Synthesis and Characterization of Silver Sulfide Nanoparticles ...

3 downloads 0 Views 2MB Size Report
[email protected]pegasus.cc.ucf.edu [email protected]pegasus.cc.ucf.edu, [email protected] S.R. MISHRA. Department of Physics, The University of Memphis, Memphis, TN ...

Journal of Sol-Gel Science and Technology 23, 151–164, 2002 c 2002 Kluwer Academic Publishers. Manufactured in The Netherlands. 

Synthesis and Characterization of Silver Sulfide Nanoparticles Containing Sol-Gel Derived HPC-Silica Film for Ion-Selective Electrode Application S. SHUKLA AND S. SEAL∗ Advanced Materials Processing Analysis Center and Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, USA [email protected] [email protected], [email protected]

S.R. MISHRA Department of Physics, The University of Memphis, Memphis, TN 38152, USA Received December 7, 2000; Accepted July 5, 2001

Abstract. Silver sulfide nanoparticles dispersed in sol-gel derived hydroxypropyl cellulose (HPC)-silica films have been successfully synthesized using H2 S gas diffusion method. This is the first attempt to produce silver sulfide nanoparticles using this technique. Ag2 S nanoparticles are generated through reaction of H2 S gas with AgNO3 precursor dissolved in the HPC-silica matrix. Transmission electron microscope (TEM) and atomic force microscope (AFM) analysis reveal nanoparticles size distribution from 2.5 nm to 56 nm for H2 S gas exposed sample. The surface chemistry of Ag2 S nanoparticles and sol-gel derived HPC-silica matrix is confirmed by X-ray photoelectron spectroscopy (XPS). The negative shifts in the core-level XPS Ag (3d) binding energy of Ag2 S nanoparticles are attributed to Ag : S surface atomic ratio exhibited by these nanoparticles with varying processing conditions. Following processing and characterization, suitability of the present method to produce silver sulfide ion-selective electrode is demonstrated by depositing Ag2 S nanoparticles on a graphite rod. The high reponse function of the electrode is due to the presence of nanoparticles. Keywords: sol-gel, silver sulfide, nanoparticles, XPS or ESCA, ion-selective electrode Introduction The physicochemical properties as well as the magnetic and optical properties of nanometer size materials are very unique and are different from their bulk counterparts [1]. Besides, the requirement for higher functionality, increased memory density, and higher speed are driving the need for smaller dimensions in various microelectronic industries. Nanomaterials, thus, hold promising future as precursors for building components of chips [2], for the development of sensors [3], for generating very small active elements in magnetic ∗ To

whom all correspondence should be addressed.

recording [4], and also as functional biological materials [5]. Work is under progress in our laboratory to synthesize and characterize sol-gel derived Hydroxypropyl Cellulose (HPC)-silica films containing sulfide and metallic nanoparticles of group IB elements. In our earlier communications, the synthesis and characterization of sol-gel derived HPC-silica films containing Au [6] and CuS [7] nonoparticles have been reported. The present technique is a three-step process by which sulfide/metallic nanoparticles embedded in a sol-gel derived HPC-silica film can be synthesized. In the first step, the selected precursor is dissolved in alcoholic solution containing both silica sol and organic

152

Shukla, Seal and Mishra

polymer to prepare a dipping solution. In the second step, a precursor film is formed on the glass substrate by dip-dry method. In the third step, this solid polymeric film containing homogeneous distribution of precursor molecules is exposed to H2 S gas to form sulfide/metallic nanoparticles. Heating the sol-gel derived HPC-silica film at high temperature can further control the particle size distribution in the nanometer range. Synthesis of Ag2 S 2-D and 3-D quantum crystals by reverse micelles [8–12] and its application as a photosensitizer for photographic processes [13] have been reported earlier in the literature. In the present article, we demonstrate and discuss the procedure for the synthesis of sol-gel derived HPC-silica film containing silver sulfide nanoparticles by the above-mentioned technique. Subsequently, morphological and chemical characterization were carried out using TEM, AFM, and XPS. In this article, we further deposit Ag2 S nanoparticles on a graphite rod for ion-selective electrode applications. The advantage of using ion-selective electrode for sensing particular ion(s) in the solution lies in the fact that in many instances time-consuming sample preparation such as filtration, distillation or extraction can be avoided. Ion Selective electrode (ISE) is a chemical sensor, which forms an electrochemical half-cell. Under equilibrium, the potential difference developed across the sensing and the reference electrode is a measure of the activity of the reactive species. In the present case, Ag2 S nanoparticles were deposited on a graphite electrode for detection of Ag+ ions. The dependence of EMF of the cell on the silver ion activity follows the Nernst equation: E = E 0 + S · log a[Ag+ ]

Experiments Synthesis of Sol-Gel Derived HPC-Silica Film Containing Ag2 S Nanoparticles Silver sulfide nanoparticles dispersed in sol-gel derived HPC-silica films were produced by gas diffusion technique. The general procedure for the preparation of sol-gel derived HPC-silica film containing sulfide/metallic nanoparticles is described schematically in Fig. 1, (referred as method-I). A dipping solution for Ag2 S precursor film was prepared by dissolving AgNO3 (0.5 g) and silica sol (0.3 g) in 100-ml anhydrous ethanol containing 0.5 g of HPC, Fig. 1(a). The silica sol was prepared from the hydrolysis and condensation of tetraethoxylane (TEOS) under acidic condition. In 10 g of TEOS, 2.0 g of water, 1.5 g of anhydrous methanol, and 100 mg of HCl (35%) were added and the resulting solution was stirred at room temperature for 1 hour to prepare the silica sol. Precursor film was then formed on a glass plate by dipping method, Fig. 1(b). After dipping the glass plate in the dipping solution, the film was dried immediately (10–15 min) by airflow at room temperature, (referred as “no H2 S/not heated” sample). Ag2 S nanoparticles dispersed in sol-gel derived HPC-silica film were obtained by exposing this precursor film to H2 S gas, Fig. 1(c), generated by the reaction between potassium sulfide and hydrochloric acid, diluted with distilled water (referred as “H2 S/not heated” sample), reaction (2). K2 S(S) + 2HCl(I) → 2KCl(aq) + H2 S(g) ↑

(2)

Ag2 S nanoparticles were produced via reaction (3):

(1)

2AgNO3(S) + H2 S(g) → Ag2 S(S) + 2HNO3(g) ↑ (3)

where, E = measured electrode potential, E 0 = standard potential of the system, S = slope, and a[Ag+ ] = activity of silver ions being measured. The solid-state membrane electrodes for the detection of monovalent silver ions based on Ag2 S have been reported in the literature [14]. Silver sulfide is an ion-conducting material, as are many silver salts. At room temperature, it exhibits much larger silver ion conductivity than corresponding halide compounds. Due to its silver ion conductivity, this material shows a Nernstian response to silver ions in the solution, and is thus well suited for ion-selective electrode applications.

The sol-gel derived HPC-silica film containing Ag2 S nanoparticles, after H2 S gas exposure step, was heated to 150–200◦ C (at the rate of 10◦ C/min) for 1 hour in a vacuum furnace at 1×10−4 Torr. The sample was cooled down to room temperature inside the furnace, (referred as “H2 S/heated” sample). In another experiment, silver sulfide powder particles were produced via precipitation method. In this method, H2 S gas was directly bubbled through a solution of AgNO3 dissolved in ethanol (referred as method-II). After bubbling the gas through the solution, black powder of silver sulfide particles precipitated out.

Synthesis and Characterization of Silver Sulfide Nanoparticles

153

Figure 1. Schematic diagram illustrating the procedure for the preparation of sol-gel derived HPC-silica film containing sulfide/metallic nanoparticles-(a) preparation of dipping solution, (b) formation of precursor film on glass substrate, and (c) formation of sulfide/metallic nanoparticles by H2 S gas exposure.

Characterization of Sol-Gel Derived HPC-Silica Film Containing Ag2 S Nanoparticles The structural analysis of Ag2 S powder (produced via method-II) was conducted with a Rigaku X-ray Diffraction (XRD) Technique. The particle size analysis of Ag2 S powder particles was carried out by JEOL Scanning Electron Microscope (SEM), while that of Ag2 S nanoparticles in H2 S/not heated and H2 S/heated samples was carried out by TEM and AFM. The surface chemistry of the sol-gel derived HPCsilica film containing Ag2 S nanoparticles, deposited on SiO2 substrate (method-I) and Ag2 S powder particles (method-II) was studied using a 5400 PHI ESCA (XPS) spectrometer at a base pressure of 10−10 Torr using Alkα X-radiation (1486.0 eV, linewidth 0.7 eV). Power

of 350 watts was used for the analysis. Both survey and high-resolution narrow spectra were recorded with electron pass energies of 44.75 eV and 35.75 eV respectively. The latter scans were conducted for core-level binding energy (B.E.) of major elements such as Ag, S, C and O. The B.E. of the Au 4f7/2 at 84.0 ± 0.1 eV was used to calibrate the B.E. scale of the spectrometer. Any charging shifts produced by the samples were carefully removed by adjusting the C (1s) B.E. of the C ---- H bond within the HPC polymer at 285.0 eV [15]. A method described by Sherwood was adopted to remove non-linear backgrounds from the spectra [16]. Non-linear least square curve fitting was performed using a Gaussian/Lorentzian peak shape after the background removal [16, 17] using a commercial peakfit software.

154

Figure 2.

Shukla, Seal and Mishra

Schematic diagram illustrating the experimental set-up used for an ion-selective electrode test.

Preparation of Ion-Selective Electrode Lastly, the method-I (without heating) described earlier was used for the preparation of an ion-selective electrode. For this purpose, a graphite rod, 0.5 mm in diameter and 40 mm in length, was used in place of silica plate for depositing silver sulfide nanoparticles. After the deposition, the graphite rod was placed in an electrochemical cell, Fig. 2, to detect the activity of the silver ions in the solution, with saturated calomel electrode as a reference electrode. AgNO3 solution of 10−6 M concentration was used as an electrolyte at the start of the test for measuring the activity of the silver ions. The overall cell reaction: + 4Hg(I) + 4Cl− (aq) + Ag2 S(s) + 2Ag(aq)

→ 2Hg2 Cl2(s) + 4Ag(s) + S2− (aq)

(4)

The concentration of the silver ions in the solution was increased in steps by controlled addition of drops of pure AgNO3 . The difference in the potential of the two electrodes was continuously monitored as the concentration of silver ions in the solution was increased. As the EMF values often drift, values were noted only

after allowing sufficient time, after the addition of each drop, to stabilize the potential difference. The total concentration range of 10−6 to 10−2 M of Ag+ ions was spanned. The response of Ag2 S nanoparticles deposited on a graphite electrode was then obtained by plotting a graph of EMF as a function of activity of silver ions in the solution.

Results Ag2 S powder produced via method-II was analyzed by XRD and the corresponding diffraction pattern is shown in Fig. 3. The major peaks are from (−121), (112), (031), (−112), (120), (111), (122), (−213), and (−123) orientation as identified by comparing the peaks with PDF card of JADE SCAN software. This indicates the formation of Ag2 S powder by method-II. SEM analysis of silver sulfide powder, produced by method-II, indicated the formation of spherical Ag2 S particles with the average particle size of 1µm. This precipitated powder (method-II) exhibited bulk behavior and was used as a reference for XPS analysis. In this investigation, we form silver sulfide nanoparticles within the sol-gel derived HPC-silica film,

Synthesis and Characterization of Silver Sulfide Nanoparticles

Figure 3.

155

Typical powder X-ray diffraction pattern for Ag2 S powder produced by method-II.

deposited on the glass substrate, by gas diffusion technique (method-I). Exposure of the sol-gel derived HPCsilica film, containing uniform distribution of AgNO3 molecules, to H2 S gas resulted in the formation of Ag2 S nanoparticles via reaction (3). Figure 4 shows TEM images of precipitated Ag2 S nanoparticles within the solgel derived HPC-silica film, at two different magnifications, produced immediately after H2 S gas-exposure step. The particles are observed to be uniformly distributed within the sol-gel derived HPC-silica film indicating the effectiveness of HPC polymer to distribute AgNO3 precursor molecules uniformly in the solid matrix. Further observation reveals that nanoparticles, as indicated by arrows in Fig. 4(b), are not exactly spherical, but have abrupt edges. This indicates that nanoparticles formed after the H2 S gas exposure are likely to be faceted in nature. The particle size distribution of Ag2 S nanoparticles, produced after H2 S gas exposure, is presented in Fig. 5. The nanoparticles size distribution appears to be rather wide. This is due to relatively large AgNO3 precursor concentration (0.5 g) used in the present investigation. Minimum and maximum nanoparticle size of 2.5 nm

and 56 nm are respectively noted through TEM analysis with an average size of 26 nm. The average size distribution of precipitated nanoparticles and their surface morphology under different processing conditions were further examined using AFM analysis. Typical AFM topographs of H2 S/not heated and H2 S/Heated samples are presented Fig. 6(a) and (b) respectively. The average nanoparticles size in Fig. 6(b) appears to be larger than that in Fig. 6(a). Thus, heating the sol-gel derived HPCsilica film containing Ag2 S nanoparticles at 150–200◦ C for 1 hour resulted in the growth of nanoparticles. Actual measurements indicated (approximately) average nanoparticles size of 50 nm and 200–300 nm in H2 S/not heated and H2 S/Heated samples, respectively. The average nanoparticles size measurements for the H2 S/not heated sample made via AFM and TEM analyses are thus in reasonable agreement with each other. It can also be noted from Fig. 6(b) that nanoparticles appear to have pyramid-like shape (see the insert in Fig. 6(b) at lower-left corner). Nanoparticles in Fig. 6(a) also exhibit pyramid like shape, which is not

156

Shukla, Seal and Mishra

Figure 4. TEM images- (a) at low magnification and (b) at high magnification, of Ag2 S nanoparticles dispersed in the sol-gel derived HPC-silica film, obtained after H2 S gas exposure step (method-I). In (a), Ag2 S nanoparticles marked by arrows appear to have sharp edges indicating their faceted nature.

visible at the given scale of magnification due to their very small size. The faceted nature of these nanoparticles was however deduced from the abrupt changes in the edges of the nanoparticles in H2 S/not heated sample presented in Fig. 4. Thus, both TEM and AFM analysis confirm the faceted nature of nanoparticles in the present investigation.

Broad scan XPS measurements for the sol-gel derived HPC-silica film under different processing conditions are presented in Fig. 7. Similar scan for Ag2 S powder produced by method-II is shown in Fig. 7. Narrow and high-resolution XPS scans of Ag (3d) and S (2p) (Figs. 8 and 9) were acquired to confirm the formation of Ag2 S nanoparticles within the sol-gel derived

Synthesis and Characterization of Silver Sulfide Nanoparticles

157

Figure 5. Particle size distribution of Ag2 S nanoparticles dispersed in the sol-gel derived HPC-silica film after H2 S gas exposure step (method-I) obtained by TEM analysis. Minimum and maximum particle size of 2.5 nm and 56 nm are detected with the average Ag2 S nanoparticles size of ∼26 nm.

HPC-silica film (method-I) and Ag2 S powder particles (method-II). Both the B.E. and the FWHM values for Ag and S core lines and their relative surface atomic concentrations are tabulated in Table 1. In Fig. 7, Ag is observed to be present on the surface of sol-gel derived HPC-silica film throughout the processing. Exposing this film containing AgNO3 to H2 S gas results in sulfur pick up at the surface of the film, Fig. 7(a). From Table 1, the relative surface atomic concentration ratio of Ag : S in the H2 S/not heated sample is noted to be 2.7 : 1. This suggests the formation of Ag2 S nanoparticles within the sol-gel derived HPCsilica matrix (see Figs. 4 and 6(a)) after the H2 S gas exposure step. Heating this film at 150–200◦ C for 1 hour did not eliminate S peak from the surface, Fig. 7(b), indicating the integrity of silver sulfide (Ag : S = 1.9 : 1 (ideal value = 2 : 1), see Table 1). Silver sulfide powder synthesized by method-II also shows the prominence presence of silver and sulfur on the surface, Fig. 7(c). The corresponding relative Ag : S surface atomic ratio of 3.6 was found to be larger than the ideal value of 2 : 1

(see Table 1). However, XRD analysis does indicate the formation of Ag2 S within the bulk of powder particles obtained via method-II. The overall broad scan XPS analysis, however, does suggest the formation of Ag2 S nanoparticles by both the methods I and II. As indicated in Fig. 8(c), the Ag 3d5/2 B.E. is measured to be 368.0 eV (FWHM = 1.1 eV) in Ag2 S powder (prepared by method-II). This is in well agreement with the literature value reported for bulk Ag2 S compound [18]. The formation of Ag2 S powder is further supported by S 2p3/2 B.E. value of 161.4 eV (see Table 1) [18]. Hence, Ag2 S powder produced by method-II exhibits bulk behavior. However, Ag 3d5/2 B.E. for Ag2 S nanoparticles (synthesized by method-I) is observed to be shifted slightly to lower B.E. values (i.e. negative shift, see Fig. 8(a) and (b)) relative to the bulk (powder) value of 368.0 eV, Fig. 8(c). In H2 S/not heated sample, Ag 3d5/2 B.E. is observed at 367.9 eV (FWHM = 1.4 eV: Fig. 8 (a)) and for H2 S/heated sample, it is measured at 367.8 eV (FWHM = 1.3 eV: Fig. 8(c)). Thus, from Fig. 8,

158

Shukla, Seal and Mishra

Figure 6. Atomic force micrographs of (a) H2 S/not heated and (b) H2 S/heated samples. The average nanoparticles size in (a) is ∼50 nm and in (b) is ∼200–300 nm. In (b), nanoparticles appear to have pyramid-like shape indicating their faceted nature and is described schematically in the insert at lower-left corner.

Synthesis and Characterization of Silver Sulfide Nanoparticles

159

Figure 7. XPS survey spectra for Ag2 S nanoparticles dispersed in the sol-gel derived HPC-silica film undergoing different processing steps (method-I)- (a) H2 S/not heated sample, (b) H2 S/heated sample, and (c) Ag2 S powder produced by method-II.

it is evident that for Ag2 S nanoparticles, Ag 3d5/2 B.E. is decreased and FWHM is increased with respect to the respective bulk values. For Ag2 S nanoparticles, S 2p3/2 B.E. level shows similar trend as illustrated in Fig. 9. For H2 S/not heated, H2 S/heated, and Ag2 S powder samples, the S 2p3/2 B.E. is respectively observed at 161.0 eV (FWHM = 1.4 eV, Fig. 9(a)), 160.8 eV (FWHM = 1.3 eV, Fig. 9(b)), and 161.4 eV (FWHM = 1.1 eV, Fig. 9(c)). Interestingly, after heating the sol-gel derived HPC-silica film containing Ag2 S nanoparticles at high temperature, a new peak appears at 163.5 eV, Fig. 9(b). The exact origin of this peak is not yet fully understood and was not detected in our earlier investigation on CuS and Au nanoparticles [6,7]. Thus, Ag 3d5/2 and S 2p3/2 XPS core-lines do indicate the formation of Ag2 S nanoparticles in the present investigation and exhibit negative shifts relative to the bulk values. In order to confirm and validate the chemistry of the matrix, in which Ag2 S nanoparticles are deposited, narrow and high-resolution XPS scans of C 1(s) and Si (2p) scans were acquired. Figure 10(b)–(d) represent C (1s) spectra obtained under different processing

Figure 8. XPS Ag (3d) spectra for (a) H2 S/not heated sample (method-I), (b) H2 S/heated sample (method-I), and (c) Ag2 S powder (method-II). The Ag 3d5/2 B.E. level is observed at 367.9 eV in (a), 367.8 eV in (b) and 368.0 eV in (c). (Note: The particle size in the figure indicates average particle size).

conditions. The C 1(s) spectrum related to HPC powder is also included for reference in Fig. 10(a). In Fig. 10, the lower peak, corresponding to C ---- H bond was adjusted to 285.0 eV for charge correction [15]. For the HPC polymer, Fig. 10(a), the two peaks at higher B.E. values are located at 286.4 eV and 288.0 eV. These values correspond to C ---- O ---- H, C ---- O ---- C and C == O bonds in the polymer [19]. This indicates the presence of HPC polymer within the film during processing. The observed B.E. valuues of these peaks in Fig. 10 are listed in Table 2. However, the intensity of C ---- O ---- H peak is decreased after heating, (compare Fig. 10(d) with Fig. 10(c)), due to elimination of O ---- H group from the polymer after heating at 150–200◦ C for 1 hour. Further, the Si(2p) spectrum (Fig. 11) shows the presence of silica within the matrix (Si 2p3/2 B.E.∼ 102.6 eV).

160

Shukla, Seal and Mishra

Table 1. XPS core-level B.E. and FWHM (±0.1 eV) values obtained for Ag 3d5/2 and S 2p3/2 B.E. levels in Ag2 S nanoparticles and powder particles, respectively processed via methods-I and II (see the text for the details). C(1s) B.E. value of 284.6 eV related to C ---- H bond in HPC polymer is taken as a reference. The relative atomic concentrations of Ag and S, calculated by considering the height of the respective major peaks in the XPS survey spectrum, are also included. Ag 3d5/2 Samples

S 2p3/2

B.E. (eV)

FWHM (eV)

% Rel. At. Conc.

B.E. (eV)

FWHM (eV)

% Rel. At. Conc.

H2 S/not heated

367.9

1.4

10.9

161.0

1.4

4.0

H2 S/heated

367.8

1.3

13.7

160.8

1.3

7.4

Ag2 S powder

368.0

1.1

47.8

161.4

1.1

13.4

Figure 9. XPS S (2p) spectra for (a) H2 S/not heated sample (method-I), (b) H2 S/heated sample (method-I), and (c) Ag2 S powder (method-II). The S 2p3/2 B.E. level is observed at 161.0 eV in (a), 160.8 eV in (b) and 161.3 eV in (c).

Narrow and high-resolution O 1(s) scans were acquired (Fig. 12) to detect any presence of metallic oxides within the sol-gel derived HPC-silica film. The O 1(s) spectrum of HPC powder sample is also included, Fig. 12(a), for reference. From Fig. 12(b)– (d), for no H2 S/not heated, H2 S/not heated, and

Figure 10. XPS C (1s) spectra for (a) HPC powder, (b) no H2 S/not heated sample (method-I), (c) H2 S/not heated sample (method-I), (d) H2 S/heated sample (method-I).

H2 S/heated samples, the C 1(s) spectra are consistently observed at 532.4 eV. O1s for HPC polymer is measured at 532.8 eV, Fig. 12(a). This suggests that the O1(s) spectra observed for sol-gel derived HPCsilica films are mainly originating from the organic polymer.

Synthesis and Characterization of Silver Sulfide Nanoparticles

161

Table 2. XPS core-level B.E. values obtained for deconvoluted C 1s peaks, presented in Fig. 11, under different processing conditions. The respective values for HPC powder are also included in the table. Samples

C H B.E (eV)

C O H, C O C B.E (eV)

C O B.E (eV)

HPC powder

285.0

286.4

288.0

No H2 S/not heated

285.0

286.4

288.3

H2 S/not heated

285.0

286.3

287.7

H2 S/heated

285.0

286.1

287.5

Figure 12. XPS O (1s) spectra for (a) HPC powder, (b) no H2 S/not heated sample (method-I), (c) H2 S/not heated sample (method-I), (d) H2 S/heated sample (method-I). The O (1s) B.E. level is observed at 532.8 eV in (a) and 532.4 eV in (b)–(d).

Figure 11. XPS Si (2p) spectra for (a) no H2 S/not heated sample (method-I) and (b) H2 S/heated sample (method-I). The Si 2p3/2 B.E. level is observed at 102.7 eV in (a) and 102.6 eV in (b).

Discussion A new method for the preparation of sol-gel derived HPC-silica film containing Ag2 S nanoparticles is reported here. In the earlier investigations, we reported the preparation of sol-gel derived HPC-silica films containing Au [6] and CuS [7] nanoparticles by this technique, where respective chlorides (viz. HAuCl4 · 3H2 O and CuCl2 ) were used as precursors. However, in the present investigation, due to insoluble nature of silver chloride in alcoholic solution, AgNO3 is used as a precursor for the preparation of Ag2 S nanoparticles dispersed in sol-gel derived HPC-silica film. The free energy calculations (G) for the reaction (3) further justified the selection of the precursor. G value for

the reaction (3) (at room temperature) is calculated to be −87.5 KJ/mol, while that for similar reaction involving AgCl as a precursor is calculated to be +111.7 KJ/mol. In method-I, Ag2 S nanoparticles are formed within the sol-gel derived silica film containing HPC, an organic polymer, which is responsible to dissolve precursor molecules (AgNO3 ) uniformly throughout the film (Fig. 4). Various other designs of matrices for the formation of nanoparticles have been reported earlier. For example, a thin film silica with layer structure prepared by converting TEOS to silica in LB membrane [20], clay [21, 22], and organic polymer such as poly(acrylonitrile-styrene) [23] have been listed as a matrix material. The chemical structure of HPC, a derivative of cellulose, is shown in Fig. 13. The solubility limit of precursor molecules in silica films is reported to increase with increase in the amount of HPC [24]. Owing to the presence of hydroxyl and ether groups in the chemical structure of this polymer,

162

Shukla, Seal and Mishra

Figure 13. Chemical structure of Hydroxypropyl Cellulose (HPC), a compatibilizer, used in the present investigation to dissolve uniformly the AgNO3 precursor in the solid matrix.

Fig. 13, HPC is compatible with silica as well as with precursor molecules through hydrogen bonding. This forms a molecular hybrid, distributing precursor molecules uniformly throughout the matrix [7]. This is the key step in the present technique for forming Ag2 S nanoparticles in the sol-gel derived HPC-silica film after the H2 S gas exposure step (Fig. 4). Presence of HPC also controls the agglomeration by capping the OH molecules on the particle surface. In our earlier investigation, we successfully demonstrated the formation of copper sulfide nanoparticles with minimum nanoparticle size of 2.5 nm and Au nanoparticles with average nanoparticles size of

Suggest Documents