Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Facile size-controllable synthesis of single crystalline \b{eta}-MnO2
  nanorods under varying acidic strengths   A simple one-pot hydrothermal synthesis of single crystalline beta-MnO2
nanorods with diameters in the range of 10-40nm is reported. During the
synthesis process, the acid molarities were varied from 1.1M down to 0.2M in
steps of 0.3M while keeping the other reaction parameters constant, resulting
in gradual transformation of the size of beta-MnO2 from micro to the nanoscale
dimension. The as-synthesized nanorods exhibit soft ferromagnetic behavior and
possess a high catalytic activity with an onset potential of -0.17V in
facilitating the oxygen reduction reaction (ORR).

27
The categories of this paper are: comp-gas
14
The categories of this paper are: math-ph
30
The categories of this paper are: q-alg
6
The categories of this paper are: astro-ph
5
The categories of this paper are: gr-qc
7
The categories of this paper are: hep-th
21
The categories of this paper are: adap-org
26
The categories of this paper are: bayes-an
23
The categories of this paper are: solv-int
9
The categories of this paper are: nlin
2
The categories of this paper are: math
3
The categories of this paper are: physics
24
The categories of this paper are: dg-ga
0
The categories of this paper are: hep-ph
31
The categories of this paper are: ao-sci
29
The categories of this paper are: funct-an