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TiO2 inverse opal structures with facile decoration of precious metal nanoparticles for enhanced photocatalytic activity
Journal Pre-proof TiO2 inverse opal structures with facile decoration of precious metal nanoparticles for enhanced photocatalytic activity
Filipp Temerov, Bright Ankudze, Jarkko J. Saarinen PII:
S0254-0584(19)31284-2
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122471
Reference:
MAC 122471
To appear in:
Materials Chemistry and Physics
Received Date:
14 August 2019
Accepted Date:
21 November 2019
Please cite this article as: Filipp Temerov, Bright Ankudze, Jarkko J. Saarinen, TiO2 inverse opal structures with facile decoration of precious metal nanoparticles for enhanced photocatalytic activity, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys. 2019.122471
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof
TiO2 inverse opal structures with facile decoration of precious metal nanoparticles for enhanced photocatalytic activity
Filipp Temerov*, Bright Ankudze, Jarkko J. Saarinen Department of Chemistry, University of Eastern Finland P.O. Box 111, FI-80101 Joensuu, Finland *Corresponding author: [email protected]
Abstract TiO2 inverse opal (IO) structures were fabricated by an infiltration method that were functionalized with gold, silver, and gold-silver core-shell metal nanoparticles. The photocatalytic activity of the TiO2 IO structures with metal nanoparticles was characterized by gas-phase conversion of acetylene into carbon dioxide. All metal nanoparticle functionalized structures showed an increase in photocatalytic activity via surface plasmon resonance coupling of the incident light. The highest photocatalytic activity was observed with gold-silver core-shell nanoparticle functionalized TiO2 IO structure (increase of 62% compared to reference TiO2 IO) followed by gold (53%) and silver (39%) nanoparticles.
Keywords: TiO2 inverse opal, noble metal nanoparticles, photocatalysis Declarations of interest: none
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1 Introduction Titanium dioxide (TiO2) is a well-known semiconductor with photocatalytic activity. Fujishima and Honda [1] demonstrated in a seminal work in 1972 water splitting into hydrogen and oxygen by irradiating TiO2 electrode with an ultraviolet A (UVA) light that triggered an extensive research of photocatalysis. TiO2 is non-toxic and has a good stability. However, anatase crystalline form has a rather large bandgap of 3.2eV (~387 nm) in the UVA range. The large band gap and high recombination rate of photoexcited electrons and holes restrict a wide scale utilization of TiO2. Doping with metal and non-metal ions, loading of noble metals, preparation of TiO2 based nanostructures (nanotubes, nanorods, nanobelts) have been main strategies to improve photocatalytical properties of TiO2 [2–4]. It is believed that enhanced photocatalytic activity in the visible range can be used for solar-driven artificial photosynthesis and solar cells with a higher efficiency that may provide solutions for transformation from the current fossil fuel-based economy into a more sustainable solar-driven economy. A promising approach is to use noble metal nanoparticles for functionalization of TiO2 as they form a Schottky barrier. This significantly enhances the separation of photogenerated carries and promotes the interfacial electron transfer process [5]. Moreover, noble metal nanoparticles, due to localized surface plasmon resonance (LSPR), can themselves interact with light in the visible range [6]. For example, Ingram et al. synthesized Au/TiO2 and Ag/TiO2 nanoparticles with LSPR for a more efficient photocatalytic water splitting [7]. Boppella et al. prepared TiO2 inverse opal/graphene oxide/Au nanocomposite for an efficient hydrogen evolution [8]. It is well known that different TiO2 shape configurations such as nanotubes [2], nanobelts [3] and nanosheets [4] have a higher photocatalytic activity than regular TiO2 nanoparticles. Recently, TiO2 inverse opal (TiO2 IO) photonic crystal structures have attracted attention due to their properties such as photonic band gap, slow light photons, and localized
Journal Pre-proof photons [9]. TiO2 IO contains a three-dimensional ordered porous structure with a large specific surface area and optical properties associated with photonic crystals. Furthermore, TiO2 IO photocatalytic activity can be enhanced by doping with noble metals such as silver or gold along with the long interaction times due to slow light effect in photonic crystals. Typical optical dyes such as methyl orange (MO) and methylene blue (MB) have been primary photocatalytic activity characterization indicators for indirect methods based on a color transformation [10]. Unfortunately, since the incident light itself can bleach the dye and induce an invalid result and moreover, nanoparticles may detach and migrate from substrate into the solution during immersion in the solution, such methods should not be used for photocatalytic activity characterization [11]. Therefore, a better alternative for photocatalytic activity measurements is to use a gas- phase method based on mineralization of organic compounds. Additionally, this method is close to real application conditions and it also removes all mechanical stresses on the surface compared to the traditional solution-based approaches with optical dyes [10] in which the surfaces are immersed into a liquid. In the present study, TiO2 IO structures were prepared and loaded with three different types of nanoparticles for enhanced photocatalytic activity. Three types of nanoparticles were prepared by hydrothermal method; gold, silver and gold-silver core-shell nanoparticles (AuNPs, AgNPs and Au/AgNPs). Nanoparticles were coated into TiO2 IO on glass substrate by casting on three solutions of nanoparticles. Finally, the photocatalytic activity was measured using the in-house built gas-phase photoreactor.
2 Materials and methods 2.1 Synthesis of polystyrene spheres Styrene (Acros Organics, 99%) distilled in deionized water (Millipore), ammonium persulfate (APS, Sigma-Aldrich, 98.0+%), and sodium dodecyl sulfate (SDS, Sigma-Aldrich, 98.5+%)
Journal Pre-proof were used for the synthesis of polystyrene spheres that was performed following the procedure by Erolla [12] with minor changes. Synthesis of polystyrene spheres with an average diameter 400 nm was performed in two steps. All reagents were degassed before loading into the reaction mixture. First, 200 mg of SDS as a capping agent was added to 160 ml of deionized water in a reaction vessel. Reaction mixture was stirred using an anchor-like mixer at 500 rpm and heated up to 70°C. Then, 20g of styrene was added to the reaction mixture and kept for 30 min at the same temperature. After that, 200 mg of APS in 20 ml of deionized water was added into the reaction vessel as an initiator. Reaction proceeded for 20h under nitrogen atmosphere. In the second step, 150 mL of distilled water and 30 ml of colloidal PS solution prepared in the first step were loaded in a three-neck flask. Then 25 mg SDS was added and the system was purged with nitrogen gas until the temperature was raised to 70°C. When the temperature reached 70°C, 28 mg of APS dissolved in 20 ml of deionized water was added to the reaction vessel. After that, 21.6 g of styrene were added slowly to the flask using a syringe. The system was polymerized at 70°C under nitrogen atmosphere for 20 h.
2.2 Synthesis of TiO2 inverse opal films Titanium (IV) isopropoxide (TTIP, Sigma-Aldrich, ≥99%), hydrochloric acid (HCl, Merck, 37%), and ethanol (Altia, 99.5%) were used to prepare titanium precursor. TiO2 inverse opal (TiO2 IO) films were prepared in three steps; First, self-assembly of PS spheres was followed by infiltration of TiO2 precursor that was calcinated to obtain IO structure. Glass plates (Thermo Scientific) treated with piranha solution served as the surface for TiO2 IO films deposition. PS film was self-assembled on microscopic glass substrate by the vertical deposition. 1 ml of PS solution was diluted in 20 ml of distillated water in a small beaker. Microscope glass slides were immersed vertically and attached by clips so that they do not touch the bottom and the walls of the beaker. The evaporation took place in a laboratory oven
Journal Pre-proof in the temperature range of 60–65°C for 2 days. Then TiO2 sol was prepared by mixing TTIP (1 ml), ethanol (1 ml) and 0.1M HCl (3 ml). The resulting mixture was stirred for 1 h before use. After that precursor was diluted 2 times with ethanol. The microscopic glasses with PS films were dipped in precursor solution for 5 min and then dried in air for 24h for hydrolysis and formation of TiO2. After drying was completed, the resulting composites were calcined using the following protocol; heating from room temperature to 250°C in 250 min that was then kept for 120 min followed by an increase from 250°C to 550°C in 300 min with a final holding time of 300 min for complete removal of PS spheres and to convert TiO2 into anatase crystalline phase. Finally, the samples were cooled down to room temperature.
2.3 Synthesis of Au, Ag, and Au/Ag nanoparticles Gold (III) chloride trihydrate (HAuCl4 ·3 H2O,), sodium citrate (TSC, Sigma-Aldrich, ≥99%), and silver nitrate () were used without further purification for the synthesis of nanoparticles. Preparation of Au NPs. Au NPs were prepared by a hydrothermal method, similar to the earlier reported work [13]. An aqueous solution of HAuCl4 · 3H2O (10 ml, 0.1%, Alfa Aesar, ≥99.9%) was added to deionized water (100 ml) and heated to boiling point. Sodium citrate (TSC, Sigma-Aldrich, ≥99%) solution (2 ml, 1%) was then added and the resulting mixture was kept for 15 min under stirring. Upon cooling down to room temperature, the Au NPs were obtained in the solution Preparation of Ag NPs. Silver nanoparticles were prepared by citrate reduction of Ag (I) ions in aqueous medium according to the previously reported study [14]. AgNO3 (60 ml, 1.0 mM, Sigma-Aldrich) aqueous solution was heated to the boiling point under reverse condenser and then sodium citrate (3 ml; 2%) was added upon intensive magnetic stirring. Brown dispersion containing Ag NPs was formed within 15 min of boiling. The obtained suspension was stirred
Journal Pre-proof under reflux for additional 30 min to complete the reaction. Finally, a green-gray Ag colloid was obtained. Preparation of Au/Ag NPs. Au/Ag NPs were synthesized by reduction of AgNO3 and HAuCl4 · 3H2O with citrate, based on the reported procedure [15]. For Au-Ag alloy NPs, HAuCl4 · 3H2O (1 ml, 0.01 M) and AgNO3 (1 ml, 0.01 M) were dissolved in 78 ml of boiled deionized water upon rapid stirring. Then, 8 ml of sodium citrate (0.1 M) were quickly added into the reaction mixture. Solution was kept at 100 °C for 15 min with continuous stirring. The synthesized NPs were washed and centrifuged for several times before they were dispersed in water and kept in the dark at a room temperature.
2.4 Nanoparticle coating into TiO2 inverse opals The microscopic glasses with TiO2 IO were carefully immersed to the beaker with the diluted metal nanoparticles solution so that all TiO2 IO film was covered by the solution. For each solution the used volume was the same:1 ml of nanoparticle solution was diluted with 20 ml of water. The glass slides did not touch the bottom or the walls of the beaker. Then, the solution was slowly evaporated in the fume hood for two days and the nanoparticles were coated into the pores in the IO structure. The obtained structure was dried in an oven (T=100°C) for 6h and then washed with ethanol and deionized water. Finally, TiO2 IO with Au, Ag and Au/Ag were obtained for further characterization. Sodium ions are known to be catalytic poison that can influence TiO2 photocatalytic activity by altering the crystallographic properties or by acting as recombination centers [16]. Here sodium ions are only present in the preparation of the metal nanoparticles, which are coated from a dilute solution onto TiO2 IO. Hence, they do not change the crystallinity of the calcined TiO2 IO. Therefore, the total sodium ion concentration in metal NP coated TiO2 IO structures is negligible.
Journal Pre-proof 2.5 Characterization methods The morphology and average structural sizes of synthesized Au NPs, Ag NPs, Au/Ag NPs and TiO2 IO, TiO2 IO-Au NPs, TiO2 IO-Ag NPs and TiO2 IO-Au/Ag NPs were acquired with Hitachi S-4800 FE-SEM (field emission scanning electron microscope). SEM images of the TiO2 IOs and TiO2 IO with metal nanoparticles were observed after dispersing a small amount of the sample on a carbon tape. Scanning transmission electron microscope (STEM) images of the metal nanoparticles were obtained after drying a significantly diluted sample on a copper grid coated with Lacey carbon film. The elemental identification of the TiO2 IO with metal nanoparticles were acquired with energy dispersive X-ray spectroscopy (EDS) using the Noran system Six (NSS) software. A PerkinElmer Lambda 900 UV/Vis/NIR spectrometer was employed to obtain the absorption spectrum of the metal nanoparticles. The PerkinElmer Lambda 900 UV/Vis/NIR spectrometer fitted with a 150 mm integrating sphere was used to measure the reflectance spectra of the TiO2 IO, TiO2 IO-Au NPs, TiO2 IO-Ag NPs and TiO2 IO-Au/Ag NPs. An in-house built (University of Eastern Finland, Department of Chemistry, Joensuu campus) gas-phase photoreactor was used for photocatalytic activity characterization as shown schematically in Fig. 2. The diameter of the used reactor was 145 mm with a volume of 15.4 cm3. The reactor was equipped with online CO2 concentration detector (Vaisala GMP343), temperature and humidity sensor (Thorlabs TSP01), and pressure meter (Wika PGT10, USB mode). All detectors were USB connected to the PC for a real-time, online measurements. Acetylene (C2H2) was used as a test analyte that was oxidized into CO2. Such approach has been shown valid for analyzing photocatalytic response before [17-19]. A mixture of acetylene and technical air was formed in the mixing chamber followed by injection into the batch-type reactor chamber. The gas flow through the reactor was continuous until the CO2
Journal Pre-proof concentration become constant. The ultraviolet UVA excitation was performed through the UVA transparent quartz glass reactor window from a high-intensity (100W) UVA lamp (UVP Black-Ray® B-100AP High Intensity, peak emission at 365 nm) located 17 cm above the reactor window. Microscopic glass substrates of TiO2 IO with and without metal nanoparticles were placed into the photoreactor. The UVA light excited the photocatalytic activity of TiO2 followed by decomposition of C2H2. As a result, acetylene was oxidized into CO2 that was measured using an optical CO2 concentration detector. All data collected from the CO2 detector was used directly without any refinement since the reaction temperature, humidity and pressure were kept constant during the measurement time.
3 Results and discussion 3.1 Characterization of metal nanoparticles Figure 3 (A, C, E) represents the TEM images of Au NPs, Ag NPs and Au/Ag NPs as synthesized. The average diameters of metal nanoparticles were around 40 ± 5 nm excluding the agglomerated nanoparticles that were observed with average diameter of 80 ± 10 nm. EDS mapping analysis was performed to confirm the composition of metal nanoparticles and each element in the Au/Ag NPs. Figure 3 (B, D, F) shows STEM images of metal nanoparticles, which were used for elemental mapping analysis. The mapping profiles show that Au NPs and Ag NPs fully consist of metallic Au and Ag respectively (red and yellow dots). It can be clearly seen that, in case of Au/Ag NPs, both Au and Ag metals were distributed homogeneously across the particle (red and yellow dots together). Surface plasmon resonance (SPR) was analyzed by UV–Vis absorption spectroscopy for metal nanoparticle characterization. Figure 3 (G) represents normalized extinction spectra of Au, Ag, and Au/Ag NPs. SPR characteristic peaks at about 520 nm and
Journal Pre-proof 430 nm correspond to Au and Ag NPs, respectively [20]. The red shift occurs from SPR absorption peak of Ag NPs towards the Au NPs for Au/Ag NPs when the ratio of Au to Ag increases. The SPR peak at 475 nm for Au/Ag presented in the Figure 3 (G) matches well with the composition alloy NPs [15].
3.2 Characterization of TiO2 IO and TiO2 IO with metal nanoparticles The polystyrene spheres with diameter of 400 nm were self-assembled as a template on the microscopic glass (Fig. 4 C). The PS spheres have a highly ordered structure that was formed during the slow evaporation of solution. As soon as calcination process was completed, TiO2 IO structures were observed (Fig. 4 A). TiO2 IO had a well-ordered honeycomb structure in anatase crystalline phase. However, the surface contained some defects and cracks, which may slightly affect the photocatalytic activity [21]. The pore size was around 350 nm whereas PS spheres had a diameter of 400 nm. This indicates that during the high temperature treatment PS spheres shrank approximately 12–13%. The crack formation mainly derives from shrinking and infiltration process. However, it should be noted that the overlayers were not observed from the top views of the structures. Noble metal NP coating on the surface of TiO2 IO can work as electron trap to promote the separation of photo-generated electrons and holes, and thus enhance photocatalytic activity. Moreover, noble metals can extend the response region of TiO2 from UVA to visible range via SPR. Figure 4 (B, D, E) shows the SEM images of TiO2 IO with gold, silver and gold/silver nanoparticles respectively. In all cases, the metal nanoparticles were uniformly distributed and firmly anchored. It can be clearly seen that nanoparticles with smaller diameter decorated walls of the TiO2 IO holes while bigger or agglomerated particles coated inside the cavities. All samples had the same morphology as TiO2 IO without metal nanoparticles indicating that metal NP coating process does not affect on the TiO2 IO morphology. Reflectance spectra of all samples were measured normal to the FCC (111) plane surface (Figure 4 F). TiO2 IO without NPs exhibits a photocatalytic activity only in the UVA region with a wavelength below 380–385 nm, which can be attributed to the intrinsic band gap of the TiO2. Moreover, a broad absorption peak (400–600) nm was observed with TiO2 IO with metal nanoparticles samples. This broad peak can be assigned to the sub-band gap state of the special TiO2 nano-ordered structure [22]. Compared to the TiO2 without metal nanoparticles,
Journal Pre-proof all other samples showed a strong absorption from 400 nm to 600 nm due to presence of metal NPs. However, the peaks of metal NPs SPR overlap with the sub-band of TiO2 [23].
3.3 Photocatalytic activity The hot electron transfer, resonant photon scattering, and near-field electromagnetic enhancement are possible energy-transfer mechanisms in a plasmonic photocatalyst systems [24, 25]. Near-field electromagnetic enhancement usually can be seen at wavelengths where the plasmon resonance and semiconductor absorption overlap [26]. In our case we can exclude effect of the resonant photon scattering since it normally occurs in the large plasmonic nanostructures [27]. We assume that the hot electron transfer mechanism was the major contribution for the observed enhanced photocatalytic activity of the TiO2 IO with metal NPs. Fig. 5(a) displays the measured photocatalytic activity of the TiO2 IO structures with metal NPs compared to the reference TiO2 IO structure. All metal NPs increase the photocatalytic activity: the highest enhancement was observed with Au/Ag core-shell nanoparticles (62%) followed by Au NPs (53%) and Ag NPs (39%). Furthermore, Fig. 5(b) shows the cyclic activation of the TiO2 IO structures with Au/Ag metal NPs showing the reproducibility and stability of the results. LSPR in bimetallic Au/Ag nanoparticles were studied using UV-Vis spectroscopy with varying metal content [28]. It was observed that bimetallic Au/Ag nanoparticles show a tunable LSPR resonance with broad absorption peaks. Our findings agreed with the predictions in Ref. [28] as observed from Fig. 3 (G): the Au/Ag NPs displayed the broadest absorption peak that was also the case in Fig. 4 (F) when loaded into the TiO2 IO structure. It is believed that the wider and stronger absorption in Au/Ag NPs results in a stronger coupling of the incident light into the nanoparticles that is followed by increased photocatalytic activity. The observed photocatalytic process can be explained according to the diagram presented in Fig. 6. Metal NPs absorb incoming visible light and electrons get photoexcited simultaneously metal ions (M+) are formed due to the LSPR effect (Eq. 1). The photoexcited electrons transfer from metal nanoparticles onto TiO2 IO surface by a Schottky barrier at the M/TiO2 interface owing to the larger work function. The electric field in the space charge promotes the transportation of excited electrons and enhances the charge separation between the photoexcited electrons and metal ions (Eq. 2). The injected electrons can be transferred to the abundant molecular oxygen to form superoxide anion radicals (O2 ∙-) (Eq. 3) and
Journal Pre-proof subsequent protonation gives (HO2∙) radicals (Eq. 4). H2O2 produces by the mixing of HO2∙ radicals and the trapped electrons (Eq. 5) following by formation of hydroxyl radicals (OH∙) (Eq. 6). However, these active species will result in the degradation and mineralization of acetylene. At the same time, the metal ions are also reactive radical species and they can directly convert C2H2 into CO2 [29]. The overall reactions (1) – (7) are given below: Me NPs + hν → Me NPs*
(1)
Me NPs* + TiO2 → Me NPs+ (h+) + TiO2(e-)
(2)
TiO2 (e-) + O2 → TiO2 +O2∙-
(3)
O2∙- + H+ → HO2∙
(4)
e- + HO2∙ + H+ → H2O2
(5)
H2O2 + e- → OH∙ + OH-
(6)
Me NPs+ (h+) + 2OH∙ + C2H2 + 2O2
→ Me NPs + 2CO2 + 2H2O
(7)
4 Conclusions In conclusion, three types of metal (Au, Ag, Au/Ag) NPs were successfully synthesized and loaded into TiO2 IO for enhanced photocatalytic activity. All prepared materials were characterized for their photocatalytic activity by using an in-house built gas-phase photoreactor. TiO2 with metal NPs showed a higher photocatalytic activity than TiO2 without metal NPs. The highest photocatalytic activity was observed with Au/Ag NPs (increase of 62%) followed by Au NPs (53%) and Ag NPs (39%). The observed enhancement in photocatalytic activity can be explained by hot electron transfer mechanism. It is believed that such metal NP doped TiO2 IO structures will find applications as enhanced photocatalysts with significant potential for solar-driven applications.
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Acknowledgements FT wishes to thank the Finnish Cultural Foundation for a research grant. JJS acknowledges the Faculty of Science and Forestry at the University of Eastern Finland for the financial support (grant no. 579/2017).
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Figure captions Figure 1. A schematic representation of the fabrication of TiO2 IO loaded with metal NPs. Figure 2. A schematic illustration of the used gas-phase reactor. Figure 3. SEM images of (A) Au NPs, (C) Ag NPs and (E) Au/Ag NPs, EDS mapping SEM images of (B) Au NPs, (D) Ag NPs and (F) Au/Ag NPs and (G) absorption spectra of metal nanoparticles. Figure 4. SEM images of (A) PS self-assembly, (B) TiO2 IO without nanoparticles, (C) TiO2 IO with Au NPs, (D) TiO2 IO with Ag NPs, (E) TiO2 IO with Au/Ag NPs and (F) reflectance spectrum of structures (green line is TiO2 IO, black line is TiO2 IO with Ag NPs, red line is TiO2 IO with Au NPs and blue line is TiO2 IO with Au/Ag NPs). Figure 5. (A) Photocatalytic activity of TiO2 IO structures with metal NPs, and (B) cycling degradation curves for TiO2 IO with Au/Ag NPs. Figure 6. Schematic diagram of a charge transfer process in TiO2 IO with metal NPs.
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Preparation of TiO2 inverse opal structures with facile deposition of metal (gold, silver and gold-silver) nanoparticles for enhanced photocatalytic activity
Filipp Temerov*, Bright Ankudze, Jarkko J. Saarinen Department of Chemistry, University of Eastern Finland P.O. Box 111, FI-80101 Joensuu, Finland *Corresponding author: [email protected]
Declarations of interest: The authors declare that they have no competing interest to declare.
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Highlights
TiO2 inverse opal (IO) structures were prepared by an infiltration method.
Gold, silver, and gold/silver nanoparticles (NPs) were coated onto TiO2 IOs.
Enhanced photocatalytic activity was observed with metal nanoparticles.
Au/Ag core-shell NPs increased photocatalytic activity by 62%.
Au and Ag NPs increased photocatalytic activity by 53% and 39%, respectively.
Filipp Temerov, Bright Ankudze, Jarkko J. Saarinen PII:
S0254-0584(19)31284-2
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122471
Reference:
MAC 122471
To appear in:
Materials Chemistry and Physics
Received Date:
14 August 2019
Accepted Date:
21 November 2019
Please cite this article as: Filipp Temerov, Bright Ankudze, Jarkko J. Saarinen, TiO2 inverse opal structures with facile decoration of precious metal nanoparticles for enhanced photocatalytic activity, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys. 2019.122471
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof
TiO2 inverse opal structures with facile decoration of precious metal nanoparticles for enhanced photocatalytic activity
Filipp Temerov*, Bright Ankudze, Jarkko J. Saarinen Department of Chemistry, University of Eastern Finland P.O. Box 111, FI-80101 Joensuu, Finland *Corresponding author: [email protected]
Abstract TiO2 inverse opal (IO) structures were fabricated by an infiltration method that were functionalized with gold, silver, and gold-silver core-shell metal nanoparticles. The photocatalytic activity of the TiO2 IO structures with metal nanoparticles was characterized by gas-phase conversion of acetylene into carbon dioxide. All metal nanoparticle functionalized structures showed an increase in photocatalytic activity via surface plasmon resonance coupling of the incident light. The highest photocatalytic activity was observed with gold-silver core-shell nanoparticle functionalized TiO2 IO structure (increase of 62% compared to reference TiO2 IO) followed by gold (53%) and silver (39%) nanoparticles.
Keywords: TiO2 inverse opal, noble metal nanoparticles, photocatalysis Declarations of interest: none
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1 Introduction Titanium dioxide (TiO2) is a well-known semiconductor with photocatalytic activity. Fujishima and Honda [1] demonstrated in a seminal work in 1972 water splitting into hydrogen and oxygen by irradiating TiO2 electrode with an ultraviolet A (UVA) light that triggered an extensive research of photocatalysis. TiO2 is non-toxic and has a good stability. However, anatase crystalline form has a rather large bandgap of 3.2eV (~387 nm) in the UVA range. The large band gap and high recombination rate of photoexcited electrons and holes restrict a wide scale utilization of TiO2. Doping with metal and non-metal ions, loading of noble metals, preparation of TiO2 based nanostructures (nanotubes, nanorods, nanobelts) have been main strategies to improve photocatalytical properties of TiO2 [2–4]. It is believed that enhanced photocatalytic activity in the visible range can be used for solar-driven artificial photosynthesis and solar cells with a higher efficiency that may provide solutions for transformation from the current fossil fuel-based economy into a more sustainable solar-driven economy. A promising approach is to use noble metal nanoparticles for functionalization of TiO2 as they form a Schottky barrier. This significantly enhances the separation of photogenerated carries and promotes the interfacial electron transfer process [5]. Moreover, noble metal nanoparticles, due to localized surface plasmon resonance (LSPR), can themselves interact with light in the visible range [6]. For example, Ingram et al. synthesized Au/TiO2 and Ag/TiO2 nanoparticles with LSPR for a more efficient photocatalytic water splitting [7]. Boppella et al. prepared TiO2 inverse opal/graphene oxide/Au nanocomposite for an efficient hydrogen evolution [8]. It is well known that different TiO2 shape configurations such as nanotubes [2], nanobelts [3] and nanosheets [4] have a higher photocatalytic activity than regular TiO2 nanoparticles. Recently, TiO2 inverse opal (TiO2 IO) photonic crystal structures have attracted attention due to their properties such as photonic band gap, slow light photons, and localized
Journal Pre-proof photons [9]. TiO2 IO contains a three-dimensional ordered porous structure with a large specific surface area and optical properties associated with photonic crystals. Furthermore, TiO2 IO photocatalytic activity can be enhanced by doping with noble metals such as silver or gold along with the long interaction times due to slow light effect in photonic crystals. Typical optical dyes such as methyl orange (MO) and methylene blue (MB) have been primary photocatalytic activity characterization indicators for indirect methods based on a color transformation [10]. Unfortunately, since the incident light itself can bleach the dye and induce an invalid result and moreover, nanoparticles may detach and migrate from substrate into the solution during immersion in the solution, such methods should not be used for photocatalytic activity characterization [11]. Therefore, a better alternative for photocatalytic activity measurements is to use a gas- phase method based on mineralization of organic compounds. Additionally, this method is close to real application conditions and it also removes all mechanical stresses on the surface compared to the traditional solution-based approaches with optical dyes [10] in which the surfaces are immersed into a liquid. In the present study, TiO2 IO structures were prepared and loaded with three different types of nanoparticles for enhanced photocatalytic activity. Three types of nanoparticles were prepared by hydrothermal method; gold, silver and gold-silver core-shell nanoparticles (AuNPs, AgNPs and Au/AgNPs). Nanoparticles were coated into TiO2 IO on glass substrate by casting on three solutions of nanoparticles. Finally, the photocatalytic activity was measured using the in-house built gas-phase photoreactor.
2 Materials and methods 2.1 Synthesis of polystyrene spheres Styrene (Acros Organics, 99%) distilled in deionized water (Millipore), ammonium persulfate (APS, Sigma-Aldrich, 98.0+%), and sodium dodecyl sulfate (SDS, Sigma-Aldrich, 98.5+%)
Journal Pre-proof were used for the synthesis of polystyrene spheres that was performed following the procedure by Erolla [12] with minor changes. Synthesis of polystyrene spheres with an average diameter 400 nm was performed in two steps. All reagents were degassed before loading into the reaction mixture. First, 200 mg of SDS as a capping agent was added to 160 ml of deionized water in a reaction vessel. Reaction mixture was stirred using an anchor-like mixer at 500 rpm and heated up to 70°C. Then, 20g of styrene was added to the reaction mixture and kept for 30 min at the same temperature. After that, 200 mg of APS in 20 ml of deionized water was added into the reaction vessel as an initiator. Reaction proceeded for 20h under nitrogen atmosphere. In the second step, 150 mL of distilled water and 30 ml of colloidal PS solution prepared in the first step were loaded in a three-neck flask. Then 25 mg SDS was added and the system was purged with nitrogen gas until the temperature was raised to 70°C. When the temperature reached 70°C, 28 mg of APS dissolved in 20 ml of deionized water was added to the reaction vessel. After that, 21.6 g of styrene were added slowly to the flask using a syringe. The system was polymerized at 70°C under nitrogen atmosphere for 20 h.
2.2 Synthesis of TiO2 inverse opal films Titanium (IV) isopropoxide (TTIP, Sigma-Aldrich, ≥99%), hydrochloric acid (HCl, Merck, 37%), and ethanol (Altia, 99.5%) were used to prepare titanium precursor. TiO2 inverse opal (TiO2 IO) films were prepared in three steps; First, self-assembly of PS spheres was followed by infiltration of TiO2 precursor that was calcinated to obtain IO structure. Glass plates (Thermo Scientific) treated with piranha solution served as the surface for TiO2 IO films deposition. PS film was self-assembled on microscopic glass substrate by the vertical deposition. 1 ml of PS solution was diluted in 20 ml of distillated water in a small beaker. Microscope glass slides were immersed vertically and attached by clips so that they do not touch the bottom and the walls of the beaker. The evaporation took place in a laboratory oven
Journal Pre-proof in the temperature range of 60–65°C for 2 days. Then TiO2 sol was prepared by mixing TTIP (1 ml), ethanol (1 ml) and 0.1M HCl (3 ml). The resulting mixture was stirred for 1 h before use. After that precursor was diluted 2 times with ethanol. The microscopic glasses with PS films were dipped in precursor solution for 5 min and then dried in air for 24h for hydrolysis and formation of TiO2. After drying was completed, the resulting composites were calcined using the following protocol; heating from room temperature to 250°C in 250 min that was then kept for 120 min followed by an increase from 250°C to 550°C in 300 min with a final holding time of 300 min for complete removal of PS spheres and to convert TiO2 into anatase crystalline phase. Finally, the samples were cooled down to room temperature.
2.3 Synthesis of Au, Ag, and Au/Ag nanoparticles Gold (III) chloride trihydrate (HAuCl4 ·3 H2O,), sodium citrate (TSC, Sigma-Aldrich, ≥99%), and silver nitrate () were used without further purification for the synthesis of nanoparticles. Preparation of Au NPs. Au NPs were prepared by a hydrothermal method, similar to the earlier reported work [13]. An aqueous solution of HAuCl4 · 3H2O (10 ml, 0.1%, Alfa Aesar, ≥99.9%) was added to deionized water (100 ml) and heated to boiling point. Sodium citrate (TSC, Sigma-Aldrich, ≥99%) solution (2 ml, 1%) was then added and the resulting mixture was kept for 15 min under stirring. Upon cooling down to room temperature, the Au NPs were obtained in the solution Preparation of Ag NPs. Silver nanoparticles were prepared by citrate reduction of Ag (I) ions in aqueous medium according to the previously reported study [14]. AgNO3 (60 ml, 1.0 mM, Sigma-Aldrich) aqueous solution was heated to the boiling point under reverse condenser and then sodium citrate (3 ml; 2%) was added upon intensive magnetic stirring. Brown dispersion containing Ag NPs was formed within 15 min of boiling. The obtained suspension was stirred
Journal Pre-proof under reflux for additional 30 min to complete the reaction. Finally, a green-gray Ag colloid was obtained. Preparation of Au/Ag NPs. Au/Ag NPs were synthesized by reduction of AgNO3 and HAuCl4 · 3H2O with citrate, based on the reported procedure [15]. For Au-Ag alloy NPs, HAuCl4 · 3H2O (1 ml, 0.01 M) and AgNO3 (1 ml, 0.01 M) were dissolved in 78 ml of boiled deionized water upon rapid stirring. Then, 8 ml of sodium citrate (0.1 M) were quickly added into the reaction mixture. Solution was kept at 100 °C for 15 min with continuous stirring. The synthesized NPs were washed and centrifuged for several times before they were dispersed in water and kept in the dark at a room temperature.
2.4 Nanoparticle coating into TiO2 inverse opals The microscopic glasses with TiO2 IO were carefully immersed to the beaker with the diluted metal nanoparticles solution so that all TiO2 IO film was covered by the solution. For each solution the used volume was the same:1 ml of nanoparticle solution was diluted with 20 ml of water. The glass slides did not touch the bottom or the walls of the beaker. Then, the solution was slowly evaporated in the fume hood for two days and the nanoparticles were coated into the pores in the IO structure. The obtained structure was dried in an oven (T=100°C) for 6h and then washed with ethanol and deionized water. Finally, TiO2 IO with Au, Ag and Au/Ag were obtained for further characterization. Sodium ions are known to be catalytic poison that can influence TiO2 photocatalytic activity by altering the crystallographic properties or by acting as recombination centers [16]. Here sodium ions are only present in the preparation of the metal nanoparticles, which are coated from a dilute solution onto TiO2 IO. Hence, they do not change the crystallinity of the calcined TiO2 IO. Therefore, the total sodium ion concentration in metal NP coated TiO2 IO structures is negligible.
Journal Pre-proof 2.5 Characterization methods The morphology and average structural sizes of synthesized Au NPs, Ag NPs, Au/Ag NPs and TiO2 IO, TiO2 IO-Au NPs, TiO2 IO-Ag NPs and TiO2 IO-Au/Ag NPs were acquired with Hitachi S-4800 FE-SEM (field emission scanning electron microscope). SEM images of the TiO2 IOs and TiO2 IO with metal nanoparticles were observed after dispersing a small amount of the sample on a carbon tape. Scanning transmission electron microscope (STEM) images of the metal nanoparticles were obtained after drying a significantly diluted sample on a copper grid coated with Lacey carbon film. The elemental identification of the TiO2 IO with metal nanoparticles were acquired with energy dispersive X-ray spectroscopy (EDS) using the Noran system Six (NSS) software. A PerkinElmer Lambda 900 UV/Vis/NIR spectrometer was employed to obtain the absorption spectrum of the metal nanoparticles. The PerkinElmer Lambda 900 UV/Vis/NIR spectrometer fitted with a 150 mm integrating sphere was used to measure the reflectance spectra of the TiO2 IO, TiO2 IO-Au NPs, TiO2 IO-Ag NPs and TiO2 IO-Au/Ag NPs. An in-house built (University of Eastern Finland, Department of Chemistry, Joensuu campus) gas-phase photoreactor was used for photocatalytic activity characterization as shown schematically in Fig. 2. The diameter of the used reactor was 145 mm with a volume of 15.4 cm3. The reactor was equipped with online CO2 concentration detector (Vaisala GMP343), temperature and humidity sensor (Thorlabs TSP01), and pressure meter (Wika PGT10, USB mode). All detectors were USB connected to the PC for a real-time, online measurements. Acetylene (C2H2) was used as a test analyte that was oxidized into CO2. Such approach has been shown valid for analyzing photocatalytic response before [17-19]. A mixture of acetylene and technical air was formed in the mixing chamber followed by injection into the batch-type reactor chamber. The gas flow through the reactor was continuous until the CO2
Journal Pre-proof concentration become constant. The ultraviolet UVA excitation was performed through the UVA transparent quartz glass reactor window from a high-intensity (100W) UVA lamp (UVP Black-Ray® B-100AP High Intensity, peak emission at 365 nm) located 17 cm above the reactor window. Microscopic glass substrates of TiO2 IO with and without metal nanoparticles were placed into the photoreactor. The UVA light excited the photocatalytic activity of TiO2 followed by decomposition of C2H2. As a result, acetylene was oxidized into CO2 that was measured using an optical CO2 concentration detector. All data collected from the CO2 detector was used directly without any refinement since the reaction temperature, humidity and pressure were kept constant during the measurement time.
3 Results and discussion 3.1 Characterization of metal nanoparticles Figure 3 (A, C, E) represents the TEM images of Au NPs, Ag NPs and Au/Ag NPs as synthesized. The average diameters of metal nanoparticles were around 40 ± 5 nm excluding the agglomerated nanoparticles that were observed with average diameter of 80 ± 10 nm. EDS mapping analysis was performed to confirm the composition of metal nanoparticles and each element in the Au/Ag NPs. Figure 3 (B, D, F) shows STEM images of metal nanoparticles, which were used for elemental mapping analysis. The mapping profiles show that Au NPs and Ag NPs fully consist of metallic Au and Ag respectively (red and yellow dots). It can be clearly seen that, in case of Au/Ag NPs, both Au and Ag metals were distributed homogeneously across the particle (red and yellow dots together). Surface plasmon resonance (SPR) was analyzed by UV–Vis absorption spectroscopy for metal nanoparticle characterization. Figure 3 (G) represents normalized extinction spectra of Au, Ag, and Au/Ag NPs. SPR characteristic peaks at about 520 nm and
Journal Pre-proof 430 nm correspond to Au and Ag NPs, respectively [20]. The red shift occurs from SPR absorption peak of Ag NPs towards the Au NPs for Au/Ag NPs when the ratio of Au to Ag increases. The SPR peak at 475 nm for Au/Ag presented in the Figure 3 (G) matches well with the composition alloy NPs [15].
3.2 Characterization of TiO2 IO and TiO2 IO with metal nanoparticles The polystyrene spheres with diameter of 400 nm were self-assembled as a template on the microscopic glass (Fig. 4 C). The PS spheres have a highly ordered structure that was formed during the slow evaporation of solution. As soon as calcination process was completed, TiO2 IO structures were observed (Fig. 4 A). TiO2 IO had a well-ordered honeycomb structure in anatase crystalline phase. However, the surface contained some defects and cracks, which may slightly affect the photocatalytic activity [21]. The pore size was around 350 nm whereas PS spheres had a diameter of 400 nm. This indicates that during the high temperature treatment PS spheres shrank approximately 12–13%. The crack formation mainly derives from shrinking and infiltration process. However, it should be noted that the overlayers were not observed from the top views of the structures. Noble metal NP coating on the surface of TiO2 IO can work as electron trap to promote the separation of photo-generated electrons and holes, and thus enhance photocatalytic activity. Moreover, noble metals can extend the response region of TiO2 from UVA to visible range via SPR. Figure 4 (B, D, E) shows the SEM images of TiO2 IO with gold, silver and gold/silver nanoparticles respectively. In all cases, the metal nanoparticles were uniformly distributed and firmly anchored. It can be clearly seen that nanoparticles with smaller diameter decorated walls of the TiO2 IO holes while bigger or agglomerated particles coated inside the cavities. All samples had the same morphology as TiO2 IO without metal nanoparticles indicating that metal NP coating process does not affect on the TiO2 IO morphology. Reflectance spectra of all samples were measured normal to the FCC (111) plane surface (Figure 4 F). TiO2 IO without NPs exhibits a photocatalytic activity only in the UVA region with a wavelength below 380–385 nm, which can be attributed to the intrinsic band gap of the TiO2. Moreover, a broad absorption peak (400–600) nm was observed with TiO2 IO with metal nanoparticles samples. This broad peak can be assigned to the sub-band gap state of the special TiO2 nano-ordered structure [22]. Compared to the TiO2 without metal nanoparticles,
Journal Pre-proof all other samples showed a strong absorption from 400 nm to 600 nm due to presence of metal NPs. However, the peaks of metal NPs SPR overlap with the sub-band of TiO2 [23].
3.3 Photocatalytic activity The hot electron transfer, resonant photon scattering, and near-field electromagnetic enhancement are possible energy-transfer mechanisms in a plasmonic photocatalyst systems [24, 25]. Near-field electromagnetic enhancement usually can be seen at wavelengths where the plasmon resonance and semiconductor absorption overlap [26]. In our case we can exclude effect of the resonant photon scattering since it normally occurs in the large plasmonic nanostructures [27]. We assume that the hot electron transfer mechanism was the major contribution for the observed enhanced photocatalytic activity of the TiO2 IO with metal NPs. Fig. 5(a) displays the measured photocatalytic activity of the TiO2 IO structures with metal NPs compared to the reference TiO2 IO structure. All metal NPs increase the photocatalytic activity: the highest enhancement was observed with Au/Ag core-shell nanoparticles (62%) followed by Au NPs (53%) and Ag NPs (39%). Furthermore, Fig. 5(b) shows the cyclic activation of the TiO2 IO structures with Au/Ag metal NPs showing the reproducibility and stability of the results. LSPR in bimetallic Au/Ag nanoparticles were studied using UV-Vis spectroscopy with varying metal content [28]. It was observed that bimetallic Au/Ag nanoparticles show a tunable LSPR resonance with broad absorption peaks. Our findings agreed with the predictions in Ref. [28] as observed from Fig. 3 (G): the Au/Ag NPs displayed the broadest absorption peak that was also the case in Fig. 4 (F) when loaded into the TiO2 IO structure. It is believed that the wider and stronger absorption in Au/Ag NPs results in a stronger coupling of the incident light into the nanoparticles that is followed by increased photocatalytic activity. The observed photocatalytic process can be explained according to the diagram presented in Fig. 6. Metal NPs absorb incoming visible light and electrons get photoexcited simultaneously metal ions (M+) are formed due to the LSPR effect (Eq. 1). The photoexcited electrons transfer from metal nanoparticles onto TiO2 IO surface by a Schottky barrier at the M/TiO2 interface owing to the larger work function. The electric field in the space charge promotes the transportation of excited electrons and enhances the charge separation between the photoexcited electrons and metal ions (Eq. 2). The injected electrons can be transferred to the abundant molecular oxygen to form superoxide anion radicals (O2 ∙-) (Eq. 3) and
Journal Pre-proof subsequent protonation gives (HO2∙) radicals (Eq. 4). H2O2 produces by the mixing of HO2∙ radicals and the trapped electrons (Eq. 5) following by formation of hydroxyl radicals (OH∙) (Eq. 6). However, these active species will result in the degradation and mineralization of acetylene. At the same time, the metal ions are also reactive radical species and they can directly convert C2H2 into CO2 [29]. The overall reactions (1) – (7) are given below: Me NPs + hν → Me NPs*
(1)
Me NPs* + TiO2 → Me NPs+ (h+) + TiO2(e-)
(2)
TiO2 (e-) + O2 → TiO2 +O2∙-
(3)
O2∙- + H+ → HO2∙
(4)
e- + HO2∙ + H+ → H2O2
(5)
H2O2 + e- → OH∙ + OH-
(6)
Me NPs+ (h+) + 2OH∙ + C2H2 + 2O2
→ Me NPs + 2CO2 + 2H2O
(7)
4 Conclusions In conclusion, three types of metal (Au, Ag, Au/Ag) NPs were successfully synthesized and loaded into TiO2 IO for enhanced photocatalytic activity. All prepared materials were characterized for their photocatalytic activity by using an in-house built gas-phase photoreactor. TiO2 with metal NPs showed a higher photocatalytic activity than TiO2 without metal NPs. The highest photocatalytic activity was observed with Au/Ag NPs (increase of 62%) followed by Au NPs (53%) and Ag NPs (39%). The observed enhancement in photocatalytic activity can be explained by hot electron transfer mechanism. It is believed that such metal NP doped TiO2 IO structures will find applications as enhanced photocatalysts with significant potential for solar-driven applications.
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Acknowledgements FT wishes to thank the Finnish Cultural Foundation for a research grant. JJS acknowledges the Faculty of Science and Forestry at the University of Eastern Finland for the financial support (grant no. 579/2017).
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Figure captions Figure 1. A schematic representation of the fabrication of TiO2 IO loaded with metal NPs. Figure 2. A schematic illustration of the used gas-phase reactor. Figure 3. SEM images of (A) Au NPs, (C) Ag NPs and (E) Au/Ag NPs, EDS mapping SEM images of (B) Au NPs, (D) Ag NPs and (F) Au/Ag NPs and (G) absorption spectra of metal nanoparticles. Figure 4. SEM images of (A) PS self-assembly, (B) TiO2 IO without nanoparticles, (C) TiO2 IO with Au NPs, (D) TiO2 IO with Ag NPs, (E) TiO2 IO with Au/Ag NPs and (F) reflectance spectrum of structures (green line is TiO2 IO, black line is TiO2 IO with Ag NPs, red line is TiO2 IO with Au NPs and blue line is TiO2 IO with Au/Ag NPs). Figure 5. (A) Photocatalytic activity of TiO2 IO structures with metal NPs, and (B) cycling degradation curves for TiO2 IO with Au/Ag NPs. Figure 6. Schematic diagram of a charge transfer process in TiO2 IO with metal NPs.
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Preparation of TiO2 inverse opal structures with facile deposition of metal (gold, silver and gold-silver) nanoparticles for enhanced photocatalytic activity
Filipp Temerov*, Bright Ankudze, Jarkko J. Saarinen Department of Chemistry, University of Eastern Finland P.O. Box 111, FI-80101 Joensuu, Finland *Corresponding author: [email protected]
Declarations of interest: The authors declare that they have no competing interest to declare.
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Highlights
TiO2 inverse opal (IO) structures were prepared by an infiltration method.
Gold, silver, and gold/silver nanoparticles (NPs) were coated onto TiO2 IOs.
Enhanced photocatalytic activity was observed with metal nanoparticles.
Au/Ag core-shell NPs increased photocatalytic activity by 62%.
Au and Ag NPs increased photocatalytic activity by 53% and 39%, respectively.