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Article

The Sensitization of TiO2 Thin Film by Ag Nanoparticles for the Improvement of Photocatalytic Efficiency

by
Vytautas Kavaliūnas
*,
Paulius Čeplikas
,
Mantas Sriubas
and
Giedrius Laukaitis
Department of Physics, Kaunas University of Technology, Studentų St. 50, LT-51368 Kaunas, Lithuania
*
Author to whom correspondence should be addressed.
Appl. Sci. 2022, 12(11), 5725; https://doi.org/10.3390/app12115725
Submission received: 15 May 2022 / Revised: 30 May 2022 / Accepted: 2 June 2022 / Published: 4 June 2022
(This article belongs to the Section Chemical and Molecular Sciences)
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Abstract

:
The formation of Ag nanoparticles on the surface of TiO2 (AgNP/TiO2) to enhance photocatalytic efficiency was studied. The Ag nanoparticles (AgNP) size, form, and distribution dependence on the initial thickness of Ag thin films, annealing temperature, and time were analyzed. The optimal annealing temperature of 400 °C and annealing time of 60 min were chosen to form AgNP from the initial Ag thin films with a thickness of 5, 7.5, and 10 nm. The formation of AgNP was done on amorphous TiO2 (a-TiO2), which crystallized into the anatase phase after the annealing. The photocatalytic efficiency (k–degradation rate constant, Defi–degradation efficiency) was evaluated by the photodegradation of Rhodamine B aqueous solution. The results suggested that the highest photocatalytic efficiency of Rhodamine B aqueous solution was reached where the average diameter (DA) of AgNP was ~38 nm (k38 = 0.017 min−1, Defi_38 = 63.5%), compared to 27 and 82 nm (k27 = 0.012 min−1, Defi_27 = 51.2% and k82 = 0.011 min−1, Defi_82 = 52.1%, respectively). The acquired results did not show clear correlation between the size and distribution of the AgNP on the TiO2 surface and photocatalytic efficiency. Nevertheless, the results suggest that AgNP can enhance the photocatalytic efficiency of TiO2 thin films (kTiO2 = 0.008 min−1, Defi_TiO2 = 36.3%).

1. Introduction

Hazardous organic contaminants from many sources, such as farms and factories, are frequently found in wastewaters and can cause environmental issues [1]. Therefore, photocatalytic degradation of various organic compounds in water has been investigated in the last few decades [2]. Various materials and their composites can decompose organic dyes in water: binary oxides (e.g., TiO2, ZnO, ZrO2), ternary oxides (e.g., SrTiO3, LiTaO3, NaTaO3), solid solutions (e.g., βAgAl1₋xGaxO2, (AgNbO3)1₋x(NaNbO3)x), nanocomposite materials (e.g., TiO2 nanorods, ZnO nanorods, TiO2/ZnS) [3], or hybrid photocatalysts (Bi3TaO7/Ti3C2, Bi2S3/Bi2O3/Bi2O2CO3) [4,5]. Of the many different photocatalysts, TiO2 has been the most widely studied and used in many applications because of its strong oxidizing ability to decompose organic pollutants superhydrophilicity, chemical stability, long durability, nontoxicity, low cost, and transparency to visible light [6]. When TiO2 or another photocatalyst absorbs a photon with energy equal to or higher than the band gap energy (Eg), electrons (e⁻) jump from the valence band (VB) to the conduction band (CB), and an electron-hole (h+) in the valence band forms. The e- and h+ pair can recombine or move to the TiO2 surface. These charge carriers react with the surface hydroxyl group or water and dissolved oxygen to produce hydroxyl (OH), peroxide (H2O2), and superoxide (O2⁻) radical anions. Later, the ·OH radicals react with the organic molecules to form CO2, H2O, NO3−, and NH4+ compounds [7]. However, the major drawbacks of TiO2 as a photocatalyst are wide band gap, short recombination time of charge carriers in bulk or on the surface, and reusability if it is in powder form [8]. This limits the application of TiO2 in the visible light region, affects photocatalytic efficiency, and slows down commercialization. Researchers proposed many methods to solve these problems: heterojunction formation [9], doping [10], noble metal deposition, inorganic acids modification, dye sensitization, or metal ion implantation [11]. For example, an increase in photocatalytic efficiency was observed when TiO2 thin films were doped with a low amount of Cu (0.6 wt.%; kCu = 0.022 min−1), followed by Mg (0.9 wt.%; kMg = 0.019 min−1) and Ni (0.5 wt.%; kNi = 0.013 min−1) [8]. The metal nanoparticles/semiconductor heterojunctions also have great potential in high-performance photocatalysis (plasmonic photocatalysis) [12]. The benefits of metal nanoparticles/semiconductor heterojunctions emerge via metal-semiconductor junction and localized surface plasmon resonance (LSPR). The metal/semiconductor junction extends the e⁻ and h+ separation time and increases the charge transfer rate. The LSPR enhances visible light response, UV light absorption, local heating effect, and reduces e⁻ and h+ diffusion length, etc. [13]. Different effects can be obtained depending on the wavelength of the light. [13,14]. After UV light irradiation, only the semiconductor is excited. Metal nanoparticles act as sinks for photo-induced electrons due to the Schottky barrier [15]. It prolongs the lifespan of photoelectrons, i.e., slows down the recombination process [16]. After VIS light irradiation, LSPR occurs. Three outcomes are possible for LSPR: photon scattering, plasmon resonance energy transfer, and hot electron transfer [14]. These processes increase charge carrier concentration or, in other words, photocatalytic efficiency. After UV-VIS light irradiation, a synergetic effect of UV and VIS light occurs [17].
Thin films technology could solve the reusability problems. Thin films of the desired material are deposited in various ways: spin coating [18], chemical vapor deposition (CVD) [19], e-beam evaporation, magnetron sputtering [20], etc. The magnetron sputtering technique has advantages against chemical methods such as controllable structure, stoichiometry, and thickness, good adhesion, homogeneity, low concentration of impurities, etc. The properties of thin films can be controlled by changing sputtering power, gas pressure, the ratio between oxygen and argon, the distance between sample and target, or even the deposition angle [21,22,23,24,25]. The nanoparticles can also be formed using various techniques, but solid-state dewetting is the simplest and probably cheapest method [26]. Pre-existing defects, such as holes, grain boundaries, and thin-film edges, are common in thin films. When such metallic thin films are heated to high enough temperatures, they often dewet the substrate and form isolated islands. The flat films start to form a rim at the grain boundaries, the edges of the film, or other defects. The mass diffuses from the triple point of grain boundaries (or from the defect site) to the flat film resulting in a rim on the surface [27]. The holes expand and develop a thickened surface around them (edge retraction) due to the local curvature gradient at their edges. The net curvature decreases as the rim gets thicker, and edge retraction slows down. The result is a thick rim and valley behind it [28]. From this moment, two outcomes, pinch-off and fingering instability, are possible [29]. The rim cuts off the rest of the mass during the pinch-off process. On the other hand, the rim starts to form finger-like structures of the film during the fingering process if coalescence of the holes occurs. The last part of this evolution is Rayleigh instability, which causes the fingers or wire-like structures to decompose into nanoparticles [30]. The kinetics of the dewetting process and the properties of nanoparticles mainly depend on the temperature, annealing time, and thickness of thin films.
The aim of this paper is to evaluate photocatalytic efficiency by investigating the photodegradation of Rhodamine B aqueous solution under UV light using prepared AgNP/TiO2 structures as photocatalysts. For this reason, AgNPs were formed on the surface of TiO2 thin films using the solid state dewetting technique. Later, the size, distribution, structure, and photocatalytic activity of AgNPs on TiO2 thin films were analyzed.

2. Materials and Methods

2.1. The Preparation of TiO2 Thin Films

TiO2 thin films (hTiO2 = 100 nm) were deposited on glass substrates using a reactive magnetron sputtering technique (PVD-75 Kurt J. Lesker). Before the deposition, a high vacuum (up to 4 × 10−3 Pa) was reached using a dual-stage rotary vane and turbo-molecular pumps. Then, the chamber was filled with Ar and O2 gases (99.999% purity) with a ratio of 20% and 80%, respectively, while ~1.87 Pa pressure was reached. Two Ti targets (99.995% purity) with a diameter of 50.8 mm were used for the deposition. A 1.6 nm/min growth rate using a DC power supply at 250 W sputtering power was achieved.

2.2. Formation of Ag Nanoparticles on the TiO2 Surface

The Ag thin films (hAg = 5; 7.5; 10 nm) were deposited on the surface of TiO2 (Ag/TiO2) by using a magnetron sputtering technique. After creating a high vacuum (up to 4×10−3 Pa), the chamber was filled with a high purity Ar gas until a working pressure of ~2.4 Pa was reached. One Ag target (99.995% purity) with a diameter of 50.8 mm was used for the deposition of Ag thin films. A growth rate of 2.5 nm/min for Ag thin films was reached using a DC power supply at 31 W sputtering power. AgNPs on the TiO2 surface (AgNP/TiO2) were formed using the solid-state dewetting (SSD) process. Thus, Ag/TiO2 structures were annealed immediately after deposition in a vacuum chamber (2.4 Pa Ar gas pressure) at different temperatures (Ta) and times (ta). Since the formation of AgNPs was carried out in a vacuum, the oxidation of silver nanoparticles should be negligibly low. Furthermore, previous research suggests that the concentration of Ag+1 and Ag+3 oxidation states were not higher than 17%, even though the calcination was done in a furnace under constant Ar gas flow [31].

2.3. Morphological and Structural Analysis

The crystal structure of deposited TiO2 thin films was investigated by an X-ray diffractometer (XRD) Bruker D8 Discover (Bruker, Billerica, MA, USA) at 2θ in a range of 20° to 70° (with a 0.01° step) using a Cu Kα (λ = 1.54059 Å) radiation. The peak analysis was carried out by using a “Match!” crystallographic analysis software. The morphology of TiO2 thin films and AgNP/TiO2 structures were analyzed by a scanning electron microscope (SEM) Hitachi S-3400N (Hitachi High-Technologies Corporation, Tokyo, Japan). The obtained surface images were analyzed by “ImageJ” software to evaluate the parameters of formed AgNPs. The optical properties of prepared samples were measured by a UV-Vis spectrophotometer USB4000 (Ocean Optics Inc., Rochester, NY, USA) in a range of 250 to 800 nm (with a 0.2 nm step) wavelength, and the spectra were analyzed by “OceanArt” software.

2.4. Photocatalytic Efficiency Evaluation

The photocatalytic efficiency was evaluated by the photodegradation of RhB (rhodamine B) (10−5 Mol/L) solution under UV-C (254 nm) light irradiation. The samples were immersed in the beaker with 50 mL of RhB solution and placed on the magnetic stirrer under the UV-C light irradiation. Additionally, oxygen gas was introduced into the solution during the measurements, with a 3.6 L/h flow rate, to slightly enhance the photodegradation of RhB. The measurements were conducted in the following order. First, the setup was left in the dark for 30 min to reach the equilibrium between the photocatalyst and RhB solution. Then, the specimen of ~3 mL of RhB was taken, and the light absorbance was measured (which corresponds to the C0). The subsequent measurements were taken under UV-C irradiation every 20 min. The obtained absorbance values corresponded to C1, C2, C3, etc. The taken RhB solution was returned to the beaker after each measurement. The light absorbance was measured by a UV-Vis spectrophotometer USB4000 (Ocean Optics Inc., Rochester, NY, USA) in the 500 nm–600 nm range, and the spectra were analyzed using the “OceanArt” software. The degradation rate constant (k) was calculated using this formula [32]:
k = − l n ( C i C 0 ) t ,
where C 0 is initial concentration of RhB, and C i is concentration of RhB at time t .
While the degradation efficiency Defi was calculated using the formula [33]:
D e f i = C 0 + C i C 0 ⋅ 100 % ,

3. Results and Discussion

3.1. The Synthesis of the AgNP/TiO2 Structures

The kinetics of the SSD process depend mainly on the three factors: annealing temperature, annealing time, and initial thickness of thin films [34]. Thus, the mentioned dependencies were analyzed. The Ag thin films with an initial thickness of 10 nm were deposited on the TiO2 surface and annealed at different temperatures (Ta): 300, 350, 400, and 450 °C for 60 min to observe the SSD dependence on the annealing temperature. The highest circularity of AgNPs was 0.918 (0—line, 1—perfect circle) for Ag/TiO2 structures annealed at 400 °C, according to the analysis of the surface images of AgNP/TiO2 (Figure 1). The coverage area and average diameter were 23.18% and 81.97 nm, respectively. Furthermore, the highest ordering (size distribution and coverage) of AgNPs was observed for Ag thin films annealed at 400 °C. Thus, the optimal annealing temperature for Ag thin films with an initial thickness of 10 nm is 400 °C. Furthermore, Serrano et al. found that thicker Ag thin films require higher annealing temperatures for the complete transition from thin films to nanoparticles through the dewetting process [35]. Based on their study, the highest ordering was achieved when Ag thin films of 10 nm thickness were annealed at 400 °C, while further increase in temperature resulted in agglomeration of nanoparticles to larger 3D structures.
Further analysis of AgNPs formation was done by annealing the Ag thin films of 10 nm thickness at 400 °C using different annealing times, i.e., 10, 20, 40, and 60 min. (Figure 2). After 10 min of annealing, the fractal-like structure was observed. This is the result of the fingering process [36]. The Ag thin films dewetted into smaller AgNPs with an average diameter of ~53.38 nm, when increasing the annealing time to 40 min. After 60 min of annealing, AgNPs agglomerated into bigger islands with an increased average diameter of ~81.97 nm. Therefore, 60 min of annealing time was considered optimal for the evenly distributed formation of AgNPs.
Given these results, further analysis was done with AgNPs obtained by annealing Ag/TiO2 structures with initial Ag thin film thickness of 5, 7.5, and 10 nm at 400 °C temperature for 60 min. The results were AgNP/TiO2 structures where the average diameters of AgNPs were 27, 38, and 82 nm, respectively (Figure 3 and Table 1). The surface resistance of the deposited Ag thin films was measured, resulting in the sheet resistance of 16.96 kΩ for Ag thin films with a thickness of 10 nm. Thus, the Ag thin film was continuous, whereas Ag thin films with a thickness of 5 and 7.5 nm exhibited an insulating behavior, suggesting that thinner Ag thin films were discontinuous. It agrees with the Huang et al. study, where the sheet resistance drastically increases with a thickness lower than 8 nm for Ag thin films [37]. Moreover, these findings explain the size dependence of AgNPs on the initial thickness of Ag thin films. It is known that the number of holes is inversely proportional to the thickness of continuous films, whereas discontinuous thin films have holes naturally, and the lower number of holes in thicker films results in the formation of enlarged AgNPs.
The as-deposited TiO2 and Ag/TiO2 did not show any particular peaks (Figure 4) that are attributed to the crystal phase of TiO2, except the Ag/TiO2 (with initial Ag thin film thickness of 10 nm) showing the corresponding peak of Ag{111} facet at 38.16° [38]. However, after the SSD process, peaks corresponding to the TiO2 anatase phase appeared at 25.43°, 38.17°, 48.18°, 54.04°, and 55.15°, with an Ag peak at 44.44° [38,39,40]. It suggests that the amorphous-to-anatase TiO2 phase transition occurred during the annealing process, which might positively affect the photocatalytic efficiency of prepared structures [41].
The LSPR effect can be observed in the transmittance spectra of AgNP/TiO2 structures (Figure 5). The transmittance of visible light reached minimum values in the 550–650 nm range. It can be seen that the LSPR effect increased with the size of AgNPs on the surface of TiO2 thin films.

3.2. Photocatalytic Efficiency

The evaluation of photocatalytic efficiency for prepared AgNP/TiO2 was done by the photodegradation of RhB solution under UV-C light irradiation. The RhB solution has the highest light absorbance at ~550–560 nm wavelength. Thus, this range was taken during the analysis. The light absorbance at 0 min attributes to the initial concentration of RhB (C0/C0), and the latter light absorbance attributes to the decrease in concentration after 20, 40, 60, and 80 min and is considered as C1/C0, C2/C0, C3/C0, C4/C0 respectively. The 50% decolorization of RhB was reached in ~50 min for AgNP/TiO2, where an average AgNPs diameter was ~38 nm (Figure 6a), whereas the 50% decolorization of RhB by using the AgNP/TiO2 with an average AgNPs diameter of 27 and 82 nm was reached in ~59 min and 58 min, respectively. Considering the TiO2 as a stand-alone photocatalyst, the same result was reached after ~78 min, and for RhB (without the photocatalyst) only after 100 min.
The degradation efficiency was evaluated after 60 min of UV light irradiation (Figure 6b). Clear dependence between AgNPs diameter and degradation efficiency was not observed. The peak value of photocatalytic efficiency was found for AgNP/TiO2, where an average AgNPs diameter was ~38 nm (k38 = 0.017 min−1, Defi_38 = 63.5%), whereas degradation rate constant and degradation efficiency for AgNPs of <dAg> = 27 nm and <dAg> = 82 nm were k27 = 0.012 min−1, Defi_27 = 51.2% and k82 = 0.011 min−1, Defi_82 = 52.1%, respectively. It was also found that photocatalytic efficiency of AgNP/TiO2, where <dAg> = 38 nm (k38 = 0.017 min−1, Defi_38 = 63.5%) was 1.7 times higher than photocatalytic efficiency of pure TiO2 thin films (kTiO2 = 0.008 min−1, Defi_TiO2 = 36.3%).
Comparison of the results with the literature is complicated due to different experimental conditions, i.e., different initial concentrations of RhB, different active areas of photocatalysts, different light sources, irradiance, etc. Therefore, the results obtained by other authors are contradictory. For example, the 50% decolorization of RhB was reached after ~30 min for anatase thin films by Wang et al. and for TiO2/β-SiC foam by Alle et al., while Wannapop et al. reached 50% decolorization of RhB for TiO2 nanorods only after 4 h [42,43,44]. Nevertheless, our results suggest that AgNPs can enhance the photocatalytic efficiency of TiO2 thin films. In addition, they are similar to Vezirogl et. al. results [45]. Authors also found that TiO2 thin films decorated with AgNPs (<50 nm) have 1.7 times higher photocatalytic efficiency than bare TiO2 thin films.
The photodegradation of RhB using a photocatalyst consists of a series of chemical reactions (Figure 7): the oxidation reaction on the surface of TiO2 with a photogenerated hole–H2O, OH⁻ → OH; the reduction reactions on the surface of AgNPs with a photogenerated electron–O2 → O2−; followed by the degradation of RhB solution itself–H+ →·HO2−/·OH and ·HO2− → CO2, H2O, NOx, etc. The oxidation and reduction reactions occur on the surface of TiO2 (Figure 7a), considering the photodegradation of an aqueous solution of RhB using TiO2 as a photocatalyst. To this matter, there is a higher probability of charge carrier recombination.
AgNPs usually act as electron scavengers [46]. It means that the photogenerated electron is captured (trapped) in AgNPs, where a reduction reaction occurs, and photogenerated holes in TiO2 take place in the oxidation reaction (Figure 7b) [47,48,49,50]. Thus, small and densely packed AgNPs on the surface of TiO2 should increase the probability of charge carrier separation resulting in higher photocatalytic efficiency. However, such behavior was not observed in the experiments. It can be explained by excessive AgNPs amount on the TiO2 surface [46]. Very densely packed AgNPs (DA = 27 nm, n = 398 µm−2) might decrease the photocatalytic activity due to the blocking effect of incoming UV light to the TiO2 surface, which leads to the decreased generation of electrons. Moreover, very densely packed AgNPs might serve as recombination sites. On the other hand, a low number of trapping sites could be created if the density of AgNPs is low (DA = 82 nm, n = 42 µm−2).

4. Conclusions

The AgNPs formation and investigation to enhance the photocatalytic efficiency of TiO2 thin films were investigated in this study. The highest circularity of 0.918 of AgNPs was observed after annealing the Ag/TiO2 at 400 °C. This study suggests that an optimal annealing time of 60 min is required to get the highest possible distribution of AgNPs when annealing Ag films of 10 nm. A shorter annealing process results in fractal-like structures. Moreover, the analysis of AgNPs formation dependency on the initial thickness (hAg) of Ag thin films suggests that thin films with a thickness lower than 10 nm are discontinuous. The results show that the density of AgNPs is inversely proportional to the thickness of Ag thin films: hAg = 5 nm, D = 398 µm−2; hAg = 7.5 nm, D = 229 µm−2; hAg = 10 nm, DA = 42 µm−2. The photodegradation experiments revealed that the photocatalytic efficiency increases for the AgNP/TiO2 structure compared to stand-alone TiO2 (kTiO2 = 0.008 min−1; Defi_TiO2 = 36.3%). The highest efficiency of Defi_38 = 63.5% (k38 = 0.017 min−1) was observed for the AgNP/TiO2, where an average AgNPs diameter was 38 nm, whereas for the AgNPs with DA = 27 nm—Defi_27 = 51.2% (k27 = 0.012 min−1) and DA = 82 nm—Defi_82 = 52.1% (k82 = 0.011 min−1). The nonlinear dependence of degradation efficiency on AgNPs size could be expanded by AgNPs density variation. AgNP could block UV light and act as recombination sites when the density of AgNPs is very high, whereas a small number of trapping sites could be created when the density of AgNPs is low.

Author Contributions

V.K.: Formal analysis, Investigation, Writing—original draft, Writing—review and editing, Visualization. P.Č.: Formal analysis, Investigation. M.S.: Conceptualization, Methodology, Writing—original draft, Writing—review and editing, Supervision, Data curation. G.L.: Conceptualization, Methodology, Resources, Funding acquisition, Project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This project has received funding from European Social Fund (project No 09.3.3-LMT-K-712-01-0162) under grant agreement with the Research Council of Lithuania (LMTLT).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to express their gratitude to the following individuals for their expertise and contribution to the manuscript: Kristina Bočkutė, Paulius Palevičius, Marius Kaminskas, Žilvinas Rinkevičius, and Minvydas Ragulskis.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. The diameter (d) distribution of AgNPs on the TiO2 surface and SEM images of AgNP/TiO2 structures obtained by SSD process (ta = 60 min, hAg = 10 nm) at different Ta: (a,e) 300 °C; (b,f) 350 °C; (c,g) 400 °C; (d,h) 450 °C.
Figure 1. The diameter (d) distribution of AgNPs on the TiO2 surface and SEM images of AgNP/TiO2 structures obtained by SSD process (ta = 60 min, hAg = 10 nm) at different Ta: (a,e) 300 °C; (b,f) 350 °C; (c,g) 400 °C; (d,h) 450 °C.
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Figure 2. The diameter (d) distribution of AgNPs and SEM images of AgNP/TiO2 structures obtained by SSD process (Ta = 400 °C, hAg = 10 nm) using different ta: (a,e) 10 min; (b,f) 20 min; (c,g) 40 min; (d,h) 60 min.
Figure 2. The diameter (d) distribution of AgNPs and SEM images of AgNP/TiO2 structures obtained by SSD process (Ta = 400 °C, hAg = 10 nm) using different ta: (a,e) 10 min; (b,f) 20 min; (c,g) 40 min; (d,h) 60 min.
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Figure 3. The diameter (d) distribution of AgNPs and SEM of AgNP/TiO2 structures obtained by SSD process (Ta = 400 °C, ta = 60 min) using different hAg: (a) 5 nm; (b) 7.5 nm; (c) 10 nm, and (d) the average NP diameter (d) dependence on hAg.
Figure 3. The diameter (d) distribution of AgNPs and SEM of AgNP/TiO2 structures obtained by SSD process (Ta = 400 °C, ta = 60 min) using different hAg: (a) 5 nm; (b) 7.5 nm; (c) 10 nm, and (d) the average NP diameter (d) dependence on hAg.
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Figure 4. XRD of (a) Ag/TiO2 structures (before SSD) and (b) AgNP/TiO2 structures (after SSD).
Figure 4. XRD of (a) Ag/TiO2 structures (before SSD) and (b) AgNP/TiO2 structures (after SSD).
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Figure 5. (a) Transmittance spectra of AgNP/TiO2 structures and (b) LSPR dependency on <dAg>.
Figure 5. (a) Transmittance spectra of AgNP/TiO2 structures and (b) LSPR dependency on <dAg>.
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Figure 6. The photodegradation of RhB under UV-C light irradiation by using an AgNP/TiO2 as a photocatalyst with different NP parameters. (a) the kinetics of the photodegradation process and (b) the degradation efficiency (Defi) of RhB after 60 min of irradiation.
Figure 6. The photodegradation of RhB under UV-C light irradiation by using an AgNP/TiO2 as a photocatalyst with different NP parameters. (a) the kinetics of the photodegradation process and (b) the degradation efficiency (Defi) of RhB after 60 min of irradiation.
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Figure 7. The photodegradation of RhB in an aqueous solution using TiO2 (a) and AgNP/TiO2 (b) as a photocatalyst. CBM—conduction band minimum; VBM—valence band maximum; Eg—energy band gap; EF—Fermi energy level; χ T i O 2 —electron affinity; hν—photon energy; e−—electron; h+—hole.
Figure 7. The photodegradation of RhB in an aqueous solution using TiO2 (a) and AgNP/TiO2 (b) as a photocatalyst. CBM—conduction band minimum; VBM—valence band maximum; Eg—energy band gap; EF—Fermi energy level; χ T i O 2 —electron affinity; hν—photon energy; e−—electron; h+—hole.
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Table
Table 1. The parameters of AgNPs on the surface of TiO2.
Table 1. The parameters of AgNPs on the surface of TiO2.
The Initial Thickness of Ag Thin Films (hAg)5 nm7.5 nm10 nm
Average area (A), nm270513246003
Average diameter (DA), nm273882
Density (n), µm−239822942
Coverage (SC), %283023
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Kavaliūnas, V.; Čeplikas, P.; Sriubas, M.; Laukaitis, G. The Sensitization of TiO2 Thin Film by Ag Nanoparticles for the Improvement of Photocatalytic Efficiency. Appl. Sci. 2022, 12, 5725. https://doi.org/10.3390/app12115725

AMA Style

Kavaliūnas V, Čeplikas P, Sriubas M, Laukaitis G. The Sensitization of TiO2 Thin Film by Ag Nanoparticles for the Improvement of Photocatalytic Efficiency. Applied Sciences. 2022; 12(11):5725. https://doi.org/10.3390/app12115725

Chicago/Turabian Style

Kavaliūnas, Vytautas, Paulius Čeplikas, Mantas Sriubas, and Giedrius Laukaitis. 2022. "The Sensitization of TiO2 Thin Film by Ag Nanoparticles for the Improvement of Photocatalytic Efficiency" Applied Sciences 12, no. 11: 5725. https://doi.org/10.3390/app12115725

APA Style

Kavaliūnas, V., Čeplikas, P., Sriubas, M., & Laukaitis, G. (2022). The Sensitization of TiO2 Thin Film by Ag Nanoparticles for the Improvement of Photocatalytic Efficiency. Applied Sciences, 12(11), 5725. https://doi.org/10.3390/app12115725

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