In Situ Synthesis of Gold Nanoparticles without Aggregation in the Interlayer Space of Layered Titanate Transparent Films

Summary

Here, we present a protocol for the in situ synthesis of gold nanoparticles (AuNPs) within the interlayer space of layered titanate films without the aggregation of AuNPs. No spectral change was observed even after 4 months. The synthesized material has expected applications in catalysis, photo-catalysis, and the development of cost-effective plasmonic devices.

Abstract

Combinations of metal oxide semiconductors and gold nanoparticles (AuNPs) have been investigated as new types of materials. The in situ synthesis of AuNPs within the interlayer space of semiconducting layered titania nanosheet (TNS) films was investigated here. Two types of intermediate films (i.e., TNS films containing methyl viologen (TNS/MV²⁺) and 2-ammoniumethanethiol (TNS/2-AET⁺)) were prepared. The two intermediate films were soaked in an aqueous tetrachloroauric(III) acid (HAuCl₄) solution, whereby considerable amounts of Au(III) species were accommodated within the interlayer spaces of the TNS films. The two types of obtained films were then soaked in an aqueous sodium tetrahydroborate (NaBH₄) solution, whereupon the color of the films immediately changed from colorless to purple, suggesting the formation of AuNPs within the TNS interlayer. When only TNS/MV²⁺ was used as the intermediate film, the color of the film gradually changed from metallic purple to dusty purple within 30 min, suggesting that aggregation of AuNPs had occurred. In contrast, this color change was suppressed by using the TNS/2-AET⁺ intermediate film, and the AuNPs were stabilized for over 4 months, as evidenced by the characteristic extinction (absorption and scattering) band from the AuNPs.

Introduction

Various noble metal nanoparticles (MNPs) exhibit characteristic colors or tones due to their localized surface plasmon resonance (LSPR) properties; thus, MNPs can be used in various optical and/or photochemical applications^1-4. Recently, combinations of metal oxide semiconductor (MOS) photocatalysts, such as titanium oxide (TiO₂) and MNPs, have been thoroughly investigated as new types of photocatalysts^5-14. However, in many cases, very small amounts of MNPs exist on the MOS surface, because most MOS particles have relatively low surface areas. On the other hand, layered metal oxide semiconductors (LMOSs) exhibit photocatalytic properties and have a large surface area, typically several hundred square meters per unit g of an LMOS^15-17. In addition, various LMOSs have intercalation properties (i.e., various chemical species can be accommodated within their expandable and large interlayer spaces)^15-20. Thus, with a combination of MNPs and LMOSs, it is expected that relatively large amounts of MNPs are hybridized with the semiconductor photocatalysts.

We have reported the first in situ synthesis of copper nanoparticles (CuNPs)²¹ within the interlayer space of LMOS (titania nanosheet; TNS^16-30) transparent films through very simple steps. However, the details of the synthetic procedures and the characterization of the other noble MNPs and TNS hybrids have not yet been reported. Moreover, the CuNPs within the TNS layers were easily oxidized and decolorized under ambient conditions²¹. As such, we focused on gold nanoparticles (AuNPs), because AuNPs are widely used for various optical, photochemical, and catalytic applications, and it is expected that they will be relatively stable against oxidation^{3-5,7,8,10-14,28,31,32}. Here, we report the synthesis of AuNPs within the interlayer space of TNS and show that 2-ammoniumethanethiol (2-AET⁺; Figure 1 inset) works effectively as a protective reagent for AuNPs within the interlayer of TNS.

Protocol

Caution: Always use caution when working with chemicals and solutions. Follow the appropriate safety practices and wear gloves, glasses, and a lab coat at all times. Be aware that nanomaterials may have additional hazards as compared to their bulk counterpart.

1. Preparation of Regents

1. Prepare the methyl viologen aqueous solution by dissolving 0.0012 g of 1,1'-dimethyl-4,4'-bipyridinium dichloride (methyl viologen; MV²⁺) in 20 ml of water to give 0.2 mM MV²⁺.
2. Prepare the gold(III) chloride aqueous solution by dissolving 0.1050 g of gold(III) tetrachloride trihydrate (HAuCl₄• 3H₂O) in 10 ml of water to give 25 mM HAuCl₄.
3. Prepare the sodium borohydride aqueous solution by dissolving 0.03844 g of sodium tetrahydroborate (NaBH₄) in 10 ml of water to give 100 mM NaBH₄.
4. Prepare the 2-ammoniumethanethiol aqueous solution by dissolving 0.2985 g of 2-ammoniumethanethiol chloride salt (2-AET⁺) in 25 ml of water to give 100 mM 2-AET⁺.

2. Synthesis of TNS Colloidal Suspensions

NOTE: Titania nanosheets (TNS; Ti_0.91O₂) were prepared according to the well-established procedure reported previously^22,23,30.

1. Prepare the starting material of layered cesium titanate Cs_0.7Ti_1.825O₄ by calcining a stoichiometric mixture of Cs₂CO₃ (0.4040 g) and TiO₂ (ST-01; 0.5000 g) at 800 °C for 20 hr²². Repeat this twice.
2. Prepare the protonated layered titanate (H_0.7Ti_1.825O₄·H₂O) by repeatedly treating 0.8142 g of cesium titanate with an HCl (100 mM, 81.42 ml) aqueous solution by using a shaker (300 Hz) for 12 hr.
3. Prepare the exfoliated layered titanate (TNS) colloidal suspensions by stirring the protonated titanate powder (0.0998 g) vigorously (500 rpm) with 25 ml of a 17 mM tetrabutylammonium hydroxide (TBA⁺ OH⁻) aqueous solution for about 2 weeks at ambient temperature under dark conditions. The resulting opalescent suspension contains exfoliated titania nanosheets (TNS; 1.4 g/L, pH = 11~12).

3. Synthesis of TNS Films²¹

1. Preparation of TNS cast films (c-TNS)
   1. Pre-clean glass substrates (~20 x 20 mm²) through ultrasonic treatments using an ultrasonic cleaner (27 kHz) in 1 M aqueous sodium hydroxide (NaOH) for 30 min.
   2. Rinse the substrates with 5-10 mL of ultrapure water (<0.056 µS cm^-1).
   3. Dip a glass substrate in a 0.1 M aqueous hydrochloric acid (HCl) for 3 min and rinse with 5-10 ml of ultrapure water.
   4. Clean the substrates through ultrasonic treatments (27 kHz) in pure water for 1 hr, and then rinse with pure water. Dry with a hairdryer for 2-3 min (until dry).
   5. Cast the colloidal suspension of TNS on the glass substrate in 300 µl aliquots.
   6. Dry at 60 °C for 2 hr using a dry oven to give the c-TNS film.
2. Preparation of Sintered TNS Film (s-TNS)
   1. To achieve thermal fixation of the TNS components on the glass substrate (s-TNS film), sinter the obtained c-TNS film in air at 500 °C for 3 hr (heating from 25 to 500 °C at a rate of 6.8 °C/min) using the oven.
   2. Repeat the sintering process twice.
3. Preparation of Films
   1. When the s-TNS films are immersed in solution, position the deposited s-TNS film so that it faces the top for all experimental procedures.
   2. Carry out all experiments under dark conditions by covering the setup with aluminum foil to avoid the photoreaction of TNS.
4. Preparation of Methyl Viologen (MV²⁺) Intercalated TNS Films (TNS/MV²⁺)
   1. Immerse an s-TNS film in an aqueous solution of MV²⁺ dichloride salt (0.2 mM, 3 ml) in a Petri dish for 7 h at room temperature (RT) under dark conditions.
   2. Rinse the obtained samples with ultrapure water (5-10 ml) and dry in air at 60 °C using an oven in the dark for ~1 hr.
5. Preparation of Au(III) Intercalated TNS Films (TNS/Au(III))
   1. Immerse a TNS/MV²⁺ film in an aqueous solution of HAuCl₄ (25 mM, 3 ml) in a Petri dish for 3 hr at RT under dark conditions.
   2. Rinse the obtained samples with ultrapure water (5-10 ml) and dry in air at 60 °C using an oven in the dark for ~1 hr.
6. Synthesis of AuNP within the Interlayer Space of TNS Films (TNS/AuNP)
   1. Immerse a TNS/Au(III) film in an aqueous solution of NaBH₄ (0.1 M, 5 ml) in a Petri dish for 0.5 hr at RT under dark conditions.
   2. Dry the obtained films in air at 60 °C using an oven in the dark for ~1 hr.
7. Preparation of 2-AET⁺ Intercalated TNS Films (TNS/2-AET⁺)
   1. Immerse an s-TNS film in an aqueous solution of 2-AET⁺ Cl⁻ (0.1 M, 3 ml) in a Petri dish for 24 hr at RT.
   2. Rinse obtained films with ultrapure water (5-10 ml) and dry in air at 60 °C using an oven in the dark for ~1 hr.
8. Au(III) and 2-AET⁺ Co-Intercalated TNS Films (TNS/2-AET⁺/Au(III)).
   1. Immerse a TNS/2-AET⁺ film in an aqueous solution of HAuCl₄ (25 mM, 3 ml) for 3 h at RT.
   2. Rinse the obtained films with ultrapure water (5-10 ml) and dry in air at 60 °C using an oven in the dark for ~1 hr.
9. Synthesis of AuNP within the Interlayer Space of TNS/2-AET⁺ Films (TNS/2-AET⁺/AuNP).
   1. Immerse a TNS/2-AET⁺/Au(III) film in an aqueous solution of NaBH₄ (0.1 M, 5 ml) in a Petri dish for 0.5 hr at RT under dark conditions.
   2. Rinse the obtained films with ultrapure water (5-10 ml) and dry in air at 60 °C using oven in the dark for ~1 hr.
10. Characterizations
    1. Carry out X-ray diffraction (XRD) analyses²¹ using a desktop X-ray diffractometer with monochromatized Cu-K_α radiation (λ = 0.15405 nm), operated at 30 kV and 15 mA.
    2. Take energy dispersive X-ray spectrometry (EDS) spectra²¹.
    3. Employ a multichannel photodetector or steady-state ultraviolet-visible (UV-Vis) absorption spectrophotometer to record UV-Vis absorption spectra for the prepared samples using transmittance mode²¹.

Results

Two types of precursor films were used in this study (i.e., with and without the protective reagent (2-AET⁺) within the interlayer of TNS). In the absence of 2-AET⁺, 1,1'-dimethyl-4,4'-bipyridinium dichloride (methyl viologen; MV²⁺) was used as an expander of the interlayer space, because MV²⁺-containing LMOSs have been frequently used as intermediates in the guest exchange method for preparing LMOSs^{16,17,21,33-36}.

Discussion

This manuscript provides a detailed protocol for the in situ synthesis of gold nanoparticles (AuNPs) within the interlayer space of TNS films. This is the first report of the in situ synthesis of AuNPs within the interlayer space of TNS. Moreover, we found that the 2-AET⁺ works as an effective protective reagent for AuNPs within the interlayer of TNS. These methods hybridized AuNPs and TNS transparent films. TNS films with good optical transparency²¹ were synthesized through sinter...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Methyl viologen dichloride	Aldrich Chemical  Co., Inc.	1910-42-5
Tetrabutylammonium hydroxide	TCI	T1685
cesium carbonate	Kanto Chemical Co., Inc.	07184-33
anatase titanium dixoide	Ishihara Sangyo Ltd.	ST-01
hydrochloric acid	Junsei Chemical Co., Ltd.	20010-0350
sodium hydroxide	Junsei Chemical Co., Ltd.	195-13775
Tetrachloroauric(III) acid trihydrate	Kanto Chemical Co., Inc.	17044-60
sodium tetrahydroborate	Junsei Chemical Co., Ltd.	39245-1210
2-ammoniumethanethiol hydrochloride	TCI	A0296
Ultrapure water (0.056 µS/cm)			Milli-Q water purification system (Direct-Q® 3UV, Millipore)
Microscope slide (Thickness: 1.0–1.2 mm)	Matsunami glass Co., Ltd.