LaTiO2N formed using our methods are rather nanoporous; providing plenty of opportunities for use in catalytic and sensing applications. In this work we used this material for using light to split water (in a process called photoelectrochemical water splitting).
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Nitrogen can play a "good guy" when locked into a solid that taps sunlight
Nitrogen can play a "good guy" when locked into a solid that taps sunlight
In this string of publications we showed that nitrogen; that ever present gas that constitutes almost 78% of dry air in our atmosphere, is a friend when locked inside a solid. TiO2, a very well respected "light harvester" (fancily called photoactive material), does even better with nitrogen in it. For introducing nitrogen into the material we use a rather simple process wherein the material is merely "cooked" in an ammonia containing atmosphere. The cookware used is called "alumina boat" and the stove used is called a "tubular furnace". Of course the cooking is often done at temperature that are somewhat hot (>400C).
We also show that making TiO2 rather spongy (i.e porous) is simple. We just use basic colloidal science which you perhaps already know; to achieve this. The basic idea is to use some "soaps" that come together while holding onto Ti containing units. When cooked; out comes spongy TiO2 (called nanoporous TiO2 due to the tiny pores it contains). This porous material soaks up nitrogen rather efficiently and hence "doping" nitrogen into this form of TiO2 is a piece of cake (figuratively speaking of course).
Materials scientists (i.e our breed) are rarely satisfied with a first order investigation of the above mentioned kind. So we went onto study various combinations of N doped TiO2 with (i) Ag (essentially painted on these materials), (ii) Fe as a co-dopant. In another set of experiments we went beyond TiO2 and showed that Cu2O can be a competitive light harvester as well; interestingly particles with sharp edged seemed to help; once again nitrogen was a friend here.
Ag coated TiO2 (doped with nitrogen) looks nifty and also does a good job with light harvesting and conversion. Interestingly use of Ag not just enhances light absorption but also helps with light conversion (likely due to small silver oxide/TiO2 junctions on these tiny surfaces!).The technique used for making these meso- and nano-porous materials is sufficiently generic and could be useful for several others materials too.
1. Refs: Mingming Zou, Fengqiang Xiong, Ayyakannu Sundaram Ganeshraja, Xiaohua.Feng, Chuanxi.Wang, Tiju Thomas and Minghui Yang, Visible light photocatalysts (Fe, N):TiO2 from ammonothermally processed, solvothermal self-assembly derived Fe-TiO2 mesoporous microspheres", Materials Chemistry and Physics (2017; just accepted) 10.1016/j.matchemphys.2017.04.035
2. Zou Mingming, Honghong Liu, Lu Feng, Tiju Thomas, and Minghui Yang, "Enhanced visible light photocatalytic activity in N-doped edge-and corner-truncated octahedral Cu 2O", Solid State Sciences 65, 22-28 (2017)
3. Zou Mingming, Honghong Liu, Lu Feng, Fengqiang Xiong, Tiju Thomas, and Minghui Yang. "Effect of nitridation on visible light photocatalytic behavior of microporous (Ag, Ag2O) co-loaded TiO2", Microporous and Mesoporous Materials 240 (2017): 137-144.
In separate investigations we found that oxynitride materials which are systems that contain specific atomic ratios of oxygen and nitrogen are rather finicky. They really care about the starting materials and what fraction of the tapped sunlight is available to us depends very much on how we begin the process. Some physical property studies of these materials showed that this was because the number of charge carriers in these oxynitrides vary dramatically based on the starting materials. In fact making these systems is currently an art; and we certainly enjoy this art form
How one goes about making LaTiO2N determines the efficiency with which the light harvested is useful for splitting water.
** We are currently looking at both the theoretical/computational and experimental aspects of this art. In particular we are developing computational approaches to see how these materials behave when heated up. In particular material interfaces, their evolution, and emergent electronic properties are currently being pursued by Kousika Anbalgam and Santosh who are graduate students in the group.
Ref: 1. Ref: Wan, Lipeng, Feng-Qiang Xiong, Yue Li, Tiju Thomas, Ruxin Che, and Minghui Yang. "Low Defect Density, High Surface Area LaNbON2 Prepared via Nitridation of La3NbO7" Materials Letters 188, 212-214 (2017). http://dx.doi.org/10.1016/j.matlet.2016.11.012
2. Xiong, Feng-Qiang, Lipeng Wan, Yue Li, Tiju Thomas, Francis Joseph DiSalvo, and Minghui Yang, "Crucial role of donor density in the performance of oxynitride perovskite LaTiO2N for photocatalytic water oxidation reaction", ChemSusChem (2017) 10, no. 5 (2017): 930-937.
Fun bonus fact:
Not surprisingly the magnetic ordering of these materials seems to be very sensitive to the amount of nitrogen in them. Briefly speaking the "when, where, how, and how-much" of magnetic and crystallographic transitions depend on presence of N in materials. We showed this using nickel chromates as the parent material.
There is also evidence that these interesting dynamics in chromates has to do with the fact that the chromium atom gets into a "confused state" and exists between multiple oxidation states. Also any asymmetries in the crystal get quenches with increasing nitrogen; this correlates with substantial increase in covalent bonding in this system. Typically all magnetic transitions shift downwards in temperature with increase in nitrogen content. This correlates well with increases magnetic frustration in these doped chromates, with increase in nitrogen content.
Specific heat capacity which is a measure of how much heat is required to raise the temperature of the object by 1C is a nice measure of transitions occurring n materials. This is n fact the way by which we explore crystallographic and magnetic transitions in these N doped chromates.
Ref: Xin Liu, Nan Yin, Tiju Thomas, Minghui Yang, Junhu Wang and Quan Shi, "Effect of nitrogen substitution on the structural and magnetic ordering transitions of NiCr2O4", RSC Advances 6, 112140-112147 (2016); DOI: 10.1039/C6RA22773B
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