Controlled synthesis, optical and electronic properties of Eu3+ doped yttrium oxysulfide (Y2O2S) nanostructures

Journal of Colloid and Interface Science 336 (2009) 889–897

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Controlled synthesis, optical and electronic properties of Eu3+ doped yttrium oxysulfide (Y2O2S) nanostructures Jagannathan Thirumalai a, Rathinam Chandramohan a,*, Sushil Auluck b, Thaiyan Mahalingam c, Subbiah R. Srikumar d a

Department of Physics, Sree Sevugan Annamalai College, Devakottai 630 033 (T.N), India Department of Physics, Indian Institute of Technology, Kanpur 208 016, UP, India c School of Physics, Alagappa University, Karaikudi 630 003, TN, India d Department of Physics, Kalasalingam University, Krishnankoil 626 190, TN, India b

a r t i c l e

i n f o

Article history: Received 16 December 2008 Accepted 8 April 2009 Available online 21 April 2009 Keywords: Hydrothermal synthesis Oxysulfide Nanostructures XPS Photo-induced impedance

a b s t r a c t Single crystalline Eu3+ doped yttrium oxysulfide (Y2O2S) nanocrystals, nanosheets, nanobelts, nanotubes, nanorods and nanowires have been successfully prepared via precursors of Y(OH)3 nanostructures in high yields and purities by a convenient hydrothermal method under mild conditions. Comprehensive structural, morphological and spectroscopical studies have been carried out on the nanometre scale. The as-prepared samples are characterized using X-ray photoelectron spectra (XPS), to investigate the elementary states on the surfaces. A significant shift (0.22–0.36 eV) in the optical spectra of the Y2O2S:Eu3+ system corresponding to the fundamental absorption and charge transfer bands, respectively, with respect to the bulk counterpart. The zero and one-dimensional (1D) nanostructures are good candidates for investigating size-induced opto-electronic properties of functional oxysulfides. In order to identify the origin and nature of the electronic transitions observed in the visible region, the photo-induced impedance measurements have been extended to the zero and 1D nanostructures. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction In recent years, nanostructured materials, such as nanocrystals, nanosheets, nanobelts nanotubes, nanorods and nanowires, have been widely investigated due to their potential applications in many different areas, such as catalysis, optoelectronics, environmentally friendly pigments, building nanodevices, nanosensors, and in fully understanding the dimensionally confined transport phenomena in functional nanomaterials [1–4]. These systems, with two restricted dimensions, not only offer opportunities for investigating the dependence of the electronic transport, but also the optical and mechanical properties during size confinement and dimensionality. The ability to control and manipulate the physical and chemical properties of materials, as we desire is one of the challenging issues in chemistry and materials science. By controlling the reaction parameters, such as molar ratio between capping agent and metallic precursor, temperature, reaction time and the order of addition of reactants, a reasonable control of the size and morphology can be achieved [5,6]. During recent years, lanthanide-doped luminescent nano-sized materials have received much attention for their wide applications on high-resolution displays, integrated optical systems, substitute for organic dyes, solid-state * Corresponding author. E-mail address: [email protected] (R. Chandramohan). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.04.042

lasers and especially biological labels. Trivalent-europium doped yttrium oxysulfide (band gap 4.2–4.8 eV) is an important phosphor system extensively applied in colour televisions, high-resolution displays, memory devices, afterglow phosphors and so on [7–10]. Photo-induced mechanism in semiconductor nanostructures has acquired considerable interest for it can unravel details pertaining to configuration of surface state(s) and its relaxation geometry [11,12]. Recent studies on the preparation of rare-earth hydroxide and oxysulfide nanostructures have shown that 1D nanostructures can be prepared by a hydrothermal method, which leads us to believe that 1D nanostructured hydroxides might be prepared via hydrothermal treatment of their counterpart oxides and oxysulfides in an autoclave [13–15,11]. This paper report results on the systematic study that has been carried out to investigate the controlled formation of hydrothermal technique to obtain general and widely applicable approach for the synthesis of zero and onedimensional rare-earth doped oxysulfide nanostructures of scientific and technological importance. The synthesis of hydroxides and the conversion of rare-earth doped oxysulfide nanostructures are discussed in detail [11]. The advantages of this synthesis, which include simplicity, convenient and innovative route for the synthesis of nanostructures of functional hydroxide, oxide and oxysulfide materials of various structure types and as well as their optical properties, further functionalization, and other properties are

J. Thirumalai et al. / Journal of Colloid and Interface Science 336 (2009) 889–897

3. Results and discussion 3.1. Formation, structure, and morphology of the Y2O2S:Eu3+ nanostructures XRD patterns obtained from the Y(OH)3 and Y2O2S:Eu3+ products are shown in Fig. 1(a) and (b). All peaks can be perfectly indexed as the pure hexagonal phase and they are in good agreement with standard Y(OH)3 ([P63/m, (176)], JCPDS #83-

(100)

(001)

A Nanoplates/ Bulk nanoparticles A7

(302)

(112) (310) (131) (400)

(002) (300)

(210)

(201)

(110) (101)

(b)

(200) (111)

A1

Intensity (a.u.)

A2 A3

A4

nanosheets A8

Intensity (a.u.)

(a)

(100)

The preparation of Y2O2S:Eu3+ nanostructures is similar to the method reported by our group [11]. But this work has been extended to synthesis of various nanostructures like nanocrystals, nanosheets, nanobelts nanotubes, nanorods and nanowires. In a typical synthesis, 0.9 mM of Y(NO3)3  6H2O were dissolved in deionized water to form a clear solution and the pH of the solution was adjusted to 7–13 by adding NaOH solution under vigorous stirring. The mixture was then transferred to a Teflon-lined stainless steel autoclave and heated at 100–180 °C for 12–48 h. A white precipitate was collected, purified, and dried in air at ambient temperature. The as prepared Y(OH)3 nanostructures were first dispersed into (0.1 mM) of Eu(NO3)3  6H2O aqueous solutions, then stirred for 2 h at 70–80 °C, followed by a subsequent sulfuration process under a vigorous stirring that leads to doped Y(OH)3 nanostructures that could further be converted to the formation of Eu3+ doped yttrium oxysulfide (Y2O2S:Eu3+) nanocrystals, nanosheets, nanobelts nanotubes, nanorods and nanowires at 600 °C for 2 h under inert (Ar or N2)/CS2/sulfur/carbon atmosphere. For comparison purposes polycrystalline Y2O2S:Eu3+ (sample A) was prepared through a conventional solid-state reaction method [16].

(203) (114) (211)

2.1. Synthesis of rare-earth (Eu3+) doped yttrium oxysulfide nanostructures

(112) (201) (104) (202)

All reagents were analytical grade and were used without further purification in the experiment.

The X-ray diffraction (XRD) patterns were recorded on Philips X’Pert PRO system from PanAlytical diffractometer with Cu Ka radiation (k = 0.15406 nm) and a scanning rate of 5° min1. Field emission scanning electron microscope (FESEM) measurements were carried out using Hitachi S-4300SE model. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) were taken with a JEOL 2000 EX II operated at 200 kV. Samples for TEM were prepared by dropping a diluted suspension of the sample powders onto a standard carbon-coated (20–30 nm) film on a copper grid (230 mesh) and air-dried. XPS measurements were performed using a LAS-3000 surface analysis system (RIBER, France) and Al Ka X-rays (1486.6 eV, width 0.5 eV). Room temperature photoluminescence (PL) spectra were recorded with a VARIAN Cary Eclipse spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Photo-induced Impedance measurements were carried out using SOLATRON instrument on both dark and UV (254 and 365 nm) shined conditions. The as-synthesized powder samples were made by compacting the powder samples to solid discs at 10 mm diameter and 1 mm thickness and the pellet/disc is held across the silver contacts like a two-electrode system. All the measurements were performed at room temperature.

(110) (103)

2. Materials and methods

2.2. Characterization

(102) (003)

studied. The experimental results have confirmed that the hydrothermal method offers a very powerful means of fabricating well-crystallized nanostructures in future. They should exhibit unique nanoscale surface characteristics such as anomalous optical properties and electronic transition behaviors, which leads to novel opto-electronic properties.

(101)

890

nanobelts A9 nanotubes A10

A5

nanorods A11 nanowires

A6

A12 0 20

40

60

2θ (in degrees)

80

10

20

30

40

50

60

70

80

2θ(degrees)

Fig. 1. (a) XRD patterns of the Y(OH)3 nanostructures: (A1) nanocrystals, (A2) nanosheets, (A3) nanobelts, (A4) nanotubes, (A5) nanorods, and (A6) nanowires. (b) XRD patterns of the Y2O2S:Eu3+ bulk and nanostructures obtained by hydrothermal treatment by varying the temperature and pH of the reaction mixture.

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2042; lattice constant: a = b = 6.261 Å, c = 3.544 Å) and Y2O2S:Eu3+  (½P 3m1; ð164Þ, JCPDS #24-1424; lattice constant: a = 3.784 Å, c = 6.589 Å) data. It is observed that for the Y2O2S:Eu3+ nanostructures, there is a marginal decrease (0.35%) in crystallographic unit-cell volume that tends to contract due to the increase in surface area of the crystallites. This may lead to a decrease in the lattice constant. No peaks attributable to other types of yttrium hydroxide and oxysulfide are observed in the XRD patterns, indicating the high purity of the phases obtained. Based on these experimental results (Table 1), the various morphologies of nanocrystals to nanowires of hydroxides and oxysulfides are obtained. Fig. 2(a)–(f) shows the TEM, HRTEM, and electron diffraction measurements of the synthesized Y(OH)3 nanocrystals, nanosheets, nanobelts, nanotubes, nanorods and nanowires are seen to be single-crystalline in nature. The resulting morphologies and synthesis conditions of Y(OH)3 are discussed in Table 1. Fig. 3(a) shows the SEM image of a polycrystalline bulk sample. This shows a hexagonal morphology that is consistent

891

with crystal structure of Y2O2S:Eu3+ having the particle size ranges between 1 and 2 lm. The oxysulfide nanostructures was further examined by TEM and HRTEM. Fig. 3(b)–(j) shows the TEM, HRTEM, and electron diffraction images of Y2O2S:Eu3+ nanostructures. These nanostructures are synthesized according to the conditions given in Table 1. As can be seen from the TEM images of Fig. 3(b), that most of the nanoplates have a hexagonal morphology with sizes in the range 15–30 nm along with some agglomerated nature of spherical particles. Nanosheets (Fig. 3(c)) were found to be the main products in the lower pH range (7 ± 8). This may be attributed to the two-dimensional growth tendency of the Y(OH)3 nanostructures at lower pH. The oxysulfide nanobelts with a (1 1 0) growth direction were obtained by calcinating Y(OH)3 nanobelts and are shown in Fig. 3(d). These nanobelts are able to clearly seen from the SEM images in Fig. 3(e). These nanosheets and nanobelts curls from the edge, indicating a possible rolling process for the formation of the nanotubes. Also Fig. 3(f) represents the typical image of yttrium oxysulfide nanotubes with an open

Table 1 Optimal experimental conditions and resulting morphologies Y(OH)3 and Y2O2S:Eu3+ nanostructures. S. No.

Sample

Experimental Conditions

Resulting morphologies

T (°C)

pH

T (h)

Shape

Size

1.

Y(OH)3

A1 A2 A3 A4 A5 A6

100 140 180 120–140 140–160 180

7–8 7–8 8–9 12–13 13 13

48 12 24 24 24 48

Hexagonal/spherical nanocrystals Nanosheets Nanobelts Nanotubes Nanorods Nanowires

15–20 nm (spherical) 30–40 nm (hexagonal) t  15 nm, W  50 nm t  25 nm, L 6 2 lm D  20 nm, L  200 nm D  15 nm, L  80 nm D  12 nm, L P 200 nm

2.

Y2O2S:Eu3+

A7 A8 A9 A10 A11 A12

600 600 600 600 600 600

– – – – – –

2 2 2 2 2 2

Hexagonal/spherical nanocrystals Nanosheets Nanobelts Nanotubes Nanorods Nanowires

15 nm (spherical)  20–40 nm (hexagonal) t  15 nm, W  70 nm t  25 nm, L 6 2 lm D  10 nm, L  200 nm D  10 nm, L  70 nm D 6 15 nm, L P 250 nm

Fig. 2. Typical TEM images of Y(OH)3 nanostructures from the sample A1 to A6: (a) nanocrystals, (b) nanosheets, (c) nanobelts, (d) nanotubes (formation of nanotubes from nanosheet-like pattern), (e) nanorods and (f) nanowires.

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Fig. 3. (a) SEM image of hexagonal-shaped bulk Y2O2S:Eu3+ (A). The TEM images of Y2O2S:Eu3+ nanostructures from the sample A7 to A12: (b) nanocrystals, (c) nanosheets, (d) nanobelts, (e) SEM image of nanobelts having thickness of single-nanobelt is around 30 nm, (f) nanotubes, (g) nanorods, (h) nanowires, (i) a close up of the boxed area in (h) shows a structure of single-nanowire, and (j) HRTEM image of Y2O2S:Eu3+ nanowire.

end. The central part of the cylindrical portion is light gray and the two peripheries are black, confirms the formations of single nanotube with a growth along [1 0 1] direction. The TEM also reveals that aggregate of several nanotubes in the typical diameter range of 10–20 nm and the length of the nanotubes are estimated to be around 200 nm. Rod-shaped Eu3+ doped Y2O2S in Fig. 3(g), with uniform diameters of 10–20 nm and lengths up to 70–150 nm can be readily obtained (refer Table 1). The morphology of the of Y2O2S:Eu3+ nanowires are shown in Fig. 3(h), where it is seen that the oxysulfide nanowires retain the original shape after calcina-

tion. Fig. 3(i) shows a enlarged view of typical Y2O2S:Eu3+ nanowires. It is confirmed from the SAED and HRTEM images of nanorods and nanowires (Fig. 3(g) and (j)) reveal a hexagonal single-crystal phase of the yttrium oxysulfide with a [1 1 0] growth direction, and which are single crystalline in nature. Also through the fast Fourier transform (FFT) pattern of the border region (Fig. 3(i), inset), reveals the nanowires grow along the (1 1 0) direction. Furthermore, the morphology of these nanostructures is likely to be a near-quantum structure nature. It is having high surface-tovolume ratio, also plays a major role in the density of singly ionized

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oxygen vacancies and the charge state of this defect, due to the existence of surface depletion, and thus in controlling the emission intensity [18,19]. X-ray photoelectron spectroscopy (XPS) is known to probe the surface of the particles. The analysis of low energy electrons, which is strongly scattered in materials and consequently have a small escape depth, enables information to be obtained from the top few atomic layers of a surface. Therefore, in order to further prove that the as-synthesized bulk and 1D nanostructures of Y2O2S:Eu3+ samples are determined by XPS, to investigate the elementary states. This paper discussing the XPS results only for the bulk and 1D nanowires of oxysulfide system as a comparison to identify the presence of elementary states of in the nano-oxysulfides. Fig. 4(a) shows the XPS survey spectra of the hexagonal Y2O2S:Eu3+ bulk and 1D nanostructures, under identical conditions. Fig. 4(b) and (c) shows the enlarged regions of the XPS spectra of the hexagonal Y2O2S:Eu3+ bulk and 1D nanowires, obtained in the range of 0–40 eV and 150–400 eV, respectively, under identical conditions. The XPS spectrum of Fig. 4(b) shows the bulk and nano-sample, displaying the existence of the lower binding energies of 18 eV for S 3s, 19 eV for Eu 5p, 23 eV for O 2s and 24 eV for Y 4p in good agreement with the literature [20,21]. However, for the Y2O2S:Eu3+ bulk and nanostructures, Y 3d at 156 and 158 eV, Y 3p at 298.5 and 310.5 eV, Y 2s at 393.6 eV, Eu 4p at 289 eV, S 3p at 164.5 eV and S 2s 228 eV peaks are also observed. The Y 2s and S 2s peaks are very weak as compared to the Y 3d and S 2s peaks (Fig. 3(c)). The com-

40000

A detailed study related to the change in size of the nanostructures is explained already through an energy diagram in Figs. 4 and 5 [11,16,17]. Owing to the clear change in size dependent effects of nanomaterials because of their abnormal behavior and in tailoring the properties of these materials, which mainly come from the high surface/volume ratio in nanosystems. This may offer deep insight for understanding the important physical and chemical phenomena and as well as the quantum confinement effect. Photoluminescence (PL) excitation and emission spectra of bulk and nanostructures of Y2O2S:Eu3+ samples are shown in Fig. 5(a) and (b). It is important to mention that luminescence data related to nanocrystals, nanosheets, nanobelts nanotubes, nanorods and nanowires of both the oxysulfide systems show some interesting results. The spectral blue-shift in the excitation bands for the nanostructures from the bulk can be rationalized by considering them may be attributed to the size-dependent effect [11,12,16]. The PL excitation spectra of the oxysulfide system essentially comprising two parts viz., fundamental absorption band (260 nm) and Eu3+–X2 ligand (X = O/S) CTB (320 nm) show significant blue shifts, respectively, with respect to the bulk counterpart. The

O KLL

Y 3p

O 2s and Y 4p Eu 4d Y 3d

Intensity (arb. units)

50000

3.2. Photoluminescence properties and surface state relaxation processes of Y2O2S:Eu3+ nanostructures

O 1s

(a) 60000

position estimated by XPS using the relative sensitivity factors of Y, O, S and Eu, also revealed excess oxygen in the samples.

30000

20000 10000

0

0

200

400

600

800

1000

Binding energy (eV)

-5

0

5

10

15

20

25

Binding energy (eV) Fig. 4. XPS spectra of (a) survey XPS spectrum of Y2O2S:Eu the range of 0–40 eV and 150–400 eV, respectively.

3+

30

35

40

150

S 2p1/2

Y 3p3/2 Y 3p1/2

Y 2s

Y 3d5/2 Y 3d3/2

Eu 4p1/2

(c)

Intensity (arb. units)

O 2s and Y 4p Eu 5s

Intensity (arb. units)

S 3s and Eu 5p

(b)

S 2s

200

250

300

350

400

Binding energy (eV)

bulk and nanowires and (b) and (c) show the enlarged regions of the weak and strong structure of XPS spectra in

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Energy (eV)

(b)

2.15

2.06

1.98

J=2

J=1

1.91 5

D0

1.83 7

FJ

1.77

J=4 A A7

Energy (eV) 6.19

A8 3.10

4.13

f-f Eu

3+

Intensity (a.u.)

(a)

- O2-/S2-

A9 A10 A11

A A12 Intensity (a.u.)

575

600

A7

A8

625

650

675

700

Wavelength (n.m.)

(c)

Energy (eV) 2.15

A9 A10

2.06

J=1

1.98

J=2

1.91 5

D0

1.83 7

FJ

1.77

J=4 A

A11 A7

A12 300

400

A8 Intensity (a.u.)

200

Wavelength (n.m.)

A9 A10 A11 A12 575

600

625

650

675

700

Wavelength (n.m.) Fig. 5. Panel (a) shows the Y2O2S:Eu3+ room temperature photoluminescence excitation spectra (kem = 626 nm) of bulk and nanostructures. f–f indicates Eu3+ lines from higher levels and CTB indicates the Eu3+–ligand charge transfer band. (b) Photoluminescence emission spectra at (kexc = 260/320 nm) corresponding to various 5D0 ? 7FJ (J = 1, 2, 4) transitions revealing identical Stark-splitting patterns for both bulk and nanostructures with respect to the variation in concentration of the samples.

blue-shift in the optical spectra for the oxysulfide system corresponding to the fundamental absorption band and Eu3+–X2 ligand (X = O/S) CTB show significant blue shifts (0.22–0.36 eV), respectively, with respect to the bulk counterpart. Fig. 5(b) and (c) gives the emission spectrum of Y2O2S:Eu3+ excited at direct-band (kexc = 260 nm) and CTB (kexc = 320 nm) consists of sharp lines ranging from 580 to 710 nm which are associated with the transitions from the excited level of 5D0 to the levels of 7FJ (J = 1, 2 and 4) of Eu3+, respectively. Among several luminescence transitions of Eu3+, the 5D0 ? 7F1 (590 nm) emission transition is mainly magnetically allowed (a magnetic-dipole transition). It is structure independent while 5D0 ? 7F2(626 nm) and 5D0 ? 7F4 (700 nm) are hypersensitive forced and weak electric-dipole transitions,

respectively, being allowed only at low symmetries with no inversion center. These transitions are known to be hypersensitive to crystal-structure and chemical surroundings [11,22,23]. While comparing with the emission spectrum related to direct and charge transfer bands shows a broader emission corresponding to the samples A7, A8 and A9, may be due to the variation in reaction parameters (Table 1). The emission spectrum obtained at 260 nm excitation, characteristic of direct band emission in yttrium oxysulfide, is shown in Fig. 5(b), peak arising from a 5 D0 ? 7F2 transition at 626 nm is observed, suggesting the absence of oxide impurity. From this emission spectrum it is clearly indicated that the Eu3+ ions have been effectively distributed into the Y2O2S matrix.

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(a)

254 nm

10E+6

RS

C1

W1 Ws

365 nm

8E+6 Z" (ohms)

R1 6E+6

Dark 4E+6 I

2E+6

II 0

(b) 12E+5

RS

254 nm

10E+5

C1

C2

R1

R2

1E+7 2E+7 Z' (ohms) W1 Ws

(c) 12E+5

4E+5

Dark

2E+5

I

C1

C2

R1

R2

0

W1 Ws

365 nm

III 6E+5

Dark

II

4E+5

III 2E+5

II m=0.5636 0

Z" (Ohms)

6E+5

RS

254 nm

8E+5

m=1.082

m=0.7561

Z" (Ohms)

I

4E+7

10E+5

365 nm 8E+5

3E+7

m=1.2507

0

m=0.7266 m=0.5366

0 0

5E+5 10E+5 15E+5 20E+5 25E+5 30E+5

5E+5 10E+5 15E+5 20E+5 25E+5 30E+5

Z' (Ohms)

Z' (Ohms) 3+

3+

Fig. 6. Nyquist impedance fitted plot of bulk (a) Y2O2S:Eu (sample A) and (b) and (c) Y2O2S:Eu nanocrystals and nanosheets according to Table 1, under dark and UV excitation conditions (254 nm and 365 nm) and the corresponding electrical-impedance equivalent circuits.

3.3. Photo-induced impedance analysis on Y2O2S:Eu3+ nanostructures The surfaces of semiconductor nanostructures play an important role in determining their optical properties. Relaxation of surface states (SS) with the combination of electric-field resonance and their role in augmenting various photo-chemical properties such as electro-luminescence, solar cells, etc., have been well demonstrated [12,24,25]. In photo-induced impedance spectroscopy, Nyquist plot is of more practical significance especially when employed for nano-material characterization. In order to identify the origin and nature of the electronic transitions observed in the visible region, the photo-induced impedance measurements for these samples both under UV exposure (254 and 365 nm radiation) and without UV (dark) conditions were made. Figs. 6(a)–(c) and 7(a)–(d) show the Nyquist type impedance plot of Y2O2S:Eu3+ bulk and nanostructures. Nyquist type impedance plot shows a simple semicircle, accompanied by a small semicircle with an adjoining straight-line. Figs. 6(b) and (c) and 7(a)–(d) discuss the Nyquist type impedance plot accommodate a semicircular pattern can be rationalized by considering a resistor– capacitor (RC) parallel net-work. Whilst the second semi-circular region in the middle frequency (roughly 60–10 Hz) would correspond to relaxation of surface states. Different models have been proposed for explaining contribution from surface state(s) on the impedance spectra for several semi-conductor electrode systems [12,26–28]. The linear portion (Warburg pattern) having profound

dependence on excitation energy on the slope would correspond to an electron/charge transfer process between core and surface state(s) in the vicinity of conduction band-edge and subsequent relaxation through diffusion of charge carriers/defects. Detailed discussions related to Warburg impedance processes are already given [11,16]. It should be noted that from the photo-induced impedance process, the magnitude of charge carriers (i.e. the total charge), associated with surface states as reflected in the photoinduced impedance data can be obtained from the following relation:

e0 Nss ¼ Q ss ¼

Z

V1

C ss dV

ð1Þ

V0

From Table 2, the optical excitation causes substantial increase in both Rss and Css values corresponding to resistance and capacitance values of surface states suggesting significant increase in photogenerated carriers. This may stem from photo-induced relaxation of surface sates. As a result, UV exposure influences the surface state relaxation in these nanoscale materials. This can in turn influences the ligand to metal charge transfer transitions of Eu3+ doped yttrium oxysulfide nanostructured materials. Moreover, because of the surface-related nature, such Eu3+ doped yttrium oxysulfide nanostructures have special potential in designing environmental sensors, biomolecule labeling devices, high-resolution displays, blinking studies, etc., to design this quantum structure for gainful application in opto-electronic devices. Owing to their excellent

J. Thirumalai et al. / Journal of Colloid and Interface Science 336 (2009) 889–897

I 10E+5

C2

R1

R2

W1 Ws

(b) 12E+5

6E+5

Dark

I

III

2E+5

C2

R1

R2

W1 Ws

III

6E+5

Dark

II

4E+5 2E+5

m=0.7351

m=0.7601 m=0.5204

0 0

0

5E+5 10E+5 15E+5 20E+5 25E+5 30E+5

m=0.5902 0

5E+5 10E+5 15E+5 20E+5 25E+5 30E+5

Z' (Ohms) RS

(c) 12E+5

Z' (Ohms)

C1

C2

254 nm R1

10E+5

365 nm

W1 Ws

(d) 12E+5

254 nm

365 nm

8E+5

II

2E+5 0 0

R1

R2

W1 Ws

I

Dark

4E+5

0

II

m=0.5055 0

5E+5 10E+5 15E+5 20E+5 25E+5 30E+5

C2

III

6E+5

2E+5

m=0.5365

C1

m=0.7120

I

4E+5

Z" (Ohms)

Dark

III

m=1.095

m=0.7890

6E+5

RS

10E+5

R2

8E+5

Z" (Ohms)

C1

365 nm

8E+5

II

4E+5

RS

254 nm

10E+5

m=1.062

365 nm

8E+5

Z" (Ohms)

C1

m=1.072

RS

254 nm

m=1.055

(a) 12E+5

Z" (Ohms)

896

5E+5 10E+5 15E+5 20E+5 25E+5 30E+5

Z' (Ohms)

Z' (Ohms)

Fig. 7. Nyquist impedance fitted plot of Y2O2S:Eu3+ nanostructures: (a) nanobelts, (b) nanotubes, (c) nanorods, and (d) nanowires, under dark and UV excitation conditions (254 nm and 365 nm) and the corresponding electrical-impedance equivalent circuits. Warburg impedance pattern with variation in slope (m) upon UV shining.

Table 2 Warburg impedance parameters values of Y2O2S:Eu3+ nanostructured system. Morphology and sample

UV condition

Rsc  106 (X cm2)

Csc (lF)

Rss (X cm2)

Css (lF)

WsR  106 (X cm2)

WsT (s)

WsP

Nanocrystals (A7)

254 nm 365 nm No UV

2.99 2.19 1.63

1.76E11 1.65E11 2.07E11

313,700 302,421 285,350

1.58E8 2.65E8 7.44E9

4.02 3.50 1.56

89.34 74.32 10.62

0.4775 0.3680 0.2665

Nanosheets (A8)

254 nm 365 nm No UV

2.72 2.33 1.80

1.95E11 1.85E11 1.91E11

335,746 304,612 258,080

1.85E8 2.85E8 7.21E9

4.12 3.54 1.52

92.97 77.89 10.08

0.4684 0.3483 0.2265

Nanobelts (A9)

254 nm 365 nm No UV

2.76 2.36 1.85

1.92E11 1.80E11 1.99E11

386,676 305,621 288,050

1.88E8 2.58E8 7.20E9

4.01 3.55 1.53

92.98 78.89 10.03

0.4744 0.3384 0.2565

Nanotubes (A10)

254 nm 365 nm No UV

2.94 2.26 1.95

1.86E11 1.60E11 2.01E11

343,725 308,521 254,050

1.78E8 2.64E8 7.36E9

4.18 3.62 1.69

90.06 76.88 12.48

0.4847 0.3814 0.2595

Nanorods (A11)

254 nm 365 nm No UV

2.90 2.32 1.98

1.80E11 1.55E11 2.17E11

385,721 302,421 225,350

1.65E8 2.23E8 6.21E9

4.08 3.44 1.60

88.15 79.19 11.67

0.4965 0.3468 0.2860

Nanowires (A12)

254 nm 365 nm No UV

2.96 2.36 1.85

1.92E11 1.80E11 1.99E11

385,647 305,621 288,050

1.88E8 2.58E8 7.20E9

4.01 3.55 1.53

92.98 78.89 10.03

0.4744 0.3384 0.2565

Rss and Css indicate resistance and capacitance values of surface states. Different Warburg parameters WsR (X), WsT (s), WsP indicate bulk (ohmic) resistance, Warburg resistance, Warburg time constant and order of the process, respectively.

hydrophilicity, these nanostructures, when used as effective confined templates, can be easily functionalized by introducing functional atoms/groups to their inner and outer surfaces through a very simple chemical modification method. Hence, the results

provide an effective route to synthesize oxysulfide nanostructures and are helpful for investigating the optic characters in the nanostructures, and promote the potential biomedical and display applications in oxysulfide nanophophors.

J. Thirumalai et al. / Journal of Colloid and Interface Science 336 (2009) 889–897

4. Summary Based on a facile hydrothermal method, the Y2O2S:Eu3+ nanostructures, such as nanocrystals, nanosheets, nanobelts nanotubes, nanorods and nanowires were successfully obtained from yttrium hydroxides. The structural and morphological studies confirm that the as-obtained oxysulfide product is single-crystalline with a hexagonal phase. Room temperature photoluminescence spectra showed that, the Y2O2S:Eu3+ nanostructures showed a significant blue-shift in both the fundamental absorption edge and Eu3+–X2 (ligand X = O/S) charge transfer (excitation) bands. The photo-induced impedance studies indicate that the energy dependent shift in the slope of the Warburg impedance pattern is regulated by a diffusion mechanism and increases the capacitive-impedance. This has been related to photo-induced modification in surface state relaxation processes. As plain as day, these surface state relaxation processes are absent in the bulk systems. Acknowledgments One of the authors J. Thirumalai wishes to thank the Director of Collegiate Education, Chennai for providing fellowship and gratefully thank NIIST (CSIR), Trivandrum, IGCAR, Kalpakkam and IICT (CSIR), Hyderabad for extending instrumentation facilities. References [1] X.F. Duan, C.M. Lieber, Adv. Mater. 12 (2000) 298.

897

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