Different control agents are utilized during the polyol process to control the reduction rate of silver and form a coordination bond to prohibit silver atoms from the accumulation onto (100) facet. The common anion used was chloride ions which can produce silver chloride and control the nucleation process of AgNWs30. Effect of controlling agentsDifferent control agents are utilized in the polyol process, to control the reduction rate of silver and make a coordination bond to prohibit any silver atoms from accumulation. The chloride ions support the multi-twinned structures to keep all particles in small size and reducing the reaction rate through competing with Ag+ via coordination bond31. The current study investigates the impact of hydrochloric acid and copper chloride as control agents in the polyol process using different molecular weights of PVP.Using hydrochloric acid as a controlling agentFigure 1a shows broadness and high intensity of the shoulder that may be due to an increase in the AgNWs diameter32. The broad shoulder emerged at 700 nm shows that AgCl seed size is smaller33. This leads to longer length of AgNWs. Whilst using PVP-1.3M with HCl, the shoulder peak surfaced is blue-shifted to 575 nm, this indicates a larger AgCl seeds and shorter length of AgNWs is obtained34. PVP-1.3M enhances the half peak width of the surface plasmon, indicating the formation of AgNWs, as shown in Fig. 1b35,36. Using mixed capping agents, only one wide peak is observed at 308 nm in Fig. 1c, which may correspond to the surface plasmon absorption band of small nanoparticles37. The results of UV–Vis spectrum shows that the AgNWs could be obtained using PVP-40K and PVP 1.3-M, while the findings in Fig. 1c may demonstrate that the AgNWs could not be attained using HCl.Fig. 1UV–Vis spectra of AgNWs using HCl with different molecular weights of PVP (a) PVP-40K, (b) PVP-1.3M, (c) volume ratio 1:1 of PVP-40K and PVP-1.3M.The synthesis of AgNWs is confirmed by the XRD patterns. The peaks at 37.63°, 43.92°, 64.83°, and 81.20° are indexed to (111), (200), (220), and (222), respectively37, as presented in Fig. 2. The peak of (111) facet is attributed to the longitudinal of AgNWs, while (100) facets indicate poor capping agent of PVP on AgNWs surface of the (100) facets37. XRD of AgNWs prepared by HCl with PVP-40K in Fig. 2a, shows several additional diffraction peaks (*) corresponding to the existence of AgCl in the final yield34. The ratio between the planes (111) and (200) is low, indicating low existence of AgNWs38,39. The lattice constant using PVP-40K is 4.12 Ã… and reduces to 4.11 Ã… using PVP-1.3M, which matches the reference JCPDS file 04-078340. The intensity ratio between the planes (111/200) is 2.83 with appearance of (100) plane for samples prepared using PVP-1.3M, as shown in Fig. 2b. Using mixed of PVP in Fig. 2c is inappropriate, since the plane (100) shows higher intensity than that of plane (111), which means that the majority of AgNWs are not covered with PVP41, as well as the growth direction is low in the plane (111).Fig. 2XRD patterns of a synthesized using HCl with different molecular weights of PVP (a) PVP-40K, (b) PVP-1.3M (c) volume ratio 1:1 of PVP-40K and PVP-1.3M.FT-IR spectra are utilized to study the interaction between the pyrrolidone ring and the silver surface42. The intensity of the coordination interaction between PVP and Ag+ could be evaluated by the occurrence of band shifts in the FT-IR spectrum43. The observed band at 2970 cm-1 is assigned to the symmetric stretching vibration of CH2 in the skeletal chain of PVP, while the distinct peak at 3000–3500 cm-1 is assigned to the stretching vibration of the -OH group44. The shown bands at 1625 cm-1 and 1237.1 cm-1 are attributed to C = O stretching vibration from PVP and in-plane vibration of the C-N group, respectively45. In Fig. 1S, the characteristic peaks are well-defined related to PVP molecules and their adsorption interaction with silver’s surface46. The two distinct peaks that attributed to PVP at 3456.41 cm-1 and 1635.62 cm−1 have a blue shift, revealing that the reaction using PVP-40K has high amount of PVP47, as seen in Fig. 1Sa. In contrast, samples prepared using the high molecular weight of capping agent in Fig. 1Sb, the peak becomes narrow with low transmittance, indicating high amount of PVP in the final yield. When mixed PVP is utilized, high transmittance of stretching vibration bands at 1639.48 cm−1 and 3460.27 cm−1 which refers to low amount of PVP in the final yield, as shown in Fig. 1Sc.Figure 3 illustrates the morphologies of the final yield. AgNWs samples prepared using HCl contained high amount of silver nanoparticles. The length and diameter of AgNWs prepared using PVP-40K were 1.5 µm and 168 nm, respectively, and having a low aspect ratio of 9, as shown in Fig. 3a,b. For AgNWs prepared with PVP-1.3M as shown in Fig. 3c,d, the length of AgNWs is increased to 1.9​ µm and the diameter is decreased to 136 nm and the aspect ratio is raised to 14, which indicate the necessity of using high molecular weight of PVP to synthesize high aspect ratio of AgNWs. High Mw of PVP slows the reduction rate of Ag nanoparticles and this reflected on the growth of AgNWs21. Moreover, longer chains of PVP create a steric hindrance effect. This means blocking (100) facet of nanowires, accordingly, promoting the formation of elongated nanowires. Using mixed PVP, most of the silver nanoparticles formed as depicted in Fig. 3e,f are accumulated together with the emergence of PVP adsorbed on (100) facet. This is evidence that the crystalline surface contains active (100) facet in the final yield39.Fig. 3TEM images of synthesized AgNWs using HCl with different molecular weight of PVP (a,b) PVP-40K, (c,d) PVP-1.3M, (e,f) volume ratio 1:1 of PVP-40K and PVP-1.3M.Using copper chloride as a controlling agentUsing cations have two oxidation states can prohibit the oxidative etching on the silver’s surface31, and form long and thin AgNWs in the final yield. Figure 4 illustrates the UV–Vis spectra of AgNWs synthesized by copper chloride at different molecular weights of PVP. Three distinguishable absorption peaks of AgNWs appeared in the spectra, with high absorption as shown in Fig. 4a. The absorption peak at 360 nm of transverse plasmon mode of AgNWs in PVP-1,3M is blue shifted due to high aspect ratio of AgNWs in this reaction36. Also, the distinguished peak at 450 nm which corresponds to silver nanoparticles is blue shifted to 440 nm that may be due to substantial prevalence of nanoparticles in the final yield, as shown in Fig. 4b32. Using mixed PVP in Fig. 4c, the distinctive peaks is attributed to the surface plasmon of AgNWs are narrow, confirming that the AgNWs formed successfully. While the distinctive peak at 450 nm attributed to silver nanoparticles has lower absorbance, indicating that the prepared silver nanowires are pure with minimal amount of nanoparticles48. Fig. 4UV–Vis spectra of AgNWs prepared using CuCl2 and different molecular weights of PVP (a) PVP-40K, (b) PVP-1.3M, (c) volume ratio 1:1 of PVP-40K and PVP-1.3M.For preparing AgNWs using CuCl2 and PV-40 K, XRD peaks are attributed to silver’s crystal lattice displayed alongside (100) facet with 6.6 aspect ratio between (111) and (200) planes of AgNWs and a lattice constant of 4.1240 Å, as shown in Fig. 5a. However, using PVP-1.3 M with copper chloride, the prepared AgNWs show a high aspect ratio between (111) and (200), equal to 5.6, which refers to the high amount of AgNWs yield, as shown as in Fig. 5b. XRD patterns show the advantages of using mixed PVP, because the aspect ratio is found to be 2.5 using CuCl2 as a controlling agent (Fig. 5c), which matches with the reference JCPDS file 04–078346. All lattice constants are close to the theoretical values of AgNWs, as shown in Table 149. XRD results confirmed that using mixed PVP is effective in synthesizing AgNWs. Fig. 5XRD patterns of AgNWs prepared using copper chloride with different molecular weights of PVP (a) PVP-40 K, (b) PVP-1.3 M, (c) volume ratio 1:1 of PVP-40K and PVP-1.3M.Table 1 Lattice constants calculated for different crystalline AgNWs prepared using polyol process with different controlling agents and capping agent having different molecular weights.For FTIR spectra for AgNWs prepared using CuCl2 and PVP-40 K, all distinct peaks.at 3458.34 cm−1, and 1639.48 cm−1 are well-defined to the stretching vibration of O–H and C=O, respectively. The high transmission emphasises low existence of PVP-40 K alongside the silver’s surface, as shown in Fig. 2Sa. FTIR spectrum of AgNWs prepared using PVP-1.3 M in Fig. 2Sb, the peak attributed to the stretching vibration of the -OH group at 3448.69 cm−1 is shortened due to the formation of AgO molecules48. The bands corresponding to C=O stretching vibration and O–H in Fig. 2Sb occurred at 1660.7 and 3448.69 cm−1, respectively, became wider confirming low existence or dissolution of PVP. On the other hand, using mixed PVP, FTIR spectrum shown in Fig. 2Sc indicates sharp peaks of C=O at 1643.34 cm−1 and 3450.62 cm−1 that attributed to stretching vibration of O–H in pyrrolidone ring with low transmittance, and confirming PVP is strongly adsorbed onto the silver’s surface46.Figure 6 displays the morphologies of AgNWs synthesized using CuCl2 and different molecular weights of PVP. When PVP-40 K employed with copper chloride, AgNWs are uniform and have high degree of purity, with length of 2.275 µm, diameter of 120.57 nm, and aspect ratio of 19, as shown in Fig. 6a,b. Due to the dissolution of PVP-1.3 M, some silver nanoparticles are adsorbed onto the silver nanowires surface onto (100) facet, as shown in Fig. 6c,d. Therefore, the estimated diameter and length of AgNWs are enlarged to 259 nm and 3.3 µm, respectively and the aspect ratio is decreased to 12.7. Figure 6e,f show long, thin, and pure nanowires synthesizing using mixed PVP, and the average diameter and length of AgNWs are 242.96 nm and 3.8 µm, respectively, with aspect ratio of 15.6. In addition, using mixed PVP forms a 10 nm thick PVP layer on the (100) facet.Fig. 6TEM images of AgNWs synthesized using CuCl2 with different molecular weights of PVP (a,b) PVP-40 K, (c,d) PVP-1.3 M, (e,f) volume ratio 1:1 of PVP-40K and PVP-1.3M.Table 2 indicates higher values of aspect ratio when copper chloride is utilized. Due to the dissolution of PVP-1.3 M, more nanoparticles accumulate on AgNWs surface and widen the diameter further. Therefore, the preparation conditions of mixed molecular weights of PVP and copper chloride are chosen to prepare AgNWs and to study the effect of temperature and reducing agent. Our findings on the length, diameter and aspect ratio of AgNWs are consistent with previous reports. lau et al.24 stated that increasing the PVP molecular weights from 55 K to 1.3 M promote the growth of AgNWs in (100) direction. Moreover, using hydrothermal of biomass and active ingredients, Li et al.26 synthesized AgNWs with diameter of 77 nm and length of 10 µm. The authors argued that there is a positive correlation between PVP molecular weights and length of AgNWs.Table. 2. The lengths, diameters and aspect ratio of prepared AgNWs prepared with different reagents.Effect of synthesizing temperatureA sufficient temperature is required to offset the deficiency of thermal energy for formation of specific facets of AgNWs. Thus, studying temperature is a crucial factor to optimize the required temperature to produce pure with high aspect ratio of silver nanowires. The current work studied the temperature, above and below 150 °C by 20 °C, on the final yield and silver atom transformations in the polyol reaction. Figure 3S shows the optical absorbance of AgNWs synthesized at different temperatures, namely 130 °C, 150 °C and 170 °C. At 130 °C. It is observed that Localised Surface Plasmon Resonance (LSPR) peak of AgNWs is not detected, and only a small peak at 465 nm of silver nanoparticles is noted, as shown in Fig. 3Sa. This revealing that the temperature is not sufficient to accelerate the reduction process, and silver atoms are started to accumulate through Ostwald ripening process to form nanoparticles50,51. Meanwhile increasing the reaction’s temperature to 150 °C, the prepared AgNWs have a slightly smaller half-peak width of LSPR, as shown in Fig. 3Sb, indicating a decrease in AgNWs diameter36. Moreover, at 170 °C in Fig. 3Sc, AgNWs LSPR peaks have red-shifted to a higher wavelength, revealing an increase of AgNWs diameter at high temperature52,53.Figure 7 depicts XRD of AgNWs prepared using CuCl2 and mixed PVP at different temperatures. At 130 °C, the thermal energy is insufficient to form (111) facet and contain only (100) facet, and the final yield missed any dominant of AgNWs peaks54. In Fig. 7b, when the temperature is increased to 150 °C, XRD pattern shows that all silver facets predominate in the final yield with an intensity ratio of (111/200) facet 1.88 and lattice constant of 4.1266 Å. At 170 °C, high thermal energy is available to form AgNWs, and excessive (111) facets predominate in the final yield, as shown in Fig. 7c55.Fig. 7XRD patterns of AgNWs synthesized using CuCl2 and mixed PVP at different temperatures (a) 130 °C, (b) 150 °C, (c) 170 °C. * is attributed to AgCl.The FT-IR spectra in Fig. 4S demonstrate PVP-AgNWs interaction at different temperatures. In comparison to high temperature, the transmittance at 130 °C recording around 65% (Fig. 4Sa). This may be due to lack of enough thermal energy to make (111) facet; as a result, the PVP failed to be adsorbed onto silver’s facets, and dissolve in the washing process of the final yield. ​The spectrum in Fig. 4Sb shows a blue-shifted peak that is attributed to C=O of PVP to 1651.05 cm−1, while in Fig. 4Sc the peak has red-shifted to 1647.19 cm−1, implying a strong adsorption of PVP onto the silver’s surface46. Figure 8 discusses the morphology of AgNWs formed at different temperatures using CuCl2 and mixed PVP. Most of the silver atoms synthesized at 130 °C are nanoparticles. There is not enough thermal energy to oxidize the ethylene glycol to glycolaldehyde in which most of the silver cation remained in the solution as well as deficiency of thermal energy becomes clear in TEM in Fig. 8a,b55. As shown in Fig. 8c,d, long and thin nanowires are synthesized at 150 °C with a high aspect ratio of 15.6 which indicates the whole ethylene glycol was converted to glycolaldehyde to assist in the reduction process54,56. It is found that above the critical temperature, the aspect ratio is decreased with increasing the number of silver nanoparticles. Excessive available thermal energy in the solution becomes useless, and silver atoms start to accumulate to each other in a random way with less tendency to form uniform nanowires at 170 °C in Fig. 8e,f52. Accordingly, the optimum temperature in the reaction is 150 °C to have AgNWs with high aspect ratio.Fig. 8TEM images of AgNWs synthesized using CuCl2 and mixed PVP at different temperatures (a,b) 130 °C, (c,d) 150 °C, (e,f) 170 °C.Effect of reducing agentsTo investigate the influence of reducing agent on the final yield, the shape and average dimensions of silver nanowires are compared using diethylene glycol. The shown UV–Vis spectrum in Fig. 5S indicates two different patterns of AgNWs synthesized with different reducing agents. The characteristic peaks of AgNWs are narrow with sharp absorption in the case of using ethylene glycol, as seen in Fig. 5Sa, compared to wider and weak peaks that appeared when using diethylene glycol, as shown in Fig. 5Sb. All peaks in Fig. 5Sb are blue-shifted, which confirms the long non-uniformity of AgNWs, while wider absorption peaks for surface plasmon response indicate an increasing diameter of AgNWs34.The XRD patterns in Fig. 9 show formation of pure crystalline AgNWs prepared using ethylene glycol with an intensity ratio of (111/200) facets of 1.88 and lattice constant of 4.13 Ã…, as shown in Fig. 9a. These results agree with the reported data (2.5 and 4.0892 Ã…), respectively, from the JCPDS file 04-078357. Using diethylene glycol as a reducing agent, non-crystalline NWs were formed, as shown in Fig. 9b.Fig. 9XRD patterns of AgNWs prepared using CuCl2 and mixed PVP with different reducing agents (a) ethylene glycol, (b) diethylene glycol.Figure 10 displays the differences in FT-IR spectra of prepared AgNWs using different reducing agents. In the case of ethylene glycol in Fig. 10a, sharp peaks of stretching vibration of C=O and O–H in PVP skeletal ring, revealing that PVP is adsorbed uniformly on the silver’s surface46. Figure 10b demonstrates intense peaks below 1400 cm−1 that mentioned a significant interaction between C-N and silver surface and NO3–158. FT-IR confirms the hypothesis that silver nitrate molecules and PVP accumulate to form non-crystalline nanowires.Fig. 10FT-IR spectra of AgNWs synthesized using CuCl2 and mixed PVP with different reducing agents (a) ethylene glycol. (b) diethylene glycol.Figure 11 displays the morphology of AgNWs formed by two different reducing agents. High yield of AgNWs are formed using ethylene glycol as a reducing agent to reduce silver cations, as shown in Fig. 11a,b. TEM images in Fig. 11c,d illustrate the formation of non-crystalline silver NWs as well as nanoparticles using diethylene glycol. These particles are formed because diethylene glycol has no ability to reduce silver cations into silver atoms, which leads to the accumulation of the silver nitrate and PVP molecules into wire shapes.Fig. 11TEM images of AgNWs AgNWs synthesized using CuCl2 and mixed PVP with different reducing agents (a,b) ethylene glycol (c,d) diethylene glycol.Preparation of transparent conducting electrodeThe transparent conducting electrodes in Fig. 6S are prepared using the drop-casting method and subjected to the same treatment to purify the silver’s surface of any organic content. As the AgNWs concentrations increase on the coated substrate, the transmittance is dropped, and sheet resistance is decreased, meanwhile existence of nanoparticles reduces the electrical properties of the electrode59,60,61.The AgNWs are randomly dispersed over the glass substrate (80 µL), as seen in the SEM images of the electrodes in Fig. 12. The existence of silver nanoparticles cause scattering the incident light and reduces the electrode transmission53 as shown in Fig. 7S. The long and thin AgNWs are more suitable for optical properties in the transparent electrodes56. On the other hand, silver nanoparticles prepared at 100 °C melted and welded together to fill the gaps of AgNWs62,63. Most of fabricated electrodes at different reaction’s conditions have excessive amount of silver nanoparticles, making them unsuitable for photovoltaic applications with high sheet resistance (> 2 MΩ sq−1) and low transmissions64. Despite AgNWs prepared using CuCl2 and PVP-1.3 M (Fig. 12b) have high amount of silver nanoparticles, the sheet resistance exhibits 30 Ω sq-1 due to PVP layer was dissolved during the electrode treatment. In the case of using CuCl2 and mixed-PVP with a volume ratio of 1:1 of PVP-40 K and PVP-1.3 M (S3), pure silver nanowires prepared with around 10 nm thick PVP remained attached with AgNWs lead to high sheet resistance and high transmission (80%). Fig. 12SEM images of AgNWs electrodes with different conditions. (a) CuCl2 with PVP 40 K, (b) CuCl2 with PVP-1,3 M, (c) CuCl2 with mixed PVP (1:1) volume ratio, (d) HCl with PVP (1:1) volume ratio, (e) HCL with PVP-1,3 M, (f) HCl with PVP-40 K, (g) CuCl2 with mixed PVP (1:1) volume ratio and diethylene glycol as a solvent.The low NW-NW contact between nanowires is the reason for the high sheet resistance. S3 was further investigated using different coating amounts. 80 µL of AgNWs was not sufficient to prepare a transparent electrode, as shown in Fig. 13a as the electrode exhibits high sheet resistance. While increasing the amount of AgNWs to 200 µl (Fig. 13b), the transmission is reduced to 72% and sheet resistance records around 800 KΩ/sq.Fig. 13SEM images of AgNWs electrodes synthesized by CuCl2 and PVP (1:1) volume ratio, with different coated amounts (a) 80 µL (b) 200 µL.
