This editorial is part of our series “1000 at 1000,” highlighting the Journal of Materials Science’s most highly cited publications as part of the journal’s celebration of 1000 issues. In this issue: “Review of the anatase to rutile phase transformation”  by D.A.H. Hanaor and C.C. Sorrell from the University of New South Wales in Sydney, Australia.
Our editor-in-chief, Barry Carter, sends me almost all the submissions to this journal that feature titanium oxides, so it was no surprise when I was voluntold to write about this review article from 2011, one of our “Sapphire Prize” finalist articles [2, 3].
Hanaor and Sorrell’s “Review of the anatase to rutile phase transformation”  captured my attention from the first paragraph (emphasis mine): “The likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transformation are deduced.” I have handled over 100 submissions of review articles, and this publication has all the hallmarks of a great one: timely, authoritative, comprehensive, critical and predictive. Think to yourself about how many review articles you have read and found useful, that tick all these boxes rather than being simply a summary of publications. And how many have used their literature analysis to make predictions?
At the heart is the most important phase change in titania: above a critical temperature—one that depends on time, atmosphere, dopants and a half-dozen other factors noted by the authors—the relatively open structure of metastable anatase (Fig. 1a) densifies to the thermodynamically stable rutile (Fig. 1b). This transformation changes not only the ceramic’s morphology, but also its photocatalytic activity. About 20–30% of the 100,000 + publications on titania study photocatalysis, so the scientific impact of a clear review was certain to be widespread. “Yet up until the publication of my review paper,” Dr. Hanaor wrote to me, “there was no work comprehensively addressing this transformation and the literature relating to this was spread across a broad range of anecdotal observations.”
During his doctoral research, he saw how additives affected this transition in bulk titania, powders and films. “In seeking answers to the observed phenomena, I found a tremendous amount of literature and observations that were screaming for some sense to made from them. In creating this review, I sought to put as much information as possible into one place, in order to create coherence in the field. The goal was to better understand this transformation and help others understand it better.”
What of the predictive element of the work? The authors devised a simple model based on ion valence and its radius and classified published reports of dopants as either promoting the anatase-to-rutile transition or inhibiting it (Fig. 2). I did a cursory search of the literature before contacting the authors. Some predicted promoters, like Mg2+ and Be2+, still haven’t been systematically studied. The authors’ prediction that Ag+ would promote the transition has since been validated [4, 5]. They predicted that Pd should be a promoter, but Pd2+ (and higher valences) has been shown to be an inhibitor . However, the data for six-coordinate Pd ions places the element very close to their transition line .
Carbon and nitrogen, used to shift titania’s band gap into the visible region, were predicted to be promoters. The authors attached some caveats to their analysis related to carbon’s assumed valence state and it acting as a reducing agent as elevated temperature. Contrary to their prediction, a study using multiwalled carbon nanotubes as a carbon source suggested that carbon inhibited the transition . Nitrogen was assumed to substitute on the oxygen lattice, increase the number of oxide vacancies and thus promote the transformation. Experimental reports on the effect of nitrogen on the anatase-to-rutile transition have been mixed: a few have stated it was a promoter [9,10,11], but the majority classified it as an inhibitor [12,13,14,15,16,17,18,19,20,21,22].
Dr. Hanaor called the linear formula “too simplistic” in his correspondence with me, but he offered that the smaller lower valence cations promote the transformation as a rule of thumb. He pointed to improvements in DFT as a way forward in the study of the transformation [23,24,25,26,27,28,29,30,31].
What is next for titania? Surprisingly, Dr. Hanaor commented that despite the vast number of publications on the various polymorphs of titania, its only significant application has been in pigments. He flagged “bronze titania” (Fig. 3) as a material that merits further investigation. Also known as TiO2(B), this material contains Ti(III), Ti(IV) and additional small cations to balance the charge [32, 33], and it has been considered as a high-rate ionic conductor for lithium-ion batteries [34, 35].
Dr. Hanaor is currently teaching courses on ceramics, energy materials and nanomaterials at TU Berlin. “I believe that creating valuable insights requires looking at a topic from multiple perspectives,” he wrote. “I like to think that I inspire my students to look at problems from multiple angles and apply diverse tools to bring about progress in various fields. Personally, the work on this paper led me to appreciate the value in creating insights from literature sources.”
The ca. 50,000 downloads of Hanaor and Sorrell’s work from the Springer website is testament to the value of a well-crafted review article.
Hanaor DAH, Sorrell CC (2011) Review of the anatase to rutile phase transformation. J Mater Sci 46(4):855–874. https://doi.org/10.1007/s10853-010-5113-0
The Sapphire Prize: Celebrating the best research published in the journal’s Sapphire Anniversary Year (2010) https://www.springer.com/cda/content/document/cda_downloaddocument/JMSC+Sapphire+Prize+Flyer+Full.pdf?SGWID=0-0-45-1018737-p35692639. Accessed 06 May 2020
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