Laboratory Report 3

Laboratory Report 3: Monosorb and ATR-IR Spectroscopy Kunyi Lin Objective: 1. Using Monosorb to measure particle surface area 2. Using ATR-IR Spectroscopy to investigate solid-liquid interaction processes

Introduction to methods: 1. What is Monosorb? Monosorb is an instrument applying BET absorption isotherm for determining particle surface area.

2.What is BET ISOTHERM? BET is BRUNAUER - EMMETT - TELLER (BET) ISOTHERM, In 1938 Stephan Brunauer, Paul Emmett, and Edward Teller developed a model isotherm that takes that possibility into account. Their theory is called BET theory, after the initials in their last names. This is an isotherm for multilayer adsorption.

x is the pressure divided by the vapor pressure for the adsorbate at that temperature (usually denoted P / P0), v is the STP volume of adsorbed adsorbate, vmon is the STP volume of the amount of adsorbate required to form a monolayer and c is the equilibrium constant K we used in Langmuir isotherm multiplied by the vapor pressure of the adsorbate. The key assumption used in deriving the BET equation that the successive heats of adsorption for all layers except the first are equal to the heat of condensation of the adsorbate.

3.What is ATR-IR Spectroscopy? Attenuated total reflection infrared (ATR-IR) spectroscopy is used for analysis of the surface of materials. It is also suitable for characterization of materials which are either too thick or too strong absorbing to be analyzed by transmission

spectroscopy. For the bulk material or thick film, no sample preparation is required for ATR analysis. For the attenuated total reflection infrared (ATR-IR) spectroscopy, the infrared radiation is passed through an infrared transmitting crystal with a high refractive index, allowing the radiation to reflect within the ATR element several times.

The sampling surface is pressed into intimate optical contact with the top surface of the crystal such as ZnSe or Ge. The IR radiation from the spectrometer enters the crystal. It then reflects through the crystal and penetrating “into” the sample a finite amount with each reflection along the top surface via the so-called “evanescent” wave. At the output end of the crystal, the beam is directed out of the crystal and back into the normal beam path of the spectrometer. To obtain internal reflectance, the angle of incidence must exceed the so-called ‘critical’ angle. This angle is a function of the real parts of the refractive indices of both the sample and the ATR crystal:

Where n2 is the refractive index of the sample and n1 is the refractive index of the crystal. The evanescent wave decays into the sample exponentially with distance from the surface of the crystal over a distance on the order of microns. The depth of penetration of the evanescent wave d is defined as the distance form the crystal-sample interface where the intensity of the evanescent decays to 1/e(37%) of its original value. It can be given by:

Where l is the wavelength of the IR radiation. For instance, if the ZnSe crystal (n1=2.4) is used, the penetration depth for a sample with the refractive index of 1.5 at 1000cm-1 is estimated to be 2.0µm when the angle of incidence is 45°. If the Ge crystal (n1=4.0) is used under the same condition, the penetration depth is about 0.664µm. The depth of penetration and the total number of reflections along the crystal can be controlled either by varying the angle of incidence or by selection of crystals. Different crystals have different refractive index of the crystal material. By the way, it is worthy noting that different crystals are applied to different transmission range (ca. ZnSe for 20,000~650cm-1, Ge for 5,500~800cm1).

Experimental Procedures: 1. Monosorb i. Add TiO2 sample (>200nm) into the U-shape tube, and refill liquid nitrogen into the cup ii. Raise the cup full of liquid nitrogen to cover the U-shape tube, start to measure iii. Wait for reaction arriving at equilibrium (the signal becomes stable) iv. Record the value shown on the Monosorb, the unit of surface is M2 v. Divide surface area over the mass of particles, get the surface per unit mass 1. ATR-IR Spectroscopy i. Put the sample into detecting area ii. Start to monitor the reaction on the screen iii. Observe the interaction of solid and liquid

Results: 1.Monosorb After measurement, we gained the surface area of total TiO2 particles, 0.98 m2. Since we added 0.1 g into the U-shape tube, so the average surface area per unit mass is 0.98 0.1 =9.8 m2/g for TiO2. 2. ATR-IR Spectroscopy ATR-IR Spectroscopy showed us that when we selected certain portion of the particles, TiO2, we could see what they interact with other molecules around themselves, including vibration, and chemical reaction.

Discussion: 1. Monosorb is a very convenient instrument to measure surface area of particles, but

it utilizes the absorption of inert gas into particles to measure how much gas do the particles absorb to know surface area. It takes time to let the absorption achieving equilibrium, so it’s not that quick and instant. 2. ATR-IR spectroscopy is is one of the few available tools to investigate processes taking place at solid-liquid interfaces. Through the software, we can clearly see how solid and liquid interact with each other. It’s really convenient to observe solid-liquid reaction processes.