Translucent Electrodes with Pores on Different Length-Scales

Abstract: Method of producing electrodes from transparent conductive metal oxides with porosity in the nanometer and the micrometer range

Description: We developed these fabrication protocols to test the influence of electrode porousity on different length scales on the performance of biophotovoltaic devices (BPVs), as described in the associated publication [Preprint DOI to follow soon; published paper DOI]. BPVs are bioelectrochemical devices powerd by photosynthetic microorganisms. Since BPVs rely on light as energy input, the electrodes had to be translucent as well as porous and conductive.The metal oxide indium tin oxide (ITO) was chosen as high-performance transparent conductive oxide material with nanoparticles and non-porous electrodes (as reference) commercially available. This protocol can also be used for other nanoparticle-based materials such as ZnO, ATO, TiO2, and more.

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License: CERN Open Hardware License

Authors: Tobias Wenzel, Daniel Härtter

Requires:

Transparent electrodes with 10-100 nanometer sized pores x 1
Translucent electrodes with 5-100 micrometer sized pores x 1
(Non-porous) indium tin oxide coated PET x 1

Instruction: Reference non-porous ITO electrode

Step 1. Electron microscopy images of the non-porous commercial ITO film on plastic substrate (PET). The defects were pictured on purpose to provide the neccessary image contrast. They were not common on the film. The ITO surface is completly flat, smooth, and non-porous, by comparison, it can be used as reference for the effect of porousity on different lengthscale as given by the electrodes described in the sub-bricks of this documentation.

Transparent electrodes with 10-100 nanometer sized pores

Description: This protocol describes how to make a ITO-nanoparticle based paste, manually bladecoat electrode substrates, and sinter them to finished porous electrode films. Finished electrodes have a large chemical surface area because of its pores (between 10-100nm).

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License: CERN Open Hardware License

Notes: Here, bladecoating is used to create thick electrodes of several micrometer (5-9µm), but spincoating can alternatively used to obtain thinner electrodes. For this purpose, the paste can be diluted e.g. with ethanol.

Authors: Tobias Wenzel

Requires:

Metal oxide nanoparticle paste x 1
FTO-glass x 1
Kapton tape x 1

Instruction: Conductivity testing

Step 1. As detailed in the associated publication, we performed impedance spectroscopy. This was done to probe the conductivity laterally, and through the depth of the electrode film. Sandwich samples were built by clipping two nanoparticle film samples on ITO glass substrate on top of each other with two paper-clips. For this purpose, the sample were built as follows: ITO nanoparticle films were prepared on commercial ITO- coated glass substrates (to avoid an additional material in- terface) with the same blade coating protocol, and via spin- coating at different speeds. Commercial ITO-coated glass samples without any nanoparticle coating were annealed as the other samples, to provide an appropriate reference point despite the deterioration of ITO conductivity during the high temperature treatment. The overlap area of the clipped-together samples (= area for probing the porous films) was about 0.25 cm2. The clean end pieces of the ITO-glass substrates were contacted with silver paste and crocodile clips, and electrode impedance spectroscopy measurements were performed across the enclosed films, us- ing the following parameters on a Biologic SP-300 Poten- tiostat: Scan at 0 V from 1 MHz to 1 Hz, with 40 points per decade, a sinus amplitude of 5 mV, waiting 0.1 periods before each frequency. No imaginary parts of the Nyquist plot were found that could indicate a non-ohmic resistance behaviour of the nanoporous or microporous films. The presented resistance data in this study is thus simply an average of the (noisy but stable) real part of the measure- ments.

Step 2. The sheet resistivity of the nanoparticle film electrodes was aslo determined on a non-conductive glass substrate with four point probe (S302 Lucas Labs and Keithley 2400) to be 100 ± 10 Ohm*cm?2. The thickness of the films was found to be 8.9 ± 0.6 µm by scanning over scratches with a Dektak 6M stylus profiler.

Instruction: Porosity testing

Step 1. Electron microscopy was performed from top view and crossectional view to examine the porosity of the film, here on the order of 10-100nm. The images were taken with a Leo Gemini 1530 VP SEM with a Schottky-emitter consisting of a zirconium oxide coated tungsten cathode and an in-lens secondary electron detector.

Assembly instruction

Step 1. Cut the non-conductive glass (as in picture, used for conductivity tests) or the FTO-glass into the desired shape (here 20x30mm FTO and labeled).

Step 2. Inside a fumehood, FTO glass was taped with kapton tape as distance spacer. In contrast to the second image here, do leave the tape ends stick to the table, this will provide stability while aplying the paste, it can be cut off afterwards. ITO-particle-terpineol paste was applied in between the tape strips with a spatula or a syringe.

Step 3. Excess paste was removed by manually pushing the side of a glass pipette rod, or the edge of a microsscope slide over the spacers (blade- coating) to yield a plain fil of a thickness defined by the spacers.

Step 4. The electrode was left to settle at room temperature for ca. 20 minutes. The kap- ton tape was removed manually.

Step 5. The electrodes were annealed by heating to 500 degree Celsius on a programmable hotplate, with the following heat ramp procedure: 3h heat ramp from room temperature to 300 ?C holding the temperature for 1 minute; 10 min ramp to 325 ?C holding for 5 min; 10 min ramp to 375 ?C for 5 min; 10 min to 450 ?C for 15 min; 10 min to 500 ?C for 15 min; off.

Metal oxide nanoparticle paste

Description: This brick briefy describes how to make a paste from ITO particles that is suitable for bladecoating or spincoating to form porous electrodes.

Requires:

Electrode nanoparticle paste ingrediences: ITO (metal-oxide) nanoparticle dispersion x 1, a-Terpineol 96+% x 1

Assembly instruction

Step 1. The nanoparticle dispersion (2.5 g equivalent of nanoparticles only) were mixed with 10.7 ml terpineol. The IPA from the nanoparticle dispersion was evaporated off in a rotary evaporator at 55 ?C and vacuum pumping. The rotary evaporator simplifies the processes of strirring while heating, and extracts solvents quickly by heat and vacuum, but it is not absolutely neccessary to use this equipemnt to get ridd of post isopropanol from the ITO particle dispersion.

Step 2. Optional alteration: There is a more elaborate recipe to make a paste that is slightly more viscous and strain ressistant during annealing due to the incorporation of ethylcellulose.This receipe has originally be develloped for TiO2 films, see details in the publication by Seigo Ito et all 2007 "Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%" (https://doi.org/10.1016/j.tsf.2007.05.090). It has also been tested by me and gave good results for spincoating, but the simpler paste was usually also crack-free when bladecoating into 9um thick films. It requires the pre-preparation of two ethylcellulose solutions which have to be dissolved very slowly under stirigng over night.

Translucent electrodes with 5-100 micrometer sized pores

Abstract: How to fabricate electrodes with pores on the micrometer lengh scale templated by plastic particles.

Description: This protocol describes the procedure to fabricate large (cm-sized) porous elelctrodes, the pores of which are precisely controlled by a polymer opal (or closed-packed polymer microsphere) template.

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License: CERN Open Hardware License

Authors: Tobias Wenzel, Daniel Härtter

Requires:

Fabrication of cm-large uniform polymer opal templates x 1
ITO (metal-oxide) nanoparticle dispersion x 1
Absolute Ethanol x 1

Instruction: Porosity testing

Step 1. Optical microscope images show the glass-like transparency of the electrode material and disply the connecting pores well. Optical microscopy was conducted with an Olympus BX60 microscope and Olympus UMPlanFl 20x objec- tives. Images were taken with an AxioCam MRc 5 (Zeiss) camera.

Step 2. Electron microscopy was performed from crossectional and top view to examine the porosity of the film, here 40um holes with ca. 10um vconnections, and also porosity on the order of 10-100nm. The images were taken with a Leo Gemini 1530 VP SEM with a Schottky-emitter consisting of a zirconium oxide coated tungsten cathode and an in-lens secondary electron detector.

Assembly instruction

Step 1. Follow the sub-brick procedure to produce the polymer template, unsing the template size of your choice.

Step 2. Exchange the solvent from the ITO nanoparticle suspension (isopropanol) by ethol by drying the particles, and resuspending them (vortex and ultrasound bath) in ethanol to obtain a 10% by weight suspension. We exchanged the default solvent isopropanol (IPA) in the ITO-nanoparticle dispersion with ethanol, because we realised that ethanol has improved wetting properties for the filling of the polystyrene sphere template. The isopropanol seems to wett the polymer microspheres stronger, closing the template on the top. For the usage in experiments, this blocks the filling solution (including, e.g. living cells) to enter the structure, which is undesirable. Ethanol, in contrast, had less of a tendency to wet the tips of the template (see electron microscopy images).

Step 3. The polystyrene opals were placed on a hotplate at 45C for the filling procedure.

Step 4. They were filled with 3 times with 25 µl of ITO suspension each.

Fabrication of cm-large uniform polymer opal templates

Abstract: Deposition holder for fabrication of cm-sized templated made of micrometer-sized polystyrene beads

Description: This is a instruction to fabricate uniform cm-sized bead scaffolds of micron-sized polystyrene beads. The scaffolds can be used for the fabrication of anodes for electrochemical cells using a inverse opal method, i.e. filling the voids with a different material, removing the beads and getting the inverse pattern. The method is easy and scalable and requires only the described deposition holder, a pipette, a temperature controlled oven and a hotplate. The deposition mechanism is controlled evaporation with a circular meniscus, inspired by [Denkov, N. D. et al. Two-dimensional crystallization. Nature 361, 26–26. issn: 0028-0836 (Jan. 7, 1993)]. The round deposition basin of the device causes a rising of the water at the boundaries, creating a negative concave meniscus. After the filling of the basin with the bead suspension, the beads settle down quickly, because polystyrene is heavier than water and the micrometer beads are too big be considered as brownian particles and to diffuse around. For nm-sized beads diffusion may play a role. Anyway, the beads settle down and distribute regularly over the whole area. When the water evaporates and the water level reaches the beads in the middle, due to the concave meniscus, there is a sudden increase of surface area in the center, because the beads are still wetted by the water. The increase of surface area results in a higher evaporation rate, inducing a water flow to the center. Due to this water flow, a small hydrodynamic force pulls the beads towards the center. This directed force urges the beads to arrange in a uniform pattern. The procedure was developed for 40 µm polystyrene beads and scaffolds of 2-5 bead layers, but it is principally considered to work also for different materials, bead sizes and scaffold thicknesses.

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Requires:

Bead-suspension ingrediences: polystyrene microspheres, 40µm x 1, DI-Water x 1
Sealing O-ring: Silicone O-ring x 1
Components for deposition holder fabrication: Aluminium sheet, 4mm x 1, M3 hex-screws x 2
Substrate material: FTO-glass x 1

Assembly instruction

Step 1. Fabrication of deposition holder: A simple hand-made device used to clamp in substrate materials and deposite a uniform area of colloids in its centre via convective aseembly during drying of colloidal suspension. We used 4mm thick aluminium rectangular pieces and cut the holes on a lathe: On top: large centre hole (depending on O-ring size!), and two M3 clearance holes; Bottom: two threaded holes for M3 screws.

Step 2. Assembly of sample holder: A cleaned substrate (here FTO-coated glass, 20x30mm, 3mm thick) is fastened in place with a rubber ring that is slightly bigger than the big hole.

Step 3. Preparing the bead suspension: Here, 40µm diameter beads were used, 5% by weight in dionised water, and suspended by shaking. These beads are heavy compared to water and settle from suspension in a matter of seconds to minutes. Smaller polystyrene or PMMA beads would behave as colloids and stay suspended.

Step 4. Filling with bead dispersion: 300-500 µl of bead disperion are filled into the basin using a pipette (depends on the required thickness of the scaffold; Here for ca.3 layers of 40 µm beads). It is recommended to mix the stock bottle just before filling the pipette and to fill the device slowly to prevent bubbles. Then the device can be tilted slightly in all direction to ensure a uniform wetting of the rubber ring and hence a uniform meniscus.

Step 5. Controlled evaporation: The water evaporation rate depends on temperature and humidity. To ensure a controlled evaporation, the devices are put into a temperature controlled lab oven with 30°C over night to dry.

Step 6. Removal of samples: After the evaporation the screws and the rubber ring have to be removed carefully. The bead film is very fragile and does not stick well to the underlying substrate.

Step 7. Melting/Annealing of beads: To make the scaffold stable and to connect the beads, the samples are heated to 130°C for 10 min on a hotplate. This procedure goes along with a shrinking of the crystal, sopmetimes resulting in cracks in the scaffold. Temperature and time depend on the melting temperature of the material and the bead sizes and have to be optimized for every specific configuration.

Part: polystyrene microspheres, 40µm

Description: Polystyrene microshperes here with an average diameter of 40 µm. The microbeades used here do not posess a high degree of monodispersity and thus do not asseble into highly ordered opals, but into close-packed structures instead. It depends on the sizse and monodispersity of the microspheres/colloids what the assembly product will look like. To our estimation, any microspere in tha range of ca. 5-100µm will work with the presented method.

Supplier: Dynoseeds

Supplier catalog #: Microbeads TS 40

URL: www.micro-beads.com

Part: Silicone O-ring

Description: Pro Silicone O-Ring (nitrile), 17.12mm Bore (or similar)

Supplier: RS components

Supplier catalog #: 527-9863

Manufacturer catalog #: BS115 SIL70

URL: uk.rs-online.com

Part: DI-Water

Description: Clean, (de-Ionised or distilled) water, as routinely purified within most wet-laboratories.

Part: FTO-glass

Description: FTO coated glass was used as conductive substrate, but other substrates could be used as well. The later heating procedure prevents a range of materials to be used that are affected by heat up to 500 degree C (e.g. stainless steel will form a non-conductive oxide layer). Instead of the product referred to here, we actually used an FTO variant with a slightly higher condictuivy of 8Ohm/sq instead of 10Ohm/sq, but this is unlikely to make a difference in most experiments.

Supplier: Sigma Aldrich or Solaronix

Supplier catalog #: 242-159-0

URL: www.sigmaaldrich.com

Part: ITO (metal-oxide) nanoparticle dispersion

Description: Indium tin oxide dispersion, <100 nm particle size (DLS), 30 wt. % in isopropanol

Supplier: Sigma Aldrich

Supplier catalog #: 700460 Aldrich

Manufacturer catalog #: 50926-11-9 (CAS number)

URL: www.sigmaaldrich.com

Part: a-Terpineol 96+%

Supplier: Sigma Aldrich (SAFC supply solutions)

Supplier catalog #: W304522 Aldrich

Manufacturer catalog #: 10482-56-1 (CAS Number)

URL: www.sigmaaldrich.com

Part: Kapton tape

Supplier: Onecall/Farnell

Supplier catalog #: 1503246

Manufacturer catalog #: 051-0007 MULTICOMP

URL: uk.farnell.com

Part: Absolute Ethanol

Description: Lab-grade ethanol, here we used 96% v/v Ethanol, analytical grade.

Supplier: Fisher Scientific

Supplier catalog #: Fisher Chemical E/0555DF/17

Manufacturer catalog #: CAS Number: 64-17-5

URL: www.fishersci.co.uk

Part: (Non-porous) indium tin oxide coated PET

Description: Plain ITO on PET sheet with a surface resistivity of 100 Ohm/sq.

Supplier: Sigma Aldrich

Supplier catalog #: 639281-5EA

URL: www.sigmaaldrich.com

Part: Aluminium sheet, 4mm

Part: M3 hex-screws

Description: The screws used here were12mm long to suit 3mm thick FTO glass inside. The length depends on the thickness of the substrate on which the microspheres are going to be deposited.

Authors

Name	E-mail	Affiliation	ORCID
Tobias Wenzel	wenzel.science@gmail.com	Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, United Kingdom	0000-0001-8443-1315
Daniel Härtter	daniel.haertter@stud.uni-goettingen.de	III. Physikalisches Institut, Georg-August-Universität, 37077 Göttingen, Germany

Total bill of materials for this project

Part	Quantity	Supplier	Supplier part number	URL
ITO (metal-oxide) nanoparticle dispersion	2	Sigma Aldrich	700460 Aldrich	www.sigmaaldrich.com
a-Terpineol 96+%	1	Sigma Aldrich (SAFC supply solutions)	W304522 Aldrich	www.sigmaaldrich.com
FTO-glass	2	Sigma Aldrich or Solaronix	242-159-0	www.sigmaaldrich.com
Kapton tape	1	Onecall/Farnell	1503246	uk.farnell.com
polystyrene microspheres, 40µm	1	Dynoseeds	Microbeads TS 40	www.micro-beads.com
DI-Water	1
Silicone O-ring	1	RS components	527-9863	uk.rs-online.com
Aluminium sheet, 4mm	1
M3 hex-screws	2
Absolute Ethanol	1	Fisher Scientific	Fisher Chemical E/0555DF/17	www.fishersci.co.uk
(Non-porous) indium tin oxide coated PET	1	Sigma Aldrich	639281-5EA	www.sigmaaldrich.com