Template assisted synthesis and assembly of nanoparticles 5

Chapter Chapter MWCNT/PbS and aligned MWCNT/PbS nanocomposites In the previous two chapters we reported the preparation of homogeneous PANi/Ag2S and clay/PbS hybrid nanocomposites by decomposing silver or lead precursor within the matrix directly. In other cases, however, semiconductor nanoparticles can also been attached onto the sidewalls of one-dimensional (1D) nanomaterials, such as nanotubes and nanowires to form another type of nanocomposites. Using 1D nanomaterials as the basic building blocks, various classes of nanocomposites have been explored.1, Carbon nanotubes (CNTs) have unique structure and remarkable mechanical, electrical, magnetic, and photonic properties etc3. The small diameter (in the scale of nanometers) and the length (in the order of microns) of CNTs lead to such high aspect ratios that CNTs is now the most promising and ideal 1D systems. A combination of unique properties of CNTs with semiconductor nanoparticles opens up the potential to use CNTs for various advanced applications such as quantum wires in heterojunction devices, reinforcing materials in composites, nanoprobes in an AFM, and catalyst support in heterogeneous catalysis. Wildgoose and coworkers4 reviewed all electrochemical methods that were used to prepare metal nanoparticles supported on carbon nanotubes and illustrated the electrochemical applications. Another detailed review given by Georgakilas and coworkers5 reported that the composites of CNTs with nanoparticles can be achieved through two main 153 Chapter pathways. One way is that naked NPs are grown and/or deposited directly onto the CNT surface. In an alternative approach, NPs are preformed and connected to CNTs using covalent linkage through organic fragments6, 7. In most cases, functionalization of the side wall of CNTs is required so that nanoparticles can be attached through covalent or non-covalent bonds. During the attachment of compound semiconductor nanoclusters via covalent linkages, and the anchoring of metal complexes to oxidized nanotube surfaces, CNTs were treated with harsh acidic processing. Such harsh conditions may break the π bonding symmetry of the sp2 hybridization because of the many defects on the sidewalls, thus mitigating their unique electronic properties8. Thus in this chapter, we attempt to use noncovalent linkages but van der Waals interactions to incorporate nanoparticles onto CNTs. One of the most potential applications of carbon nanotubes includes being a field emission (FE) material in vacuum microelectronics, such as flat-panel field emission displays (FEDs), nanoprobes of AFM, microsensors etc.9-11 One of the excellent FE properties of aligned multi-wall CNTs (denoted as aCNT hereafter) is its low turn-on electric field properties and extremely high emission current. These properties have been explored by several groups.10, 12, 13 On the other hand, CNT has good mechanical stability, high thermal/electrical conductivity, and large surface/interface area, which are necessary criteria for efficient optoelectronic and sensing devices.14 For example, aCNT coated with TiO2 have been found to possess novel photocurrent responses and photo-induced electron-transfer properties.15, 16 Semiconductor and CNT hybrid materials such as CdS-17, 18, CdSe-19, CdTe-20, and CuS-CNTs21, provide the highly 154 Chapter efficient generation of photocurrents via the interaction between excited nanoclusters and conductive CNTs. Electron transfer from semiconductor to CNT was also observed in ZnS-22 and PbS-CNT23 composites. As mentioned in Chapter 4, PbS is a well-known semiconductor compound with good photoconductive properties in the infrared (IR) domain (800-3000 nm)24 and can be employed as photonic and optical switch devices25 and organic26, 27 solar cells. The effect of PbS nanoparticles to the FE and photoelectric properties of CNT have not been investigated. Although Yu23 reported the quenching of PL emission peak and attributed this to be charge transfer from PbS to MWCNT in PbS/MWCNT composites, a systematic investigation on its photoelectric property is required. As far as we know, there is also no report on the preparation and properties of aCNT/PbS nanocomposites. In Section 5.1 of this chapter we will discuss the preparation and characterization of MWCNT/PbS nanocomposites prepared by in situ decomposition of PbTB in the presence of MWCNT. In Section 5.2, the characterization results of aCNT/PbS composites prepared in the same way on aligned CNTs will be presented. Field emission and photoelectric properties of MWCNT/PbS and aCNT/PbS composites will be discussed in Section 5.3. 5.1 Preparation and characterization of MWCNT/PbS nanocomposites A similar synthetic method has been used to prepare PbS nanoparticles in situ from PbTB precursor within the matrix of MWCNT. A schematic diagram showing the 155 Chapter preparation procedure is presented in Figure 5.1 while details are given in Section 2.6.1. Commercial MWCNT was used as purchased without further functionalization and PbTB was decomposed in situ in MWCNT suspension with the aid of propylamine. A series of samples with different feed ratios of MWCNT to PbTB have been prepared as given in Table 2.10 of Chapter 2. These MWCNT/PbS nanocomposites can be obtained at high yields of ~95%. All the prepared samples have been fully characterized, but, since their properties are similar, only the characterization results of Sample will be presented here unless specifically stated. PbTB/DMF MWCNT hours, RT Propylamine MWCNT/PbS nanocomposites Figure 5.1. A schematic synthetic route of MWCNT/PbS composites. The general morphology of the MWCNT/PbS nanocomposites prepared was first analyzed using TEM. Representative TEM images (Figure 5.2a and 5.2b) revealed that most of the MWCNTs have been coated with PbS nanoparticles, while a small amount of PbS nanoparticles agglomerated into larger lumps. The higher magnification TEM image shown in Figure 5.2b shows that PbS nanoparticles on the sidewall of the MWNTs are disordered and not uniform, and the average size of PbS 156 Chapter particles is ~ 12 nm although some bigger particles of ~25 nm can be observed. Figure 5.2c gives a comparison of PbS nanoparticles grown under the same conditions in the absence of CNT. b a c Figure 5.2. Representative TEM images of (a-b) of MWCNT/PbS nanocomposites (Sample 2), and (c) PbS nanoparticles prepared without MWCNT. Crystalline PbS nanoparticles can be observed from well-resolved lattice fringes in the HRTEM images of MWCNT/PbS composites shown in Figure 5.3a. The 3.43 Å lattice spacing fits well with the (002) phase of MWCNT28 and (111) phase of PbS nanoparticles according to the database. This indicated a good lattice match between PbS and MWCNT. The lattice match could contribute to the attachment of PbS 157 Chapter nanoparticles on the wall of CNT. However, other crystalline structure with lattice spacing equals to 2.9 Å, which corresponds to (200) of PbS crystal (Figure 5.3b) was also observed. We also noticed that the parallel graphitic layer alternated with defect regions where the planes are bent, broken or interrupted (within circled areas of Figure 5.3a). The disordered CNT structures caused by defects are also confirmed by Raman spectra and will be discussed later. a b Figure 5.3. Representative HRTEM images of MWCNT/PbS composites prepared. Additional evidence for the formation of PbS nanoparticles on the MWCNT surface is provided by XRD pattern as shown in Figure 5.4. The diffraction peaks allow the identification of the sample as a mixture of cubic phase PbS (JCPDS 05-592) and MWCNT. The strongest diffraction peak at 26° arises from (111) phase of PbS and (002) phase of MWCNT28-30. On the other hand, EA results confirmed almost equal elemental ratio of S to Pb in this sample. The TEM images and corresponding XRD figures of Sample 1, Sample and Sample are presented in Appendix 3A. 158 220 200 Sample 311 222 111(002) Chapter Sample PbS 05-592 30 40 50 60 70 80 90 2θ Transmittance% Figure 5.4. XRD pattern of MWCNT/PbS nanocomposites (Sample and Sample 3) (plane labeled within brackets belong to MWCNT). CNT Sample 4000 3500 3000 2500 2000 Wavenumber cm 1500 1000 -1 Figure 5.5. A comparison of the FTIR spectrum of MWCNT with that of typical MWCNT/PbS composites prepared (Sample 2). Figure 5.5 showed a comparison of the FTIR spectra of commercial MWCNT and MWCNT/PbS composites (KBr pressed pellet). Both spectra show O-H stretching and bending vibrations peak at 3450 cm-1 and 1631 cm-1 respectively, and C-H 159 Chapter symmetrical and asymmetrical stretching peak at 2851 cm-1 and 2921 cm-1 respectively. The peak at ~ 1581 cm-1 corresponds to the IR active phonon mode of nanotubes that has been previously reported31. There is no significant change in the FTIR spectrum after PbS formation on the wall of MWCNT. This indirectly suggests that there is no covalent bonding between MWCNT and PbS nanoparticles. To further confirm the these results, the oxidation states and composition of PbS were examined by XPS measurement. A typical wide scan XPS spectrum of MWCNT/PbS nanocomposite is displayed in Figure 5.6, showing the predominant presence of carbon, oxygen, sulfur and lead. C 1s 6000 O 1s 3000 Pb 4d3/2 Pb 4d5/2 Intensity 4000 Pb 4f 5000 O 2s S2p 2000 1000 1000 800 600 400 200 Binding Energy (eV) Figure 5.6. Representative XPS wide scan spectrum for MWCNT/PbS nanocomposites (Sample 2). Figure 5.7 shows the corresponding XPS C 1s, S 2p and Pb 4f peaks. The C 1s XPS peak (Figure 5.7a) can be fitted with two components at 284.7 eV and 285.8 eV respectively. These correspond well to the backbone of CNT and its carboxylate-like surface structures as reported in earlier studies32, 33. 160 Chapter a b C 1s S 2p SA SC SB 288.0 285.0 282.0 Binding Energy (eV) c 168.0 165.0 162.0 Binding Energy (eV) 159.0 Pb 4f 4f5/2 148.0 171.0 4f7/2 144.0 140.0 136.0 Binding Energy (eV) Figure 5.7. Typical XPS peaks for (a) C 1s, (b) S 2p, and (c) Pb 4f components for MWCNT/PbS nanocomposites. The S 2p peak envelope (Figure 5.7b) consists of at least three doublet components. The intense doublets at 161.2 and 162.4 eV (denoted as SA) may be assigned to the 2p3/2 and 2p1/2 spin-orbit components originated from sulfur in PbS crystal lattice. The peaks at 168.0 eV and 169.2 eV (denoted as SB) may originate from sulfate. Earlier studies have observed a broadening of S 2p peak and attributed it to the interaction of sulfur with those of the graphite layers.21 In our case, we tentatively attributed the third component (denoted as SC) at 163.6 eV and 164.8 eV to the interactions of S with CNT. The relative intensities of these three components 161 Chapter indicate that the majority of S species in the composites was sulfide S2- (SA) and sulfate SO42- (SB). Oxidation of PbS nanoparticles in composites have been reported but was found to be limited to the surface of the composite material.34 Figure 5.7c shows the Pb 4f peak of the nanocomposite. Good fits resulted in two consistent sets of 4f doublets. The first doublet denoted as PbA appeared at 138.4 eV and 143.3 eV for the Pb 4f7/2 and 4f5/2 components respectively. The second doublet denoted as PbB was centered at 139.3 and 144.1 eV. These features may be assigned to PbS and PbSO4 accordingly due to oxidation detected in the S species above. The PbA component is found to have a slightly higher binding energy compared to that of a reference bulk PbS sample (4f peaks at 137.5 and 142.4 eV, respectively32). The absence of lead oxide (PbO, PbO2, or Pb3O4) in the Pb 4f region (expected at 138.9, 137.4 and 138.0 eV) confirmed that oxidation has primarily occurred at sulfur atoms. MWCNT/PbS MWCNT 100 Weight% 80 60 40 20 0 200 400 600 800 1000 Temperature (°C) Figure 5.8. TGA plots of MWCNT (solid line) and MWCNT/PbS nanocomposites (dot line). Figure 5.8 shows the TGA plots for MWCNT and MWCNT/PbS composites. These similarly featureless TGA plots indicated that there is no intrinsic weight loss 162 Chapter AMWCNT as grown Sample 12 0.0008 0.0006 0.0004 Current A 0.0002 0.0000 -0.0002 -0.0004 -0.0006 -0.0008 -2 -1 Bias Voltage V Figure 5.22. Characteristic I-V curves of as grown aCNT and aCNT/PbS composites on silicon wafer. From the FTIR, Raman spectra and I-V curve of aCNT/PbS composites, it can be concluded that the incorporation of PbS nanoparticles does not change the intrinsic chemical properties of aCNT. Since aligned MWCNTs were reported to exhibit attractive FE and photoelectric properties, we investigate the FE and photoelectric properties of aCNT/PbS composites in comparison to MWCNT/PbS in the following section. 5.3 Field emission and photoelectric studies of MWCNT/PbS and aCNT/PbS composites In earlier reports, metal-CNT composites have been utilized to improve the FE properties of carbon materials. For example, FE properties of CNTs could be enhanced by embedding low work function metals. Wadhawan et al48 and Kim et al49 observed improved FE properties for Cs-embedded CNTs. Similar improved FE 177 Chapter properties were also reported for Au-embedded CNTs.50 For semiconductor/CNT such as CdS(CdSe)/MWCNT, ZnS/MWCNT, CuS/MWCNT and PbS/MWCNT, optoelectronic properties were intensely studied. As far as we are aware, the performance in FE of such composites has never been reported before. Thus in this section, we carried out a study on the FE property of MWCNT/PbS and aCNT/PbS composites. More details on the FE measurements have been given in Section 2.6.3 of Chapter 2. FE property of MWCNT/PbS nanocomposites In Section 5.1, the MWCNT/PbS nanocomposites were found to display good conductivity. Therefore, it is expected that this nanocomposite could have good FE property. Figure 5.23a represents a typical FE current density (J) versus applied field (E) of MWCNT/PbS sample. The J-E characteristic of MWCNT/PbS sample is well defined and the turn-on field, defined as the field required to acquiring an emission current density of 10 μA/cm2, is 4.6 V/μm. The maximum current density of ~0.45 mA/cm2 was achieved at an applied field of ~ V/μm. The quantitative description of field emitters is usually evaluated using the framework of the Fowler-Nordheim (F-N) theory. According to the F-N theory, the FE current density can be expressed as, ⎛ ⎞ ⎟ ⎜ αA (βE )2 exp⎜ − Bφ ⎟ J= φ ⎜ βE ⎟ ⎝ (5.3) ⎠ where, φ is the work function of the emitters (eV), α is the effective emission area and β is the field enhancement factor caused by local geometry of the emitters. The 178 Chapter universal constants A = 1.54×10-6 A eV V-2 and B = 6.83×103 eV-3/2 V μm-1. It is well known that the emission density is primarily dependent on the field enhancement factor and work function of the materials. The corresponding plot of ln(J/E2) versus 1/E (so called F-N plot) of MWCNT/PbS displayed in Figure 5.23b comprises a linear region (inset in Figure 5.23b), emphasizing the quantum tunneling electron emission mechanism. We did not calculate the field enhancement factor β value because the incorporation of PbS nanoparticles may lead to changes in the work function of final product11, resulting in large differences in the emitted current. Working function measurement of MWCNT/PbS should be further investigated (e.g. UPS study). 0.5 a linear fit -8 -10 ln (J/E ) -6 Current density, J (mA/cm ) 0.4 -6 b -4 -8 ln (J/E ) 0.3 0.2 -12 -14 -10 -16 -12 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 1/E 0.1 -14 -16 0.0 -18 -1 Applied electric field, E (V/μm) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 1/E Figure 5.23. Field emission plots: (a) J-E and (b) the corresponding F-N plot of MWCNT/PbS composites (Sample 3) deposited on silicon wafer. (insert is the fitting curve in F-N linear region) One of the reasons for good FE property of MWCNT/PbS composite is due to its good conductivity. The PbS nanoparticles on the CNTs surface may also contributed to good FE behavior. It could be due to the change of work function of CNT after incorporation of PbS nanoparticles.50 On the other hand, rough surface as shown in 179 Chapter Figure 5.24 also affect the FE behavior since it is reported that a nanotube which is slightly longer than the surrounding nanotubes dominates the emission.11 Thus the reproducibility is not as good as those of aCNT/PbS composites, which will be presented in the following sub-section. Figure 5.24. SEM images of MWCNT/PbS composites (Sample 3) cast on silicon wafer. FE test of the as purchased MWCNT powder also was also carried out for comparison. However, the adhesion between MWCNTs and the silicon substrate is not as good as aligned MWCNT grown by PECVD. The current overload was observed due to short circuiting during the FE measurement. FE property of aCNT/PbS nanocomposites with regular array morphology As reported, the FE of aCNT as grown by PECVD is different from that of as purchased MWCNT powder. In the following paragraph, FE property of aCNT/PbS composites prepared in ethanol will be presented. In order to test if there is any change in the FE property of CNT, aCNT/PbS composites Sample 15 was prepared right after the FE test of the as grown aCNT. The corresponding J-E curves of aCNT and aCNT/PbS composites are shown in Figure 180 Chapter 5.25. -2 1.2 -2 a -4 0.20 -6 -10 -12 0.6 -14 -16 0.4 10 1/E 0.2 0.15 ln(J/E2) -8 Current density, J (mA/cm2) ln(J/E2) Current density, J (mA/cm2) 1.0 0.8 b -4 -6 -8 -10 -12 0.10 -14 -16 0.05 10 1/E 0.00 0.0 Applied electric field, E (V/μm) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Applied electric field, E (V/μm) Figure 5.25. FE J-E curves of (a) aCNT as grown by PECVD and (b) aCNT/PbS (Sample 15). Inserts are F-N curves of the corresponding sample. From Figure 5.25, it is obvious that the turn-on field of Sample 15 (2.9 V µm-1 from Figure 5.25b) is much lower than that of the aCNT (4.1 V µm-1 from Figure 5.25a). The F-N curves of these two samples (inserts in Figure 5.25) exhibit a linear range, in which the FE behavior obeys the F-N equation. The SEM image of Sample 15 (Figure 5.18c) has illustrated that the CNT density remained similar to that of aCNT as grown by PECVD. The emission current density of aCNT/PbS nanocomposite is 0.2 mA cm-2 when the electric field is 3.0 V µm-1. This value is much higher than that of aCNT (1.3×10-5 mA cm-2 at 3.0 V µm-1). The lower turn-on field and higher emission current are due to the improved conductivity brought about by the incorporation of PbS nanoparticles. Sample 15, however, was found to be burnt, which means short circuit could have occur after the applied voltage was increased to V µm-1. This value is much lower than the maximum applied voltage of aligned MWCNT. This could be caused by the 181 Chapter aggregated PbS nanoparticles on the aCNT surface. -10 0.00035 a -11 0.16 -4 0.14 -6 b 0.00020 Current density, J (mA/cm2) ln (J/E2) Current density, J (mA/cm2) -12 0.00025 -13 -14 0.00015 -15 0.00010 -16 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 1/E 0.00005 0.00000 0.12 0.10 ln(J/E2) 0.00030 -8 -10 -12 0.08 -14 0.06 -16 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 2.0 2.5 0.02 -0.02 -0.5 4.0 0.0 0.5 3.0 -4 c 3.5 4.0 d -6 Current density, J (mA/cm2) -8 -10 -12 -14 -16 10 1/E 0.00 0.15 ln(J/E2) 0.20 -6 ln(J/E2) 1.5 0.25 -4 0.05 1.0 Applied electric field, E (V/μm) -2 0.20 Current density, J (mA/cm2) 1/E Applied electric field, E (V/μm) 0.10 0.00 -0.00005 -0.5 0.15 0.04 -8 -10 -12 -14 0.10 -16 10 1/E 0.05 0.00 0.0 0.5 1.0 1.5 2.0 Applied electric field, E (V/μm) 2.5 3.0 Applied electric field, E (V/μm) Figure 5.26. FE J-E curves for aCNT/PbS with different PbTB feed concentration (a) Sample 13, (b) Sample 14, (c) Sample 15, and (d) Sample 16. Inserts are F-N curves of the corresponding sample. Table 5.1. Field emission characteristics of aCNT/PbS composites prepared in ethanol. Maximum current density [PbTB] Turn on field (mA/cm2) (mg/mL) (V µm-1) Sample 13 1.7 2.9 0.6 Sample 14 3.1 3.4 0.15 Sample 15 6.2 2.85 0.3 Sample 16 12.3 0.2 ACNT 4.1 1.2* * Current density of as grown aCNT when the voltage is 500 V. Sample ID 182 Chapter The FE J-E curves of aCNT/PbS samples with different PbTB initial feed concentration were shown in Figure 5.26 and the corresponding FE characteristics are listed in Table 5.1. For all samples, a lower maximum emission current density was observed because of the aggregation of PbS nanoparticles on the surface of MWCNT. For the turn-on field, we found that the well-dispersed PbS nanoparticles (SEM images in Figure 5.19a, 5.19b and 5.19c) effectively improved the local field and the turn-on field decreased to 3.4 and 2.85 V µm-1 respectively. When excess PbS nanoparticles were coated onto the surface of aCNT (Figure 5.19d), the increase of turn-on field (5 V µm-1) is expected due to the field-screening effect. Thus in summary, aCNT/PbS nanocomposites with regular array morphology provided comparatively better field emission properties, unless excess PbS nanoparticles were coated on the surface of CNTs. FE property of aCNT/PbS nanocomposites with polygonal morphology We have also investigated the FE property of aCNT/PbS composites with polygonal morphology. As shown in Figure 5.27a, the turn-on field of Sample (4.95 V µm-1) is higher than that of aCNT. The current density of Sample is 1.9×105 mA cm-2 when the electric field is 3.0 V µm-1. This value is close to that of aCNT as prepared by PECVD. The higher turn-on field and similar emission current density (compared with aCNT) are probably due to the polygonal network (Figure 5.27b). The CNTs in polygon area bent down due to the capillary force so that the height of the CNT film decreased, it is reasonable to believe that the field emission behavior is dominated by the walls of the polygon. So the decrease in maximum FE current (0.6 mA cm-2 under an applied field of about V µm-1) may be attributed to screening 183 Chapter effect caused by the increased density of CNT and PbS nanoparticles in the walls of the polygon. 0.6 a -4 b -6 0.4 -8 ln(J/E2) Current density, J (mA/cm2) 0.5 -10 -12 -14 0.3 -16 -18 0.2 0.0 0.5 1.0 1.5 2.0 2.5 3.0 1/E 0.1 0.0 Applied electric field, E (V/μm) Figure 5.27. (a) FE J-E curve of aCNT/PbS composites prepared in DMF (Sample 6, insert is the F-N curve), and (b) the corresponding SEM image. In summary, aCNT/PbS composites prepared in ethanol exhibit FE characteristics that depend significantly on their surface morphology. aCNT/PbS composites with intact regular array displayed a rather lower turn-on field and high emission current whereas those composites with polygonal network structure have a higher turn-on field. However, the maximum electric field which is applied to all aCNT/PbS composites is much lower than that of aCNT, which is due to the screening effect caused by aggregations of PbS nanoparticles on the CNTs surface. Photoelectric property of MWCNT/PbS and aCNT/PbS nanocomposites The earlier PL study on MWCNT/PbS (Figure 5.9 in Section 5.1) showed that the PL emission peak may be quenched by the presence of MWCNT. Yu et al23 have also observed a decrease in the PL emission peak intensity in MWCNT/PbS heterostructure materials. As explained by Yu et al, electron transfer from PbS to 184 Chapter MWCNT is energetically favorable when PbS is coupled to the MWCNT. They gave a schematic band diagram for the PbS/MWCNT system as shown in Figure 5.28 to illustrate the electron-hole recombination and photo-induced electron transfer process. The conduction band and the valence band energy levels of the PbS quantum dots are taken to be 3.5 and 4.9 eV51 and the Fermi level of the MWCNT is known to be 4.35.1 eV. Similar electron transfers from excited CdS to oxides18, ZnS to MWCNT22 and CuS to MWCNT21 have been monitored in earlier studies. eV CB hν VB 3.5 - - - - hν′ 4.9 + + + PbS Fermi level MWCNT Figure 5.28. Schematic band diagram of PbS/MWCNT system. “1” represents electron-hole recombination process and “2” represents photoinduced electron transfer process.23 PL measurement of aCNT/PbS was thus carried at room temperature and its spectrum was shown in Figure 5.29. The emission of PbS is almost completely quenched in the aCNT/PbS composites, suggesting that MWCNT can act as good electron acceptors and charge transfer has been magnified in the CNT-based composite system. Thus, the utilization of PbS nanoparticles through binding with aligned MWCNT is expected to display good performance in solar cell devices. Hence, we attempted to perform a preliminary photocurrent measurements using aCNT/PbS as the working electrode to test the efficiency of photocurrent production. 185 Chapter Intensity (Art Unit) Detailed experimental procedure has been given in Section 2.6.4 in Chapter 2. 800 1000 1200 1400 1600 Wavelength nm Figure 5.29. Typical PL spectrum of aCNT/PbS nanocomposite (Sample 8). When irradiated with a laser of excitation wavelength at 532 nm, we detected photocurrent generation. An open-circuit voltage of ~14 mV and a short-circuit current of 0.026 µA were recorded in these experiments. The preliminary result of photocurrent measurement of aCNT/PbS composite (Sample 8) is shown in Figure 5.30. As can be seen from this figure, the photocurrent response to laser was reproducible although the current is quite week compared with CdS/MWCNT18 and CuS/MWCNT21 system. Further investigations are required to provide more information to explain the current generation. 0.042 Current density J (μA/cm ) 0.041 0.040 0.039 0.038 0.037 50 100 150 200 Time t (s) Figure 5.30. Photocurrent response of aCNT/PbS (Sample 8). 186 Chapter 5.4 Conclusions MWCNT/PbS and aCNT/PbS nanocomposites have been synthesized through in situ decomposition of PbTB in the presence of MWCNT powder and aligned MWCNT grown by PECVD. PbS nanoparticles were found to readily attach onto the wall of CNTs through van der Waals interactions. FTIR, TGA and other characterizations indicated that the incorporation of PbS nanoparticles did not have significant effect on the intrinsic chemical properties of MWCNTs. 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Figure 5. 25, it is obvious that the turn-on field of Sample 15 (2.9 V µm-1 from Figure 5. 25b) is much lower than that of the aCNT (4.1 V µm-1 from Figure 5. 25a) The F-N curves of these two samples (inserts in Figure 5. 25) exhibit a linear range, in which the FE behavior obeys the F-N equation The SEM image of Sample 15 (Figure 5. 18c) has illustrated that the CNT density remained similar to that of aCNT... 3 .5 4 5 2.0 2 .5 0.02 -0.02 -0 .5 4.0 0.0 0 .5 3.0 -4 c 3 .5 4.0 d -6 Current density, J (mA/cm2) -8 -10 -12 -14 -16 0 2 4 6 8 10 1/E 0.00 0. 15 ln(J/E2) 0.20 -6 ln(J/E2) 1 .5 0. 25 -4 0. 05 1.0 Applied electric field, E (V/μm) -2 0.20 Current density, J (mA/cm2) 3 1/E Applied electric field, E (V/μm) 0.10 2 0.00 -0.000 05 -0 .5 0. 15 1 0.04 -8 -10 -12 -14 0.10 -16 0 2 4 6 8 10 1/E 0. 05 0.00 0.0 0 .5 1.0 1 .5 2.0... aggregation of PbS nanoparticles on the surface of MWCNT For the turn-on field, we found that the well-dispersed PbS nanoparticles (SEM images in Figure 5. 19a, 5. 19b and 5. 19c) effectively improved the local field and the turn-on field decreased to 3.4 and 2. 85 V µm-1 respectively When excess PbS nanoparticles were coated onto the surface of aCNT (Figure 5. 19d), the increase of turn-on field (5 V µm-1)... aspect ratio of CNTs increases from 10µm/30nm to 12µm /50 nm (estimated from Figure 5. 14), dense polygonal network was observed and the width of polygon caves decreases from 10-70 µm to 2 -50 µm as expected 169 Chapter 5 a b Figure 5. 14 SEM images of aCNT on silicon wafer with different aspect (L/D) ratios (a) 10µm/30nm, and (b) 12µm /50 nm a b Figure 5. 15 SEM images of polygonal networks of aCNT/PbS composites... voltage of aligned MWCNT This could be caused by the 181 Chapter 5 aggregated PbS nanoparticles on the aCNT surface -10 0.16 0.000 35 -4 0.14 a -11 -6 b 0.00020 Current density, J (mA/cm2) ln (J/E2) Current density, J (mA/cm2) -12 0.000 25 -13 -14 0.000 15 - 15 0.00010 -16 0.0 0 .5 1.0 1 .5 2.0 2 .5 3.0 3 .5 1/E 0.000 05 0.00000 0.12 0.10 ln(J/E2) 0.00030 -8 -10 -12 0.08 -14 0.06 -16 0 0.0 0 .5 1.0 1 .5 2.0 2 .5 3.0... CdS18 and ZnS22 decorated SWCNT, indicated that MWCNT/PbS composites are possible to be applied as solar cell 1 65 Chapter 5 MWCNT/PbS MWCNT as purchased a 0. 65 MWCNT/PbS MWCNT as purchased b 0.60 Absorbance Absorbance 0.60 0 .55 0 .50 0 .55 0 .50 0. 45 0. 45 0.40 0.40 400 50 0 600 700 800 1000 Intensity (a u.) 1200 1400 1600 Wavelength nm Wavelength nm c 800 1000 1200 1400 1600 Wavelength nm Figure 5. 11 UV-vis-NIR... can be evaluated by the intensity ratio I1 350 /I 158 0 of the D- and G-bands In our case, the intensity ratio I1 350 /I 158 0 of commercial MWCNT normalized with WiRETM 2.0 software is as high as 1 .5, indicating that there are many defects on the surface of commercial CNT This intensity ratio decreased to 1. 35 after the PbS nanoparticles were incorporated The decrease of the intensity ratio (10%) is not significant... that the crack of CNTs was formed on a time scale of seconds while the shrinkage and bending of CNTs takes place on a time scale of minutes.41 Thus, the shrinkage and bending of CNTs are prohibited by the fast evaporation of ethanol a b Figure 5. 18 SEM images of aCNT/PbS composites prepared in ethanol (a) Sample 14, and (b) zoom in area within circular box in (a) The effect of feed amount of PbTB to the... 158 0cm-1 are respectively assigned to O-H (stretching and bending vibration), C-H stretching (symmetric and Transmittance (a.u.) asymmetrical) and C-C vibration backbone of CNT AMWCNT as grown Sample 12 4000 350 0 3000 250 0 2000 Wavenumber cm 150 0 1000 -1 Figure 5. 20 Representative FTIR spectrum of aCNT/PbS (Sample 12) Representative Raman spectra of aCNT and aCNT/PbS composites are shown in Figure 5. 21... morphology of aCNT/PbS one by one: Effect of aspect ratio of aCNT to morphology In the following we studied the effect of aspect ratio of aCNT to polygonal network as the PECVD growth condition was changed SEM images of aCNT with different aspect ratio of CNTs were shown in Figure 5. 14 and the corresponding polygonal networks of the prepared aCNT/PbS nanocomposites (Sample 7 and 12) were shown in Figure 5. 15 . 1 65 Chapter 5 1000 1200 1400 1600 0.40 0. 45 0 .50 0 .55 0.60 0. 65 1000 1200 1400 1600 0.40 0. 45 0 .50 0 .55 0.60 0. 65 MWCNT/PbS Absorbance Wavelength nm MWCNT as purchased b 400 50 0 600. Figure 5. 5. A comparison of the FTIR spectrum of MWCNT with that of typical MWCNT/PbS composites prepared (Sample 2). Figure 5. 5 showed a comparison of the FTIR spectra of commercial MWCNT and. O-H stretching and bending vibrations peak at 3 450 cm -1 and 1631 cm -1 respectively, and C-H 159 Chapter 5 symmetrical and asymmetrical stretching peak at 2 851 cm -1 and 2921 cm -1