Introduction

Electrochemical processes take place of reactions caused or accompanied by the passage of electrical current for different purposes. In general, this process involves electron transfer between a solid conductor (an electrode) and a substance dispersed in an electrolytic medium. In this way, the electrochemical reaction can be induced in different ways: by maintaining an electrical potential for a certain time and adjusting it so that a certain current density is maintained throughout the process [1]. In many situations, it is extremely relevant to elucidate unknown electrogenerated intermediates or products to obtain information concerning the reaction mechanism adjacent to/or on the surface of a non-transparent electrode. By combining electrochemical and spectroscopic techniques such as UV-VIS and infrared absorption spectroscopy, as well as Raman microspectroscopy, real-time powerful data can be obtained to understand the reaction steps avoiding misinterpretation [2, 3]. This research area can conveniently perform ex situ investigations by using accessories for specular reflectance although in situ observations demand a very particular spectroelectrochemical (SEC) cell [4, 5].

Accessories for these purposes are easily found on the market, but often difficult to purchase due to the high cost, the exclusivity of parts from the manufacturers, and consequent low versatility. Thus, an alternative found by many research groups has been the homemade construction of the own accessories [6, 8], including recent projects using 3D printing technology [6,7,8,9,10,11,12,13,14]. This additive manufacturing technique can be considered fast, low cost, and sustainable, with a great capacity for customization of complex structures, in particular, internal structures [9,10,11]. Furthermore, many models can be easily downloaded on the Internet and modified and reproduced according to the user’s needs around the world [15,16,17,18,19,20].

This work used 3D printing technology to develop a series of accessories aiming to integrate ex situ or in situ electrochemical and spectroscopic techniques to investigate dynamic interfacial and surface phenomena, such as an accessory for specular reflectance, an SEC for UV-VIS investigation, and an SEC for Raman microspectroscopy analysis capable of acting in flux or in a stationary regime. The cells were designed to be used with the available equipment with minimum complexity for the interface, in a compatible way with common commercial electrodes, and especially as a simple, low-cost, low-time production, and sustainable open-lab alternative, allowing, with few design modifications, the capacity to be used in different instruments and conditions.

Experimental section

All chemicals and suppliers employed in this work are listed in the Supporting Information Section S.1.1.

Apparatus

The electrochemical processes were controlled by a BASi Epsilon Potentiostat/Galvanostat, using a system made of three electrodes: a homemade platinum disc electrode (⌀6 mm) as working electrode (see in the Supporting Information Section S.2 the Design and characterization) and an Ag/AgCl (3M KCl) and a platinum wire as reference and auxiliary electrodes, respectively. All accessories were designed to replace the original spectrometers sample holders. The equipment used in the investigations were Shimadzu UV-1800 for UV-VIS acquisition, Perkin Elmer Frontier for middle-infrared region (mid-IR), and a Horiba LabRAM HR Evolution for Raman microspectrometry.

Accessories design

All printable parts were designed using a free and online 3D app TinkercadTM (https://www.tinkercad.com). The models were sliced using the Ultimaker CuraTM app (https:// https://ultimaker.com/) to create G-Code instruction archives used by the homemade 3D printer (based on RepRap Ormerod). The specular reflectance module and SEC cells were made using black color ⌀1.75 mm PETg filament (3DFila, Brazil). The material was chosen based on its good chemical and high impact resistance, great durability and ease of printing—even without a heated bed—to prevent spurious contribution by undesirable scattering radiation. The 3D printing settings are described in the Supporting Information Section S.1.2. Figure 1 shows all accessories fabricated in this work.

Fig. 1.
figure 1

Lab-made 3D-printed accessories projects: (a) Specular reflectance module for ex-situ UV-VIS-NIR and FTIR measurements. (b) Spectroelectrochemical cell for in situ UV-VIS measurements. (c) Spectroelectrochemical cell for in situ Raman microspectrometry

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The specular reflectance module (Fig.1a) was designed to adapt two plane mirrors in angle. The first one deflects radiation to the sample surface—which must be placed face-down—and a second one deflects the component of radiation specularly reflected by the substrate that supports the sample back to the original direction. A similar arrangement was used to construct in situ UV-VIS SEC (Fig. 2b). In this case, the sample surface of the working electrode was vertically disposed and enclosed into a cell. A quartz window (⌀20 x 1 mm) was allowed to interact between the radiation and electrode surface. Finally, the in situ Raman SEC (Fig. 1c) was designed to obtain the working electrode surface facing upwards, capable of receiving an array of three or more electrodes and adapters to perform dynamic flux measurements. The same quartz window of UV-Vis SEC (⌀20 x 1 mm) can be used if the electrolytic medium offers a corrosion risk to the microscope objective or allows flow within the cell. More details about the accessory fabrication are described in the Supporting Information Section S.3.

Fig. 2
figure 2

Lab-made 3D-printed accessories installed in the commercial equipment: Specular reflectance module for ex situ UV-VIS-NIR and FTIR measurements coupled in a (a) Shimadzu UV-1800 UV-VIS spectrophotometer and (b) Perkin Elmer Frontier FTIR spectrometer. (c) Spectroelectrochemical cell for in situ UV-VIS measurements coupled in a Shimadzu UV-1800 UV-VIS spectrophotometer. (d) Spectroelectrochemical cell for in situ Raman microspectrometry installed in a Horiba LabRAM HR Evolution Raman microspectrometer

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Spectroelectrochemistry analysis

The accessories performance was evaluated using some materials and processes with predictable spectroscopic behavior and extensive discussions in the literature. In the specular reflectance module case, a thin film of Allura Red AC was deposited on the surface of an aluminum disc. The Supporting Information Section S.4 shows the colorant structural formula and the sample preparation procedure. The spectrums were recorded for UV-VIS and mid-IR regions. As a proof of concept for the SEC cells, an electrodeposition of Prussian Blue (PB) film in the lab-made Pt working electrode using 0.1 mol L-1 KCl solution as supporting electrolyte was performed. By the potentiostat mediation, a potential of + 0.3 V vs. Ag/AgCl (3M KCl) was applied during 200 s promoting the reaction between potassium ferricyanide and ferric chloride. Continuous absorbance measurements were taken at 700 nm for UV-VIS SEC or a spectrum in the region between 1600 and 2600 cm-1 (using 633 nm laser) every 20 s with the Raman SEC cell. The conversion of this film to the Berlin Green (BG) and Prussian White (PW) forms was carried out by the application of the appropriate potential −0.3 and 1.1 V for PW and BG, respectively.

Results and discussions

All printed pieces had dimensions that were identical to the projects they were based on, which ensured a good and reliable connection between all pluggable/threadable pieces. The accessories preserved their shape for a long time and showed no signs of moisture absorption or degradation that would hamper the integrity or purpose that they were developed. The time required for the prints was approximately 42 h (18 h for both the reflectance module and UV-VIS SEC and 6 h for Raman SEC), by an estimated cost of around 15 $ per unit. High-quality, leak-free devices Fig. 2 shows the 3D-printed reflectance module, UV-VIS SEC, and Raman SEC cells.

Allura Red AC is a synthetic azo dye whose chromophore properties are based on the presence of an azo group (R-N=N-R’), which is connected to the auxochrome hydroxy and methoxy groups through aromatic π-system rings. The colorant presented a characteristic absorption band at 500 nm on the visible region (Fig. 3a). The spectra for the mid-IR region show the characteristics bands at 3445 (hydroxy), 1500 (azo), and at 1040 and 1060 cm-1 (sulfonate groups). All the results were in agreement with the database and current literature [21, 22] showing that the accessories are very suitable for this investigation approach.

Fig. 3
figure 3

(a) UV-VIS and (b) mid-IR spectra for Allura Red AC film deposited on the surface of an aluminum mirror obtained with the lab-made 3D-printed accessory for ex situ specular reflectance measurements

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The performance of the UV-VIS and Raman SECs was evaluated by monitoring the Prussian Blue (PB) growth film process [23]. The SECs were mounted in the respective spectrometers using a three-electrode electrochemical system. The reaction was promoted from the mixture of 4.0, 1.0, and 1.0 mL of 0.1 mol L-1 potassium chloride; 10 mmol L-1 potassium ferricyanide; and 10 mmol L-1 ferric chloride solutions, respectively. After the addition of the last component, a potential equal to + 0.3 V was applied and maintained for 200 s, while continuous absorbance measurements were taken at 700 nm for UV-VIS SEC—maximum absorption for the PB or a spectrum in the region between 1600 and 2600 cm-1 (using 633 nm laser) every 20 s in the Raman case [24]. The film electrosynthesis process follows the reaction between hexacyanoferrate (II) and ferric chloride (Eqs. 1 and 2).

$${\left[{\mathrm{Fe}}^{\left(\mathrm{II}\mathrm{I}\right)}{\left(\mathrm{CN}\right)}_6\right]}^{3-}+{\mathrm{e}}^{-}\rightleftarrows {\left[{\mathrm{Fe}}^{\left(\mathrm{II}\right)}{\left(\mathrm{CN}\right)}_6\right]}^4$$
(1)
$${4\mathrm{Fe}}^{3+}+3{\left[{\mathrm{Fe}}^{\left(\mathrm{II}\right)}{\left(\mathrm{CN}\right)}_6\right]}^{4-}\longrightarrow {{\mathrm{Fe}}_4}^{\left(\mathrm{II}\mathrm{I}\right)}{\left[{\mathrm{Fe}}^{\left(\mathrm{II}\right)}{\left(\mathrm{CN}\right)}_6\right]}_3$$
(2)

In the first case, the absorption index at 700 nm—maximum absorption for the PB—during the process was monitored. Fig. 4a shows the increase in the absorption while applying the suitable potential with PB film growing at the electrode surface. In the in situ Raman monitoring (Fig. 4b), the signal at 2200 cm-1 increased due to the presence of (CN) bands between 2000 and 2200 cm-1.

Fig. 4.
figure 4

In situ film growth measurements using lab-made 3D-printed accessories for spectroelectrochemistry: (a) Scatter plot of the in situ absorption monitoring at 700 nm using UV-VIS Spectroelectrochemical cell. (b) Scattering spectra as a function of time using Raman spectroelectrochemical cell, during the electrodeposition of Prussian Blue film in the lab-made platinum working electrode surface in 0.1 mol L-1 KCl solution

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In addition, the electrochemical conversion of PB film into the Prussian White (PW) and Berlin Green (BG) redox states was also real-time accompanied. For conversion to PW and BG forms, the electrolyte medium was changed to 0.1 mol L-1 KCl solution, after washing and purging the cells with ultrapure water and the new medium. A voltammogram between −0.3 and 1.1 V was taken for electrochemical evidence of film formation and maintenance (see Supporting Information Section S.5, Fig. S.19). The film was converted to the other forms by applying a potential of −0.3 V and 1.1 V for conversion to PW and BG forms, respectively. The conversion potential was maintained during the spectra acquisition process, whose responses were taken as referring to the pure forms of those states [20]. The two distinct pairs of redox peaks correspond to the oxidation of PW to PB (close to 0.3 V, Eq. 3) and the oxidation of PB to BG (at approximately 1.0 V, Eq. 4). The redox behavior of the PB film can be expressed according to the following equations:

$${\mathrm{K}}_4{{\mathrm{Fe}}_4}^{\left(\mathrm{II}\right)}{\left[{\mathrm{Fe}}^{\left(\mathrm{II}\right)}{\left(\mathrm{CN}\right)}_6\right]}_3\rightleftarrows \kern0.5em {{\mathrm{Fe}}_4}^{\left(\mathrm{II}\mathrm{I}\right)}{\left[{\mathrm{Fe}}^{\left(\mathrm{II}\right)}{\left(\mathrm{CN}\right)}_6\right]}_3+\kern0.5em {4\mathrm{K}}^{+}+{4\mathrm{e}}^{-}$$
(3)
$${{\mathrm{Fe}}_4}^{\left(\mathrm{II}\mathrm{I}\right)}{\left[{\mathrm{Fe}}^{\left(\mathrm{II}\right)}{\left(\mathrm{CN}\right)}_6\right]}_3-{3\mathrm{e}}^{-}\kern0.5em +{3\mathrm{Cl}}^{-}\rightleftarrows {{\mathrm{Fe}}_4}^{\left(\mathrm{II}\mathrm{I}\right)}{\left[{\mathrm{Fe}}^{\left(\mathrm{II}\mathrm{I}\right)}{\left(\mathrm{CN}\right)}_6\mathrm{Cl}\right]}_3$$
(4)

Figure 5 shows the simultaneous control of the applied potential and the spectroscopic information using the UV-VIS and Raman SECs.

Fig. 5.
figure 5

In situ electrochemical conversion of Prussian Blue film in a lab-made platinum working electrode in 0.1 mol L-1 KCl solution using lab-made 3D-printed accessories for spectroelectrochemistry (a) UV-VIS reflection-absorption spectra. (b) Raman scattering spectra of the three different redox states of the film: (1) Prussian Blue by applying 0.5 V, (2) Prussian White by applying −0.3 V, and (3) Berlin Green by applying 1.1 V vs. Ag/AgCl/KCl(sat)

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One can observe when analyzing Fig. 5a.1 that the absorption band at 700 nm was assigned to the intervalence charge transfer from the Fe(II)-C to Fe(III)-N (reason of the blue color). This signal was attenuated with the reduction of the Fe (III) sites when the film was converted to the PW form by application of −0.1 V vs. Ag/AgCl (3M KCl) as can be observed in Fig. 5a.2. In highly anodic potentials, the PB was converted on BG, whose absorption band (near than 400 nm) can be associated to crystal field transitions (Fig. 5a.3) [23,24,25]. In the Raman spectroscopy investigations (Fig. 5b.1), PB forms can be characterized through the presence of (CN) bands between 2000 and 2200 cm-1. The frequency of the CN- vibrational stretching mode depends upon the oxidation state of the metal cation coordinated to the cyanide anions, and also their electronegativity. The cyanide group tends to act as a 𝜎-donor, so as the metal-carbon bond increases, by increasing the metal electron density or metal charge, the electron density from the antibonding orbital is reduced in the C≡N bond, which strengthens that bond and increases its vibrational frequency [26]. As a mixed-valence compound, PB showed two sharp bands in that interval that can be assigned to the A1g and Eg (CN) modes of the CN- anion: the lower frequency band at 2092 cm-1 can be linked to a (CN) mode associated with Fe(II) cation, whereas the higher frequency band at approximately 2156 cm-1 can be assigned to a 𝜈(CN) mode associated with Fe(III) cation. At cathodic potentials (Fig 5b.2), a completely different pattern was observed, with the appearance of (CN) scattering at 2136, 2103, and 2048 cm-1, due to the conversion to PW form. On the other hand, at anodic potentials, the Raman spectrum (Fig. 5b.3) exhibits a band at 2155 cm-1 and a broad shoulder at lower frequencies, as a consequence of the higher oxidation state of most of the Fe cations caused by the electrochemical film conversion to the BG form [27, 28]. Table 1 summarizes the main features of the 3D printing lab-made accessories for spectroelectrochemical measurements as well as useful corresponding references where more detailed information can be obtained if necessary.

Table 1 Overview of lab-made accessories for spectroelectrochemical measurements
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Conclusions

The results obtained in this work demonstrated the capacity of 3D FDM printing as a technology to produce rapid accessories prototyping for spectroscopy and spectroelectrochemistry. The cells were properly designed using PETg-based filament, being it a cheap, inert, durable, versatile, and sustainable material. Thus, this powerful technique was used to produce robust alternatives accessories for ex situ specular reflectance spectroscopy and spectroelectrochemical cells suitable for in situ Raman and UV-VIS spectroscopy measurements.

Furthermore, accessories and cells were tested using Allura Red AC dye and PB-electrodeposited films as reference materials/processes. It was possible to obtain spectroscopic measurements of the films and observe in situ growth and structural changes during the redox processes suffered by PB. The specular reflectance accessory allows adaptation to receive electrodes as a sample, and the spectroelectrochemical cells were designed to receive standard ⌀6 mm electrodes although the accessories can be easily adapted to any other needs—the projects are ideas that can be improved by linking up the demands of industry and research institutions. Finally, similar studies can be carried out in analogous processes as kinetics and reaction mechanisms estimations, the study of shorter-lived species, localized interfacial investigations, and surface modifications analysis considering the innovative designs and new analytical strategies capacity of these new tools. Reflectance measurements mediated by accessories can be appropriately performed on different materials, and are applicable to the characterization of polymers, ceramics, dyes, pigments, semiconductors, etc.