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Article

Fractal Behavior of Nanostructured Pt/TiO2 Catalysts: Synthesis, Characterization and Evaluation of Photocatalytic Hydrogen Generation

by
Anca Vasile
1,*,
Gianina Dobrescu
1,
Veronica Bratan
1,
Mircea Teodorescu
2,
Cornel Munteanu
1,
Irina Atkinson
1,
Catalin Negrila
3,
Florica Papa
1,* and
Ioan Balint
1,*
1
“Ilie Murgulescu” Institute of Physical-Chemistry of the Romanian Academy, 202 Spl. Independentei, 060021 Bucharest, Romania
2
Faculty of Applied Chemistry and Materials Science, Polytechnic University of Bucharest, 1-7 Gh. Polizu Street, 011061 Bucharest, Romania
3
National Institute of Material Physics, 077125 Magurele, Romania
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(9), 619; https://doi.org/10.3390/catal14090619
Submission received: 6 August 2024 / Revised: 28 August 2024 / Accepted: 6 September 2024 / Published: 13 September 2024
(This article belongs to the Special Issue Effect of the Modification of Catalysts on the Catalytic Performance, 2nd Edition)
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Abstract

:
The fractal characterization of supported nanoparticles is a useful tool for obtaining structural and morphological information that strongly impacts catalytic properties. We have synthesized and characterized Pt supported on TiO2 nanostructures. Triblock copolymers with thermosensitive properties were used as templating agents during the synthesis process. In addition to the several techniques used for the characterization of the materials, we carried out fractal analysis. The prepared materials showed a reduction in the band gap of TiO2 from 3.44 to 3.01 eV. The extended absorption in the 500–700 nm regions is mostly attributed to the presence of supported Pt nanoparticles. The ability of the nanostructured Pt/TiO2 catalysts to generate H2 in an aqueous solution was evaluated. The test reaction was carried out in the presence of methanol, as a hole scavenger, under simulated solar light. Pt/TiO2-3TB shows the highest rate of H2 (4.17 mmol h−1 gcat−1) when compared to Pt/TiO2-0TB (3.65 mmol h−1 gcat−1) and Pt/TiO2-6TB (2.29 mmol h−1 gcat−1) during simulated solar light irradiation. Pt/TiO2-3TB exhibits a more structured organization (fractal dimensions of 1.65–1.74 nm at short scales, 1.27–1.30 nm at long scales) and a distinct fractal behavior. The generation of hydrogen via photocatalysis can be linked to the fractal characteristics.

1. Introduction

Since the 1980s, titanium dioxide (TiO2) has been thoroughly studied as a workable photocatalyst for environmental cleanup [1] and numerous techniquesfor altering its catalytic performance have been developed. Since its introduction by Kraeutler and Bard [2], surface platinization of TiO2 has been a widely used photocatalyst modification technique due to the increased activity of the platinized TiO2 (Pt/TiO2) for a variety of photocatalytic reactions [3]. Electrochemical and time-resolved spectroscopic studies [3,4] have corroborated the theory that Pt deposits on TiO2 slow down fast charge pair recombination by acting as an electron sink (Schottky barrier electron trapping) and promote interfacial electron transfer to dioxygen or other electron acceptors. Nevertheless, not all of the documented Pt effects have been favorable [1]. The role of Pt deposits in photocatalytic reaction mechanisms appears to be far more complex than a simple electron sink. Depending on the synthesis conditions, the platinized TiO2 may refer to very different photocatalysts. Their properties should be determined by a number of factors, including the substrate (TiO2) properties, the amount of Pt loaded, Pt’s oxidation state, Pt’s particle size, dispersion, and morphology, etc. [5,6].
There are numerous investigations of Pt nanoparticle synthesis [7], as well as on the properties for different catalytic uses like hydrogenations [8,9], energy conversion [10,11], water treatment [12], and air treatment [13]. The medium in which the metal precursors are dissolved is a key parameter to be controlled during Pt nanoparticle colloidal synthesis since it affects the stabilization of nanoparticles formed, as well as the reducing agent type that will strongly affect how nanoclusters form. Hydrogen is a well-known reducing agent for the formation of Pt nanoparticles [14]. The surfactants are used as stabilizing agents to provide stability to the nanoparticles formed and to obtain their desired size and shape [15,16,17]. Polyol synthesis [18] of Pt nanoparticles using a thermal procedure [19] has been reported. Nano-catalysts have been prepared using a variety of methods, including the colloidal method. Researchers have expressed a lot of interest in the field of block copolymer-assisted nanomaterials synthesis for a variety of catalytic applications [20,21,22,23]. Heat-sensitive polymers, which undergo a lower critical solution temperature (LCST) phase transition, have received special attention due to their potential in catalysis [24] and functional materials [25,26]. Many studies on thermoresponsive polymers rely on poly(N-isopropylacrylamide) (PNIPAAm), which has an LCST at 32 °C. Heskins et al. first reported this about 40 years ago [27] and it has been extensively studied since then [28,29]. Thermoresponsive polymers have many applications, including their use as nanoparticle stabilizers [30] in catalysis, as demonstrated by Ballauff et al. [31].
Fractal theory is another useful tool for characterizing a catalyst’s surface. The fractal characterization of supported and unsupported metallic nanoparticles of Pt, Pd, Rh, and bimetallic Pt—Cu, Pd—Cu, and Pd—Ag was presented in the literature [32,33,34] and the fractal characteristics of these catalysts were correlated with their catalytic properties [34]. The concept of fractal dimension (df) proved itself to be an effective tool for describing the roughness and irregularities of materials at various scales. The fractal dimension, which describes such fractal materials, was found to be in the range of 2 ≤ df ≤ 3: low values (df = 2.0) indicate regularity and smoothness, intermediate df values indicate irregular surfaces, and df values near 3 indicate highly irregular surfaces. Fractal surfaces are more efficient than planar surfaces for diffusion-limited adsorption processes, so fractal catalytic support outperforms a non-fractal support [35]. Peng et al. [35] investigated the relationship between catalyst performance and fractal dimension using the Mahnke and Mögel model [36], and the model of Neimark [37]. They investigated the surface fractal dimensions of Pt/TiO2 pretreated at various temperatures, as well as other key factors influencing the effect of pretreatment on catalytic activity, such as platinum dispersion and strong metal support interactions (SMSIs). Yin et al. [38] investigated the impact of Pt decorating on the sensing properties of SnO2 gas sensors. In this study, fractal analysis was used to describe surface irregularities. They investigated and characterized the structural details of surface samples using the self-similarity of surfaces at various scales, and also analyzed the samples’ field emission scanning electron microscopy (FESEM) micrographs to determine the surface texture parameters and fractal dimension. Nigmatullin et al. [39] studied the electropolymerization process of aniline (PANI) with the formation of the modified film under potentiodynamic conditions and correlated the obtained parameters with the morphology of a polymer film using an alternative method, electron scanning microscopy (SEM). They determined the fractal dimension’s dependence on an external factor known as potential. To quantify the various morphologies of 2D tin disulfide (SnS2), Shao et al. [40] used fractal dimensions to assess complexity using the box-counting method. Growth conditions accurately control a wide range of morphologies, including hexagon, triangle, windmill, dendritic, and coralloid, with fractal dimensions ranging from 1.01 to 1.81. The change in the Sn source becomes an important factor in controlling various morphologies with fractal dimensions ranging from 1.01 to 1.81.
In this work, to enhance the photocatalytic hydrogen generation, we focused on the surface structure of the Pt/TiO2 photocatalyst. Specifically, we synthesized three nanostructured Pt/TiO2 materials with different surface structures, using new types of thermosensitive polymers as structure-directing agents in an aqueous medium at room temperature. Besides their characterization using SEM, TEM, XPS, XRD, CO chemisorption, and UV–vis spectroscopy techniques, the fractal analysis of prepared materials was performed. A useful method to learn about the structure and morphology of supported nanoparticles, which have a great influence on their catalytic properties, is their fractal characterization. The fractal dimension was determined using the “box—counting” method. The photocatalytic activity of Pt nanoparticles supported on TiO2 for hydrogen generation under simulated solar irradiation was investigated, and a relationship between fractal dimensions and catalytic properties was obtained. The catalytic performance of Pt/TiO2 photocatalysts is determined by a variety of factors, including the material’s structural and electronic properties, which are influenced by particle size and preparation methods.

2. Results and Discussion

2.1. Structural Properties (SEM, TEM, XRD, Fractal Analysis, XPS)

2.1.1. SEM

Figure 1 shows SEM images of the synthesized Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB catalysts, displaying the morphologies of the calcined sample. The corresponding EDS spectra and quantitative elemental analysis of some areas of Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB surfaces are shown in Supplementary Files, Figure S1.
The SEM micrographs of Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB show particles with a uniform distribution, spherical morphology, and slight agglomeration. The Pt/TiO2-3TB sample exhibited a higher degree of particle agglomeration (Figure 1b). The EDX spectra (Figure S1) revealed the presence of Pt on the TiO2 surface. Figure S1b shows that the Pt/TiO2-3TB has a higher Pt percent in this area.

2.1.2. TEM

The size and morphology of the nanoparticles were further investigated using TEM analysis. Figure 2A, B, and C show TEM images of calcined Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB, respectively.
TEM images revealed the typical structure of TiO2, which consists of irregularly shaped crystalline aggregates with a size of 15–30 nm (Figure 2). Round-shaped Pt nanoparticles are dispersed on the TiO2 support, with average sizes ranging from 1.5 to 5.0 nm, 2.0 to 4.5 nm, and 0.09 to 2.5 nm for Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB, respectively.
Platinum nanoparticles were synthesized in the presence of thermosensitive water-soluble block copolymers, previously prepared and described [5,41,42]. As previously demonstrated [41], the thermogelation properties of the blockcopolymers used in this study were determined by the hydrophobic monomer content of the side blocks and the molecular weight (MW) of the PEG middle block. At a constant PEG chain length, increasing TBAM concentrations resulted in higher hydrophobicity of the copolyacrylamide blocks. This may have an impact on the formation and distribution of platinum nanoparticles.

2.1.3. Fractal Analysis

Nanostructured Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB catalysts modified with thermoresponsive polymers can be analyzed fractally to characterize their structure. Therefore, it is interesting to determine the powder fractal dimension. The analysis of transmission electron microscopy (TEM) images is a direct method for determining the fractal dimension [43].
The “box—counting” method is one of the most useful for measuring the fractal dimension. The method relies on the fractal object’s self-similarity property. Self-similarity has a mathematical description [44,45]:
N (r/R) ~ (r/R)−D,
where D is the fractal dimension and N(r, R) is the number of boxes of size r that cover the object of linear size R; in other words, self-similarity is the property of an object that makes it appear the same when zoomed in.
Keeping the object’s maximum size constant, the box dimension is defined as the exponent D in the following relation:
N(r) = Ar−D,
where N(r) represents the number of linearly sized boxes required to cover the object in atwo-dimensional plane. For Euclidean objects, one needs a number of boxes proportional to r − D, where the exponent D is the Euclidean dimension of the plane, 2. The prefactor A, also known as lacunarity, is a measure of how the space is filled, the gap, or the object texture [46].
To define a box dimension, boxes are placed in a position and orientation that reduces the number of boxes required to cover the set. Finding the configuration that minimizes N(r) among all possible ways to cover the set with r-sized boxes is a very difficult computational problem.
In practice, to calculate the box-counting fractal dimension, one counts the number of linear size r boxes required to cover the set for a range of r values and plots the logarithm of N(r) on the vertical axis versus the logarithm of r on the horizontal axis. The slope of the straight line is −D. In theory, for each box size, the grid should be overlaid so that the fewest number of boxes are occupied. This is accomplished by the computer code rotating the grid for each box size by 90 degrees and plotting the minimum value of N(r).
To compute the fractal dimensions, we used Tru-Soft Int’l, Inc.’s Benoit software, version 1.31. The primary data obtained were used to calculate the fractal dimensions across various self-similarity domains.
The box-counting fractal dimensions of nanoparticles were calculated using modified TEM micrographs that were subtracted from the TiO2 substrate and converted from greyscale to black and white. Figure 3 illustrates TEM micrographs of Pt/TiO2-0TB (Figure 3(A1,A2)), Pt/TiO2-3TB (Figure 3(B1,B2)), and Pt/TiO2-6TB (Figure 3(C1,C2)), which were used to determine fractal dimensions.
Figure 4 shows the particle size distribution of Pt nanoparticles for Pt/TiO2-0TB (Figure 4(A1,A2)), Pt/TiO2-3TB (Figure 4(B1,B2)), and Pt/TiO2-6TB (Figure 4(C1,C2)).
Fractal dimensions of the images’ grey levels are computed from the TEM micrographs (Figure 3A–C) and fractal dimensions of the three-dimensional embedded object are obtained. The results of such calculations are shown in Table 1 and Table 2.
Pt/TiO2-3TB samples are better organized; both micrographs (Figure 3(B1,B2)) have fractal dimensions of 1.65–1.74 at short scales and 1.27–1.30 at long scales (Table 2). The short-scale fractal dimension is greater than the long-scale fractal dimension, indicating that the particles are tightly packed (agglomerations) and highly correlated at short distances; meanwhile, at long distances, the Pt nanoparticle agglomerations (packs) are dispersed.
The Pt/TiO2-0TB and Pt/TiO2-6TB samples behave differently than Pt/TiO2-3TB. This behavior can be correlated with Pt’s photocatalytic hydrogen generation activity.

2.1.4. XRD

The catalyst structures were revealed using XRD analysis. As shown in Figure 5, the XRD pattern of the Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB catalysts is essentially the same as that of the TiO2 sample, which consists of anatase (90.3%) (JCPDS card no. 00-021-1272) and rutile (9.7%) (JCPDS card no. 01-077-0441) crystalline phases. The crystalline size of anatase is 14 nm on average. For the Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB samples, the typical crystalline diameters of the rutile phase are 18, 22, and 16 nm, respectively. The absence of diffraction lines of the Pt phase on the Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB catalysts indicates that Pt is well dispersed and their crystal sizes are very small, as observed in the TEM images.

2.1.5. XPS

The surface elemental composition and chemical state of catalytic materials were evaluated using X-ray photoelectron spectroscopy (XPS). The XPS spectra revealed that the components Ti, O, Pt, and C (typical contamination) were present on the surface of the Pt/TiO2 catalysts. Table 3 shows the expected surface element contents (at.%) based on XPS measurements.
The relative atomic percentage of Pt estimated from XPS examination in the Pt/TiO2-3TB sample, 0.1 at.%, is lower than the surface Pt calculated for the Pt/TiO2-0TB and Pt/TiO2-6TB samples, 0.2 at.% (Table 3). This is highly associated with the dispersion of Pt on the catalyst surfaces as determined by CO chemisorption experiments.
Figure 6 illustrates the XPS spectra of the Pt4f (Figure 6a) and Ti2p (Figure 6b) regions for the Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB catalysts.
The binding energies (BE) for the decomposed peaks of the Ti 2p, Pt 4f, O 1s core level in the XPS spectra over the Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB samples are listed in Table 4.
From the region for Pt 4f in the XPS scans (Figure 6a), the highest doublet indicates the usual metallic state [47,48,49]. As shown in Figure 6a and Table 4, the Pt 4f7/2 and Pt 4f5/2 lines at 71.08 eV (peak A)-74.55 eV (peak C), 70.80 eV (peak A)-74.27 eV (peak D), and 71.16 eV (peak A)-74.63 eV (peak C) for Pt/TiO2-0TB, Pt/TiO2-3TB and Pt/TiO2-6TB, respectively, are attributed to metallic platinum (Pt0), while the second doublet at 73.41 eV (peak B)-76.29eV (peak D), 72.10 eV (peak B)-75.64 eV (peak F), and 73.46 eV (peak B)-76.40 eV (peak D) for Pt/TiO2-0TB, Pt/TiO2-3TB and Pt/TiO2-6TB, respectively, can be assigned to Pt2+ characteristic of platinum oxide (PtO). Furthermore, the Pt/TiO2-3TB sample exhibits the lines at 73.76 eV (peak C)-76.94 eV (peak E), which indicates the most likely Pt2+ characteristic of Pt(OH)2.
Figure 6b shows that all samples had almost identical Ti 2p spectra, with two peaks at 458.85 ± 0.1 eV and 464.63 ± 0.1 eV, originating from the Ti 2p3/2 and Ti 2p1/2 core levels, respectively, indicating a typical oxidation state of Ti4+ in the TiO2 [50].
From deconvoluted XPS spectra of all samples, the core lines for the O1s states of oxygen decompose into three peaks (not shown here) at A-530.15 ± 0.18, B-531.5 ± 0.2, and C-532.45 ± 0.2 eV (Table 4). The first binding energy indicates the lattice oxygen, O2−, the second surface adsorbed oxygen or oxygen bonded to carbon (O = C), and the third oxygen bonded to carbon (O—C) [51].
In summary, the XPS examination indicates that Pt is either metallic (Pt0) or coordinated with oxygen or hydroxyl groups (Pt2+).

2.2. Optical Absorption Properties of Catalysts

The UV–visible absorption spectra of the Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB samples were recorded at room temperature in the 200–1000 nm wavelength range. TiO2 was used as a reference sample, as shown in Figure 7.
The inclusion of Pt considerably increased optical absorption in the 300–700 nm band when compared to bare TiO2. The Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB samples showed intense absorption at visible and NIR wavelengths, which increased slightly towards shorter wavelengths. The localized surface plasmon resonance (SPR) of platinum nanoparticles typically ranges between 450 and 550 nm for larger particles [52]. The extended absorption in the 500–700 nm regions is mostly attributed to the presence of platinum. This is characteristic of supported Pt nanoparticles. The Tauc plot [53,54] is used to calculate band gap (Eg) values from reflectance [F(R)] spectra. To calculate Eg, extrapolate the linear component of the graph (αhν)1/η versus hν (where α represents absorption and hν represents light energy) to the abscissa axis. Table 5 shows the calculated band gap values for direct transitions (η = 1/2) of Pt/TiO2-0TB, Pt/TiO2-3TB, Pt/TiO2-6TB, and TiO2 as reference.
The TiO2 support’s absorption (200–400 nm) [55] dominates all UV–vis spectra. This sharp absorption band is attributed to the interband absorption of the original TiO2 [56]. Energy band gap (Eg) values changed to lower energies in the following order: Eg (TiO2) = 3.44 eV > Eg (Pt/TiO2-0TB) = 3.14 eV > Eg Pt/TiO2-3TB) = 3.06eV > Eg (Pt/TiO2-6TB) = 3.01 eV.

2.3. CO Adsorption over Metal Surfaces

CO chemisorption tests were carried out to assess the dispersion of Pt. The number of active metal sites is one of the most significant parameters for metallic catalysts. CO chemisorption and electron microscopic data were utilized to calculate active Pt site concentrations and particle size distributions, respectively. Table 6 shows the Pt dispersion, particle size, and other important parameters measured by CO pulse chemisorption.
Samples Pt/TiO2-0TB and Pt/TiO2-6TB had a substantially greater dispersion of Pt nanoparticles on the TiO2 surface (32.23% and 34.07%, respectively) than sample Pt/TiO2-3TB (15.12%), which is consistent with the smaller size of the Pt nanoparticles (Table 6) and the relative atomic percentage of the Pt element determined from the XPS examination (Table 3). Several variables, such as the metal–support interaction or the varying adsorption stoichiometry of CO/Pt, which is connected to the size of the Pt particles, as well as some morphological changes happening under reaction conditions, may result in discrepancies between the sizes of the Pt nanoparticles acquired by TEM and those obtained by CO chemisorption findings.

2.4. The Photocatalytic Testing for Hydrogen Generation

The photocatalytic behavior of all samples was investigated for 4 h at 18 °C using an aqueous nitrate solution and simulated solar irradiation (AM 1.5). Methanol was used as a sacrificial species to generate photocatalytic hydrogen. The gas that evolved from the reactor was sampled and analyzed periodically. Figure 8 shows the data on the cumulative amount of H2 produced by the photocatalytic reduction of protons on Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB. The corresponding cumulative CO2amounts from the photocatalytic reaction on Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB are shown in Supplementary Files, Figure S2.
The amount of H2 produced over all catalysts increased almost linearly with illumination time. As indicated in Figure 8, Pt/TiO2-3TB shows the highest rate of H2 (4.17 mmol h−1 gcat−1) when compared to Pt/TiO2-0TB (3.65 mmol h−1 gcat−1) and Pt/TiO2-6TB (2.29 mmol h−1 gcat−1) during simulated solar light irradiation. Beyond a specific optimum level, when additional Pt is introduced to the TiO2 surface, it leads to the blockage of the photosensitive TiO2 surface. As a result, this reduces the surface concentration of electrons and holes that are available for reaction. The narrower band gap of Pt/TiO2-3TB may account for some of its increased photocatalytic activity. Surface morphology, particle size, and crystal structure are important characteristics that influence H2 photocatalytic production via water-splitting under solar irradiation. The significant improvement in the photocatalytic performance is the result of a combination of factors, including an extended response to visible light, alignment of bands that work well together, and rapid separation of charge carriers in the material [57]. The oxidation of alcohols to radicals can constantly limit the recombination of photogenerated electrons and holes, indirectly increasing the photocatalyst’s reduction capacity. The ideal reforming process of alcohols in the overall reaction can be expressed as follows [58,59]:
CaHbOc + (2a − c) H2O → a CO2 + (2a − c + b/2) H2,
Hydrogen production is coupled with methanol oxidation up to CO2 (Figure S2). The proposed charge transfer pathway and reaction mechanism for simulated solarlight-driven photocatalytic H2 production of the Pt supported on TiO2 nanostructures photocatalysts are shown in Figure 9.
First, under irradiation, the Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB samples with narrowed bandgap (3.14, 3.06, and 3.01 eV, respectively) are excited and charge carriers are generated. Due to the difference in conduction band (CB) position, the photo-generated electrons (e−) in the CB of TiO2 can transfer to the Pt active sites, resulting in the enhanced intrinsic charge separation efficiency of TiO2 leaving holes (h+) in the valence band of TiO2. Then both methanol, as a hole scavenger, and Pt nanoparticles can act as the active sites for proton (H+) reduction, and the photo-generated h+ in TiO2 are consumed by methanol simultaneously, which therefore boosts the hydrogen production under simulated solar irradiation.
Methanol was used in the reaction solution, thus decreasing the electron-hole recombination rate and making conduction band electrons more accessible for proton (H+) reduction. Methanol molecules are oxidized to CO2 (Figure S2) in the aqueous nitrate solutions of Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB, according to the following equations [60]:
CH3OH + hν + photocatalyst → HCHO + H2,
HCHO + H2O + hν + photocatalyst → HCO2H + H2,
HCO2H + hν + photocatalyst → CO2 + H2,
Pt/TiO2-0TB and Pt/TiO2-6TB catalysts show weaker photocatalytic activity, possibly due to different contributing factors. Despite having a higher dispersion of exposed Pt on the TiO2 support and a higher atomic percentage of metallic Pt on the surface, as shown by CO chemisorptions and the XPS technique, respectively, the H2 production rate is lower in comparison with the Pt/TiO2-3TB catalyst. It is anticipated that the presence of more abundant and larger Pt oxide clusters may lead to a weaker interaction with the TiO2 surface, leading to the observed differences in activity. The different distributions and sizes of the Pt nanoparticles on the titania surface contribute to a higher hydrogen production rate and impact on the photoactivity of Pt/TiO2-3TB. These phenomena are partly related to the oxidation state and local environment of Pt on the TiO2 surface. Fractal analysis characterization shows that in the case of the Pt/TiO2-3TB catalyst, the particles form tightly packed agglomerations and show a high correlation at short distances, while the Pt nanoparticle agglomerations (packets) are dispersed at long distances. In this way, there can be a stronger interaction with the TiO2 surface, and the photo-produced electrons could be more easily transferred from the conduction band (CB) of TiO2 to the CB of Pt, contributing to the increased separation of the photo-produced charge carriers (electrons, e−, and holes, h+).
For an initial evaluation of the stability of the catalysts, UV–vis analysis was performed using the selected Pt/TiO2-3TB sample. The UV–visible absorption spectra of the Pt/TiO2-3TB sample were recorded at room temperature in the 200–1000 nm wavelength range before and after performing the photocatalytic test, as shown in Supplementary Files, Figure S3. Thus, from the results of the absorbance curves obtained on the selected sample it can be said that the material largely retains the same pattern. The characteristic absorption of the TiO2 support (in the range from 200 to ~400 nm) attributed to the inter-band absorption of the original TiO2 is dominant in both UV–vis spectra. The Pt/TiO2-3TB sample showed intense absorption at visible and NIR wavelengths, which increased slightly towards shorter wavelengths. The strong absorption with a broad maximum of around 550 nm, exhibited even after the photocatalytic test, shows the preservation of the character of the localized surface plasmon resonance (SPR) of platinum nanoparticles.
Table 7 presents a comparative analysis of the catalysts under study and other published research in the literature; however, an exact comparison is challenging due to the different experimental setups.

3. Materials and Methods

3.1. Chemicals

Titanium (IV) dioxide (P25, BET surface area 50 m2/g) and chloroplatinic acid hexahydrate (H2PtCl6·6H2O) were used without additional purification. The P(NIPAM-co-(TBAM)-PEG-P(NIPAM-co-TBAM) triblock copolymers were previously employed and detailed [5,41]. Briefly, they were prepared by the single electron transfer–living radical polymerization (SET-LRP) technique, by copolymerizing N-isopropyl acrylamide (NIPAM) and N-t-butyl acrylamide (TB) in the presence of α,ω-bis(2-chloropropionate) poly(ethylene glycol) (PEG, Mn ≈ 4000 Da) as the macroinitiator and CuCl/tris(2-dimethylaminoethyl)amine as the catalytic system, in 50:50 (vol/vol) DMF: water mixture at 20 °C, as previously described [41]. The molecular characteristics of the P(NIPAM-co-(TBAM)-PEG-P(NIPAM-co-TBAM) triblock copolymers, the approximate molecular weight (MW) of the PEG middle block, 4000 Da, and 0, 3, and 6 mol% TBAM, respectively, in the initial monomer mixture were presented earlier [5,41]. The temperatures at which the solution turns cloudy, determined visually on 0.5 wt.% triblock copolymer aqueous solutions (TC), were 40, 38, and 36 °C for the triblock copolymers with 0, 3, and 6 mol% TB, respectively.

3.2. Catalyst Preparation

Pt-supported TiO2 materials were prepared using a synthesis method adapted from the literature [42]. That is, 1.00 g of TiO2 powder was mixed with an aqueous solution containing a certain amount of triblock copolymers and calculated amounts of H2PtCl6 to obtain 1%wt Pt loading. The novel triblock copolymers, previously prepared and described [5,41], were used to control the structure and growth of the platinum nanocrystals. They present a central block of polyethylene glycol (PEG) and lateral blocks of statistical copolymers of N-isopropyl acrylamide (NIPA) and N-t-butyl amide (TB). The molar percentage of TB is 0, 3, and 6 mol%; the molar ratio between the platinum cation and triblock copolymer, calculated on the base of the monomer unit, was in all cases 1:10. The triblock copolymer, Pt4+, and TiO2 were combined in an aqueous suspension, followed by bubbling with Ar gas for 20 min to eliminate air. Subsequently, the slurry was exposed to H2 gas for 10 min. Following this, the reaction vessel was kept under stirring for 12 h in a thermostated water bath. The critical phase transition point temperatures (TC) were 40, 38, and 36 °C for the samples with 0, 3, and 6 mol% TB, respectively. The Pt-supported TiO2 materials were prepared at 4 °C below the TC. At the end of the reduction process, the slurry was filtered, dried, and calcined at 200 °C for 1 h. The synthesized samples at 36 °C, 34 °C, and 32 °C were named Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB, respectively.

3.3. Catalyst Characterization

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), fractal analysis, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), CO chemisorption, and UV–vis spectroscopy techniques were used to carry out the characterizations of the catalysts.
The synthesized Pt/TiO2 catalysts were characterized using a high-resolution microscope, FEI Quanta 3D FEG (Thermo Fisher, Waltham, MA, USA) equipped with an EDS Apollo X detector. The analyses were conducted in high vacuum mode, at an accelerating voltage of 20 kV with an Everhart–Thornley secondary electron detector. Scanning electron micrographs and elemental mappings were obtained at a work distance of 10 mm and a magnification factor between 10,000 and 100,000. The morphology, particle size, and microstructure were examined through Transmission Electron Microscopy (TEM), using the high-resolution microscope FEI Tecnai G2-F30 S-Twin (Thermo Fisher Scientific, Waltham, MA, USA). The particle size distribution was determined from TEM images.
One of the most useful methods to measure the fractal dimension is the “box-counting” method. The method uses the self-similarity property of the fractal object. To compute the fractal dimensions, we used the Benoit software, version 1.31, TruSoft Int’l, Inc. The primary data obtained were used to compute the fractal dimensions on different self-similarity domains.
The X-ray photoelectron spectroscopy (XPS) measurements were carried out with a SPECS spectrometer (Berlin, Germany) with a PHOIBOS 150 analyzer, using a non-monochromatic Al Kα radiation source (1486.7 eV) at 300 W. The charge compensation was realized by a flood gun of the Specs FG15/40 type. The acquisition was operated at a pass energy of 10 eV for the individual spectral lines and 50 eV for the extended spectra.
The powder X-ray diffraction (XRD) pattern was recorded with a Rigaku diffractometer type Ultima IV (Tokyo, Japan) in parallel-beam geometry. The diffraction patterns were obtained in the 2θ range between 10° and 80°.
The Chem-Bet-3000 Quantachrome Instrument (Boynton Beach, FL, USA) equipped with a thermal conductivity detector (TCD), was used to estimate the surface area of exposed platinum nanoparticles dispersed on TiO2 through CO pulse chemisorption performed at room temperature. Before CO chemisorption measurements, the catalysts were reduced with H2 at 400 °C for 1h. Subsequently, pulses of CO (50 µL) were fed into the carrier gas stream (He gas flow of 70 mL min−1) using a precision analytical syringe.
The interaction of Pt and TiO2 was investigated using diffuse reflectance spectroscopy. UV–vis spectra were recorded with a Perkin Elmer Lambda 35 spectrophotometer (Shelton, CT, USA) equipped with an integrating sphere. The measurements were carried out at room temperature in the 200–1100 nm range, using spectralon as a reference. The reflectance measurements were converted to absorption spectra using the Kubelka–Munk function, F(R).

3.4. Photocatalytic Testing

The photocatalytic hydrogen generation experiment was carried out using simulated solar irradiation (AM 1.5) using methanol as a hole scavenger. A photochemical reactor constructed of quartz was used for the test. The light source was provided by a solar simulator (Peccell L01) placed at the side of the photoreactor and tightly connected to the optical quartz glass (5 × 5 cm) of the photoreaction vessel. To maintain the temperature of the reactant solution at 18 °C during the test, a flow of cooling water was used. A condenser, cooled with a recirculation chiller at −5 °C, was placed on top of the photoreactor to remove water vapors. For the photocatalytic H2 evolution test, 50 milligrams of photocatalyst dispersed in 120 mL of aqueous nitrate solution containing 1 mL methanol (CH3OH) were introduced into the reaction vessel. The online gas chromatograph equipped with TCD detectors (Buck Scientific 910, Norwalk, CT, USA) monitored the amount of hydrogen generated under continuous irradiation of the solar simulator every 30 min. The hydrogen was separated on a MS 5 column, the CO2 was separated on a Hayasept column, and then they were measured for quantification.

4. Conclusions

In summary, Pt supported on TiO2 nanostructures has been successfully synthesized. New triblock copolymers with thermosensitive properties, as templating agents, under mild conditions were used. The prepared materials showed a reduction in the band gap of TiO2 from 3.44 eV to 3.14, 3.06, and 3.01 eV for Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB, respectively. The samples showed an increase in absorption in the visible light region. The extended absorption in the 500–700 nm regions is mostly attributed to the presence of supported Pt nanoparticles. The results obtained from CO chemisorption experiments are correlated with those from TEM and XPS analysis. Utilizing 2D images, the box-counting method yielded the most reliable data on the fractal properties of these samples. The determination of the fractal dimension shows significant changes in the surface topography due to the addition of block copolymers as structure-directing agents.
The ability of the nanostructured Pt/TiO2 catalysts to generate H2 in an aqueous solution in the presence of methanol, as a sacrificial agent, under simulated solar light irradiation, was evaluated. Pt/TiO2-3TB shows the highest rate of H2 (4.17 mmol h−1gcat−1) in comparison to Pt/TiO2-0TB (3.65 mmol h−1gcat−1) and Pt/TiO2-6TB (2.29 mmol h−1gcat−1). Pt/TiO2-3TB exhibits a more structured organization (fractal dimensions of 1.65–1.74 nm at short scales and 1.27–1.30 nm at long scales) and a distinct fractal behavior. The photocatalytic hydrogen generation performance could be related to this behavior.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14090619/s1, Figure S1: EDS quantitative results of some areas of Pt/TiO2-0TB (a), Pt/TiO2-3TB (b) and Pt/TiO2-6TB (c) surfaces; Figure S2: Evolved CO2 from the photocatalytic reaction catalyzed by Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB in the presence of methanol. Reaction conditions: catalyst weight, 0.050 g; temperature, 18 °C; reactant, NO3−, 120 mL; hole scavenger, methanol, 1 mL; Ar, 20 cm3 min−1; solar simulator; Figure S3: Kubelka-Munk absorption curves of Pt/TiO2-3TB, before and after photocatalytic reaction test.

Author Contributions

Conceptualization, A.V., F.P. and I.B.; software, G.D.; validation, A.V. and F.P.; formal analysis, G.D.; investigation, A.V., G.D., V.B., M.T., C.M., I.A., C.N. and F.P.; writing—original draft preparation, A.V.; writing—review and editing, A.V., F.P. and I.B.; supervision, F.P. and I.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM images of Pt/TiO2-0TB (a), Pt/TiO2-3TB (b), and Pt/TiO2-6TB (c).
Figure 1. SEM images of Pt/TiO2-0TB (a), Pt/TiO2-3TB (b), and Pt/TiO2-6TB (c).
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Figure 2. TEM images (A) Pt/TiO2-0TB, (B) Pt/TiO2-3TB, and (C) Pt/TiO2-6TB.
Figure 2. TEM images (A) Pt/TiO2-0TB, (B) Pt/TiO2-3TB, and (C) Pt/TiO2-6TB.
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Figure 3. TEM micrographs of Pt/TiO2-0TB (A1,A2), Pt/TiO2-3TB (B1,B2), and Pt/TiO2-6TB (C1,C2) used to compute fractal dimensions.
Figure 3. TEM micrographs of Pt/TiO2-0TB (A1,A2), Pt/TiO2-3TB (B1,B2), and Pt/TiO2-6TB (C1,C2) used to compute fractal dimensions.
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Figure 4. Particle size distribution of Pt nanoparticles for Pt/TiO2-0TB (A1,A2), Pt/TiO2-3TB (B1,B2), and Pt/TiO2-6TB (C1,C2) samples.
Figure 4. Particle size distribution of Pt nanoparticles for Pt/TiO2-0TB (A1,A2), Pt/TiO2-3TB (B1,B2), and Pt/TiO2-6TB (C1,C2) samples.
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Figure 5. X-ray diffraction patterns of Pt/TiO2-0TB, Pt/TiO2-3TB, Pt/TiO2-6TB, and TiO2 catalysts.
Figure 5. X-ray diffraction patterns of Pt/TiO2-0TB, Pt/TiO2-3TB, Pt/TiO2-6TB, and TiO2 catalysts.
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Figure 6. (a) Pt 4f and (b) Ti 2p XPS spectra for Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB.
Figure 6. (a) Pt 4f and (b) Ti 2p XPS spectra for Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB.
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Figure 7. Kubelka–Munk absorption curves of Pt/TiO2-0TB, Pt/TiO2-3TB, Pt/TiO2-6TB, and TiO2 as a reference, at room temperature.
Figure 7. Kubelka–Munk absorption curves of Pt/TiO2-0TB, Pt/TiO2-3TB, Pt/TiO2-6TB, and TiO2 as a reference, at room temperature.
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Figure 8. Photocatalytic hydrogen generation in the presence of methanol from the reaction catalyzed by Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB. Reaction conditions: catalyst weight, 0.050 g; temperature, 18 °C; reactant, NO3−, 120 mL; hole scavenger, methanol, 1 mL; Ar, 20 cm3 min−1; solar simulator.
Figure 8. Photocatalytic hydrogen generation in the presence of methanol from the reaction catalyzed by Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB. Reaction conditions: catalyst weight, 0.050 g; temperature, 18 °C; reactant, NO3−, 120 mL; hole scavenger, methanol, 1 mL; Ar, 20 cm3 min−1; solar simulator.
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Figure 9. Illustration of the reaction mechanism for photocatalytic H2 evolution over Pt supported on TiO2 nanostructures in the presence of methanol under simulated solar light irradiation.
Figure 9. Illustration of the reaction mechanism for photocatalytic H2 evolution over Pt supported on TiO2 nanostructures in the presence of methanol under simulated solar light irradiation.
Catalysts 14 00619 g009
Table 1. Box—counting fractal dimensions of Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB.
Table 1. Box—counting fractal dimensions of Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB.
SampleAnalyzed TEM ImageFractal
Dimension		Self-Similarity Domain (nm)Correlation
Coefficient		Observations
Pt/TiO2-0TBFigure 3(A1)1.847 ± 0.0062–1160.999Fractal behavior with a large self-similarity domain characterizing the TiO2 substrate
Pt/TiO2-0TBFigure 3(A2)1.623 ± 0.014
1.893 ± 0.009		0.1–1
1–29		0.999
0.999		Fractal behavior of Pt nanoparticles;
Fractal behavior of TiO2 substrate
Pt/TiO2-3TBFigure 3(B1)1.672 ± 0.013
1.883 ± 0.007		0.1–1
1–59		0.999
0.999		Fractal behavior of Pt nanoparticles;
Global fractal behavior of TiO2 substrate
Pt/TiO2-3TBFigure 3(B2)1.700 ± 0.026
1.849 ± 0.019		0.1–3.4
3.4–36		0.997
0.999		Pt nanoparticles fractal structure;
TiO2 fractal behavior
Pt/TiO2-6TBFigure 3(C1)1.745 ± 0.008
1.844 ± 0.008		0.2–4
4–116		0.998
0.999		Bi-modal fractal behavior of the structure
Pt/TiO2-6TBFigure 3(C2)1.579 ± 0.007
1.881 ± 0.009		0.1–1
1–37		0.999
0.999		Fractal behavior of Pt nanoparticles;
Fractal behavior of TiO2 substrate
Table 2. Box-counting fractal dimensions of Pt nanoparticles.
Table 2. Box-counting fractal dimensions of Pt nanoparticles.
SampleAnalyzed TEM ImagePt Fractal
Dimension		Self-Similarity Domain (nm)Correlation
Coefficient		Observations
Pt/TiO2-0TBFigure 3(A1)1.53 ± 0.03
1.44 ± 0.04		0.3–1.3
14.0–114.1		0.998
0.994		Short-scale correlations;
Long-scale correlations
Pt/TiO2-0TBFigure 3(A2)1.75 ± 0.02
1.06 ± 0.05		0.1–1.1
9.0–25.7		0.999
0.993		Short-scale correlations;
Long-scale correlations
Pt/TiO2-3TBFigure 3(B1)1.65 ± 0.02
1.30 ± 0.04		0.1–1.0
10.8–52.2		0.999
0.994		Short-scale correlations;
Long-scale correlations
Pt/TiO2-3TBFigure 3(B2)1.74 ± 0.02
1.27 ± 0.01		0.1–1.0
8.3–30.8		0.999
0.999		Short-scale correlations;
Long-scale correlations
Pt/TiO2-6TBFigure 3(C1)1.58 ± 0.02
1.54 ± 0.02		0.2–1
22.3–110.9		0.999
0.998		Short-scale correlations;
Long-scale correlations
Pt/TiO2-6TBFigure 3(C2)1.62 ± 0.02
1.42 ± 0.03		0.1–1
11.8–33.6		0.998
0.998		Short-scale correlations;
Long-scale correlations
Table 3. Surface element content estimated from XPS measurements on reduced Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB samples.
Table 3. Surface element content estimated from XPS measurements on reduced Pt/TiO2-0TB, Pt/TiO2-3TB, and Pt/TiO2-6TB samples.
C
(at.%)		O
(at.%)		Ti
(at.%)		Pt
(at.%)
Pt/TiO2-0TB30.9050.1018.800.20
Pt/TiO2-3TB54.3033.3012.200.10
Pt/TiO2-6TB18.0060.0021.700.20
Table 4. Binding energy (BE) for the decomposed Pt 4f, O 1s, Ti 2p core level peaks in the XPS spectra.
Table 4. Binding energy (BE) for the decomposed Pt 4f, O 1s, Ti 2p core level peaks in the XPS spectra.
SampleBE (eV)
Pt 4f7/2Pt 4f5/2O 1sTi 2p3/2Ti 2p1/2
Pt/TiO2-0TBA-71.08
B-73.41		C-74.55
D-76.29		A-530.04
B-531.38
C-532.23		A-458.75B-464.52
Pt/TiO2-3TBA-70.80
B-72.10
C-73.76		D-74.27
F-75.64
E-76.94		A-530.33
B-531.68
C-532.65		A-459.04B-464.75
Pt/TiO2-6TBA-71.16
B-73.46		C-74.63
D-76.40		A-530.11
B-531.33
C-532.19		A-458.85B-464.63
Table 5. Calculated band gap energy values (direct electronic transition type) of Pt/TiO2-0TB, Pt/TiO2-3TB, Pt/TiO2-6TB, and TiO2 as a reference.
Table 5. Calculated band gap energy values (direct electronic transition type) of Pt/TiO2-0TB, Pt/TiO2-3TB, Pt/TiO2-6TB, and TiO2 as a reference.
CatalystEg (eV)
TiO23.44
Pt/TiO2-0TB3.14
Pt/TiO2-3TB3.06
Pt/TiO2-6TB3.01
Table 6. The Pt dispersion, the corresponding particle size, and the metal surface area obtained from CO pulse chemisorption.
Table 6. The Pt dispersion, the corresponding particle size, and the metal surface area obtained from CO pulse chemisorption.
CatalystMonolayer Uptake Volume
(μmol/g)		Metal Surface Area
(m2/g)		Pt Size 1
(nm)		Pt Size 2
(nm)		Dispersion
(%)
Pt/TiO2-0TB16.520.801.171.532.23
Pt/TiO2-3TB7.750.372.492.515.12
Pt/TiO2-6TB17.460.841.101.234.07
1 from CO chemisorption measurements. 2 from TEM analysis.
Table 7. Comparative results of the studied photocatalysts with previous works reported in the literature.
Table 7. Comparative results of the studied photocatalysts with previous works reported in the literature.
PhotocatalystsSynthesis MethodExperimental
Conditions		H2 Evolution
Rate		Ref
1%Pt/TiO2Photo—reduced Pt clusters on the surfaces of TiO2 NpSolar simulator;
TEOA−10%Methanol/Water		1638.25
µmol·g·h−1		[61]
Pt-TiO2/MoSe2Impregnation chemical reduction methodXe lamp (λ ≥ 420 nm); 1:1 Methanol/Water1337
µmol·g−1·h−1		[62]
1%Pt/TiO2Photodeposition500 W- UV Hg lamp; 1:10 Methanol/Water6000
µmol·g−1·h−1		[63]
0.2%Pt/TiO2Impregnation methodUV lamps, max 254 nm);
1:3 Methanol/Water		3400
µmol·h−1·g−1		[64]
1% Pt-TiO2Incipient wetness impregnation
method.		Solar simulator (250–2000 nm, Sun) visible light);
1:4 Methanol/Water		120
µmol·h−1·g−1		[65]
2.1%Pt/TiO2PhotodepositionSolarium lamps, (300 and 400 nm) max, at 365 nm3068.8
µmol·h−1		[66]
1%Pt/TiO2Direct reduction of Pt Np on the TiO2 surface in the presence of triblock copolymersSimulated solar irradiation
AM (1.5);
1% Methanol/
Aqueous solution		4170
µmol·h−1·gcat−1		This work
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Vasile, A.; Dobrescu, G.; Bratan, V.; Teodorescu, M.; Munteanu, C.; Atkinson, I.; Negrila, C.; Papa, F.; Balint, I. Fractal Behavior of Nanostructured Pt/TiO2 Catalysts: Synthesis, Characterization and Evaluation of Photocatalytic Hydrogen Generation. Catalysts 2024, 14, 619. https://doi.org/10.3390/catal14090619

AMA Style

Vasile A, Dobrescu G, Bratan V, Teodorescu M, Munteanu C, Atkinson I, Negrila C, Papa F, Balint I. Fractal Behavior of Nanostructured Pt/TiO2 Catalysts: Synthesis, Characterization and Evaluation of Photocatalytic Hydrogen Generation. Catalysts. 2024; 14(9):619. https://doi.org/10.3390/catal14090619

Chicago/Turabian Style

Vasile, Anca, Gianina Dobrescu, Veronica Bratan, Mircea Teodorescu, Cornel Munteanu, Irina Atkinson, Catalin Negrila, Florica Papa, and Ioan Balint. 2024. "Fractal Behavior of Nanostructured Pt/TiO2 Catalysts: Synthesis, Characterization and Evaluation of Photocatalytic Hydrogen Generation" Catalysts 14, no. 9: 619. https://doi.org/10.3390/catal14090619

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