Chemical Education Journal (CEJ), Vol. 4, No. 1 /Registration No. 4-12/Received January 31, 2000.
URL = http://www.juen.ac.jp/scien/cssj/cejrnlE.html

Solid State Chemistry in General Chemistry Laboratory: The Intercalation of Hydrogen in Tungsten Oxide

Chun-Guey Wu*; Wuh-Leng Lin

Department of Chemistry, National Central University
Chung-Li, Taiwan 32054, ROC

E-mail: t610002@cc.ncu.edu.tw

Abstract: Solid state chemistry has been one of the most extensively studied subjects in the recent two decades. However, in the past, chemistry-major students did not have much training on solid state reactions and the techniques of physicochemical studies on solid state materials. This designed experiment employed a simple red-ox reaction of tungsten oxide (WO3) to demonstrate a typical solid state intercalation reaction and techniques of conductivity measurements. Some general concepts in solid state chemistry, such as intercalation, categorizing of materials based on conductivity, and charge transport properties of solid state materials are also introduced.

Introduction

At the end of the twentieth century, the rapid progress of science and technology has forced basic science educators to face a great challenge. Namely: how to educate youth both in basic knowledge as well as frontier science and technology in a limited time. The department of chemistry at National Central University was founded recently (1993). Our education and research goals were focused specially on environmental science and materials chemistry. These two important fields of chemistry were also integrated into the curriculum of our undergraduate courses. For example, chemical waste treatment [1]; the preparation of pH sensors; and the intercalation reactions in solid state materials [2] were included in the general chemistry laboratory.

Solid state chemistry was one of the bases of the materials science. In the past, most chemistry departments did not put the solid state chemistry-related experiment, especially synthesis, in the curriculum of undergraduate laboratory courses. This may be because the theory of the solid state materials is difficult for student majores in chemistry and most physicochemical experiments on solid state materials require extra instruments. Furthermore, in the past, traditional solid state chemistry had been regarded as a not quite knowledgeable science , due to the fact that preparation procedures of solid state materials are simple but the products are complicated and unpredictable in nature. Nevertheless, nowadays, solid state chemistry has become one of most extensively studied sciences amongst the chemists.Highly developed science and technology require new materials and new techniques. No matter the preparation of new materials or using new techniques to fabricate old materials, all need the knowledge and skill of solid state chemistry and experiments. In this respect, the training of experimental skills of solid state chemistry is especially important. The intercalation of hydrogen in tungsten oxide, taken from "Teaching General Chemistry: the Materials Science Companion" edited by A. B. Ellis et al. [3], was a good example to demonstrate the typical reaction of solid state materials and the modification of physicochemical and charge transport properties of solid state materials by simple chemical reactions. The preparation procedure is simple and the tremendous changes in the conductivity and color can attract students easily.

Tungsten oxide (WO3) is a well-known electrochromic (color change by applying electric field) material with diverse colors. The structure of WO3 is built by the corner sharing of WO6 octahedron units. When an atom, such as hydrogen or a metal ion, is inserted in the WO3 structure, the material (MxWO3, M= H, Ca, Sr, Ba  ..) is called tungsten bronze. Tungsten bronze owes its name to the metallic luster and is used in the production of  bronze  paints. It also provides a good example of the formation of intense and characteristic color by the reduction of metal oxides. The corresponding color of MxWO3 in different degrees of reduction of W+6 is shown in Figure 1 [3].

Figure 1: The color of Na xWO3 with different x values (degree of reduction of W).

Furthermore, the charge transport properties of WO3 also change from insulating to semiconducting to metallic conducting upon reducing W+6 and then intercalating foreign atoms. The conducting materials can be roughly divided into semiconductor, metallic conductor and superconductor, based on the temperature-dependent conductivity. The higher the temperature, the higher the conductivity is semiconductor. The conductivity of the metal decreased when the temperature increased. When the materials have no resistance at a certain temperature (generally very low), we call them superconductors. The conducting mechanisms of those conductors may be very complicated. However, a simple chemical reaction (such as reduction-oxidation) can change the material from one type of conductor to another.

Chemistry involved

The reduction-oxidation between WO3 and Zn

WO3 + x/2ZnWO3x- + x/2Zn+2

The intercalation of H+ in WO3

WO3x- + x H3O+HxWO3 + x H2O

The reduction-oxidation between HxWO3 and O2

4HxWO3 + xO24WO3 + 2xH2O

Physicochemical studies

The conducting property of a material comes from the movement of ions, electrons, or holes inside the material. Therefore, the conductivity of the material depends on the mobility and number of charge carriers. In the reduction of WO3, the W+6 was reduced to W+5, the extra electrons were located in the conduction band of WO3, and therefore increased its conductivity. The resistance (unit: Ohm) and conductivity (unit: Simen/cm or 1/ohm.cm) were commonly used to express the degree of conducting. However, the resistance depends on the size and shape of the materials, therefore, the most commonly used way to express the movement and number of charge carriers in a material is the conductivity.

In this experiment, the set-up for measuring the resistance is shown in Figure 2. The resistance was read directly from the multimeter.

 


There are many ways to measure the conductivity. The two-probe method was one of the simplest. The set-up of the two-probe conductivity measurement is shown in Figure 3 and the conductivity was calculated as follows:

 

Furthermore, the structure of WO3 changed when the W+6 was reduced to W+5, therefore, the electronic structure also changed. The variation of the energy gap between the HOMO and LUMO of WO3 also changed the color of the material.

Experimental procedure

(A) Synthesis

  1. 0.5 g WO3 is placed in a 150 mL beaker, and the color of WO3 is reconded.
    (Caution: WO3 is an irritant, handle it in the hood, avoid contact with skin and eyes)
  2. In a beaker, 50 mL 3.0 M HCl(aq) is added with stirring,and the change in WO3 after HCl(aq) was added is observed.
  3. Add 1 g of zinc powder to the beaker, observe and record the changes during the reaction. Repeat procedures 1 and 2, add different amounts of zinc powder, such as 1.2 g, 1.5 g respectively, and observe the difference between each reaction. (Caution: This step will produce hydrogen gas, which is very flammable and explosive )
  4. When the reaction finishes (no gas evolved and the color of the WO3 does not have further change ), filter the reaction mixture through filter paper,then wash the solid with distilled water and, dry in air.
  5. The formation of the intercalation compound HxWO3 is judged by the color change of the reagent WO3. It can also be obown by X-ray powder diffraction measurement.

(B) Conductivity measurement

  1. Press WO3 or HxWO3 powdey in the middle of a capillary tube, the size of the sample should be about 1 cm in length. Each end of the sample is connected with a copper wire, as shown in Figure 2. The copper wires are then connected to a multimeter and the resistance is read directly. Record the resistance of both WO3 and HxWO3. For better comparison, the sample length in each sample should be the same.

    (Caution: The packing of the sample and the contact between copper wire and sample will affect the results of the measurement.)

  2. Pack the sample into a bigger capillary tube and, do the same resistance measurement.
  3. Change the length of the sample,and do the same resistance measurement.
  4. For the conductivity measurement, WO3 or HxWO3 is pressed to a pellet (1.2 cm in diameter), then put in-between two copper plates, as shown in Figure 3. Apply a voltage to the sample and measure the current with a current meter. The conductivity could be calculated using equation 1 shown in the section on physicochemical studies.

(C) The stability of tungsten bronze

  1. Heat the as-prepared HxWO3 powder inan the oven at 100oC for 30 minutes, then measure the resistance again, record the data and color and compare it with that before heating.
  2. Measure the resistance of the samples after standing in air at room temperature for 1 week and, observe the change resistance and color.

Results of learning evaluation

Most of the chemistry freshmen not know the difference between solid state materials and molecular solids; resistance and resistivity; and the intercalation reaction and addition reaction. In addition almost all of the students do not have any experience with solid state reaction. Therefore, a learning evaluation was carried out amongst forty freshmen major in chemistry right after the experiment was done. The results were as fallows:

  1. Up to 97% of the students could distinguish between insulators, semiconductors, and conductors.
  2. Some 65% of the students were familiar with the ligand field theory and band theory, but only 10% were able to apply the ligand field theory and band structure to explain the color change in WO3 upon reduction and insertion of hydrogen ions.
  3. 50% of the students understood the relationship between current, voltage, resistance, and conductivity.
  4. 56% of the students knew how to do good conductivity measurements.
  5. 59% of the students knew the factors which affect the insertion of H+ in WO3.

Conclusion

From this designed experiment, students can learn several concepts and techniques in solid state chemistry, such as the preparation of solid state materials, solid state reactions, reduction-oxidation reactions, conductivity measurement, and the units and physical meaning of conductivity.

References

  1. Wuh-Leng Lin; Chun-Guey Wu; Kwang-Ting Liu, Chemistry, 1996, 54, 63-66.
  2. Chun-Guey Wu; Wuh-Leng Lin, Chemistry, 1999, 57, 45-48.
  3. A. B. Ellis; M. J. Geselbracht; B. J. Johnson; G. C. Lisensky; W. R. Robinson, Teaching General Chemistry: A Materials Science Companion, ACS Publication, 1993, P415.

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