Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: One-dimensional van der Waals heterostructures as efficient metal-free
  oxygen electrocatalysts   Two-dimensional covalent organic frameworks (2D-COFs) are an emerging family
of catalytical materials with well-defined molecular structures. The stacking
of 2D nanosheets and large intrinsic bandgaps significantly impair their
performance. Here, we report coaxial one-dimensional van der Waals
heterostructures (1D vdWHs) comprised of a carbon nanotube (CNT) core and a
thickness tunable thienothiophene-pyrene COF shell using a solution based in
situ wrapping method. Density functional theory calculations and in-operando
and ex-situ spectroscopic analysis show that the carbon-sulfur region in the
thienothiophene groups is the active catalytic site. The unique coaxial
structure enables controllable n-doping from the CNT core to the COF shell
depending on COF shell thickness, which lowers the bandgap and work function of
COF. Consequently, the charge transfer barrier between the active catalytic
site and adsorbed oxygen intermediates becomes lower, resulting in a dramatic
enhancement in their catalytic activity for oxygen redox reactions. It enables
a high-performance rechargeable zinc-air battery with a specific capacity of
696 mAh gZn-1 under a high current density of 40 mA cm-2 and excellent cycling
stability. 1D vdWHs open the door to create multi-dimensional vdWHs for
exploring fundamental physics and chemistry, as well as practical applications
in electrochemistry, electronics, photonics, and beyond.

2
The categories of this paper are: math
23
The categories of this paper are: solv-int
34
The categories of this paper are: chem-ph
29
The categories of this paper are: funct-an
17
The categories of this paper are: q-fin
32
The categories of this paper are: atom-ph
18
The categories of this paper are: econ
9
The categories of this paper are: nlin
8
The categories of this paper are: hep-ex
33
The categories of this paper are: plasm-ph
20
The categories of this paper are: acc-phys
27
The categories of this paper are: comp-gas
10
The categories of this paper are: q-bio
14
The categories of this paper are: math-ph
35
The categories of this paper are: mtrl-th
1
The categories of this paper are: cs