1. Introduction

Titanium dioxide (TiO₂), an example of metal-oxide type semiconductors, has been broadly studied and widely used in different applications such as a photocatalytic material for self-cleaning coatings, environmental purifiers, antifogging mirrors and many others [1, 2]. Being into consideration practical application of TiO₂, a lot of work is also dedicated obtained titania with different ions [1, 3-9]. As the sub-micrometer size TiO₂ powders are promising materials, further investigations are desired, with the tools providing reliable and complex high-resolution information about observed objects. Therefore, atomic force microscopy (AFM) along with scanning electron microscopy (SEM) [10-12] and transmission electron microscopy (TEM) [13-15] becomes an important diagnostic instrument allowing obtain the information about electrical, magnetic and thermal properties of such a small objects [16-19]. It should be underlined, that due to wide range of measurement conditions such as: temperature, environment, magnetic field in connection to very simple preparation procedure [20-21] AFM has become very popular among researches worldwide. As it provides quantitative topography information, one can extract the particle size data, however the influence of the shape of the scanning tip should be taken into account [22, 23]. Moreover, the shape of the grains may be a source of useful information. Additionally, direct tip-sample contact in both: contact and intermittent contact modes as well as various derived techniques, may be used in determination and mapping of the mechanical properties of investigated material [24-26].

The main goal of this work was investigation influence of amount of silver on the structural and optical properties of TiO₂ powder via XRD, SEM, AFM and UV-vis methods. A special emphasize was put to investigated obtained titania powders via AFM technique.

2. Materials and Methods

2.1. Materials

Titanium(IV) buthoxide (TBOT) (99+%) and titanium(IV) isopropoxide (TIPO) (99+%) were purchased from Alfa Aesar. Ethanol (96%), isopropyl alcohol, silver nitrate AgNO₃ was purchased from POCh Gliwice.

2.2. TiO₂ Powder Synthesis

TiO₂ powders were prepared by sol-gel method using as titanium precursor titanium(IV) buthoxide (TBOT) or titanium(IV) isopropoxide (TIPO). Titanium precursor was dissolved in ethanol (or isopropyl alcohol) mixed with 3.5 ml of distilled water yielding a titania sol. The solution was stirred in a plastic flask at room temperature for 4 h. During the stirring the titania powder was formed and after filtering was dried at room temperature. The TiO₂ powder was heated at 500℃ for one hour. Details about synthesis of TiO₂ are presented in Table 1.

Table 1. Details about synthesis of TiO2

2.3. Ag Doped TiO₂ Powder

Titania doped with silver were obtained by using two methods of synthesis: TiO₂-Ag-y and TiO₂-Ag-ya. In both cases such components as TIPO, ethanol, and distilled water were applied and stirred during 4 hours. Differences in both methods of synthesis were in the time and order to added AgNO₃ to the mixture. To obtain TiO₂-Ag-y AgNO₃ was stirred four hours along with other components, while in the case of TiO₂-Ag-ya AgNO₃ was added to sol mixture and stirred only 2 hours. Details of the titania-silver synthesis are presented below.

TiO₂-2-Ag-y: TiO₂-2-Ag powder was prepared by sol-gel method using as titanium precursor titanium(IV) isopropoxide (TIPO). Titanium precursor was dissolved in ethanol. Briefly, 4.5 ml of TIPO and 21 ml of ethanol were mixed with 3.5 ml of distilled water and addition of silver nitrate AgNO₃ to the obtaining sol with the molar ratio Ag/TIPO equal 1%, 5%, 10% (abbreviated herein as TiO₂-2-Ag-1, TiO₂-2-Ag-5, TiO₂-2-Ag-10). The solution was stirred in a plastic flask at room temperature for 4 h. During the stirring the titania powder was formed and after filtering was dried at room temperature. The TiO₂-2-Ag powder was heated at 500℃ for one hour.

TiO₂-2-Ag-ya: TiO₂: Ag-ya powder was prepared by sol-gel method using as titanium precursor titanium(IV) isopropoxide (TIPO). Titanium precursor was dissolved in ethanol. Briefly, 4.5 ml of TIPO and 21 ml of ethanol were mixed with 3.5 ml of distilled water yielding a titania sol. The solution was stirred in a plastic flask at room temperature for 2 h. Next addition of silver nitrate AgNO₃ to the obtaining sol with the molar ratio Ag/TIPO equal 1%, 5%, 10% (abbreviated herein as TiO₂-2-Ag-1a, TiO₂-2-Ag-5a, TiO₂-2-Ag-10a) and mixed for 2h. During the stirring the titania powder was formed and after filtering was dried at room temperature. The TiO₂ -2-Ag- ya powder was heated at 500℃ for one hour.

2.4. Preparation TiO₂ Samples on the Glass Substrate

In order to perform the AFM measurements of the TiO₂ powders, the microscope glass was used as the substrate. The surface of the glass was covered with cyanoacrylate glue and left for few second for preliminary cure. A small quantity of the powder was carefully placed on the surface of the substrate and left for few minutes to cure the glue. Afterwards, the weakly attached grains of the powder were removed using pressured air, in order to avoid them to stick to the scanning tip. Such an approach allows obtain small agglomerates and single grains on the surface.

2.5. Methods

X-ray diffraction patterns were recorded using powder on a Pulveraceous diffractometer Dron – 2. Co radiation filtrated by Fe was applied. Scanning electron microscopy (SEM) studies were performed with a tungsten cathode Vega II SBH (TESCAN) to examine the morphology of the TiO₂. UV-vis spectra were recorded as films from chloroform solution casted on the glass substrate by using Jasco V670 spectrophotometer. Atomic force microscopy (AFM) measurements were performed with Innova system from Bruker (formerly Veeco) in air, at temperature 23℃ and humidity 35% RH. Two modes were used: intermittent contact (IC-AFM) with phase imaging (PI) feature and force modulation mode (FMM). Both modes deliver morphological information about the surface of the sample. The phase feature imaging allows create a map of the tip-sample energy dissipation as the distance between them changes periodically. The energy dissipation is a complex response and may contain various components related to the surface stiffness, adhesion, presence of electrostatic field and others [27-31]. In order to provide more reliable information source, the force modulation mode derived from contact technique was used. It allows create the stiffness map of the surface[32,33]. The intermittent contact mode measurements were performed using standard MPP-11120 probes from Bruker (k=40 n/m, f_res=300 kHz, r_tip=8 nm), FMM mode measurements on the other hand were performed using MPP-31100 probes from Bruker (k=0.9 N/m, f_res=20 kHz, r_tip=8 nm). The scanning speed in both modes was applied at the level 0.2 Hz, providing good surface tracking with minimized risk of the particles attachment to the tip. The data was processed with SPIP software from Image Metrology. In order to provide better readability of the topography images, the Sobel transform data was additionally presented, as it reveals presence of fine structures and was successfully utilized in other papers [34-36].

Zeta potentials of nanoparticles were determined using analyzer Zetasizer Nano ZS Malvern Instruments. In 20 ml H₂O 0.01 g TiO₂ was put. The pH of suspensions was adjusted using 0.1 M HCl or 0.1 M NaOH.

3. Results and Discussion

3.1. Characteristic of TiO₂

3.1.1. X-ray diffraction, SEM and UV-vis

X-ray diffraction and scanning electron microscopy (SEM) were used to analyze the titania. The X-ray spectra of the both TiO₂ powders (TiO₂-1 ÷ TiO₂-5) annealed at 500℃ shows the same diffraction pattern characteristic to anatase crystalline phase (see Fig. 1a). A major peak corresponding to (101) reflections of the anatase phase of TiO₂ was apparent at the angle of 29.45°. Our X-ray results are in good agreement with other works dedicated TiO₂ [e.g. 37].

Moreover, for the titania powders TiO₂-3 ÷ TiO₂-5 traces of rutile crystalline phase was found at the angle of 32.00° (see Fig. 1b).

Pure TiO₂-1 ÷ TiO₂-5 powders were investigated by SEM technique. Morphologies of TiO₂-1 and TiO₂-2, as an examples revealed by SEM micrographs are shown in Fig. 2. TiO₂-1 ÷ TiO₂-5 samples appeared as agglomerations of smaller particles. The TiO₂ powder in all cases presented aggregates consisting of smaller particles (from 80 nm for TiO₂-4, to 200 nm for TiO₂-1) to larger particles (from 100 nm for TiO₂-4, to ~300 nm for TiO₂-1).

The grains size shape depends on the synthetic conditions and precursors used. The smallest grain sizes were observed for the reactions with TIPO. The important parameter controlling the particle formation and aggregation was the water/precursor molar ratio (see Experimental part).

Figure 1. X-ray pattern of TiO2-1 powder annealed at 500℃ (a), TiO2-4 powder annealed at 500℃ (b)

The average grain size of obtain pure TiO₂-1 was found about 240 nm, for TiO₂-2 was 170 nm, for TiO₂-3 was 110 nm, for TiO₂-4 was 90 nm while for TiO₂-5 was 160 nm.

Figure 2. SEM micrographs of titania powders: (a): TiO2-1 and (b) TiO2-2

Fig. 3a shows the diffuse reflective UV–vis spectra of pure TiO₂-1 ÷ TiO₂-5 samples. The all TiO₂ samples had strong absorption only in the UV region corresponding to its band gap energy. The obtained results are connected with the value of optical energy gap of investigated samples and are in good agreement with the results described in [37-39].

In order to investigate the visible-light absorption in detail, the Kubelka–Munk functions of all TiO₂-1 ÷ TiO₂-5 samples were calculated. The obtained results are shown in Fig. 3b.

Figure 3. Diffuse reflective UV–vis spectra of TiO2-1 ÷ TiO2-5 (a) and Kubelka–Munk function at different wavelengths of TiO2-1 ÷ TiO2-5 (b)

All investigated samples exhibited one absorption maximum band in the range 305-330 nm, depend on the method of synthesis applied to obtained TiO₂ powder. Samples TiO₂-1 and TiO₂-2 were red shifted in comparison with other investigated compounds and exhibited λ_max at 330 nm. Samples TiO₂-3 and TiO₂-5 has λ_max at about 320 nm while TiO₂-4 was blue shifted in comparison with other compounds (λ_max at 305 nm). The values of maximum of UV-vis absorption band detected from the Kubelka–Munk function are presented in Table 2.

Table 2. Absorption UV-vis properties of pure and doped with Ag TiO2 powder detected from the Kubelka–Munk function

3.1.2. AFM Study

The AFM measurements of the TiO₂ samples were performed in intermittent contact mode. Several measurements have been taken in order to provide statistically significant data. As the powder easily agglomerates, the optical camera view was used to place the scanning tip in areas covered with smallest visible objects, allowing to assume, that desired quality data will be collected with minimum risk of the tip contamination. Obtained result allowed to determine the diameter of the grains by analyzing the profile of the surface. The readouts were done in case of grains with clearly imaged both opposite edges. It should be underlined, that the impact of the shape of the scanning tip could be neglected in case of observed structures. The data obtained with AFM and SEM are consistent. The grains have regular, round and smooth shape. The example of acquired topography with its Sobel transform for TiO₂-2 is presented in Fig. 4.

In our work we investigated the influence of two titanium precursors on the structural and optical properties of synthesized TiO₂. As precursors we used TBOT (TiO₂-1) or TIPO (TiO₂-2). For details of synthesis see Table 1. Being into consideration kind of precursor used, we can conclude that the average grain size was smaller for the titania obtained from TIPO (TiO₂-2) than from TBOT. On the other hand, kind of precursor applied to obtain TiO₂ not influence on the absorption UV-vis properties and kind of crystalline phase of titania (see Table 2).

Moreover, we investigated influence of kind and amount of the solvent on the structural and optical properties of the synthesized titania. As a solvent we used ethanol or isopropyl alcohol (see Table 1). Being into consideration kind of the alcohol applied we can conclude that the average grain size was smaller for the titania obtained from isopropyl alcohol (TiO₂-4) than from ethanol (TiO₂-5). It is interesting that we observed big differences in the absorption UV-vis properties of these two materials. Along with change isopropyl alcohol (TiO₂-4) into ethanol 15 nm red shift was found in absorption UV-vis properties of TiO₂ (see Table 2). We not observed changes in the crystalline form of titania along with change isopropyl alcohol (TiO₂-4) to ethanol (TiO₂-5).

Additionally, we investigated influence of amount of ethanol (10, 21 and 42 ml) on the structural and optical properties of titania. Along with increase the amount of ethanol decrease of average grain size was found, clearly seen for the compounds TiO₂-2 (21 ml of C₂H₅OH) and TiO₂-3 (42 ml of C₂H₅OH). No big differences in absorption UV-vis properties were observed for these compounds, however along with increase the amount of ethanol from 10 ml (TiO₂-5) to 21 ml (TiO₂-2) 10 nm red shift was found (see Table 2). Moreover, along with increase the amount of ethanol from 10 ml (TiO₂-5) to 21 ml (TiO₂-2) crystalline phases of titania change from anatase and rutyl to pure anatase form.

Figure 4. AFM topography of TiO2-2 (left) Sobel transform of the topography (right), and the profile with markers used for determination of the grain radius

3.2. Characteristic of Ag Doped TiO₂

3.2.1. X-ray Diffraction and EDS and UV-vis

For the silver doped titania, X-ray diffraction and scanning electron microscopy (SEM) along with EDS were used. The size of crystallite (D) of Ag doped TiO₂, as was detected by X-ray, was about 130 Ǻ. For all the samples independ on the amount of Ag the same diffraction pattern characteristic to anatase crystalline phase was detected. A major peak corresponding to (101) reflections of the anatase phase of Ag doped TiO₂ was apparent at the angle 29.47-29.75° depend on the method of synthesis used and amount of silver in titania. Small diffraction peak at the angle 52.08° coming from Ag in TiO₂ was only observed in the case of samples with 10% w/w of Ag in titania (see Fig. 5).

Moreover, higher amount of Ag in titania was detected by X-ray in the sample TiO₂-2-Ag-10 in comparison with the sample TiO₂-2-Ag-10a. The size of crystallite (D) of Ag in TiO₂, as was detected by X-ray, was about 145 and 128 Ǻ, respectively for TiO₂-2-Ag-10 and TiO₂-2-Ag-10a powders.

Figure 5. X-ray pattern of TiO2-2-Ag-10, as an example

Moreover, the amount of Ag in titania was detected with EDS, and is good agreement with our experimental methods. The obtained results are schematically presented in Fig. 6.

Similarly as for pure powders, we analyzed Ag doped titania by UV-vis method. Fig. 7a shows the diffuse reflectance UV–vis spectra of TiO₂-2-Ag-y and TiO₂-2-Ag-ya samples. The all Ag doped TiO₂ samples had strong absorption in the UV region similar as pure powders. Moreover, weak absorption was observed in the range 450 – 800 nm and is caused the presence of Ag in titania. For the silver doped TiO₂, also the Kubelka–Munk functions were calculated. The obtained results are shown in Fig. 7b.

Figure 6. Ag weight content analysis performed with EDS

All investigated Ag doped titania samples exhibited one absorption maximum band in the range of 323-332 nm, depend on the method of synthesis applied to obtained Ag doped TiO₂ powder and amount of Ag in sample. Moreover, along with increase the amount of Ag in titania, second absorption band was observed in the range 566-622 nm (see Table 2). Additionally, increase of amount of silver in titania from 5 to 10 % w/w caused increase the absorption intensity of this band. Samples with 10% w/w of Ag in TiO₂-2 exhibited big red shifted in comparison with other investigated compounds (see Table 2).

Figure 7. Diffuse reflective UV–vis spectra of TiO2-2-Ag-y and TiO2-2-Ag-ya (a) and Kubelka–Munk function at different wavelengths of doped with Ag titania (b)

3.2.2. AFM study

The samples were measured with intermittent contact mode and additionally available phase imaging feature. The example of TiO₂-2-Ag-1AFM image is presented in Fig. 8.

Figure 8. The topography (left) and Sobel transform of the topography of measured TiO2-2-Ag-1 sample

The topographical information allowed determining the grains diameter. The measurements results are presented in Table 3.

The average diameter of Ag doped TiO₂-2 grains detected with AFM was found about in the range of 30- 200 nm, depend on the amount of silver applied (see Table 3).

Table 3. The averaged diameter values of the Ag doped TiO2-2 and powder measured with AFM

As the silver presence could be detected on the surface of the grains as it causes different level of the energy dissipation between oscillating tip and sample, the relevant data was acquired. The phase imaging feature allowed to create additional map, which combined with topography showed the placement of the Ag on the silver. Figure 9 shows 3D view of the topography with color representation of phase imaging response for TiO₂-2-Ag-1, as an example. One can see clearly the areas of two different fractions confirming presence of two materials on the surface.

Figure 9. The 3D topography image of the TiO2-2-Ag-1 with phase imaging data presented with the colors

It should be mentioned, that the interpretation of such a data should be performed carefully, as the PI response can also vary due to tip-sample distance changes caused by limited time response of the microscope (edges or tilted surfaces) as well as the changing contact area between the tip and sample, as scanned objects can have complex shapes and in some spots observed effect may be misleading. In presented data however, such a issues could be easily identified and excluded. Obtained data allowed observing the presence of Ag on the grains surface, allowing this material to be chemically active.

Additionally, the shape of single grains may allow confirm the kind of the material once detected with other method (XRD). Presented images (Fig. 10) illustrate the observation of presence of specific angles and surfaces of the grains, confirming the anatase form of TiO₂-2-Ag-1. The profiles reveal clearly repeatable structure form, with certain angles even though the grains are randomly oriented.

As mentioned before, also the mechanical properties of the sample may be mapped as the direct tip-sample contact allows measuring the interaction force.

Figure 10. Topography (left) and Sobel transform of the topography (right) as well as the profiles of investigated TiO2-2-Ag-1 sample

Utilization of force modulation mode enables recognition of soft and hard areas of investigated surface. The results of the measurements of the TiO₂-2-Ag-1 sample are shown in Fig. 11.

Figure 11. Topography (top) and the amplitude response of the force modulation mode (bottom) of investigated TiO2-2-Ag-1 sample

The data analysis requires simultaneous observation both: the topography and FMM response images, as the topographical features such as the grains and the substrate should be correlated with the stiffness map. One can see, that the in general the grains reveal higher stiffness (brighter FMM response colors) than the substrate (darker colors), as it is expected concerning the fact, that the substrate is covered with the cured glue film. It should be underlined, that topographical features can have an impact on local FMM response, as the tip-sample contact area or angle can vary. Therefore the analysis was performed in flat and horizontally oriented areas.

To obtain TiO₂-Ag titania powder with the average grain size 170 nm was used as starting material, and the crystal phase was anatase (TiO₂-2). The synthesis of TiO₂-2-Ag-y (or ya) powders with various amount of silver (1, 5 and 10%) allows the formation of solids with such important properties as: (i) extended the λ_abs. to higher wavelengths for the compounds with 5 and 10% of silver (from about 330 to 620 nm), (ii) modified the topography of powders and (iii) changed the zeta potentials of titania. In the absence of Ag TiO₂-2 had negative zeta potentials changed along with change the pH from 4 to 10 (from -11 to -34 mV, respectively). For the doped with Ag titania nanomaterials also negative zeta potentials were found, however along with increase the amount of Ag and method of synthesis titania-silver powders some differences were found. In the presence of 1% of Ag, TiO₂-2-Ag-1 had negative zeta potentials less than -16 mV at pH 4, and greater than -28 mV at pH 10. Nanoparticles TiO₂-2 with 5% of Ag exhibited higher values of zeta potentials at pH 4 than TiO₂-2-Ag-1 (-19 mV). Similar behavior was found for the compounds TiO₂-2-Ag-ya (for pH 4: -14 mV, for ph: 10 -18 mV for TiO₂-2-Ag-1a, and for pH 4: -20 mV, for ph 10: -24 mV for TiO₂-2-Ag-5a). It indicates that the amount of Ag along with method of titania-silver synthesis influences on the properties of titania. Probably ions imparted negative charge to undoped nanoparticles TiO₂-2 surfaces and increased their absolute surface potentials. Additional work is necessary to confirm our suggestions.

4. Conclusions

In this work we presented the results of the investigation of the morphological and optical properties of the TiO₂ and Ag doped TiO₂ powder. Utilized techniques such as AFM, SEM, EDS, XRD and UV-vis allowed acquire essential information about developed materials. Both: AFM and SEM provided the diameter of the grains. Obtained values are coherent. Specific features of the grains, such as the shape related to the material’s form, could be on the other hand observed using only AFM technique, which provides accurate 3D surface imaging. Such ability allowed verify that the anatase form was obtained, which was also detected with XPS. It was also shown, that the expected Ag content in grains was verified with EDS analysis. Additionally, the presence of silver spots on the surface of grains was detected using phase imaging technique. Utilization of the force modulation mode provided the information about the stiffness of the substrate and the material. It should be underlined however, that obtained data is qualitative; therefore no real value of the mechanical properties may be determined using this method. Performed UV-vis measurements allowed to learn that the increase of the silver content significantly reduces the light transmission for wavelength above 400 nm.

ACKNOWLEDGEMENTS

The authors thank dr J. Warycha for SEM and EDS measurements, dr. M. Palewicz for UV-vis measurements, Mss K. Gryzlo for zeta potential experiments and prof. W. Mielcarek for X-ray measurements.