Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Stable Co-Catalyst-Free Photocatalytic H2 Evolution From Oxidized
  Titanium Nitride Nanopowders   In the present work, a simple strategy is used to thermally oxidize TiN
nanopowder (ca. 20 nm) to an anatase phase of a TiO2:Ti3+:N compound. In
contrast to the rutile phase of such a compound this photocatalyst provides
photocatalytic activity for hydrogen evolution under AM1.5 conditions - this
without the use of any noble metal co-catalyst. Moreover the photocatalyst is
active and stable over extended periods of time (tested for 4 months).
Importantly, to achieve successful conversion to the active anatase polymorph,
sufficiently small starting particles of TiN are needed. The key factor for
catalysis is the stabilization of the co-catalytically active Ti3+ species
against oxidation by nitrogen present in the starting material.

31
The categories of this paper are: ao-sci
29
The categories of this paper are: funct-an
8
The categories of this paper are: hep-ex
35
The categories of this paper are: mtrl-th
27
The categories of this paper are: comp-gas
14
The categories of this paper are: math-ph
25
The categories of this paper are: patt-sol
12
The categories of this paper are: nucl-th
2
The categories of this paper are: math
36
The categories of this paper are: cmp-lg
18
The categories of this paper are: econ
37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
15
The categories of this paper are: nucl-ex
7
The categories of this paper are: hep-th
5
The categories of this paper are: gr-qc