A Sulfhydryl Azobenzene-Modified Polyaniline/Silver Electrode and Its Photoswitching Electrochemical Performance.

In this work, a sulfhydryl-functionalized azobenzene derivative (Azo) was synthesized and polyaniline/silver was modified (PANI/Ag) to make a nanocomposite (PANI/Ag/Azo). A series of characterization techniques like1HNMR, UV-vis absorption spectra, Raman spectra, FT-IR, XRD, SEM, TEM, and TGA was employed to study Azo, PANI/Ag, and PANI/Ag/Azo. Electrochemical properties were measured by cyclic voltammetry (CV) and galvanostatic charging/discharging (GCD). CV showed that UV and blue light had hardly any effect on PANI/Ag. However, with the prolonged exposure time of UV light, the maximum CV current density of PANI/Ag/Azo rose from 1.24 to 2.72 A g-1. Then, after 20 min of blue light irradiation, the maximum current density gradually recovered (from 2.72 to 1.26 A g-1). The GCD also obtained similar results. After formula calculation, the specific capacitance of PANI/Ag/Azo also presented a reversible trend under the alternating irradiation of UV light and blue light. All the results show that PANI/Ag/Azo has a good photoelectric response, and its electrochemical performance can be reversibly adjusted by light. This result provides a new design idea for developing electrode materials with real-time electrochemical properties.

Introduction

Conducting polymers have received considerable interest in microwave absorption,[1] supercapacitors,[2,3] vapor sensors,[4] and electromagnetic shielding[5] due to their particular structure and excellent physical and chemical properties. Especially in supercapacitors, using a conductive polymer as an electrode material has the advantages of high capacity, fast charge and discharge time, increased safety, and environmental friendliness. In numerous conducting polymers, such as polyaniline (PANI),[6−9] polypyrrole (PPy),[10−13] polythiophene (PTh),[14−16] and their derivatives have been widely studied as electrode materials. Especially, PANI is considered one of the most promising electrode materials, which is attributed to its excellent environmental stability, synthesis simplicity, unique doping/dedoping reversibility, and outstanding conductivity.[17]

However, the inferior cyclic stability of PANI limits its practical application. In addition, as electrode materials, the conductivity of a single conducting polymer is not enough to prepare supercapacitors with excellent properties. Therefore, the combination of PANI and other active materials (carbon materials, metal compounds, or other polymers) can surpass these intrinsic disadvantages of PANI and exhibit better thermal stability, catalytic and electrochemical properties. PANI composites exhibit a better application prospect in sensors,[18] supercapacitors,[19] fuel cell, and other fields compared to pure PANI. Many research works have been focused on enhancing the electronic conductivity of PANI electrodes by using metal doping. Nevertheless, out of many metal nanoparticles, silver (Ag) nanoparticles are widely used because of their high conductivity, excellent thermal stability, electrocatalytic ability, low cost compared to gold (Au).[20] In addition, many studies have also confirmed that the electrochemical performance of PANI/metal nanocomposites is much better than that of pure polyaniline. For example, Patil et al.[21] studied the electrochemical performance of PANI and Ag/PANI thin films by a dip-coating technique and found that the specific capacitance increased from 285 (for PANI) to 512 F g–1 (for Ag/PANI) at 0.9% doping of Ag. Xie et al.[22] confirmed that the capacitance value (850 F g–1 at 10 mV s–1) of PANI-Ag nano cable arrays was much higher than most PANI supercapacitors. Simultaneously, Tamboli et al.[23] found that the Ag/PANI nanocomposite has higher thermal stability than pure PANI. Although their performance has improved, it is not yet practical enough. Therefore, the development of new methods is vital to improving the electrical properties of PANI/Ag in the capacitor community.

It is of great significance to find a simple and real-time controllable method to adjust the electrochemical performance of the PANI/Ag nanocomposite. Generally, the electrochemical properties can be adjusted by preparing different morphology PANI/Ag composites, such as the nanotube structure,[24] fibrous structure,[25] and core–shell structure.[26] Alternatively, a third kind of organic or inorganic materials is added to form a ternary system. For example, Dhibar et al. fabricated Ag-PANI/MWCNT nanocomposites by adding multiwalled carbon nanotubes and obtaining the highest specific capacitance 528 F g–1 at a 5 mV s–1 scan rate.[27] Alternatively, graphene,[28] poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS),[29] Pt,[30] etc., have also been studied as additives. The electrochemical properties of the PANI/Ag material were remarkably changed in those ways. However, its electrical properties could not be real-time controlled by a simple and green method. As a clean energy, light can be easily and quickly controlled remotely.[31] Therefore, we can adjust the properties of PANI/Ag by adding light-responsive materials. Among lots of light-responsive materials, the azobenzene compound is a well-known light-induced material owing to its unique characteristics, including widely applicable performance, photochemical stability, and fast response.[32,33] It can undergo trans–cis isomerization by the method of ultraviolet/visible light irradiation[34] or heating.[35] Due to the special property, Azo is often used as an excellent light switchable molecule.[36−40] In addition, the photoisomerization of azobenzene derivatives can affect the properties of silver, and the surface plasmon resonance of silver also can affect the photoresponse of azobenzene in the composite system. Therefore, it is an interesting experiment to prepare PANI/Ag/Azo composites and control their electrical properties by light irradiation. However, no report on the optical modulated electrochemical properties of such hybrid materials has been published.

In this article, a photosensitive azobenzene derivative with sa ulfhydryl group(Azo) was synthesized and to make a polyaniline/silver/azobenzene composite (PANI/Ag/Azo) with polyaniline/silver (PANI/Ag) by self-assembly between silver and sulfhydryl. It is the first report on the synthesis of PANI/Ag/Azo adjustable nanocomposites by self-assembly to the best of our knowledge. In addition, the morphology, optical properties, thermal stability, and composition of the obtained PANI/Ag/Azo composites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible spectroscopy (UV–Vis), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and Surface-enhanced Raman spectroscopy (SERS). Also, the electrochemical properties of PANI/Ag/Azo were studied by the electrochemical workstation under different light irradiation conditions.

Results and Discussion

UV–Vis Absorption Spectra Analysis

The photochemical behavior of the Azo was investigated by UV–vis absorption spectroscopy, as shown in Figure . The Azo sample was dissolved in DMSO and then magnetically stirred under light irradiation to ensure uniform light absorption. Azo shows the maximum absorption peak at 355 nm, with a broad band at 450 nm.[41,42] The previous absorption peak is attributed to the π–π* transition of the trans-Azo, and the latter arises from the n-π* transition of the cis-Azo.[43,44] The absorption band at 355 nm further decreased, and the absorption band at 440 nm increased slightly upon continuous UV irradiation (Figure a). In the meantime, with the increase of UV irradiation time, the maximum absorption peak blue shifts. These spectra changes exhibit the isomerization of Azo molecules from the trans-to-cis structure form.[45,46] The UV–vis spectra of Azo no longer change after exposure to UV light for 20 min, suggesting that the system reaches a photo-stationary state. After exposure to UV light for 20 and 25 min, the two curves overlap, indicating that azobenzene has reached a stable state from trans-Azo to cis-Azo. On the contrary, when exposed to blue light (Figure b), the absorption band at 355 nm recovered back, which indicates that the cis-Azo transformed to trans-Azo. After blue light irradiation for 20 min, the system reaches a photo-stationary state. In addition, it can be seen from Figure c (UV–vis spectra of Azo upon repeated alternating irradiation by UV and blue light is shown in Figure S1) that the reversible photoisomerization process in DMSO solution could be repeated several times upon repeated alternating irradiation by UV and blue light. Also, the absorbance of Azo without light is higher than that of Azo after the first cycle of light, which is consistent with other references.[47−49]

Figure 1

Time-dependent absorption spectra of the Azo DMSO solution (3 × 10–5mol/L) under (a) irradiation by UV light at 365 nm and (b) illumination by blue light at 450 nm. (c) The changes in the absorption band of Azo molecules at 355 nm upon alternating irradiation interval 25 min by UV and blue light (purple represents UV light irradiation, blue represents blue light irradiation).

Raman Spectrum and IR Analysis

The Raman spectrum of Azo powder was tested. In Figure S2, the characteristic peaks of Azo at −N=N– (1434 cm–1)[50] and −SH (2535 cm–1) are observed. FT-IR spectra of the Azo, PANI/Ag, and PANI/Ag/Azo composites are shown in Figure . Meanwhile, the FT-IR spectrum of pure Azo showed absorption bands at 3452 (ν–OH), 1685 (νC=O), 1604 (ν–N=N– stretching vibration),[51] 1503 (νC=C of the benzenoid ring), and 1385 cm–1 (δsCH). Combined with the previous1HNMR and UV–vis analysis, the target product (2-mercapto-5-(p-tolyldiazenyl)benzoic acid) has been synthesized. For the FT-IR spectrum of PANI/Ag, the characteristic peaks that appeared at 3463 cm–1 were correlated with −NH stretching vibration. The absorption peaks at 1592, 1490,1300, 1125, and 813 cm–1 corresponded to the C=C stretching deformation of quinonoid, C=C stretching vibration of the benzenoid ring, C–N stretching vibration, N=Q=N (Q denotes quinonoid) stretching vibration, and C–H out of plane bending vibration, respectively. Similar IR curves were also observed by Wang et al.,[52] which indicated that PANI/Ag had been synthesized successfully. Similar absorption characteristic peaks with some additional peaks are shown in the FT-IR spectrum of PANI/Ag/Azo. The characteristic peak at 3451 cm–1 becomes sharper due to the double action of −NH stretching vibration and −OH stretching vibration. The absorption peak at 1685 cm–1 shifted to 1689 cm–1; the peaks at 1604 and 1592 cm–1 shifted to 1641 cm–1; and the peaks at 1125 and 813 cm–1 shifted to 1255 and 800 cm–1. These peak shifting phenomena proved the existence of an interaction between PANI/Ag and Azo.

Figure 2

FT-IR spectra of Azo, PANI/Ag, and PANI/Ag/Azo.

In addition, XPS was used for the PANI/Ag/Azo composite’s surface composition, see Figure S3. The whole XPS analysis of PANI/Ag/Azo was shown in Figure S3a, and it is clear that the composite consists mainly of Ag, C, N, S, and O. The Ag 3d region of PANI/Ag/Azo (Figure S3b) showed two peaks at 367.8 and 374 eV, which corresponded to the Ag 3d5/2 and Ag 3d3/2 energy levels, respectively. The result was in accord with Wang et al.′s work,[52] indicating the presence of Ag NPs. The C 1s region of PANI/Ag/Azo showed three bands (Figure S3c), the band of C–O at 286.4 eV, the band of C–N at 285.5 eV, and the band of C–C at 284.5 eV.[42] In addition, the N 1s region of PANI/Ag/Azo (Figure S3d) showed the N=N band at 400 eV, which is attributed to the overlap of (Ph) N=N (Ph). Furthermore, the N=C band at 399.8 eV is attributed to PANI.[38] These results show that the composite has been successfully prepared, and it is consistent with the results of IR spectroscopy.

XRD Analysis

The crystalline structure of the composites is studied by XRD analysis, as shown in Figure . Both XRD patterns show a broad peak at 2θ ≈ 16–20°, which was contributed to PANI’s amorphous behavior. For the XRD pattern of PANI/Ag, there were four sharp diffraction peaks appearing at 38.4, 44, 64.5, and 77.3°, which correspond to Bragg′s reflections from the (111), (200), (220), and (311) planes of Ag, respectively (JCPDS No. 04-0783). The result was in accordance with Wang′s work.[52] It is proved that the PANI/Ag composite has been successfully synthesized. Similar peaks are observed on PANI/Ag/Azo’s XRD pattern except for the Ag NPs’ crystallinity (the peaks of PANI/Ag are sharper). The result is more likely the interaction between Azo and Ag.

Figure 3

XRD of PANI/Ag and PANI/Ag/Azo composites.

SEM and TEM Analysis

The surface morphologies of PANI/Ag and PANI/Ag/Azo composites were investigated by SEM and TEM, as shown in Figure . As can be seen from Figure a,b, the PANI/Ag composite is presented in a cubic morphology.

Figure 4

(a, b) SEM images of PANI/Ag; (c, d) SEM images of PANI/Ag/Azo; (e) TEM images of PANI/Ag; (f) TEM images of PANI/Ag/Azo.

As shown in Figure c,d, PANI/ Ag/Azo has a lower dispersion compared to the Ag/PANI composite material because of the addition of Azo. There is something attached to the surface of Ag NPs, which may be that the sulfhydryl group on azobenzene has self-assembled with Ag NPs. In Figure e, in the TEM image of PANI/Ag, the darker part is Ag, and the lightly shaded part is PANI. It is seen clearly that Ag NPs appear as dark, nearly spherical on the smooth surface of PANI, and the particle size distribution of Ag NPs is relatively uniform. After the addition of Azo, there are many light-colored circular substances around the surface of Ag, which are most likely Azo (Figure f). These results indicated that the addition of Azo could change PANI/Ag’s morphologies to a certain extent, and there is an interaction between PANI/Ag and Azo. This result is consistent with IR and XRD.

TGA Analysis

The decomposition behavior and stability of PANI/Ag and PANI/Ag/Azo compositions were studied by thermogravimetric analysis. In Figure , for the PANI/Ag composite, the initial weight loss of only 0.5% was observed, which may be due to the presence of some volatile impurities or the removal of water molecules up to 135 °C.[30] The second thermal weight loss is related to the decomposition of low molecular weight oligomers or side products at 100–360 °C. However, the decomposition rate of PANI/Ag is significantly accelerated at 230–360 °C. The possible explanation is that the bonded interaction between PANI and Ag is not stable after a 350 °C temperature. Dhibar et al.[27] also observed similar TGA curves for Ag-doped PANI composites. In the last stage, at the temperature of 360–800 °C, the polymer backbone starts to break. For Azo, the final residual mass is 53% at 800 °C. The weight loss is related to removing water molecules, disintegrating carboxylate, and the decomposition of the pristine Azo backbone. Compared to PANI/Ag, it can be seen that there are four stages of thermal transition that have been observed for PANI/Ag/Azo composites. Similar to the former, at 20–135 °C, the thermal weight loss is about 2.3% due to the removal of water molecules. The second thermal transition is associated with the degradation of some impurities and carboxyl. The third thermal transition corresponds to the decomposition of Azo compounds[53] and unstable bonding interactions between PANI and Ag. The lower loss rate at this stage, which probably was due to Azo’s addition, changes PANI/Ag’s internal structure. The fourth thermal transition was attributed to the decomposition of the polymer backbone after 410 °C. The remaining masses of PANI/Ag and PANI/Ag/Azo are 55 and 62% at 800 °C, respectively, indicating that PANI/Ag’s thermal stability is improved after incorporating Azo molecules.

Figure 5

TGA curves of PANI/Ag and PANI/Ag/Azo compositions.

Electrochemical Properties

To investigate the electrochemical behavior of the photoresponsive PANI/Ag/Azo composite, electrochemical analyses of PANI/Ag and PANI/Ag/Azo composites were performed in a standard three-electrode cell setup using cyclic voltammetry (CV) in the potential range of 0–0.8 V in 1.0 M H2SO4 solution at a scan rate of 0.05 V/s. Figure a presents the CV curves of PANI/Ag/Azo at different UV irradiation times. A dramatic increase in CV current density is detected after UV irradiation 5 min, and the current intensity increased with the increasing time of UV irradiation gradually. Upon a UV irradiation of 20 min, the highest CV current density of PANI/Ag/Azo increases from 1.24 to 2.72 A g–1.

Figure 6

CV curves of (a) PANI/Ag/Azo upon UV irradiation for various times, (b) UV light-irradiated PANI/Ag/Azo upon blue light irradiation for different times, and (c) PANI/Ag after UV light and blue light irradiation at a potential scan rate of 50 mV s–1.

Conversely, upon blue light irradiation for 20 min, the highest current density of the UV-irradiated PANI/Ag/Azo decreases from 2.72 to 1.26 A g–1 (Figure b). Under the alternating irradiation of UV and blue light, the CV curves of both are similar, and the variation of current density shows an opposite trend. It indicates that PANI/Ag/Azo has a certain invertibility upon light. As a control experiment, the CV curves of PANI/Ag upon UV light irradiation for 20 min and consequent blue light irradiation for 20 min were also tested. The result was displayed in Figure c. It is clear that the CV curves of the PANI/Ag electrode show an obvious oxidation peak and reduction peak, which indicates the pseudocapacitor behavior of PANI. On the contrary, the redox peak of PANI/Ag/Azo is lighter than PANI/Ag, which may result from the joint action of Azo and PANI/Ag. In addition, none of PANI/Ag’s CV curves had shown apparent current density changing after UV and blue light irradiation. This indicates that light has almost no effect on the electrical property of the PANI/Ag electrode. The reversible photoresponsive behavior can reversibly control the electrochemical performance of PANI/Ag/Azo due to the conformational change of the Azo between trans and cis forms upon light irradiation.

Furthermore, the cis-PANI/Ag/Azo showed better electrochemical performance than trans-PANI/Ag/Azo. The reason is that the tunneling barrier length of cis-azobenzene connected Ag NPs is shorter than that of trans-azobenzene-connected Ag NPs.[38,54−56] With the increasing time of UV irradiation, more cis-Azo are exposed on the surface of Ag, leading to higher conductivity of PANI/Ag/Azo. On the contrary, upon blue light irradiation, the cis-Azo transforms into trans-Azo. Therefore, the electrochemical performance of PANI/Ag/Azo became worse gradually with the increasing time of blue light irradiation.In addition, we further investigate the electrochemical capacitor performance of PANI/Ag/Azo upon light irradiation. Figure a,c present GCD curves of PANI/Ag/Azo composite at different UV and blue light irradiation times separately, within the range of 0–0.8 V at a current density of 1 A g–1. Moreover, according to the discharge time, the specific capacitance (Cs, F g–1) of the PANI/Ag/Azo electrode can be calculated. Its specific capacitance calculation equation is as follows:where I (A), Δt (s), m (g), and ΔV (V) present the discharge current, total discharge time, mass of active material(m1 – m0),[57] and the potential change during discharge progress, respectively.

Figure 7

(a)GCD curves of PANI/Ag/Azo upon UV irradiation for varying times. (b) the calculation results of the Cs values of the sample under UV irradiation. (c) GCD curves of UV light-irradiated PANI/Ag/Azo upon blue light irradiation for varying times. (d) the calculation results of the Cs values of the sample under blue light irradiation.

The calculation results of the Cs values of PANI/Ag/Azo are shown in Figure b,d separately. We can clearly see that PANI/Ag/Azo’s specific capacitance increases from 140.33 to 270.12 F g–1 with the increasing time of UV light irradiation. Also, after that, the UV irradiated PANI/Ag/Azo was exposed to blue light for 20 min. The specific capacitance decreases from 270.12 to 159.1 F g–1. According to these data, we can know that the PANI/Ag/Azo hybrid’s specific capacitances can also be controlled by light irradiation. The specific capacitance of cis-PANI/Ag/Azo is superior to that of trans-Azo due to the trans-PANI/Ag/Azo in the assembled form (sulfhydryl self-assembly with silver) bears a relatively small surface area. Concurrently, the results of GCD are in good agreement with the CV results. In addition, we further investigated the electrochemical capacitor performance of PANI/Ag upon light irradiation (Figure S4). The PANI/Ag electrodes were irradiated alternately with UV and blue light for 20 min, and then charge and discharge experiment was carried out. From Figure S4, we can see that the GCD curves are changing under alternating UV and blue light. However, the Cs values (Table ) of the three GCD curves are similar.

Table 1

Calculation Results of Cs Values of the PANI/Ag Composite

sample types	Cs/F g–1
PANI/Ag no light	198
UV light 20 min	202.5
blue light 20 min	201.6

It was evident that light irradiation has little effect on its specific capacitance. The difference of those curves is probably caused by an experimental error. Meanwhile, it can be observed that the specific capacitance of PANI/Ag (198 F g–1) is superior to that of PANI/Ag/Azo without light irradiation (140.33 F g–1). On the one side, Azo exists in the form of a trans structure under normal conditions. There exist two possible reasons for this. On the other side, as an organic substance, Azo will not enhance PANI/Ag’s electrochemical performance without any stimulation since it does not conduct electricity.

The cycling stability is an essential factor for a well-established electroactive material. The PANI/Ag/Azo electrode’s cycling stability was tested by repeatedly measuring the GCD plots at the current density of 4 A g–1 for 1000 cycles. The inset of Figure shows the GCD curves measured at the first three and last three cycles in the 1000 times charge/discharge process. The GCD curves at the first three cycles show higher symmetry than the last three cycles, suggesting the reduction in the reversibility after the 1000 times repeated charge/discharge process. Moreover, the capacitance retention only remained about 65% during the 1000 cycles charge–discharge process. These results suggest that the PANI/Ag/Azo composite has low reversibility and cycling stability. This may be related to the repeated swelling and contraction of conjugated polyaniline. In the following research, the electrode’s stability can be improved through space limitation and structure design. For example, polyaniline can be combined with mechanically soft materials functionalized by azobenzene. The materials’ morphology can be controlled by the hard template method to form a cavity structure to buffer the volume expansion.

Figure 8

Cyclic performance of the PANI/Ag/Azo composite (no light irradiation) at a current density of 4 A g–1. Inset: first three and last three cycles of the GCD plots of the PANI/Ag/Azo composite measured in the 1000 times repeated charge–discharge process.

Conclusions

In summary, the PANI/Ag/Azo composite was prepared by the self-assembly effect for the first time, and its electrochemical performance was investigated. It was found that the composite showed excellent photosensitive electrochemical properties by the electrochemical workstation, and the photoisomerization can significantly adjust its electrochemical performance, triggering the spatial change of Azo. Notably, the specific capacitance of PANI/Ag/Azo is higher than PANI/Ag as the UV irradiation time increases. Furthermore, its particular capacitance can return gradually upon the blue light irradiation. Therefore, the PANI/Ag/Azo composite has a great potential to be used as an adjustable material for practical applications in photoresponsive supercapacitors and sensors.

Materials and Methods

Materials

Aniline, ammonium persulfate((NH4)2S2O8, 98%), hydrochloric acid(HCl, 35%), sodium hydroxide(NaOH, 96%), ethyl alcohol absolute(C2H6O, 99.7%), sodium nitrite(NaNO2, 99%), thiosalicylic acid(C7H6O2S, 97%), p-toluidine (C7H9N, 99%), and silver nitrate(AgNO3, 99.8%) were purchased from Sinopharm Chemical Reagent Co. Aniline is used by secondary vacuum distillation, and other chemicals were used without further purification. All water used in this experiment is deionized water.

Synthesis of 2-Mercapto-5-(p-tolyldiazenyl)benzoic Acid (Azo)

2-Mercapto-5-(p-tolyldiazenyl)benzoic acid was synthesized according to the following steps. A 6 mL diluted HCl (2:1) and p-methylaniline (8 mmol) were added in a 50 mL conical flask , kept ultrasonic for 5 min, and then cooled to 0–5 °C. In addition, NaNO2 (8 mmol) was added, and the temperature was held at 0–5 °C for 30 min to obtain a light yellow diazotization solution. O-Mercaptobenzoic acid (8 mmol) was added in a 20 mL NaOH solution (0.5 mol/L) and kept ultrasonic until dissolved. Next, the above solution was added to the diazotization solution by slow drops. Simultaneously, the system’s pH value was adjusted with a saturated Na2CO3 solution to make pH = 7–8. After stirring at 5–10 °C for 24 h, the bright yellow raw materials were obtained by filtration. Finally, the light yellow materials were obtained by recrystallization with ethanol as the solvent. 1H NMR (400 MHz, DMSO): δ7.21–7.98 (Ph-H, m, 7H), 3.44(SH, 1H), 2.51(CH3, 3H) (Figure S5).

Synthesis of PANI/Ag and PANI/Ag/Azo Composites

The PANI/Ag composite was synthesized by in situ chemical polymerization methods. A 10 mL AgNO3 (0.85 g) aqueous solution and 1 mL 1 mol/L nitric acid (HNO3) solution were added into a 50 mL beaker to form an oxidation solution. Next, a 40 mL sodium hyphenate (Na2HPO3·5H2O) (0.27 g) solution and 4.5 mL 1 mol/L HNO3 solution containing 0.5 g aniline were added to a 100 mL round-bottom flask with magnetic stirring. The oxidizing solution was slowly added dropwise and reacted at 40 °C in a water bath for 90 min to prepare the nano-silver adhesive system. Following this, the nanosilver adhesive system was removed from the water bath and cooled slowly to room temperature. After that, 10 mL aqueous solution containing 1.23 g ammonium persulfate ((NH4)2S2O8) and 1 mL 1 mol/L HNO3 was added in drops one by one with stirring. The reaction lasted 7 h at room temperature. Then, the precipitation was washed with acetone, ethanol, and deionized water three times. Furthermore, the final product was a blackish green precipitate of PANI/Ag nanocomposite, as obtained after drying in a vacuum oven at 60 °C for 24 h. A total of 2.04 g of Azo was dissolved in 100 mL ethanol and kept ultrasonic for 15 min. In the above steps, the reaction continued for 4 h after adding ammonium persulfate. Then, the Azo solution was added to the above reaction solution for another 3 h with vigorous stirring. Finally, the precipitate was filtered, and the filter cake was washed with acetone, ethanol, and deionized water three times. Furthermore, the product was a red-brown precipitate of the PANI/Ag/Azo nanocomposite, as obtained after drying in a vacuum oven at 60 °C for 24 h. The schematic representation of the synthesis of PANI/Ag/Azo composite is illustrated in Figure . Furthermore, photoinduced isomerization of Azo on a single Ag NP is shown in Figure .

Figure 9

Schematic representation of the synthesis of PANI/Ag/Azo composites.

Figure 10

Schematic illustration of photoinduced isomerization of azobenzene on a single Ag nanoparticle.

Material Characterization

Proton nuclear magnetic resonance (1HNMR) spectra of Azo were recorded on a Bruker ARX400 MHz spectrometer using dimethyl sulfoxide (DMSO) as a solvent with tetramethylsilane (TMS) as the internal standard at room temperature. Raman spectra of Azo were recorded with a Raman microscopic spectrometer (SD25, UK). FT-IR was recorded using an FT-IR spectrometer (NICOLET 6380, Thermo Nicolet, US). X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi spectrometer (ThermoFisher Scientific, UK). The UV–vis absorption spectra of Azo was tested on a UV5000 PC spectrophotometer (Precision Instrument Co., Ltd., Shanghai), which was recorded at room temperature in the DMSO solution.

Meanwhile, the time evolution of the Azo’s absorption spectra in DMSO upon irradiation of UV light and blue light was carried out using a xenon lamp system (CEL-HXF300). The samples’ surface morphology was examined by SEM (FlexSEM1000, Hitachi Ltd., JP) and transmission electron microscopy (TEM, FEI, G2 F20/F30, 200 kV). X-ray diffraction patterns were taken on a SIEMENS (D5000) X-ray. TGA was performed using a thermogravimetric analyzer (Q500, TA, US) at a heating rate of 10 °C/min in an Ar2 atmosphere. CV and GCD were performed on a CHI660E (Shanghai Chen Hua, China) workstation using a standard three-electrode cell setup. The electrode that is coated with the sample, platinum wire, and saturated silver chloride electrode (Ag/AgCl) were used as a working electrode, opposite electrode, and reference electrode, respectively.