J. Phys. Chem. C 2009, 113, 5329–5335
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Cubic Pd16S7 as a Precursor Phase in the Formation of Tetragonal PdS by Sulfuration of Pd Thin Films Pablo Diaz-Chao,* Isabel J. Ferrer, Jose R. Ares, and Carlos Sanchez MIRE Laboratory, Dept. Fı´sica de Materiales, C/ Francisco Toma´s y Valiente 7, UniVersidad Auto´noma de Madrid (UAM), Cantoblanco, 28049 Madrid, Spain ReceiVed: December 3, 2008; ReVised Manuscript ReceiVed: January 26, 2009
Experimental results are reported showing that sulfuration of metallic Pd thin films gives rise to Pd16S7, which finally, transforms into tetragonal PdS. The whole process has been investigated by measuring the film electrical resistance while increasing its temperature and the sulfur vapor pressure. At temperatures T ≈ 460 K and pressures PS2 ≈ 10-6 mbar, Pd is already fully transformed into Pd16S7. Obtained Pd16S7 nanocrystalline films (50 nm crystallite size) have been characterized (morphological, structural, optical, and transport properties at room temperature (RT)) for the first time. They show a cubic structure with a lattice parameter a ) 8.921 ( 0.009 Å, an electrical resistivity F ) 7.2 × 10-4 Ω · cm, and a Seebeck coefficient S ) 11.8 µV/K. Their optical absorption coefficient is almost constant for photon energies in the range 1-3 eV. On increasing the temperature and sulfur pressure, Pd16S7 gives rise to nanocrystalline PdS thin films (30-40 nm crystallite size) with an n-type semiconducting behavior, having an electrical resistivity F ) 5.6 × 10-2 Ω · cm and an a Seebeck coefficient S ) -268 µV/K at RT. The optical absorption measurements indicate that the absorption edge of PdS is due to two allowed direct transitions with EG ≈ 1.45 and 1.70 eV, respectively. Finally, obtained results are discussed on the light of thermodynamic calculations, which confirm the sequence of sulfides formation described in the paper. The possible interest of PdS in thermoelectric applications is emphasized. 1. Introduction Nowadays, palladium sulfides are used in industrial applications due to their catalytic activity in different processes.1-3 Technological applications under patent have also been found in photographic and lithographic films and lithographic plates.4,5 Palladium sulfide grown in polymer matrix has even been developed for the manufacture of semiconductors.6 However, and in spite of this wide range of applications, our fundamental knowledge of the different palladium sulfides seems to be quite limited. A few thermodynamic studies have been carried out,7-10 where thermodynamic properties (enthalpies and entropies) of several palladium sulfides are given. Electrical, photoelectrochemical, and transport properties, as well as their optical absorption spectra have also been barely investigated.2,11,12 Furthermore, there also exist few investigations dealing with the formation mechanisms of the different Pd sulfides, mainly in the form of thin films, something that is important in order to understand the material properties. Nevertheless, the field appears to be very promising from the experimental point of view. In fact, different techniques (photochemical and thermal vapor deposition and aerosol-assisted CVD13,14) have proved to be fruitful in the preparation of PdS thin films. In particular, we have recently reported on the growth of PdS thin films by direct sulfuration of Pd thin films.11,15 In these reports, we emphasize that palladium sulfide (PdS) thin films have remarkable properties to be used as a photovoltaic11 and thermoelectric materials15 due to their high optical absorption coefficient, band gap energy, Seebeck coefficient, and electrical resistivity. However, in those investigations, PdS thin films were obtained by prolonged and “blind” sulfuration of Pd, and it was not * To whom correspondence should be addressed. Phone: +34 914974777; Fax: +34 914978579; E-mail:
[email protected].
possible to obtain information on the transformation process. Now, and in order to obtain further knowledge on that transformation, in situ electrical measurements have been performed during short period sulfurations of metallic Pd layers. Cubic Pd16S7 has been found to be an intermediate phase in the process Pd f PdS, and it has been thoroughly investigated. Its main structural, optical, and transport properties are now reported. Moreover, it is shown that PdS thin films of excellent characteristics are obtained with short sulfuration times. A brief discussion on the whole process leading from Pd to PdS based on present available thermodynamic and kinetic information is presented. 2. Experimental Section Metallic palladium thin films were deposited onto glass substrates by the thermal evaporation method. Direct sulfuration of Pd films by heated sulfur powder was used to investigate their transformation process. The sulfuration system consists of two furnaces to independently heat the sulfur source and the sample, so that the temperature of the sulfur powder (TSU) determines the total pressure of the vapor (PT), the sample temperature (TSA), and the composition of the vapor surrounding the film,16 mainly the S2 partial pressure (PS2). Figure 1 shows the time evolution of TSU, TSA, PT, and PS2 during a typical experiment. TSA was increased linearly at a rate of 100 K/h up to 600 K while TSU rises up to 350 K. At TSA ) 475 K, the vapor in the sample compartment is mainly composed of S2, reaching maximum pressure, PT ) PS2 ) 5 × 10-4 mbar, at TSA ) 600 K. The enrichment of the sulfur vapor in S2 molecules favors the sulfuration process, since the S2 molecule is the active species in metal sulfuration processes.17 The meaning of the continuous vertical lines indicating samples from “a” to “d”
10.1021/jp810631t CCC: $40.75 2009 American Chemical Society Published on Web 03/12/2009
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Figure 1. Total sulfur vapor pressure (PT) and S2 partial pressure (PS2) versus sulfuration time. Dashed lines correspond to the temperatures of the system (sample, TSA, and sulfur source, TSU) during the process. The times at which samples “a”, “b”, “c”, and “d” are obtained are indicated by the continuous vertical lines.
will be explained in the Results Section. The film is placed in the sample compartment between two ceramic foils of the sample holder, which is provided with four stainless steel electrical contacts. The four-probe method is used to measure the electrical film resistance in situ during the process. Further details about the experimental setup are reported in ref 18. Structural characterization of the films was done by X-ray diffraction (XRD) in grazing-angle configuration at an incidence angle of 1.7°, in a Siemens D5000 automated X-ray diffractometer (Cu KR radiation, λ ) 1.5406 Å). Crystallite size was calculated from the XRD patterns by application of the Scherrer Formula.19 Films thickness was measured in a profilometer Dektak IIA ((10 Å). Stoichiometric analyses of the samples were accomplished by energy dispersive X-ray (EDX) in an EDX INCA x-sight analyzer (incident electron energy of 10 keV), associated to a scanning electron microscope (SEM, Hitachi S-3000N). The morphology characterization was performed in an Oxford instruments field emission gun scanning electron microscope (SEM-FEG) JEOL JSM 6500F. Electrical resistivity (F) and Seebeck coefficient (S) of the films were also measured “ex-situ” at room temperature (RT) by the Van der Pauw20 method and by a differential method,21 respectively. Optical transmittance (O) and reflectance (R) measurements in a Perkin-Elmer Lambda 9 spectrophotometer were carried out in normal incidence configuration at RT and used to calculate the absorption coefficient (R) through the following equation:22
O)
(1 - R)2e-Rd 1 - R2e-2Rd
(1)
3. Results 3.1. Metallic Pd Films. Pd films, obtained as reported in ref 11 were characterized before sulfuration. The thickness of the as grown metallic Pd films was 40 ( 5 nm. Cubic Pd phase was identified from the XRD measurement (Figure 2). A crystallite size of D ) 15 ( 2 nm and a lattice parameter of a ) 3.900 ( 0.005 Å were calculated. This latter result is in good agreement with the bulk Pd lattice parameter (a ) 3.890 Å).23 Measurements of resisivity and Seebeck coefficient at room temperature give values of 1.3 × 10-4 Ω · cm and 5.8 µV/K, respectively. These values of F and S differ from those reported for bulk Pd (F ) 1 × 10-5 Ω · cm and S ) -9.3 µV/K, respectively).24 Main characteristics of Pd films are given in Table 1 together with those from sulfurated films.
Figure 2. XRD patterns from the initial Pd layers and from samples “a”, “b”, “c”, and “d” (from bottom to top). XRD pattern of PdS from ref 11 is also given.
3.2. Sulfuration Process. It is well-known that, during the formation of metallic sulfides by sulfuration of metallic films, the electrical film resistance suffers significant changes related to the different processes involved in the sulfide formation mechanisms.15,18,25 In Figure 3, the time evolution of the in situ resistance of Pd films during its sulfuration is shown normalized (RN ) R(t)/R0) to its initial value, R0. The sample and sulfur powder temperatures go from RT to 600 and 350 K, respectively, during the same period as shown in Figure 1. Details of the RN variations during the first three hours of the process are depicted in the inset of Figure 3. Initially (t e 1.0 h), the metallic character of the Pd film gives rise to a slight linear increase in the normalized resistance up to RN ) 1.05 at TSA ) 355 K. From this result, a temperature coefficient of resistance (TCR) for metallic Pd thin films of (7.5 ( 0.5) ×10-4 K-1 has been calculated, which agrees well with that of bulk Pd (TCR ) 8.50 × 10-4 K-1).26 At longer times (t > 1.0 h) RN versus sulfuration time reveals the structural and/or compositional changes of the original Pd film. To investigate their causes, we have quenched several films (indicated as samples “a”, “b”, “c”, and “d” in Figures 1 and 3) from the operation temperature down to RT. The cooling rate of the films was ∼4 °C/min for samples “a” and “b” and ∼9 °C/min for samples “c” and “d” to avoid structural or compositional changes. Finally, the properties of samples “a”, “b”, “c”, and “d” are investigated at RT as described in the following sections. 3.3. Formation of Cubic Pd16S7: Sample “a”. Once a temperature of 355 K is reached (t ≈ 1.0 h), the reaction of the film with sulfur vapor seems to start, as suggested by the rapid rise in RN up to a value of 2.8 at T ) 396 K (1.50 min). Then, the film is quenched, and sample “a” is obtained (see inset in Figure 3). This sample is ∼1.2 times thicker (see Table 1) than the original Pd film, which proves that some reaction between the sulfur atmosphere and the Pd film has taken place. XRD pattern of sample “a” is given in Figure 2. Although no clear identification of crystalline phases is deduced from this pattern, three diffraction peaks can be distinguished: one narrow peak at ∼38.1° and two broader peaks at ∼39.0° and ∼45.0° (marked in Figure 2, sample “a” as (•) and (*), respectively). The overlapping between the narrowest and one of the broad peaks (38.1° at 39.0°, respectively) could be thought of as the
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TABLE 1: Properties of the Samples Obtained by Sulfuration of Pd Thin Films Measured at RTa sample
phase
crystallite Size (nm)
lattice parameter (Å)
Fb (Ω · cm)
Sc (µV/K)
atomic ratio S/Pd
d/d0d
1.3 × 10 6.9 × 10-4 7.2 × 10-4 1.8 × 10-2
5.8 1.9 11.8 -136
0.48 ( 0.05 0.54 ( 0.05 0.88 ( 0.05
1.0 1.2 1.6 2.9
5.6 × 10-2
-268
1.00 ( 0.05
2.9
6.0 × 10-2
-250
1.1 ( 0.05
3.5
-4
Initial a b c
Pd (cubic) Pd + Pd16S7 Pd16S7 (cubic) PdS (tetragonal)
15 ( 2
3.900 ( 0.005
51 ( 5 31 ( 3
d
PdS (tetragonal)
37 ( 4
ref 11
PdS (tetragonal)
50 ( 10
8.921 ( 0.009 a ) 6.408 ( 0.009 c ) 6.611 ( 0.009 a ) 6.432 ( 0.009 c ) 6.601 ( 0.009 a ) 6.429 ( 0.002 c ) 6.610 ( 0.006
a Characteristics of PdS films from ref 11 are also included for comparison purposes. b Electrical resistivity. c Seebeck coefficient. d Thickness of the film (d) relative to that of the original Pd film (d0).
Figure 3. Evolution of the in situ film electrical resistance (R(t)) normalized to its initial value (R0) during the sulfuration (open circles). A detailed view of first period of the process is shown in the inset. The sample and sulfur source temperatures are given by dashed lines. Vertical continuous lines indicate the times at which samples are obtained.
formation of just one unique phase (Pd16S7) with a bimodal crystallite size distribution. This hypothesis however should be discarded, since the lattice parameter of both peaks should be the same and thus both the narrowest and the broad peaks would be expected to be centered at the same diffraction angle. As a consequence, we have associated the peak at 38.1° to the formation of the phase Pd16S7, whose most intense diffraction peak is located at 37.68°.23 This association is reinforced by the crystallization of such a phase in sample “b” (see Section 3.4). The two broader peaks, although less clear, could be identified with an expanded Pd lattice (∼3%) with a smaller crystallite size. Neither Pd4S nor Pd3S phases are observed in the diffractogram taken from this sample. From EDX measurements, a S/Pd atomic ratio of 0.48 ( 0.05 is obtained, which supports the formation of the Pd16S7 phase. Therefore, according to the previous XRD pattern, a small amount of metallic Pd coexists with the Pd16S7 phase, which seems to be poorly crystallized. A SEM image (Figure 4) shows a grain size of ∼25 nm. Although the electrical resistivity of film “a” increases and its Seebeck coefficient decreases (6.92 × 10-4 Ω · cm and 1.94 µV/K, respectively) in comparison with those of metallic Pd, the sample keeps a metallic-like behavior. This point is supported by optical measurements. In Figure 5a, the variation of the reflectance with photon energy for sample “a” is shown. From its comparison with reflectivity measurements of metallic Pd reported in ref 27, it can be seen that both present similar behavior and values in the photon energy range measured. The absorption coefficient of sample “a” is shown in Figure 5b to be compared with that of the rest of the samples (“b”, “c”, and “d”).
3.4. Crystallization of Cubic Pd16S7: Sample “b”. As it is observed in the inset of Figure 3, RN remains constant for ∼30 min (despite the TSA increase up to T ≈ 450 K), then it decreases down to 2.3 at T ) 460 K. At this moment, the film is quenched, and sample “b” is obtained. The thickness of sample “b” grows up to 1.6 times the thickness of the initial film and ∼1.3 times that of the previous stage. The film appears to be exclusively formed by cubic Pd16S7 as clearly identified by XRD (Figure 2). From this pattern, a crystallite size of 51 ( 5 nm and a lattice parameter of a ) 8.921 ( 0.009 Å have been calculated, which agrees well with the reported lattice parameter (a ) 8.9300 Å).23 A SEM image of sample “b” (Figure 4) exhibits a grain size very similar to that of sample “a” (∼25 nm) and smaller than the crystallite size, which suggests a columnar shape of the grains. EDX compositional results show a S/Pd atomic ratio of 0.54 ( 0.05, which is higher than the one obtained in the previous sample (sample “a”) and also higher than the nominal ratio of this phase (7/16 ) 0.44). This higher atomic ratio can be due to the existence of a high density of Pd vacancies. In fact, it has been repeatedly emphasized that metallic chalcogenide thin films28 grown by different procedures are usually p-type conductors due to metallic vacancies. We have measured the Seebeck coefficient of film “b”, and a value of S ) 11.8 µV/K has been obtained, which clearly corresponds to a p-type conductor. The electrical resistivity exhibited by the film is 7.15 × 10-4 Ω · cm. The reflectance and optical absorption coefficient of sample “b” appear to be lower than those of film “a” (Figure 5a and b) although showing similar tendencies as a function of hν. This change could be due to the transformation of the small amount of Pd existing in sample “a” and the improved crystallization of sample “b”. In any case, results in Figure 5 suggest that the Pd16S7 phase shows metallic-like behavior. 3.5. Formation of PdS: Samples “c” and “d”. Finally, the last increment of the resistance RN is completed in a two-step process (Figure 3): the first one up to TSA ) 589 K (sample “c”) and the second one where the sample temperature reaches 600 K and remains constant until the end of the stage (sample “d”). XRD patterns of both samples (Figure 2) show PdS as the unique well-crystallized phase,23 although peaks of higher intensity are shown by sample “d”. Therefore, it can be concluded that a full transformation of Pd16S7 into PdS is associated with the RN increase that takes place on passing from sample “b” to sample “c”. The calculated crystallite size and the lattice parameters, as well as the thickness increment, show similar values in both stages “c” and “d” (see Table 1). These similarities indicate that no significant differences exist in the structural properties of both samples. Moreover, final thicknesses of samples “c” and “d” are almost twice than that of sample
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Figure 4. SEM-FEG images of samples “a”-“d”. Images of samples “c” and “d” in larger scales are included to show long-range features due to the sulfuration process.
conductivity turns out to be n-type as shown by the sign of the thermoelectric coefficients, whose absolute values are also highly increased, -136 and -268 µV/K in samples “c” and “d”, respectively. The optical measurements carried out on these two films (Figure 5b) show the semiconductor character of the crystallized PdS phase that was already hinted by the change of the transport properties. The absorption coefficient of sample “d” takes values lower than those exhibited by sample “c”, although both samples show similar variation with photon energy. Similar optical spectra have been obtained from PdS nanoparticles and reported in ref 2. 4. Discussion
Figure 5. (a) Optical reflectance (R) spectra of samples “a” and “b”. (b) Optical absorption coefficient (R) of samples “a”-“d”.
“b”, as expected taking into account the stoichiometric ratios of Pd16S7 and PdS and their respective parameters (see Table 1). However, results from EDX show a S/Pd atomic ratio of 0.88 ( 0.05 in sample “c” and 1.00 ( 0.08 in sample “d”. The morphology shown by both samples (Figure 4) also exhibits significant differences, although the grain size seems to be the similar (∼30 nm). It must be emphasized that the stoichiometry is improved in sample “d” matching the nominal stoichiometry of the PdS phase. The resistivities of samples “c” and “d” are 1.83 × 10-2 Ω · cm and 5.62 × 10-2 Ω · cm, respectively, which are 2 orders of magnitude higher than that of Pd16S7 (sample “b”). The
According to the experimental results, several stages can be established in the sulfuration process of metallic Pd layers. The first one involves the formation of a poorly crystallized Pd16S7 phase, coexisting with a remaining metallic Pd layer. The complete crystallization of such a sulfide phase (Pd16S7) takes place in the following stage. Subsequently, the transformation of a Pd16S7 phase into semiconducting PdS occurs. Finally, PdS crystallizes and reaches the nominal stoichiometric value (S/ Pd ) 1). To explain the previous sequence of phases, a review of the published data on the thermodynamics of the Pd-S system has been carried out. As far as we know, only two papers reporting on the thermodynamic properties of this system that include the Pd16S7 phase have been published (Taylor;7 Zubkov et al.10), even though they do not agree in the values of standard enthalpies of formation of some Pd sulfides. Unfortunately, Zubkov et al. do not compare their results with those of Taylor, who considers the reported data up to its publication date. Moreover, Zubkov et al. only report the formation enthalpies,
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+ S2 f 14/3Pd16/7S
(8)
2 /3Pd4S
+ S2 f 8/3PdS
(9)
2 /7Pd4S
+ S2 f 8/7PdS2
(10)
8 /3Pd4S
Figure 6. Calculations of the change in the Gibbs Energy (a) for the reactions described in eqs 2-6 (expressed per mol of S2) and (b) for those described in eqs 14-18 (expressed per mol of Pd).
whereas Taylor also gives the formation entropies. Therefore, our thermodynamic study of the Pd-S system has been carried out considering the data reported by Taylor.7 Once the sulfur vapor is available, the possible reactions that may initiate the sulfuration process are described by eqs 2-6:
8Pd + S2 f 2Pd4S
(2)
6Pd + S2 f 2Pd3S
(3)
32 /7Pd
+ S2 f 2Pd16/7S
(4)
2Pd + S2 f 2PdS
(5)
Pd + S2 f PdS2
(6)
It is important to emphasize that the Gibbs Energy (∆G) of these reactions must be expressed per mole of S2 because the S2 molecule is the limiting species at the start of the process. This criterion is valid, since the initial reaction between sulfur and palladium takes place at the surface of the sample, where the Pd is available. The dependence of ∆G on temperature for each reaction is plotted in Figure 6a, and it shows that Pd4S is the most stable phase and reaction 2 is the most probable one. Once the formation of a thin layer of Pd4S has occurred, subsequent transformation of such sulfide into another compound with higher sulfur content may take place by its reaction with sulfur vapor:
6Pd4S + S2 f 8Pd3S
(7)
The comparison of the ∆G calculated values of these reactions (Figure S1) show that the formation of Pd16S7 (reaction 8) is the one with the most negative ∆G but still higher than that of the formation of Pd4S (reaction 2). This means that, from a thermodynamic point of view, the Pd4S would continue its formation from Pd and S2 until Pd is exhausted and afterward Pd16S7 would form from Pd4S. However, provided that sample “a” (first stage) shows the coexistence of Pd16S7 and metallic Pd and not the formation of Pd4S, it can be concluded that reaction 8 is faster than reaction 2. At this point, two main possible formation processes are suggested: First, formation of Pd4S (reaction 2) could be achieved simultaneously to the phase Pd16S7 from metallic Pd (reaction 4), since their values of ∆G are very similar (∆G (Pd16S7) ∼ 4% higher than ∆G (Pd4S)). Therefore, nucleation of both phases (Pd4S and Pd16S7) at the same time could be expected, but faster formation of Pd16S7 might occur due to a higher reaction rate. Second, formation of Pd4S may take place at the film surface, but if its growth kinetics is slower than that of reaction 8, subsequent transformation of the Pd4S sulfide into Pd16S7 might occur at the surface. Pd4S would be consumed, and Pd16S7 would coexist with metallic Pd. Unfortunately, both former alternatives are indistinguishable from our experimental results. Once the Pd16S7 is completely formed, the reaction with sulfur vapor gives rise to the PdS phase, as the calculation of ∆G of reactions 11 and 12 proves (see Figure S2).
14 /9Pd16/7S
14 /25Pd16/7S
+ S2 f 32/9PdS
(11)
+ S2 f 32/25PdS2
(12)
Finally, further sulfuration of PdS, described by reaction 13, does not occur because it is not thermodynamically spontaneous (Figure S3).
2PdS + S2 f 2PdS2
(13)
To understand the final formation of the PdS (which had been previously discarded), the possible global reactions should be expressed per mole of Pd because the initially available Pd is finally depleted and hence it is the limiting reactant for the global process.
Pd + 1/8S2 f 1/4Pd4S
(14)
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Pd + 1/6S2 f 1/3Pd3S
(15)
Pd + 7/32S2 f 7/16Pd16/7S
(16)
Pd + 1/2S2 f PdS
(17)
Pd + S2 f PdS2
(18)
The values of ∆G of eqs 14-18 (Figure 6b) show that the PdS phase (reaction 17) is the most thermodynamically stable, as observed at the end of the sulfuration process. In conclusion, the thermodynamic study of the Pd-S system explains the final formation of PdS from sulfuration of Pd films. Moreover, it also supports the existence of intermediate phases during the formation process. As concerns the sulfuration mechanism, it is well-known that the formation of transition metal sulfides and oxides involves cation vacancy migration from the surface to the bulk of the sample.29 Just in the cases where cation valence is +4 or +5 (Ti, Zr, etc.), anion vacancy diffusion occurs.30,31 According to this fact, diffusion of Pd vacancies is expected to take place in the formation mechanism of the Pd sulfide thin films. Under this assumption, the formation of the initial sulfide takes place at the surface of the film by the reaction of the sulfur vapor with a sulfide layer already formed by sulfur adsorption, leading to a bilayer structure (sulfide/metal). In fact, in the first stage (sample “a”), the remaining metallic Pd is thought to be under the formed sulfide Pd16S7. Besides, provided that both phases crystallize in a cubic structure, the expansion observed in the Pd lattice as well as the contraction of the crystallized Pd16S7 phase could be explained by the corresponding lattice mismatch. In any case, after the complete transformation of Pd into Pd16S7, its subsequent crystallization takes place (second stage). Although this process of crystallization involves the increase of the resistivity (Fb/Fa ) 1.04, see Table 1), the decrease of 20% observed in RN in this stage (Figure 3) is explained as due to the increment of the thickness (db/da ) 1.3):
(RN)b Fb da ) × ) 0.8 (RN)a Fa db
(19)
The resulting Pd16S7 crystallized phase exhibits metallic-like behavior, as shown by the values of resistivity and Seebeck coefficient. As far as we know, this is the first time that the transport properties of this phase are reported. Our experimental results regarding this phase shed some light on the observation of Taylor,7 who pointed out the difficulty in obtaining Pd16S7, since the previous works on the Pd-S system did not report its formation. In this work, we describe the P and T conditions to obtain such a phase. The increase of 2 orders of magnitude in RN from the second to the third stage of the sulfuration curve is related to the transformation of Pd16S7 into PdS. This transformation is also reflected in the room temperature resistivity values of samples “c” and “d”: almost 2 orders of magnitude higher than that of sample “b” (Table 1). Furthermore, the thermoelectric coefficient of samples “c” and “d” turns to negative and takes values of hundreds of microvolts per kelvin. All these transport properties are typical of an n-type semiconductor. On the other hand, the variation observed in the EDX results between the third and fourth stages points to a decrease in the density of point lattice
Figure 7. Optical absorption of PdS nanoparticles from ref 2 and absorption coefficient of sample “d” vs photon energy. Note the different hν scales.
defects in the films, as it is also suggested by the changes in resistivities and Seebeck coefficients.32 The analysis of the accomplished optical measurements give some additional and relevant information on the semiconducting characteristics of PdS. In the first place, we must say that the hump observed in the optical absorption coefficient of PdS (Figure 5b) at hν ∼1.5 eV was tentatively attributed to a sub-band transition in ref 11. However, we have noticed that the same structure appears in the absorbance measurements of PdS nanoparticles (
