Basic Principles of UV-Visible Diffuse Reflectance Spectroscopy

Which range of light does UV-visible spectroscopy utilize?

The wavelength range of ultraviolet (UV) light is 10-400 nm, and the range for visible light is 400-760 nm. Light with wavelengths greater than 760 nm is considered infrared (IR) light. The range of wavelengths from 10 to 200 nm is referred to as far ultraviolet (UV) light, while the range from 200 to 400 nm is referred to as near ultraviolet (UV) light. UV-visible spectrophotometers typically utilize near ultraviolet (UV) and visible light, with a typical testing range from 200 to 800 nm.

Figure 1:UV-Visible Spectrum

What can UV-Visible Diffuse Reflectance Spectroscopy (UV-Vis DRS) do?

UV-Visible Diffuse Reflectance Spectroscopy (UV-Vis DRS) can be used to study the light absorption properties of solid samples, as well as the structure, oxidation state, coordination state, and coordination symmetry of transition metal ions and their complexes on catalyst surfaces.

What is diffuse reflection?

When a beam of light is incident on a powdery crystalline surface, part of the light undergoes specular reflection on the surface of each grain; another part of the light is refracted into the interior of the surface grains, partially absorbed, and then emitted to the interior grain interface, where it undergoes reflection, refraction, and absorption again. This process repeats several times, and finally, the radiation is reflected in all directions from the surface of the powder, which is called diffuse reflection.

Figure 2:Diffuse Reflectance Spectrum Diagram

Basic Principles of UV-Visible Spectroscopy

For UV-visible spectroscopy, whether it's UV-visible absorption or UV-visible diffuse reflectance, the fundamental reason is often electronic transitions.

For organic compounds, electronic transitions include n-π, π-π transitions, which will be introduced in UV-visible spectrophotometry.

For inorganic compounds:

1. In the transition metal ion-ligand system, one part acts as the electron donor while the other as the acceptor. Upon light excitation, charge transfer occurs, with electrons absorbing photons of certain energies and transferring from the donor to the acceptor, resulting in absorption spectra in the UV region. Among these, when the charge transfers from the metal (M) to the ligand (L), it is called MLCT (Metal-to-Ligand Charge Transfer); conversely, when the charge transfers from the ligand to the metal, it is called LMCT (Ligand-to-Metal Charge Transfer).

2. When transition metal ions themselves absorb photons and undergo transitions within their internal d orbitals (d-d transitions), causing absorption bands due to coordination field, the required energy is lower, exhibiting absorption spectra in the visible or near-infrared region.

3. Surface plasmon resonance of noble metals: Noble metals can be considered as free electron systems, with their optical and electrical properties determined by conduction band electrons. In the theory of metal plasma, if certain electromagnetic disturbances cause non-zero charge density in some regions of the plasma, electrostatic restoring forces occur, leading to oscillations in charge distribution. When the frequency of the electromagnetic wave matches the plasma oscillation frequency, resonance occurs. This resonance, on a macroscopic level, manifests as absorption of light by metal nanoparticles. Surface plasmon resonance of metals is an important factor determining the optical properties of metal nanoparticles. Due to internal plasmon resonance excitations in metal particles or interband absorption, they exhibit absorption bands in the UV-visible region.

Testing Method of UV-Visible Diffuse Reflectance Spectroscopy - Integrating Sphere Method

An integrating sphere, also known as a light integrating sphere, is a hollow spherical shell designed to collect light. The inner wall of the integrating sphere is coated with a white diffuse reflecting layer (usually MgO or BaSO4), and the diffuse reflection at each point on the inner wall of the sphere is uniform. The intensity of light produced by the light source S at any point B on the sphere wall is the sum of the intensities of light produced by multiple reflections. The purpose of using an integrating sphere is to collect all the diffuse reflected light, and the principle of measuring diffuse reflectance spectra using an integrating sphere is as follows: Since the sample absorbs UV-visible light more strongly than the reference (usually BaSO4), the signal of the diffuse reflected light collected by the integrating sphere is weaker. This difference in signal can be transformed into a UV-visible diffuse reflectance spectrum. Using an integrating sphere can avoid differences in diffuse reflection caused by the process of light collection.

Figure 3: Measurement of Diffuse Reflection Including Specular Reflection Using an lntegrating Sphere

The Law of Diffuse Reflection (K-M Equation)

The law of diffuse reflection describes the optical relationship of a monochromatic light beam incident on an object that both absorbs and reflects light.

Key points:

1. In practice, as seen from the integrating sphere method above, what is usually measured is not the absolute reflectance R∞, but rather a relative reflectance compared to a standard sample (usually BaSO4).

2. The diffuse reflection of the sample is wavelength-dependent.

3. In a dilute species, F(R∞) is proportional to the species concentration, similar to Lambert's Law (to be discussed in UV-visible spectrophotometry).