The reaction of Cu+ and PhC ≡ CH is illustrated in the Scheme 3. Before and after reaction, phenylacetylene and PhC2Cu have little effect on conductivity of solution (other substances, such as excess OAc−, remain unchanged, their effects on conductivity can be ignored). For a dilute solution, the conductivity L of a strong electrolyte is proportional to its concentration, and the total conductivity of the solution is equal to the sum of the conductivity of the electrolyte constituting the solution. With increasing reaction time, Cu+ and phenylacetylene formed PhC2Cu precipitate rapidly, and the conductivity of the solution decreases gradually. Therefore, the conductivity meter was used to measure the change of conductivity with time in this reaction process to track the change of reactant concentration with time.
The following kinetic model is often used to describe the second-order reaction rate equation of solid-liquid system (Ho et al. 2006),
where Ct is the concentration at the time of t, C0 the initial concentration, k is the rate constant. And Lt \(\propto\) Ct. So, plot with 1/Lt ~ t, if a straight line is obtained, it can be determined that the reaction is a second-order reaction. Plots of 1/Lt vs. t at the different temperature are shown in Fig. 1.
Obviously, these lines (1a-1c) have a very good linear relationship, relationship coefficients are more than 0.98 (r > 0.97). Now, it can be determined that the reaction is a pseudo-second-order reaction. The reaction rate constant k of the reaction at different temperatures can be obtained from the slope of the straight line (Table 1)
Table 1
Reaction rate constant at different temperatures
T/K | 298 | 303 | 308 |
k | 3.00×10− 5 | 4.91×10− 5 | 6.64×10− 5 |
According to Arrhenius formula, plot with ln(k/[k]) ~ 1/T (1d), the slope s of the line is -7.30×103 K, and the activation energy Ea = -s × R = 7.30 × 8.315 × 103 = 60.7 kJ ∙ mol− 1. It is worth noting that, because of the poor stability of Cu+ in solution, if the reaction is carried out at a temperature higher than 308 K, the reaction characteristics will no longer conform to the above-mentioned second-order reaction model due to the occurrence of side reactions.
The possible reaction mechanism between Cu+ and phenylacetylene is illustrated in the Scheme 4. SE2 reaction occurred between Cu+ and phenylacetylene, transition state (I) was formed firstly; then protons leaved to form phenylethynylcopper.
The X-ray diffraction (XRD) patterns of P and P* samples are shown in Fig. 2, as seen in the Fig. 2, due to their poor crystallinity, their XRD peaks are not very smooth. However, their main diffraction peaks do not show significant differences. Therefore, it can be determined that P and P* have the same composition.
Figure 3 shows typical SEM images of P and P* in the different scale. It is clearly shown that both the two materials are nanowire structure. Their widths are about 30 nm, the difference was not significant. However, the length of P is much smaller than that of P*, in other words, P has smaller particle size, which means that P may have better catalytic activity in heterogeneous catalytic reactions (Becker et al. 2011).
The optical properties of as-prepared P and P* samples were measured by Uv–vis DRS. As shown in Fig. 4a, in the range of 250–480 nm, both P and P* nanowires exhibit highly strong absorption, while in the range of 480–800 nm, their light-harvesting ability to visible light decreased rapidly. This means that enhancing their absorption capacity to visible light may be one of the focuses in the future research. The band gap of values of P and P* nanowires are approximately 2.20 eV in Fig. 4b, correspondingly, and their absorption edge are 563 nm.
Figure 5 represents FT–IR spectrum of P recorded in the range of 3500 − 500 cm− 1. As shown in Fig. 5, the peaks at 1928 cm− 1 are attributed to the C ≡ C bridging modes (Chui et al. 2005). And the peaks at 1595, 1480 and 1442 cm− 1 are attributed to the stretching vibration of the C = C band of phenyl groups. While 748 and 686 cm− 1 are the characteristic absorption peaks of bending vibration of benzene ring. Furthermore, P nanowire have no absorption peaks at 650 cm− 1 (ascribed to -C ≡ C-H) proving that hydrogen was substituted by copper completely (Krikor et al. 1987). Compared with P*, the infrared characteristic absorptions of P show a certain red shift. This phenomenon may be attributed to the surface effect of the material. With the decrease of particle size, the proportion of surface atoms increases. Compared with bulk atoms, the distance between surface atoms is larger, and the force constant of corresponding surface atoms is smaller, resulting in the decrease of surface phonon frequency. In this way, the relaxation effect of surface atoms may lead to the red shift of infrared spectrum.
The Raman spectrum of P nanowire at about 1006, 1186 and 1604 cm− 1 are the characteristic lines of benzene ring (Fig. 6). The Raman band at 1936 cm− 1 may be attributed to a second-order transition enhanced by Fermi resonance with v(C ≡ C) (Garbusova et al. 1973). Other lines (536, 790 cm− 1) may be ascribed to the vibrations origin from the coordination interaction between copper and phenylacetylene. It is worth noting that the positions of these lines show a “red shift” compared with the previous reports10, this may be due to the smaller size of P nanowires.
As shown in Fig. 7, transient photocurrent responses of P and P* nanowires were measured under intermittent visible light irradiation. Notably, the photocurrent density of P nanowire was about 110 µA/cm2, which is 20 times higher than that of P* at the same condition. This could mean that P nanowire has a better application potential in the field of photoelectric conversion.
The photocatalytic activities of P and P* nanowires were examined by degrading MV and glyphosate under visible light irradiation (300W xenon lamp was used as the visible light source). Before the irradiation, adsorption-desorption equilibrium was established by stirring in the dark for 30 min. As shown in Fig. 8, in the process of photodegradation, P nanowires exhibit a better catalytic activity than that of P*. As seen in Fig. 8, after 30 minutes of illumination, the degradation rates of MV and glyphosate were 97% and 70%, respectively. Glyphosate is more difficult to degrade than methyl violet, after 120 minutes of illumination, their degradation rates were 69% and 63% respectively.
The photocatalytic activities of P and P* nanowires were examined by degrading MV and glyphosate under visible light irradiation (300W xenon lamp was used as the visible light source). Before the irradiation, adsorption-desorption equilibrium was established by stirring in the dark for 30 min. As shown in Fig. 8, in the process of photodegradation, P nanowires exhibit a better catalytic activity than that of P*. As seen in Fig. 8, after 30 minutes of illumination, the degradation rates of MV and glyphosate were 97% and 70%, respectively. Glyphosate is more difficult to degrade than methyl violet, after 120 minutes of illumination, their degradation rates were 69% and 63% respectively.