Tiju ThomasDepartment of Metallurgical and Materials Engineering
Indian Institute of Technology Madras
Sardar Patel Road, Chennai 600036, Tamil Nadu, India
Contactph. no: +91 8056456442
email id: firstname.lastname@example.org or email@example.com
Hydrogen is truly the best fuel; it can offer almost 141 kJ for each gram of it! Diesel and petrol/gasoline merely have 1/3rd of this calorific value. However widespread use is fraught with many difficulties.
The challenges currently with usage of hydrogen are to do with safety concerns in gas storage and transportation, and low rate of production leading to non-viability of technologies at the point-of-use.
Another global concern of immediate relevance involves heavy-metal (say mercury i.e. Hg) ion pollution. Viable processes which can simultaneously remove and result in beneficiation of the contaminants are almost never reported.
It is in the above context that we have developed a single-step, in situ co-reduction approach which has the dual advantage of (i) Hg contaminant removal, and (ii) room temperature hydrogen production. The key component resulting in hydrogen generation is a nano-amalgam that is produced within the "dirty" water. The hydrogen production rate (720 mL/min for 0.5 g-Al salt) is far superior to what would be expected from the use of pure hydrides, and/or using bulk amalgams at room temperature.
The method we have developed is simple, chimie douce (i.e soft chemical), hence potentially affordable, and capable of providing a means of beneficiating Hg contaminated water present in effluents from certain industries (for example, industries which uses chlor-alkali process). The in situ co-reduction approach helps in bypassing the usual rate limiting steps. Given the potential that exists in scale down and up, this approach offers a method to address the long standing challenge of point-of-use hydrogen availability.
Think about it - using dirty water as a source of fuel need'nt be in science fiction anymore!
For more information please read our patents and paper referred here. If you are interesting in commercializing this invention of ours we would be happy to talk to you.
Ref: Abdul Malek, Edamana Prasad, and Tiju Thomas, "Chimie douce hydrogen production from Hg contaminated water, with desirable throughput, and simultaneous Hg-removal", International Journal of Hydrogen Energy (2017; https://doi.org/10.1016/j.ijhydene.2017.05.082)
"Hydrogen generation from waste water via galvanic corrosion of in-situ formed aluminum amalgam" (2016) (Indian Patent application no. 201641027502)
Ultra-small stable quantum dots (essentially extremely tiny particles that confine even the free electrons in them rather well) are very interesting for a wide range of applications. However using them for applications requires them to be of essentially the same size which is rather hard. However Niya Mary from our group discovered a size focusing mechanism in ceramc systems (ZnO to be precise). Bhusankar from the group has extended this result to copper oxide as well. However unlike ZnO; down at this size regime Cu seems to flip-flop between two different oxidation states. Fascinatingly the usual formula that works for quantum dots does not work for our materials; we suspect this has to do with the nature of Cu in these systems (i.e it being of multiple oxidation states; namely a mix of +1 and +2).
Ongoing efforts indicate that the size-focusing approaches we have developed are primarily dependent on specific interactions between the Cu atom and the surface-active agent (called hard-hard-acid-base interactions) used in this process; the solvent we use matters too.
In fact there are ongoing theoretical and computation efforts to develop predictive models for this process so that the experiments may be intelligently designed.
Ref: Bhusankar Talluri, Edamana Prasad, and Tiju Thomas, "Ultra-small (r<2 nm), stable (>1 year), mixed valence copper oxide quantum dots with anomalous band gap", arXiv:1706.01261 (2017)
** Some more stories from our work benches are underway. Please do return to keep yourself posted.
This is how the device is made (Thanks to Aneesh from IITB and Dr. Maity from Tezpur University). The idea here was to make a device that can sense (i.e sniff!) methane; which is a hydrocarbon that is rather hard to detect. However detecting it would be nice since beyond a certain concentration it can be explosive in air.
This is how the low dimensional material structure relevant to this "sniffer" is made. The device material is essentially just slender graphene oxide (essentially just a carbon paper that is a few atomic layers thick with a few oxygen atoms here and there) along with zinc oxide tiny particles (nanoparticles). Appropriate heat treatment is very essential to get a good device material (in other words; they better be cooked well - like in case of food under or over cooking is problematic).
At the site of action - methane - an otherwise difficult to detect molecule is oxidized (essentially burnt up); the electronic signatures of which are helpful in sniffing out its presence. ZnO seems to aid and accelerate this oxidation process resulting in higher sensitivity of this device and quicker response.
Ref: Argha Sarkar, Santanu Maity, Aneesh M. Joseph, S Chakraborty, and Tiju Thomas, "Methane-Sensing Performance Enhancement in Graphene Oxide/Mg:ZnO Heterostructure Devices" Journal of Electronic Materials (2017); DOI: 10.1007/s11664-017-5619-1
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
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).
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.
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
A simple chemical probe was invented for detecting heavy transition metal ions (Hg2+, Cd2+, Cu2+, and Pb2+) in water. Later on, they were gettered and converted in to a industrially useful catalyst.
Ref: ACS Sustainable Chem. Eng., 2016, 4 (6), pp 3497-3503
A novel chemically-induced nanorod to quantum dot transition is reported in ZnO. This transition is achieved using co-surfactants in a marginally polar solvent in chimie douce (soft chemical) conditions. This is different from the physical instability driven transitions reported so far in metal nanowires and polymers. A suitable mechanism for the observed phenomenon has been proposed. There is ongoing effort to understand the precise physics underlying this phenomena.
Ref: RSC Adv., 2015, 5, 15154-15158
The electric and optical properties of a solid whose chemical composition is Na 1/2Bi1/2TiO3 (NBT) is modified using application of electric field alone. Interestingly the shifts in atomic positions that are induced due to applied electric field is irreversible!
Ref: J. Appl. Phys. 117, 244106 (2015); http://dx.doi.org/10.1063/1.4923222
TiO2, a material which usually does not absorb visible light, is made in a highly porous form. Tungsten (W) and nitrogen were incorporated in the material using some simple approaches to result in TiO2 that absorbs in the visible region! Furthermore this material was used for purifying water, using the sunlight they were capable of harvesting, owing to the fact that they could now absorb in the visible!
Ref: Solid State Sciences Volume 54, April 2016, Pages 49-53
All of us have used batteries and electric wires of some sort. Most of us know that a flow of electric charges (called 'current') occurs through the wire, when the two ends of the wire are rigged up to the battery. What happens when you use a insulating wire? It turns out that the battery still does its job of setting up a gradient of electric potential across the wire, but the wire just does not respond! (by definition, this is what insulators do) The electrons making up the wire however are however displaced from their equilibrium position. That is to say, they are displaced by a bit, in response to the applied field. This means that there is a net displacement of electric charges within the wire, in response the the applied field, even though there is no observable current. If you remove the field now, the charges would go back to their equilibrium positions, leading to a "discharge" event. Insulating materials that can be charged and discharged this way are called dielectrics. These materials can be used as reservoirs of charges; in conventional electrical engineering, these reservoirs of charge are called "capacitors". For more about this subject, do consult the wikipedia article on "capacitors".
Many crystalline ceramics (which simply means materials whose cooking requires high temperatures) can be charged and discharged this way. Most well known and commercially used dielectrics contain lead, which is is extremely toxic, and damages the human brain. One of our projects concerns materials that can be used to replace these lead containing dielectrics.
With regards to lead-free dielectrics, the material of choice for us is a bit complicated, and has the longish name "sodium bismuth titanate". You may want to remember it, if you want to impress your friends!
To sodium bismuth titanate, we add an element called Europium. It turns out that this mix of materials emits red light very efficiently, when excited suitably (electrically or using some other light source). Our colleague, Prof. U. Ramamurty, was curious to study the mechanical behavior of these new ceramics we had made. To our surprise, we found that the mechanical properties and optical properties follow trends that mirror one another! This was fascinating to us, and we reported this, along with a plausible explanation for the observed phenomenon. For more information about this, please do consider reading the paper we wrote.
Ref: Solid State Communications 173, 38–41 (2013)
Zinc oxide (ZnO) is a common material used in paints. It also turns out that its electrical properties make it suitable for making sensors, transistors, solar cells, etc. Recently we reported the world's smallest ZnO particles! These tiny chunks of ZnO are merely 2 nm in radius! By the way, this means that these particles have a radius that is 37500 times smaller than the nominal breadth of human hair (75 micrometers).
It so happens that We are also the first ones to report a curious phenomenon called digestive ripening (DR), in this ZnO. DR is a fascinating process wherein particles of different sizes evolve together, to give rise to particles of same size! This phenomenon has been reported in a few other material sustems; its fundamental reason remains a puzzle till this day (although there exist some nice theories, one of which is consistent with our observations).
Our discovery of DR in ZnO is the first report of DR in any oxide material; it also turns out to be the fastest and the lowest temperature DR reported so far! What is mor.... we achieve this using simple kitchen chemistry!
Ref: Ceramic International (2014) (DOI: 10.1016/j.ceramint.2014.05.116)
If you look at the periodic table, and focus on the last few elements, you will discover the lanthanide and actinide elements. We specialize in using lanthanides in designing and making novel optical materials. The lanthanide element we chose for this project is Europium (Eu).
Eu has an interesting feature - it can either lose 2 or 3 electrons, to become a positively charged ion. Whether it loses 2 or 3 electrons depends upon the chemical environment we choose for the Eu atom. Typically stabilizing Eu in the +2 state is hard, since it would rather lose all 3 of its outermost electroncs. Hence in order to arrest Eu in its +2 state, we often requirevery reducing atmospheres (i.e atmospheres wherein the Eu cannot get rid of all 3 of its electrons). This is quite a challenge. However very recently, we devised a simple process by means of which we succeeded in stabilizing Eu in its 2+ state, using an open-air process!
The general idea is to use a variant of what happens in the internal combustion engine of your two/four wheeler. If the amount of air available for combustion is insufficient for the fuel, you are likely to create a very reducing environment (this is wel known). This general idea is used to achieve a single step, rapid process (~5-10 minutes) for stabilization of Eu in its 2+ state. In doing so, we got a very good blue light emitting material (Eu2+ in CaAl2O4); this material is both cheap and robust. Please do read the paper cited here, to learn more.
Ref: Journal of Alloys and Compounds, 589, 596–603 (2014)
2 decades ago, in late 1980's Y2O3 hit headlines, not due to what it did, but due to a material for which it was used as a starting precursor. Y2O3 was one of the starting materials for synthesis of a compound called yttrium barium copper oxide (YBa2Cu3O). This resulting material became one of the most well studied, high temperature superconductor of that era.
In this paper, we take a step back to analyze Y2O3, and ways of making it. We developed a simple, one-step protocol for the synthesis of Y2O3 - it turns out that we managed to engineer this material for both photoluminescent (PL) and thermoluminescent (TL) applications. Photoluminescence simply means getting light out, by exciting the material using light (usually of higher frequency). Thermoluminescence means getting light out of the material by heating it.
In this work we show that by simply switching one experimental parameter (which happens to be an organic fuel in our case), we could make either photoluminescent or thermoluminescent material. Specifically when a organic compound, ethylene diamine tetracetic acid (EDTA) was used in the reaction mix, a nice thermoluminescent Y2O3 was obtained; but when its disodium derivative (Na2-EDTA), we obtain a better photoluminescent material! The resultant material is inevitably made of smaller particles (10-30 nanometers). Use of Na2-EDTA results in slightly larger particles, hinting at the role of sodium in grain growth of Y2O3. The physics of defects of Y2O3 was explored using analysis of the thermoluminescence data.
Ref: Journal of Alloys and Compounds 585, 129-137 (2014)
Y2O3 is a very remarkable material, that made it big for two reasons: (i) it is used in the cathode ray tube screens in televisions, and (ii) it was used to make the mostimportant superconductor of the 1980s. When used as a TV screen phosphor, Eu is added to (i.e "doped into") Y2O3.
In this work, we grew tiny rods of Y(OH)3: Ni2+ and tiny particles (nanoparticles) of Y2O3:Ni2+ phosphors. When heated Y(OH)3 rods readily decomposes to yield Y2O3 particles that are all stuck to each other. Using a technique called "electron paramagnetic resonance" we showed that the Ni dopant has 6 neighboring atoms, which aids in efficient blue emission.
Further more, we subjected these novel optical materials to gamma radiation (the kind that caused the birth of "Hulk"!). Typically the impact of radiation is studied by heating the radiated material. At 195 and 230 degree C, the irradiated material glows. The nature of the glow was used to extract the physics of the defects in the irradiated Y2O3. The physical parameters associated with the radiation-induced defects are activation energy (E), frequency factor (s) and order of kinetics (b). Finally we found that the material is quite stable against irradiation, and hence suitable for measuring radiation in a nuclear environment (using a technique called radiation dosimetry).
Ref: Journal of Luminescence (2014)
If you look at the periodic table, and focus on the last few elements, you will discover the lanthanide and actinide elements. We specialize in using lanthanides in designing and making novel optical materials. The lanthanide element we chose for this project is Europium (Eu).
Eu has an interesting feature - it can either lose 2 or 3 electrons, to become a positively charged ion. Whether it loses 2 or 3 electrons depends upon the chemical environment we choose for the Eu atom. Typically stabilizing Eu in the +2 state is hard, since it would rather lose all 3 of its outermost electroncs. Hence in order to arrest Eu in its +2 state, we put it in competition against Cr in a material called CaAl2O4. In this situation, when the synthesis parameters are appropriately chosen, we not only arrest Eu in its 2+ state using an open-air process, we also get remarkably good blue luminescence from the system! The best blue luminescence was seen when a combination of fuels (called ODH and urea) were used in 1:5 ratio. The material we made compares favorably with the most popular commercial blue phosphor (called BAM: Eu2+).
Ref: paper submitted (under review)
Wires and rods of gold (Au) have some very unique properties, due to the way in which electrons in them interact with light. Furthermore when these wires are arranged in a periodic manner, light bounces off them in a manner that is very sensitive to the incident angle. This provides an opportunity to make sensors out of these systems.
In this project, we explore several high and "low" tech (hence low cost) methods to make such structures. The highlight of this project is that we succeeded in coaxing gold particles to self-organize along lines! While our technology has its own limitations (incomplete coverage, and low quality of assembly), we are making progress for sure.
The high tech methods for making these structures involves use of electron and ion guns (literally!). By shooting electron and ions carefully, gold nanostructures with desired geometries have been carved out! For more details, please do contact me!
Cobalt ferrite (CoFe2O4) is an engineering material which is used for applications such as magnetic cores, magnetic switches, hyperthermia based tumor treatment. It has also found use in contrast agents for magnetic resonance imaging. Use of ferrites in practical applications hinges on our ability to control size, solution behavior, and overall magnetic behavior. Having ferrites that are bio-compatible is an added bonus.
In this work we discovered that cobalt ferrite made by us showed an "inverse relationship" between the lattice constant and crystallite size. Usually as crystallite size decreases, lattice constant is found to increase. However in the cobalt ferrite made by us, we observe a decrease in lattice constant with decrease in crystallite size! In earlier reports where such an inverse trend was reported, researchers had employed vacuum based techniques to make cobalt ferrite. We observe this relatively exotic trend in cobalt ferrite made using low-temperature solution chemistry. We have developed a hypothesis for explaining the observation (it involves some stoichiometric arguments). However our hypothesis is yet to be tested. Further more, our results highlight ways in which size, and magnetic behavior can be optimized.
The title would be quite accurate even if we said "Soft chemistry based organic photovoltaics".
In a solar cell, the electron-hole pair generated when light is incident on the junction needs to be efficiently (i) separated (in space), to ensure that they do not recombine, and (ii) collected at the electrodes. We use a low temperature approach to make a large array of nano-needles of a material called ZnO. This material is known to be a good collector of electrons. The large array of ZnO nanoneedles means that we have a large surface area over which electrons can be effectively collected. We achieve excellent current densities in this device, but sadly observe low voltage supplies coming off the device (technically this device parameter is called "Open circuit voltage"). We are chugging our way through, and sooner than you realize, we may have a cool device in hand. So do check back with us, if you happen to bump into this story!
By the by, this story became an invited contribution in IEEE Photovoltaics (Boston conference, July 2013). Kudos to Arun and his team who made this possible! Please click here to see the poster Arun presented when he was away, sharing this story with materials scientists in Boston.
Waste waters generated in industries and homes have a combination of negatively (anionic) and positively (cationic) charged species. A material called activated carbon is often used to get rid of these contaminants. The commonly used Aquaguard too uses activated carbon. Several RO-UV (short for "reverse osmosis-ultra violet") also use activated carbon. The problem is that this material is a bit too expensive, since its manufacture involves vacuum techniques. Hence it is desirable to come up with alternate materials (called adsorbents), which would be able to adsorb (immobilize) both anionic and cationic contaminants.
In this study, we focus on anionic (Congo Red, Orange G) and cationic (Methylene Blue, Malachite Green) dyes which are widely found in effluents from textile, leather, fishery, and pharmaceutical industries. Their carcinogenic (cancer causing), mutagenic (mutation causing), genotoxic (gene disrupting), and cytotoxic (cell death inducing) impact on mammalian cells is well-established. Remember human beings are mammals too! Hence these dyes, beyond a certain minimal concentration, harms you and me.
We show that combining ZnO, (Zn0.24Cu0.76)O and cobalt ferrite to make an "adsorbent bed" works well. This system efficiently adsorbs the model anionic and cationic pollutants. This adsorbent system works well even when complex mixtures of these pollutants is used. All adsorbent phases are synthesized using room temperature, high yield (~96-100%), green chemical processes. This makes the process eco-friendly and economically viable.
In this paper, we do a comprehensive analysis of the thermodynamics and kinetics involved in the adsorption process. We also study the material properties that are responsible for the observed adsorption behavior.
PS: Niya presented a poster at the Royal Society of Chemistry's Water Quality workshop (Aug 13-14, 2013 in Bangalore). The poster she presented can be found here. This poster contains a brief over view of our work on both photocatalysts, and adsorbents.
The nexus of mechanical and optical behavior of materials is one of the least explored areas of materials science. And yet, there are lots of interesting stories to be unravelled here! The following is one among them.
Eu doped Na0.5Bi0.5TiO3 (NBT) is an efficient emitter of red light. In such optical materials, both electrical means or light based excitation can be used to elicit visible light (red, in this case). Eu atom is the light emitter in this material. When Eu in NBT is increased beyond a certain point, a saturation of emission intensity is observed. At this doping level, we also find a restoration of elastic modulus (to undoped values)!! This concurrent effect is used to develop a simple model (we call it the "phase segregation model") which is helpful in explaining the observed intensity saturation.
(paper accepted in "Solid State Communications". Please write to me, if you wish to have a copy of the same)
Simply speaking, thermoluminescence (TL) is emission of light when an object is heated. Photoluminescence is light emission that occurs when a material is excited with light (usually wavelength of excitation source is lower than that the emitted light). Both these phenomena are quite sensitive to defects in the materials.
In this work, we show that an efficient light emitting material (phosphor) called Y2O3 can be made both photoluminescent (PL) and thermoluminescent (TL) by simply switching one experimental parameter during the synthesis process. There is a low-cost method for oxide nanomaterial synthesis called solution combustion technique. This technique usually involves a combustible mixture of a fuel and an oxidizer (just like chemistry involved in fire crackers!). We show that by simply switching the fuel used for synthesis, luminescence property can be tuned for different applications! Detailed investigation of influence of fuel nature on structural and luminescence characteristics of Y2O3 and their correlations have been explored. We also hypothesize the possible reasons for the observed spectral characteristics in detail.
Photocatalysis has been touted as one of the most promising ways to remedy water. However despite its 80 year old legacy, its industrial use has been very limited. Also technologies based on photocatalysis have not gained any popularity in the developing world, despite significant related academic activity in these parts of the world. Here we identify issues that have prevented the translation of this technology, especially in the context of the developing world.
We present our paradigm for a comprehensive, multi-disciplinary approach for improving the industrial viability of photocatalytic water treatment (especially in the developing world). Combining soft and green chemical principles seems essential for making these technologies economically feasible and socially relevant. In this paper, we talk about various design factors that go into comprehensive water remediation. Issues such as nanotoxicity, chemical yield, cost, and ease of deployment in reactors are important, while designing new photocatalysts. Several issues are presented using examples from literature. Some promising technology directions are hence identified.
ref: "Combining 'chimie douce' and green principles for the developing world: improving industrial viability of photocatalytic water remediation", Tiju Thomas and Nagaraju Kottam, Chemical Engineering Science (2013) (Letter to the Editor)
Note: Please write to me in case you need this paper. I will be happy to give you a copy.
Developing a single semiconducting material that can effectively tap sunlight, and generate easily separable "excitons" is the holy-grail in photovoltaic technologies. An exciton is essentially a Couloumbically bound pair of electron and a hole. These excitons, which are generated when semiconductors absorb sunlight, can be split (this is much trickier than it sounds!) and sent along opposite directions to generate a non-zero electric current, if the material system is part of a closed electric circuit. This is how a solar cell works. This also helps us understand why engineers would like a semiconductor which absorbs over the entire solar spectrum.
For making a good solar cell we need a semiconductor which (i) absorbs over the entire solar spectrum, and (ii) gives exciton sufficient time (called the "exciton lifetime" in textbooks) so that charge separation is realistically possible. Developing materials of this kind are promising from the point of view of solar cells. We set out making materials of this kind, and succeeded in finding a material that has almost full solar spectrum absorption (it is Cu doped ZnO made in a specific fashion). We do not know how promising the exciton life time etc are at the moment. We now wish to use this material system for making solar cells.
Currently we are using this material for another promising environmental application; namely, detoxification of water. The basic principle is the following: light-generated excitons in these systems can be split in order to kick-start a set of electron-transfer reactions in polluted waters, within which these photocatalysts are dispersed. The elctron-transfer reactions, if carefully directed, can result in complete oxidation of the toxic organic poisons in the polluted water, which in turn should help in the detoxification of water. In this case, the semiconductor is called a photocatalyst, since it uses light and facilitates oxidation of toxic organic wastes, without getting modified during the detoxification process.
We have achieved this, using semiconductor nanoparticles that were made using soft-chemical routes (<60 degrees C). In fact our most efficient "photocatalyst" is made at room temperature, without the aid of any furnace or oven! This essentially means that we have a photocatalyst which can be made at very low cost, and with very little carbon foot-print (which means minimum pollution). In addition, these nanoparticles are expected to be environmentally benign and biologically inert, since they belong to the size-regime where these particles are known to be non-toxic. Further more, we have developed synthetic paradigms and rules for photocatalyst design, the combination of which is expected to be helpful in making photocatalysts viable for use in the developing world.
In order to know the details of our work, please feel free to talk to me.
Ref: This work has been communicated, and should be in print soon.
Perovskite is essentially calcium titanate (CaTiO3). You can find the way in which atoms arrange themselves within a perovskite, by going to images.google.com and typing "crystal structure + perovskite". Notice how the atoms sit with respect to one another. The way atoms sit with respect to one another determines the "crystal structure" of a crystalline substance.Perovskite based materials (which means those materials that have similar crystal structure, but different atomic composition) have been used extensively in electronic devices due to their ability to store charges. This is because they tend to be superb insulators, with high dielectric constants (dielectric constant is a measure of response of a material to an applied electric field). We are interested in using materials related to the perovskite, to make novel optical materials, with a very interesting set of optical properties.
You will need to visit this page again, 5 months from now, if you wish to know what these novel properties are!
Zinc oxide (ZnO) is a beautiful material, with many applications. Believe it or not, it is used in cosmetic preparations such as sunscreen lotions. This material is a wide band gap semiconductor, which simply means that it behaves "almost" like an insulator. Electrochemistry can be used to determine the conditions under which one can deposit ZnO, starting from a dilute zinc nitrate aqueous solution. A while ago, Prof. Kristin Poduska's group found that, depending on the deposition conditions, one essentially ends up growing a "rectifying junction" or an "Ohmic junction",instead of a thin layer of resistive ZnO (as is predicted by the Pourbaix diagram --- which is a kind of phase diagram). This is because these electrochemical conditions enable deposition of not just ZnO, but some combination of Zn and ZnO in a sequential manner. The evidence suggests that electrodeposition of oxides is a much more complicated process than was previously thought!
People have been using ZnO electrodeposits to make devices such as solar cells, but the results have'nt been promising. Electrodeposition is attractive for making devices since it is much cheaper, less energy intensive, and easier to scale up to an industrial scale. Our results on these electrodeposits, suggest that there is substantial lateral heterogeneity in case of these oxide electrodeposits (which simply means neighboring regions of the electrodeposits behave quite differently). These heterogeneities can be probed using optical and electrical means. In fact, we find that such laterial heterogenity is intrinsic to ZnO electrodeposits. It is very likely that this heterogeneity is intrinsic to oxide electrodeposition in general (this statement is not yet verified!). Evidence suggests that lateral heterogeneities are the most likely culprits for the observed poor performance of devices made using ZnO electrodeposits.
For more details see: ECS Solid State Letters Vol. 1, P35-37 (2012). doi: 10.1149/2.002202ssl
Gallium nitride (GaN) is a wide band gap material (which means it is practically an insulator) with plentiful promise in high speed and high power electronics, and optical engineering applications. The problem however is that this material is very expensive. Sophisticated techniques currently used to make this material include methods such as metal organic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE). These methods are not only energy intensive, but also expensive and result in slow growth of this material. Thankfully these techniques result in high quality gallium nitride (GaN), which is essential for engineering applications. The cost however is prohibitive, and the technical expertize and the carbon-footprint is certainly a non-neglible problem. The question is: would it be possible to come up with a technique using which several grams (~30 grams!) of high quality GaN can be made in a short time (say 1-4 hours).
A reliable way of making nitrides involves the use of ammonothermal methods, wherein ammonia is used as the nitrogen source. At high temperatures, ammonia cracks and becomes an excellent source of reactive nitrogenous species. These reactive species can be made to react with Gallium, to yield GaN. The principle seems straightforward, however in practice there is a substantial challenge here. A crust of GaN forms on the surface of Ga, when ammonothermal reaction is carried out using Ga and reactive nitrogenous species. This means that only Ga present on the surface is converted to GaN; and the rest of the Ga remains unreacted and unused. This leads to wastage of most of the gallium that is put within the reaction chamber. Considering that gallium is a precious metal, this is a problem that needs to be addressed. We averted this problem by resorting to some clever chemistry. Bismuth (Bi) came to our rescue here. A Ga-Bi mixture can be used as the starting material, instead of pure Ga. When this was done, a simple wetting-sinking mechanism kicked in during the GaN synthesis reaction, which ensured the consumption of all the Ga. Briefly, what Bi does is "coat" the new GaN particles which form on the melt-surface. The particles formed on the surface becomes heavier due to Bi-loading, and sink to the bottom; exposing fresh Ga, which then reacts. This continues till all of Ga is consumed. This also ensures that the reactions were rapid (<4 hours), and high yield (~100%)!! Hence this technology circumvented a central problem associated with GaN synthesis using ammonothermal technique. Now we are capable of making several grams (~30 grams) in just 4 hours, at a cost that is 10% the market cost!
Further more, we used a variant of this technology to make efficient light emitting materials, such as Europium doped GaN. Europium is a rare earth element, and sits cosily within the GaN matrix. The f electrons of Eu do not participate in chemical bonding with the lattice, which ensures that the f-f electron transitions are similar (in many ways) to the atomic spectra of Eu! The lattice/matrix within which Eu is sitting should be good at harnessing incident light, and transfering the energy absorbed to the rare earth ion, which is the light emitter. Not all lattices are capable of doing this. In fact coming up with a good optical lattice for rare earth dopants is non-trivial. For example, even in GaN, the slightest amount of oxygen could destroy its ability to be a good host. We got the conditions just right to obtain good red emission (wavelength ~ 620 nm) from Eu:GaN synthesized using ammonothermal techniques. This material can now be put to good use in devices such as light emitting diodes and lasers. You are welcome to talk to us in case you would like to make a device out of this material.
Ref: Journal of Crystal Growth, Vol. 316, 90-96 (2011).
Nanoparticles are small chunks of matter whose sizes are typically <100 nanometers. To give you a sense of how tiny these particles can be: we are talking about sizes that are roughly one-thousandth the breadth of your hair! Do you think you can see such tiny particles using your eye? Usually study of such tiny particles require microscopes that are much better than the ones you have used in middle and high school. Microscopes that you used in your high school biology lab are called optical microscopes. Study of nanoparticles often require electron microscopes, which are much more powerful, and hence capable of probing much smaller chunks of matter.
Gallium nitride particles made using ammonothermal methods are a few microns large (about one-tenth to one-hundredth the breadth of your hair). In principle, these particles can be broken down to give nanoparticles that are about 20-30 nm. The idea we had was to use rare earth doped GaN made using ammonothermal methods (remember: these materials are excellent light emitters) as the starting material to make nanoparticles. A popular technique for making nanoparticles from these micron-sized particles is essentially by "chopping" them repeatedly. The advantage of such chopping techniques (formally called "nanoscission") is that it is easily scalable, and hence commercially viable. However we found that such chopping methods resulted in loss of light emitting property; however it did result in particles of the right size regime. We found that nanoscission techniques, which are very successful and hence popular for making catalysts on an industrial scale, very often fail when it comes to optical materials.
In order to circumvent the problem, we turned to another popular industrial process called ball milling. It turned out that ball milling could retain the emissive properties of RE:Ga, as opposed to nanoscission techniques. This might be because nanoscission techniques results in flatter particles, with much higher surface area per unit mass of the materials. In general, these extra surfaces provide opportunities for excitons (Coumbically bound electron-hole pairs, generated when you sufficiently excite the optical material) to recombine, without transferring their energy to the light emitting ion (rare earth ion, in this case). Hence we found that physical particle size reduction of optical materials is better done, when one uses the ball-mill approach, as opposed to the nanoscission method.
We were eventually able to engineer the ball-mill process to achieve Er doping of GaN. Er doping in GaN is particularly tricky, since Er levels get easily quenched in GaN matrix. For example, while making Er:GaN using ammonothermal synthesis, the slightest amount of oxygen in the reaction chamber can completely quench the Er transitions. However we found that samples made using our ball-mill approach gave Er emissions that were much more stable that Er:GaN obtained using chemical methods!
Please note that these physical methods have yields approaching 100%, which make these techniques viable for large scale industrial production of nano-GaN based phosphors.
Ref: Journal of Crystal Growth Vol. 311, 4402-4407 (2009).
MRS Proceedings http://dx.doi.org/10.1557/PROC-1202-I09-12
In the Tit-bit titled "Nanosizing RE: GaN while retaining optical properties", we saw that it is indeed possible to minimize the size of nitride particles while retaining the optical properties. This is done using a carefully designed ball-milling process (which is basically a clever way of hammering!). To enhance the engineering relevance of these nanoparticles, it is desirable to "paint" large surfaces with these tiny particles. This would enable use of these particles in devices such as surface conduction electron displays (SEDs). One of the ways to "paint" a substrate uniformly would be by using electric fields.
The idea of using electric fields to obtain particle coverage on a large surface area seems like an exotic idea at first, but the principle has been around since the early days of colloid chemistry and electrochemistry. Of course, getting the conditions just right to get a really flat layer of these particles is not that. That really requires quite a bit of hit and trial. The operational principle is the following: these nanoparticles, if carefully dispersed in a liquid will result in a stable colloid. These nanoparticles pick up a surface charge, while they bounce around in the solution. One can hence direct these particles by application of electric fields. We eventually succeeded getting fairly crackfree thin films of these nanoparticles. We are in conversations with some display-tech companies who seem interested in using our phosphors, and this large scale deposition technology.
What we have seen so far is that rare earth doped nitrides can be excited to obtain emissions from the f-f inner shell transitions of rare earth ions. However a few questions remain: (i) why is that GaN is such a good host?, and (ii) how is that the energy absorbed by GaN matrix is transferred to the RE ion? Ways to probe mechanisms of this kind include pressure and temperature dependent photoluminescence. Photoluminescence simply means that light is used to excite material suitably, so that it emits well. The emission spectrum is collected and analyzed using an appropriate spectrometer. Pressure and temperature dependent PL essentially means is that you "squeeze" or heat the material, while doing the photolumiescence experiment. Variations from the room temperature, atmospheric pressure photoluminescence spectra of the material is helpful in elucidating luminescence mechanisms.
Typically heating the sample reduces the emission intensity of the material. This phenomena is called thermal quenching. However we noticed that the thermal quenching can be completely suppressed by application of pressure! This observation strongly supports a certain model of RE excitation, wherein incident light results in the formation of excitons (essentially bound electron-hole pairs), which then get "pinned" to the neighbourhood of the RE ions. This pinning is responsible for energy transfer between the exciton and the RE ion. The application of compressive hydrostatic pressure (~6.8 GPa) results in stronger localization of bound exciton on Eu3+ ion trap; which in turn enhances the efficiency of energy transfer between the exciton and the RE ion, while reducing possibility of energy back transfer between Eu3+ ion 4f-shell electrons and GaN host. A combination of these factors is responsible for observed suppression of luminescence thermal quenching by the applied hydrostatic pressure.
Further more, it was found that the Eu ions create different electronic states within the forbidden gap of GaN (think of them as electronic energy levels that should'nt exist, but are created because we have added Eu into the lattice). We also notice a few less efficient Eu3+ ions excitation pathways (other that the exciton mediated path described above) due to shallow energy levels in the forbidden gap. In simpler terms What this means is: addition of Eu within the GaN matrix, results in addition of levels within the forbidden energy gap, which provides alternate means by which conduction electrons can excite the RE ions.
An interesting outcome of our collaborative studies with Prof. Jadwicienzak's group was the discovery of the high radiation hardness in our powders. We discovered that optical properties of our Eu:GaN powders remain unaffected when 2 MeV oxygen ions (visualize them has Oxygen ions moving at about one-hundredth the speed of light!) are bombarded on to our powders with a fluence of 1.7×1012 to 5×1013 cm-2 (fluence = number of particles per unit area). For these experiments, the powder was embdedded in a matrix of another material called potassium bromide (KBr). Our studies suggest that Eu-doped GaN powder phosphor can be considered for devices meant for use in high radiation environments. To put our results in perspective, it is helpful to know that the alternative technology (Eu:GaAs) results in a system that has radiation tolerance that roughly similar or less than our system. But our system has an additional advantage of being free of arsenic, which is a toxic element.
Have you ever played with a slinky? If you have never seen one, please click here: http://www.jimrigby.org/wp-content/uploads/2012/03/slinky.jpg . To see a video of how slinky stretches, click here : http://www.youtube.com/watch?v=vwjoUmJ4J1w . Why is the slinky behaving that way? Slinky is essentially a spring with a very low spring constant; which means that when you stretch a slinky, the restorative force generated by the slinky is'nt much. Hence slinky is essentially a "soft spring".
Solids can be thought of as atoms packed together, which are attached to one another using "springs". The reason why atoms behave as if they are attached via "springs" is because they are bonded chemically to one another. Under some circumstances, solids can behave like slinkies (at least in some specific directions). As you might imagine, such "soft modes" can result in large scale displacement of atoms, which in some cases may result in an entirely new arrangement of atoms.
Why am I saying this? Power diodes are electronic devices that carry a few amperes per square cm. This is a large current density. Wide band gap material called silicon carbide (SiC) is a popular choice for making power diodes. However these diodes were seen to fail faster than was initially predicted, especially considering their thermal and chemical stability. What people observed was that the electronic properties of these diodes were getting modified over time. Think of it as some "aging" occuring in the device. It was soon learnt that it was because of stacking faults (a mislignment in the arrangement of the atoms) being created at the junction of the device. This observed phenomenon did not have an explanation, which is why it piqued our interest. Along with Prof. Umesh Waghmare (JNCASR), I demonstrated that there are "slinky like" modes (formally called "soft modes"), which are responsible for stacking fault expansion in these systems. The soft mode is also responsible for making the faulted structure thermodynamically more stable than the perfect SiC crystal. This is why SiC power diodes failed faster than was originally predicted. For a more technical account on this work, please do consider reading our paper.
Ref: Physical Review B 77, 121203(R) (2008)
> Plasmonic wires (researcher: Aneesh Joseph, collaborator: Prof. Manoj Varma CeNSE)
> Bulk heterojunction solar cells using inorganic (ZnO) nanosponge layers made using "kitchen chemistry"! (researcher: Arun Dhumal, collaborator: Prof. Praveen C. Ramamurthy)
> Simple synthesis of microwave absorber materials
> Green chemical synthesis of ultrasmall ZnO QDs
> Electron transfer dynamics in photocatalytic systems/oxide-electrolyte interfaces (collaborator: Prof. K. V. Adarsh, IISER-Bhopal)
> Ab-initio studies for zeroing-in on design rules for photocatalysts. (collaborator: Prof. Prasenjit Ghosh, IISER-Pune)
> ZnO Quantum dots (QDs) and QD-composites as inclusions in liquid crystals (collaborator: Prof. S. Kumar, Raman Research Institute)