The focus of this study is to broaden the application of Confocal Imaging System in the study of liquid/gas and liquid/solid interfaces. Through rationally designed experiments, the aggregation behavior of water-soluble porphyrin (TSPP) at the liquid/air interface was studied by utilizing the high longitudinal resolution of confocal Raman microscopy.Significant differences in the behavior of TSPP in the ontology and at the interface were found. At the same time, the composition of the solution near the electrode surface during the electrochemical oxidation of methanol was profiled. Taking advantage of its high lateral resolution, Raman analysis of thin gold layers deposited on glassy carbon by means of the surface enhancement effect Imaging research, discover the difference between optical imaging and Raman imaging.

  In recent years, various spectroscopic techniques using light as a means of excitation and detection have been developing rapidly with the continuous improvement of instrument performance and the introduction of new experimental methods.And new experimental methods have been developed rapidly and become indispensable for the characterization and study of interfacial structures at the molecular level.and have become indispensable means to characterize and study interfacial structures at the molecular level.They have become indispensable tools for characterizing and studying interfacial structures at the molecular level in situ. They are useful in identifying the molecular species involved in interfacial processes (e.g., chemical and electrochemical reactions).They have made remarkable achievements in identifying molecular species involved in interfacial processes (e.g., chemical and electrochemical reactions), studying the orientation of interfacial species, and determining the composition and thickness of surface membranes.In situ spectroscopic methods, in-situ spectroscopy has been used to identify the molecular species involved in interfacial processes (e.g. chemical and electrochemical reactions), to study the orientation of interfacial species, and to determine the surface membrane composition and thickness. Among the in situ spectroscopic methods, Raman spectroscopy.Among the in situ spectroscopic methods, Raman spectroscopy can provide the microstructural information of interfacial molecules more conveniently. However, in the absence of surface enhancement or resonance enhancement.However, in the absence of surface enhancement or resonance enhancement, the intensity of Raman scattering of monolayer species is lower than that of conventional (resonance-enhanced) spectroscopy.scattering of monolayer species without surface enhancement or resonance enhancement is lower than the detection sensitivity of conventional (non-confocal microscopy) Raman spectroscopy [ 1].without surface enhancement or resonance enhancement.The development of Raman spectroscopy has been greatly limited by the discovery of epitopes in the mid-1970s. Surface-enhanced Raman scattering (SERS) was discovered in the mid-1970s.Surface Enhanced Raman Scatt ering (SERS) discovered in the mid-1970s, has the effect of significantly enhancing surface (or surface) scattering.effect was discovered in the mid-1970s, which can significantly enhance the Raman scattering signals of surface species and is interface-sensitive.It is interface-sensitive. This phenomenon has attracted much attention since its discovery. However, this effect is onlyon a few special substrates (Au, Ag and Cu) is strong enough to be of practical interest [2].The resonance-enhanced Raman spectroscopy is also Resonance-enhanced Raman spectroscopy can also significantly enhance the Raman signal, but the effect itself is not interface-sensitive.This effect is not interface-sensitive per se, and the native species also contribute to the Raman signal [ 3].The contribution of ontogenetic species to the Raman signal is also significant [ 3]. How to extract the weak interfacial signals from the interference of strong body signals is a key issue in interfacial studies.signals from the interference of strong body signals is the focus of interface research. In the past decades, various techniques have been developed to study the interfacial processes.In the past decades, various techniques have been developed to study interfacial processes. In recent years, confocal microscopy has been widely used in Ramaninstrument has been widely used, which brings a breakthrough in the application of Raman spectroscopy in interfacial science.This paper will firstly briefly introduce the confocal microscopy technique. In this paper, we will firstly briefly introduce the principle and characteristics of confocal microscopy system, and list the applications of this technique in liquid/gas interfaces.This paper will first briefly introduce the principle and characteristics of confocal microscopy system, and list some applications of this technique in liquid/gas and solid/liquid interfaces and Raman imaging.The paper will first briefly introduce the principle and characteristics of the confocal microscope, and list some applications of this technique in liquid/gas and solid/liquid interfaces and Raman imaging.

    Raman spectroscopy is measured using the L abRam Type I confocal Raman microscope from Dilor and the THR-1000 Raman microscopy from SPEX. The common feature of both systems is the efficient filtering of Rayleigh lines and reflected light using notch filters.So just one monochromator. In addition, it is equipped with a CCD detector, which is therefore highly sensitive. In addition, due to the confocal microscope, the sampling volume of the instrument is only micron-level, which greatly reduces the influence of the signal of the solution phase on the adsorbed species signal on the electrode surface, thereby further improving the detection sensitivity of surface species. The lens is an Olympus 50x long-focal length objective, and the slit of the spectrometer can be rooted.Set up according to different experimental needs. The excitation light is the 6321 8 nm excitation line provided by the He-NE laser and the 488 nm excitation line provided by the Ar+ laser, respectively.

Principles of confocal Raman spectroscopy

    The principle of confocal technology was proposed as early as 1957, but it was not used for optical section analysis until 1967. In 1977, this technique began to be used in Raman spectroscopy. However, it was not until the nineties that confocal microscopy was really widely used in Raman [4]. The working principle of confocal Raman microscopy is shown in Figure 1, that is, the laser beam is focused on the sample surface through the incident pinhole (H1), the irradiated point on the sample surface is imaged at the probe pinhole (H2), and its signal is collected by the detector after H2 (the optical path is shown by the solid line); When the laser is defocused on the sample surface, most of the signal at the sample is blocked by H2 (the optical path is shown by the dotted line), and there is no way to reach the detector through the pinhole. When we move the sample up and down in the direction of the laser incidence, we can focus the laser on different layers of the sample, so that the acquired signal will also come from different layers of the sample, so that the sample can be profiled. It can be seen that the biggest feature of this structure is that it can effectively exclude the interference from other layer signals outside the focal plane, so as to effectively exclude the influence of the solution body signal on the layer signal that needs to be analyzed. Benefiting from this, we obtained a weak electrochemical system.Enhances Raman signaling of monolayer adsorbed species on metal surfaces[5]. Theory and experiments show that the larger the numerical aperture of the microscope lens and the smaller the diameter of the probe pinhole, the better the confocal performance of the instrument [6]. At the same time, confocal microscopy systems themselves have a high horizontal spatial resolution, which depends only on the magnification of the microscope lens and the wavelength of the laser used. This article will focus on the application of confocal Raman spectroscopy in various interface studies.

Study of porphyrin aggregation processes at liquid/gas interfaces

    Due to the special role of porphyrins and their derivatives in photosynthesis, porphyrins have been extensively studied in the past few decades. The aggregate of porphyrin plays a particularly important role in the process of energy transfer and transformation. In this study, the characteristics of confocal Raman spectroscopy were used to study the aggregation behavior of porphyrins at the interface. PORPHYRINS ARE WATER-SOLUBLE TETRAP-SULFONATOPHENYL PORPHYRIN, T SPP, WHICH HAVE THREE FORMS IN AQUEOUS SOLUTION, FREE T SPP, PROTONATED H4T SPP2, and T SPP aggregate. The three forms of T SPP have different absorption wavelengths in solution, namely 413, 434, and 490 nm [ 7]. The excitation line at 488 nm was used to selectively excite the resonant Raman signal of the aggregate to provide sufficient sensitivity to detect the monolayer information. The concentration of T SPP solution is 10-5 mol# L-1 and the electrolyte is 10-3 mol# L-1 NaClO4. The pH of the solution is adjusted at 4 such that only protonated T SPP exists in the solution. Figure 2a shows the absorption spectrum of the solution body, and the presence of only a single peak of 434 nm in the Soret region indicates that there are no detectable aggregates in the solution. Moreover, when the laser is focused on the liquid/gas interface and the solution body of the above solution, we also find that there is no obvious Raman signal of aggregates in the body, and the Raman spectrum is characterized by protonated T SPP, which verifies the results of the absorption spectrum, indicating that only protonated T SPP exists in the body. However, the Raman spectral signal obtained at the liquid/gas interface is significantly different from that obtained in the body, as shown in Figure 2(b). The Raman strength of the ontology is much lower than the signal of the interface, and the latter in the figure is 5 times stronger. At the same time, the frequency and relative intensity of some peaks in the spectrum changed. In the body, the peak frequencies of 1 240, 1 327, 1 548 cm - 1 all underwent significant displacement, and a new peak of 988 cm - 1 appeared. In the low wavenumber region, the peak intensities of 245 and 317 cm - 1 changed significantly. The appearance of a peak of 988 cm - 1 and abnormal enhancement of the signal in the low wavenumber region are characteristic of aggregates [7]. Since the polygamite is absorbed at 490 nm, it can be excited by the excitation line at 488 nm to produce a resonant Raman process, which significantly enhances the signal on the surface.The above results show that the aggregation behavior of T SPP in the interface region and in the bulk is significantly different. The reason, first of all, is most likely that the concentration of protonated T SPP at the interface is higher, and the chance of T SPP colliding with each other and aggregating is greater than in the ontology, so that T SPP aggregates can be generated at the interface. Of course, we cannot completely rule out another possibility, that is, in solution, protonated T SPP can already form aggregates, which will produce salting out under the condition of admixture ions, resulting in an increase in the concentration of aggregates at the interface and enhanced Raman signal. However, under such conditions, the signal of the aggregate should also be detected in the liquid phase. So, the first is more likely. In both cases, the enrichment of aggregates at the interface leads to further interaction between them to generate larger aggregates, further enhancing the low-wavenumber signal. We have seen this phenomenon in experiments that change the concentration of NaClO4 solutions

Fig 2 ( a)  Absorption  spectra of 10- 5 mol# L- 1 TSPP +10- 3 mol # L- 1 NaClO4 ( pH =  41 0)    ( b ) Raman spectra from the bulk solution and the liquid/ gas interface. Excitation line: 488 nm

It should be noted that such studies are possible only on confocal systems. Traditional Raman acquisition devices collect all Raman signals in the light zone into the detector, making it impossible to distinguish the source of the signals. Especially when the signal of the interface is enhanced, it will mask the contribution of other layer signals. In a confocal microscopy system, the collected signal can be divided so that the source of the signal can be distinguished by focusing so that it is from different layers. It should be noted that due to the limitations of instrument sensitivity, current studies must also take advantage of resonance enhancement effects. However, we believe that with further improvement in instrument performance, it will be possible to study signals without enhanced monolayers. In addition, it is possible to detect unenhanced signals using multiple internal reflections for such systems, but experimental implementation is relatively difficult.

detects changes in solution composition during electrochemical reactions

The above examples illustrate the advantages of Raman spectroscopy in the study of the liquid/gas interface. For confocal systems, we can also use its characteristics to monitor the changes in the solution composition in the interface area of the reactive electrode surface in real time. We chose a methanol oxidation process that is important in electrocatalysis and fuel cell research. It is known that the total reaction of methanol anodizing is

CH3OH+H2O    CO2+6H⁺+6e⁻

It can be expected that the oxidation of methanol will lead to a decrease in the pH of the liquid layer near the electrode surface and a decrease in the concentration of methanol. Using the confocal Raman microscopy system with the characteristics of profile analysis, the laser can be focused on the electrode surface, so that the Raman signal mainly comes from the electrode surface to adsorb species, and when the laser is focused on the electrode surface, the Raman signal is mainly from the bulk solution, so by changing the distance between the laser focusing point and the electrode surface, we can obtain the concentration gradient relationship of each species in the solution with the distance d from the electrode surface during the reaction process. In the experiment, we can not only detect the intermediate products (CO) of dissociation and adsorption of the system, but also monitor the change of solution composition [8]. Figure 3 shows the spectral peaks of SO4HSO and methanol obtained in 1mol#L-1CH3OH 011mol#L-1H2SO4 solution at 019V with d. The vibration peaks of 980, 1 018 and 1 050 cm - 1 are SOHSO and C) O)H, respectively. It can be seen

When d is 0, the spectral peak intensity of HSO is significantly greater than that of SO-, and the spectral peak intensity of methanol is lower, indicating that due to the oxidation of methanol, methanol near the electrode surface is consumed in large quantities, and a large amount of H is generated, resulting in a decrease in the pH of the solution. When d increased to 20 Lm, the ratio of HSO and SO- peak intensity changed rapidly, but the peak intensity of HSO was still slightly greater than that of SO-, and the spectral peak intensity of methanol also increased, after that, the peak intensity of these three peaks changed slowly, and when d reached 100 lm, the peak intensity of HSO was slightly lower than that of SO-, indicating that the pH value at this position was closer to the bulk solution. We found that even at 100 Lm from the electrode surface, the peak intensity of 1 018 cm - 1 representing methanol was much lower than the pre-reaction intensity, indicating a large methanol consumption. By comparing the integrated intensity ratios of SO- and HSO- spectral peaks near the electrode surface and in the solution body, we can also calculate the pH near the electrode surface at the corresponding potential [ 9] . In addition, by using the confocal characteristics, we can also focus the laser on the electrode surface, keep the focusing conditions unchanged, and detect the changes of surface species at different reaction potentials. Similarly, we found that at high reaction potentials, the local methanol concentration decreased, while the pH of the solution decreased due to the large amount of H produced by the oxidation of the methanol. The above studies show that Raman spectroscopy can be used as a multifunctional interface research and analysis technique.

Fig.3 Spatial resolved Raman spectra in 1 mol # L- 1CH3OH+ 01 1 mol# L-  1 H2SO4 , showing the concen- tration gradient in the vicinity of the electrode sur- face. The roughness factor of Pt is about 200 . The potential was held at 01 9 V.  Excitation line: 63218 nm

 Raman imaging

From the above studies, it can be seen that confocal microscopy systems can provide high longitudinal (Z) resolution. The microscope features of the system also have a high transverse (X-Y) resolution. Fujishima et al. developed surface-enhanced Raman imaging (SER I) using confocal Raman microscopy and SER S technology, and studied SER S active silver surface adsorbents and SERI images of self-assembled membranes [10]. The spatial resolution of this technique depends on the magnification of the microscope lens and the wavelength of the laser used. Under current instrument conditions, i.e. utilizing a 50x telephoto lens and 6321 8 nm excitation light, the spatial resolution is approximately 11 5 Lm. Experiment with us

Probe molecules with a large Raman scattering section and strong and stable surface interaction can be selected first, such as pyridine. If we want to detect the A1 vibration peak (1 012 cm-1) of pyridine, we can set the spectrum range to 990 ~ 1 030 cm - 1. By scanning the sample stage, the integral Raman intensity at each point is recorded, and then the intensity and scanning range are used as grayscale maps to give the corresponding Raman image of the substrate. The electrode substrate used in our experiment is a thin layer of SERS activity gold deposited on a glassy carbon electrode with a scanning range of 30@ 30 Lm2. The result is shown in Figure 4. Although the surface of the deposition observed under a normal microscope is quite uniform, we can see from the Raman image that the Raman signal varies considerably between points on the gold surface. Bright spots indicate areas with strong Raman signals, dark spots indicate areas with weak Raman signals, and it can be seen that the intensity difference between the light and dark areas of the image can be up to 10 times. This shows that we should not only use surface topography to judge the uniformity of surface SERS strength, but should effectively judge by Raman imaging. In addition to the application of judging SER S activity, this technology will also have a wide range of applications in the study of conductive polymers, L-B membranes and self-assembled membrane electrodes, as well as electrode passivation films and micro-area corrosion. Due to the limitation of optical diffraction limits, the spatial resolution of this technique can only be at the sub-micron level, which cannot be compared with scanning probe technology. However, the recent emergence and further development of near-field optical Raman microscopy technology that breaks through the optical diffraction limit and has a spatial resolution of tens of nanometers will surely bring a new revolution in Raman spectroscopy technology, so that Raman spectroscopy can be more widely used in field research.

Fig.4  ( a) Surface Raman image ( 30 @ 30 Lm) of the elec-trodeposited gold surface on a glassy carbon electrode for the ring breathing mode of pyridine at-018V ( vs.  SCE) .  ( b) Surface enhanced Raman spectra of the bright region ( A) and the dark region ( B) . The gold surface was deposited at - 013 V ( vs. SCE) for 5s. Solution:  0101 mol# L- 1 pyridine + 01 1 mol# L- 1 KCl . Excitation line: 63218 nm

References

1.B Pet tinger . in Adsorpt ion of M olecules at M etal Electrodes, J Lipkowski and P N R oss, Eds. New York: VCH 1992, 285

2.A Campion and P K ambhampati. Chem . S oc. Rev . , 1998, 27: 241

3.T G Spiro an d P St ein. A nnu . Rev . Phys . Chem  . , 1977, 28: 501

4.S Sharonov , I Nabiev , I Chourpa et al. J . Raman Sp ec trosc . , 1994, 25: 699

5.Z Q T ian, J S G ao , X Q Li et al. J. R aman Spectrosc . , 1998, 29: 703

6.B Ren (任斌) . The Doctor. s Degree Disser tation , X iamen Univ . (厦门大学博士学位论文) , 1998

7.D L A kins, H R Zhu and C G uo . J . Phys. Chem . , 1996, 100: 5420

8.X Q Li ( 李筱琴) . T he M aster. s Degree T hesis , Xiamen Univ . (厦门大学硕士学位论文) , 1998

9.B Ren, X Q Li, C X She et al. Electrochim . A cta , 2000 ( in press)

10.X M Y ang , D A T ryk , K Has himot o et al. J. R aman Spectrosc . , 1998, 29: 725

Application of Confocal Microprobe Raman Spectroscopy in the Analysis of Interfaces

Bin REN, Xiao qin LI, Yong XIE, Weny un HU and Zhongqun T IAN

Dep art ment of Chemistr y and State Key Lab. f or Physical Chemistry of Solid Surf aces, Xiamen University , 361005 Xiamen

Abstract   

Confocal micro probe Raman spectroscopy has been applied to the study of liquid/ g as and solid/ liquid interfaces. With the help o f the r esonance Raman effect, the aggr eg atio n process of TSPP has been mo nitor ed at the liquid/ gas interface using Raman spectroscopy, which is shown to have very different behav ior compared with that in the bulk. The solution layer analysis has been performed in the interfacial region above a Pt electrode under reaction, and the solution composit ion is monito red in situ during the electrochemical reaction.Taking the adv antage of high spatial  resolution of the confocal microscope, the surface enhanced Raman imaging has been performed on the electrodeposited gold surface.

Keywords  

Confocal microprobe Raman spectroscopy , Liquid/ Gas interface, Solid/ Liquid interface