
==== Front
STAR Protoc
STAR Protoc
STAR Protocols
2666-1667
Elsevier

S2666-1667(24)00424-6
10.1016/j.xpro.2024.103259
103259
Protocol
Protocol for the preparation of TiO2-modified boron-doped diamond photoelectrode via electrophoretic deposition and its photoelectrochemical study
Quilumbaquin Wendy 1
Castillo-Cabrera G. Xavier 13
Borrero-González Luis J. 2
Espinoza-Montero Patricio J. pespinoza646@puce.edu.ec
14∗
1 Escuela de Ciencias Químicas, Pontificia Universidad Católica del Ecuador, Quito 170525, Ecuador
2 Laboratorio de Óptica Aplicada, Escuela de Ciencias Físicas y Matemática, Pontificia Universidad Católica del Ecuador, Quito 170525, Ecuador
∗ Corresponding author pespinoza646@puce.edu.ec
3 Technical contact

4 Lead contact

10 8 2024
20 9 2024
10 8 2024
5 3 103259© 2024 The Author(s)
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Summary

Electrophoretic deposition is a straightforward method for synthesizing high-quality photoanodes. We present a protocol for synthesizing a TiO2-modified boron-doped diamond photoanode (BDD/TiO2) via electrophoretic deposition, detailing the chemical and electrochemical treatments applied to the bare BDD electrode prior to use. We provide a step-by-step guide for performing photoelectrochemical characterization under both dark and light conditions and describe an optical technique for investigating band-gap energy.

For complete details on the use and execution of this protocol, please refer to Quilumbaquin et al.1

Graphical abstract

Highlights

• Preparation and characterization of BDD-based photoelectrodes by electrochemistry

• High-quality thin-film BDD/TiO2 photoelectrode preparation using electrophoresis

• Electrochemical and photoelectrochemical characterization of BDD/TiO2 photoelectrode

• Optical properties of BDD/TiO2 using diffuse reflectance spectroscopy

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

Electrophoretic deposition is a straightforward method for synthesizing high-quality photoanodes. We present a protocol for synthesizing a TiO2-modified boron-doped diamond photoanode (BDD/TiO2) via electrophoretic deposition, detailing the chemical and electrochemical treatments applied to the bare BDD electrode prior to use. We provide a step-by-step guide for performing photoelectrochemical characterization under both dark and light conditions and describe an optical technique for investigating band-gap energy.

Subject areas

Chemistry
Material sciences
Environmental sciences
==== Body
pmcBefore you begin

Stable photoelectrodes are the cornerstone of many photoelectrocatalytic processes such as in water decontamination.2 While several techniques for preparing highly stable and non-toxic photoelectrodes have been reported,3,4 there remains a dearth of primary procedures for synthesizing photoanodes based on boron-doped diamond (BDD) substrates. BDD is an electrode commonly used as anode exhibiting p-type semiconducting behavior.5 Its active electrochemical surface demonstrates remarkable stability in charge transfer reactions and exhibits low background currents.6 Given these features, BDD emerges as a promising candidate for surface modification via chemical or physical methods with stable photocatalysts to form what is known as a photoelectrode, with superior optical and electrochemical performance.

This protocol delineates a straightforward procedure for preparing a photoanode based on BDD modified with the commonly used titanium dioxide photocatalyst, through a chemical-assisted method.7,8 Furthermore, guidance is provided on how to accurately characterize photoelectrodes using electrochemical techniques in both light and dark conditions.

Preparation of the reagents and equipment

A complete list of reagents and equipment can be found in the ‘‘key resources table”. All the reagents were used without further purification.

Cleaning of boron-doped diamond (BDD) electrode

Timing: ∼30 min

This is the very first step for the modification and characterization of the unmodified (bare) BDD electrode to ensure a clean working surface free of organic impurities, dust, or moisture.1. Submerge the BDD electrode (3.0 × 1.5 × 0.2 cm3) in acetone reactive grade, ensuring complete coverage of the surface by the liquid.

Note: For bipolar electrodes (BDD on both sides), it’s recommended to position the electrode at an angle, touching the walls of the beaker at both ends, for the next step.

2. Ultrasonicate the electrode submerged in acetone for 10 min at 20°C and 40 kHz (Figure 1A).Figure 1 Cleaning and activation of bare BDD electrode

(A) Ultrasonication-assisted cleaning of the BDD surface in acetone prior to use.

(B) Electrochemical activation of the BDD electrode in 0.5 mol L−1 HClO4 passing a current of 0.05 A.

3. Collect the electrode and wash it at least three times with deionized water (DI). Allow it to dry at room temperature.

4. HClO4 aqueous solution: Carefully add 4.3 mL of 70% perchloric acid (HClO4) to a volumetric flask containing water. Mix thoroughly and allow the solution to cool to room temperature. Then, dilute to 100 mL with water (0.5 mol L−1).

5. Repeat steps 1 and 2 using 0.5 mol L−1 HClO4 and DI exclusively, thereby concluding an ultrasound-assisted cleaning cycle.

Alternatives: Acid cleaning can be with any strong acid in a moderate concentration range, typically from 0.1 to 0.5 mol L−1.

Activation of bare BDD electrode

Timing: ∼30 min

Before conducting any tests, ultrasound-assisted cleaning is followed by surface activation. This step is crucial because BDD is a carbonaceous material with various lattice terminations resulting from sp3/sp2 hybridization ratios during synthesis. These terminations include oxygen-terminated (O-terminated) and hydrogen-terminated (H-terminated) surfaces. The subsequent electrochemical activation step is outlined below. For more detailed guidance on BDD, readers can consult Einaga’s work.66. Assemble a two-electrode cell with BDD and Pt mesh, and carefully add enough 0.5 mol L−1 HClO4 to immerse the entire working surface of the electrode completely (Figure 1B right).

Alternatives: Electrochemical activation in an acidic medium can be with any strong acid in a moderate concentration range, typically from 0.1 to 0.5 mol L−1.

7. Connect the positive terminal of a DC power supply to the BDD electrode and the negative terminal to the Pt mesh.

Note: Ensure that the distance between the electrodes is approximately 1 cm.

8. Apply a current of 50 mA for 30 min. During this time, observe the evolution of oxygen on the BDD surface and hydrogen on the Pt surface (Figure 1B left).

Alternatives: The activation time and applied current can be adjusted as needed to achieve the desired outcome based on concurrent characterization requirements. Troubleshooting 1.

CRITICAL: An anodic activation is required because it results in a surface with oxygenated terminations, which is desirable for enhanced binding with the photocatalyst (TiO2).

9. Retrieve the electrode and thoroughly rinse it with DI at least three times. Allow it to air dry at room conditions.

Preparation of redox couple for BDD characterization

Timing: ∼30 min

10. Before modification, it’s crucial to characterize the bare electrode in a suitable electrolytic medium to establish its initial state. This step involves preparing the redox couple for investigation in the appropriate electrolytic medium.a. Na2SO4 aqueous solution: weigh 0.7031 g of Na2SO4 (99%) and dissolve it in DI under magnetic stirring at 20°C. Ensure thorough mixing until fully dissolved, then adjust the volume to 50 mL volumetric flask (0.1 mol L−1).

b. KCl aqueous solution: weigh 3.6903 g of KCl (99%) and dissolve it in DI under magnetic stirring at 20°C. Ensure thorough mixing until fully dissolved, then adjust the volume to 50 mL volumetric flask (1.0 mol L−1).

c. Ferri/ferrocyanide redox couple in KI solution: weigh 0.0326 g of K3[Fe(CN)6] (99%) and 0.0416 g of K4[Fe(CN)6]·3H2O (98.5%) and dissolve them in 1.0 mol L−1 KCl under magnetic stirring at 20°C to obtain a yellowish aqueous solution. Ensure thorough mixing until fully dissolved, then adjust the volume to 50 mL volumetric flask (2.0 mmol L−1) (Figure 2A).Figure 2 Chemicals preparation and electrochemical setup arrangement

(A) Ferri/Ferro cyanide redox couple preparation (2.0 mmol L−1).

(B) Preparation of TiO2 nanopowder suspension in 2.5% (v/v) alcoholic mixture.

(C) Electrochemical setup arrangement: The cell was confined into a Faraday cage and comprised a BDD working electrode (WE), Pt mesh as counter electrode (CE), and Ag/AgCl as reference electrode (RE).

CRITICAL: We strongly encourage readers to prepare a sufficient quantity of freshly made solutions at the time of conducting experiments to avoid concentration gradients and related issues that can occur with stored solutions.

Preparation of suspension of TiO2 nanoparticles

Timing: ∼20 min

11. Degussa P25 TiO2 suspension: Degussa P25 TiO2 nanospheres were employed to prepare the suspension for modifying the BDD electrode (Figure 2B).a. Mix 1.25 mL of isopropanol with 48.75 mL of DI, shaking until fully blended and homogenized (2.5%(v/v))

b. Weigh 1.25 g of Degussa P25 TiO2 and add it to the aforementioned alcoholic mixture. Ultrasonicate the mixture for 30 min at 20°C and 40 kHz to attain a well-dispersed white mixture of the photocatalyst (2.5%(w/v)). Troubleshooting 2.

Assembly of electrochemical setup for electrode characterization

Timing: ∼10 min

12. Assemble a standard three-electrode system for electrode characterization, consisting of BDD as the working electrode, Pt mesh as the counter electrode and Ag/AgCl as the reference electrode in a quartz cell. The electrodes should be positioned approximately 1 cm apart.

13. Enclosed the cell within a Faraday cage to minimize electronic noise and ensure precise control over light and dark conditions (Figure 2C).

14. Position the light source approximately 10 cm away from the cell.

Preparation of spectroscopy arrangement

Timing: ∼30 min

To record the diffuse reflectance spectra (DRS), the experiments were conducted using a well-positioned spectroscopic arrangement mounted on a X-Y platform equipped with micrometric screws (Figure 3).15. A 75 W PowerArc PTI Horiba lamp was used as the light source, with the light guided to the sample via an optical fiber.

16. Connect a SMA collimator to the optical fiber to ensure a uniform and directed illumination.

17. Adjust lenses with focal lengths of 40 mm, 60 mm, and 150 mm to precisely focus and direct the light onto the sample and to collect the reflected light.

18. A Horiba Micro HR monochromator (Czerny-Turner mounting, Automatic with 140 mm of focal length) was used, along with a Horiba DSS-SIGA (2.2)020A dual sensor (Si/InGaAs).

Alternatives: Diffuse reflectance experiments can be carried out using a traditional UV-vis spectrophotometer equipped with an integration sphere. For further details on suitable spectroscopic arrangements, readers may find the works of Blitz and Frei useful.9,10

Figure 3 Multifunctional optical setup for recording DRS spectra: This setup includes a 75 W Xe arc lamp as the light source guided towards the sample by an optical fiber

The signal was detected by a Si/InGaAs dual sensor.

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER	
Chemicals, peptides, and recombinant proteins	
	
Acetone	Sigma-Aldrich	CAS 67-64-1	
Sodium sulfate	Sigma-Aldrich	CAS 7757-82-6	
Perchloric acid	Sigma-Aldrich	CAS 7601-90-3	
Potassium ferricyanide	Sigma-Aldrich	CAS 13746-66-2	
Potassium ferrocyanide	Sigma-Aldrich	CAS 14459-95-1	
Degussa P25 titanium dioxide nanopowder	Sigma-Aldrich	CAS 13463-67-7	
BDD/Nb	Metakem	Germany	
	
Software and algorithms	
	
Origin Pro 8.9	OriginLab Corporation	Massachusetts, USA	
	
Others	
	
Electrochemical workstation	CH Instruments	1230C	
DC Power supply	BK Precision	1760A	
Germicidal LED lamp	Sylvania germicidal	21 W UV-C	
75 W PowerArc Xe lamp	Horiba	PTI Horiba	
Collimator	Horiba	SMA collimator	
Lenses 40-60-150 mm	Horiba	N/A	
Monochromator	Horiba	Horiba Micro HR	
X-Y platform	Horiba	N/A	
Dual sensor (Si/InGaAs)	Horiba	Czerny-Turner	

CRITICAL: Perchloric acid is a corrosive chemical that can cause skin burns and eye damage. Handle it inside the fume hood using personal protective equipment.

Materials and equipment

Reagents

HClO4 aqueous solution

Reagents	Final concentration	Amount	
HClO4	0.5 mol L−1	4.3 mL	
DI	N/A	100 mL	
Total	N/A	N/A	
Note: The HClO4 aqueous solution must be freshly prepared on the day of the experiments.

Na2SO4 aqueous solution

Reagents	Final concentration	Amount	
Na2SO4	0.1 mol L−1	0.7031 g	
DI	N/A	50 mL	
Total	N/A	N/A	
Note: The Na2SO4 aqueous solution must be freshly prepared on the day of the experiments.

Ferri/ferrocyanide redox couple in KI solution

Reagents	Final concentration	Amount	
KI	1.0 mol L−1	3.6903 g	
K3[Fe(CN)6]	2.0 mmol L−1	0.0326 g	
K4[Fe(CN)6]·3H2O	2.0 mmol L−1	0.0416 g	
DI	N/A	50 mL	
Total	N/A	N/A	
Note: The Ferri/ferrocyanide redox couple in KI aqueous solution must be freshly prepared on the day of the experiments.

Degussa P25TiO2 suspension

Reagents	Final concentration	Amount	
Degussa P25 TiO2	2.5% (w/v)	1.25 g	
Isopropanol	2.5% (v/v)	1.25 mL	
DI	N/A	48.75 mL	
Total	N/A	N/A	
Note: The Degussa P25 TiO2 suspension must be freshly prepared on the day of the experiments.

Step-by-step method details

Characterization of bare BDD electrode

Timing: ∼60 min

Characterization is conducted through conventional electrochemical techniques under dark conditions to gather data regarding the initial state of the BDD electrode post-cleaning and activation. The specific conditions employed are outlined below.1. Cyclic Voltammetry (CV) in redox couple: assemble the three-electrode cell containing the ferri/ferro aqueous solution.Note: BDD serves as the working electrode, platinum mesh as the counter electrode, and Ag/AgCl as the reference electrode.

Note: Ensure the electrodes are positioned approximately 0.5 cm away from the BDD electrode (Figure 4A).

a. Bubble the K3[Fe(CN)6] / K4[Fe(CN)6] redox couple solution with nitrogen for 3 min to generate an inert atmosphere, primarily to mitigate oxidation by dissolved oxygen.

b. Perform cyclic voltammetry scans on the K3[Fe(CN)6] / K4[Fe(CN)6] redox couple, starting from the open circuit potential (OCP), typically at 0.27 V vs. Ag/AgCl for this system.

c. Conduct 10 scans at a rate of 100 mV s−1 within a working window spanning from −0.4 to 0.8 V vs. Ag/AgCl for complete stabilization of the system.

d. Collect the electrode and wash it with DI at least three times and air dry.CRITICAL: The working window should be adjusted to ensure that the entire voltammogram i.e., the oxidation and reduction peaks, is visible.

Note: For this quasi-reversible reaction it is optimal to obtain a peak separation (ΔEp = Ec–Ea) of approximately 0.2 V vs. Ag/AgCl.

Figure 4 Electrochemical characterization and BDD/TiO2 preparation

(A) Electrochemical setup for recording CV profiles, EIS spectra, and photocurrent under both dark and light conditions.

(B) Two-electrode arrangement for electrophoretic deposition, with the electrodes positioned 1 cm apart.

(C) Electrophoretic deposition performed with a 4.8 V bias.

(D) Solvent evaporation at 100°C.

(E) Annealing of the BDD/TiO2 photoanode at 200°C.

(F) Final modified BDD/TiO2 photoanode.

2. CV in electrolytic medium: Repeat the same assembly described in the first step in 0.1 mol L−1 Na2SO4.a. Perform CV in the electrolytic solution, starting from the OCP, typically at 0.40 V vs. Ag/AgCl for this system.

b. Execute 10 consecutive scans at a rate of 100 mV s−1 within a defined potential range spanning from −1.4 to 2.1 V vs. Ag/AgCl.

c. Collect the electrode and wash it with DI at least three times and air dry.

Note: The working window should be adjusted to identify the region where oxygen and hydrogen evolution occurs at the BDD electrode.

3. Electrochemical impedance spectroscopy (EIS): assemble the three-electrode cell containing the K3[Fe(CN)6] / K4[Fe(CN)6] aqueous solution.a. Configure the impedance-potential parameters at OCP, ensuring that the frequency range over which the potential oscillates is broad, typically from 1 to 10000 Hz. Additionally, set the amplitude of the waveform to 5 mV.

b. Collect the electrode and wash it with DI at least three times and air dry.

CRITICAL: Depending on the workstation being used, ensure that the appropriate program for impedance-potential analysis is selected, typically labeled as AC impedance-potential.

4. Generate the Nyquist plot, where the real component of the impedance is represented on the abscissa axis (Z′) and the negative of the imaginary component of the impedance (-Z″) corresponds to the ordinate axis.

Note: As the redox reaction of the K3[Fe(CN)6] / K4[Fe(CN)6] pair involves a single electron transfer, the Nyquist plot manifests as a semicircular arc. The radius of this arc correlates directly with the resistance to charge transfer (RCT) in ohms.

Electrophoretic deposition of TiO2 on BDD

Timing: ∼30 min

Electrophoretic deposition is a critical step in fabricating the as-desired photoelectrode, requiring precise control over the parameters outlined below to achieve effective thin film modification.5. Assemble a two-electrode cell by incorporating the as-prepared TiO2 suspension, comprising BDD and aluminum plate keeping a distance of 1 cm (Figure 4B).

6. Connect the positive terminal of a DC power supply to the aluminum plate and the negative terminal to the bare BDD electrode.

7. Apply a 4.8 V DC bias potential and gently immerse the two-electrode array into the suspension under moderate magnetic stirring (Figure 4C).

8. Keep the electrodes in the suspension for 15 s and remove them slowly and carefully to obtain high-quality TiO2 film.

9. Carefully transfer the as-prepared photoelectrode into a crucible, ensuring that the modified surfaces avoid contact with the walls of the crucible, only resting against its edges.

10. Place the crucible on a hot plate and heat it to 100°C for 5 min to facilitate solvent evaporation (Figure 4D).

11. Finally, transfer the crucible into a furnace and anneal the photoanode at 200°C for 12 min (Figure 4E) to enhance the adhesion of the photocatalyst onto the BDD surface (Figure 4F).

CRITICAL: Ensure moderate stirring of the suspension during electrophoretic deposition, typically at 400 rpm to prevent any build-up of the photocatalyst. Troubleshooting 3.

Note: The annealing process can be carried out in an air atmosphere without any issues because the commercial TiO2 nanoparticles we used do not undergo any parasitic reactions during annealing. The purpose of annealing is solely to enhance the adhesion between the TiO2 and the BDD substrate.

Characterization of the BDD/TiO2 photoanode

Timing: ∼100 min

Characterization is performed using conventional electrochemical techniques. Additionally, light conditions are incorporated, along with an optical non-electrochemical technique to gather data regarding the state of the BDD/TiO2 photoanode. Troubleshooting 4.12. CV in redox couple: assemble the three-electrode cell containing the ferri/ferro aqueous solution.Note: BDD/TiO2 serves as the working electrode, platinum mesh as the counter electrode, and Ag/AgCl as the reference electrode.

Note: Incorporate the light source approximately 10 cm away from the cell.

a. Conduct 10 CV scans on the K3[Fe(CN)6] / K4[Fe(CN)6] redox couple, starting at the OCP, using the same scan rate and working window for consistency. Perform these scans under dark conditions.

b. Switch on the light source and keep the cell under illumination for 4 min to activate the BDD/TiO2 photoanode.

c. Repeat the 10 CV scans as described in (a) while keeping the light source activated.

d. Collect the photoanode and wash it with DI several times and air dry.

13. CV in electrolytic medium: assemble the same cell containing as-prepared Na2SO4 aqueous solution, incorporating the light source at 10 cm away from the cella. Perform 10 CV scans on the sodium sulfate solution, starting at the OCP at the same scan rate and working potential window, under dark conditions.

b. Switch on the light source and maintain the cell under illumination for 4 min to ensure proper activation of the BDD/TiO2 photoanode.

c. Repeat the 10 CV scans while keeping the illumination activated.

d. Remove the photoanode and wash it with plenty of DI.

14. Photocurrent: assemble the same cell configuration for CV in the electrolytic medium.a. Select optimal potentials near the onset of the oxygen evolution reaction based on the CV profile previously recorded.

b. Program current versus time technique at as-selected potentials and adequate time range. For this study, we opt for a duration of 210 s.

c. Initiate the program for a duration of 120 s to ensure the solid-liquid junction stabilizes adequately.

d. Pause the program and allow an additional 2 min under dark conditions to establish a new initial state at the photoanode surface.

e. Following this interval, play the program for 10 s to record the baseline signal without illumination.

f. Halt the program once more and activate the illumination for 2 min to ensure the proper activation of the photoanode surface.

g. Once the 2-min illumination period has concluded, proceed to run the program for 10 s to capture the photocurrent signal.

h. Pause the program and deactivate the light for 4 min to allow for a return to equilibrium, then, repeat steps (e) to (g) until the completion of the 210-s assay.

i. Remove the photoanode and wash it several times.

15. EIS: assemble the three-electrode cell containing the K3[Fe(CN)6] / K4[Fe(CN)6] aqueous solution, and incorporate the light source.a. Repeat the EIS assay under dark conditions to obtain data for the BDD/TiO2 photoanode.

b. Switch on the light source and maintain the cell under illumination for 4 min to ensure proper BDD/TiO2 photoanode activation.

c. Perform the EIS assay as outlined earlier, then analyze and compare the impedance results with the non-illumination data to ensure consistency.

d. Remove the photoanode and wash it several times and air dry.

16. Diffuse reflectance spectroscopy: the BDD/TiO2 photoelectrode was accurately positioned using a sample holder on the X-Y platform.a. The light is guided by the optical fiber. The reflected diffuse light is collected through a pair of lenses (60 and 150 mm) and focused on the monochromator that is coupled to the Si/InGaAs detector. The signal recorded by the detector is digitized with a data acquisition system (SpectrAcq3, Horiba) controlled by software (SynerJY, Horiba).

b. The signal entering the monochromator is maximized using a XY translation stage assembly with micrometric screws. Then, the spectrum of the reference is recorded and finally, the spectrum of the sample is recorded.

17. Band-gap energy calculation: the band gap energy was determined using the Tauc method and the Kubelka-Munk function. The energy-dependent absorption coefficient (α) can be expressed according to Equation 1:(Equation 1) (αhν)1γ=B(hν−Eg)

Here, hν is the photon energy, B is a constant, Eg is the band-gap energy, and γ is a value that depends of the electronic transition. For BDD and TiO2, which are indirect transition semiconductors,γ equals 2.a. Divide the reflectance data of the sample by the reflectance data of the reference to obtain R∞, according to Equation 2.(Equation 2) R∞=RsampleRreference

b. Calculate the Kubelka-Munk function F(R∞) using Equation 3.(Equation 3) F(R∞)=(1−R∞)22R∞

c. Replace the Kubelka-Munk function with the absorption coefficient in Equation 1 to derive Equation 4.(Equation 4) [F(R∞)hν]12=B(hν–Eg)

d. Graph [F(R∞)hν]12 versus hν to obtain the characteristic Tauc plot (Figure 5F).Figure 5 Evaluation of electrochemical and photoelectrochemical active surface area and diffuse reflectance spectroscopy

(A–C) Cyclic voltammetry in (A-B) 2.0 mmol L−1 [Fe(CN)6]3-/4- in 1.0 mol L−1 KCl, (C) 0.1 M mol·L−1 Na2SO4.

(D) Photocurrent response in 0.1 M mol·L−1 Na2SO4.

(E) Nyquist plot of bare BDD and BDD/TiO2 photoanode.

(F) Band gap and UV-vis diffuse reflectance spectra of BDD/TiO2 photoanode.

e. Draw a straight line in the linear region of the Tauc plot, extending it to the x-axis. The intersection point corresponds to the band-gap energy.Note: Readers will find the work of Makuła and coworkers useful for further analysis of reflectance data.11

Expected outcomes

This protocol outlines the fabrication of a BDD electrode modified with TiO2 nanoparticles via electrophoretic deposition method, aiming for potential application in the degradation of persistent organic compounds. In our study, this modification significantly enhanced the performance of the BDD/TiO2 photoanode compared to the bare BDD electrode. Specifically, the electroactive area increased from 4.98 to 10.82 cm2 after modification. The current density also improved from 0.11 mA cm−2 in the absence of light to 0.23 mA cm−2 under UV-light, as shown in Figures 5A and 5B. This enhancement leads to higher reaction efficiency and better electron transfer capabilities. Moreover, a slight potential shift in the oxidation peak was observed, from 1.6 to 1.5 V vs. Ag/AgCl, as a result of modification by photocatalyst (Figure 5C). The transient photocurrent response (Figure 5D) demonstrated a substantial increase from 1.8 μA cm−2 to 214 mA cm−2, favoring charge transfer and inhibiting recombination. On the other hand, Nyquist analysis (Figure 5E) indicated a decrease in the semicircle radius, reflecting a reduction in RCT. This improvement enhances the internal mobility of the carriers and increasing the active sites. Finally, the diffuse reflectance spectrum (Figure 5F) confirmed that the band gap of the BDD/TiO2 photoanode aligns with theoretical predictions, validating the modification process.

Limitations

The electrophoretic technique for synthesizing photoanodes emerges as a relatively simple method. Nevertheless, achieving a high-quality thin film of the photocatalyst mandates rigorous oversight. This entails mastering operand parameters, from dipping into suspension to substrate characteristics, to ensure optimal outcomes. Double-sided BDD substrates were demonstrated to be problematic in both chemical-assisted modification and characterization. In this protocol, no additional treatment was applied to commercial TiO2 nanoparticles. Researchers may find it beneficial to finely grind the nanopowder using an Agate mortar prior to preparing the suspension. This step could enhance the stability of the suspension for further modifications. Furthermore, incorporating mechanical dipping could enhance the quality of the film. Relying solely on manual dipping introduces variability depending on the individual performing the immersion.

Under light conditions, both double-sided BDD and BDD/TiO2 electrodes present challenges in characterizations. Since BDD is not a transparent substrate, illuminating both sides becomes necessary to maximize energy conversion efficiency. In this protocol, a commercial germicidal UV lamp was utilized, lacking polarization in its light source. Moreover, prolonged usage of this lamp may lead to heating, potentially affecting the results. To minimize these issues, we chose to position the lamp away from the cell. However, researchers may find it advantageous to employ polarized light sources with high stability across a broad electromagnetic spectrum to address these concerns effectively.

Troubleshooting

Problem 1

Despite the usefulness of electrochemical activation for generating oxygen-terminated lattices in BDD, this activation is temporary. Over time and with repeated experiments, the surface tends to revert to its equilibrium state.

Potential solution

Electrochemical reactivation under the same conditions should be beneficial for regenerating the desired lattice termination, ensuring accuracy during experiments. For highly oxygenated or hydrogenated surfaces of BDD, oxygen and hydrogen plasma treatments, respectively, could be advantageous. The choice depends on the intended application of the photoelectrode. For further detailed guidance on these treatments, readers can consult Williams and Yang’s works.12,13

Problem 2

Due to the high concentration of TiO2 nanoparticles suspension needed for electrophoretic deposition, achieving optimum stability requires careful control when slowly adding the nanopowder to the alcoholic mixture. Vigorous magnetic stirring is necessary to prevent the accumulation and settling of TiO2 powder.

Potential solution

Finely grinding the TiO2 nanopowder using an Agate mortar can reduce particle accumulation and improve the suspension’s stability for subsequent depositions.

Problem 3

Obtaining a TiO2 film is straightforward, but determining the optimal parameters for achieving a high-quality thin film is labor-intensive. After preparing a stable suspension, the procedure demands rigorous control over manual dipping and retrieval, complemented by adequate magnetic stirring. Moreover, the immersion time and applied bias are crucial factors influencing the film’s quality.

Potential solution

Mechanical dipping is advantageous over manual dipping because it allows for precise control over the speed of immersion and withdrawal. Additionally, suspending magnetic stirring at the moment of immersion and during the electrophoretic process can improve the homogeneity of the film. The immersion time generally ranges from 10 to 20 s, with an applied bias of 4.5–5.0 V.

Problem 4

Using a UV germicidal lamp as a light source for photoelectrochemical analysis is disadvantageous because, in long-term experiments, it tends to generate heat, which subsequently heats the cell.

Potential solution

Positioning the UV lamp away from the cell can minimize the heating effect, and in most cases, using a thermal bath can help maintain a stable temperature within the cell.

Resource availability

Lead contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Patricio J. Espinoza-Montero (pespinoza646@puce.edu.ec).

Technical contact

Questions about the technical specifics of performing the protocol should be directed to and will be answered by the technical contact, G. Xavier Castillo-Cabrera (gxcastillo@puce.edu.ec).

Materials availability

All materials generated in this study are available upon request from the lead contact.

Data and code availability

• This article includes all data generated or analyzed during this study.

• Any additional information required to analyze the data reported in this paper is available from the lead contact upon request.

• This article does not report any original code.

Acknowledgments

The authors thank the 10.13039/501100011749 Pontificia Universidad Católica del Ecuador for funding through the project "Preparation of boron-doped diamond (BDD) photoelectrodes modified with bismuth semiconductors and their application in photoelectrocatalysis," code: QINV0402-IINV529020100 .

Author contributions

Conceptualization, P.J.E.-M. and W.Q.; methodology, P.J.E.-M., W.Q., and G.X.C.-C.; investigation, W.Q., G.X.C.-C., L.J.B.-G., and P.J.E.-M.; writing – original draft, W.Q., G.X.C.-C., and P.J.E.-M.; review and editing of the final version of the manuscript, P.J.E.-M. and L.J.B.-G.; funding acquisition, P.J.E.-M. W.Q., G.X.C.-C., L.J.B.-G., and P.J.E.-M. conceived and performed the experiments; P.J.E.-M. and L.J.B.-G provided reagents; P.J.E.-M. provided expertise and feedback.

Declaration of interests

The authors declare no competing interests.
==== Refs
References

1 Quilumbaquin W. Castillo-Cabrera G.X. Borrero-González L.J. Mora J.R. Valle V. Debut A. Loor-Urgilés L.D. Espinoza-Montero P.J. Photoelectrocatalytic degradation of high-density polyethylene microplastics on TiO2-modified boron-doped diamond photoanode iScience 27 2024 109192 10.1016/j.isci.2024.109192
2 Castillo-Cabrera G.X. Espinoza-Montero P.J. Novel trends in mixed oxide electrodes for photoelectrocatalytic wastewater treatment Curr. Opin. Electrochem. 44 2024 101448 10.1016/J.COELEC.2024.101448
3 Abegunde O.O. Akinlabi E.T. Oladijo O.P. Akinlabi S. Ude A.U. Overview of Thin Film Deposition Techniques AIMS Materials Science 6 2019 174 199 10.3934/MATERSCI.2019.2.174
4 Altomare M. Nguyen N.T. Naldoni A. Marschall R. Structure, materials, and preparation of photoelectrodes Photoelectrocatalysis: Fundamentals and Applications 2023 83 174 10.1016/B978-0-12-823989-6.00005-9
5 Terashima C. Hishinuma R. Roy N. Sugiyama Y. Latthe S.S. Nakata K. Kondo T. Yuasa M. Fujishima A. Charge Separation in TiO2/BDD Heterojunction Thin Film for Enhanced Photoelectrochemical Performance ACS Appl. Mater. Interfaces 8 2016 1583 1588 10.1021/acsami.5b10993 26756353
6 Einaga Y. Boron-Doped Diamond Electrodes: Fundamentals for Electrochemical Applications Acc. Chem. Res. 55 2022 3605 3615 10.1021/acs.accounts.2c00597 36475616
7 Sigcha-Pallo C. Peralta-Hernández J.M. Alulema-Pullupaxi P. Carrera P. Fernández L. Pozo P. Espinoza-Montero P.J. Photoelectrocatalytic degradation of diclofenac with a boron-doped diamond electrode modified with titanium dioxide as a photoanode Environ. Res. 212 2022 113362 10.1016/j.envres.2022.113362
8 Pacheco-álvarez M.O. Rodríguez-Narváez O.M. Wrobel K. Navarro-Mendoza R. Nava-Montes de Oca J.L. Peralta-Hernández J.M. Improvement of the degradation of Methyl Orange Using a TiO2/BDD composite electrode to promote electrochemical and photoelectro-oxidation processes Int. J. Electrochem. Sci. 13 2018 11549 11567 10.20964/2018.12.70
9 Blitz J.P. Diffuse Reflectance Spectrocopy Modern Techniques in Applied Molecular Spectroscopy 1998 Wiley & Sons, Inc 185 217
10 Frei R.W. Roland W.) MacNeil J.D. Diffuse Reflectance Spectroscopy Environmental Problem Solving 2017 CRC Press)
11 Makuła P. Pacia M. Macyk W. How To Correctly Determine the Band Gap Energy of Modified Semiconductor Photocatalysts Based on UV-Vis Spectra J. Phys. Chem. Lett. 9 2018 6814 6817 10.1021/acs.jpclett.8b02892 30990726
12 Yang N. Foord J.S. Jiang X. Diamond Electrochemistry at the Nanoscale: A Review Carbon 99 2016 90 110 10.1016/j.carbon.2015.11.061
13 Williams O.A. Daenen M. D’Haen J. Haenen K. Maes J. Moshchalkov V.V. Nesládek M. Gruen D.M. Comparison of the growth and properties of ultrananocrystalline diamond and nanocrystalline diamond Diam. Relat. Mater. 15 2006 654 658 10.1016/j.diamond.2005.12.009
