Surface enhanced Raman scattering substrate with high-density hotspots fabricated by depositing Ag film on TiO₂-catalyzed Ag nanoparticles

Highlights

• SERS activity of TiO₂-catalyzed Ag NPs is improved ∼400 times by sputtering Ag film.

• SERS substrates with high-density hotspots are obtained by combined method.

• Enhancement factor of the SERS substrates reaches to ∼1.2 × 10⁷.

• High-density hotspots is responsible for the improvement of SERS activity.

Abstract

Surface enhanced Raman scattering (SERS) has been developed as a powerful tool for detection of ultra-trace molecules, thus at foundation of a number of important technologies, such as chemical analysis and biosensor. Photocatalytic growth of Ag nanoparticles (NPs) by TiO₂ film is a cost-effective, facile and surfactantless way to fabricate SERS substrates with high stability and reproducibility, but the SERS activity still needs to be improved. In this work, we report a facile way to improve SERS activity of TiO₂-catalyzed Ag NPs by additionally depositing a layer of Ag film using magnetron sputtering. Combining with the advantages of liquid and vapor environment, the Ag NPs size increase to the resonance size without decrease of density, so the spacing of Ag NPs decrease and high-density hotspots can be formed. By optimizing the sputtering time of Ag films, the SERS activity of the Ag NPs/films is improved by a factor of ∼400. This work not only establish a low-cost method for fabricating high-quality SERS substrate, more importantly, propose a novel way to prepare plasmonic nanostructures with high-density hotspots.

Introduction

Surface enhanced Raman scattering (SERS), which occurs on rough metallic (typically Au and Ag) surfaces or nanoparticles (NPs), has been intensely explored as a highly sensitive technique for detection of ultra-trace or single molecule [1], [2], [3]. For the purpose of SERS application in chemical analysis and biosensing, a key step is fabrication of high-quality SERS substrates, which needs to be highly sensitive, reproducible, stable over time, easy to be fabricated, and cost-effective [4]. In the past decades, many methods have been explored to fabricate various nanostructures for SERS substrates, such as cycling oxidation-reduction of Ag electrode surface [5], deposition of metallic island films [6] or Ag nanorods [7], lithography for ordered Ag nanostructures [8], [9] or three-dimensional metal nanostructures [10], [11]. Though these methods are potential for fabrication of SERS substrate, there is a necessity to be further improved in the viewpoint of practical applications. For example, lithography is capable of fabricating a SERS substrate with highly controlled hotspots, but its high cost is obstructive to commercial application. Extremely high SERS activity was claimed to achieve on Ag-decorated three-dimensional nanostructures [12], [13], [14], but the reproducibility needs to be improved. Therefore, it is necessary to explore the way for commercial applications.

Because of the excellent photocatalytic activity, TiO₂ films have been widely used to grow Ag NPs on the surface of TiO₂ films by reducing the Ag⁺ in AgNO₃ aqueous solution under the irradiation of ultraviolet (UV) light [15], [16], [17], [18], [19]. The TiO₂-catalyzed Ag NPs can be used as SERS substrate with the following advantages. First, photocatalytic reduction is a cost-effective method facilitating fabrication of TiO₂-catalyzed Ag NPs, in which the semiconductor (TiO₂) and Ag NPs synergically contribute to Raman enhancement. Second, the TiO₂-catalyzed Ag NPs prepared by the photocatalytic reduction are surfactant free, thus preventing against the contamination in SERS measurement from the agents, which are usually necessary in synthesis of noble metallic NPs by other solution-phase reactions, such as the chemical synthesis of Ag NPs in colloids. Third, the TiO₂-catalyzed Ag NPs are stable for long-time use (>1 year) [16]. More importantly, the TiO₂-catalyzed Ag NPs is very uniform in size and controllable, thus leading to a high reproducibility in acquiring Raman signals. For example, Li et al. [17] found the spot-to-spot variance in Raman signals on the TiO₂-catalyzed Ag NPs as small as ∼10%, which is a good performance among various SERS substrates. Despite of above advantages, the TiO₂-catalyzed Ag NPs are still absent of enough attention, because the SERS activity needs to be improved when they are used as SERS substrates.

It is well known that the electromagnetic hotspots, which mainly occurred in the nanogaps less than 10 nm, make the most important contribution to the Raman enhancement in SERS substrates [20], [21], [22], [23]. By investigations into the TiO₂-catalyzed Ag NPs, the spacing between adjacent Ag NPs was found too large to form hotspot, thus producing a relatively low SERS activity [17]. Systemic study revealed that the liquid environment is a major factor resulting in the wide gaps between the Ag NPs [24]. In the liquid environment, Ostwald ripening is the dominative mechanism controlling the photocatalytic growth of Ag NPs, as schematically shown in Fig. 1(a), and then leading to a wide distribution of Ag NPs in size, as shown in Fig. 1(c), and a lower density, thus obstructing the formation of high-density hotspots. This shortage cannot be overcome in the photocatalytic growth of TiO₂-catalyzed Ag NPs. Therefore, we have to explore other methods for improving the SERS activity of the TiO₂-catalyzed Ag NPs.

Differing from the photocatalytic growth of Ag NPs in liquid environment, OR-dominated growth of Ag NPs is limited at room temperature in physical vapor deposition (PVD), such as magnetron sputtering. After nucleation, the Ag NPs deposited by magnetron sputtering could uniformly grow up without obvious OR process, as schematically shown in Fig. 1(a) and the SEM image in Fig. 1(d). High-density hotspots have been obtained in deposition of Ag films at the percolation threshold [25], but the sizes of Ag NPs is at a level of 10–30 nm, which is too small to produce a stronger Raman resonance under excitation of visible light. Moreover, there is a difficulty in technology controlling the growth of Ag NPs at the percolation threshold. After the point of percolation threshold, coalescence of Ag NPs will take place, and then resulting in formation of Ag film, which exhibits a rather low SERS activity, as shown in Fig. 1(b).

The different growth modes between vapor and liquid environment make us think of a novel way to fabricate SERS substrates with high-density hotspots by combining the photocatalytic growth and magnetron sputtering. It can be expected that the limited OR growth of Ag NPs in vapor environment will increase the size of TiO₂-catalyzed Ag NPs without decrease its density, leading to the formation of high-density hotspots.

In this work, we report our attempt to fabricate a new SERS substrate with high-density hotspots by depositing a layer of Ag film on the TiO₂-catalyzed Ag NPs using magnetron sputtering method. The enhancement of Raman signals was studied as a function of deposition time of Ag films, and an enhancement factor as large as ∼1.2 × 10⁷ was obtained by optimizing the deposition time of Ag films. This method was demonstrated easily to control the SERS activity of TiO₂-catalyzed Ag NPs at a relatively high level, which is comparable with the SERS substrates fabricated by lithography or other methods. In addition, we studied the dependence of hotspots on the deposition time and gave a discussion on the enhancement mechanism in the SERS substrates.