2.1. Physicochemical Characterization of d-TiO2-Pt/Cu and Magnetic Fe3O4@SiO2/d-TiO2-Pt/Cu Photocatalysts
The preparation of d-TiO2
photocatalysts was based on the hydrothermal method in the oxidative environment (addition of 20–75 mol% of HIO3
). The electron paramagnetic resonance (EPR) analysis confirmed the presence of titanium vacancies with g value of 1.995 after calcination. The signal at EPR spectra for bulk Ti3+
was not observed for pure TiO2
sample, see in Figure 1
The general physicochemical and photocatalytic characteristics of the obtained defective d-TiO2
-Pt/Cu and Fe3
-Pt/Cu samples, i.e., BET surface area, pore volume, calculated bandgap (Eg) and phenol degradation efficiency in UV-Vis and Vis light are presented in Table 1
and Table 2
The BET surface area of pure TiO2
obtained from Titanium(IV) butoxide (TBT) hydrolysis in water and defective TiO2
samples was similar and ranged from 167 to 172 m2
. The specific surface area of the metal-modified d-TiO2
samples fluctuated from 166 to 101 m2
and depended on the type and amount of metallic species deposited on d-TiO2
surface. The samples modified with Pt NPs revealed a higher BET surface area of about 148 m2
compared to d-TiO2
modified with copper oxide (101 m2
), and bimetallic Pt/Cu NPs (152 m2
). The relations between photoactivity in UV-Vis and Vis light range versus BET surface area are also shown in Table 1
and Table 2
. The obtained results indicated that not so much the surface area but rather the presence of Ti defects and modification with metal nanoparticles caused the enhanced photoactivity of the obtained photocatalysts. Moreover, as shown in Table 2
, the addition of surface-modifying metal nanoparticles, as well as further deposition of d-TiO2
-Pt/Cu on magnetic matrice, did not affect the magnitude order of the BET surface area, which remained in the range of 101 to 172 m2
_20-Cu0.1 and d-TiO2
The energy bandgaps for all samples were calculated from the plot of (Kubelka–Munk·E)0.5
versus E, where E is energy equal to hv, and summarized in Table 1
. The samples consist of defective TiO2
exhibited narrower bandgap of 2.7–2.9 eV compared to TiO2
-Pt0.05 photocatalysts. Moreover, for all metal-modified defective photocatalysts, the bandgap value, calculated from Kubelka–Munk, transformation did not change, compared to d-TiO2
matrice, indicating surface modification than doping [11
The XRD patterns for selected d-TiO2
-Pt/Cu and Fe3
-Pt/Cu samples are presented in Figure 1
and Figure 2
, with a detailed phase composition and crystalline sizes for all photocatalysts being listed in Table 3
and Table 4
. Peaks marked “A”, “R”, and “B” corresponds to anatase, rutile, and brookite phases, respectively. Both crystalline structures (anatase and brookite) appeared for pure TiO2
prepared by the sol–gel method. For Pt-modified TiO2
anatase was the major phase, whereas brookite existed as the minor phase. The average crystallite size of anatase was 5–6 nm. The preparation of d-TiO2
photocatalysts proceeded in the oxidative environment. The introduction into the crystal structure of various types of defects promotes the transformation of anatase to rutile at lower temperatures. Therefore, for the samples obtained in the presence of HIO3
as the oxidizing agent, after the annealing process the percentage of anatase (the most intense peak at 25° 2θ, with the (101) plane diffraction, ICDD’s card No. 7206075) was decreased in favor of (110) rutile, with the peak at 31° 2θ (ICDD’s card No. 9004141), even below the anatase to rutile phase transformation temperature [35
]. For the samples d-TiO2
_75 and d-TiO2
_75-Pt0.05, the dominant phase was rutile with a crystallite size of about 6 nm. Further, the surface modification with plasmonic platinum and semi-noble copper did not cause changes in anatase crystallite size, remaining about 5–6 nm size. The percentage of the brookite phase increased to 8.5% and 13% for d-TiO2
_20-Pt0.1/Cu0.1, and d-TiO2
_20-Pt0.1 samples, respectively. It resulted from the additional thermal treatment after metal nanoparticles deposition on the photocatalyst surface. Moreover, Pt and Cu modification of TiO2
did not cause the shift of the peaks in the XRD pattern. The presence of platinum and copper deposited on TiO2
was not approved (no peaks for platinum or copper) due to low content (0.05–0.1 mol%) and nanometric size.
The XRD analysis of Fe3
-Pt/Cu confirmed the formation of a magnetic composite, and, as observed in Figure 3
and Table 4
, there was no significant difference between the diffraction patterns of the obtained magnetic photocatalysts modified with Pt/Cu NPs. The presence of pure magnetite, with diffraction peaks at 30.2°, 35.6°, 43.3°, 57.3°, and 62.9° 2θ corresponding to (220), (311), (400), (511), and (440) cubic inverse spinel planes (ICDD’s card No. 9005813) was confirmed for all Fe3
-Pt/Cu magnetic photocatalysts. The decrease in Fe3
peaks intensity was caused by the formation of tight non-magnetic shell on the core surface, which was previously described by Zielińska-Jurek et al. [26
]. The broad peak at 15–25° 2θ corresponds to amorphous silica [38
]. The content of the magnetite crystalline phase varied from 21% to 28% for Fe3
_20-Pt0.05 and Fe3
_20-Cu0.1, respectively. At the same time, TiO2
crystallite size and anatase to rutile phase content ratio remained unchanged for Fe3
_20 and Fe3
_20-Pt/Cu samples. No other crystalline phases were identified in the XRD patterns, which indicated the crystal purity of the obtained composites.
The photoabsorption properties of metal-modified defective d-TiO2
samples were studied by diffuse reflectance spectroscopy, and exemplary data are shown in Figure 4
. Comparing to pure TiO2
photocatalyst, introducing platinum as a surface modifier caused an increase of absorption in the visible light region, however, without shifting a maximum, as presented for sample TiO2
–Pt0.05. Modification of defective d-TiO2
with Pt and Cu was associated with a further increase of Vis light absorbance and proportional to the amount of the deposited metal. Moreover, the deposition of Pt caused a more significant absorbance increment than the same modification with Cu species.
Defective d-TiO2-Pt/Cu deposited on Fe3O4@SiO2 core were characterized by extended light absorption ranged to 700 nm. It could be observed that the described absorption properties in the Vis light for metal-modified TiO2 and absorption properties of final composites have been preserved.
The presence of Localized Surface Plasmon Resonance (LSPR) peaks for Pt and Cu were confirmed based on DR-UV/Vis spectra measurements with pure TiO2
as a reference (see in Figure 4
c). Platinum surface plasmon resonance was observed at the wavelength of about 410–420 nm [33
]. Electron transfer between Cu(II) and valence band of titanium(IV) oxide could be confirmed by absorption increment from 400 to 450 nm. The typical LSPR signal for zero valent copper at 500–580 nm was not observed, suggesting that Cu is mainly present in its oxidized forms [41
To confirm the presence of noble metal and semi-noble metal NPs on defective TiO2
surface, the XPS analyses for the selected photocatalysts and deconvolution of Pt 4f and Cu 2p were performed, and the results are presented in Figure 5
. Platinum species deposited on the titania surface were designated by deconvolution of Pt 4f peak into two components: Pt 4f7/2
and Pt 4f5/2
. According to the literature, Pt 4f7/2
peak, with binding energies in the range of 74.2 to 75.0 eV, refers to the Pt0
, while Pt 4f5/2
peak, appearing at 77.5–77.9 eV is assigned to Pt4+
]. The main peaks for Cu 2p appeared as Cu 2p3/2
and Cu 2p1/2
at 934 eV and 952 eV. Both of those peaks are commonly attributed to Cu+
]. Obtained data indicated that both Pt and Cu species were successfully deposited on the titania surface.
Moreover, the presence of Pt NPs at the surface of the magnetic nanocomposites was also confirmed by microscopy analysis. As presented in Figure 6
, the formation of SiO2
shell, with a thickness of about 20 nm, tightly covering magnetite nanoparticles was observed. Platinum nanoparticles with a diameter of about 10–20 nm were evenly distributed on the d-TiO2
2.2. Photocatalytic Activity of d-TiO2-Pt/Cu and Fe3O4@SiO2/d-TiO2-Pt/Cu Photocatalysts
The effect of Pt and Cu presence on the properties of defective d-TiO2
photocatalysts was evaluated in reaction of phenol degradation under UV-Vis and Vis light irradiation. The results, presented as the efficiency of phenol degradation as well as phenol degradation rate constant k, are given in Figure 7
and Figure 8
. Additionally, the effect of the electron (e?
), hole (h+
), hydroxyl radical (?
OH), and superoxide radical (?
) scavengers were investigated and presented in Figure 9
Among analyzed metal-modified photocatalysts, TiO2–Pt0.05 revealed the highest phenol degradation in UV-Vis light. After 60 min of irradiation, about 76% of phenol was degraded. After introducing plasmonic platinum and semi-noble copper species as a surface modifiers, UV-Vis photoactivity of defective d-TiO2 samples increased to 59%. The degradation rate constant k increased to 1.47 × 10?2 min?1 compared to d-TiO2_20 (0.79 × 10?2 min?1), and d-TiO2_75 (0.52 × 10?2 min?1) photocatalysts. Nonetheless, the most significant changes were observed during the photocatalytic process in visible light (λ > 420 nm). Modifying with 0.05 mol% of Pt, the surface of almost inactive in Vis light d-TiO2_75 resulted in three-times higher photocatalytic activity under visible light. Therefore, a highly positive effect of metal surface modification of defective d-TiO2 photocatalyst surface was noticed. It resulted from better charge carriers’ separation and decreasing the electron-hole recombination rate. Moreover, the narrower bandgap of the defective d-TiO2 (in comparison with pure TiO2) and modification with Pt possessing surface plasmon resonance properties, could also enhance visible light absorption and consequently led to photocatalytic activity increase.
Pure magnetite, coated with inert silica, did not affect the photocatalytic process. Furthermore, the Fe3
composite modified with Pt NPs, and bimetallic Pt/Cu NPs revealed the highest photocatalytic activity in Vis light range. The phenol degradation rate constant in Vis light was 2-times higher for Fe3
-Pt/Cu compared to Fe3
sample. However, the obtained magnetic photocatalysts had similar photocatalytic activity in UV-Vis light, almost regardless of the surface modification of d-TiO2
with noble metals. It probably resulted from larger Pt particles (~20 nm) deposition at the surface of Fe3
composite than for TiO2
–Pt0.05 with particles size of about 2–3 nm. Previously, we have reported that the size of noble metal nanoparticles, especially platinum, deposited on the TiO2
surface strictly depends on the semiconductor surface area, as well as its crystal lattice defects [33
]. Fine metal particles are produced on the TiO2
surface with a developed specific surface area with a high density of oxygen traps and nucleation sites, and the highest photocatalytic activity is noticed for Pt-modified photocatalyst, where the size of Pt is below 3 nm [33
]. In the present study, Pt nanoparticles’ average diameter was about 20 nm as a result of the deposition of Pt ions and their reduction on formed particles’ defects. Therefore, the lower metal/semiconductor interface resulted in a decrease in photocatalytic activity under UV-Vis light irradiation.
For the final stability and reusability test, the most active defective photocatalyst was selected. For sample d-TiO2
_20/Pt0.1/Cu0.1, three 1-h-long subsequent cycles of phenol degradation under UV-Vis light were performed. The obtained results are presented in Figure 9
There was no significant change in phenol degradation rate constant after the second and third cycles. Thus, the analyzed photocatalyst revealed good stability and reusability.
Furthermore, the reactive species were investigated to understand the photocatalytic reaction mechanism. Benzoquinone (BQ), silver nitrate (SN), ammonium oxalate (AO), and tert-butanol (t-BuOH) were used as superoxide radical anions (·O2?
), electrons (e?
), holes (h+
), and hydroxyl radicals (·OH) scavengers, respectively. Obtained results, presented as phenol degradation rate constant k, in comparison to the photodegradation process without scavengers, are presented in Figure 10
. The most significant impact on phenol degradation reaction in the presence of metal-modified d-TiO2
was observed for superoxide radicals. After introducing to the photocatalyst suspension BQ solution, the phenol degradation efficiency was significantly inhibited. A slight decrease was also observed in the presence of SN as an electron trap. On the other hand, the addition of AO and t-BuOH did not decrease the phenol degradation rate.
Modification of TiO2
resulted in the shift of the valence band as was revealed from the analysis of Mott–Schottky plot, where the relation between applied potential vs. Csc?2
is presented (see in Figure 11
). According to the intersection with E axis the flat band potential was estimated. In the case of pure titania it equals to ?1.2 V, whereas for d-TiO2
_20-Pt0 the value of ?1.13 was reached. In order to prepare energy diagram of both materials given in Figure 12
, the values of bandgap energy was taken into account. As could be seen, for the modified material the position both the conduction and valence band are shifted. According to Monga et al. [46
] the Schottky barrier formed at the metal-TiO2
interface affecting the efficiency of e- transfer. The lowering of the CB band edge is in accordance with the literature indicating that the work function of the metal prone decrease of the CB location. Then, the Schotky barrier is decreased at the metal/semiconductor heterojunction. As a result, the transfer of the photoexcited electron from metal NPs to titania is facilitated and plays important role in photocatalytic activity improvement. The introduction of titanium defects to the TiO2
crystal structure also resulted in narrowing the bandgap from 3.2 to 2.7 eV.
Based on the presented results, a schematic mechanism of UV-Vis phenol degradation in the presence of metal-modified defective Fe3
-Pt/Cu photocatalyst was proposed and shown in Figure 12
. After irradiation of the photocatalyst surface with UV-Vis light, electrons from the Pt are injected to the conduction band of titania and then utilized in oxygen reduction to form reactive oxygen radicals. The path of phenol degradation led through several intermediates, such as benzoquinone, hydroquinone, catechol, resorcinol, oxalic acid, and finally, to complete mineralization to CO2
]. An analysis of possible charge carriers’ impact revealed that for photoactivity of d-TiO2
-Pt/Cu, they are responsible for mainly generated superoxide radicals. The phenol degradation mechanism proceeded by the generation of reactive oxygen species, e.g., ?
, which attacked the phenol ring, resulting in benzoquinone and hydroquinone formation confirmed by high-performance liquid chromatography (HPLC) analyses. Moreover, during the photoreaction, the concentration of formed intermediates decreased, which suggests mineralization of recalcitrant chemicals to simple organic compounds.