Template assisted synthesis and assembly of nanoparticles 4

Chapter Chapter Organo-clay/PbS nanocomposites Amongst the many media used in composite materials, those based on clays have been quite widely investigated. This is probably because the starting clay materials are cheap and easily available, while their intercalation chemistry has been known for a long time. It is well known that after intercalation, the polarity and solubility of clay will change accordingly. Clay modified by organic reagents is called organo-clay. The polarity of the organo-clay changes from hydrophilic to hydrophobic, which helps in the further preparation of organo-clay related composites. Clay/polymer nanocomposites occupied the majority of clay related nanocomposites. Relatively fewer examples of clay/inorganic nanocomposites were reported. Han1, have reported the preparation of clay/CdS and other clay/metal sulfide nanocomposites by onepot hydrothermal reaction. In their reaction, they mixed water-soluble metal-thiourea complex with aqueous clay suspension and claimed that the complex cations will be introduced into the interlayers of clay through ion exchange reaction (refer to Figure 1.1). After heating under hydrothermal condition, this aqueous suspension produced CdS pillared clay. However, the clay/CdS nanocomposites prepared by this method were not well dispersed and the CdS particles formed were ~ 100 nm in size. In this chapter, we propose to improve the distribution and the size control of nanoparticles in clay composite by using organo-clay as the matrix and alkanethiol as the capping agent. We demonstrate the advantage of using precursor approach to prepare nanoparticles in situ within the clay layers, in this case PbS nanoparticles. PbS is a narrow band gap IV-VI semiconductor with a band gap of 0.41 eV. In PbS, 118 Chapter the excitons have large bulk Bohr exciton radius (aB = 18 nm) because of its small carrier effective masses (m* = 0.1me), and large optical dielectric constant (ε = 17). The strong quantum confinement regime is thus easily achieved, and the ground-state absorption edge can be tuned over a wide wavelength range (typically from near infrared to visible). As a result, PbS quantum dots (QDs) with narrow size distribution exhibit a finestructured absorption spectrum, blue shift of absorption onset, and large third-order nonlinear optical susceptibilities. Thus far, PbS related nanocomposites such as poly(ethylene oxide)/PbS, polyvinyl alcohol/PbS3, , polystyrene/PbS5, sulfonated polystyrene/PbS6, polythiourethane (PTU)/PbS7, polyvinyl acetate/PbS8, poly(vinyl pyrrolidone)/PbS9, polyacrylamide/PbS10, polyacrylonitrile/PbS11, poly(2-methoxy-5(2’ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV)/PbS12, 13 et al14 have been prepared and their properties such as linear and nonlinear optical properties, refractive index, PL emission were investigated. Since clays have a wide range of applications as adsorbents, electrode supporting materials, catalyst etc., it is expected that the organoclay/PbS nanocomposites will provide more synergistic physical and materials properties, which combine the properties of clay and PbS nanoparticles. 4.1 Optimization of preparation method The prepared PbTB precursor was found to decompose neatly at ~180°C from TGA study as shown in Figure 4.1. In earlier reports of our research group, we have confirmed that PbTB can be decomposed by amine at room temperature while the PbS nanoparticles formed can be stabilized by the addition of amine or thiol.15 119 Chapter 100 PbTB Weight % 80 60 40 20 0 200 400 600 800 1000 Temperature °C Figure 4.1. TGA plot of PbTB precursor in N2 environment. As the first attempt in the synthesis of PbS nanoparticles in organo-clay, we heated a mixture of PbTB and organo-clay I30p dispersion in toluene to 85°C in an oil bath. The modified clay - I30p obtained from Nanocor Inc. contains octadecylamine (ODA) molecules as organic modifier to enlarge the clay interlayer distance. In order to prevent aggregation of PbS particles, we added DDT as capping agent in this experiment. The reaction proceeded as indicated by a color change from light brown to light reddish brown and finally to dark brown. In the isolated products, some PbS cubic particles of ~100 nm in size were observed outside the clay sheets as shown in Figure 4.2. At the same time, smaller PbS nanoparticles of ~10 nm in size were dispersed both inside and outside of the interlayers of the organo-clay. The formation of large PbS particles (circled areas in Figure 4.2) was still observed even though the DDT to PbTB ratio was ten times in excess. Thus, the challenge remains mainly on how to efficiently bring the PbTB decomposition to occur at the clay interlayers and to control the particle growth. 120 Chapter Figure 4.2. Representative TEM image of organo-clay/PbS composites prepared by heating PbTB and I30p with DDT. We recognized that the reaction should be slowed down in order to allow better diffusion of reagents into the inter clay layers. Thus a suitable solvent DMF was added to dissolve PbTB, and the mixture of PbTB in clay dispersion was stirred for some time to give a more homogeneous reaction medium. A short chain amine, namely propylamine, was then added to initiate the reaction at room temperature. From Figure 4.3, it can be seen that smaller PbS nanoparticles of ~ 25 nm were formed indeed, but there were still some aggregations (in circled areas) observed outside the interlayers of clay. Figure 4.3. Representative TEM image of organo-clay/PbS nanocomposites prepared with the addition of propylamine at room temperature. 121 Chapter We noted that the color of the solution changed to reddish brown which indicated the formation of PbS seeds and the reddish brown color deepened gradually upon the addition of propylamine in the earlier experiment. We decided to add DDT to impede the growth of nanoparticles at pre-determined interval has been initiated (this period is denoted as Reaction Time I, varied from 10 minutes to hours as seen in Section 4.4). The solution was then stirred further for some time (this period is denoted as Reaction Time II, varied from 30 minutes to 20 hours) before ethanol was added to quench the reaction. The improved synthetic route is schematically shown in Figure 4.4 and detailed procedures are described in Section 2.5. PbTB/DMF Organo-clay I30p Reaction Time I Propylamine Reaction Time II Dodecanethiol Organo-clay/PbS nanocomposites Figure 4.4. Schematic optimized synthetic route for organo-clay/PbS nanocomposites. Detailed characterization results of the organo-clay/PbS nanocomposites prepared with different feed ratio of PbTB to organo-clay will be discussed in Section 4.2. Using our improved method, PbS nanoparticles with uniform sizes can be formed inside the clay interlayers. We propose that DDT, being capped onto the surface of the nanoparticles, has helped to carry these PbS nanoparticles into the clay interlayers. Thus 122 Chapter the effect of DDT to the reaction was investigated by varying the feed ratio of DDT to PbTB and the results are described in Section 4.3. The organic modifier ODA in I30p is supposed to have changed the nature of clay interlayers to hydrophobic. We believe that it is through van der Waals interactions between ODA and DDT that the formed PbS nanoparticles were hold within the interlayers of the organo-clay. The details of this investigation will be discussed in Section 4.3. The organo-clay/PbS nanocomposites prepared were found to exhibit near infrared PL emission. It is expected that the PL peak position is dependent on the size of PbS nanoparticles and hence attempts to tune the particle sizes were carried out in Section 4.4 by changing parameters such as the relative feed amounts of PbTB, DDT and propylamine, as well as the reaction time before and after DDT was added (Reaction Time I and II). The detailed conditions for the various series of samples prepared are listed in Tables 2.2 to 2.6 in Chapter 2. 4.2 Characterization of organo-clay/PbS nanocomposites Representative TEM images in Figure 4.5 showed the typical morphology of organoclay/PbS nanocomposites prepared with increasing PbTB feed ratio. The detailed experimental feed ratios are given in Table 2.2. It was found that uniform PbS nanoparticles were well dispersed in organo-clay interlayers and most of the particles were found within the layers of the organo-clay. It is worthy to note that no obvious aggregation was observed although the reaction time is long enough (20 hours) for Ostwald ripening to occur. This is due to the space constraints within the clay interlayers and also the capping effect of DDT. Comparing with previous publications1, 2, 16, the 123 Chapter morphology and dispersity of our organo-clay/PbS nanocomposites are much more smaller and uniform than other reported particles size (> 100 nm) in clay/MS (M = Cd, Co, Ni, Zn, Pb) composites1. The sizes of our prepared PbS nanoparticles were too small to be estimated from these TEM images. a b d c e Figure 4.5. TEM images of organo-clay/PbS nanocomposites with increasing PbTB feed ratios (a) Sample 1, (b) Sample 2, (c) Sample 3, (d) Sample 4, and (e) Sample 5. (Sample details refer to Table 2.2) The HRTEM images shown in Figures 4.6 allowed a clearer observation of the morphology of the PbS nanoparticles. The crystalline PbS shows regular lattice plane with interplanar distance of 2.90 Å (insert of Figure 4.6a), which is in agreement with 124 Chapter the (200) planes of standard cubic PbS. The size histogram shown in Figure 4.6c indicates the relatively narrow size distribution of PbS nanoparticles with an average size of 3.8 ± 0.9 nm. Clay, as well as all the organic complexes studied exhibited electron diffraction ring patterns that can be correlated with the diffraction behavior17. The SAED pattern (Figure 4.6b) exhibited polycrystalline diffraction dots that can be indexed to cubic structure of PbS nanoparticles, while the diffraction rings indicated the crystallinity of organo-clay. The d spacing values calculated from SAED are 4.6 Å, 2.6 Å, and 2.1 Å. a c 25 b Fraction % 20 15 10 10 Diameter (nm) Figure 4.6. Typical HRTEM images of organo-clay/PbS nanocomposites (a) Sample with insert showing the lattice of one PbS nanoparticle, (b) the corresponding SAED pattern, and (c) histogram showing the size distribution. Figure 4.7 showed the XRD diffraction patterns of organo-clay/PbS nanocomposites prepared with increasing PbTB feed ratio. Both the clay diffraction peaks18, 19, 20, 21 and PbS diffraction peaks (JCPDS 05-592 cubic phase: labeled in brackets in Figure 4.7) were observed, In general, the PbS diffraction peaks become stronger as the content of Pb 125 Chapter increases in these samples. The much broaden PbS diffraction peaks suggested the nano- (400) (311) (222) (220) 130 (111) (200) * 020, 110 001 size of these PbS particles. Clay:PbTB=1:5 * 20 * * 10 60 80 Clay:PbTB=1:2 * * * * 40 15 20 * 20 40 60 80 Clay:PbTB=1:1 25 30 20 40 60 80 Clay:PbTB=2:1 2θ 20 40 60 80 Clay:PbTB=3:1 20 40 60 80 PbS 05-592 20 40 60 80 I30p 20 40 2θ 60 80 Figure 4.7. XRD diffraction patterns of organo-clay/PbS nanocomposites with increasing PbTB feed ratio (Samples to 5, refer to Table 2.2 in Chapter 2). Peaks labeled in brackets were assigned to PbS JCPDS 05-592, and those without brackets were due to the organo-clay I30p. The patterns of standards are depicted at the bottom for comparison. Due to the extreme small size of PbS particles prepared (5-7 nm), increasing amount of PbS did not have an obvious effect on the interlayer distance of the organo-clay. The presence of higher order peaks (2θ = 20°, 2θ = 34.9°, 2θ = 54°, 2θ = 62°) confirmed that the layered structure of the organo-clay layers is not disrupted by the incorporation of the PbS nanoparticles. The SAED results above (Figure 4.6b, d = 4.6 Å, 2.6 Å) are consistent with the XRD peaks of organo-clay at 2θ = 20°, 2θ = 34.9°. On the other hand, 126 Chapter the value d = 2.1 Å from SAED corresponded to the (220) phase of PbS nanoparticles. Interestingly, in addition to the standard patterns of PbS and I30p, a series of peaks with regular intervals were also observed between 5o and 30o in most of the organo-clay/PbS samples (labeled with * in Figure 4.7). We tentatively assign these peaks to the organized self-assembled organic chains and a detail discussion will be given in Section 4.3. Table 4.1 showed the EA results of Samples to 5. The results demonstrates that the as-prepared organo-clay/PbS composites contain quite a substantial amount of organic species as compared to the reported composition of montmorillonite (Si: 31.16%, O: 50.17%, Al: 13.28%, Mg: 3.25%, Na: 1.20%, K: 0.40% and Ca 0.54%)22, indicating that the organic species was introduced. It is noted that the experimental Pb weight percentages, while increasing from Samples to 5, were consistently lower than the expected values estimated from the feed ratios (refer to Table 4.1). This deviation may probably be caused by a loss of small PbS nanoparticles in solution during the purification process since thiolated PbS nanoparticles are partially soluble in DMF and ethanol. Table 4.1. EA results of organo-clay/PbS nanocomposites prepared with increasing PbTB feed ratio as presented in Table 2.2. Sample ID C (%) H (%) N (%) S (%) Pb (%) 16.75 18.69 20.81 21.88 28.28 3.70 3.94 4.26 4.29 5.21 1.64 1.36 1.16 1.04 40, Samples 14 to 16), the PL peak positions were observed to remain unshifted at ~ 1300 nm, suggesting that the amount of DDT added has probably reached 139 Chapter its critical concentration. When insufficient DDT was added (i.e. [DDT]/[PbTB] < 2, Sample and Sample 9), the PL emission peaks will red-shift to 1288 nm or even disappear completely (Sample 8) due to much aggregation of the PbS nanoparticles. An explanation for the observed red-shift may be given as follows: the DDT molecules are acting both as the capping agent for PbS nanoparticles as well as a stabilizing agent on the PbTB-propylamine complex46. When the amount of DDT was increased, the DDT stabilizing role dominates over its capping effect, thus affecting the decomposition leading to lesser amount of nuclei formed. The growth of this small amount of nuclei therefore leads to larger PbS particles after a long reaction time. Hence, the corresponding PL emission peaks shifted to longer wavelength. Thus in summary, these control experiments provided strong evidences to support that both ODA and DDT play important roles in the formation of organo-clay/PbS nanocomposites. We believe that the driving force to hold PbS nanoparticles within the clay interlayers is the interdigitation of long chains through van der Waals interactions between organic groups. 4.4 Tunable optical property of organo-clay/PbS nanocomposites It is well known that the absorption band edge in PbS nanocrystals is dependent on the nanocrystal size, with the band edge blue-shifting as the size of the nanocrystal decreases.47 This offers excellent size tunability across the near infrared (NIR) region and can be used in electroluminescent devices such as light-emitting diodes (LEDs)48, 49, 50 and optical devices such as optical switches51 (because of high third-order nonlinear optical properties). In optoelectronics, size tunability permits spectral control of 140 Chapter absorption for photovoltaic52, stimulated emission, and electroluminescence applications. In this section, several approaches have been used to tune the PL peak position of these composites. Effect of the amount of DDT As discussed in the previous section, the amount of DDT has resulted in quite significant shifts in the PL emission peak position. Another set of nanocomposites with varying amount of DDT was prepared, this time using shorter Reaction Time I (i.e. the interval time between the addition of propylamine and DDT is 30 minutes instead of hours). This series of samples are denoted as Series II, Samples 17 to 22, and the detailed conditions of sample preparation are given in Table 2.4 of Chapter 2. Figure 4.17 showed the corresponding PL peak positions of these samples while the TEM and XRD analyses of these nanocomposites were summarized in Appendix 2C and 2D. 1387 Sample 22 1360 Sample 21 1289 1194 Sample 20 Sample 19 1153 Sample 18 1241 Sample 17 1000 1200 1400 1600 Wavelength nm Figure 4.17. PL spectra of organo-clay/PbS nanocomposites prepared with increasing amount of DDT (Series II, from bottom: Samples 17 to 22). 141 Chapter From Figure 4.17, we observe a similar trend for the shift in PL peak positions as in Series I. Thus, the PL peak was blue-shifted slightly (Samples 17 to 18) followed by a continual red-shift (Samples 19 to 22) as the amount of DDT was increased. Although the width of the peak also increased (FWHM increased from 180 nm to 300 nm), these PL peak positions are observed from ~ 1150 to ~ 1390 nm when [DDT]/[PbTB] changed from to 50. Therefore, PL emission can be tuned within a wide wavelength range by adjusting the amount of DDT added. It was noted that, comparing to Figure 4.16, the corresponding PL peak positions of Series II samples appeared at shorter wavelengths in general. Thus, in the following sub-section, we will discuss the influence of Reaction Time I to PbS particles size. Effect of Reaction Time I Typically, the particle size and hence the PL peak position can be tuned through controlling the particles growth time. We investigated this effect and prepared a series of samples (Series I, Samples 23 to 27; details refer to Table 2.5 of Chapter 2; TEM and XRD figures are presented in Appendix 2E and 2F) by varying the duration of Reaction Time I (interval between addition of propylamine and DDT). The corresponding PL spectra were shown in Figure 4.18. We found that the PL emission peak red shifted from 1130 to 1264 nm while the FWHM of the emission peak remained almost the same as 150 nm. Since DDT is acting as a capping agent, PbS particles will grow rapidly before the addition of DDT. Therefore, when other reaction conditions are the same, a longer Reaction Time I will produce larger PbS crystals seeds. The corresponding PL peak maxima are thus shifting to higher wavelengths. 142 Chapter Sample 27 2hours Sample 26 1hour Sample 25 45mins Sample 24 30mins Sample 23 10mins 1000 1200 1400 1600 Wavelength nm Figure 4.18. PL spectra of organo-clay/PbS nanocomposites prepared with increasing Reaction Time I (Series I, from bottom: Samples 23 to 27). Figure 4.19 showed the PL spectra of another two series of organo-clay/PbS samples prepared with different Reaction Time I. Difference between these two series of samples is in the [DDT]/[PbTB] ratio. This ratio is fixed at for Samples 28 to 30 (Series II, sample details refer to Table 2.6 of Chapter 2) while the ratio is 10 for Samples 31 to 33 (Series III, sample details refer to Table 2.7 of Chapter 2). In these two cases, similar trend was observed for the peak position shift, thus PL peak position is red-shifted from 1170 nm to 1330 nm through the control of Reaction Time I. Comparing to the PL peak positions of Series II samples (Figure 4.19a), the corresponding peak positions for Series III samples (Figure 4.19b) consistently appeared at higher wavelengths. This is as same as what we expected from the higher amounts of DDT used. 143 Chapter a Sample 30 2hours 1254 Sample 33 2hours b 1319 1221 1299 Sample 32 90mins Sample 29 90mins 1279 1186 Sample 31 30mins Sample 28 30mins 1000 1200 1400 Wavelength nm 1600 1000 1200 1400 1600 Wavelength nm Figure 4.19. PL spectra of organo-clay/PbS nanocomposites prepared with increasing Reaction Time I: (a) Series II, from bottom: Samples 28 to 30, (b) Series III, from bottom: Samples 31 to 33. Effect of Reaction Time II In another series of samples, the time interval was varied after DDT was added (Reaction Time II) (Samples 34 to 39, sample details given in Table 2.8 of Chapter 2). The PL spectra of these composites are shown in Figure 4.20, suggesting again a red shift in the peak position. When Reaction Time II was controlled within hours (Samples 34 to 37), the PL peak position of the nanocomposites almost remain the same. This indicated the good capping function of DDT on the PbS nanoparticles. When the Reaction Time II was increased to 10 hours, Ostwald ripening occurred and larger PbS nanoparticles continued to grow while smaller ones eventually redissolved (Samples 38 and 39). In this case, the PL emission peak of nanocomposites can be tuned from 1180 to 1300 nm. The TEM and XRD results of these samples are given in Appendix 2G and 2H. 144 Chapter Sample 39 20hours Sample 38 10hours Sample 37 2hours Sample 36 1hour Sample 35 30mins Sample 34 5mins 1000 1200 1400 1600 Wavelength nm Figure 4.20. PL spectra of organo-clay/PbS nanocomposites prepared with increasing Reaction Time II (from bottom: Samples 34 to 39). Effect of the amount of propylamine At the last comparison, the amount of propylamine added was also varied to study its effect. Figure 4.21 showed the PL spectra of nanocomposites prepared with varying amount of propylamine (Sample 40 and Sample 41, sample details refer to Table 2.9 of Chapter 2). The emission peak maxima red-shifted from 1185 nm to 1285 nm as the molar ratio of propylamine to PbTB was increased from to 4. Increasing amount of propylamine may be related to a faster decomposition of the PbTB precursor and hence a faster growth of the PbS crystals. Thus, the PL emission peak can be tuned by the amount of propylamine used in the reaction. 145 Chapter Sample 41 PbTB:propylamine=1:2 Sample 40 PbTB:propylamine=1:4 1000 1200 1400 1600 Wavelength nm Figure 4.21. PL spectra of organo-clay/PbS nanocomposites prepared with different amount of propylamine (from bottom: Sample 40 and Sample 41). Table 4.4. PL emission peak position range of organo-clay/PbS nanocomposites Parameters PL emission position [clay]:[PbTB] = 2:1~1:5 1280nm to 1480nm [clay]:[DDT] = 1:2 ~ 1:250 1150nm to 1390nm [PbTB]:[propylamine] =1:1~1:4 1150nm to 1300nm Reaction time I & II: 10mins~20hours 1120nm to 1300nm In summary, we demonstrated in this section the possibility of tuning the PL peak position of the organo-clay/PbS composites from ~ 1100 nm to ~ 1500 nm through varying the amount of PbTB, DDT, propylamine and Reaction Time I and II separately. The PL emission peak range was listed in Table 4.4. 4.5 Conclusions In conclusion, PbS nanoparticles have been successfully formed in situ within the organo-clay matrix at room temperature. The embedded PbS nanoparticles were found to 146 Chapter have narrow size distribution with average particle sizes of ~ 5-7 nm. 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Physical Review B 1996, 54, (24), 17628-17637. 152 [...]... 1996, 12, (18), 44 21 -44 29 25 Katti, K S.; Sikdar, D.; Katti, D R.; Ghosh, P.; Verma, D Polymer 2006, 47 , (1), 40 3 -41 4 26 Madejova, J Vibrational Spectroscopy 2003, 31, (1), 1-10 27 Snyder, R G.; Strauss, H L.; Elliger, C A Journal of Physical Chemistry 1982, 86, (26), 5 145 -5150 28 Macphail, R A.; Strauss, H L.; Snyder, R G.; Elliger, C A Journal of Physical Chemistry 19 84, 88, (3), 3 34- 341 29 Li, Y Q.;... interlayers of clay organo-clay I30p organo-clay/PbS composite organo-clay/PbS mixture 100 Weight% 90 80 70 60 0 200 40 0 600 800 1000 Temperature C Figure 4. 12 TGA plots of organo-clay/PbS nanocomposites (Sample 2) and a mixture of I30p and PbS nanoparticles at similar PbS percentage It have been reported that the PL of PbS nanoparticles is rather sensitive to surface property and chemical environment 44 Because... 41 PbTB:propylamine=1:2 Sample 40 PbTB:propylamine=1 :4 1000 1200 140 0 1600 Wavelength nm Figure 4. 21 PL spectra of organo-clay/PbS nanocomposites prepared with different amount of propylamine (from bottom: Sample 40 and Sample 41 ) Table 4. 4 PL emission peak position range of organo-clay/PbS nanocomposites Parameters PL emission position [clay]:[PbTB] = 2:1~1:5 1280nm to 148 0nm [clay]:[DDT] = 1:2 ~ 1:250... as in Table 4. 2 Sample 5 Sample 4 Sample 3 Sample 2 Sample 1 I30p 40 00 3800 3600 340 0 3200 3000 Wavenumber cm 2800 2600 240 0 800 600 40 0 -1 Sample 5 Sample 4 Sample 3 Sample 2 Sample 1 I30p 2000 1800 1600 140 0 1200 1000 Wavenumber cm -1 Figure 4. 8 FTIR spectra of organo-clay I30p and organo-clay/PbS nanocomposites prepared with different PbTB feed ratio (Samples 1 to 5) 128 Chapter 4 Table 4. 2 Assignments... W S Journal of Physical Chemistry B 20 04, 108, (30), 11001-11010 41 Tjong, S C Materials Science & Engineering R-Reports 2006, 53, (3 -4) , 73-197 42 Lee, S J.; Han, S W.; Choi, H J.; Kim, K Journal of Physical Chemistry B 2002, 106, (11), 2892-2900 43 Kim, Y H.; Lee, D K.; Jo, B G.; Jeong, J H.; Kang, Y S Colloids and Surfaces a-Physicochemical and Engineering Aspects 2006, 2 84, 3 64- 368 44 Sashchiuk,... most of the long-chain DDT-capped PbS particles would prefer to stay outside of the interlayers as we expected 138 Chapter 4 The function of DDT in the formation of organo-clay/PbS nanocomposites The effect of DDT on the size and distribution of PbS nanoparticles in organo-clay matrix was also investigated The amount of DDT was varied (Series I, Samples 8 to 16, as details in Table 2.3 of Chapter 2) and. .. samples while the TEM and XRD analyses of these nanocomposites were summarized in Appendix 2C and 2D 1387 Sample 22 1360 Sample 21 1289 11 94 Sample 20 Sample 19 1153 Sample 18 1 241 Sample 17 1000 1200 140 0 1600 Wavelength nm Figure 4. 17 PL spectra of organo-clay/PbS nanocomposites prepared with increasing amount of DDT (Series II, from bottom: Samples 17 to 22) 141 Chapter 4 From Figure 4. 17, we observe... occurred and larger PbS nanoparticles continued to grow while smaller ones eventually redissolved (Samples 38 and 39) In this case, the PL emission peak of nanocomposites can be tuned from 1180 to 1300 nm The TEM and XRD results of these samples are given in Appendix 2G and 2H 144 Chapter 4 Sample 39 20hours Sample 38 10hours Sample 37 2hours Sample 36 1hour Sample 35 30mins Sample 34 5mins 1000 1200 140 0... Wavelength nm Figure 4. 20 PL spectra of organo-clay/PbS nanocomposites prepared with increasing Reaction Time II (from bottom: Samples 34 to 39) Effect of the amount of propylamine At the last comparison, the amount of propylamine added was also varied to study its effect Figure 4. 21 showed the PL spectra of nanocomposites prepared with varying amount of propylamine (Sample 40 and Sample 41 , sample details... Sample 6 was a preparation of PbS nanoparticles in DDT without the clay matrix, while Sample 7 was a preparation of organo-clay/PbS without DDT (Details of feed ratios are listed in Tables 2.2 and 2.3) The presence of the ordered series of diffraction peaks at low angle in the XRD of Sample 6 (Figure 4. 10a) together with the absence of such peaks in that of Sample 7 (Figure 4. 10b) clearly confirmed that . Si-O structure of the clay, were assigned as in Table 4. 2. 40 00 3800 3600 340 0 3200 3000 2800 2600 240 040 00 3800 3600 340 0 3200 3000 2800 2600 240 0 Sample 1 Sample 2 Sample 3 Sample 4 Wavenumber. 140 120 100 80 60 40 20 0 0 200 40 0 600 800 1000 a Intensity Binding Energy (eV) O 2s Al 2p Al 2s Si 2s Pb 4f Si 2p b 130 Chapter 4 shown in Figure 4. 9b) and that of N 1s peak with Pb 4d. composite. 1000 1200 140 0 1600 Wavelength nm Sample 2 147 2 Sample 3 Sample 4 1387 1 342 Sample 5 13 14 Sample 6 1277 Figure 4. 13. PL spectra of PbS nanoparticles (Sample 6, top-most) and organo-clay/PbS