Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Hydrogen sensing by sol-gel grown NiO and NiO:Li thin films   Hydrogen sensors have been prepared using nickel oxide (NiO) and
lithium-doped nickel oxide (NiO:Li) thin films, deposited on glass substrates
by the sol-gel spin coating technique. The surface morphology, structure,
optical and electrical properties of the obtained films were studied. Hydrogen
sensing results are presented for three operating temperatures (140, 160, and
180 {\deg}C) and for hydrogen concentrations ranging from 1000 to 15000 ppm in
synthetic air. The NiO and NiO:Li (2% and 8% doping concentrations) sensors
show maximum responses for the operating temperature of 180 {\deg}C. When
tested at different hydrogen concentrations in air, the lithium-doped NiO
sensors showed a higher response than the undoped NiO films.

10
The categories of this paper are: q-bio
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The categories of this paper are: hep-th
12
The categories of this paper are: nucl-th
36
The categories of this paper are: cmp-lg
3
The categories of this paper are: physics
21
The categories of this paper are: adap-org
30
The categories of this paper are: q-alg
14
The categories of this paper are: math-ph
35
The categories of this paper are: mtrl-th
33
The categories of this paper are: plasm-ph
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The categories of this paper are: dg-ga
20
The categories of this paper are: acc-phys
32
The categories of this paper are: atom-ph
11
The categories of this paper are: hep-lat
28
The categories of this paper are: alg-geom
13
The categories of this paper are: quant-ph