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J. Phys. Chem. B 2000, 104, 9396-9403
Ultrafast Electronic Relaxation Dynamics in Layered Iodide Semiconductors: A Comparative Study of Colloidal BiI3 and PbI2 Nanoparticles A. Sengupta,† K. C. Mandal,‡ and J. Z. Zhang*,† Department of Chemistry, UniVersity of California, Santa Cruz, California 95064, and EIC Laboratories, Inc., 111 Downey Street, Norwood, Massachusetts 02062 ReceiVed: March 15, 2000; In Final Form: June 9, 2000
We report direct measurements of ultrafast electronic relaxation dynamics in BiI3 colloidal nanoparticles using femtosecond transient absorption spectroscopy and compare with PbI2, a similar layered iodide semiconductor. The BiI3 nanoparticles are prepared using colloidal chemistry methods in different solvents, including ethanol, 1-butanol, acetonitrile, water, and in aqueous poly(vinyl alcohol) (PVA) matrix as well as in inverse micelles. The particle sizes and shapes are determined using low- and high-resolution transmission electron microscopy. The peak positions of the electronic absorption spectra of both PbI2 and BiI3 can be explained using a particle-in-a-rectangular-box model. The absorption peaks are found to blue shift, and at the same time, the particle size decreases upon aging under light for PbI2. These changes are proposed to be caused by breakdown of initially formed large single-layered particles into small multilayered particles. The differences and similarities in their electronic absorption spectra are also addressed. The electronic relaxation dynamics in BiI3 nanoparticles are directly monitored, and the relaxation is found to be sensitive to solvent and insensitive to particle size, similar to that of PbI2. The dynamics are somewhat dependent on the probe wavelength but independent of excitation intensity. Also similar to PbI2, there appear to be oscillatory features at early times with a period changing with solvent but not with particle size. These features seem to be characteristic of these layered iodide semiconductor nanoparticles. For BiI3, however, the oscillation period is slightly shorter and overall relaxation is faster than that in PbI2 in the same solvent. The electronic relaxation is much faster in aqueous solution containing PVA and in inverse micelles with no oscillations observed at early times. The results suggest that the surface play a major role in the electronic relaxation process of both BiI3 and PbI2. The influence of particle size is relatively minor in the size range studied (2-100 nm), probably because the relaxation is dominated by surface characteristics that do not vary significantly with size. It could also be that the size is much larger than the exciton Bohr radius (0.61 nm for bulk BiI3), and thereby spatial confinement is not significant in affecting the relaxation process.
Introduction In semiconductor materials, spatial confinement of charge carriers in multilayered or multi-quantum-well structures has many potential utilities.1 Several layered semiconductor materials such as PbI2, BiI3, HgI2, Bi2S3, and Sb2S3 showed interesting optical and electronic properties.2-5 Layered semiconductor nanoparticles have potential applications in photovoltaics, detectors, sensors, photocatalysis, lubrication, nonlinear optics, and photoelectrochemistry. They have advantages including enhanced transport properties (within a layer), less defects due to nearly ideal surfaces with almost no dangling bond perpendicular to the layer, and strong absorption in the visible due to small band gaps.6-8 In these layered materials, the quantum size effect is expected to be strongly anisotropic given the anisotropic nature in the directions parallel and perpendicular to the C-axis. Recently we have studied ultrafast electronic relaxation dynamics in lead iodide, PbI2, semiconductor colloidal nanoparticles.9 The results suggest that surface effect dominates over size (in the size range studied, 3-100 nm) in the electronic relaxation process. Significant photodecomposition was observed in PbI2 nanoparticles in different solvents, along with noticeable blue shift of the peak positions, decrease in particle size, and † ‡
University of California. EIC Laboratories, Inc.
increase in the optical absorption in the UV-vis region. At early times, the electronic relaxation showed what appeared to be oscillations with a period changing with solvent but not size. The nature of the oscillation was unclear; the mechanism of photodegradation and the associated changes in optical absorption were not fully understood. To help address some of the unresolved issues, we have conducted a study of a similar layered semiconductor system, bismuth iodide (BiI3). A comparative study between BiI3 and PbI2 helps to gain some new insight into the electronic relaxation and photodegradation mechanisms. BiI3 has a crystal structure very similar to PbI2 except that one-third of the Pb2+ sites are vacant for BiI3.10 BiI3 has a rhombohedral structure with a layer of metal sandwiched between two hexagonally closed packed layers of iodine. The extra charge on the Bi3+ ions makes them occupy only two-third of the octahedral holes and one-third of the octahedral sites remain unoccupied, which give rise to a honeycombed bismuth plane. Each anion is thus interacting with three cations within a “sandwich” as well as weakly interacting with the adjacent “sandwich”. BiI3 is a promising material for non-silver and thermally controlled photographic applications.11-14 The band gap of bulk BiI3 was reported as ∼2 eV.15-18 The optical properties and stacking faults of BiI3 single crystals have been studied in detail.17,19 Different types of excitonic transitions near the fundamental absorption edge have been observed,
10.1021/jp000980+ CCC: $19.00 © 2000 American Chemical Society Published on Web 09/13/2000
Colloidal BiI3 and PbI2 Nanoparticles including direct and indirect excitonic bands and a series of sharp absorption bands due to stacking faults in the crystals.17 Colloidal BiI3 nanoparticles and BiI3 clusters in geolite LTA have been synthesized and the quantum confinement effect has been studied spectroscopically.10,18,20 Electronic relaxation dynamics in BiI3 nanoparticles have not been reported. In this paper we report the first study of the electronic relaxation dynamics in BiI3 nanoparticles using femtosecond transient absorption spectroscopy and compare with PbI2.9 In the following sections, we first provide some experimental details on the synthesis of BiI3 colloidal nanoparticles as well as on their characterizations using microscopy and spectroscopic techniques. A comparison of the ground-state electronic absorption spectra of colloidal BiI3 and PbI2 nanoparticles is given, followed by presentation and analysis of transient absorption data on electronic relaxation dynamics. The observations indicate that the electronic relaxation process in BiI3 colloidal nanoparticles is sensitive to the surface but insensitive to particle size in the range studied (2-100 nm). Possible interpretations of the results are provided. Experimental Section The preparation of PbI2 nanoparticles in various solvents was discussed in detail in previous works.1,9 Similar procedure was used to prepare BiI3. However, due to low solubility of Bi(NO3)3‚5H2O in water, 10% HNO3 was used in the preparation of a 0.01 M stock aqueous solution of Bi(NO3)3‚5H2O. Typically, 4 mL of the 0.01 M aqueous solution of bismuth nitrate was added to 100 mL of the solvent of interest. This solution was then vigorously stirred as 4 mL of a 0.05 M aqueous solution of potassium iodide solution was rapidly injected in it by syringe to generate clear orange-red solutions of BiI3 nanoparticles. The environment of the BiI3 nanoparticles was kept anionic by having [I-] at a slightly higher concentration than that required for a [I-]:[Bi3+] ratio of 3:1 in solution. Various solvents, such as water, ethanol, 1-butanol, and acetonitrile, were used, and the stability of BiI3 nanoparticles was somewhat dependent on the solvent used. The particle sizes were determined by using low- and high-resolution transmission electron microscopy (TEM). The particles in the freshly prepared colloidal solutions of BiI3 nanoparticles in different solvents had a broad size distribution. The particles in acetonitrile were not very stable under room light. BiI3 nanoparticles in 1-butanol were more stable under light and photodegraded into smaller particles after 1-2 weeks. But the degradation rate was much slower and the increase in optical density (OD) was much smaller over time compared to that observed in PbI2. The particles aged under dark were stable in all solvents for several weeks and TEM taken after 4-6 weeks showed clear evidence of degradation of larger particles into smaller ones, even though the peak positions and the OD did not change significantly in the dark. The BiI3 nanoparticles in aqueous solution containing PVA (MW 86 000-100 000) were prepared following the procedure of ref 21 with some modification. Typically a saturated solution of BiI3 nanoparticles was prepared in the presence of 0.5% aqueous PVA. A bright red colloidal solution of BiI3 with slight opalescence was obtained, and TEM measurements showed the presence of nanoparticles with a large size distribution (30100 nm). BiI3 nanoparticles in inverse micelles were prepared following the procedure developed for ZnS nanoparticles in ref 22 with modifications. Typically, 8 g of surfactant AOT (sodium
J. Phys. Chem. B, Vol. 104, No. 40, 2000 9397 dioctylsulfosuccinate salt) was dissolved in 100 mL of heptane. Aqueous solution of 1 mL of 0.01 M Bi(NO3)3‚5H2O solution was slowly added to this mixture to obtain a clear solution containing inverse micelles. Then 1 mL 0.05 M aqueous KI was added dropwise to obtain lemon yellow to bright yellow colloidal solution of BiI3 nanoparticles in a clear inverse micelle solution. The estimated radius of the water droplet formed in the inverse micelle system using the following equation22,23 was calculated to be 8.375 Å:
R ) 3VwW/R0
(1)
where Vw is the volume of a single water molecule (30.15 Å3), W is the ratio of water to surfactant, and R0 is the surface area of the anionic headgroup (60 Å2). The high-resolution TEM data show that the particles in the inverse micelle solution mostly have a size distribution of 1.5-2.5 nm in diameter, which is consistent with our calculations of radius of the water droplet. The electronic absorption spectra were measured in a HewlettPackard diode array spectrophotometer (8452 Å) with 2-nm resolution. Preliminary fluorescence measurements on a PerkinElmer fluorometer (LS50B) showed no observable fluorescence at room temperature for the BiI3 nanoparticles. TEM measurements were taken using low-resolution JEOL-100 CX and highresolution JEOL-200 CX transmission electron microscopes. The transient absorption experiments were performed using a femtosecond Ti-sapphire laser system, involving a pumpprobe scheme, as described previously.24 Pulses of 40 fs duration (5 nJ/pulse energy) at a repetition rate of 100 MHz were generated and amplified in a Ti-sapphire regenerative amplifier using chirped-pulse amplification. The final output laser pulses of 200 fs duration with energy of 250 µJ/pulse, centered at 780 nm were generated at 1 kHz repetition rate. This amplified output was doubled using a 1 mm KDP crystal to generate 390 nm pulses (20 µJ/pulse), which were used to excite (pump) the sample. The pump power was attenuated using neutral density filters to avoid generating any signal from the pure solvent, due to multiphoton ionization. The remaining 780 nm light was used to generate a probe pulse, using white light generation in a quartz window in the wavelength region of 650-950 nm for monitoring the photogenerated electrons. The time delay between the pump and probe pulses was controlled by a translation stage. The sample was contained in a quartz cell with 1 cm optical path length, and the OD of the sample was around 1.5 at 390 nm. The photodecomposition of BiI3 mentioned earlier was relatively insignificant during each set of dynamics measurements and thus did not affect the dynamics data observed. Results Electronic Absorption Spectra and Particle Size of Colloidal BiI3 Nanoparticles. Figure 1 shows the electronic absorption spectra of freshly prepared BiI3 nanoparticles in acetonitrile, 1-butanol, and ethanol. In ethanol, the spectrum has two main peaks around 326 and 390 nm and a shoulder at 430 nm, while in 1-butanol the peaks are around 290, 340, and 380 nm. In acetonitrile the spectrum consists of three major peaks around 290, 358, and 472 nm. Even though some of these peak positions are very close to those observed in the groundstate electronic absorption spectra of colloidal solutions of PbI2 nanoparticles, some new peaks are observed in the electronic absorption spectra of BiI3 nanoparticles in different solvents. Figure 2A shows the electronic absorption spectrum of BiI3 nanoparticles in aqueous PVA with a new peak at 498 nm. Similar to the peak observed at 414 nm in the spectrum of PbI2
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Figure 1. Ground-state electronic absorption spectra of freshly prepared BiI3 nanoparticles in 1-butanol (A), acetonitrile (B), and ethanol (C).
Figure 2. The ground-state electronic spectra of BiI3 nanoparticles in aqueous solution containing PVA (A) and in inverse micelles (B, freshly prepared).
in aqueous PVA,9 this peak is red shifted compared to the absorption peaks in other solvents. To the blue of this peak appear several higher energy bands that are broadened. Figure 2B shows the electronic absorption spectra of BiI3 nanoparticles in inverse micelle. The spectrum has two wellresolved peaks at around 332 and 402 nm, which are again different from those in other solvents. TEM pictures in Figure 3 show the sizes of BiI3 nanoparticles in different solvents under different conditions. Figure 3A shows particles freshly prepared in acetonitrile. Figure 3B and C reveal particle sizes in 1-butanol, a few days old and aged under dark for a few weeks, respectively. The particles have a broad size distribution in both A and B. Particles of small size of ∼2-4 nm, medium size of 20-30 nm, and large size up to 100 nm were observed, and the approximate ratio of the particle size distribution was 10:4:2 for small:medium:large. However it varied somewhat from sample to sample. For the aged sample C, the particles are smaller (2-8 nm) and have a narrower distribution. Figure 3D shows a high-resolution TEM image revealing a particle size distribution of 1.5-2.5 nm for BiI3 in inverse micelles. TEM pictures of BiI3 nanoparticles in PVA (not shown) show sign of degradation under electron beam. Ultrafast Transient Absorption. Figure 4A shows the transient absorption decay profiles of photoexcited BiI3 nanoparticles in acetonitrile, probed at 720 and 790 nm, following excitation at 390 nm. Figure 4B shows the time evolution on a longer time scale for the same sample as used for Figure 4A, probed at 720 nm. Following an instrument-response limited rise (
