Investigation of Optical and Photocatalytic Properties of Bismuth

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The Investigation of Optical and Photocatalytic Properties of Bismuth Nanospheres Prepared by a Facile Thermolysis Method Zhi Wang, Chunli Jiang, Rong Huang, Hui Peng, and Xiaodong Tang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 20 Dec 2013 Downloaded from http://pubs.acs.org on December 22, 2013

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The Journal of Physical Chemistry

The Investigation of Optical and Photocatalytic Properties of Bismuth Nanospheres Prepared by a Facile Thermolysis Method

Zhi Wang,

†

†

†

Chunli Jiang, Rong Huang,

†

†

Hui Peng * and Xiaodong Tang

†

Key Laboratory of Polarized Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200241, P. R. China

*Hui Peng, Present Address: Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, 500 Dong-Chuan Road, Shanghai 200241, China, Telephone Number: +86 13611646152, E-mail: [email protected].

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ABSTRACT: In this work, pure phase bismuth nanospheres have been successfully prepared by an thermolysis of bismuth acetate in oleylamine. The size distribution of the bismuth nanospheres was improved by quickly quenching the reaction as revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Ultraviolet-near-infrared (UV-NIR) absorbance spectrum of the bismuth nanospheres showed camel-like shapes located at 425 nm and 575 nm, which could be ascribed to the effect of the surface plasmon resonance and light scattering. Due to the absorption in the visible range, the prepared bismuth nanospheres showed good photocatalytic properties to the degradation of RhB.

Keywords: quenching, absorbance, surface plasmon resonance, degradation

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1. Introduction Interest in exploiting the unique properties of metal nanostructures continues to grow nearly unabated, due to its unique size and shape dependent optical, magnetic, and catalytic properties, and potential applications in catalysis,1-3 biolabeling,4 photonics,5 and information storage6 etc. In particular, bismuth nanostructures stand out as an important example for its special properties and potential applications different from bulk materials. Bulk bismuth exhibits semimetal behavior with a very small indirect band overlap and special electronic properties resulting from its small electron effective-mass, low charge carrier density (105 times smaller than conventional metals at 4.2 K), and long mean free path.7-10 Therefore, the dimensionally restricted bismuth and its different morphologies in nanostructures gives much promise to study finite-size effect,11 quantum confinement,12 magnetoresistance,8 semimetal-to-semiconductor transition10 and thermoelectric effects.13 These size-induced semimetal to semiconductor transition and related quantum confinement effects are potentially useful for optical and electro-optical device applications.14-15 In general, physical16-17 and chemical18-20 methods as well as electrodeposition21-22 have been developed to prepare various bismuth nanostructures such as nanoparticles,20 triangular nanoplate,23 nanotubes,24-25 nanowires26 and nanospheres.27 Among these nanostructures, nanospheres gain increasing attention because they can be used to construct 3D crystalline lattices which exhibit complete photonic band gaps extending over the entire optical regime.28-30 More recently, Xiang et al31 and Yella et al32also has reported the good catalysis properties of bismuth nanospheres (NSs) for the growth of SnS2 nanotubes and germanium nanowires, respectively. In the reported works for the chemical preparation of bismuth NSs, a reductant is necessary to reduce Bi3+ to zerovalent bismuth. Ethylene 3



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glycol (EG) or poly(ethylene glycol) are widely used as such aim.27,33-36 J. Wang et al have obtained bismuth NSs in a mixture solution of acetone and of poly (vinyl pyrrolidone) (PVP) by reducing BiO3+ with EG.35 Similar procedure has also been reported for preparing uniform bismuth NSs ranging from 100 nm to 600 nm.27

The optical properties of bismuth nanoparticles with a diameter less than 20 nm have been widely investigated. The absorption peak of colloidal bismuth nanoparticles at 253 nm which was firstly reported by Gutierrez et al was associated with the surface plasmon resonance band (sprb) supported by the calculation in the same diameter range based on Mie theory.16 This conclusion was followed in the late reports.37-38 While, Foos et al found the absorption maximum located at 196 nm instead of 253 nm for the bismuth nanoparticles with diameters ranging from 3.2 nm to 8.0 nm.18 More recently, Velasco-Arias et al pointed out that the reported absorption of bismuth nanoparticles (8-20 nm) in the UV range was not the sprb and caused by the presence of Bi(III) species in the colloids.39 They also pointed out that sprb should not necessarily be expected in the UV−visible region (220−600 nm) in small quasi-spherical semiconducting bismuth NPs without the induction of a higher free carrier concentration.39 But Toudert et al40 and J. McMahon et al41 theoretically illustrated that sprb can be tunable in the whole near-ultraviolet, visible, and near-infrared range by changing the size and shape of bismuth nanoparticles as well as the dielectric constant of the environment. Although the sprb of bismuth nanoparticles less than 20 nm is still in questionable, it is clear that the sprb of bismuth nanoparticles is dependent on the size and shape. So it is interesting to investigate the optical property of bismuth NSs which are larger than 20 nm and have a fine sphere shape.

Herein, we report a facile approach to prepare well separated bismuth NSs with a size range of 30 4


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~ 60 nm through reproducible thermolysis of an insoluble bismuth acetate precursor in oleylamine. The as-prepared bismuth NSs have been characterized by XRD, SEM, TEM and UV-NIR electronic absorbance spectroscopy. The results show that the prepared bismuth NSs have different optical properties from those of bismuth nanoparticles less than 20 nm. New absorption bands were observed in the visible and near infrared range. The photocatalytic properties of prepared bismuth NSs were also investigated.

2. Experimental Section 2.1 Chemicals Bismuth acetate (Bi(OOCCH3)3) were purchased from Alfa Aesar chemistry Co., Ltd. Oleylamine (mass concentration: 80 ~ 90%) were purchased from Aladdin Chemistry Co., Ltd. Toluene were from Shanghai Lingfeng Chemical Reagent Co., Ltd. All the chemicals are of analytical grade or better and used without further purification.

2.2 Preparation of well-separated bismuth NSs In a typical synthesis procedure, 1 mmol of Bi(OOCCH3)3 were slowly added to a three-necked flask contained 5 ml of oleylamine forming a suspension at room temperature. After being vacuumized by an oil-pump for 30 min to remove water from oleylamine under stir, the three necked flask was filled with high purity nitrogen (99.9 %). Then, the heating process was conducted in silver sand heated by a heating mantle with stirrer which is controlled by a proportional integral derivative (PID) controller device. The surface of the white bismuth acetate powder gradually turned black and the insoluble precursor finally disappeared while the temperature increased to 315 °C. The whole heating process lasted for 2.5 h. After reaction, the mixture was naturally cooled to room temperature or poured into 50 5



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ml cooled toluene (-6 °C). The product was collected by centrifugation, then washed with toluene, chloroform and absolute alcohol thoroughly. Finally the product collected by centrifugation at 6000 r/min was dried in vacuum. Here the sample was prepared by natural cooling process denoted as S1, the sample quenching by toluene at -6 °C denoted as S2.

2.3 Characterization The crystalline phase of prepared samples was characterized by a Bruker D8 X-ray diffractometry using Cu Kα radiation source (λ=1.5418 Å). The XRD scans were collected from 20° to 70° (2θ), with a step of 0.02° and a data collection time of 0.2 s. The JCPDS PDF database42 was utilized for phase identification.

The morphologies of these samples were investigated by SEM (JSM7500SF, JEOL, Japan) and TEM (Tecnai 20U-TWIN). The UV-NIR absorbance spectra were measured on a Perkin-Elmer Lambda-900 spectrophotometer by dispersing the samples in anhydrous ethanol. The IR transmittance measurements were performed over the frequency range from 400 to 4000 cm-1 using a Fourier transform infrared spectrometer (Bruker Vertex 80 V) with the KBr pellet technique.

2.4 Photocatalytic activity measurements The photocatalytic behavior of bismuth NSs was examined by photocatalytic degradation of RhB. For this, 20 mg of the samples were placed in a 100 mL triangular flat bottom flask containing 50 mL of 2 × 10-5 M RhB aqueous solution, and then the flask was sealed with a plug. The visible light was provided by a 200 W iodine tungsten lamp with irradiation wavelength ranging from 380 to 830 nm. The distance between the lamp and the flask was controlled to be 15 cm. The change of RhB concentration as a function of the optical absorption maximum was monitored by using a UV-vis 6


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spectrophotometer (TU-1901, Beijing Purkinje General) with intervals for a total irradiation time of 9.5 h.

3. Results and discussion 3.1 Morphology and crystal structure To study the difference among the morphologies of the samples prepared under different experimental conditions, scanning electron microscopy (SEM) measurement was performed. Figure 1 represents the results. As shown in Figure 1a, the sample S1 has a good shape of nanospheres. But the sizes of these nanospheres are not even. The diameters range from 10 nm to 70 nm. In order to narrow the size distribution, a fast cooling processing was adopted by pouring the reaction mixture into cooled toluene (-6 °C). Because the melting point of bismuth is 271 °C,27 the bismuth produced in the reaction was in liquid state at the reaction temperature of 315 °C. A fast cooling rate of the temperature by pouring the high temperature solution containing bismuth liquid to the cooled toluene may result in a relatively narrow size distribution. Figure 1b shows the SEM images of the bismuth NSs obtained by this fast cooling process. It is clear that the size distribution of obtained bismuth was improved. The diameter of these nanospheres varied from 30 nm to 60 nm. Figure 2 gives the TEM images and size distribution to confirm the good shape and relatively narrow distribution of spherical bismuth NSs. The selected area electron diffraction (SAED) patterns shown in the inset of Figure 2a and 2b indicates that the bismuth NSs were highly crystalline.

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Figure 1. SEM images of bismuth NSs. (a) sample S1 prepared by natural cooling process, (b) sample S2 prepared by quenching with cooled toluene (~ -6 °C) (i.e., scale bar = 100 nm).

18

(c) Distribution (%)

18

Distribution (%)

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15 12 9 6

(d)

12 9 6 3

3 0

15

0 0 10 20 30 40 50 60 70 80 90 100

Diameter (nm)

0 10 20 30 40 50 60 70 80 90 100 Diameter (nm)

Figure 2. TEM images of (a) sample S1 prepared by natural cooling process, (b) sample S2 prepared by quenching with cooled toluene (i.e., scale bar = 100 nm). Inset: SAED pattern taken from the sample S1 and S2. (c, d) the corresponding size distributions of the sample S1 and S2.

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To further identify the purity and crystallinity of the prepared bismuth NSs, the powder X-ray diffraction (XRD) measurements of the sample S1 and S2 were carried out. As shown in Figure 3, It can be seen that all of the XRD patterns can be readily indexed to a rhombohedral phase [space group: R3m (166)] of elemental Bi (JCPDS no 85-1329) without any other impurity phases identified and are in good agreement with the SAED patterns. These data indicate that the preparation of pure bismuth NSs can be achieved by the thermolysis of bismuth acetate in oleylamine effectively. It also illustrates that the fast cooling process by using cooled toluene does not change the crystallinity of obtained bismuth NSs.

(012)

Intensity (Counts)

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(202) (110) (113) (104) (015)

(003) (101)

(107) (122) (024) (116) S2

S1 20

30

40

50

60

70

2Theta (°) Figure 3. XRD diffraction patterns of the bismuth NSs.

3.2 Formation Mechanism In order to understand the formation mechanism of bismuth NSs, FT-IR spectroscopy was used to investigate the chemical composition of the surface of the bismuth NSs. Curve a in Figure 4 gives the spectrum of oleylamine which is in agreement with the previous reports.43-44 For example, The 9



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characteristic bands of terminal methyl asymmetric inplane stretching νas(CH3, ip) and methyl asymmetric C-H stretching νas(CH2) appeared at 2922 cm-1 and 2852 cm-1. The band around 1629 cm-1 was assigned to the combined motion of NH2 scissoring and N-H bending. Curve b in Figure 4 is the spectrum of bismuth NSs which is same to that of oleylamine. So, it is reasonable to consider that bismuth NSs were capped by oleylamine. It is reported the absorption band position of νas(CH3, ip) and νas(CH2) provide insight into the local molecular environment of the alkyl chains on the surface of metal nanoparticles.43, 45 In the spectrum of oleylamine, νas(CH3, ip) and νas(CH2) located at 2922 cm-1 and 2852 cm-1, respectively. While, both of them shifted to lower wavelength, i.e. νas(CH3, ip) = 2919 cm-1 and νas(CH2) = 2850 cm-1, as shown in the inset of Figure 4. These small shift might be due to the constraint of the molecular motions caused by the formation of a close-packed oleylamine layer on the surface of bismuth NSs. This phenomenon has also been reported in the dodecanethiol-capped Ag nanocrystals46 and oleylamine–capped Ag nanoparticles.43 It is known that oleylamine can used as stabilizer and reducing agent for the synthesis of nanoparticles.47 In our developed method, bismuth ions might act as an oxidizing agent and reduced by oleylamine to form liquid bismuth “nanoparticles” due to the high temperature used. These liquid bismuth “nanoparticles” could aggregate to form nanospheres. That is the reason why the fast cooling process can improve the size distribution. Chen et al43 suggested that the oxidation of oleylamine produced nitriles and imines which passivated the Ag nanoparticle surface, but our results of FT-IR spectra illustrate that bismuth NSs were mainly capped by oleylamine.

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a

Transmittance

b

2700

2800 2900 3000 3100 -1 Wavenumber (cm )

a

b

500

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumbers (cm ) Figure 4. FT-IR spectra of (a) oleylamine and (b) the sample S2 5

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Extinction Efficiency

S1 S2 Bismuth acetate

4

4

3

3

2

2 120 nm 100 nm 80 nm 60 nm 40 nm 20 nm

1

oleylamine 0 400

600

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1400

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Scattering Efficiency

B

A

Absorbance

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0

300 400 500 600 700 800 900 300 400 500 600 700 800 900

Wavelength (nm)

Wavelength (nm)

Figure 5. (A) The optical absorption spectra of bismuth NSs, comparing with bismuth acetate and oleylamine in atmosphere at room temperature. (B) Extinction efficiencies (left panel)and scattering efficiencies (right panel) for bismuth NSs with diameters 20, 40, 60, 80,100 and 120 nm dispersed in ethanol.

3.3 Optical properties of bismuth NSs To investigate the optical properties of the prepared bismuth NSs, optical absorption measurement were first performed in atmosphere. The results are shown in Figure 5A. For the both samples S1 and S2, an absorption band appeared at 265 nm in the ultraviolet. In the visible wavelength range, the spectra 11



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are featured by an absorption band located at 425 nm and another absorption band around 575 nm, forming a camel-like structure. As mentioned in the introduction section, the small absorption peak at 265 nm is generally regarded as the surface plasmon resonance band (sprb) of small bismuth nanoparticles (less than 20 nm) in the sample. While, Velasco-Arias et al illustrated that this absorption peak was caused by Bi(III) species in the colloids.39 So the absorption spectra of the starting materials, bismuth acetate and oleylamine, were measured and given in Figure 5A. It is clear that bismuth acetate has an absorption band in the range of 213 nm to 316 nm, which indicates that the peak at 265 nm was caused by the presence of Bi (III) species. The peaks located at 425 nm and 575 nm are seldom reported due to previous works mainly focusing on the bismuth nanoparticles less than 20 nm. Recently, J. McMahon et al. simulated the localized surface plasmon resonances (lspr) of spherical bismuth nanoparticles in vacuum as a function of the nanoparticles size by using Mie theory.41 They found that the lspr red shifted with the increase of the size. When the size of bismuth nanoparticles was around 200 nm, the camel-like shape of the absorption bands located at ~360 nm and ~700 nm was observed.41 Obviously, the size of 200 nm is much larger than ours. However, the simulation was performed under a condition of vacuum and our spectra were measured in the solvent of ethanol. Toudert et al theoretically illustrated that the sprb of nano-bismuth was sensitive to the dielectric function of the surrounding medium.40 The resonance wavelength red shifted with the increase of dielectric constant. So we calculated the extinction efficiencies based on Mie theory by considering the dielectric constant of the surrounding medium , as shown in Figure 5B. It is clear that the extinction band red shifted with the increase of the nanosphere size. When the size reached 100 nm, a camel-like extinction bands appeared. The scattering efficiencies were also calculated. It can be seen that the contribution of scattering to the extinction band increases with the size and become more and more significant. So we believe that the 12


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surface plasmon resonance and light scattering both contribute to the observed camel-like absorption bands in our case. 3.4 Photocatalytic behavior of bismuth NSs

1.0 A

1.0

0h 0.5 h 1.0 h 1.5 h 2.0 h 2.5 h 3.5 h 5.5 h 7.5 h 9.5 h

0.8 0.6 0.4

B 0.8

Degrading rate

Normalized Absorbance

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0.2 0.0 450

0.6 0.4

In the absence of Bi NSs In the presence of Bi NSs

0.2 0.0

500

550

600

650

0

2

Wavelength(nm)

4

6

8

10

Time(h)

Figure 6. (A) Absorption spectra of RhB after exposure to visible light irradiation as a function of reaction time, (B) The degrading rate of blank RhB solution and RhB solution with bismuth NSs (S2) as a function of irradiation

The photocatalytic properties of bismuth nanospheres were evaluated by the photo degradation of RhB solution under visible irradiation. The degrading rate (D) of RhB can be expressed by the following equation,

A −A  D =  0 × 100%   A0 

(1)

where A0 is the absorbance of RhB solution measured at a wavelength of 554 nm before irradiation and A is the absorbance after the irradiation. The results of these photocatalytic evaluations are represented in Figure 6A, which shows the absorbance curves of RhB solution in the presence of bismuth NSs as a function of irradiation time. It is clear that the absorbance peak intensity of RhB solution decreased gradually under irradiation. Figure 6B also shows the degrading rate of RhB as a function of irradiation time in the absence and presence of bismuth NSs. In the absence of bismuth NSs, the degrading rate 13



increased slowly and only reached 18% after 9.5 h irradiation. While in the presence of bismuth NSs, the degrading rate increased quickly in the first 1 hour, then maintained a relatively slow rate and reached 92% after 9.5 h irradiation. This result illustrates that the prepared bismuth NSs shows a good catalytic property to the photodegradation of RhB due to the surface plasmon band of bismuth nanospheres shifted to the visible wavelength range. In order to confirm the observed photocatalytic properties of bismuth NSs, the XRD measurement was carried out after photocatalysis. The results are shown in Figure 7. It can be seen that no other diffraction peaks appeared. It is well known that bismuth metal is easy to be oxidized. So the good stability of bismuth NSs in our case is due to the closed-packed oleylamine layer as indicated by the FT-IR spectra.

(012)

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(202) (110) (113) (104) (015)

(003) (101)

(107) (122) (024) (116)

a

b 20

30

40

50

60

70

2Theta (°) Figure 7. XRD diffraction patterns of the bismuth NSs before (a) and after (b) 9.5 h irradiation.

4. Conclusions A facile thermolysis route has been successfully developed for the preparation of bismuth NSs 14


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with a diameter in the range of 30 nm to 60 nm with high yield. The optical properties of prepared bismuth NSs were characterized. The bismuth NSs with such size showed two absorption peaks at 425 nm and 575 nm, forming a camel-like spectral shape. The results of Mie calculation suggests that both the surface plasmon resonance and light scattering contributed to the observed camel-like absorption bands. The photocatalytic degradation experiments showed that the synthesized bismuth NSs have a good capacity to photo-degrading RhB under visible light irradiation.

Acknowledgments This work was supported by State Key Basic Research Program of China (Grant No. 61176011), National Basic Research Project (Grant No. 2013CB922301), KLIFMD-2011-06, Shanghai Pujiang Program (Grant No. 11PJ1403000, 11PJ1402900), Innovation Program of Shanghai Municipal Education Commission (Grant No. 12ZZ041), NCET-11-0143 and PCSIRT.

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Investigation of Optical and Photocatalytic Properties of Bismuth

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