1. Introduction
Titanium dioxide (TiO
2) is known as one of the most popular photoactive materials. It has emerged as an excellent photocatalyst for environmental applications due to its low cost, low toxicity, outstanding chemical stability, and unique photochemical properties. However, there are some properties which need to be improved for practical applications under visible light—e.g., the large band gap energy (∼3.2 eV for anatase) and fast recombination of photo-generated electron-hole pairs [
1].
Up to now, many papers have reported different strategies in order to enhance the photoactivity of TiO2, such as impregnation with dye sensitizers and doping with nonmetal and metal elements.
Dye sensitizers such as quinizarin and zinc protoporphyrin are commonly used to modify the TiO
2 surface. These chromophore compounds are able to absorb visible light and excite electrons. The excited electrons then migrate to the conduction band of TiO
2, leading to the formation of reactive oxygen species (ROS) [
2,
3,
4].
On the other hand, doping with nonmetals, including N, F, S, C, and P, has been explored to extend the light absorption of TiO
2 into the visible-light region. Nitrogen has been proven and considered as an effective dopant to narrow the band-gap energy due to its atomic size comparable to that of oxygen, high electronegativity and ionization energy, marked thermal stability, and cost-effectiveness [
1,
5].
Meanwhile, the incorporation of noble metals (such as Au, Ag, Pt, Cu, and Pd) onto the surface of N-TiO
2 is also a favorable strategy to overcome the problem of fast recombination of the photo-generated electron-hole pairs and to improve the charge transfer [
6,
7,
8,
9]. Among them, Ag is the most suitable candidate for industrial applications due to its relatively low cost and easy preparation [
10,
11,
12].
In addition, Ag nanoparticles also have been known to display strong cytotoxicity toward a wide range of bacteria, including
Escherichia coli [
13],
Staphylococcus aureus [
14],
Acinetobacter baumannii [
15], etc. The Ag nanoparticles can simply come into contact with the cell surfaces and destroy the membranes to inactivate bacteria. In another way, the inactivation process can be initiated by interaction of the bacteria with ROS (
•O
2−,
•OH) in the photocatalytic system [
16,
17]. Therefore, in addition to enhancing the photoactivity of N-TiO
2, Ag doping is also expected to result in an antibacterial effect for selected microorganisms.
Typically, when Ag/N-TiO
2 catalysts are illuminated by light, electron-hole (e
−, h
+) pairs form, and then the interfacial charge transfers to Ag nanoparticles through the formation of Schottky barriers can suppress recombination of electrons and holes and extend their lifetimes in the Ag/N-TiO
2 system. The surrounding O
2 molecules can capture the electrons to form superoxide anion radicals (
•O
2−), and H
2O molecules can be oxidized in the presence of holes to form hydroxyl radicals (
•OH), as shown in
Figure 1. These ROS (
•O
2−,
•OH) have strong oxidation potentials and can degrade numerous organic, frequently bio-resistant materials into harmless products [
18,
19,
20].
Gao et al. prepared Ag/N-TiO
2 via hydrothermal method with various Ag concentrations. The photocatalytic activity of an as-prepared sample was examined in Rhodamine B (RhB) solution under visible-light irradiation. It was found that the photocatalytic performance of Ag/N-TiO
2 was affected by the concentration of Ag nanoparticles. It was reported that in the first period, the photocatalytic activity increased upon increasing Ag content, and then dropped above the optimal Ag content [
10]. Gaidau et al. synthesized Ag/N-TiO
2 grains by using electrochemical method. The photocatalytic experiment with the Orange II dye demonstrated that the photocatalytic activities of TiO
2 under visible light could be improved by the synergistic effect of N doping and Ag modification [
14]. Sun et al. successfully fabricated an Ag/N-TiO
2 catalyst via an in situ calcination procedure, with titanium nitride (TiN) and silver nitrate (AgNO
3) as starting materials. The catalyst displayed an enhanced light absorption and a red shift of the optical edge compared to pure TiO
2 and N-TiO
2. Under visible-light irradiation, a superior Methylene Blue (MB) degradation over Ag/N-TiO
2 was also found compared to N-TiO
2 [
21]. However, these methods also involved special equipment or complex preparation processes. It is still necessary to develop more facile and efficient approaches. In addition, using dyes as organic model compounds could interfere with the photocatalytic performances of the catalysts studied due to their own light absorption and sensitizer properties. Therefore, other model compounds such as chemical probes or organic pollutants which do not absorb visible light must be utilized in the photocatalytic assessment of the catalysts.
In this work, N-TiO
2 catalysts with hollow and non-hollow structures were fabricated via different methods such as co-precipitation and sol–gel procedures [
22]. Titanium isobutoxide–urea (NT-U, sol–gel) and titanium isopropoxide–ammonium hydroxide (NT-A, co-precipitation) were used as catalyst precursors. In the designation of the catalysts, NT indicates N-doped TiO
2, while U and A represent the nitrogen sources (urea and ammonium hydroxide).
Ag nanoparticles were also decorated on the surface of N-TiO2 via a facile photo-deposition method. Various concentrations of Ag were applied to investigate the photocatalytic efficiency under visible light. In addition, numerous measurements, including diffuse reflectance spectra (DRS), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy–energy dispersive X-ray spectroscopy (TEM–EDS), Brunauer–Emmett–Teller (BET) surface area, and inductively coupled plasma (ICP) spectroscopy were used for material characterization.
In order to assess the photocatalytic performances of the catalysts, coumarin was used as a chemical probe to analyze the formation of both
•OH and other reactive species (photo-generated electron or •O
2−) under visible light [
22]. Furthermore, to evaluate the performance of photocatalytic degradation regarding emerging contaminants in the environment, 1,4-hydroquinone (1,4-HQ) was also used as a model organic pollutant, which is a major benzene metabolite and commonly found in the industries of pharmaceutics and personal care [
23,
24]. Lastly, antibacterial effects of the catalysts were evaluated by using bioluminescence method in the presence of
Vibrio fischeri strain. These bioluminescent bacteria are Gram-negative and commonly found in the marine environment [
25].