**3. Results and discussion**

As shown in **Figure 2**, the specific morphology of TiO<sup>2</sup>

nanotubes fabricated at different voltages [21].

**substrates based on TiO2**

ture of the TiO2

of TiO2

40 Raman Spectroscopy

Innovative SERS active platforms based on TiO2

water mixture (volume ratio 50:50) with 0.27 M NH4

Ag nanoparticles (from 0.01 up to 0.03 mg/cm2

by controlling the conditions of anodic polarization (type of electrolyte, voltage and anodization time), because there is a direct linear relationship between anodization voltage and the average diameter of the nanotubes formed. In general, the diameter and wall thickness of the nanotubes increase with anodic voltage [20, 21]. The possibility of preparing nanotubes of different size, shape and wall thickness provides control over the geometrical surface area and specific surface area, which are important parameters when developing new substrates for SERS applications.

**Figure 2.** Effect of anodic voltage Vmax on average diameter and wall thickness of nanotubes. The inserts show top views

for investigating various organic probe molecules such as pyridine (Py), mercaptobenzoic acid (MBA), organic dye 5-(4-dimethylaminobenzylidene) rhodamine (DBRh) and rhodamine 6G (R6G). For this purpose, simple electrochemical methods were applied: anodic oxidation of Ti foil (0.25 mm-thick, 99.5% purity, Alfa Aesar) in an optimized electrolyte: a glycerol/

10 up to 30 V. They led to the formation of nanoporous titanium oxide structures, and subsequently, to the preparation of specific metal nanostructures on surfaces thereof during PVD processes (magnetron sputtering, evaporation at high and low vacuum). Before the Ag deposition, all of the samples were annealed in air at 650°C for 3 h in order to transform the struc-

technique: the evaporation method in a low vacuum (p = 3 × 10−3 Pa) with a JEE-4X JEOL

**2. Fabrication, surface and structure characterization of SERS** 

 **nanotubes**

NT from amorphous to crystalline [19, 22].

nanotubes can be relatively easily modified

NT with noble metal deposits (Ag) were used

F under different constant voltages from

) were deposited using the sputter deposition

For our studies, we used an optimized electrolyte based on a mixture of glycerol and water (volume ratio 50:50) with 0.27 M NH4 F. The growth of nanotubes under a constant voltage is perpendicular to the metal substrate, as shown in **Figure 3**. STEM images reveal nanotubes that are hollow in shape, separated from each other, and that feature a characteristic

**Figure 3.** STEM images of a cross-section of a TiO2 nanotubular layer before and after heat treatment at 650°C (20 V); insert shows a top view of the nanotubes obtained at 25 and 10 V.

"columnar structure." A typical surface morphology of the nanotubes is shown in the inset to **Figure 3** (10 and 25 V). Usually, the as-grown porous anodic layers exhibit poor adhesion to the Ti substrate, and so, to improve their adhesion and mechanical stability, the samples were annealed in air at 650°C for 3 h. The heat treatment in this temperature range does not cause any visible changes in the diameter or shape of the TiO2 nanotubes, but in the annealing process an interfacial region is formed between the Ti substrate and the TiO2 nanotubes that stabilizes the entire TiO2 NT/Ti substrate system. Moreover, the heat treatment results in a change in the nanotube structure, from amorphous (directly after anodization) to crystalline: anatase [19, 21, 24]. All these factors are crucial for the suitability of the nanotubes as substrates when preparing SERS-active adsorbates.

After annealing, the above structures proved to be important for designing active substrates for SERS spectroscopy, where a large surface area and a stabilized structure are required [19]. The free-standing nanotubes adorned with Ag nanoparticles formed a layer of natural nanoresonators (antennas), which repeatedly enhanced Raman scattering [14, 18]. However, the geometrical factors of the Ag-n deposit and their relation to the geometry of the nanotubes are not yet well understood. Therefore, the details of the SERS mechanism should be carefully considered.

agglomerated and form rings. As a result of silver evaporation at a low vacuum, the plasmonic

PVD processes: (a) magnetron sputtering, (b) evaporation at a low vacuum, and (c) evaporation at a high vacuum.

nanotube layers (top-views) formed at 25 V and loaded with 0.01 mg Ag/cm2

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walls of the nanotubes and forming a thin, solid coating around the tubes (**Figure 4b**). After evaporation at a high vacuum, where the process of silver deposition is very slow (0.13 nm/min), spherical Ag nanoparticles are formed (see **Figure 4c**). The diameter of these particles is below

nanoresonators having a strictly defined geometry and surface development. This would ensure the formation of highly active places—"hot spots," i.e., gaps and cavities serving as surface plas-

uum evaporation at low and high vapor pressures. Signals from the Ag MNN, Ti LMM and O KLL Auger transitions are clearly visible. These results suggest that O is bound to Ti, and there are areas where the oxidized Ti substrate is not fully covered by the Ag deposit, which

mon resonators that significantly increase the intensity of the electromagnetic field. **Figure 5** shows typical AES spectra for samples with an Ag deposit (0.01 mg/cm2

nanotubes coated with a plasmonic metal (Ag) could act as antenna-

nanotube layer, tightly covering the

) after vac-

after different

nanoparticles are distributed homogenously in the TiO2

is consistent with the SEM microscopic observations.

50 nm. Densely-packed TiO2

**Figure 4.** SEM images of TiO2

In general, there are two important mechanisms underlying SERS. The first, and the dominant, mechanism toward large SERS enhancement factors (EF ) is that of electromagnetic field enhancement, where localized surface plasmons (LSPs) in the metallic nanostructure increase the Raman signal intensity. The other contribution to SERS EF is the chemical enhancement mechanism, where the charge transfer between the adsorbed molecule and the metal plays a critical role in enhancing and modifying the modes of molecular vibration [25]. Both enhancement mechanisms can operate simultaneously when using TiO2 nanoporous structures in the form of freestanding nanotubes with a suitable, carefully prepared deposit of silver nanoparticles.

**Figure 4** shows SEM images of TiO2 nanotube layers (top-views) formed at 25 V and loaded with 0.01 mg/cm2 of Ag. A careful inspection of **Figure 4a** reveals that the magnetron-deposited Ag-n is located on the tops and side walls of the nanotubes, while the silver particles become Titanium (IV) Oxide Nanotubes in Design of Active SERS Substrates for High Sensitivity… http://dx.doi.org/10.5772/intechopen.72739 43

"columnar structure." A typical surface morphology of the nanotubes is shown in the inset to **Figure 3** (10 and 25 V). Usually, the as-grown porous anodic layers exhibit poor adhesion to the Ti substrate, and so, to improve their adhesion and mechanical stability, the samples were annealed in air at 650°C for 3 h. The heat treatment in this temperature range does not

a change in the nanotube structure, from amorphous (directly after anodization) to crystalline: anatase [19, 21, 24]. All these factors are crucial for the suitability of the nanotubes as

After annealing, the above structures proved to be important for designing active substrates for SERS spectroscopy, where a large surface area and a stabilized structure are required [19]. The free-standing nanotubes adorned with Ag nanoparticles formed a layer of natural nanoresonators (antennas), which repeatedly enhanced Raman scattering [14, 18]. However, the geometrical factors of the Ag-n deposit and their relation to the geometry of the nanotubes are not yet well understood. Therefore, the details of the SERS mechanism should be carefully considered. In general, there are two important mechanisms underlying SERS. The first, and the domi-

enhancement, where localized surface plasmons (LSPs) in the metallic nanostructure increase

mechanism, where the charge transfer between the adsorbed molecule and the metal plays a critical role in enhancing and modifying the modes of molecular vibration [25]. Both enhancement

of freestanding nanotubes with a suitable, carefully prepared deposit of silver nanoparticles.

Ag-n is located on the tops and side walls of the nanotubes, while the silver particles become

ing process an interfacial region is formed between the Ti substrate and the TiO2

nanotubes, but in the anneal-

) is that of electromagnetic field

is the chemical enhancement

nanoporous structures in the form

nanotube layers (top-views) formed at 25 V and loaded

of Ag. A careful inspection of **Figure 4a** reveals that the magnetron-deposited

NT/Ti substrate system. Moreover, the heat treatment results in

nanotubular layer before and after heat treatment at 650°C (20 V);

nanotubes

cause any visible changes in the diameter or shape of the TiO2

nant, mechanism toward large SERS enhancement factors (EF

the Raman signal intensity. The other contribution to SERS EF

mechanisms can operate simultaneously when using TiO2

**Figure 4** shows SEM images of TiO2

with 0.01 mg/cm2

substrates when preparing SERS-active adsorbates.

that stabilizes the entire TiO2

42 Raman Spectroscopy

**Figure 3.** STEM images of a cross-section of a TiO2

insert shows a top view of the nanotubes obtained at 25 and 10 V.

**Figure 4.** SEM images of TiO2 nanotube layers (top-views) formed at 25 V and loaded with 0.01 mg Ag/cm2 after different PVD processes: (a) magnetron sputtering, (b) evaporation at a low vacuum, and (c) evaporation at a high vacuum.

agglomerated and form rings. As a result of silver evaporation at a low vacuum, the plasmonic nanoparticles are distributed homogenously in the TiO2 nanotube layer, tightly covering the walls of the nanotubes and forming a thin, solid coating around the tubes (**Figure 4b**). After evaporation at a high vacuum, where the process of silver deposition is very slow (0.13 nm/min), spherical Ag nanoparticles are formed (see **Figure 4c**). The diameter of these particles is below 50 nm. Densely-packed TiO2 nanotubes coated with a plasmonic metal (Ag) could act as antennananoresonators having a strictly defined geometry and surface development. This would ensure the formation of highly active places—"hot spots," i.e., gaps and cavities serving as surface plasmon resonators that significantly increase the intensity of the electromagnetic field.

**Figure 5** shows typical AES spectra for samples with an Ag deposit (0.01 mg/cm2 ) after vacuum evaporation at low and high vapor pressures. Signals from the Ag MNN, Ti LMM and O KLL Auger transitions are clearly visible. These results suggest that O is bound to Ti, and there are areas where the oxidized Ti substrate is not fully covered by the Ag deposit, which is consistent with the SEM microscopic observations.

**Figure 5.** Typical Auger survey spectra taken at the surface of Ag/TiO2 NT layers obtained at Vmax = 25 V and covered with the same amount of Ag deposit (0.01 mg/cm2 ). The Ag MNN signal for a pure Ag reference sample is also shown.

In order to gain further insight into the chemical state of the samples before and after the silver deposition processes, XPS measurements were performed for those samples having a small amount of Ag (0.01 mg/cm2 ) (see **Table 1**). The deconvolution of the main XPS signals for Ti2p3/2 and O1s suggests that titanium is bound to oxygen and forms titanium oxide IV. Highresolution XPS spectra confined to the Ag range gave the binding energies of Ag3d5/2 peaks located at 367.9 (high vacuum), 368.3 (magnetron sputtering) and 368.4 eV (low vacuum), respectively, which is consistent with the literature [26–30] and also with our reference data. This implies that the Ag agglomerates and single nanoparticles located on the tops and in the deeper parts of the TiO2 nanotubes are metallic silver. A small shift in the Ag3d5/2 and Ti2p3/2 peaks (see **Table 1**) for the samples after functionalization by PVD methods (±0.1–0.4 eV) may suggest that the XPS signals of Ag are modified by an interaction with the TiO<sup>2</sup> nanoporous substrate in relation to the position of the Ag standard peak. This interaction may induce a shift in the Fermi level in the deposited silver, in particular, if single nanoparticles are supported on the oxide carrier – the SMSI effect. Such effects have been reported by Goodman et al. and Lopez et al. [31, 32] for gold nanoparticles on TiO2 supports, and were also observed in our recent work, where a ZrO2 nanoporous layer was covered with Ag nanoparticles [33].

**Table 1.** XPS results for titania nanotube layers before and after silver nanoparticles deposition by PVD methods at low

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45

NT (25 V)/Ti substrates fabricated by: Ag magnetron

and high vacuums.

**Figure 6.** Average SERS spectra of Py adsorbed on Ag-n/TiO2

sputtering, Ag evaporation method at a low vacuum, Ag thermal evaporation method at a high vacuum.

**Figure 6** shows the SERS spectra of pyridine (Py) adsorbed at TiO2 NT/Ti platforms fabricated at 25 V and covered with the same average amount of Ag-n (0.01 mg/cm2 ) by the three different procedures: magnetron sputtering, and vacuum evaporation at low and high vapor pressures. The spectra presented were averaged from 20 measurements on each sample tested. The SERS spectra of pyridine are dominated by two bands: at ~1010 and at ~1034 cm−1 originating from the aromatic ring vibrations of this molecule. Moreover, on the spectra recorded, there are also others bands clearly visible at 1150 and 1220 cm−1, which are also characteristic of Py adsorbed on a standard silver surface [34, 35]. The SERS measurements revealed that the distribution of Ag nanoparticles on the nanotubular substrate affects SERS intensity (compare Titanium (IV) Oxide Nanotubes in Design of Active SERS Substrates for High Sensitivity… http://dx.doi.org/10.5772/intechopen.72739 45


**Table 1.** XPS results for titania nanotube layers before and after silver nanoparticles deposition by PVD methods at low and high vacuums.

In order to gain further insight into the chemical state of the samples before and after the silver deposition processes, XPS measurements were performed for those samples having a small

Ti2p3/2 and O1s suggests that titanium is bound to oxygen and forms titanium oxide IV. Highresolution XPS spectra confined to the Ag range gave the binding energies of Ag3d5/2 peaks located at 367.9 (high vacuum), 368.3 (magnetron sputtering) and 368.4 eV (low vacuum), respectively, which is consistent with the literature [26–30] and also with our reference data. This implies that the Ag agglomerates and single nanoparticles located on the tops and in the

peaks (see **Table 1**) for the samples after functionalization by PVD methods (±0.1–0.4 eV) may

substrate in relation to the position of the Ag standard peak. This interaction may induce a shift in the Fermi level in the deposited silver, in particular, if single nanoparticles are supported on the oxide carrier – the SMSI effect. Such effects have been reported by Goodman

ent procedures: magnetron sputtering, and vacuum evaporation at low and high vapor pressures. The spectra presented were averaged from 20 measurements on each sample tested. The SERS spectra of pyridine are dominated by two bands: at ~1010 and at ~1034 cm−1 originating from the aromatic ring vibrations of this molecule. Moreover, on the spectra recorded, there are also others bands clearly visible at 1150 and 1220 cm−1, which are also characteristic of Py adsorbed on a standard silver surface [34, 35]. The SERS measurements revealed that the distribution of Ag nanoparticles on the nanotubular substrate affects SERS intensity (compare

suggest that the XPS signals of Ag are modified by an interaction with the TiO<sup>2</sup>

et al. and Lopez et al. [31, 32] for gold nanoparticles on TiO2

**Figure 5.** Typical Auger survey spectra taken at the surface of Ag/TiO2

**Figure 6** shows the SERS spectra of pyridine (Py) adsorbed at TiO2

at 25 V and covered with the same average amount of Ag-n (0.01 mg/cm2

) (see **Table 1**). The deconvolution of the main XPS signals for

). The Ag MNN signal for a pure Ag reference sample is also shown.

nanotubes are metallic silver. A small shift in the Ag3d5/2 and Ti2p3/2

nanoporous layer was covered with Ag nanoparticles [33].

nanoporous

supports, and were also observed

NT layers obtained at Vmax = 25 V and covered

NT/Ti platforms fabricated

) by the three differ-

amount of Ag (0.01 mg/cm2

44 Raman Spectroscopy

with the same amount of Ag deposit (0.01 mg/cm2

deeper parts of the TiO2

in our recent work, where a ZrO2

**Figure 6.** Average SERS spectra of Py adsorbed on Ag-n/TiO2 NT (25 V)/Ti substrates fabricated by: Ag magnetron sputtering, Ag evaporation method at a low vacuum, Ag thermal evaporation method at a high vacuum.

**Figure 4**). A roughly two-fold increase in SERS intensity occurs when the titania nanotubes are adorned with Ag-n after the magnetron sputtering process. Three kinds of Ag particles can be distinguished: those accumulated on the tops of the nanotubes, forming "rings," and single particles (see. **Figure 4**) separated from each other and/or those Ag particles produced in the high vacuum process, which are located on the tops ("mouth") of the nanotubes. The latter do not yield such a strong SERS effect. Apparently, the key factor in the SERS activity of Ag-n *is the size and mode of the specific surface area of the silver particles* formed during a specific vacuum process. While the SERS intensity reached was not very high, well-reproducible and good-quality SERS spectra were obtained.

bands at 1004 and 1034 cm−1. The band at 1004 cm−1 is due to the ring breathing mode (ν<sup>1</sup>

notation), whereas that at 1034 cm−1 is due to symmetric triangular ring deformation (ν12) [34, 35]. The SERS spectral intensity increases distinctly with the amount of Ag metal deposit, which correlates with a change in surface topography (compare **Figure 7**). This effect is most likely related to an increase in the number of narrow gaps between the silver particles themselves (locations particularly active in SERS spectroscopy) with an increasing amount of Ag deposit.

Our previous experiments show that the SERS enhancement factor for free-standing TiO2 nanotubes adorned with silver nanoparticles is strictly related to the size of the nanotubes.

EF = ISERS /Iref × hcref/Nsurf (1)

where ISERS and Iref are the Raman intensities obtained from the SERS and normal Raman (NR) investigations, respectively, cref stands for the concentration of pure Py in the NR measurements, and h is the depth-of-focus of the laser beam. The average number of adsorbed molecules of Py per geometrical surface area unit participating in the SERS measurements (Nsurf) was calculated assuming that the adsorbed molecules are spheres closely packed on a plane to form a hexagonal lattice. AFM measurements were performed to determine the geometrical

cated at 10 V up to 25 V (see **Figure 2**), there was an increase in the SERS enhancement factor

**Figure 8.** SERS spectra of pyridine adsorbed at the surface of nanotubes (10 V) annealed in air at 650°C for 2 h and

surface area, see **Figure 9a**. More details can be found in the publication [15].

) for the pyridine (Py) probe molecule was successfully esti-

Titanium (IV) Oxide Nanotubes in Design of Active SERS Substrates for High Sensitivity…

, magnetron sputtering) on nanotubes fabri-

. A reference spectrum for pure Ag (electrochemically roughened silver

The SERS enhancement factor (EF

of from 105

to 106

coated with silver deposit: 0.01, 0.02, 0.03 mg/cm2

surface) is also given.

mated using the following formula (1):

For the same amount of Ag deposit (0.02 mg/cm2

(see **Figure 9b**).

, Wilson

47

http://dx.doi.org/10.5772/intechopen.72739

The next step was to repeat the same measurements with a nanotubular substrate prepared at an anodization of 10 V to produce nanotubes having a smaller diameter, thereby increasing the specific surface area of the SERS-active silver. **Figure 7** shows typical SEM images of a platform for SERS measurements, based on TiO2 nanotubes (10 V): a surface of nanotubes of titania on a Ti substrate after deposition of Ag nanoparticles by the evaporation method in a low vacuum of 0.01 (a), 0.02 (b), and 0.03 (d) mg Ag/cm2 . For the smallest amount of Ag (0.01 mg∙cm−2), the silver nanoparticles tend to gather on the tops of the nanotubes and on their side walls (c). Characteristic specific structures of Ag-n are formed around the nanotubes, consisting of agglomerates of Ag nanoparticles. Increasing the amount of silver (0.02 and 0.03 mg∙cm−2) leads further to a visible development of the Ag surface area, up to the formation of silver nanoparticle agglomerates having characteristic slits of from several to a few dozen nanometers.

**Figure 8** shows the spectrum of pyridine adsorbed on Ag-n/TiO2 NT (10 V) platforms from an aqueous solution of 0.05 M pyridine +0.1 M KCl. The spectra are dominated by two strong

**Figure 7.** Top view of titania dioxide nanotubes (10 V) annealed for 2 h in air at a temperature of 650°C after silver deposition: 0.01 (a), 0.02 (b), 0.03 mg/cm2 (d). A cross-sectional view of a nanoporous layer after deposition of Ag – 0.01 mg/cm2 is also given, left side (c).

bands at 1004 and 1034 cm−1. The band at 1004 cm−1 is due to the ring breathing mode (ν<sup>1</sup> , Wilson notation), whereas that at 1034 cm−1 is due to symmetric triangular ring deformation (ν12) [34, 35]. The SERS spectral intensity increases distinctly with the amount of Ag metal deposit, which correlates with a change in surface topography (compare **Figure 7**). This effect is most likely related to an increase in the number of narrow gaps between the silver particles themselves (locations particularly active in SERS spectroscopy) with an increasing amount of Ag deposit.

**Figure 4**). A roughly two-fold increase in SERS intensity occurs when the titania nanotubes are adorned with Ag-n after the magnetron sputtering process. Three kinds of Ag particles can be distinguished: those accumulated on the tops of the nanotubes, forming "rings," and single particles (see. **Figure 4**) separated from each other and/or those Ag particles produced in the high vacuum process, which are located on the tops ("mouth") of the nanotubes. The latter do not yield such a strong SERS effect. Apparently, the key factor in the SERS activity of Ag-n *is the size and mode of the specific surface area of the silver particles* formed during a specific vacuum process. While the SERS intensity reached was not very high, well-reproducible and

The next step was to repeat the same measurements with a nanotubular substrate prepared at an anodization of 10 V to produce nanotubes having a smaller diameter, thereby increasing the specific surface area of the SERS-active silver. **Figure 7** shows typical SEM images of a platform

Ti substrate after deposition of Ag nanoparticles by the evaporation method in a low vacuum

the silver nanoparticles tend to gather on the tops of the nanotubes and on their side walls (c). Characteristic specific structures of Ag-n are formed around the nanotubes, consisting of agglomerates of Ag nanoparticles. Increasing the amount of silver (0.02 and 0.03 mg∙cm−2) leads further to a visible development of the Ag surface area, up to the formation of silver nanoparticle agglomerates having characteristic slits of from several to a few dozen nanometers.

an aqueous solution of 0.05 M pyridine +0.1 M KCl. The spectra are dominated by two strong

**Figure 7.** Top view of titania dioxide nanotubes (10 V) annealed for 2 h in air at a temperature of 650°C after silver

(d). A cross-sectional view of a nanoporous layer after deposition of

nanotubes (10 V): a surface of nanotubes of titania on a

. For the smallest amount of Ag (0.01 mg∙cm−2),

NT (10 V) platforms from

good-quality SERS spectra were obtained.

46 Raman Spectroscopy

for SERS measurements, based on TiO2

deposition: 0.01 (a), 0.02 (b), 0.03 mg/cm2

is also given, left side (c).

Ag – 0.01 mg/cm2

of 0.01 (a), 0.02 (b), and 0.03 (d) mg Ag/cm2

**Figure 8** shows the spectrum of pyridine adsorbed on Ag-n/TiO2

Our previous experiments show that the SERS enhancement factor for free-standing TiO2 nanotubes adorned with silver nanoparticles is strictly related to the size of the nanotubes. The SERS enhancement factor (EF ) for the pyridine (Py) probe molecule was successfully estimated using the following formula (1):

$$\mathbf{E}\_p = \mathbf{I}\_{\text{SERS}} / \mathbf{I}\_{\text{nd}} \times \mathbf{h} \mathbf{c}\_{\text{nd}} / \mathbf{N}\_{\text{surf}} \tag{1}$$

where ISERS and Iref are the Raman intensities obtained from the SERS and normal Raman (NR) investigations, respectively, cref stands for the concentration of pure Py in the NR measurements, and h is the depth-of-focus of the laser beam. The average number of adsorbed molecules of Py per geometrical surface area unit participating in the SERS measurements (Nsurf) was calculated assuming that the adsorbed molecules are spheres closely packed on a plane to form a hexagonal lattice. AFM measurements were performed to determine the geometrical surface area, see **Figure 9a**. More details can be found in the publication [15].

For the same amount of Ag deposit (0.02 mg/cm2 , magnetron sputtering) on nanotubes fabricated at 10 V up to 25 V (see **Figure 2**), there was an increase in the SERS enhancement factor of from 105 to 106 (see **Figure 9b**).

**Figure 8.** SERS spectra of pyridine adsorbed at the surface of nanotubes (10 V) annealed in air at 650°C for 2 h and coated with silver deposit: 0.01, 0.02, 0.03 mg/cm2 . A reference spectrum for pure Ag (electrochemically roughened silver surface) is also given.

**Figure 9.** (a) Typical AFM image of silver nanoparticles (0.020 mg/cm2 ) deposited by the magnetron sputtering technique on TiO2 oxide layers (20 V)—top view; (b) SERS enhancement factor EF as a function of the final formation voltage Vmax of the titania nanoporous layers. EF estimated for Py band at ~1010 cm−1, Ag deposited by magnetron sputtering technique—0.02 mg/cm2 .

The hundredfold increase in SERS enhancement shown in **Figure 9b** is apparently due to a combination of two factors: a change in the specific surface area of the nanotubes with formation voltage [15], and a change in the distribution and size of the Ag nanoparticles with nanotube size [23]. These geometrical factors affect the properties of the silver "nanoresonators" produced on the tops and side walls of the TiO2 nanotubes. The present results confirm the importance of the size of the geometrical surface area of the TiO2 nanotubes for the plasmonic properties of the Ag-n deposit itself and, consequently, for the properties of the Ag/TiO2 NT composite materials.

**Figure 10.** SERS spectra of p-mercaptobenzoic acid (a) and DBRh dye (b) recorded at the surface of 10 V nanotubes.

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49

**Figure 11.** Left: SERS spectrum for R6G molecules adsorbed from an 10−7 M aqueous solution on a surface of TiO2

(magnetron sputtering). Right: SEM images showing the

NT nanotubes (25 V) with a silver deposit of 0.02 mg/cm2

morphology and cross-sectional view of the SERS active platform.

**Figure 10** shows SERS spectra of two other probe molecules: p-mercaptobenzoic acid (a) and DBRh dye (b) recorded on the platforms, which were characterized by an enhancement factor larger than 106 . In both cases, the spectra are of good quality and high intensity. They show that the background is not very high, and quite "flat" for both molecules. Our SERS experiments confirmed the good reproducibility of such substrates obtained by adorning TiO<sup>2</sup> nanotubes with Ag metal clusters and particles, which effectively support plasmon resonance [18].

**Figures 11** and **12** show R6G spectra taken with our Ag-n/TiO2 NT platform covered with a solution containing 10−7 or 10−9 mol/l R6G. A rhodamine molecule is often used when the relationship between the local electromagnetic field enhancement and a large SERS signal is explored, which makes it possible to measure Raman spectra from a single molecule located on an Ag particle or individual Ag nanoparticles [36–39]. The characteristic peaks at ~970, 1150, 1220, 1300, 1340, 1410, 1500 and 1600 cm−1 correspond to the Raman lines for R6G [3, 36]. In particular, the bands which are usually assigned to aromatic C-C stretching vibrations of R6G molecule are clearly visible. It can be seen that the SERS spectrum of R6G adsorbed on our Ag substrate exhibits enough intensity for this molecule to be detected even at a small concentration in an aqueous solution (below 10−7 mol/l). As a result of this reduced concentration of R6G, some bands are suppressed (compare **Figures 11** and **12**). This sensitivity of our

Titanium (IV) Oxide Nanotubes in Design of Active SERS Substrates for High Sensitivity… http://dx.doi.org/10.5772/intechopen.72739 49

**Figure 10.** SERS spectra of p-mercaptobenzoic acid (a) and DBRh dye (b) recorded at the surface of 10 V nanotubes.

The hundredfold increase in SERS enhancement shown in **Figure 9b** is apparently due to a combination of two factors: a change in the specific surface area of the nanotubes with formation voltage [15], and a change in the distribution and size of the Ag nanoparticles with nanotube size [23]. These geometrical factors affect the properties of the silver "nanoresonators"

properties of the Ag-n deposit itself and, consequently, for the properties of the Ag/TiO2

**Figure 10** shows SERS spectra of two other probe molecules: p-mercaptobenzoic acid (a) and DBRh dye (b) recorded on the platforms, which were characterized by an enhancement factor

that the background is not very high, and quite "flat" for both molecules. Our SERS experiments confirmed the good reproducibility of such substrates obtained by adorning TiO<sup>2</sup>

tubes with Ag metal clusters and particles, which effectively support plasmon resonance [18].

a solution containing 10−7 or 10−9 mol/l R6G. A rhodamine molecule is often used when the relationship between the local electromagnetic field enhancement and a large SERS signal is explored, which makes it possible to measure Raman spectra from a single molecule located on an Ag particle or individual Ag nanoparticles [36–39]. The characteristic peaks at ~970, 1150, 1220, 1300, 1340, 1410, 1500 and 1600 cm−1 correspond to the Raman lines for R6G [3, 36]. In particular, the bands which are usually assigned to aromatic C-C stretching vibrations of R6G molecule are clearly visible. It can be seen that the SERS spectrum of R6G adsorbed on our Ag substrate exhibits enough intensity for this molecule to be detected even at a small concentration in an aqueous solution (below 10−7 mol/l). As a result of this reduced concentration of R6G, some bands are suppressed (compare **Figures 11** and **12**). This sensitivity of our

. In both cases, the spectra are of good quality and high intensity. They show

nanotubes. The present results confirm the

) deposited by the magnetron sputtering technique

estimated for Py band at ~1010 cm−1, Ag deposited by magnetron sputtering

as a function of the final formation voltage

nanotubes for the plasmonic

NT platform covered with

NT

nano-

produced on the tops and side walls of the TiO2

**Figure 9.** (a) Typical AFM image of silver nanoparticles (0.020 mg/cm2

oxide layers (20 V)—top view; (b) SERS enhancement factor EF

composite materials.

technique—0.02 mg/cm2

Vmax of the titania nanoporous layers. EF

.

larger than 106

on TiO2

48 Raman Spectroscopy

importance of the size of the geometrical surface area of the TiO2

**Figures 11** and **12** show R6G spectra taken with our Ag-n/TiO2

**Figure 11.** Left: SERS spectrum for R6G molecules adsorbed from an 10−7 M aqueous solution on a surface of TiO2 NT nanotubes (25 V) with a silver deposit of 0.02 mg/cm2 (magnetron sputtering). Right: SEM images showing the morphology and cross-sectional view of the SERS active platform.

development of this type of SERS platform may involve using Ti of varying degrees of purity, and alloys of Ti, where the impurities and alloying elements will have an impact on the formation of nanoporous oxides layers by means of doping process (a change in the electron structure of the titanium oxide). As a consequence, this could lead to a significantly stronger enhancement of the intensity of the SERS spectra due to the occurrence of modified interactions between the doped titanium oxide and the metal nanoparticles, effectively supporting surface plasmons resonance (Ag, Au, Cu). All those factors should be taken into account when

Titanium (IV) Oxide Nanotubes in Design of Active SERS Substrates for High Sensitivity…

We are most grateful to Prof. Robert Nowakowski from the Institute of Physical Chemistry PAS (Poland) for his cooperation (AFM measurements) and many helpful discussions. This

, Marcin Hołdyński<sup>1</sup>

3 Faculty of Material Science and Engineering, Warsaw University of Technology, Warsaw,

[1] Sharma B, Frontiera RR, Henry AI, Ringe E, Van Duyne RP. SERS materials, applications, and the future. Materials Today. 2012;**15**:16-25. DOI: 10.1016/S1369-7021(12)70017-2 [2] Schlucker S. Surface-enhanced Raman spectroscopy: Concepts and chemical applications. Angewandte Chemie, International Edition. 2014;**53**:4756-4795. DOI: 10.1002/

[3] Tian F, Bonnier F, Casey A, Shanahan AE, Byrne HJ. Surface enhanced Raman scattering with gold nanoparticles: Effects of particle shape. Analytical Methods. 2014;**6**:9116-9123.

[4] Fan M, Andrade GFS, Brolo AG. A review on the fabrication of substrates for surface enhanced Raman spectroscopy and their applications in analytical chemistry. Analytica

Chimica Acta. 2011;**693**:7-25. DOI: 10.1016/j.aca.2011.03.002

1 Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland

and Maria Janik-Czachor1

, Tomasz Płociński<sup>3</sup>

,

http://dx.doi.org/10.5772/intechopen.72739

51

planning future analytical applications of new modified SERS substrates.

work was financially supported by the Institute of Physical Chemistry PAS.

\*, Jan Krajczewski2

\*Address all correspondence to: mpisarek@ichf.edu.pl

, Andrzej Kudelski2

2 Faculty of Chemistry, University of Warsaw, Warsaw, Poland

**Acknowledgements**

**Author details**

Marcin Pisarek1

Poland

**References**

anie.201205748

DOI: 10.1039/c4ay02112f

Mirosław Krawczyk<sup>1</sup>

**Figure 12.** SERS spectrum of R6G molecules adsorbed from a 10−9 M aqueous solution on a surface of TiO2 NT nanotubes (25 V) with a silver deposit of 0.05 mg/cm2 (thermal evaporation method at a high vacuum).

fabricated SERS substrates is apparently related to the homogeneous Ag distribution on the surface of the regular nanoporous TiO2 structures; this should make it possible to produce specific morphologies that are extremely useful in Raman investigations.
