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A review of growth mechanism, structure and crystallinity of anodized TiO_2 nanotubes

机译：阳极氧化TiO_2纳米管的生长机理，结构和结晶性研究进展

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摘要

This paper reviews the state of the art of anodized titanium dioxide nanotubes (TiO_2 NTs), with an emphasis on the growth mechanism leading to their formation and the effect of heat treatment on their structure and properties. The discussion is primarily focused on TiO_2 NTs grown in fluoride containing electrolytes, although the mechanism of formation of NTs in fluoride free solutions via Rapid Breakdown Anodization (RBA) is briefly covered. After an initial overview of progress made on the synthesis of anodized TiO_2 NTs the review provides an analysis of the factors affecting the anodizing process (fluoride concentration, electrolyte type, applied potential and anodizing time). Details of the current-time transient, the chemistry of the process and the chemical composition of the anodic films are described which provide key information to unveil the nanotube growth mechanism. The main debate is whether NTs growth in fluoride containing solutions occurs via field-assisted plastic flow (i.e. a constant upward displacement of the oxide to form the NTs) combined with field-assisted ejection of the Ti~(4+) ions (i.e. ions are ejected into the electrolyte without oxide formation) or via field-assisted dissolution (i.e. preferential dissolution at the pore base where the field is stronger) or whether both processes play a role. Whenever anodization takes place in organic solutions the experimental evidence supports the plastic flow model, whereas in aqueous media field-assisted (and chemical) dissolution occur. The mechanism of rib formation on the walls of the NTs is also reviewed, and it clearly emerges that the applied potential and water content in the electrolyte are key factors in determining whether the NTs are ribbed or smooth. There also appears to be a relationship between the presence of ribs and the evolution of oxygen bubbles at the anode. The impact of thermal treatment on the properties of the NTs is also described. A variety of crystalline structures are present in the NTs (i.e. anatase or rutile). depending on the heat treatment temperature and atmosphere and the resulting electrical properties can be varied from dielectric to semi-metallic. A heat treatment temperature limit ranging from 500 to 800 C exists, depending on preparation history, above which sintering of nanoscale titania particles occurs leading to collapse of the NTs structure. Future work should aim at using annealing not just to influence the resulting crystalline phase, but also for generating defects to be exploited in specific applications (i.e. photocatalysis, water splitting and photovoltaics).
机译：本文回顾了阳极氧化二氧化钛纳米管（TiO_2 NTs）的技术现状，重点介绍了导致其形成的生长机理以及热处理对其结构和性能的影响。尽管简要介绍了通过快速击穿阳极氧化（RBA）在无氟溶液中形成NTs的机理，但该讨论主要集中于在含氟电解质中生长的TiO_2 NTs。在初步概述了阳极氧化TiO_2 NTs的合成进展后，本综述对影响阳极氧化工艺的因素（氟化物浓度，电解质类型，施加电势和阳极氧化时间）进行了分析。描述了当前时间瞬态，工艺化学和阳极膜化学成分的详细信息，这些信息提供了揭示纳米管生长机理的关键信息。主要争论是含氟溶液中NTs的生长是否通过场辅助塑性流（即氧化物不断向上移动以形成NTs）与Ti〜（4+）离子（即离子）的场辅助喷射相结合而发生。或通过场辅助溶解（即在电场较强的孔基础处优先溶解）或两种过程均起作用而将其喷射到电解质中而不形成氧化物）。每当在有机溶液中发生阳极氧化时，实验证据就会支持塑性流动模型，而在水性介质中会发生现场辅助（和化学）溶解。还回顾了NTs壁上肋形成的机理，并且清楚地表明，电解质中的施加电势和水含量是确定NTs是否带肋或光滑的关键因素。肋的存在与阳极处的氧气气泡的发展之间似乎也存在关系。还介绍了热处理对NTs性能的影响。在NT中存在多种晶体结构（即锐钛矿或金红石）。根据热处理温度和气氛以及所产生的电性能，可以从介电到半金属变化。取决于制备历史，存在500至800℃的热处理温度极限，在该温度之上，发生纳米级二氧化钛颗粒的烧结，从而导致NTs结构的塌陷。未来的工作应旨在利用退火不仅影响最终的结晶相，而且还要产生将在特定应用（例如光催化，水分解和光伏）中利用的缺陷。

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  • 来源
    《Materials Science & Engineering》 |2013年第12期|377-406|共30页
  • 作者

    D. Regonini; C.R. Bowen; A. Jaroenworaluck; R. Stevens;

  • 作者单位

    Materials Research Centre, Department of Mechanical Engineering, University of Bath, Claverton Down Road, BA2 7AY, UK,Laboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology, Uberlandstrasse 129, CH-8600 Duebendorf Switzerland;

    Materials Research Centre, Department of Mechanical Engineering, University of Bath, Claverton Down Road, BA2 7AY, UK;

    National Metal and Materials Technology Center, 114, Thailand Science Park, Paholyothin Road, Klong 1, Klong Luang, Pathumthani 12120, Thailand;

    Materials Research Centre, Department of Mechanical Engineering, University of Bath, Claverton Down Road, BA2 7AY, UK;

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  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

    TiO_2; Nanotubes; Anodization; Growth mechanism; Annealing; Grain growth;

    机译：TiO_2;纳米管;阳极氧化;生长机理;退火;晶粒长大;

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