TiO2 Coatings for Degradation of Organics by Photocatalysis

TiO2 Coatings for Degradation of Organics by Photocatalysisby David Sharp

During my placement I was part of an investigation into the treatment of water using photocatalysis after activating titanium dioxide nanoparticles under UV light. This involved investigating curing and deposition techniques of coatings with a view to integrating this concept into future factory builds and design.

IntroductionTitanium dioxide (TiO2) nanoparticles can act as photo catalysts under certain conditions. They can be sintered at high temperatures onto a surface when heated in various ways including ovens, hot plate or near infrared (NIR) machines. If this surface is then irradiated with ultra-violet (UV) light it causes an electron from the valence metallic band to be excited into the conduction band. When this electron undergoes relaxation an electron (e-) and a hole (h+) pair are formed. If the pair survive recombination they will produce a highly reactive species and will cause further reactions. The h+ in the valence band will oxidise an anion to form a hydroxyl radical (.OH) whilst the e- can reduce surface absorbed oxygen molecules to yield superoxide radicals (O2

.-). These are an extremely reactive species. The hydroxyl radical is the main species that participates in my investigations by degrading dye that has contaminated water supplies.

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Photocatalysis Process Involving TiO2

TiO2 Coatings for Degradation of Organics by Photocatalysis

The dye we predominantly investigated was indigo carmine (C16H8N2Na2O8S2). This dye is used as a model dye and has other uses such as in jeans, milk and biscuits. However in high concentrations it is an irritant and can cause permanent injury to the cornea and conjunctiva.

During my placement I had the opportunity to experience many new and advanced processes such as making TiO2 pastes, coating the substrate, sintering the sample, testing the sample by dye degradation, scaling up the testing to use the SPECIFICS water evaluation technique (SWET), running a line trial, using 3D modelling software to design and print a sample holder to aid testing, running thermogravimetric analysis (TGA), as well as making up aerogel precursors and taking these supercritical to make aerogels. My placement was at SPECIFIC research labs (sustainable product engineering centre for innovative functional industrial coatings) at the Baglan Bay Innovation Centre (BBIC). However most of my time was spent across the road in SPECIFIC’s PMRC (pilot manufacturing resource centre) building where they aim to take new concepts and scale them up to industrial proportions where prototypes can be tested. My mentors for the placement were Mr. Ashley Pursglove and Dr. Rachel Woods who are both technology transfer fellows at SPECIFIC.

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Indigo Carmine Molecule

Baglan Bay Innovation Centre Pilot Manufacturing Resource Centre

TiO2 Coatings for Degradation of Organics by Photocatalysis

Testing and the Set UpTo make the samples I cut the substrate to a size of 80x80mm, the materials we used were either: glass plates, glass fibre mesh (GFM), stainless steel mesh (SSM), an extremely fine stainless steel mesh (FSSM) or a ‘self-cleaning’ grout. Next I made the the TiO2 paste that is the coating. The paste is made up from a mixture of water, polymer binder and TiO2 nanoparticle powder. The TiO2 nanoparticles are in the P25 form which is seen as the best photoactive blend between the rutile phase and anatase phase of TiO2. The paste must be viscous but also have flowable characteristics. Next you must secure a piece of white roll to a cl ip board and secure the substrate to the white roll by using sellotape or scotch tape. I put the tape at the top and bottom of the sample as I could use the top piece of tape to pour the paste onto it in excess without compromis ing the sample. I used a glass rod to coat the substrate with the paste using the draw down method. Whilst spreading the paste you must push down firmly onto the substrate whilst maintaining a constant and steady speed to ensure the draw provides a thin coating with complete coverage. The thickness of the coating is negligible as only the top layer of nanoparticles will be activated under the UV so to save money and supply of the coating the thinnest coating possible is desirable. Next comes the sintering stage where you must place the sample in an oven or on a hotplate for an hour at 500oC, this is to remove any water or polymer binder from the coating to ensure full dispersion of the nanoparticles which maximises surface area. You can also sinter using an NIR oven, I will elaborate on this practise later in my results. The substrate will be left with a thin TiO2 coating in the P25 form and is ready for testing.

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Samples from top left to bottom right: SSM non-sintered, GFM, FSSM & SSM sintered

‘Self-cleaning’ Grout Sample

TiO2 Coatings for Degradation of Organics by Photocatalysis

To test the samples I arranged the apparatus in the following set-up. I used a shaker plate to act as the base of the equipment, it shook the dish containing the dye and sample at a constant rate for the duration of the test, this is designed to keep the water constantly disturbed to allow thorough mixing of the solution during the breaking down of the dye. This means regardless of where I took a sample of the solution the absorbance value for the dye was representative for the test situation. Next a clamp stand is used to hold the curved 6 UV lamp holder. The holder is curved to allow maximum UV photons to penetrate and be absorbed by the entirety of the sample to maximise efficiency of the photocatalysis. The UV lamps provide light intensity at a constant strength as well as being kept the same distance above the sample regardless of which test was running to control any variables. The dimensions of the petri dish containing the dye and sample was also kept consistent throughout. Next the sample was washed before testing to remove any excess

dust or any non-adhered nanoparticles that could remain in suspension within the solution and could affect any absorbance value recorded. One more step before testing was to place the sample in 100ml of distilled water in the petri dish to take a zero value for absorbance of the solution. The water sample was transferred in a cuvette and placed in the UV/VIS machine shown left. Careful attention was paid to ensure that no fingermarks, smudges or air bubbles were present on the parallel sides of the cuvette that the absorbance reading was taken across so a true value was measured. The solution I used was 100ml

and 10ppm of indigo carmine dye, this means 10mg of indigo carmine was dissolved in one litre of water, these values were also kept constant. An initial value was recorded before the sample and dye were placed under the lamps, this absorbance value was usually around 0.375 at 610nm wavelength. The test was run until the absorbance value at 610nm was zero or three concordant values were recorded in succession with an discrepancy of around 0.002 being allowed. Values for absorbance were recorded every minute for the first ten minutes then every two minutes from thereafter. The degradation showed a first order decay so this plan was suitable.

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FSSM Sintered GFM Sintered SSM Sintered

Perkin Elmer UV/VIS Lambda XLS

TiO2 Coatings for Degradation of Organics by Photocatalysis

Another method of testing involves using a portable UV/VIS with an Ocean Optics optical fibre to record the results automatically as shown above however we encountered a few problems during my placement and could not use this system to record enough results. In this set up a sample holder is required which allowed me the opportunity to use Inventor, a 3D modelling and design computer program, to design a new stand and clamp to hold the sample. I used a MakerBot 3D printer to print my design and the stand will now be used in future testing.

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Testing Set Up

3D Design & Printing of a Sample Stand with a Clamp

TiO2 Coatings for Degradation of Organics by Photocatalysis

During another stage of the placement I was required to run thermogravimetric analysis (TGA). This was to ensure the in-house paste we used for the coatings had the same make-up as a bought-in paste from BASF. For our testing to be valid the results from the TGA instrument must have matched each other. I gained experience in setting up the analysis program as well as loading the crucible in which the paste was placed for analysis.

In the last couple of weeks of the project it was time that we scaled up the testing we had done from 80x80mm samples to both 220x100mm and 1000x1000mm samples. This involved running both a SWET test and a line trial. The SWET allows 2 types of substrates to be tested simultaneously as three 220x100mm samples of each substrate are placed in the machine whilst 6 litres of dye is run over the samples and under UV lamps. The line trial involved the coating of 1000x1000mm GFM samples using a roller coater and drying over before being placed under the NIR to sinter the samples. The samples produced are then tested in direct sunlight on the ‘big rig’ where the sun provides both the UV to commence the degradation and the solar power to provide charge to run the pump that ensures the dyed water is irrigated over the sample and collected at the bottom and re-fed through the rig until the water has been completely cleaned. Although this was quite spectacular, in practice due to time constraints and weather we were unable to record any results from the samples that were made in the line trial. Despite this, it was an invaluable experience to see industry on a larger scale and have an insight into the ultimate goal of this project.

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Thermogravimetric Analysis

SWET Sample 220x100mm

Line Trial Showing Left to Right: Roller Coated, Drying Oven & NIR

‘Big Rig’ with 1x1m Sample During a Run

TiO2 Coatings for Degradation of Organics by Photocatalysis

Investigating the MethodAnother part of my placement was investigating stages within the method to obtain information that would optimise the process of making a sample. The first test was to establish which temperature provides the best sintering temperature to run at for an hour to maximise photo activity of the sample. I can suggest this is due to the best samples having the greatest surface area. I believe how quickly the polymer binder is evaporated off will determine the dispersion of the nanoparticles.

The results show a clear trend with only one exception. As the sintering temperature increases from 300oC to 600oC the rate at which degradation occurs increases with only the 500oC sample providing an exception. I think that this is due to either a faulty sample, incomplete coverage of the coating or physical damage to the coating; this means repeats are needed. The disadvantage to this data is that all of these samples were heated on the hot plate and as the rate is increasing up to 600oC, you need to test samples above this temperature to find the level at which the temperature is at its optimum, however the hot plate is limited to 600oC. The way to get around this is to use the Carbolite oven which can reach temperatures of up to 1100oC. We set a cap of 800oC to the samples as we know above this the samples will undertake permanent damage. Before I finished the placement I made up samples of both GFM and SSM from 300oC to 800oC in the oven at 50oC intervals, however I was unable to test them due to time constraints

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Testing of Different Sintering Temperatures on GFM

TiO2 Coatings for Degradation of Organics by Photocatalysis

The next test was into the effectiveness of of the types of paste used to coat each substrate. The stock paste used in all of the tests so far is of the P25 type of TiO2 a mix of both the anatase phase and rutile phase of TiO2 and is generally thought to provide the best photo activity throughout the scientific world. Anatase however is the most photoactive phase of TiO2 so we decided it would be best for the research to compare a paste made up of pure anatase to a paste made of a P25 mix. I coated a sample of GFM and SSM with both anatase paste and P25 paste, these were the results:

The initial rates of each substrate shows P25 providing the faster degradation. However the final points of each curve contradict one another. With the SSM, the P25 is clearly faster and at every point the curve is ahead of the anatase paste, however with the GFM the anatase paste overtakes the P25 paste, this is a confusing result as it is not what we would expect. More testing is required to find if this is a genuine trend or an anomaly of a sample.

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L: Samples I Made for Further Testing After My PlacementR: Polymer Binder Instantly Combusting Due to Heat at 750oC

Anatase Paste vs. P25 Paste

TiO2 Coatings for Degradation of Organics by Photocatalysis

Another test we decided to carry out was one into ‘self-cleaning’ grout. We theorised that if it claimed to be self-cleaning it must contain a photocatalyst. To investigate the potential in using grout for water treatment we made a 0%, 5%, 10%, 15% by weight P25 and grout mixture and put them to the test under our standard conditions. We would expect that the rate would increase as the percentage of P25 in the grout did. However we would also expect to see an increase in the amount that the tiles would break up due to ablation as we increased P25 due to poor binding between the grout and TiO2 the results are shown below:

As you can see the results are as expected, the rate of degradation of the dye increases as the percentage of P25 increase with the purple 15% line on the bottom and blue 0% line on the top. The decrease in the 0% grout can be explained by adsorption of dye onto the surface and also the presence of a photocatalyst as was theorised. A trend can also be seen in the final point of the curves being further from zero as the percentage increases. Again this can be explained by the 15% tile ablating leaving grout in suspension which affected the absorbance value even when all of the dye has broken down. Although potential can be recognised from the results massive ablation cannot be allowed to occur so more testing is still required.

The final investigation into the method concerns the sintering technique, do we use a hot plate, Carbolite oven or NIR? As shown by my previous results the hot plate is useful but is limited to 600oC which is not desirable especially if the optimum temperature lies between 600oC and 800oC. The NIR is what is used to sinter the largest scale samples as it is the easiest to sinter large samples within a short timeframe making it the most time efficient, however when using the NIR we found it burnt/melted the GFM so further testing is required into the viability of this process.

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Grout Samples Degradation Tests

TiO2 Coatings for Degradation of Organics by Photocatalysis

ResultsThe thermogravimetric analysis (TGA) results are shown below:

These results show that the in-house paste is very similar to the BASF sent paste, this lends credibility to the results which are valid so the retests are not required. This was a positive result.

Another method I used to confirm the accuracy of the results was to degrade acetone in the Fourier transform infrared (FTIR) spectrometer. The acetone is degraded because it is an organic compound like the dye, however the acetone is in a gaseous phase after injection into the tubing of the machine. The same principle would apply that the faster the acetone concentration reduced or the carbon dioxide concentration increased the more effective the sample is. The graph below is to show how as time goes on the acetone is degraded to carbon dioxide due to photocatalytic reactions after the activation of the sample surface by UV photons.

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Water Loss

Polymer Binder Being Removed

TiO2 Remains TiO2 Remains

TGA Analysis BASF (left) vs. In-house (right)

Acetone Degradation Using FTIR

TiO2 Coatings for Degradation of Organics by Photocatalysis

The results for all the FTIR samples are shown below. As you can see the best sample is the SSM that has been coated twice. I believe this is because the test is in the gaseous phase, a thicker coating would improve the diffusion of the acetone within the sample for degradation however with an aqueous organic this would be a disadvantage as a second coa t wou ld be was ted product. The four very similar samples are the P25 on glass, SSM both one layer of coating and two layers, and the BASF P25, all have an almost identical degradation pattern. The differences are so slight retesting must be completed to allow certainty in any conclusions drawn between them. The four NIR samples performed the worst, this is a surprising result as it seems to suggest oven sintering is much more effective than NIR sintering. Again I will be cautious in any conclusion drawn as these are preliminary results but certainly interesting ones. The poor performance by the NIR samples could potentially be caused by too high NIR intensity that caused damage to the substrate or has caused the P25 to turn partially to rutile which is less photoactive which can happen at high temperatures. Alternatively, the intensity may have been too low and not all of the polymer binder has been removed causing the surface area of the P25 to be less than is achievable on the samples.

We saw earlier that the grout samples we had made up gave us the results we would expect, the samples with a higher percentage of P25 were more photoactive but how did they compare to the rest of our samples?

The Grout samples are shown in orange where the other samples are shown in grey. This clearly shows the grout has been outperformed by most of the other samples. However this is the first test done with the grout so with a bit of optimisation this method does show potential and testing into it will continue.

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FTIR All Degradation Results

Grout vs. Other Degradation Testing

TiO2 Coatings for Degradation of Organics by Photocatalysis

The graph above shows a directly comparable test as the conditions were constant for all of the samples I made. This is the major test I carried out during my project and it gives clear and incredibly useful results. The SSM with P25 has outperformed all of the other samples, next is also the SSM with anatase, so I can say with certainty that the SSM is the best substrate to coat out of all the substrates used so far. Despite having the fastest initial rate, third is the FSSM with P25 which comes in before both GFM samples. The final 4 samples are the grout samples. This means I can suggest that the order of effectiveness is; the best sample is the SSM then FSSM followed by GFM and finally the grout. The other comment I can make is that the adhesion of the coating to the substrate is excellent in the SSM, FSSM and GFM samples as all of the samples have got close enough to zero that we can be happy with them. Overall, the SSM with P25 has ultimately performed the best.

The next step is to measure and record the results in a scaled up test, this is what I did by running them in the SWET this time with a different dye, brilliant yellow. The test was run with three 220x100mm samples of each substrate simultaneously. The results show a familiar trend as again the SSM outperforms the GFM which would go along with the conclusion that the SSM with P25 is the best substrate and coating combination.

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All Samples Comparable Degradation Testing

SWET Results

TiO2 Coatings for Degradation of Organics by Photocatalysis

Further TestingA new type of coating was being experimented with as I finished my placement and testing will continue on it into the future. I have initial results for sol-gel coatings and the results are intriguing as they seem to outperform the P25 which is used as standard at the moment. Sol-gels are solids produced by fabricating titanium oxides. Monomers are made into a solution which acts as a precursor to the growth of a network of titanium dioxide nano-structures.

Another test is in the pipeline where the dyes are tested in a salt solution to mimic sea water as realistically the water that is going to be treated will not always be fresh or distilled water. This brings in the question of rust, which obviously should not be a problem with SSM and GFM. However, it must be tested to examine whether the samples will behave in the same way.

Finally an exciting new development is the exploration of aerogels. I was fortunate enough to be involved in an initial production of some silicon dioxide aerogels during my placement. Aerogels are around 99.8% void space and the substance they are made from acts as a fine scaffolding that has an incredibly high surface area, up to 3,000 square metres per gram. That means a one cubic inch aerogel will have a bigger surface area than a football field. They have been found to hold up to 4,000x its own weight. They also have truly special thermal properties. Due to its lightness it can be held by a blow torch flame and it is such a good insulator, a flower sustains no damage.

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Sol-gel Testing

Aerogels

TiO2 Coatings for Degradation of Organics by Photocatalysis

The possibilities for aerogels are endless, however in photocatalysis they could revolutionise water treatment as their surface area would increase the rate of degradation ten fold. Whether they could be left as cylinders or crushed to a powder to adhere to a substrate is unknown. However, the machine needed to make them costs $80,000 and despite their relatively low cost to make the machinery is very expensive and it is a very slow and inefficient batch process as an alcogel precursor must be made and solvent exchanged with methanol for a week beforehand. In the vessel the methanol is exchanged with liquid carbon dioxide at 100 bar pressure, the samples are then taken supercritical before being degassed slowly to preserve the complex structure. The samples must be taken supercritical because if the liquid to gas boundary is crossed then an aerogel will splinter and be crushed under its own capillary forces. A supercritical fluid is a gas with liquid properties. We believe a precursor has been found to form the alcogel required to form a titanium dioxide aerogel, testing is set to commence shortly and the results are eagerly anticipated.

Personal ExperienceThe placement for me has been invaluable and given me the opportunity to experience a working lab. I have taken part in projects and experiments containing complex chemistry and learned the practical things you cannot read in a text book about the workings of a research environment. I must thank Dr. Ian Mabbett for handling the paperwork to allow me to complete this project, but mostly Dr. Rachel Woods and Mr. Ashley Pursglove whose passion for their project was infectious. I could not help but be drawn into the excitement and enthusiasm that comes along with a project which has the potential to save many lives and be very successful. I am sure this project will succeed and am proud to be able to say I was a brief part of it. More than anything else though, it has re-affirmed my desire to follow this field as it has shown me the possibility to make a real difference and revolutionise a practise performed worldwide is available for those who are ready to work hard for it.

ReferencesTreatment of Methyl Orange by Photocatalysis Floating Bed by Enqiang Wang, Qiaoli Zheng, Shihong Xu & Dengxin LiPorous Titanium Dioxide Coatings Obtained by Anodic Oxidation for Photocatalytic Applications by Hernán Traid & María VeraEffect of metal-doping of TiO2 nanoparticles on their photocatalytic activities toward removal of organic dyes by M. KhairySynthesis of Silica Aerogel by Supercritical Drying Method by Tomasz Błaszczyński , Agnieszka Ślosarczyk & Maciej Morawski

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Aerogel Production - Supercritical Vessel & Aerogel Results