VKC ScienceChemistry for written exams Telegram

UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2019 · Paper I

Q3(a). Conductivity of ZnO heated in vacuum and in oxygen

  • 10 marks
  • Answered
Question

Zinc oxide is a semiconductor that loses oxygen atoms in a vacuum but gains additional oxygen atoms when heated in oxygen. Its conductivity increases when it is heated in a vacuum but decreases when it is heated in oxygen. Account for these observations.

Answer in brief

ZnO is an n-type, metal-excess semiconductor (Zn_(1+δ)O). Losing oxygen leaves electrons that enter the conduction band, so conductivity rises; taking up oxygen withdraws these electrons as O²-, so conductivity falls.

Key points

  • the filled valence band is built mainly from O 2p orbitals, the empty conduction band from Zn 4s orbitals. Stoichiometric ZnO has very few carriers.
  • In vacuum oxygen leaves the lattice as O₂; the two electrons of each departing O²- stay in the crystal: O²-(lattice) → 1/2O₂(g) + 2e⁻.
  • the extra Zn sits in interstitial sites or beside oxide vacancies, and its electrons occupy donor levels just below the conduction band and are easily excited into it.
  • More electrons in the conduction band means more charge carriers: the conductivity increases. Conduction is by electrons (negative carriers), so the oxide is n-type.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. Pure ZnO is a wide band gap solid: the filled valence band is built mainly from O 2p orbitals, the empty conduction band from Zn 4s orbitals. Stoichiometric ZnO has very few carriers.
  2. In vacuum oxygen leaves the lattice as O₂; the two electrons of each departing O²- stay in the crystal: O²-(lattice) → 1/2O₂(g) + 2e⁻.
  3. The solid becomes Zn_(1+δ)O (or ZnO₁₋ₓ): the extra Zn sits in interstitial sites or beside oxide vacancies, and its electrons occupy donor levels just below the conduction band and are easily excited into it.
  4. More electrons in the conduction band means more charge carriers: the conductivity increases. Conduction is by electrons (negative carriers), so the oxide is n-type.
  5. In oxygen the reverse happens: 1/2O₂(g) + 2e⁻ → O²-(lattice). Oxygen fills the vacancies and removes electrons from the conduction band; conductivity falls.
  6. Hence the conductivity of ZnO depends on the oxygen pressure over it: σ falls as p_(O₂) rises.

Application. the same response is used in ZnO and SnO₂ gas sensors: adsorbed oxygen traps electrons and a reducing gas (CO, H₂) releases them, changing the resistance. The colour change of ZnO, white when cold and yellow when hot, also comes from loss of oxygen on heating.

Practising this answer? Write it in a Mains test and get it checked by VKC. Civil Services Mains test series · Telegram

Model answer to Q3(a), 2019 Paper I: solution page 12
Solution page 12 of 40
Read the whole 2019 solution2019 question paper

All parts of Question 3