, optoelectronics, and energy storage devices[6].

In the production of nanocellulose-based films, the added electro-conductive filler should be preferably considered to render the obtained film electroconductive. The fillers currently used for conferring electro-conductivity to nanocellulose-based films mainly include graphite, carbon black, carbon fiber,carbon nanotubes, and graphene, as well as polypyrrole and polyaniline. Among these electro-conductive fillers,graphene is one of the most promising candidates owing to its unique nanostructure and excellent physical properties. The most common route obtaining bulk quantities of graphene begins with the oxidation of graphite to graphene oxide (GO). However, the abundance of oxygen groups imparts GO with low electrical conductivity. Therefore, prior to the desired application, it is generally necessary to reduce GO.Currently, GO reduction may be performed through chemical methods and/or heat treatment processes[7-9].However, owing to the toxicity of chemical reductants such as hydrazine and the high temperature (＞500℃)required in the thermal reductions, the usual chemical and thermal reductions are not compatible with the current electronic and chemistry technologies, and hence, their extensive applications have been limited.Therefore, in addition to improving the current reduction processes, other effective reduction methods,including microwave and photocatalytic reductions,are considered feasible for the production of highquality reduced GO (RGO) nanosheets. Concerning the photocatalytic reduction processes, Xu et al[10] showed that Au, Pt, and Pd metallic nanoparticles adsorbed on GO sheets can reduce them with ethylene glycol in a catalytic process. Moreover, photocatalytic reduction of GO sheets using metal oxide semiconductors such as TiO2 was investigated[11-12]. Thus, the feasibility of photocatalytic reduction of GO and its application potential should be further explored.

Nanocrystalline cellulose (NCC), also called cellulose nanocrystals (CNC), is typically a rigid rodshaped monocrystalline cellulose domain (whisker)with widths of approximately 5～80 nm and lengths of 200～500 nm. In our previous works, processes for the production of NCC from cotton microcrystalline cellulose (MCC)[13] and recycled paper fiber[14] were developed, and the application of NCC as a reinforcing phase in nanocomposite films was demonstrated[6].Moreover, graphite[15], carbon nanotubes[16], and GO[17] were employed as fillers for the production of electro-conductive cellulosic paper. In the present work, photocatalytic reduction of GO was achieved in the presence of titanium dioxide (TiO2) as a catalyst.The microstructure of the as-prepared TiO2-RGO nanocomposite was characterized using Fouriertransform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA),scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Subsequently, the obtained TiO2-RGO nanocomposite was used as an electro-conductive filler for the production of NCC films via vacuum filtration. The rheological behavior of NCC suspensions and the electro-conductivity and mechanical properties of the NCC films as a function of the added TiO2-RGO nanocomposite were systematically investigated. This work offers an effective route for the production of electro-conductive NCC films, which may hold significant potential as transparent flexible substrates for future electronic device applications.

2 Experimental

2.1 Materials

Commercial MCC powder provided by Shanghai Tonnor Material Science Co., Ltd. (Shanghai, China)was used as the raw material for the extraction of NCC.Dialysis bags (Mw cut off 10,000) were purchased from Hangzhou Mike Chemical Instrument Co., Ltd.(Hangzhou, China). Graphite powder (particle size ＜20 µm) was purchased from Hangzhou Xinhua Paper Co., Ltd. Sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl, 37%), potassium permanganate (KMnO4),sodium nitrate (NaNO3), hydrogen peroxide (H2O2,30%), and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich. Distilled water was used for all the experiments.

2.2 Preparation of NCC

Rod-shaped NCC was prepared from cotton material according to the procedure described in our previous work[13]. Briefly, 10 g of MCC powder was gradually added to 100 mL of 65.0 wt% sulfuric acid aqueous solution. The hydrolysis was performed at 50℃for 2 h under continuous stirring. Subsequently,deionized water was added to terminate the hydrolysis.Subsequently, the resulting mixture was centrifuged at 11,000 r/min for 10 min to separate the NCC, which was thereafter washed with deionized water and repeatedly centrifuged for four times. The obtained colloidal suspension of NCC was filtered with deionized water in a dialysis bag for one day to a constant pH value of 7.0.Finally, the suspensions were subjected to freeze-drying and vacuum drying prior to the subsequent processing.

敦 宁(1980-)，男，河北石家庄人，河北大学政法学院副教授，法学博士，硕士生导师，研究方向：刑法学；

2.3 Photocatalytic reduction of GO

二是多种形式为补充。一是法院与某家律师事务所开展合作。如上城、余杭两家法院分别与一家律师事务所合作，由该律师事务所承接法院的调解案件。律所调解成功的案件，以当事人撤诉或法院出具调解文书形式结案。二是律师个人参与法院调解。律师个人作为某法院的特邀调解员按照法院安排参与调解工作，或者加入浙江“在线矛盾纠纷多元化解平台”上设立的网上律师调解中心，承接调解工作。调解结果亦由法院出具相关法律文书。三是律所自行设立律师调解工作室，直接接受当事人的委托开展调解工作。

2.4 Preparation of NCC/TiO2-RGO nanocomposite film

In a typical procedure, TiO2-RGO suspension (0.5 mg/mL) was added to the NCC suspension[20] in an ultrasonication bath for 10 min at room temperature,according to the mass ratio of NCC to TiO2-RGO of 99∶1. To prepare the NCC/TiO2-RGO nanocomposite film, each NCC/TiO2-RGO nanocomposite suspension was vacuum-filtered, followed by dissolution of the membrane filter and subsequent drying at room temperature to obtain dry films.

2.5 Characterization and analysis

五要建立健全水资源合理配置、高效利用、有效保护的规划体系，抓紧完善相关配套法规、制度和标准，细化各项政策举措，大力推进水资源管理法制化进程。

1) Resistance type: used to simulate lighting equipment etc;

The XRD patterns of the GO, RGO, and TiO2-RGO nanocomposite samples were characterized using an X-ray diffractometer (X’TRA-055, ARL, Switzerland)with Cu Kα radiation (λ=0.154 nm) at 50 kV and 100 mA.

Scattered radiation was detected in the range 2θ=3°～80°, at a scan rate of 3°/min. TGA (PerKin Elmer, USA) was used to investigate the thermal weight loss of all the samples. The samples were analyzed under nitrogen atmosphere and heating up to 800℃ at a heating rate of 20℃/min.

The electro-conductivity of the NCC film filled with various TiO2-RGO samples is shown in Fig.7. Upon adding 1% GO as a conductive filler, the conductivity of the composite NCC film became almost 0 S/m.The electro-conductivity of the composite NCC film increased slightly for the ratio of RGO to TiO2 of 10∶0(1% RGO as the conductive filler). Moreover, when the ratio of RGO to TiO2 was 9∶1, the electro-conductivity of the composite NCC film was 9.3 S/m, indicating the low reduction efficiency of pure RGO without TiO2.With the increase in the proportion of TiO2 added to the photocatalytic reaction, the electro-conductivity of the composite NCC-based film decreased gradually.However, TiO2 is non-conductive.With the increase in TiO2 content, the amount of catalyst gradually increased, whereas the proportion of the conductive filler RGO in the composites relatively decreased,leading to the decrease in the electro-conductivity of the NCC film. Therefore, in the current experiment,when the ratio of RGO to TiO2 was 9∶1, the electroconductivity of the NCC film appeared to achieve the desired value.

The molecular analyses of pure GO, RGO, and TiO2-RGO nanocomposites were performed using FT-IR spectra (Nicolet 5700 spectrometer Thermo Fisher Scientific, USA). The FT-IR spectra in the range 500～4000 cm-1 were obtained at a resolution of 0.09 cm-1,for which the samples were palletized with KBr powder.

The morphologies of all the samples were observed using SEM (FEI, SIRION 200) and TEM (JSM-2100,JEOL, Japan). The rheological properties of all the suspensions were evaluated using a cylinder rotary rheometer (Physica MCR301, Anton Paar, Austria)at 25℃. The electro-conductivity of the film samples was measured using the four-point probe method(SZ2258C). The mechanical properties of the NCC/TiO2-RGO nanocomposite film were evaluated using a universal tester for high-strength materials.

3 Results and discussion

3.1 Preparation of electro-conductive NCC film

3.3.1 Effect of TiO2-RGO on the rheological behavior of the NCC suspensions

Fig.1 Schematic illustration of the general process for fabrication of an electro-conductive NCC film

3.2 Effect of photocatalytic reduction on the microstructure of RGO

3.2.1 FT-IR spectra

A GO sample was prepared from graphite powder by following the modified Hummers method[18-19]. For the photocatalytic reduction of the GO sheets, first,0.09 g GO powder was dispersed in 200 mL ethanol(C2H5OH, Merck, ＞99.9%). The mixture was first sonicated (Qsonica Q1376, USA) for 1.5 h at 100 W to obtain GO. Subsequently, 0.01 g TiO2 powder was added and sonication was continued for 0.5 h at 100 W.Subsequently, the suspension was slowly stirred for 1 h in a black box and irradiated with a 110 mW/cm2 mercury lamp (peak wavelengths at 275, 350, and 660 nm) for 1.5 h at room temperature (25℃). The suspension was slowly stirred during the reduction process to ensure uniform irradiation of the TiO2-RGO suspension.

Fig.2 shows the FT-IR spectra of GO, RGO, and TiO2-RGO samples, which were obtained to determine the changes in the various oxygen-containing functional groups. The characteristic peak at 1726 cm-1 corresponds to the C=O stretching of COOH groups located at the edges of the oxidized graphite for GO and the broad overlapped peaks in the range 2300～3700, 1726,and 1622 cm-1 indicates that GO contains numerous—OH, —COOH, C=O, and other oxygen-containing functional groups. After reduction, for RGO, the peak around 1726 cm-1 was weakened, whereas for the TiO2-RGO composite, the peak in this vicinity almost disappeared, indicating that, after the GO reduction under UV irradiation, a small portion of the C=O functional groups on the GO was removed, and the reduction under UV irradiation was limited. With regard to TiO2-RGO, most of the C=O groups on the GO base were effectively removed when the photocatalyst TiO2 was added for the photocatalytic reduction,which indicates enhanced reduction efficiency compared with that of UV reduction. The characteristic peaks at 1622 cm-1 and 3400 cm-1 are attributed to the vibrational absorption of the O—H functional group, and the characteristic peak at 3400 cm-1 was absorbed by the vibration of the C—OH functional group. The apparent signs of weakening at 3400, 1726, and 1622 cm-1 for TiO2-RGO indicate that most of the —OH and —COOH groups on the GO substrate were removed via the photocatalytic reduction, but some oxygen-containing groups remained in RGO.In addition, the residual OH bands of the spectrum may partly originate from the high and strongly bound humidity content of GO[21]. The vibrational absorption peak of the C=O functional group at 1726 cm-1 can be regarded as the characteristic peak of GO rather than RGO, which is consistent with the result reported in the literature and may indicate that GO was effectively reduced to RGO[22].

Fig.2 FT-IR spectra of GO, RGO, and TiO2-RGO samples

3.2.2 XRD analysis

3.3.2 Effect of TiO2-RGO on the electro-conductivity of the NCC film

Fig.3 XRD patterns of GO, RGO, and TiO2-RGO samples

Fig.4 TGA of GO, RGO, and TiO2-RGO samples

3.2.3 Thermal stability analysis

The thermal decomposition behaviors of the GO, RGO,and TiO2-RGO samples were comparatively evaluated,and the corresponding curves are shown in Fig.4. It can be observed that, for the GO, RGO, and TiO2-RGO samples, the process of thermogravimetry can be roughly divided into three main stages. In the initial stage in the temperature range of 20～120℃, the surface moisture and interlayer moisture evaporation of GO, RGO, and TiO2-RGO naturally caused weight loss with the increase in temperature. The weight losses were 18.9%, 16.8%, and 13.5% for GO, RGO,and TiO2-RGO, respectively, and the weight loss rate was mainly affected by the degree of dryness. The second major weight loss phase was in the temperature range of 120～390℃, which was due to the thermal decomposition of the functional groups on the GO,RGO, and TiO2-RGO substrates. During this stage,the weight losses were 46.5%, 44.6%, and 36.2%,respectively, for GO, RGO, and TiO2-RGO, indicating that a large number of oxygen-containing functional groups such as —OH, —COOH, and C=O on GO were effectively reduced and removed. The weight loss rate of TiO2-RGO was much lower than those of GO and RGO, which is mainly attributed to the fewer functional groups on TiO2-RGO and also indicates that adding TiO2 enhanced the photocatalytic reduction and thermal stability significantly. The third stage was the gradual decomposition of graphene carbon skeleton in the temperature range of 390～660℃, and the final residual rates were 5.0%, 5.8%, and 12.9%,respectively, for GO, RGO, and TiO2-RGO. The TiO2-RGO composite contains a small amount of TiO2, resulting in its relatively high residual rate. In contrast, we can conclude that the thermal stability of GO is relatively poor and GO has strong water absorption. There was no significant improvement in the thermal stability of RGO obtained without the photocatalytic reaction with TiO2, indicating its low reduction efficiency. When nano-TiO2 was added to the photocatalytic reaction, the obtained TiO2-RGO composite had a remarkably improved thermal stability,indicating that GO was effectively reduced to RGO[25].

3.2.4 TEM images

The liquid micro-morphologies of GO and TiO2-RGO were determined using TEM (JSM-2100, JEOL, Japan)and the results are shown in Fig.5. After oxidative stripping, the multiple layers of graphite powder were decomposed into a single layer or a few layers of graphene sheet, which appeared brighter and less overlapped. From the TEM image of TiO2-RGO, it can be observed that RGO did not agglomerate or recombine in the layer after photocatalytic reduction,and the monolayer two-dimensional honeycomb structure did not change. In contrast, the carbon atoms showed a trend of further decreasing layers.Simultaneously, a small amount of nano-TiO2 was compounded on the carbon atom layer of RGO,indicating the successful and effective preparation of TiO2-RGO nanocomposites. The GO and TiO2-RGO samples obtained in this study are similar to those reported in the previous work[26].

Fig.5 TEM images of GO and TiO2-RGO samples

3.3 Application of TiO2-RGO nanocomposite in NCC film

The general process for the fabrication of an electroconductive NCC film is illustrated in Fig.1. NCC was initially isolated from the hydrolysis of MCC with sulfuric acid and thereafter used as the matrix for the production of the electro-conductive composite film. Subsequently, a modified Hummers method was introduced to prepare GO. Subsequently, GO was photocatalytically reduced in the presence of TiO2 as a catalyst. Finally, the electro-conductive NCC film was fabricated via vacuum filtration using the TiO2-RGO nanocomposite as an electro-conductive filler and NCC as the base material. In general, NCC, TiO2-RGO nanocomposite, and the target electro-conductive NCC film are composed of recyclable bio-based and carbon-based materials, which can largely satisfy the requirements of sustainable development strategies.Thus, the proposed concept not only broadens the application field of NCC, but also provides a theoretical and practical route to the research and development of electro-conductive NCC films.

In order to design the proper IECF length (LIECF) for generating different OAM modes, we calculate the Δneff using COMSOL software and the corresponding beat length (Λ) at a wavelength of 1550 nm, as listed in Table 1, where Λ can be expressed as Λ = λ/Δneff.

Fig.6 shows the rheological properties of NCC, NCC/GO, and NCC/TiO2-RGO composite sols. As the shear rate increased, the viscosity of the three sols experienced a significant decrease. This was probably due to a large number of hydrogen bonding interactions between the solvated NCC particles; therefore, the NCC sol formed a three-dimensional network of cross-linked structures, exhibiting good thixotropy.The addition of 1% GO to the NCC sol had a limited effect on the rheological behavior. When 1% TiO2-RGO was added to the system, the viscosity of the NCC/TiO2-RGO sol decreased compared with that of the NCC sol. In other words, the addition of the TiO2-RGO composite helped improve the dispersibility of the NCC/TiO2-RGO composite sol because RGO has a large π-conjugated system and can bind to small or high molecules with conjugated systems to enhance their dispersibility through π-π interactions and thus improve the dispersibility of the composite sol[27].

Fig.6 Viscosity curves of the NCC/TiO2-RGO composite sol

The XRD patterns of the GO, RGO, and TiO2-RGO nanocomposites samples are shown in Fig.3. The strong diffraction peak at 2θ=10° corresponds to the characteristic XRD peak of GO. However, the intensity of the diffraction peak of RGO was slightly weakened, which may be due to the poor reduction.The characteristic peak of the TiO2-RGO composite evidently decreased, indicating that UV irradiation had a certain reduction effect on GO. The addition of the photocatalyst nano-TiO2 to the light catalytic reaction could improve the reduction efficiency. The weaker diffraction peaks of the XRD curves of TiO2-RGO at 2θ=25°, 48°, 55°, and 62° are the characteristic diffraction peaks of TiO2[23], further illustrating the presence of TiO2 in the TiO2-RGO composite. In addition, the broad shoulder peak around 16°～17° is interpreted to be due to the incomplete intercalation and absorbed water[24].

事实上，关于企业高管薪酬的研究长久以来一直是研究中的热点问题。之所以会出现市场对于高管薪酬的质疑，很大程度上是因为公众认为不公平导致，高管给自己发放高薪酬被人们广为诟病。因此，国内企业必须根据市场供给情况建立一个“合理”的高管薪酬体系。这个体系使企业高级管理人员的薪酬在具有市场竞争优势的同时，又能够将起到激励约束的作用，恰当地将高级管理人员的绩效考核结果(包括企业的业绩增长、风险管控等)与薪酬挂钩起来。

Fig.7 Effect of the ratio of RGO to TiO2 on the electroconductivity on the NCC film

3.3.3 Effect of TiO2-RGO on the mechanical properties of the NCC film

在数据造假方面，传统的方法是选择该学科的一个专家，将文章送审，让学科专家验证数据真实性。互联网环境下，可以在百度的“知道”栏目上发布提问，或在博客、微博、论坛上发起讨论，经一番交锋，集众人之智慧，数据的真实性就自然水落石出。

Fig.8 Effect of addition of TiO2-RGO on the elastic modulus of the conductive NCC/TiO2-RGO film

The mechanical properties of the NCC/TiO2-RGO nanocomposite film were evaluated using an universal tester for high-strength materials. Various loadings of TiO2-RGO (with the control of the TiO2-RGO photocatalytic ratio of 9∶1) were measured, and the results are shown in Fig.8. It can be observed that the mechanical properties of the electro-conductive NCC film generally show a tendency to increase first and thereafter decrease with the addition of TiO2-RGO.Specifically, in the absence of TiO2-RGO, the elastic modulus of the NCC film was 3498 MPa, which increased to 3998 MPa with the addition of 1% TiO2-RGO. This is because the hydrogen bonds between NCC and RGO molecules enhance the interaction between the two substances, thus further improving the mechanical properties of the conductive NCC/TiO2-RGO film. Nevertheless, with the continuous increase in the content of TiO2-RGO, the elastic modulus of the conductive NCC/TiO2-RGO film decreased gradually,which may be largely attributed to the distribution of the conductive fillers. Fig.9 shows the tensile strength of the conductive NCC/TiO2-RGO film. The tensile strength of the NCC film was 16.0 MPa, whereas the tensile strength of the NCC film with 1% TiO2-RGO reached 18.1 MPa. Overall, the addition of TiO2-RGO can lead to enhanced mechanical properties of the conductive NCC/TiO2-RGO film; however, the addition level should be strictly controlled.

Fig.9 Effect of addition of TiO2-RGO on the tensile strength of the conductive NCC/TiO2-RGO film

Fig.10 SEM images of the conductive NCC/TiO2-RGO film

3.3.4 Morphology of the NCC film filled with TiO2-RGO

The morphologies of all the samples were observed using SEM (FEI, SIRION 200). Fig.10 shows the SEM surface morphologies of the NCC film and the conductive NCC/TiO2-RGO film. The NCC film(Fig.10(a)) and the conductive NCC/TiO2-RGO film(Fig.10(b)) are easily distinguishable. The surface of the conductive NCC/TiO2-RGO film was relatively smooth. In contrast to the apparent rod-shaped particles and relatively large gap observed on the NCC film (Fig.10(c)) at high magnification, the highmagnification surface of the conductive NCC/TiO2-RGO film (Fig.10(d)) displayed a uniform and compact surface, indicating that TiO2-RGO was favorable for improving the dispersion of the NCC sol. Moreover,the compatibility between the two parts appeared to be ideal. The addition of the TiO2-RGO nanocomposites also helped to improve the barrier properties of the composite NCC film.

CORS在山东沂源儒林河初设中的应用及思考………………………………… 崔居峰，张 盼，谢 振(14.55)

4 Conclusions

A flexible transparent electro-conductive film was prepared via vacuum filtration with ultrasonic dispersion,using NCC as the matrix and TiO2-RGO as the electroconductive filler. The TiO2-RGO nanocomposite was successfully obtained via photocatalytic reduction,and subsequently, the suitable conditions for the film fabrication were discussed. The microstructure,electrical conductivity, and mechanical properties were comprehensively investigated. The results showed that the composite electro-conductive films exhibited the desired conductivity and mechanical properties when RGO∶TiO2=9∶1 and the TiO2-RGO loading was 1%.The optimum electrical conductivity was 9.3 S/m, and the elastic modulus and tensile strength reached 3998 MPa and 18.1 MPa, respectively.SEM analysis showed that the surface of the electro-conductive NCC/TiO2-RGO film was smooth,uniform, and compact. This work offers a route for the production of electro-conductive NCC films, which may hold significant potential as transparent flexible substrates for future electronic device applications.

Acknowledgments

This work was financially supported by the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY14C160003, LQ16C160002),the National Natural Science Foundation of China (Grant No.31100442), the Public Projects of Zhejiang Province(Grant No. 2017C31059), Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Zhejiang Open Foundation of the Most Important Subjects (Grant No. 2016KF01), and 521 Talent Cultivation Program of Zhejiang Sci-Tech University (Grant No. 11110132521310).

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