高导电耐1．3V vs RHE腐蚀纳米非化学计量比氧化钛Magneli粉体混合物

Highly conductive nano－sized Magnéli phasestitanium oxide （TiOx）

Aditya F． Arif

Ratna Balgis

Takashi Ogi

Ferry Iskandar

Akihiro Kinoshita

Keitaro Nakamura

Kikuo Okuyama

Abstract

Despite the strong recent revival ofMagnéli phase TiOx as a promising conductive material， synthesis of Magnéliphase TiOx nanoparticles has been a challenge because of the heavy sintering nature of TiO2 at elevatedtemperatures． We have successfully synthesized chain－structured Magnéli phases TiOx with diameters under 30 nm using athermal－induced plasma process． The synthesized nanoparticles consisted ofa mixture of several Magnéli phases．A post－synthesis heat－treatment wasperformed to reduce the electrical resistivity without changing the particlemorphology． The resistivity of the heat－treated particle was as low as 0．04 Ω．cm， with a specific surface area of52．9 m2 g−1． The effects of heat－treatment on changes in the crystalstructure and their correlation with the electron conductivity are discussedbased on transmission electron microscopy images， X－ray diffraction spectra，and X－ray adsorption fine structure spectra． Electrochemical characterizationusing cyclic voltammetry and potentiodynamic scan shows a remarkable electrochemical stability in a strongly oxidizingenvironment．

Figure 1． Illustration of （a） TiOx NP  preparation using RF induction thermal plasma method， and （b） postsynthesis heat  treatment to improve the electrical conductivity．

Figure 2． SEM and TEM images showing the  chain structure and the lattice structure of the NPs， and particle size  distribution of （a） TiOx－A， （b） TiOx－B， and （c） TiOx－C． Red boxes in the TEM  images indicate discontinuous lattices． Some lattice spaces can be associated  with reduced titanium oxides， such as Ti3O5 ［002］ （4．73 Å）， Ti2O3 ［202］ （2．11  Å）， Ti4O7 ［004］ （2．48 Å）．

Figure 3． XRD spectra and the main peaks  assignment of TiOx－A， TiOx－B， and TiOx－C． TiOx－A shows the greatest extent of reduction， indicated by the  domination of the Ti2O3 phase． The broad peak indicated by 1 is assigned to  Ti3O5， Ti4O7， Ti8O15， and TiO2 while 2 is assigned to Ti3O5 and TiO2． Samples with a greater extent of  reduction tend to have darker colors． A complete list of the identified  species is provided in the supporting information．

Figure 4． SEM， TEM images， and particle  size distribution of heat－treated （a） TiOx－A， （b） TiOx－B， and （c）TiOx－C． A  clear lattice structure can be observed for the heat－treated samples， showing  Ti4O7 ［022］ （2．8 Å）， Ti4O7 ［122］ （2．9 Å）， and higher oxidation states TiOx  （likely Ti8O15 ［10 11］ for 3．63 Å）．

Figure 5． （a） Fraction of Ti3＋ and Ti4＋  in the samples based on XAFS analysis．  The Magnéli phase composition of each sample was determined based on the XRD  spectra of （b） TiOx－A， （c） TiOx－B， and （d） TiOx－C． （i） is the spectra of  as－synthesized nanoparticles and （ii） is the spectra of heat－treated  nanoparticles．

要使用这么高端的表征方式，这个表征难度挺大。

Table 1． Electrical resistivity of the  samples before and after heat－treatment． The  electrical resistivity of the samples significantly decreased after heat  treatment． TiOx－A has the highest electrical conductivity．

折合电导值为25 S／cm

Figure 6． （a） Cyclic voltammogram of  heat－treated TiOx－A in oxygen－saturated  HCl 1 M solution between 0 and 1．4 V／RHE at a potential sweep of 50 mV／s．  The voltammogram shape did not significantly change after 1000 cycles，  especially in the pseudocapacitive region． （b） Potentiodynamic curve of  heat－treated TiOx－A in oxygen－saturated HCl 1 M solution between 0 and 1．5  V／RHE． The curve shows a tendency of TiOx－A to passivate with a high  break－down potential．

Pseudocapacitive：赝电容

break－down potential：击穿电位

Compared to the cyclic voltammogram of Ti4O7  particles， the current densities exhibited by TiOx－A were lower， likely  because of the lower electrical conductivity as a consequence of containing several  Magnéli phases． However， TiOx－A possesses much better stability over  potential cycling than Ti4O7．

1．4 V／RHE is the maximum theoretical  potential of a fuel cell during start and stop conditions． This potential was  chosen as the cyclic voltammetry reverse potential to represent a severe  environment that might be imposed to TiO x NPs in their application as  conductive material．

Figure S4． （a） cyclic voltammogram and  （b） potentiodynamic curve of Ti4O7 in oxygen－saturated 1M HCl． （c） XRD  spectra of Ti4O7 sample showing Ti4O7 peaks （Ref． ICDD No． 50－787） and （d）SEM  images of Ti4O7 particles

Table S4． Comparison of electrochemical  characteristics of TiOx－A and Ti4O7

不能使用Eeq和Tafel来判断实际的应用级别腐蚀速率。

合成过程对实验结果影响非常大。

Conclusions

In summary， we have demonstrated the  synthesis of Magnéli phases titanium oxide NPs via an RF induction thermal  plasma method． Nanoparticles with low electrical resistivity were successfully  obtained after applying a post－synthesis heat treatment to the as－synthesized  NPs． In particular， we found an interesting phenomenon in the formation of  new crystal structures after the heat treatment and suggested mechanisms for  the crystal structure changes based on the evidence from XAFS and XRD  analyses． The resistivity values of the heat－treated samples were related  with the Magnéli phase content， especially Ti4O7， estimated from the XRD  spectra． Further investigation of this phenomenon is encouraged for a better  understanding of the solid－state transformation of Magnéli phase titanium  oxide． The electrochemical characterization shows a remarkable stability of  the heat－treated TiOx－A NPs in a strongly oxidizing environment， which opens  a wider opportunity for the application of plasma－synthesized TiOx NPs．