Facile Synthesis of Electroconductive AZO @ TiO 2 Whiskers and Their Application in Textiles

1Key Laboratory of Science and Technology of Eco-Textiles, Jiangnan University, Ministry of Education, Wuxi 214122, China 2College of Textile & Clothing, Jiangnan University, Wuxi 214122, China 3State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science and Laboratory of Advanced Materials, Fudan University, Shanghai 200438, China 4Institute of Orthopaedics, The First Affiliated Hospital of Soochow University, Suzhou 215006, China 5Wuxi Entry-Exit Inspection and Quarantine Bureau, Wuxi 214101, China


Introduction
With the rapid development of nanotechnology, a great number of nanomaterials and nanostructures have been applied in diverse electronic and related fields.In recent years, many researchers have focused on the preparation of TiO 2 with different morphologies for special functional applications [1], such as nanotubes [2], nanorods [3][4][5], and nanowires [6].Rod-like TiO 2 whiskers are more likely to disperse uniformly compared with nanoparticles, guaranteeing the good inner homogeneity [7].Besides, the pathway for electron is readily formed as the rod materials overlap with each other, thus improving conductivity.However, the high electric resistance of TiO 2 limits its application.One effective approach to reduce the electric resistance is to dope electroconductive elements onto TiO 2 [8][9][10].You et al. [11] prepared monodispersed Cu-doped TiO 2 nanorods with tunable lengths (2-30 nm), diameters (2-5 nm), and doping concentrations (1.7-3.2%)through a low temperature hydrolytic route.The surface-functionalized method was employed to synthesize TiO 2 @CdS core-shell nanorods using citric acid as an agent by Das and De [12].Nevertheless, the reported works only focus on the photoelectron chemical behavior; the conductivity of the TiO 2 whiskers coated with the conductive material has rarely been studied.
Al doped ZnO (AZO) is promising as a cost-effective replacement for other transparent conductors, such as In 2 O 3 : Sn (ITO), SnO 2 : Sb (ATO), and SnO 2 : F (FTO), due to its high thermal stability, low price, and nontoxicity characteristics [13,14].P-type doping of ZnO can be achieved via the addition of Al as the dopant [15].AZO powders have been successfully prepared by a simple chemical coprecipitation method by Zhang et al. [16] and the photocatalytic and electrical properties were improved compared to those of ZnO.Wu et al. synthesized aluminum and gallium codoped ZnO powders (AGZ), which led to fine grain size ranging from 14 to 28 nm and low resistivity of 2.518 × 10 3 Ω⋅cm [17].The conductivity is caused by the replacement of Zn 2+ with Al 3+ , releasing excessive electrons into the conduction band.
In this study, the Al doped ZnO materials coatings onto TiO 2 whiskers were synthesized by a facile hydrothermal method.The microstructure, morphology, and electrical performance of the AZO@TiO 2 whiskers were investigated.The properties of the polypropylene nonwoven fabrics modified with the AZO@TiO 2 whiskers were further studied.with the molar ratio of (Ti)/(K 2 CO 3 ) = 4 were firstly mixed in distilled water.After dispersing in ultrasonic cleaner (100 W) for 30 min, the mixture was heated and blended with magnetic stirrer.When it became sticky, the dope was transferred into drying oven.After grinding process, the dried powders were performed in a muffle furnace at 1000 ∘ C for 10 h in order to form K 2 Ti 4 O 9 whisker bunches and then boiled for 8 h in deionized water to disperse into the sole whiskers.After the HCl (60 mol/L) treatment for 6 h in the thermostatic water bath, the precursor was annealed at 1000 ∘ C for 5 h with the speed of 5 ∘ /min to obtain TiO 2 whiskers.

Preparation of AZO
@TiO 2 Whiskers.Firstly, Zn(NO 3 ) 2 ⋅6H 2 O was dissolved in distilled water with an amount of Al(NO) 3 ⋅9H 2 O further added into the Zn(NO 3 ) 2 solution to obtain the desired solution A. The suspension was made with TiO 2 nanoparticles and 100 mL distilled water.After the ultrasonic treatment for 30 min, it was poured into a three-necked flask.A mixed acid solution of A was added to the suspension drop by drop at 65 ∘ C with stirring (300 rpm).The pH value was maintained with the buffer solution during the process.After a 120 min reaction, the white precipitation was filtered and washed with distilled water and dried in a drying oven at 70 ∘ C.After the sintering, the electroconductive AZO@TiO 2 whiskers were obtained.

Preparation of Antistatic Fabric.
After cleaning with detergents and washed with distilled water, all the polypropylene nonwoven fabrics (40 cm × 45 cm) were dried in the oven for 24 h.The synthesized size, consisting of sticking agent (Guangdong Gongchenrihua Co., Ltd., China) and thickening agent (Shenzhen Yongzhiqiang Chemical Technology Co., Ltd., China), was mixed with 0.1, 0.2, 0.3, 0.4, and 0.5 wt% electroconductive AZO@TiO 2 whiskers.The polypropylene nonwoven fabrics were prepared with the mixture by surface coating manually.After coating, the fabrics were dried in vacuum oven for 1 h.

Characterization.
The TiO 2 whiskers were fabricated from TiO 2 and K 2 CO 3 nanoparticles in a stable reactor, which could be heated up to 1000 ∘ C with a controllable heat rate of 10 ∘ /min by an automatic tube furnace (GSL 1600X, Hefei Kejing Materials Technology, China).
The morphology of the electroconductive AZO@TiO 2 whiskers was examined by scanning electron microscope (SEM) (SU-1510, Hitachi, Japan) operated at 10 kV.The samples were gold-sputtered on the surface to ensure the electrical conductivity and observed in vacuum environment to eliminate air impact.XRD analysis was performed using the X-ray powder diffraction (XRD) (D8 Advance, Bruker, Germany) with monochromated Cu-K radiation ( = 1.54183Å) across a 2 range of 10-80 ∘ with a rate of 0.1 s/step.The component of electroconductive AZO@TiO 2 whiskers was conducted with the X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo Fisher Scientific, America), employing a soft X-ray source of Al-K (ℎV = 1486.6eV) and operating at 150 W.
The surface resistivity of the electroconductive AZO@TiO 2 whiskers was manifested by the four-probe meter (SZT-2A, Tongchuang Electronic, China).The antistatic property of nonwoven fabric was measured by fabric induced electrostatic measuring instrument (YG (B) 342D, Darong Textile Instrument, China).

Orthogonal Test for Electroconductive AZO@TiO 2
Whiskers.Orthogonal design is one of the most effective and time-saving methods for the studies involving multiple variables in order to find out which factors (or variables) influence to the most extent properties of the target product [18].Besides, it provides a powerful way to find an optimal combination of a smaller number of arrays to obtain the optimum thus reducing more experiments associated with the analysis [19,20].Based on our previous experimental results, the main factors, including calcination temperature, pH, doping ratio (Al/Zn), and coating ratio (Zn/Ti), affected the formation process of electroconductive AZO@TiO 2 whiskers.In this study, the L 9 (2 3 ) orthogonal array of the Taguchi method [21] was adopted to investigate these factors and optimize the synthetic conditions.The factors and levels are listed in Table 1.Table 2 and Figure 1 present the experimental results of the L 9 (2 3 ) orthogonal array.The surface resistivity of resultant AZO@TiO 2 whiskers was measured.The surface resistivity at level I for factor  (, , or ) is as follows:   where , , , and  represent the numbers of levels for factors , , , and  and   ,   ,   , and   represent the ranges, which affect the influences of levels on the experimental result.According to the value of  in Table 2, the influence of four factors on the electroconductivity of AZO@TiO 2 whisker decreases in the order: doping ratio > pH > calcinations temperature > coating ratio.Doping ratio and pH value are the dominant factors over others.
Our previous studies showed that the conductivity was enhanced with the increase of coating ratio (Sn/Ti) when TiO 2 nanoparticles were encapsulated by ATO [22].In this study, we continue to optimize the doping ratio to find the lowest surface resistivity as shown in Table 3.The lowest surface resistivity was approached when the coating ratio was 45 at%.Nevertheless, when the coating ratio continues to increase, the electroconductivity was deteriorated, which indicates that the coating ratio has been saturated.
In the factor of doping ratio, the surface resistivity of AZO@TiO 2 whiskers decreases at first and then increases steadily at the optimal level of 2.5 at%.Al 3+ is ready to substitute Zn 2+ position to form negatively charged lattice defects as the radius of Al 3+ is smaller than Zn 2+ .Therefore, a free electron emerges.The conductivity would increase with the implement of Al 3+ contents when doping ratio is below 2.5 at%.The excess Al would form ZnAl 2 O 3 rather than replace Zn 2+ lattice site, which increases the resistivity.According to the report of Chen et al. [23] and Lu et al. [24], ZnAl 2 O 4 would lead to the increasing resistivity of AZO.
The pH value also has a big impact on the conductivity of AZO@TiO 2 whiskers, as it offers the opportunities for the suitable reactive environment.According to reaction equation (2), Zn(OH) 4 2− is prepared in alkaline conditions and uniformly coated on the surface of TiO 2 whiskers.With further dehydration reaction, the H 2 O is removed as shown in reaction equation (3): The influences of calcination temperature and coating ratio are relatively small that the resistivity fluctuated between 18 KΩ⋅cm and 26 KΩ⋅cm.The optimal condition of resistivity was as follows: doping ratio = 2.5 at%, pH = 8, calcination temperature = 400 ∘ C, and coating ratio = 45 at%.Due to the fact that the optimal experiment is out of nine groups in Table 2, we carried out another experiment according to the optimal condition and minimal surface resistivity of 3.6 KΩ⋅cm was approached.

Properties of
Electroconductive AZO@TiO 2 Whiskers.The electroconductive AZO@TiO 2 whiskers are yellow in color and small in size, as revealed in Figure 2(a).Figure 2(b) shows the SEM image of AZO@TiO 2 whiskers.AZO@TiO 2 whiskers are rod-like, with a diameter of 280-340 nm and a length of 1.95-3.75m.Furthermore, coating layer of AZO locates on the surface of TiO 2 whiskers, where the electrons could be freely spread among whiskers when they are connected with each other and readily conduct electricity within the materials.
XPS analysis was employed to further confirm the chemical composition of the synthetic whiskers.Figure 2(d) illustrates that synthetic whiskers consist of Zn, C1s, O, and Ti elements.Among them, the binding energy of C 1s peak was at 284.8 eV and used as the reference standard.
With scanning over the corresponding XPS spectra area, the binding energies for the Zn 2p region at around 1000 eV were analyzed.The peak located at 1044.43 eV was the same as Zn 2p

Antistatic Fabric Coated with AZO@TiO 2 Whiskers.
The coating method is of special interest in fabrication of nanostructured materials such as nanocomposite coatings carrying a variety of functionalities [29].A layer of electroconductive AZO@TiO 2 whiskers was coated on the surface of polypropylene nonwoven fabrics.Compared with the uncoated polypropylene nonwoven fabrics in Figure 3(a), the electroconductive AZO@TiO 2 whiskers were coated onto the surface (Figure 3(b)).The influence of the content of AZO@TiO 2 whiskers on the antistatic property of fabrics was shown in Figure 3(c)I.With increased AZO@TiO 2 whiskers, the half-life voltage of fabrics decreased sharply at first.When the content is more than 4 wt%, the tendency to decrease is slow, and the half-life voltage drops to 1.47 s.This suggests that the modified fabrics have good antistatic property.When the content is 8 wt%, the half-life voltage is 0.401 s (<0.5), which indicates that the coated fabrics reach the index of excellent antistatic property (the red line).Meanwhile, antistatic property after washing is very important for surface-modified fabrics.If it is poor, the antistatic property does not remain for long [30].Changes in the half-life voltage with different amount of AZO@TiO 2 whiskers after washing for 20 times are shown in Figure 3(c)II, similar to Figure 3(c)I.In addition, when the content of AZO@TiO 2 whiskers reached 10 wt%, the half-life voltage of surface-modified fabrics after washing for 20 times was 0.481 s (<0.5) still meeting the requirement of excellent antistatic property.The application of fiber materials is limited in some particular environment due to the high resistance.The electroconductive AZO@TiO 2 whiskers with unique rodshaped structure and excellent conductivity own a wide future application in the functional textiles.

Conclusion
In summary, the rod-like electroconductive AZO@TiO 2 whiskers were successfully fabricated by coating Al doped ZnO onto TiO 2 whiskers.The optimal synthetic condition 6 Journal of Nanomaterials for electroconductive AZO@TiO 2 whiskers was obtained with doping ratio of 2.5 at%, pH value of 8, calcination temperature of 400 ∘ C, and coating ratio of 45 at%.The electroconductive AZO@TiO 2 whiskers are light yellow with a length to diameter ratio of 5.6.The polypropylene nonwoven fabrics modified with AZO@TiO 2 whiskers exhibited excellent antistatic performance and laundering durability, indicating wide application of AZO@TiO 2 whiskers in the antistatic textiles.

Figure 1 :
Figure 1: The trends between factors and levels and surface resistivity.

Figure 3 :
Figure 3: (a) Pure polypropylene nonwoven fabrics; (b) polypropylene nonwoven fabrics coated with AZO@TiO 2 whiskers; (c) the relationship between the content of AZO@TiO 2 whiskers and fading period of half-life voltage before washing (I) and after washing for 20 times (II).
2.1.Materials.TiO 2 nanoparticles with an average diameter of 250 ± 35 nm were purchased from Jianghu Chemical Industry Co., Ltd., China.Al(NO) 3 ⋅9H 2 O, Zn(NO 3 ) 2 ⋅6H 2 O, and NaOH were purchased from Sinopharm Chemical Reagent Co. Ltd., China.All the chemicals and reagents used in this study were of analytical grade.Preparation of TiO 2 Whiskers.The TiO 2 whiskers were prepared by two-step synthesis.Generally, TiO 2 nanoparticles and K 2 CO 3

Table 1 :
Factors and levels of orthogonal design.Symbols , , , and  represent factors of coating ratio, doping ratio, pH, and calcination temperature.Symbols 1, 2, and 3 represent concentration levels of each factor.
The arrangements of columns , , , and  were decided by orthogonal design for 4 (factor) * 9 (run number); every row of run number represents one experimental replicate; every run was carried out twice.

Table 3 :
Coating ratio and surface resistivity.
[5],28] the other one located at 1021.53 eV was assigned to Zn 2p 1/2 , while those of Zn 3d, Zn 3p, and Zn 3s were located at 10.08 eV, 88.08 eV, and 139.08 eV, respectively.These peak positions agree with the reference values, suggesting a normal state of AZO in AZO@TiO 2 whisker[27,28].The binding energy of O 1s peak was at 530.08 eV, clarifying the existence of TiO 2 whisker.The Ti 2p 3/2 and Ti 2p 1/2 peaks also had obvious double peaks at binding energies of 458.53 eV and 475.33 eV, respectively, indicting a normal state of TiO 2 in AZO@TiO 2 whisker[5].