Titanium(IV) oxide is commonly used in photocatalysis. However, it has some drawbacks, e.g., a high rate of electron-hole recombination and a wide bandgap. In here, the surface of anodic titanium(IV) oxide (ATO) was modified with metal nanoparticles (gold, silver, and copper) in order to enhance its photoelectrochemical (PEC) and photocatalytic (PC) properties. SEM analysis revealed that Au, Ag, and Cu nanoparticles obtained on an ATO surface by chemical methods had an average diameter of 50 ± 10 nm, 30 ± 6 nm, and 25 ± 3 nm, respectively. Enhancement of photoelectrochemical water-splitting current efficiency in the wavelength range of 300–400 nm was observed due to the occurrence of the Schottky barriers. However, the nanoparticles had no effect on the current efficiency in the range of 400–600 nm which meant that the surface plasmon resonance (SPR) effect was not observed. A rate of methyl red photodecomposition was improved after the modification of the ATO surface. The best results were obtained for ATO decorated with gold nanoparticles.
Titanium(IV) oxide is an n-type semiconductor that is known from its good stability, high resistance to corrosion, nontoxicity, biocompatibility, and low cost of production. Due to these properties, TiO2 is widely used in medicine, industry, and the military. Recently, titanium(IV) oxide nanostructures, like nanoparticles [
As is well known, TiO2 is commonly used in photocatalysis due to its ability to absorb the UV irradiation. It is possible because of the value of its bandgap, which is 3.2 eV and 3.0 eV for anatase and rutile, respectively [
Despite the advantages mentioned above, TiO2 has some crucial drawbacks that limit its utilization in photocatalytic processes. Titanium(IV) oxide has a wide bandgap which allows absorbing light only from the UV range. On the other hand, the solar spectrum consists of approximately 5% of UV and 40% of visible light. That is why TiO2 cannot be used as a photocatalyst under sunlight irradiation which would have been preferable in terms of lowering processing costs. Additionally, the rate of electron-hole recombination is high and significantly decreases photocatalytic efficiency. In order to extend potential applications of TiO2, numerous modifications have been performed that expand the spectrum of the light absorption and enhance the efficiency of photocatalysis and photoelectrocatalysis [
The decoration of the TiO2 surface with metal nanoparticles can be made by using photo- or electrodeposition, precipitation, and ion-exchange techniques. The most commonly used metals are noble metals (Au, Ag, and Pt [
On the other hand, the second phenomenon that can occur at the metal-semiconductor interface is the surface plasmon resonance (SPR). Surface plasmons are defined as the collective oscillation of conduction electrons (surface plasmons) at the interface between a metal and other materials. Under light irradiation, charge-density oscillations might occur and the formation of the electric field can be observed [
Silver, gold, and copper nanoparticles, which are characterized by the presence of absorption bands at 400 nm, 520 nm, and 580 nm, respectively, are of particular interest [
Photoelectrochemical (PEC) or photocatalytic (PC) properties of anodic materials modified with metal nanoparticles (where: DSSC dye-sensitized solar cell).
Material | Deposited metal nanoparticles | PEC or PC performance | Experimental details | Ref. | |
---|---|---|---|---|---|
Before deposition | After deposition | ||||
Anodic TiON nanotubes | Ag | 6 mA cm−2 | 14 mA cm−2 | As 1.0 V SCE in 0.1 M KOH (PEC) | [ |
Anodic TiO2 nanotubes | Cu | 0.055 mA cm−2 | 0.747 mA cm−2 | As 1.0 V vs. SCE in 0.1 M Na2S and 0.1 M Na2SO3 (PEC) | [ |
Anodic TiO2 nanotubes | Au | Enhancement of 20.5% in photocurrent | DSSC (PEC) | [ | |
Anodic TiO2 nanotubes | Ag | 9.1 mA cm−2 | 12.2 mA cm−2 | DSSC (PEC) | [ |
Anodic TiO2 nanotubes | AgCu | ~30 |
~70 |
0.5 V vs. Ag/AgCl/0.1 M KCl (PEC) | [ |
12% enhancement in phenol degradation efficiency observed | 60 min of UV irradiation (PC) | ||||
Anodic TiO2 nanotubes | Au | 6.95 |
217.7 |
Photocatalytic H2 production (PC) | [ |
In this work, we present the photoelectrochemical and photocatalytic properties of anodic titanium(IV) oxide decorated with chemically deposited metal nanoparticles. The anodic materials were compared in terms of the morphology, amount of nanoparticles on the oxide surface, generated photocurrent at different wavelengths and applied potentials, and rate of methyl red decolorization. Additionally, we showed whether the SPR effect or Schottky barrier occurs during surface modification of anodic TiO2 under the proposed experimental conditions.
Titanium foil (99.5% purity, 0.25 mm thick) was polished electrochemically, and then chemically [
The morphology of ATO layers was characterized using a field emission scanning electron microscope (SEM, Hitachi S-4700).
Copper nanoparticles (CuNPs) were deposited using a SILAR method [
In order to confirm the presence of gold and silver nanoparticles, the postreaction solutions were analyzed by using a UV-Vis spectrophotometer (Evolution 220, Thermo Fisher Scientific). UV-Vis spectra of Au and Ag nanoparticles were recorded against distilled water in the range of 200–780 nm.
Photoelectrochemical measurements were performed using a three-electrode cell with a quartz window, where the nanostructured ATO layer was used as a working electrode (WE), a platinum foil was used as a counter electrode (CE), and a Luggin capillary with a saturated calomel electrode (SCE) was used as a reference electrode. The generated photocurrents were measured using a photoelectric spectrometer (Instytut Fotonowy, Poland) equipped with the 150 W xenon arc lamp and combined with a potentiostat [
Photodegradation of methyl red, MR (pure p. a., POCH S.A.), was carried in a UV reactor (Instytut Fotonowy, Poland) consisting of 20 UV-A lamps (160 W). All experiments were performed using a 5 mg L−1 dye solution in 0.01 M HCl. For the photodegradation tests, 10 mL of the MR solution was used. During experiments, at given reaction intervals, the concentration of dye was determined spectrophotometrically (Evolution 220 UV-Vis Spectrophotometer, Thermo Scientific) in the range of 200–600 nm. The percentage of dye loss (DEG%) was determined using the following equation:
Typical morphology of anodic TiO2 layers obtained by three-step anodization at the potential of 40 V is shown in Figure
Top view of nanostructured ATO layers ((a), insert – a cross-sectional view) with deposited copper (b), gold (c), and silver (d) nanoparticles.
As shown in Figure
EDS spectra of ATO layers modified with metal nanoparticles (a). UV-Vis spectra of chemically synthesized gold and silver nanoparticle solutions (b).
SEM top view of the ATO layer modified with silver nanoparticles (a). Corresponding distributions of titanium (b), oxygen (c), and silver (d) on the ATO surface.
The ATO layers decorated with noble metal nanoparticles were used as photoanodes for photoelectrochemical water splitting. It was expected that additional maxima in the photocurrent density vs. wavelength spectra (AuNPs ~ 520 nm, AgNPs ~ 400 nm, and CuNPs ~ 570 nm) might be observed as a result of a modification of the anodic TiO2 surface with gold, silver, and copper nanoparticles. However, for all tested anodic materials, there was no significant increase observed in the generated photocurrent in the wavelength range of 400–600 nm. In the case of a photoanode modified with copper nanoparticles, the metal surface could have oxidized and, therefore, the SPR effect was not observed [
Photocurrent density vs. wavelength spectrum measured at 1 V vs. SCE in the wavelength range of 200–800 nm for the ATO layers decorated with gold, silver, and copper nanoparticles.
The complex photoelectrochemical behavior of the tested photoanodes was studied at the potential range of 0–1 V vs. SCE and wavelengths ranging from 300 to 400 nm. The photocurrent densities as a function of incident light wavelength and applied potential were recorded for anodic TiO2 samples annealed at 500°C and decorated with CuNPs, AuNPs, and AgNPs (Figure
Photocurrent density as a function of incident light wavelength and applied potential recorded in 0.1 M KNO3 for the ATO layers annealed at 500°C (a) and decorated with copper (b), gold (c), and silver (d) nanoparticles. IPCE values obtained at 0, 0.5, and 1 V vs. SCE for bare ATO and ATOs decorated with noble metals (e).
In order to verify the photocatalytic effectiveness of ATO layers decorated with metal nanoparticles in decomposition of MR, photodegradation tests were performed. Prior to photocatalytic tests, a series of standard solutions to generate a calibration curve was prepared at the MR concentration range of 0–5 mg L−1. The typical UV-Vis spectra obtained in 0.01 M hydrochloric acid are presented in Figure
UV-Vis spectra of methyl red solutions (0–5 mg L−1) prepared in 0.01 M HCl (a). Calibration curve used for MR determination (b). Photocatalytic degradation of 5 mg L−1 MR under 350 nm UV irradiation in the presence of ATO samples decorated with different nanoparticles. MR concentration (c) and ln(C) (d) vs. irradiation time curves.
Percentage of dye loss (DEG%), and decomposition rate of MR in the presence of ATO-based photocatalysts.
Sample | DEG (%) | k (h−1) |
---|---|---|
ATO | 75 ± 2 | 0.24 ± 0.01 |
ATO CuNPs | 76 ± 2 | 0.22 ± 0.02 |
ATO AuNPs | 84 ± 2 | 0.30 ± 0.01 |
ATO AgNPs | 79 ± 1 | 0.25 ± 0.01 |
MR solution | 12 ± 2 | 0.02 ± 0.01 |
The synthesized Au-ATO photocatalysts have been used multiple times in order to determine the stability of such materials. As can be seen in Figure
ln(C) vs. irradiation time curves of dye decomposition in the presence of the AuNP-ATO photocatalyst (a). MR degradation percentage obtained using the AuNP-ATO photocatalyst in the photocatalytic process (b).
In this study, the effect of the ATO surface decorated with Cu, Au, and Ag nanoparticles on photoelectrochemical and photocatalytic properties were investigated. Nanoparticles were synthesized by a chemical method, and the presence of metals on TiO2 was confirmed by EDS analysis. All obtained materials were used as photoanodes in photoelectrochemical water splitting and as photocatalysts in a methyl red decomposition process. It was found that in studied conditions the Schottky barrier is formed between the metal/ATO interface, which leads to enhancing photocurrent generation in the range of 300–400 nm. However, in the visible light spectrum (400–600 nm) no additional effect from SPR was observed. Nevertheless, deposition of noble metal nanoparticles on the ATO surface resulted in improved efficiencies in both studied processes. It was shown that deposition of gold nanoparticles resulted in high photocurrent densities (100
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge financial support from the Jagiellonian University. The SEM imaging was performed in the Laboratory of Field Emission Scanning Electron Microscopy and Microanalysis at the Institute of Geological Sciences, Jagiellonian University, Poland.