Enhanced Photocatalytic Efficiency of TiO₂ Membrane Decorated with Ag and Au Nanoparticles

Featured Application

The photocatalysis could be used for water treatment and other environmental engineering.

Abstract

Ag and Au nanoparticles (NPs) were decorated on the surface of TiO₂ membranes by two methods, i.e., hydrothermal synthesis and photoreduction. The size of Ag and Au NPs on the surface of TiO₂ membranes was dependent on the method of preparation and varied from 2 nm–10 nm. The photocatalytic performance of the TiO₂ particle, TiO₂ membrane and the Ag/Au-decorated TiO₂ membrane was tested for the catalytic degradation of Rhodamine B (RhB) and Escherichia coli (E. coli) under irradiation of visible light. The experiment results showed that both Ag- and Au-decorated TiO₂ membranes exhibited excellent photocatalytic activity in the visible light region. Among the prepared materials, Ag-decorated TiO₂ membranes prepared by photoreduction showed the highest activity, which could be attributed to the local surface plasmon resonance (LSPR) effect of the noble metal.

In recent years, many thin film materials, including carbon film, graphene and thin SnO₂ membranes, have been studied and have shown excellent physical and chemical properties [1,2,3]. Various TiO₂ nanomaterials have also been reported, such as TiO₂ nanopowders, nanowires, nanotubes, nanoflowers, etc. [4,5,6]. Among them, TiO₂ membrane have gained considerable attention due to their outstanding catalytic performance, which could be attributed to the large surface area and low thickness of the material [7,8]. TiO₂ nanoparticles (NPs) with a large specific area have higher photocatalytic efficiency because the photocatalytic process occurs on the surface.

High photocatalytic efficiency is the main evaluation method for photocatalysts, which is important to degrade organics in short time periods [9,10]. According to the research, photocatalysts with low photocatalytic efficiency degraded organics within a couple of hours, such as nitrogen-doped TiO₂ thin film [11], TiO₂ nanotubes [12] and BiOCl nanowire arrays [13]; high photocatalytic efficiency like that of BiOCl membranes could completely degrade RhB within 2 min under UV light [14].

Further modification of TiO₂ can improve its response to visible light, inhibit the recombination of photogenerated carriers and, consequently, enhance the photocatalytic performance. Various materials, including metals (Au, Ag, Pt, Cu), non-metals (C, N, S), metal oxides (WO₃, Fe₂O₃, SiO₂), etc., have been used to dope TiO₂ [8,15,16,17]. These doping materials can produce vacancies and interstitial or substitutional defects that change the photocatalytic properties of TiO₂. The band gap of TiO₂ is narrowed notably after doping with noble metals (Au, Ag, Pt) [18]. Because the noble metals have lower Fermi levels than TiO₂, the photogenerated electrons of the conduction band will not recombine with the holes, but instead transfer to the noble metal particles on the surface of TiO₂ [19,20,21,22]. Hence, the photogenerated electrons and holes are effectively separated, and the photocatalytic activity is improved [23,24,25]. Moreover, the noble metal NPs can more easily attract organic molecules than semiconductors. Therefore, the noble metal NPs play the role of the electron carrier in the system of TiO₂ doped with noble metals, which is an effective method to improve the photocatalytic properties of TiO₂.

Herein, we report a TiO₂ membrane decorated with Ag and Au NPs, which exhibited extremely high catalytic efficiency. The TiO₂ membrane was prepared by a hydrothermal method [26]. A certain amount (8 mL) of TiCl₃ was mixed with ethylene glycol (30 mL), and the mixture was stirred for 7 h at room temperature until the reaction completed. Then, doubly-distilled water (1 mL) was added, and the mixture was transferred into a stainless steel reactor and kept at 150 °C for 4 h. The products were separated by centrifugation at 16,000 rpm, washed with ethanol three times and dried in a vacuum at 50 °C for 48 h. TiO₂ membrane was decorated with Ag and Au NPs by the hydrothermal and photoreduction methods, respectively. The mixture of the TiO₂ membrane (0.4 mg) and AgNO₃ or HAuCl₄ (0.015 mol/L, 0.4 mL) was processed by 365-nm UV light or hydrothermal reaction (180 °C, 6 h). The final products were centrifuged at 10,000 rpm, washed with doubly-distilled water three times and dried in a vacuum at 50 °C for 24 h. The sample descriptions are given in

Table 1. Processes for TiO₂ membranes.

Name	Sample Description	Process Method	Ti (At%)	Ag (At%)	Au (At%)
S0	TiO2 membrane		--	--	--
SAg-UV	TiO2 membrane with Ag	UV	48.2	1.1	--
SAg-HY	TiO2 membrane with Ag	hydrothermal reaction	47.7	2.0	--
SAu-UV	TiO2 membrane with Au	UV	48.1	--	1.2
SAu-HY	TiO2 membrane with Au	hydrothermal reaction	47.6	--	2.3

.

TiO₂ membranes decorated with Ag and Au NPs were observed and characterized by transmission electron microscope (TEM, Talos F200S, Thermo fisher, Waltham, MA, USA) and X-ray diffraction (XRD, PHILIPS P W 3O4O/60X′PertPRO, Amsterdam, The Netherlands). Scanning electron microscope energy dispersive spectromete (SEM EDS, JSM-7500F, JEOL Ltd., Tokyo, Japan) analysis showed the real composition of the prepared samples, as given in Table 1. Ultraviolet-visible light (UV-Vis) diffuse reflectance spectroscopy was measured on a Perkin Elmer UV-Vis spectrometer (Lambda 750 S, Perkin Elmer, Waltham, MA, USA) equipped with a Labsphere RSA-PE-20 integration sphere. The electrochemical behaviors of the TiO2 membranes, SAg and SAu, were examined by electrochemical impedance spectroscopy (EIS, RST5200F, shiruisi Instrument, Zhengzhou, China). The photocatalytic performance of the TiO₂ membranes, S_Ag and S_Au, was evaluated by the degradation reaction of Rhodamine B (RhB). A Xe lamp with optical filters (14 V, 14–21 A, visible light) was used as the light source, and the distance between the lamp and the interface of the solution was 12 cm (0.1 W/m²). The test was carried out by adding the prepared photocatalyst (0.4 mg) to the solution of RhB (5 mg/L, 250 mL) and measuring the absorbance at 552 nm (i.e., the maximum absorption wavelength of RhB) by UV-visible spectrophotometry every 5 min. The concentration of RhB in the solution was calculated from the absorbance, and the photocatalytic degradation efficiency was calculated as follows:

$$\eta =\frac{C_0-C_t}{C_0}$$

(1)

where C₀ is the initial concentration of RhB and C_t is the concentration of RhB after an irradiation time of t. Escherichia coli (E. coli) cells were treated by S_Ag under visible light irradiation (0.1 W/m²) for 10 min. Then, the morphology of the native and treated E. coli cells was examined by SEM. The cells were fixed according to the standard protocol by reduction with glutaraldehyde and oxidation with OsO₄, then dehydrated stepwise with ethanol of increasing concentration (50–100%).

The prepared TiO₂ membrane was decorated with Ag/Au NPs by either hydrothermal synthesis or photoreduction. The morphology and structure of the resulting material were observed and characterized by TEM. Figure 1 shows the diameter histogram and the TEM images of the S_Ag and S_Au samples prepared under different conditions.

The ultrathin TiO₂ membrane was two atoms thick and Ag or Au NPs were dispersed on the membrane surface, as shown in Figure 1. The particle size ranged from 2 nm–10 nm. The particle size of the decorated Ag was the smallest (ca. 2 nm) when S_Ag was prepared by photoreduction with ultraviolet light (Figure 1). Similarly, the particle size of the decorated Au was also the smallest (ca. 2 nm) in S_Au prepared by photoreduction with ultraviolet light (Figure 2D). Hydrothermal synthesis at 180 °C furnished Au particles of ca. 10 nm in size on the TiO₂ membrane. Ag or Au NPs prepared by the photoreduction method were dispersed evenly within the TiO₂ network. However, the NPs aggregated together with hydrothermal synthesis. Generally, Ag and Au NPs were successfully decorated on the TiO₂ membrane.

The S_Ag and S_Au samples exhibited some adsorption in the visible light region as shown in Supplementary Material S1, which could be attributed to the local surface plasmon resonance (LSPR) of the Ag and Au NPs. The LSPR effect on the surface of noble metal NPs is caused by the collective shock of conductive electrons generated by electromagnetic radiation. The size and shape of the noble metal NPs can change the surface density of the electromagnetic field and are thus important factors for the LSPR effect [27]. Obviously, the S_Ag sample prepared by photoreduction had a strong absorption in the visible light region. For TiO₂, the absorption in the UV region resulted only from its wide band gap (3.2 eV). After the Ag and Au NPs were introduced, the light response range of TiO₂ clearly extended to the visible region. The Ag and Au NPs on the TiO₂ membrane could transfer free elections generated by the LSPR effect to the valence band of TiO₂ and consequently enhance the photocatalytic activity of TiO₂ under visible light [28,29,30].

Figure 2 shows the photocatalytic degradation of RhB with P25 (TiO₂ particle with size 25 nm) and the decorated TiO₂ membrane. The degradation efficiency of RhB was evaluated by the first order reaction, which is presented in Supplementary Material S2. The catalytic efficiency of photocatalysts was ranked as S_Ag-UV > S_Ag-HY > S_Au-UV > S_Au-HY > S₀ > S_P25. When P25 was used as the catalyst, the degradation of RhB under irradiation of visible light was very limited. Nevertheless, the degradation efficiency was improved notably when the TiO₂ membrane was used in the photodegradation reaction. The decorated TiO₂ membrane enhanced the degradation efficiency further. S_Ag exhibited higher catalytic efficiency than S_Au. Besides, S_Ag/S_Au prepared by UV irradiation exhibited higher degradation efficiency than hydrothermal synthesis. In addition, the recycling experiment of S_Ag-UV showed that it could be used over 50 times and that the photocatalytic efficiency of S_Ag-UV had not reduced significantly, which is presented in Supplementary Material S3.

Mechanistically, in the photocatalytic reaction, photogenerated electrons and holes on the TiO₂ membrane work as the active species to decomposing organic compounds. The existence of Ag and Au NPs on the TiO₂ membrane forms a Schottky junction on the surface of TiO₂, which promotes charge separation and improves the quantum efficiency. Generally, Ag shows a stronger LSPR effect than Au. In addition, the absorption range of TiO₂ extended to the visible light region due to the LSPR effect of the Ag and Au NPs. Since the LSPR sensitization effect of plasmonic NPs depends mainly on their size and shape, the LSPR effect is strengthened when the size of the Ag and Au NPs increases [31,32]. Hence, the S_Au and S_Ag samples prepared by photoreduction showed higher efficiency in the photodegradation of RhB. During this process, RhB was first broken down into benzoic acid and phthalic acid and then into ethanedioic acid, while the organic carbon content remain unchanged [33]. Finally, the RhB was degraded into small molecules, such as CO₂, NO_x and H₂O, and finally, oxidized completely, with the organic carbon content decreased significantly [34,35,36].

The degradation of RhB by photocatalysts has been reported by many groups. TiO₂ and Fe₂O₃ co-doped graphene aerogel can remove 97.7% RhB within 60 min under visible light irradiation [37]. SiO₂@C-doped TiO₂ hollow spheres can degrade 61.7% RhB after 100 min under UV irradiation [38]. Ag/TiO₂ composites can degrade 80% RhB after 120 min under UV irradiation [39]. Compared with the above photocatalysts, the Ag decorated TiO₂ membrane can almost degrade 90% RhB within 30 min under visible light irradiation. Therefore, Ag/Au-decorated TiO₂ membranes showed excellent photodegradation efficiency.

The prepared TiO₂ membrane was characterized as being twos atoms thick [26]. The specific surface area was greatly increased compared with P25 TiO₂. The target molecules were more easily absorbed on TiO₂ membrane. The degradation of the target molecules experienced a dynamic adsorption-photocatalysis-desorption process. The electron and hole produced under irradiation reacted with the target molecules on the enlarged surface; thus, the recombination of the electron and hole was also inhibited. The process is presented in Figure 3. As a result, the photocatalytic efficiency of the TiO₂ membrane was enhanced significantly compared to P25 TiO₂.

The photocatalytic mechanism is demonstrated below with S_Ag as an example. The S_Au has photoresponse in the visible light region due to the LSPR effect of Au NPs. Analyzing by EIS, the Au NP-doped TiO₂ membrane represented lower resistances than the pure TiO₂ membrane, which could make the charge transfer at the interface more rapid and effective. The charges on the surface of Au NPs are subjected to collective shock under irradiation of visible light and become free elections, which results in a strengthened local electric field. Under the local electric field, free electrons then rapidly jump to the conduction band (CB) of TiO₂ and overcome the Schottky junction, which results in the separation of photogenerated electrons and holes and thus improves quantum efficiency. Generally, pure TiO₂ has a wide band gap. Under light irradiation, the excitation of an electron would take place if the light energy is greater than the band gap of the catalysts. The wide band gap of pure TiO₂ is known to be 3.2 eV. By using Mott−Schottky plots and XPS spectra, the band gap from the valence band to the conduction band for Ag-decorated TiO₂ deceased to 2.6 eV. Therefore, the material showed photocatalytic activity under visible light irradiation. The photogenerated electrons can react with the O₂ on the surface of catalyst to form ·O₂⁻, and the holes of Au can combine with H₂O on the catalyst surface to form ·OH, both of which are strongly oxidative. These oxidative species can then oxidize the RhB molecule on the surface of Au NPs.

The photocatalysis degradation is divided into three stages. They are the formation of both the hole and electron under light excitation, the production of radical species and the reaction of the radical species with the targets (pollutants, bacteria, etc.). Electron paramagnetic resonance (EPR) has confirmed the separation of the hole and electron [40]. The production of radical species along time was also proven by the different adduct species detected with EPR [41]. The degradation product of the targets was detected by some groups [42]. The silver content is closely related to the separation of the hole and electron and the production of radical species, as described above. As a result, silver will affect photocatalysis and degradation efficiency significantly.

Au or Ag NPs experience an evolution during the photocatalytic process. A moderate growth of particle size was observed. From the statistics of the TEM images, the particle size of Ag and Au NPs increased from 2.3 ± 0.1 nm–13 ± 0.3 nm and from 2.4 ± 0.1 nm–17 ± 0.4 nm after photocatalysis, respectively. On the other hand, the photocatalytic activity did not change in the catalyst recyclability test (Supplementary Material S3), showing that Au or Ag NPs maintain a dominant metallic state.

The surface morphology of the E. coli cells was examined by SEM. Figure 4A shows that the native E. coli cells had a rod-like structure with regular wrinkles, which was changed after the photocatalytic degradation. Figure 4B shows that some depressions appeared on the surface of cells treated with P25 TiO₂, and irregular wrinkles appeared on the cells treated with S_Au.

The results suggested that the cell surface was damaged and the cell membrane structure was altered. The outer membrane of Gram-negative bacteria is composed of lipopolysaccharide (LPS), peptidoglycan and phospholipid layers. The mechanical strength of the cell membrane depends on peptidoglycan, and LPS is responsible for the surface structure and morphology [43,44,45]. The beta 1,4-link between the two amino sugars of N-acetyl glucose amine and N-acetyl acid forms the LPS. By connecting four peptide side chains on the N-acetyl cell wall acid molecules, the peptides in between are again joined by bridges to form a reticulate structure that is very strong. Therefore, the cell membrane plays a vital role in maintaining cell morphology. The outermost layer of the cell membrane is LPS, which is exposed to the environment and subjected to the attack of free radicals during photocatalytic degradation. The cells still maintained the original rod shape after treatment with P25 TiO₂ and S_Au, which indicated that LPS was damaged, but peptidoglycan remained intact. The peptidoglycan layer and the outer membrane form a strong network to maintain the morphology of the cell [46,47]. In contrast, after treatment with S_Ag, the E. coli cells exhibited a fusiformis structure, and the structure of the native cells was destroyed, which indicated that the outer membrane and the peptidoglycan layer were fully damaged, but the inner membrane remained intact. In addition, E. coli photo-killing analysis was done by E. coli inactivation, suggesting that the E. coli treated with S_Ag inactivation was decreased within 3.5 h under visible light, which is presented in Supplementary Material S4.

The outstanding performance of the TiO₂ membrane as a photocatalyst stems from the material’s large surface area and small thickness. However, the quantum efficiency of the TiO₂ membrane is not high. Decorating the TiO₂ membrane with Ag and Au NPs can extend the photo response to a broader range and improve the photocatalytic properties. In this work, the TiO₂ membrane was prepared by hydrothermal synthesis, and then, Ag and Au NPs were decorated on the surface of the TiO₂ membrane by hydrothermal synthesis or photoreduction. The size of the Ag and Au NPs prepared by photoreduction was ca. 2 nm and the smallest among all material samples(Supplementary Material S5). The size of the noble metal NPs is important for the local surface plasmon resonance (LSPR) sensitization and affects the quantum efficiency. Under visible light, S_Ag and S_Au showed excellent photocatalytic performance. Silver NPs can produce a stronger LSPR effect than Au NPs [31,32]. The TiO₂ membranes decorated with Ag NPs were found to have good photocatalytic properties in the degradation of RhB and E. coli under irradiation of visible light and presumably may find use in the photocatalytic degradation of more organic compounds and microorganisms.