For chemical reactions in liquid state, such as catalysis, understanding of dynamical changes is conducive to practical applications. Solvation of copper salts in aqueous solution has implications for life, the environment, and industry. In an ongoing research, the question arises that why the color of aqueous CuCl2 solution changes with solution concentration? In this work, we have developed a convenient and efficient in situ surface enhanced Raman scattering technique to probe the presence of many intermediates, some of them are responsible for color change, in crystallization of aqueous copper chloride solution. The versatility of the novel technique was confirmed in the identification of five intermediates states in the transition from CdS to MoS2 nanowires in solution. The facile in situ method is expected to be widely applicable in probing intermediate states in a variety of chemical reactions in solution.
In an investigation of growing nanowires through sequential cation exchange1, it was noticed that the color of aqueous CuCl2 solution changes from light blue green to turquoise, green then dark green upon the increase in solution concentration. The natural question is what causes the change in color. Copper chloride is widely used for sterilizing bacteria in swimming pool2. In fact, the light blue green color of swimming pool water can often be attributed to the presence of Cu2+. In addition, copper chloride is a frequent oxidizing agent compound usually used for oxidizing phenol in chemical industry3. For electroless copper plating4, the research on intermediates is also critical for the improvement of the manufacturing process. Solvation of copper salts in aqueous solution has implications for life, the environment, and industry. In view of its importance in practical applications on top of the curiosity about the cause of a commonly observed physical phenomenon, we have carried out an investigation on the changes in aqueous CuCl2 solution upon the variation in solute concentration. A convenient and efficient in situ SERS technique was developed and the presence of many intermediates, some of them are responsible for color change, was readily detected. It is further revealed that intermediates undergo changes even under supersaturated conditions.
For chemical reactions in liquid state, understanding of dynamical changes is conducive to practical applications. As a prime example, research into catalysis, which is critically important in the production of most industrially important chemicals, is based on the reach to transition state with less energy or forming intermediates not produced naturally in the presence of catalysts. Among various transition metal catalysts, copper is considered to be most prominent and promising because of its versatility, low cost, and low toxicity5. In addition, copper is one of the most common transition elements in biological systems, a normal human can have 80 to 120 mg in their body6, and copper is present in a large number of enzymes, many involved in electron transfer, activation of oxygen, and oxygen transport7,8. Crichton8 showed that the Cu (II) four-coordinate complexes are of square planar and the Cu (I) four-coordinate complexes are of tetrahedral structure. Cu (II) is a transition metal ion with d9 (t2g6eg3) configuration and have significant Jahn-Teller effects. Identification of by-products9,10 and intermediates11,12,13 is an crucial step for providing complete picture of reaction pathways. In situ study of reactions provides a unique view of dynamic reactions in real time as they occur14,15,16,17,18. In the present study, a facile in situ surface-enhanced Raman scattering (SERS) technique was developed for the investigation of dynamical changes in copper chloride dihydrate crystallization process in CuCl2 aqueous solution. The intermediates in the crystallization of CuCl2 in water are clearly identified. The SERS substrates used are gold nanoparticles with two different average sizes on Si and PI film substrates. Copper chloride aqueous solution drop was first dripped on SERS substrates with a micropipette. In situ SERS observation was then carried out to detect the intermediates of Cu2+, Cl− , and H2O during dynamic change from aqueous states to solid state. Crystallization was found to undergo different vibrational energy state changes before and after supersaturation in ionic solution. Here we show a facile method to investigate the dynamic change of bonding of transition metal ions, Cu2+, and emphasize the in-situ SERS observation as a facile technique for the observation of dynamic changes of intermediates in a variety of chemical reactions in solution. The versatility of the novel technique was confirmed in the identification of five intermediates states in the transition from CdS to MoS2 nanowires in solution.
SERS was first discovered in 197419, Fleishman et al. utilize noble-metal surface to enhance the intensity and sensitivity of Raman spectroscopy. To increase sensitivity level and the applicability of SERS, a variety of nanostructures on surfaces with different morphologies20,21 were fabricated. In general, the sensitivity level critical to the enhancement behavior of SERS can be described by the enhancement factor (EF). The maximum SERS EF is a maximum localized charge density wave amplified on a specific positions (hot-spot) and the average SERS EF is an average charge density wave amplified on the surface. When the value of average SERS EF is as large as 107–108, it can be considered as a functional SERS substrate and is capable of detecting single-molecule22,23. The implication is that such a substrate24 is useful for precise in situ study. The ability to enhance Raman spectrum is fundamentally correlated not only with the precise details of the structure on the surface but also with the metallic material properties.
Figure 1a shows the photographs of CuCl2•2(H2O) (Alfa Asear, 99.999%) aqueous solutions with different mole concentrations, from left to right are pure water, solutions of 0.5 M, 0.75 M, 1 M, 2.5 M, 5 M, 7.5 M, 10 M, 12.5 M, 15 M and crystalline CuCl2•2(H2O). As shown in Fig. 1a, the solution color changes with concentration and if the concentration is higher than 10 M copper chloride dihydrate, it becomes supersaturated. An intriguing question is why the color of solution changes drastically from light blue to dark green with the increase in copper chloride concentration from 0.5 M to 7.5 M. Figure 1b shows UV/Visible transmittance spectra for copper chloride aqueous solutions (0.5 M, 0.75 M, 1 M, 2.5 M, 5 M, 7.5 M, 10 M, 12.5 M, and 15 M). It is apparent that the electronic states change continuously with the increase in copper chloride mole concentrations. The transmittance peaks change from 482 nm with broad full width at half maximum (FWHM) 176.6 nm at 0.5 M to 532 nm with broad FWHM 34.9 nm at 15 M. Interestingly, even under supersaturation conditions (10 M, 12.5 M, and 15 M), the wavelength, intensity, and FWHM of the absorption peaks still exhibit changes. It can be inferred that even when the solution is supersaturated, the addition of more solute (CuCl2) to the solution (CuCl2 aqueous solutions) still leads to the change in electronic states of the solution. Figure 1c shows the SERS spectra of copper chloride aqueous solutions (0.5 M, 0.75 M, 1 M, 2.5 M, 5 M, 7.5 M, 10 M, 12.5 M, and 15 M. It is seen that when concentration changes from 1 M to 2.5 M, a small peak at 289 cm−1 emerges. It is a bond between Cu2+ and Cl1− and the presence of the bonds is the reason for solution color changes from blue (482 nm) to turquoise (504 nm)24. After the concentration is raised to 15 M, 214 cm−1 and 406 cm−1 peaks emerge indicating the formation of crystalline CuCl2•2(H2O) in solution24.
The intermediate states of copper chloride aqueous solutions have been investigated with density functional theory, ab initio methods, extended X-ray absorption fine structure (EXAFS), Minuit X-ray absorption near-edge structure (MXAN) and UV-vis-near IR optical absorption spectroscopy previously25,26,27,28. To capture the dynamic reaction of those Cu2+ and Cl− during the concentration change from 1 M copper chloride aqueous solution to higher concentration, supersaturation and condensation, a drop of 1 M copper chloride aqueous solution was dispensed on the SERS substrate. The solution gradually evaporates at room temperature and eventually becomes crystalline CuCl2•2(H2O). The total evaporation process takes about 20 min and allows in-situ Raman measurements (514 nm, green laser) at 20 s interval. The main changes were detected to occur at about 920 s to 1200 s (Fig. 2). In the beginning24 (920 s), the total solution is full of Cu(H2O)62+ (Supplementary Fig. S7, stage I), as the times go by, a broad 289 cm−1 peak was generated (980 s), indicating the presence of a strong band of Cu-Cl bonds24. It is likely that that there is a complex mixture of CuCl+, CuCl2, CuCl3−, and CuCl42−25. The complex mixtures (Supplementary Fig. S7, stage II) contribute to a broadened 289 cm−1 peak at 980 s and change to CuCl42− and CuCl2•2(H2O) at 1040 s (Supplementary Fig. S7, stage III). The 108 cm−1 peak indicates a long bond stretching mode for linear Cl-Cu-Cl in CuCl2•2 (H2O), the 214 cm−1 peak is ascribed to a Cu-Cl bond in CuCl2•2 (H2O) (ag), and the 406 cm−1 peak corresponds to Cu-O bond in CuCl2•2 (H2O). The 188 cm−1 peak indicates Cu-Cl bond in CuCl42- (ag) and 289 cm−1 peak corresponds to Cu-Cl bond in CuCl42– (b2g). At 1040 s there is a 188 cm−1 peak indicating the presence of Cu-Cl bond in the tetrahedral structure CuCl42− (ag) (Supplementary Fig. S7, stage III). From 1040 s to 1080 s, the Cu2+ sustains stereoisomer compounds change with the same surrounding Cl−, as the same CuCl42− compound but with the structural change from tetrahedral to square planar. At 1080 s, the compound transforms to CuCl2•2 (H2O) as the reaction is completed (Supplementary Fig. S7, stage IV).
In Fig. 3, the dynamic changes of crystallization from 1 M copper chloride aqueous solution is captured with different wavelength (632.8 nm, red) laser pump. The difference from changes shown in Fig. 2 is the additional observation of short duration vibrational states: 547 cm−1, 468 cm−1 and 568 cm−1. The three vibrational states can further reveal the mechanism of CuCl2•2 (H2O) crystallization in supersaturated solution. Peak at 547 cm−1 indicates that there is a Cu-O bond and the bond stretching vibration is like a dicopper(III) complex [Cu2(μ-O)2(Et3CY)2](SbF6)229 at 290 s. At 320 s, peak at 468 cm−1 indicates a Cu-O bond30 and peak at 568 cm−1 indicating Cu-O bond like in CuO thin film31. The peaks not obvious in Fig. 2 but evident in Fig. 4 are corresponding to those vibration states more sensitive to the probing of 632.8 nm laser irradiation than that of 514 nm laser. It indicates that during crystallization, oxygen atoms bond with different angles and lengths to Cu (II) atom in CuCl2−.
Figure 4 summarizes vibration modes detected during the drying process of copper chloride aqueous solution (1 M) by real time SERS. SERS peaks appear at different times and concentrations can be differentiated into four stages. Stage I is the initial state with low solute concentrations. In this stage the bonds with Cu2+ are too weak for detection although the presence of Cu(H2O)62+ was detected by optical absorption spectroscopy reported in ref. 32. Stage II is signaled by the appearance of 289 cm−1 peak indicating the solute concentration is high enough so that the corresponding bonds are detectable by SERS. Stage III represents supersaturation stage with the appearance of vibration mode of 214 cm−1 as seen in Fig. 1 (c). Other modes, such as 108 cm−1, 188 cm−1, 289 cm−1 and 406 cm−1 are also detected under green laser irradiation. Additionally, transition states exhibited by 547 cm−1, 468 cm−1, and 568 cm−1 peaks appear at different time and are only evident under red light laser irradiation. In the final stage (stage IV), copper chloride aqueous solution is crystallized to form CuCl2•2 (H2O) and CuCl42− (square planar) compounds and SERS peaks would no longer change with time.
Figure 5 shows the real time change in SERS signals in the transition of a single CdS to MoS2 nanowires by cation exchange obtained with in situ observation at room temperature (Supplementary Fig. S8a). The results represent the first in situ observation of cation exchange32,33 process. In the SERS spectra, in the beginning (0 s), the 299 cm−1 peak corresponds to first order longitudinal optical mode (1LO) of CdS. At 340 s, the E2g1 (391 cm−1) and A1g (399 cm−1) signals of 2-D MoS2 structure emerge (Supplementary Fig. S9d). Overall, five intermediate states emitting 454 cm−1, 470 cm−1, 638 cm−1, 750 cm−1, and 798 cm−1 signals were identified. Both the 454 cm−1 and 470 cm−1 peaks arise after 40 s reaction and at 100 s reach the maximum intensities (Supplementary Fig. S8b). The two peaks can be ascribed to two phonon vibration mode, 2LA(M)34 and A2u35. At 100 s, 750 cm−1 peak indicates the presence of 2E2g1 vibration mode36. The 798 cm−1 and 638 cm−1 peaks appear at 220 s and 260 s and represent Mo-O-Mo vibrations37 and A1g(M) + LA(M) mode34, respectively. The in situ data indicate that in the transition from CdS to MoS2 by cation exchange at room temperature, five intermediate states are detected by the novel method. In addition, the differences between in plane vibration (E2g1) and out of plane vibration (A1g) of MoS2 during cation exchange (Supplementary Fig. S9) vary with the time and are 10 cm−1 (120 s), 11 cm−1 (200 s), 10 cm−1 (280 s), and 8 cm−1 (340 s) perhaps indicating the dynamical change in strain36.
514 nm laser (green laser) with 5 nm FWHM was also used as a light source to pump the CdS NW on SERS substrate. The green laser tends to excite CdS NW to emit green light since the band gap of CdS is around 2.4 eV (~516 nm). As a result, the Raman signals are obscured. It indicates that the method is constrained by the type of laser probe used as well as the band gap of the semiconductor nanowire.
In summary, we have developed a convenient and effective in situ SERS method to probe the intermediate states in the crystallization of copper chloride aqueous solution. From optical transmittance measurement and Raman spectroscopy, it is elucidated that the appearance of 289 cm−1 peak, corresponding to various Cu-Cl bonds, accompanies the change of solution color from light blue to green. Several intermediate states are disclosed by the in situ SERS under green and red laser irradiations. It is further revealed that intermediates undergo changes even under supersaturated condition. The versatility of the novel technique was confirmed in the identification of five intermediates states in the transition from CdS to MoS2 nanowires in solution. The facile in situ SERS technique is expected to be widely applicable for investigating the intermediate states in a variety of chemical reactions, which are of both fundamental and practical importance.
How to cite this article: Tan, C.-S. et al. Intermediates in the cation reactions in solution probed by an in situ surface enhanced Raman scattering method. Sci. Rep. 5, 13759; doi: 10.1038/srep13759 (2015).
The authors acknowledge the support from the Ministry of Science and Technology of Taiwan under grant numbers MOST 103-2221-E-007-003 and MOST 102-2633-M-007-002.
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The Journal of Physical Chemistry C (2016)