First, the origin of the color of matter
The color of matter is the result of its reflection. White light is a mixture of light and is made up of a mixture of various shades of light. If a substance appears yellow in a white light environment (such as nano zinc oxide), it is because this object absorbs some or all of the blue light. The color of the material is due to its selective absorption of light of different wavelengths.
Different colors of light have different wavelengths, and different materials absorb different wavelengths of colored light. Substances can only selectively absorb radiation that has energy equal to the sum of vibrational energy, rotational energy change, and electron motion energy change of the material. In other words, even the same substance can have different colors if it can be at different energy levels. For example, zinc oxide, either in normal or nano form, is yellow at high temperatures and light at low temperatures. The reason is that at different temperatures, the energy of transition between molecular energy and electron energy of zinc oxide is different, and therefore, the absorption of various color lights is different.
Second, the structure of coarse zinc oxide and nano-zinc oxide, and the resulting intramolecular energy difference
The direct or indirect zinc oxide of coarse particles is an ionic crystal. In general, zinc atoms and oxygen atoms exist in the form of ion bonds. Due to the coarser particles, the number of oxygen atoms and zinc atoms in each particle is quite large, and the number of the two atoms is the same (see ZnO for molecular formula 1:1). However, for nano-ZnO, the particles are quite fine, which increases the number of unbonded atoms on the surface of the particles. In other words, nano-ZnO can no longer be regarded as an ideal crystal with an infinite number of ideal crystal planes. On its surface, there are disordered intercrystalline structures and crystal defects. The presence of these atoms, which are different from the central part of the surface, makes them highly capable of reacting with other substances, which is what we commonly call activity.
Studies have shown that in nano-ZnO, there are at least three states of oxygen, they are lattice oxygen (inside the particles), surface adsorption of oxygen and hydroxyl oxygen (--OH), and the amount of zinc in the particles is greater than that of oxygen. The quantity is not a 1:1 condition. This is completely different from ordinary zinc oxide. Oxygen vacancies exist on the surface of nano-zinc oxide, there are many dangling bonds, and it is easy to react with other atoms to react. This is also the basic principle of the application of nano-zinc oxide as an active agent in rubber and catalysts.
Due to the above difference between nano zinc oxide and ordinary zinc oxide. The energy levels of the transitions of the molecular energy and the electron energy in the particles are different, and therefore their colors are also different. Normal zinc oxide is white, and nano zinc oxide is slightly yellow.
Third, the nano-zinc oxide changes with time and environmental humidity, the color change
For nano-zinc oxide, due to the presence of adsorbed oxygen and hydroxyl oxygen on the surface of the particles, the amount of these two kinds of oxygen will change with time, such as the adsorption of moisture and the re-adsorption and exfoliation of oxygen in the air. The change in the amount of these two kinds of oxygen will inevitably cause the change of the molecular and electronic energy level in the particles, and the absorption of light will also be different. Therefore, the color of the nano-zinc oxide becomes lighter.
Fourth, the relationship between the color and purity of nano-zinc oxide
Pure nano-zinc oxide, its color is pure yellow, it looks bright color.
When the nano zinc oxide contains impurities such as iron, manganese, copper, and cadmium, the color of the zinc oxide will be felt in the yellowish soil, because of the oxidation of iron, manganese, copper, cadmium, etc. The materials are all colored materials. After being mixed with each other, several shades of light are blended to show white soil. The change in color of nano zinc oxide (or active zinc oxide, light zinc oxide) over time will be masked by the earth color, making the color appear to change minimally; when the nano zinc oxide contains high levels of impurities The color will become very deep, and it is impossible to observe the change of its color with time.
As mentioned before, the color of a substance is caused by its selective absorption of external light. Therefore, when we compare the color of zinc oxide, it is best to observe in the outdoor bright place. Selecting a different environment for comparison will give you a different comparison result, which also reflects the interestingness of light reflection.
Fifth, another explanation of the color of nano zinc oxide
Nano-zinc oxide is obtained by calcining basic zinc carbonate. In this process, if the basic zinc carbonate is not completely decomposed, the color of the nano zinc oxide will appear white because the basic zinc carbonate is pure white. In addition, production in the South and North, or in wet and dry weather, can also affect color. Because the nano-zinc oxide reacts with moist air and carbon dioxide to form basic zinc carbonate, the reverse reaction of the calcination process occurs. How much this change affects the quality of the product, it is still difficult to determine because basic zinc carbonate itself is also catalytic, suitable for use in desulfurizers and rubber industries; while in the feed industry, basic zinc carbonate has and oxidation The same function of zinc, it is also a feed additive, at the same time, in the feed industry, our concern is mainly whether the content of heavy metals meets the standard requirements.