Two-step deposition of Ag nanowires/Zn2SnO4 transparent conductive films for antistatic coatings

Silver nanowire (AgNW) networks play an important role in the transparent conductive electrodes or antistatic coatings. In this work, we describe a facile two-step method to fabricate AgNWs/Zn2SnO4 composite films. Long AgNWs with a high aspect ratio were prepared through a modified polyol method, in which the organic octylamine hydrochloride rather than the commonly used inorganic chlorides was used as the shape-controlling agent. The AgNW networks were fabricated on the glass substrate, on which the Zn2SnO4 film was deposited, forming robust AgNWs/Zn2SnO4 composite films. The as-prepared composite films have strong adhesion, high thermal stability, low sheet resistance (5–15 ohm sq−1) and high light transmittance (85–80%), indicating a promising application prospect for transparent conductive electrodes and antistatic coatings.


Introduction
Electrostatic charges can easily accumulate on the surfaces of many insulators, such as glass, ceramics, plastics, rubber and paper, which are harmful on many occasions. For example, static charges oen bring about the electrostatic adhesion of dust, and discharging may lead to breakdown of electronic components, and in some situations can even cause combustion or an explosion. [1][2][3] To solve the issue of charge accumulation, some conductive materials, such as 2-D layered MXENE, 4 conductive polymers, 5-10 conductive oxides, [11][12][13][14][15][16] carbon materials, [17][18][19] and metal nanowires, 20-28 are adopted as the antistatic materials to be coated on the surface of some insulating materials. Among these materials, the conductive polymers have relatively poor conductivity and stability, thus they are not suitable for actual applications especially in harsh space environmental conditions. 29 Conductive oxides such as indium-tinoxide (ITO), [30][31][32] uorine-doped SnO 2 (FTO), 33,34 antimony-doped SnO 2 (ATO) [35][36][37] and aluminium-doped ZnO (AZO) 38,39 have good stability and conductivity, but their applications are hindered by their vacuum-based preparation methods. The magnetron sputtering method cannot meet the requirements of new exible and stretchable electronic products and large-area space-cras for antistatic coatings. 12,13 Graphene and carbon nanotubes are good antistatic materials, but their preparation costs are high and their conductivity is relatively poor. [17][18][19] By contrast, metal copper and silver nanowires (AgNW), especially the latter, are of higher electrical conductivity, promising thermal stability, and low preparation cost, become the ideal candidate for the antistatic coating materials.
Like the conductive oxides, the conductive AgNW networks are of high light transmittance, which widen their application as the conducting layer for antistatic coatings and even as the transparent and conductive electrodes for optoelectronic devices. [20][21][22][23][24][25] Unlike the conductive oxides, the adhesion between AgNWs and the substrate is poor, thus an inorganic or organic matrix is needed to form a composite lm with AgNWs. The introduction of the matrix not only improves the adhesive force of the AgNWs, but also increases the stability of AgNWs and the electroconductivity of the antistatic layer. For instances, coated with TiO 2 nanoparticle lm, the stability of AgNWs against oxygen is greatly improved since the oxygen is isolated outside. The electron conductivity of TiO 2 helps to collect and transport the electron between the substrate and the AgNW network, namely, to improve the short-range conductivity. 40 In addition to TiO 2 , however, only a few oxides such as ZnO, 41 MoO 3 , 42,43 WO 3 , 44 SnO 2 (ref. 45 and 46) and AZO 47,48 have been adopted as the inorganic matrix for AgNWs, and more inorganic matrices still need to be developed. Herein, we demonstrate a new inorganic matrix, i.e. Zn 2 SnO 4 , as the matrix for AgNWs networks. In this work, long AgNWs with a high aspect ratio were prepared through a modied polyol method, in which the organic octylamine hydrochloride rather than the common inorganic chlorides was used as the shape-controlling agent. The conductive networks were prepared by spraying our homemade AgNWs on glass substrates. On the AgNW networks, the Zn 2 SnO 4 lm was deposited by spin-coating a Zn-Sn-O precursor solution, followed by annealing at 200 C. The asprepared AgNWs/Zn 2 SnO 4 composite lms showed high adhesion, good conductivity, high transmittance and excellent thermal stability, indicating that these AgNWs/Zn 2 SnO 4 composite lms have promising prospects in the eld of antistatic coatings.

Preparation of AgNWs
First, two stock solutions were prepared: (A) 2.5 mM octylamine hydrochloride, (B) 200 mM AgNO 3 in ethylene glycol. Next, 0.4 g PVP and 40 mL of EG were loaded into a 100 mL of ask. The solution was stirred at 100 C, and 2 mL of the solution A and 10 mL of the solution B were added into the ask to form a homogeneous solution. The solution was poured into a 100 mL of Teon liner of a stainless autoclave and was put in a pre-heated oven (115 C) and was incubated for 11 h. Aer that, the autoclave was took out and cooled to the room temperature. The as-prepared AgNWs were puried by ltering and then were collected and redispersed in ethanol.

Preparation of the Zn 2 SnO 4 precursor solution
0.2752 g zinc acetate was dissolved in a mixture of 0.5 mL of nbutyric acid, 0.5 mL of n-butylamine and 10 mL of ethanol by magnetic stirring and heating at 100 C, then the solution was cooled to the room temperature. Aer that, 0.1422 g tin dichloride was added to form a mixed Zn-Sn-O precursor solution. The as-prepared Zn 2 SnO 4 precursor solution was stored for future use.

Fabrication of AgNWs/Zn 2 SnO 4 composite lms
The glass substrates were washed in deionized water and ethanol for 30 min to remove the dust and impurities, and were dried by the nitrogen ow. The AgNW dispersion was spraycoated onto the glass substrate by a spray gun. The spray distance and pressure were 8.0 cm and 150 kPa, respectively. The density of nanowires in the AgNWs network was controlled by the spray time. Finally, Zn 2 SnO 4 precursor solution was spincoated (2600 rpm, 20 s) onto the AgNW networks and then was annealed at 200 C for 5 min to form the AgNWs/Zn 2 SnO 4 composite lm. Note that the thickness of Zn 2 SnO 4 thin lm can be changed by adjusting spin-coating rate or tuning the concentration of Zn 2 SnO 4 precursor solution (see Table S1 in ESI †), and the thickness of Zn 2 SnO 4 layer in the AgNWs/ Zn 2 SnO 4 composite lms used for all characterizations is about 24 nm only if otherwise pointed out.

Characterizations
The UV-vis transmission spectra were collected on a Metash UV-5200 spectrophotometer, recorded from 400-1000 nm. The Xray diffraction (XRD) pattern was measured on a Bruker D8 Focus diffractometer equipped with a Cu Ka radiation (l ¼ 0.15405 nm) source. The SEM images were acquired on a Hitachi-4800 eld-emission scanning electron microscope (SEM, Hitachi, Ltd., Japan), operated at an accelerating voltage of 10 kV. The optical images were captured on a Phenix HD digital camera (MC-D800U). The sheet resistance was measured by the four-point probe method with a Keithley 2400 source meter (sheet resistance ¼ measured resistance Â 4.532). Variable temperature experiment was carried out on a ET9000 electric transport measurement instrument (East Changing Inc., China) equipped with a 280 temperature controller and a Cryodyne refrigerator (CTI-Cryogenics Helix Technology Corporation, USA), accompanied by the transmission measurement at 550 nm on a Metash UV-5200 spectrophotometer.

Results and discussion
Silver nanowires of high purity were prepared by our modied polyol method, in which organic octylamine hydrochloride, instead of commonly used inorganic chlorides, was adopted as the shape-controling agent. Fig. 1a-c show the morphologies of as-prepared AgNWs, indicating that these AgNWs have an average diameter of $130 nm and an average length of $95 mm. Long AgNWs with high aspect ratios is benecial for lowering the sheet resistance and improving the transparency of the AgNW networks. The XRD pattern in Fig. 1d, including the peak positions and the intensity ratio of these peaks are consistent with those described in literature, 49 indicating that these AgNWs may have a pentagonally twinned structure, as is conrmed in Fig. 3c.
In order to fabricate better AgNW networks, three types of solution-based deposition methods, namely drop-casting, spincoating and spray-coating, have been tried. As shown in Fig. 2a, the AgNWs cannot be uniformly distributed on the glass substrate using the drop-casting method, causing a inhomogeneous resistance distribution in the conducting lm. We also did not get well distributed AgNW networks by the spin-coating method (see Fig. 2b). When the concentration of AgNWs ink is low, it is difficult to spin-coat AgNWs on the glass substrate due to centrifugal action, and the coffee ring may form, 50 which will severely affect the conductivity of the network. If the concentration of AgNWs ink is high, the conductivity of the lm will be good, but the light transmittance will be poor. Moreover, the AgNWs trend to agglomerate during the spin-coating process, resulting in a rough surface of the thin lm and a high resistance. Compared with the above two methods, the spraycoating, a well-known solution processing method that widely used in industrial laboratories for lm preparation, is a more effective way for distributing AgNWs, especially on some irregular surfaces. 51 As Fig. 2c shows, the glass substrate with the AgNW networks prepared by a spray-coating method has a relatively smooth surface. It was observed that the AgNWs are uniformly distributed on the glass and overlaped each other (Fig. 3a), forming a conductive network. More importantly, the density of AgNWs in the network can be tuned simply by altering the spray time, accompanied by a simultaneous regulation of the resistance and transmission of the network. Generally, the as-prepared pristine AgNW networks cannot be used directly as the antistatic coating due to their weak adhesion with the substrate. The AgNWs are usually added into an inorganic or organic matrix rst and then are casted onto the substrate to form a composite lm, which can signicantly improve the adhesion between the AgNW networks and the substrate. Recently, a few inorganic oxides such as TiO 2 , ZnO, MoO 3 , WO 3 , SnO 2 and AZO have been investigated for xation of AgNWs on some substrates. Herein, we use a new inorganic matrix, Zn 2 SnO 4 , to anchor AgNW networks on glass substrate. Different to the routine one-step processing method, the preparation of AgNW/Zn 2 SnO 4 herein is divided into two steps, i.e. spray-coating the AgNWs rst and then spin-coating the Zn 2 SnO 4 layer. The upper Zn 2 SnO 4 layer was prepared by spincoating the mixed Zn-Sn-O precursor solution, followed by a low-temperature sintering process at 200 C for 5 min. During sintering, zinc and tin salts hydrolyze to form a ne, smooth and uniform transparent Zn 2 SnO 4 thin lm (see Fig. S1 †), covering the AgNW networks. Owing to the low sintering temperature, the as-prepared Zn 2 SnO 4 semiconductor lm possesses an amorphous structure and contains lots of oxygen vacancy defects, 52 which has a high electron mobility of  Zn 2 SnO 4 , 53 helping to transfer electrons between the glass substrate and the AgNW networks. XRD patterns of as-prepared AgNW/Zn 2 SnO 4 composite thin lm and the corresponding thin lm annealed at 450 C for 30 min are shown in Fig. S2. † It was found that Zn 2 SnO 4 matrix thin lm annealed at 450 C for 30 min is amorphous. If pristine Zn 2 SnO 4 thin lm were annealed at 600 C for 1 h in air, XRD characteristic peaks for cubic Zn 2 SnO 4 can be clearly observed in Fig. S3, † which is consistent with the literature report. 54 Note that XRD patterns of ZnO or SnO 2 impurities are not observed. To further conrm homogenous composition for as-prepared AgNW/Zn 2 SnO 4 composite lm, we measured SEM image and the corresponding elemental maps for as-prepared AgNW/Zn 2 SnO 4 composite lm, as shown in Fig. S4. † It was clearly observed that Zn and Sn elements have a homogeneous distribution, indicating that ZnO or SnO 2 impurities should not exist. The sheet resistance showed a neglectable increase from 7.11 ohm sq À1 to 8.62 ohm sq À1 aer the deposition Zn 2 SnO 4 (see Fig. 4a). No obvious alteration in AgNW distribution is observed aer deposition of the upper Zn 2 SnO 4 layer ( Fig. 3a and b), except that the AgNWs are closely anchored on the glass surface by an ultra-thin Zn 2 SnO 4 layer (Fig. 3c), forming a rm AgNWs/Zn 2 SnO 4 composite lms. Fig. 3d shows the optical picture of AgNW networks before and aer Zn 2 SnO 4 deposition, indicating a neglectable decline of transmission (from 91.48% to 89.73%) for the visible light (see Fig. 4b). It should be pointed out that the right one in Fig. 3d is a sample which had passed by a tapepeeling test and its conductivity remained unchanged. No discernible damage was observed for the AgNWs/Zn 2 SnO 4 composite lm, whereas the AgNW networks on the le sample can be easily peeled off from the glass substrate and leaves the AgNW networks on the tape, revealing a signicant improvement of adhesion aer Zn 2 SnO 4 deposition.
Thermal test was also carried out to investigate the stability of AgNW networks and AgNWs/Zn 2 SnO 4 composites, which is of high importance for their future application. As shown in Fig. 5, both pristine AgNW networks and AgNWs/Zn 2 SnO 4 composite  lms are stable below 280 C when the thermal treatment is set at 10 min. Above this point, the pristine AgNW networks begins to lose their conductivity (totally at 300 C), whereas the AgNWs/ Zn 2 SnO 4 composite lm is still stable until the temperature reaches 330 C and totally lose its conductivity at 380 C. On the other hand, if the temperature is set at 275 C, the pristine AgNW networks are stable in 15 min, and then begin to lose their conductivity. As shown in Fig. 6c, the rising of the sheet resistance is attributed to the partial melting of some silver nanowires, leaving disconnected silver nanorods or nanoparticles. When the thermal treatment comes to 35 min, the AgNW networks almost lose their conductivity, indicating fusion of most AgNWs in the networks. By contrast, the AgNWs in AgNWs/Zn 2 SnO 4 composite did not show distinct variation in morphology in 50 min (Fig. 6b), showing higher thermal stability than that of the pristine AgNW networks. Above this point of time, however, very small silver nanoparticles start to form around the nanowires (Fig. 6d), shrink slowly and disappear in the end, accompanied by a loss in conductivity. The electrical conductivity of a AgNWs/Zn 2 SnO 4 composite lm at low temperatures was also investigated. As shown in Fig. 7, the resistance of the AgNWs/Zn 2 SnO 4 composite lm is stable above À23 C, and then drop dramatically (20.44 ohm sq À1 to 4.51 ohm sq À1 ) at about À33 C until it reaches a platform again at about À53 C. This unnormal curve may result from our test sets, test xtures, poor thermal conductivity of glass substrate and two-probe based test method herein. Anyway, the lowtemperature test proves that the AgNWs/Zn 2 SnO 4 composite lm has a better electrical conductivity at low temperatures. Based on these variable temperature experiments, it can be concluded that our AgNWs/Zn 2 SnO 4 composite lm has a wide usable temperature range of À103 to 330 C, which can meet the requirements of thermal stability for antistatic coatings in various elds. In addition, the stability of AgNWs/Zn 2 SnO 4 composite lm against ultraviolet light illumination could be improved by using a previously reported method. 55

Conclusions
In summary, long silver nanowires with high aspect ratios were prepared by a modied polyol method using organic octylamine hydrochloride as the shape-controling agent. Transparent and conductive AgNWs/Zn 2 SnO 4 composite lms were fabricated by employing a two-step (spray-coating AgNWs and spin-coating Zn 2 SnO 4 ) method on the glass substrate, which exhibited high electric conductivity (5-15 ohm sq À1 ) and light transmittance (85-80%). The thermal stability and adhesion of AgNWs/ Zn 2 SnO 4 composite lms were dramatically improved by deposition of Zn 2 SnO 4 matrix. The as-prepared transparent conductive AgNWs/Zn 2 SnO 4 composite lms can be a good substitute for the commercial transparent conductive metal oxides, such as ITO, FTO, and AZO. More importantly, these robust and stable AgNWs/Zn 2 SnO 4 lms have a wide usage temperature range of À103 to 330 C, indicating a promising application prospect for antistatic coatings.

Conflicts of interest
There are no conicts to declare.