Comparative Photo-Electrochemical and Photocatalytic Studies with Nanosized TiO₂ Photocatalysts towards Organic Pollutants Oxidation

Abstract

The size of TiO₂ can significantly affect both its photocatalytic and photo-electrochemical properties, thus altering the photooxidation of organic pollutants in air or water. In this work, we give an account of the photo-electrochemical and photocatalytic features of some nanosized TiO₂ commercial powders towards a model reaction, the photooxidation of acetone. Cyclic voltammograms (CV) of TiO₂ particulate electrodes under UV illumination experiments were carried out in either saturated O₂ or N₂ solutions for a direct correlation with the photocatalytic process. In addition, the effect of different reaction conditions on the photocatalytic efficiency under UV light in both aqueous and gaseous phases was also investigated. CV curves with the addition of acetone under UV light showed a negative shift of the photocurrent onset, confirming the efficient transfer of photoproduced reactive oxygen species (ROSs), e.g., hydroxyl radicals or holes to acetone molecules. The photocatalytic experiments showed that the two nano-sized samples exhibit the best photocatalytic performance. The different photoactivity of the larger-sized samples is probably attributed to their morphological differences, affecting both the amount and distribution of free ROSs involved in the photooxidation reaction. Finally, a direct correlation between the photocatalytic measurements in gas phase and the photo-electrochemical measurements in aqueous phase is given, thus evincing the important role of the substrate-surface interaction with similar acetone concentrations.

1. Introduction

Semiconductor photocatalysts have been intensively investigated because of their applicability in the treatment of pollutants in both air and water phases. Titanium dioxide (TiO₂), a wide band gap semiconductor (3.0–3.2 eV), exhibits an excellent reactivity that enables it to photo-abate organic pollutants, ubiquitous in the environment from both natural and anthropogenic origin [1,2,3]. Moreover, the last decade has witnessed a rapid growth on the nanotechnology field, leading to the increasing use of TiO₂ nanopowders in industrial and commercial products [4,5]. The extremely small size of nanopowder crystallites confers them a high specific surface area, which may result in a prospectively greater reactivity compared to larger sized powders [6].

Up to now, widespread attention has been directed to the development of TiO₂ powders with very small crystallite size in an effort to enhance the photocatalytic activity and process efficiency. However, great concerns have risen in the last years on the effective toxicity of nanoparticles. Particles with diameters below 100 nm fall into the classification of “ultrafine” materials (nanoparticles) [7], and some studies reported that these particles are linked with several adverse health effects [8,9]. Ultrafine particles may be highly harmful for humans because of their capability to deeply penetrate lungs and cell membranes [10] causing lung tumors, inflammations, fibrosis, DNA damage [11] and cytotoxicity [12,13]. In order to substitute nano-sized particles in industrial and environmental remediation processes, the current research is investigating the photocatalytic efficiency of TiO₂ materials possessing larger crystallite size. In particular, some reports have successfully demonstrated the interesting catalytic performance of commercial sub-micro-sized TiO₂ powders sold as pigmentary materials [14,15]. The efficiency has been tested towards the photodegradation of (i) both NO_x and volatile organic compounds (VOCs) in gas phase and (ii) some organic dyes dissolved in water [16,17].

Parallel to this, nanostructured TiO₂ electrodes have also been extensively studied as photoanodes for photo-electrochemical water splitting [18,19,20,21] and water remediation [22,23,24,25,26]. One peculiar feature of the electrochemical measurements is that the oxidation and reduction reactions can be carried out separately and studied independently. In this way, it becomes possible to gain information on the limiting reaction rate of the individual processes, leading to a clear account of the changes and modifications that could occur on the TiO₂ surface. In several reports, the photo-electrochemical properties of TiO₂ nanoelectrodes were investigated with the aim to study their electronic structure and photoefficiency through electrochemical measurements [27,28,29]. In particular, the research on the electrochemical properties of TiO₂ electrodes pointed out that the photoefficiency depends on the particle morphology, including size and shape [30,31,32,33]. In fact, nanoparticle shape and preferential surface orientation of its facets play an important role in the electrochemical oxidation because these factors affect not only the reactants but also the interfacial charge transfer. In comparison with their larger counterparts, it is likely that the nano-sized powders suffer from an increased charge recombination rate at the surface, due to the large density of surface states that could act as carrier trapping sites; on the contrary, the larger-sized powders could sustain a built-in electric potential within the particles, which is able to generate a space charge region that facilitates charge separation, thus preventing surface recombination. Consequently, hindering the recombination of photogenerated charge carriers is essential for improving the efficiency of net charge transfer at the semiconductor/electrolyte interface (SEI). In this respect, many factors can influence the overall electrochemical response, including electrode thickness, surface area and particle size. The correct control of these parameters is thus essential.

The purpose of this paper is to compare the properties of four nanosized TiO₂ commercial powders through photo-electrochemical and photocatalytic measurements of the degradation of acetone. In particular, we chose to investigate the oxidation of acetone on TiO₂ as a model reaction to probe its photocatalytic activity. In order to get a correct comparison between electrochemical and photo-electrochemical experiments, the electrochemical analysis was conducted in both saturated N₂ and O₂ solutions. This way, it is possible to independently monitor the reduction and oxidation processes. In addition, to gain further mechanistic information, photo-electrochemical analyses were also conducted for different acetone solution concentrations, investigating the influence of the amount of pollutant on the photoefficiency.

2. Results and Discussion

2.1. Characterization of the Titania Powders

The surface area of nano-sized powders, analyzed by means of the BET method, is higher than that of the larger-sized samples, as expected [14]. P25 and PC105 show a surface area of 50 and 80 m²/g, respectively, while 1077 and AH-R show much lower values (~12 m²/g). The XRD spectra reveal the presence of the pure anatase phase for all the quoted samples but P25, which contains a 75:25 ratio of anatase-to-rutile phase [34]. The morphological aspects of all samples were examined by HR-TEM (Figure 1). Nanometric powders (P25 and PC105) exhibit the typical average particle size of 20–30 nm. Furthermore, the nanosized materials are homogeneous with well-packed crystalline particles. In parallel with this, a distinctive morphology could be noticed in the larger-sized samples (1077 and AH-R). The average crystallite size of the anatase, calculated via Scherrer’s formula, falls into the 100–130 nm range. In all cases, the analyses of the fringe patterns (either as such or by means of the FFT elaboration) reveal the presence of the (101) family of planes ascribable to the anatase phase (ICDD card n. 21-1272).

Finally, the average thickness of the respective films (as measured by profilometry in two different sections of the TiO₂ annealed film) was about 3 µm for both nanometric and sub-micrometric samples.

It is well known that surface hydroxyls are crucial species for the photocatalytic process. In particular, photo-generated holes may react with water molecules adsorbed on semiconductor surfaces, resulting in the formation of trapped holes as OH radicals [14]. To show the prospective formation of these radicals, FTIR spectra of the samples in the ν(OH) spectral range in air (black lines) and after outgassing at RT (red lines) are reported in Figure 2. All the materials in air exhibit two complex absorption bands, respectively located in the 3000–3450 cm⁻¹ range and at ν ≥ 3600 cm⁻¹. On the basis of the spectral behavior and of the literature data [35,36], the former broad peak can be ascribed to the stretching mode of all H–bonded OH groups present at the surface of the various solids, whereas the latter corresponds to the stretching mode of all the Ti–OH species free from hydrogen bonding interactions. Comparing the spectra of smaller-sized samples with that of larger-sized TiO₂, it is evident that nano-sized samples are characterized by a significantly higher amount of hydroxyl species. In particular, PC105 shows the highest amount of OH groups among the samples studied, where the absorbance of PC105 has been halved. Moreover, nano-sized surface samples are characterized by an uneven distribution of terminal Ti–OH species in comparison with their micro-sized counterparts (1077 and AH-R).

After outgassing at RT (Figure 2, red lines) physisorbed water molecules are removed and, as a consequence, the absorbance of the envelope related to H–bonded OH groups decreases significantly. For the larger-sized TiO₂, the band related to Ti–OH shifts toward lower frequency upon outgassing, indicating that physisorbed water alters the reactivity of Ti–OH groups. In particular, a band at lower frequency points to the presence of hydroxyl groups with a more acidic character.

2.2. Photo-Electrochemical Analysis

In photo(electro)catalysis, two separate processes take place simultaneously: the oxidation process based on photogenerated hole transfer and the reduction process with photogenerated electrons, which can be separately studied, including their kinetics, by electrochemical methods. Moreover, it is of great interest to correlate the photo-electrochemical and photocatalytic behavior of the smaller TiO₂ samples (P25 and PC105), in comparison with that of larger ones (AH-R and 1077).

Regarding the oxygen reduction process, Berger et al. experimentally established that the accumulation region onset reflects the conduction band and surface state properties and that the associated current is correlated to the active surface area [37,38]. Figure 3 shows the voltammograms in the dark for TiO₂ samples in a 0.1 M HClO₄ aqueous solution in the presence and the absence of oxygen. In the absence of oxygen, the voltammetric response is dominated by the accumulation region appearing at potentials below −0.2 V vs. Ag/AgCl. As expected, the capacitance of the nanocrystalline electrodes is much larger than that of the sub-microcrystalline ones, as the interfacial area of the nanocrystalline electrodes is expected to be much larger for the same electrode thickness. The shown reductive currents are directly associated to the accumulation of electrons in the TiO₂ film. Nanometric samples (P25 and PC105) show higher reduction currents in agreement with the literature, [39] which is likely related to the higher internal surface area of these samples with respect to sub-microsized ones. In any case, the onset of the oxygen reduction current is similar in all cases and coincides with the onset of the accumulation region.

Prior to presenting and discussing the photo-electrochemical results, it is interesting to obtain information on the double layer structure in each of these types of electrodes. In the case of nanocrystalline electrodes, the existence of significant band bending within the particles is discarded, as their mesoporous nature and small particle diameter precludes the formation of a sizable in-built electric potential gradient. However, this is not necessary the case of microcrystalline electrodes.

For testing this in the case of a larger-sized TiO₂ electrode, the potential dependence of capacitance was analyzed using the Mott–Schottky (M–S) theory in the case of the AH-R sample. A linear behavior in the C_sc⁻² vs. E plot would point to the existence of a space charge layer within each particle. In addition, both the donor concentration, N_D, and the flatband potential, E_fb, of the semiconductor electrode can be estimated. The M–S equation can be written as:

$$C_{SC}^{-2}=\left(\frac{2}{{\mathsf{\epsilon }\mathsf{\epsilon }}_0\mathrm{e}{N}_D}\right)\left(E-E_{fb}-\mathrm{kT}/\mathrm{e}\right)$$

(1)

where C_sc represents the differential capacitance of the space charge layer. The flat band potential is given by extrapolation to C⁻² = 0. Figure 4 shows the M–S plot for the TiO₂ electrode AH-R. A good linear correlation is found in a relatively wide potential range. This is interesting as it is compatible with the large particle size of AH-R sample sustaining a space charge layer. The value of E_fb results to be around −0.6 V. This behavior is mainly determined for the particle size in the case of particulate electrodes, and it can thus be anticipated for electrodes of this kind.

The behavior of the different electrodes toward photooxidation processes was investigated. Figure 5 shows voltammograms under UV illumination as a function of the acetone concentration for all the samples under scope. The response in the absence of acetone is attributed to the photooxidation of water, while in its presence, the photooxidation of the organic compound should prevail. The latter can be deduced from the negative shift of the photocurrent onset observed upon the addition of acetone, which indicates that acetone photooxidation is easier than that of water, as acetone scavenges holes more easily than water [40], thus reducing the overpotential for its oxidation.

Figure 6 corresponds to the stationary photocurrent density (at 0.6 V) vs. acetone concentration plots for the same electrodes. The oxidation photocurrent increases with the concentration of acetone up to 10 mM and then decreases until a near-negligible photocurrent at 1 M of acetone. The values of the photocurrent under otherwise the same conditions depend on the TiO₂ type, but the characteristic trend is similar for all samples. The maximum current recorded using 10 mM acetone for the nano TiO₂ samples, P25 and PC105, is in the range of 7–9 mA/cm², whereas for 1077 and AH-R electrodes the values are slightly lower (5.5–6 mA/cm²). The adsorption of acetone on the surface of the TiO₂ film might explain the trend observed for the photocurrent as the acetone concentration in the electrolyte is increased. The addition of acetone (<10 mM) increases the photocurrent because either free or trapped holes react easily with the acetone adsorbed on TiO₂ allowing for an enhancement of the oxidative process. However, a further increase of the concentration of acetone gradually decreases the observed photocurrent maybe according to a strong adsorption of the pollutant that diminishes the formation of OH˙. For acetone concentrations higher than 1 M, very low photocurrents can be recorded probably due to a complete inhibition of the anodic process by blocking the TiO₂ electrode surface.

Therefore, the oxidative activity of the prepared TiO₂ electrodes reaches a maximum at lower acetone concentration values around 10 mM. These observations are in agreement with the inhibition of the formic acid photooxidation on TiO₂ resulting from the addition of relatively small amounts of acetone [41]. It is remarkable that the photocurrent onset is located at slightly more negative potentials in the case of nanosized samples indicating a lower recombination rate. In the case of microcrystalline electrodes (AH-R), the flat band potential as determined from the Mott–Schottky plot is located at potentials more negative (by more than 0.2 V) than the onset potential, which unveils that substantial recombination occurs even in the presence of the organics.

To gain more information about the potential activity of these powders as photo-electrocatalysts, in Figure 7, the current density under illumination for the different TiO₂ electrodes corresponding to the photooxidation of acetone (dashed line) together with the current density in the dark corresponding to the reduction of oxygen (solid line) are plotted together. According to the Wagner–Traud principle, the intersection point gives the value of the photocurrent (acetone oxidation) and the dark current (oxygen reduction) under open circuit conditions (i.e., under the typical conditions of suspended photocatalysts). This approach allows us to correlate both photo-electrochemical and photocatalytic measurements, as the latter is a particular case of the former where under open-circuit condition, the rates (or currents) of the anodic and cathodic processes are matched [42]. Figure 7 shows that the P25 powder gives the fastest acetone oxidation of all the samples checked. PC105 and AH-R would have very similar photoactivities, while 1077, would show the lowest photoefficiency. The significant differences found among the different TiO₂ powders highlight that P25, thanks to its peculiar properties, like high surface area, the presence of rutile and the particle morphology, happens to show the best photoefficiency. In particular, we believe that P25 efficiency is highly enhanced by the presence of both phases. The offset between the energetic locations of the conduction band and valence band edges for rutile and anatase should favor charge separation and diminish recombination. In general, there are two opposite factors that should play a key role in determining the photoactivity of powdered TiO₂ samples and electrodes: (i) a high interfacial area in nanocrystalline samples, which is particularly desirable in the case of substances present at low concentrations; and (ii) the existence of a space charge in microcrystalline samples, diminishing recombination and enhancing photoactivity. Because of the existence of these opposing factors, it is understandable that particular nanocrystalline and microcrystalline samples could exhibit similar photoactivities. Thus, according to our electrochemical measurements, the photoefficiency of the studied TiO₂ samples follows the trend: P25 > PC105 ≈ AH-R > 1077.

2.3. Photodegradation Results

Photocatalytic processes are based on the high oxidative potential of photogenerated holes either free or at the catalyst surface. When the TiO₂ is irradiated with UV light, electrons are promoted to the conduction band with the formation of holes on the valence band, leading to charge separation. These highly reactive species can promote oxidation and reduction reactions. The reaction of radical species generated through the capture of holes together with the direct transfer of holes, are the main steps in the photodegradation of many organic compounds, like acetone, adsorbed on the surface of TiO₂.

In Figure 8, the degradation of acetone on the four studied TiO₂ samples is reported. The photoefficiency of AH-R is lower than the activity obtained to P25 and PC105 (nano-sized), that is, it follows the sequence P25 ≈ PC105 > 1077 > AH-R with an inversion of the two larger-sized samples behavior as previously observed in the photo-electrochemical measures. Smaller-sized samples show the best photocatalytic performance, leading to a complete degradation of acetone within 1 h. A reason for the different relative photoactivity of AH-R and 1077 could be due to morphological differences, which reflect into a distribution of free ROSs including hydroxyl radicals, involved in the photooxidation as confirmed by FTIR characterization. It is important to mention that the particle size of the TiO₂ can affect strongly the adsorption of acetone species. Generally, the smaller the particle size of TiO₂ leads to the larger surface area and higher adsorption of acetone species, which in turn facilitates their degradation by photoproduced ROSs on the surface of TiO₂. In addition, in nanosized TiO₂ system, the yield of photoproduced hydroxyl radicals and their diffusion length could be further enhanced compared to sub-micro-sized TiO₂ system.

As previous reported by Bianchi et al. [14], the photodegradation of acetone in gas phase is highly influenced by the amount of OH available on the catalytic surface. The presence of high yield of OH groups on the surface of TiO₂ (see the spectra reported in Figure 2) can lead to higher yield of photogenerated reactive oxygen species which in turn enhances the oxidation of acetone. Thus, nanosized samples have the highest photocatalytic activity, because of the high degree of hydroxylation of their surfaces, as shown by the IR spectra. However, larger-sized TiO₂ still exhibits a photodegradation comparable to nanometric samples. To shed light on this issue, Figure 9 reports the XPS spectra for both microcrystalline samples. Peak I is O1s attributed to bulk O while peaks II and III would correspond to bridging and terminal OH groups [43]. The ratio OH/O^2- is 0.32 and 0.12, respectively, for 1077 and AH-R. This dissimilarity between the two larger-sized powders confirms the fundamental role of •OH radicals in the photocatalytic process and explains the faster photooxidation with the 1077 sample (see Figure 8).

2.4. Comparison between the Photo-Electrocatalytic Experiments in Aqueous Phase and Photocatalytic Experiments in Gas Phase

Once all information from the measurements has been scrutinized, we shall attempt to semi-quantitatively compare both processes in different reactive media, i.e., photo-electrocatalytic in aqueous phase (PEC) and photocatalytic in gas phase (PC). Presumably, the features of each media might affect the photoactivity of acetone oxidation. In order to evince these differences, we need to use comparable units for the reaction rates in both cases; as reference magnitude, we will use the open-circuit current density for PEC measurements ($j_{PEC}$) as estimated in Figure 7, whereas we shall calculate an apparent open-circuit current density for PC measurements ($j_{PC}$) from kinetic data in Figure 8. It is worth mentioning that the analysis is pertinent in terms of the substrate concentration as our experimental conditions are comparable in both phases, provided the acetone concentration is ca. 7 mM in gas phase and 10 mM in aqueous phase. To estimate $j_{PC}$ and compare it with $j_{PEC},$ we will make the next assumptions:

(i)

In gas phase, the acetone is fully mineralized, thus exchanging 16 electrons with the photocatalyst. This assumption is not relevant, as even with partial mineralization we could estimate $j_{PC}$.

(ii)

The total real area of the photocatalyst is exposed to the reactive medium. Thus, the surface area ($S_{BET}$) and amount ($m$) of photocatalyst are relevant parameters when estimating $j_{PC}$.

(iii)

The value of $j_{PEC}$ (aqueous phase) should be renormalized with respect to the surface area of the electrode in order to compare its magnitude with $j_{PC}$ (gas phase). This normalization implicitly assumes the electrolyte permeates all the nanostructured thin film electrodes studied.

Hence, the apparent current density in gas phase will be given by

$$j_{PC}=\frac{zFv}{S_{BET}m}$$

(2)

where $z$ is the number of exchanged electrons (16 for full acetone mineralization); $F$ is the Faraday constant (96,485 C·mol⁻¹), and $v$ is the reaction rate (mol·s⁻¹) estimated from the slope of curves in Figure 8 in the time range from 0 to 40 min. In addition, the normalized open-circuit current density in aqueous phase with respect to the real surface are will be

$$j_{PEC}=j\left(\frac{S}{p{S}_{BET}m}\right)$$

(3)

where $j$ is the open-circuit current density values taken from Figure 7, $S$ is the geometric exposed area of the electrodes (1 cm²), and $p$ is the porosity of the film (taken as 0.50; this means only half of the total surface area is exposed to the electrolyte, as particles merge during electrode preparation and its real area is hence reduced). The amount of catalyst is estimated assuming all doctor bladed dispersion is deposited on the FTO substrates. In Figure 10, we gather all estimated current densities and their relationship.

As observed, there is a reasonably good linear correlation between $j_{PC}$ and $j_{PEC}$, which could be used to predict the behavior in one phase given the same in the other phase. It should be noted that the correlation is valid for this particular reaction and set of photocatalysts, and by no means can it be extended to any gas phase reaction. In any case, this correlation points out that the nature of the media (gas or liquid) where the photocatalytic degradation of acetone takes place is not significantly relevant; this could be explained in terms of the acetone interaction with the titania surface, that is governing the reaction mechanism [40,44]. Interestingly, the activity ordering of the photocatalysts seems to be reversed with respect to prior measurements, as sub-micro sized powders (1077 and AH-R) now apparently appear more active than the nanosized powders (PC105 and P25); however, we should note this is only stemming from the definition of current densities, as we have expressed them in terms of the surface area, thus catalysts with larger values, will yield lower apparent current densities. Interestingly, the linear relationship between (apparent) current densities could be expressed as

$$j_{PC}=\frac{zF}{p}\left(\frac{v/m}{j/m^{\prime }}\right)j_{PEC}$$

(4)

where $m^{\prime }$ is the mass of the deposited photocatalyst in the electrodes. As observed, the relationship does not depend on the surface area of the photocatalyst; nonetheless, the equation scales with the mass-normalized ratio of the gas phase rate and the aqueous phase current density.

3. Experimental Section

3.1. Reagents and Chemicals

Acetone is a Fluka product at high purity grade. Four commercial titanium dioxide samples, two nanometric (AEROXIDE^® TiO₂ P25 by Evonik Ind. and PC105 by CrystalGlobal) and two sub-micrometric (1077 by Kronos and AH-R by Hundsman) were chosen as photocatalysts and used without any pre-treatment or activation.

3.2. Materials Characterization

The surface area of all samples was determined by conventional N₂ adsorption (BET) at 77 K using a sorptometer instrument (Costech Mod. 1042). X-ray photoelectron spectra (XPS) were taken in an M-probe apparatus (Surface Science Instruments). The source was monochromatic Al Kα radiation (1486.6 eV). The energy scale was calibrated with reference to the 4f_7/2 level of freshly evaporated gold sample at 84.00 ± 0.1 eV. The accuracy of the reported BE can be estimated to be ±0.2 eV. The morphology of the catalysts was investigated by high resolution electron transmission microscopy (HR-TEM). TEM images were recorded using a JEOL 3010-UHR instrument (acceleration potential: 300 kV; LaB₆ filament), with samples dry dispersed on lacey carbon Cu grids. Absorption/transmission IR spectra have been obtained on a Perkin–Elmer FT-IR System 2000 spectrophotometer equipped with a Hg–Cd–Te cryo-detector in the 7200–580 cm⁻¹ range at a 2 cm⁻¹ resolution (number of scans ~20). For IR analysis, powder catalyst has been compressed in self-supporting discs (of about 10 mg·cm⁻²) and placed in a homemade quartz cell, equipped with KBr windows and connected to a conventional high-vacuum line. Spectra were recorded at room temperature (RT) both in air and after prolonged outgassing at RT.

3.3. Preparation of Electrodes

TiO₂ electrodes were prepared by spreading an aqueous slurry of commercial nanosized TiO₂ over a freshly cleaned conductive glass (fluorine-doped tin oxide, FTO glass). In order to have the same thickness of active material on the FTO surface, the suspension was prepared by adding 0.25 or 0.5 g of TiO₂ powder, respectively, for nanoparticles and sub-microparticles of TiO₂, to a mixture of distilled H₂O (1.25 mL for nanopowders and 1.00 mL for sub-micropowders), 15 µL of acetylacetone (Aldrich), and 15 µL of Triton X100 (Aldrich). In general, 15 µL of this suspension was dropped to an FTO glass, and a thin film of TiO₂ was obtained using the doctor blade method. Afterwards, the films were annealed and sintered for 2 h at 450 °C in air. The average thickness (in µm) was measured by means of a Profilometer Instrument (KLA-Tencor Alpha Step D-100) in two different portions of the TiO₂ annealed film.

3.4. Photo-Electrochemical Measurements

The measurements were carried out using a computer-controlled potentiostat (Autolab PGSTAT 30), and a 1000 W Hg-Xe arc lamp (Newport) as an illumination source. The experiments were performed in acidic medium, by using a 0.1 M HClO₄ aqueous solution in a conventional three-electrode photo-electrochemical cell equipped with a quartz window. The counter and reference electrodes were a Pt wire and an Ag/AgCl/KCl (3M) electrode, respectively. All the potentials are quoted against this reference electrode. Before each measurement, the cell was cleaned with an acidic and concentrated KMnO₄ solution overnight, cleaned with a H₂O₂/H₂SO₄ (1:1 vol.) solution and then thoroughly rinsed with distilled water. Finally, the voltammetric cell was boiled in distilled water with a few drops of H₂SO₄. The working solution was bubbled with N₂ for 20 min prior to the electrochemical measurements and then a nitrogen flux was passed over the solution to avoid the oxygen entry. All cyclic voltammograms (CVs) were recorded between −0.7 and 0.8 V vs. Ag/AgCl at a scan rate of 20 mV·s⁻¹. For studying the reduction process, the electrolyte solution was saturated with O₂ by bubbling the gas for 30 min. When the oxidation process was studied, CV measurements of the TiO₂ electrodes were recorded under UV-A illumination using several acetone concentrations (from 1 mM to 1 M) in the N₂-purged electrolyte solution.

3.5. Photocatalytic Set-Up

The photocatalytic degradation of acetone in air was conducted in a Pyrex glass cylindrical reactor with a diameter of 200 mm and an effective volume of 5 L. The amount of catalyst (in the form of powder deposited from a 2-propanol slurry on a flat glass disk) used in the tests was 0.05 g [45]. The gaseous mixture in the reactor was obtained by mixing hot chromatographic air, humidified at 40%, and a fixed amount of volatilized pollutant, in order to avoid condensation. The initial concentration of acetone in the reactor was 400 ppm. The photon source was a 500 W iron halogenide lamp (Jelosil, model HG 500) emitting in the 315–400 nm wavelength range (UV-A) at 30 W·m⁻². Acetone tests lasted for 2 h. The actual concentration of pollutant adsorbed on TiO₂ in the reactor was determined directly by micro-GC sampling.

4. Conclusions

Photo-electrochemical and photocatalytic studies were conducted on nano- and sub-microsized TiO₂ commercial samples, comparing them for the photodegradation of acetone, a typical indoor pollutant. It was found that a high concentration of pollutant inhibits photo-electrochemical degradation. Importantly, both smaller- and larger-sized TiO₂ nano-samples revealed that the optimal concentration for the oxidation of acetone was close to 10 mM. Nanometric samples were found to exhibit higher photocurrents than sub-microsized ones. The potential photocatalytic activity of the samples was evaluated by comparing the j-E curves obtained in the presence of oxygen in the dark and in the presence of acetone (10 mM) under UV illumination. The best efficiency was obtained for nanometric P25, which generates an equivalent photocurrent under open circuit conditions higher (0.08 mA/cm²) than in the other ones. Otherwise, both nano-sized PC105 and larger-sized AH-R have the similar values of photocurrent (0.03 mA/cm²), resulting more active than the other larger-sized sample (1077). It is remarkable that a nanocrystalline and a microcrystalline sample display similar efficiency. This is a result of the existence of two separate factors: increased surface area (in smaller-sized samples) and the existence of an operating space charge layer (in larger-sized samples). Among the different TiO₂ samples, PC105 and P25 exhibit the best results in photocatalytic gaseous oxidation. On the contrary, 1077 has better photoefficiency than AH-R, as shown by the faster oxidation rate. This feature could be explained by the amount of photoactive “free” Ti-OH sites, which are probably responsible for photooxidation. In summary, it is important to highlight that larger-sized samples have different behaviors depending on the nature of the catalytic processes. In photo-electrochemical analysis, not only the degree of hydroxilation but also the morphology and the particle size play an important role while for the photocatalysis in gas phase, OH radicals have a fundamental role. In this sense, larger-sized TiO₂ samples could be valuable candidates for the photooxidation of VOC in gas phase. Finally, a direct correlation between gas phase and aqueous phase measurements has been attempted, showing that both correlate pretty well; this evinces the role of the media in this particular reaction and with this set of photocatalysts does not seem relevant.