# Mechanism of WS2 Nanotube Formation Revealed by in Situ/ex Situ Imaging

## TL;DR

This study reveals a new mechanism for forming WS2 nanotubes using in situ imaging, showing how oxide cores recede during high-temperature reactions.

## Contribution

A new 'receding oxide core' mechanism is identified, complementing the existing 'surface-inward' model for WS2 nanotube growth.

## Key findings

- The 'receding oxide core' mechanism becomes prominent at temperatures above 900 °C.
- H2S diffusion slows as WS2 layers form, leading to anisotropic volatilization of the oxide core.
- Fresh WS2 layers form within the cavity created by oxide vapor reactions.

## Abstract

Multiwall WS2 nanotubes have been synthesized
from W18O49 nanowhiskers in substantial amounts
for more
than a decade. The established growth model is based on the “surface-inward”
mechanism, whereby the high-temperature reaction with H2S starts on the nanowhisker surface, and the oxide-to-sulfide conversion
progresses inward until hollow-core multiwall WS2 nanotubes
are obtained. In the present work, an upgraded in situ SEM μReactor with H2 and H2S sources
has been conceived to study the growth mechanism in detail. A hitherto
undescribed growth mechanism, named “receding oxide core”,
which complements the “surface-inward” model, is observed
and kinetically evaluated. Initially, the nanowhisker is passivated
by several WS2 layers via the surface-inward reaction.
At this point, the diffusion of H2S through the already
existing outer layers becomes exceedingly sluggish, and the surface-inward
reaction is slowed down appreciably. Subsequently, the tungsten suboxide
core is anisotropically volatilized within the core close to its tips.
The oxide vapors within the core lead to its partial out-diffusion,
partially forming a cavity that expands with reaction time. Additionally,
the oxide vapors react with the internalized H2S gas, forming
fresh WS2 layers in the cavity of the nascent nanotube.
The rate of the receding oxide core mode increases with temperatures
above 900 °C. The growth of nanotubes in the atmospheric pressure
flow reactor is carried out as well, showing that the proposed growth
model (receding oxide core) is also relevant under regular reaction
parameters. The current study comprehensively explains the WS2 nanotube growth mechanism, combining the known model with
contemporary insight.