Titanium Dioxide and UV Air Purifier

Hello community of Instructable,

I hope you are all well in the emergency circumstances we are living in this moment.

Today I am bringing to you an applied research project. In this Instructable I will be teaching you how to build an air purifier working with a TiO2 (Titanium Dioxide) photocatalyc filter and UVA LEDs. I will tell you how to make your own purifier and I will also show you an experiment. According to scientific literature this filter should remove bad odors and kill bacteria and viruses in the air that goes through it, including coronavirus family.

In this research paper you can see how this technology can be used effectively to kill bacteria, fungi and viruses; they actually quote a research of 2004 titled The Inactivation Effect of Photocatalytic Titanium Apatite Filter on SARS Virus, in which the researchers state that 99.99% of severe acute respiratory syndrome viruses were killed.

I would like to share this project as I believe it could be a particularly interesting one because it tries to solve a serious problem and because its multidisciplinary: it brings together notion of chemistry, electronics and mechanical design.

The steps:

1. Photocatalysis with TiO2 and UV light

2. Supplies

3. 3D Design of the air purifier

4. Electronic circuit

5. Solder and assemble

6. The device complete

7. The stinky shoe purification effort

In this section I will explain the theory behind the reaction.

Everything is summarized graphically in the image above. Below I will explain the image.

Basically, the photon with enough energy arrives in the molecule of TiO2 in the orbit where an electron is spinning. The photon hits the electron hard and makes it jump away from the valence band to the conduction band, this jump is possible because TiO2 is a semiconductor and because the photon has enough energy. The energy of the photon is determined by its wavelength according to this formula:

E = hc/λ

where h is the Plank Constant, c is the speed of light and λ is the wavelength of the photon, which in our case is 365nm. You can calculate the energy using this nice online calculator. I our case it is E=3,397 eV.

Once the electron jumps away there is a free electron and a free hole where it once was:

electron e-

hole h+

And these two in turn are hit by some other molecules that are parts of the air which are:

H2O molecule of water vapor

OH- Hydroxide

O2 molecule of oxygen

A few redox reactions happen (learn more about them in this video).

Oxidation:

Water vapor plus a hole gives hydroxyl radical plus hydrated hydrogen ion: H2O + h+ → *OH + H+(aq)

Hydroxide plus a hole gives hydroxyl radical: OH- + h+ → *OH

Reduction:

oxygen molecule plus an electron gives superoxide anion: O2 + e- → O2-

This two new things formed (hydroxyl radical and superoxide anion) are free radicals. A free radical is an atom, molecule or ions with a single unpaired electron, this is crazy unstable as said in this very funny Crush Course video.

Free radicals are the main responsible for many chain reactions that happen in chemistry, for example polymerization, which happens when monomers joins one to the other to form a polymer, or in other words to make what we more broadly call plastic (but that's another story).

O2- hits big bad odor molecules and bacteria and breaks their carbon bonds forming CO2 (carbon dioxide)

*OH hits big bad odor molecules and bacteria and breaks their hydrogen bonds forming H2O (water vapor)

The union of the free radical to carbon compounds or organisms is called mineralization and this is exactly where the killing is happening.

For further information I have attached the PDF of the scientific papers that I quoted in the intro.

To make this project you'll need:

- 3D printed case

- 3D printed lid

- laser cut anodized aluminum 2mm thick

- silk screen (optional, eventually I didn't use it)

- 5 pieces of high power UV LED 365nm

- PCB stars with 3535 footprint or LEDs already mounted on a star

- thermal double-sided adhesive tape

- TiO2 Photocatalyst Filter

- Power supply 20W 5V

- EU connector 5/2.1mm

- Fan 40x10mm

- thermal shrieking tubes

- countersunk head M3 bolts and nuts

- 5 1W 5ohm resistors

- 1 0.5W 15ohm resistor

- small wires

I have added the links for purchasing some stuff but I am not running any affiliate program with the vendors. I put the links only because if somebody would like to replicate the air purifier in this way can have an idea of the supplies and costs.

You can find the whole assembly file in format .x_b in the achieve.

You may notice that I had to optimize the case for 3D printing. I made the walls thicker and I decided not smooth the angle at the base.

The heatsink is laser cut and milled. There is a 1mm lowering on the 2mm anodized aluminum (RED ZONE) that allows for better bending. The bending has been done manually with pliers and vise.

A friend of mine made me notice that the pattern on the front of the case is similar to the tattoo that Leeloo wears in the movie the The Fifth Element. Funny coincidence!

The electronic circuit is very easy. We have a constant voltage power supply of 5V and in parallel we are going to place 5 LEDs and a fan. Through a bunch of resistors and with some math calculations we decide how much current we would feed into the LEDs and into the fan.

THE LEDs

Looking at the LED datasheet we see that we can drive them up to 500mA maximum, but I decided to drive them at half power (≈250mA). The reason is that we have a small heatsink, which is basically the aluminum plate at which they are attached to. If we drive the LED at 250mA the forward voltage of the LED is 3.72V. According to the resistance that we decide to put on that branch of the circuit we obtain the current.

5V - 3.72V = 1.28V is the voltage potential we have on the resistor

Ohm law R = V/I = 1.28/0.25 = 6.4ohm

I will use the commercial value of resistance of 5ohm

Power of the resistor = R I^2 = 0.31W (I have actually used 1W resistors, I left some margin because the LED could heat up the area quite a bit).

THE FAN

The fan suggested voltage is 5V and 180mA current, if driven with this power it can move air at the flow rate of 12m3/h. I noticed that going at this speed the fan was too noisy (27dB), so I decided to lower a bit the voltage supply and the current supply to the fan, to do so this I used a resistor of 15ohm. To understand the value needed I used a potentiometer and I saw when I would have around half of the current, 100mA.

Power of resistor = R I^2 = 0.15W (I've used 0.5W resistor here)

So the actual final flow rate of the fan results 7.13 m3/h.

I have used thin cables to join the LEDs together and make the whole circuit and soldered everything as organized as possible. You can see that the resistors are protected inside heat shrink tubing. Be aware that you have to solder the anode and the chatode of LEDs to the right poles. The anodes goes to one resistor end and the cathodes goes to GND (-5V in our case). On the LED there is an anode mark, find the location of it looking it up in the LED datasheet. LEDs are attached to the heatsink with thermal double-sided adhesive tape.

I have actually used a DC connector (the transparent one) to easily remove the whole block shown in the first picture (heatsink, LEDs and fan), however this element can be avoided.

The black 5/2.1 EU DC main power supply connector has been glued in an hole that I drilled manually.

The side holes I made in the lid to fix the lid with screws to the case were also drilled manually.

Making all the soldering in that small space was a little challenge. I hope you'll enjoy embracing it.

Congratulations! Just plug it and start purifying air.

The air flow rate is 7.13 m3/h so a room of 3x3x3m should be purified in around 4h.

When the purifier is on I have noticed that out of it comes an odor that reminds me of ozone.

I hope you have liked this Instructable and if you are even more curios there is an extra section about an experiment I made.

If you are not willing to build your own air purifier but you would like just to get it straight away you could buy it on Etsy. I made a couple so feel free to visit the page.

Bye and take care,

Pietro

In this extra section I would like to show a little funny experiment that I did with the purifier.

Initially I put a very stinky shoe - I assure you it really smelt bad - in an hermetic acrylic cylinder with a volume of 0.0063 m3. What should make that shoe that smelly are big molecules containing sulfur and carbon and also bioeffluents and bacteria coming from the foot that was wearing that shoe. What I was expecting to see when I turned the purifier on was the VOC to reduce and CO2 to increase.

I left the shoe there in the cylinder for 30min in order to reach the "stink balance" inside the container. And through a sensor I noticed a massive increase in CO2 (+333%) and VOC (+120%).

At minute 30 I placed inside the cylinder the air purifier and I turned it on for 5min. I noticed a further increase in CO2 (+40%) and VOC (+38%).

I removed the stinky shoe and I left the purifier turned on for 9min and CO2 and VOC were keeping to increase dramatically.

So according to this experiment something was happening inside that cylinder. If VOC and bacteria are being destroyed through the process of mineralization the theory tells us that CO2 and H2O is formed, so one could say that it is working because the experiment shows that CO2 is keep forming, but why also VOC kept increasing? The reason may be that I used the wrong sensor. The sensor I used is the one shown in the picture and from what I understood it estimates CO2 according to a percentage of VOC using some internal algorithms and also reach VOC saturation easily. The algorithm, which is developed and integrated into the sensor module interpreted the raw data, e.g. metal oxide semiconductor resistance value, in CO2 equivalent value by doing the comparison test against NDIR CO2 gas sensor and Total VOC value based on the comparison test with instrument FID. I think that I did not used equipment sophisticated and accurate enough.

Anyway it has been funny to try to test out the system this way.