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Picosecond laser induced fragmentation of coarse Cu2O particles into nanoparticles in liquid media
Accepted Manuscript Title: Picosecond laser induced fragmentation of coarse Cu2 O particles into nanoparticles in liquid media Author: Mokhtar Ali Nagarjuna Remalli Fahem Yehya Anil Kumar Chaudhary Vadali V.S.S. Srikanth PII: DOI: Reference:
S0169-4332(15)02439-3 http://dx.doi.org/doi:10.1016/j.apsusc.2015.10.041 APSUSC 31522
To appear in:
APSUSC
Received date: Revised date: Accepted date:
6-6-2015 1-10-2015 6-10-2015
Please cite this article as: M. Ali, N. Remalli, F. Yehya, A.K. Chaudhary, V.V.S.S. Srikanth, Picosecond laser induced fragmentation of coarse Cu2 O particles into nanoparticles in liquid media, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.10.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
HIGHLIGHTS Coarse Cu2O particles are fragmented into nanoparticles in liquid media using
ip t
picosecond laser. Fragmented particles retained the crystal structure.
cr
Lattice strain in fragmented particles in ethanol is more than in those fragmented in
us
water.
d
M
an
Laser fragmentation in liquid media allows continuous production of oxide nanoparticles.
te
Picosecond laser induced fragmentation of coarse Cu2O particles into nanoparticles in
Ac ce p
liquid media
Mokhtar Alia, Nagarjuna Remallia, Fahem Yehyab,c, Anil Kumar Chaudharyb,§, Vadali V. S. S. Srikantha,* a
School of Engineering Sciences and Technology (SEST), University of Hyderabad, Hyderabad
500046, India
§
Author
to
whom
correspondence
should
be
addressed.
Electronic
mail:
[email protected]; Phone No.: +91 40 23138807 *
Author to whom correspondence should be addressed. Electronic mail: [email protected];
Phone No.: +91 40 23134453 1 Page 1 of 15
b
Advanced Centre of Research in High Energy Materials (ACRHEM), University of Hyderabad,
Hyderabad 500046, India c
Department of Mathematics and Physics, Faculty of Education, Al-Baida University, Al-Baida
ip t
38018, Yemen Abstract
cr
Micron sized cuprous oxide (Cu2O) particles are easily fragmented into nanosized (5-10 nm)
us
particles using picosecond (ps) laser (wavelength = 532 nm) pulses. The coarse Cu2O particles are first synthesized by reducing copper chloride with the aid of honey. These particles are then
an
dispersed in liquid media (double distilled water or ethanol) and exposed to ps laser pulses to obtain well-dispersed nanosized Cu2O particles. By using this method of fragmentation,
M
morphology of the particles can be altered while retaining their crystal structure. The innate
d
nature of this method allows continuous production of nanoparticles from coarser particles.
1. Introduction
te
Keywords: picosecond laser; fragmentation; cuprous oxide; nanoparticles.
Ac ce p
Cuprous oxide (Cu2O) is considered as an important material system owing to its usefulness in diverse applications [1-5]. Similar to other nanomaterials, nanosized Cu2O is expected to have unique chemical and physical properties and therefore outperform its bulk counterpart in applications. However, synthesis of Cu2O and especially nanosized Cu2O is not straight forward [1,6]. In this context, Cu2O nanoparticles are often produced by laser induced breakdown at solid Cu-liquid (such as water) interface [7-9]. However, the final product is often not Cu2O alone but a composite of Cu, CuO and Cu2O phases. The oxide phases in the product could be controlled by varying the laser intensity (oxidation rate increases with laser intensity), adjusting the laser focus (defocused laser beam at the interface results in Cu2O whilst the focused beam results in
2 Page 2 of 15
CuO) and controlling the pH of the liquid medium (greater the basic nature of the medium greater is the oxidation of Cu). In order to overcome the difficulties in obtaining bulk amounts of phase pure Cu2O nanoparticles, in this work, easy fragmentation of micron sized Cu2O particles
ip t
into nanosized Cu2O particles using picosecond (ps) laser pulses in water and ethanol media is elucidated. The discussion includes the role of medium in the final outcome, the influence of
cr
irradiation time and the structural evolution of the fragmented particles. This work paves a way
us
to use laser fragmentation as an inexpensive method for continuous production of large volumes of other useful nanomaterials, especially oxides.
an
2. Experimental work
Synthesis procedure used to obtain coarse polycrystalline Cu2O particles is similar to our
M
previously reported work [6]. The synthesis procedure is briefly discussed here. Three separate
d
solutions were prepared by dissolving ~5 ml of edible honey, ~3.41 g of copper chloride
te
dehydrate (CuCl2·2H2O) and ~4 g of sodium hydroxide (NaOH) pellets in 100 ml of distilled water each. The three solutions were then mixed together and vigorously stirred for 1 h under
Ac ce p
room conditions until a homogeneous resultant reddish brown solution as shown in Fig. 1(a) was obtained. The solution was then allowed to reflux for 1 h leading to precipitation. The resultant precipitate was collected, washed using distilled water and then ethanol to remove any supernatant and finally dried at 120 °C for 2 h to obtain Cu2O particles. 1 g of these particles was then uniformly dispersed in 1 l of double distilled water using ultrasonication for 20 min. 80 ml of this solution was used in each laser fragmentation experiment. Picosecond laser (model EKSPLA- PL-2250; 532 nm, 10 Hz, 30 ps) was used for the fragmentation experiments. Laser energy of ~30.9 mJ (90% of the total power) was used for laser irradiation which was carried out for 1 h. The choice of 532 nm laser was owing to the
3 Page 3 of 15
anticipation that the material under consideration would exhibit a very strong absorption in the visible region. On the other hand, the use of ultrashort pulses (pico or femto seconds) helps in avoiding rapid heating and the associated violent surface material evaporation when the laser
ip t
interacts with a solid material. The use of ultrashort pulses minimizes the process of thermal conduction to the surroundings and in turn helps in the production of small sized particles
cr
through fragmentation. The maximum fluence of the laser used in this work (for the laser energy
us
of 38.8 mJ) was ~146.6x103 J/cm2. However, a lower fluence would be more desirable because it can minimize thermal effects and material damage, if any. On the other hand, too low fluence
an
may not aid in any local melting of the material when the laser interacts with the material. The point of focus of the laser was adjusted slightly below the surface of water in order to
M
avoid any refraction effect which reduces the penetration depth of the laser in the solution and
d
normally decreases the fragmentation efficiency. Additionally, during the laser irradiation, the
te
solution was under stirring to maximize the movement of the particles, which is necessary to enhance the efficiency of fragmentation. Figure 1(b) shows the photograph of the solution
Ac ce p
dispersed with fragmented (i.e., after laser interaction) Cu2O particles. A change in color of the solution is the first indication of fragmentation. Ethanol was also used as the medium in place of distilled water and the above mentioned laser fragmentation experiment was carried out.
4 Page 4 of 15
Figure 1. Photographs of Cu2O solution (a) before and (b) after its exposure to ps laser pulses. Morphological study of the samples was carried out using transmission electron microscope
ip t
(TEM) (model FEI Technai G2 S-Twin). Crystal structure of the samples was studied by x-ray diffraction (XRD) and electron diffraction techniques. The diffraction patterns were recorded
cr
using a Cu Kα line (λ=1.54 Å) in 2θ range of 20-80° (where θ is the angle of diffraction).
us
Bruker’s AXS Model D8 Advance System was used to carry out the XRD experiments. 3. Results and discussion
an
Figure 2 shows the x-ray diffractogram of as-synthesized coarse Cu2O particles. The diffraction peak positions at 29.75, 36.74, 42.63, 61.89, 73.82 and 78.48° are indexed to (110),
M
(111), (200), (220), (311), and (222) crystal planes, respectively of face centered cubic (FCC) Cu2O. The inset of Fig. 2 shows the plane view secondary electron micrograph of the coarse
d
Cu2O particles. It can be clearly observed from the micrograph that the Cu2O particles have
te
spherical morphology. The Cu2O particles are observed to have sizes in the micron and sub-
Ac ce p
micron range which is expected in the case of chemical synthesis methods. Figure 3 shows the x-ray diffractogram of fragmented Cu2O particles. The diffraction peaks are indexed to FCC Cu2O like in the case of as-synthesized particles. This clearly indicates that the fragmented particles have retained the original crystal structure. However, in this case the diffraction peaks are broarder (in comparison to as-synthesized Cu2O particles’ case) indicating a decrease in the average crystallite size.
5 Page 5 of 15
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Figure 2. X-ray diffractogram of as-prepared coarse Cu2O particles. Inset shows the secondary
Ac ce p
te
d
M
an
electron micrograph of the Cu2O particles.
Figure 3. X-ray diffractogram of Cu2O particles obtained after 1 h of laser (energy of ~30.9 mJ) fragmentation in water.
Transmission electron micrograph and the corresponding electron diffraction pattern of the fragmented Cu2O particles are shown in Fig. 4. A continuous network of uniformly sized (~7 nm) nanoparticles can be clearly observed from Fig. 4(a). The electron diffraction pattern (Fig. 4(b)), which is a ring pattern indicates polycrystalline nature of the fragmented particles. The diffraction rings in the pattern are indexed to the crystal planes of FCC Cu2O. However, the most important part of this work is concerning the morphology and crystallinity of the fragmented 6 Page 6 of 15
Cu2O particles (in distilled water and ethanol) when the laser irradiation was for 30 min for the same laser energy of 30.9 mJ. These experiments have also helped in understanding the fragmentation mechanism. TEM micrographs and the corresponding electron diffraction patterns
d
M
an
us
cr
ip t
of the fragmented Cu2O particles obtained in these experiments are shown in Fig. 5.
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Figure 4. (a) Transmission electron micrograph and (b) the corresponding electron diffraction
Ac ce p
pattern of the Cu2O particles obtained after 1 h of ps laser (energy of ~30.9 mJ) fragmentation in water.
The micrographs (Figs. 5(a) and (c)) show that there is a mixed morphology of fragmented spherical particles and nanowires in both distilled water and ethanol cases. In the case of water as the medium (i.e., in Fig. 5(a)), it can be clearly observed that the fiber-like features are constituted by smaller features which indicates that the fragmentation of the fibers has already begun in 30 min. In the case of water as the medium, all the diffraction rings (Fig. 5(b)) are indexed to the crystal planes of FCC Cu2O. This result is similar to the one discussed in the case of the fragmentation in water for 1 h. However, in the present case, the intensity of diffraction
7 Page 7 of 15
rings is faint (in comparison to those observed in Fig. 4(b)) indicating a difference in nature of crystallites diffracting the electrons. This is definitely supported by the mixed morphology (Fig. 5(a)) observed in this case. In the case of fragmentation in ethanol, three diffraction rings (Fig.
ip t
5(d)) are indexed to monoclinic CuO whilst rest are indexed to FCC Cu2O. However, x-ray diffraction peaks (Fig. 6) corresponding to CuO could not be identified most probably owing to
Ac ce p
te
d
M
an
us
cr
the presence of only very small volume percentage of CuO in the sample.
Figure 5. (a) Transmission electron micrograph and (b) the corresponding electron diffraction pattern of the fragmented Cu2O particles in water; (c) Transmission electron micrograph and (d) the corresponding electron diffraction pattern of the fragmented Cu2O particles in ethanol; The 8 Page 8 of 15
time of ps laser (energy of ~30.9 mJ) irradiation is 30 min. Figure 6 shows the x-ray diffractograms of fragmented Cu2O particles in distilled water and ethanol. The diffraction peaks in both water and ethanol cases are indexed to FCC Cu2O.
ip t
Surprisingly, the diffraction peaks are sharper and intense in the case of fragmented Cu2O particles in both water and ethanol media. In order to analyze this aspect, the highest intensity
cr
(111) diffraction peak (Fig. 7(a)) of as-synthesized Cu2O particles is compared with that of
us
fragmented Cu2O particles (Figs. 7(b) and (c)). It is evident from Fig. 6 and Fig. 7 that the (111) diffraction peaks of the fragmented Cu2O particles are sharper and intense in comparison that of
an
as-synthesized Cu2O particles. This clearly shows that the crystallinity of the fragmented particles is better in comparison to the as-synthesized particles. In Fig. 7(a) symmetric peak
M
broadening is observed, whereas in Fig. 7(b) and Fig. 7(c) asymmetric peak broadening is
d
observed. All the diffraction peaks in Figs. 7(b) and (c) are shifted towards left and right with
te
respect to the peaks in Fig. 7(a), respectively. This is due to the tensile and compressive residual
Ac ce p
stresses induced in fragmented particles in water and ethanol, respectively.
9 Page 9 of 15
Figure 6. XRD patterns of Cu2O nanoparticles produced by ps laser fragmentation in (a) water
an
us
cr
ip t
and (b) ethanol. The time of ps laser (energy of ~30.9 mJ) irradiation is 30 min.
M
Figure 7. (111) diffraction peaks of (a) as-synthesized Cu2O particles (b) Cu2O particles fragmented in distilled water and (c) Cu2O particles fragmented in ethanol. The time of ps laser
te
d
(energy of ~30.9 mJ) irradiation is 30 min.
Modified Williamson-Hall (W-H) method [10-13] was applied on the x-ray diffraction data
Ac ce p
(Fig. 6) to further understand the crystallinity of the samples after fragmentation. Modified W-H method takes into account the contribution of lattice strain in broadening the x-ray diffraction peaks. The average crystallite sizes as determined by modified W-H method are 9 and 12 nm for Cu2O particles fragmented in water and ethanol (30 min laser irradiation), respectively. However, it should be noted that in the case of fragmentation in ethanol, the peak broadening is more which is a clear indication that the strain in the fragmented particles in ethanol is more in comparison to the strain in particles fragmented in water. Please see Supplementary Data for more details. The electron micrographs of Cu2O particles fragmented at different experimental conditions
10 Page 10 of 15
clearly indicate a two-step fragmentation mechanism. It is apparent that nanowires are first formed followed by their fragmentation into small particles as the laser irradiation continued. The results also show that fragmentation process is faster in water than in ethanol. This is due to
ip t
the difference in physical, optical and chemical properties of ethanol and water [14,15]. Based on the above presented morphological and crystallinity data of the fragmented Cu2O particles and
cr
the available literature about the fragmentation mechanism of semiconducting particles [16-20],
us
the following fragmentation mechanism for Cu2O is proposed. Under the influence of focused ultra-speed laser pulses like ps laser pulses, the dynamic semiconducting particle’s surface in a
an
liquid medium experiences (when the focused laser spot is almost equal to the particle size) a great increase in temperature and pressure. At the beginning, the randomly moving (due to
M
stirring) micron sized Cu2O particles dispersed in distilled water are exposed to the laser
d
irradiation pulses for 30 min. The repetition rate of the ps laser pulses was maintained at 10 Hz
te
which means that the time between two pulses is 100 ms which is smaller than the particle rotation time. Therefore, when ps laser pulses irradiate a particle in liquid medium, there is a
Ac ce p
high probability to expose the same particle to several laser pulses. As a result, the rotation of the particles allows the laser pulses to partially remove longer and uniform part of the irradiated coarser particle which form nanowires in the initial step. The length of nanowires depends on the dynamic motion of particles in the solution. i.e., the time consumed to fragment a particle is considered when the particle crosses the focused laser spot. However, the diameter of the nanowires depends on the laser energy and pulse duration. The electron micrographs clearly reveal that some part of particles are totally fragmented while some are either partially fragmented or remain intact with their initial morphologies. Figs. 5(a) and (c) clearly show the presence of mixed morphology. In the next step, the nanowires are subdivided to nanoparticles
11 Page 11 of 15
due to long irradiation time. The results of two different types of nanoparticles in water and ethanol can be explained using dielectric nature of the solvent. The dielectric constant of water is high (~80.4) whereas the dielectric constant of ethanol is low (~24.3). Therefore, the
ip t
nanoparticles produced by laser pulses experience some sort of a strong force due to dielectric nature of water and follow faster fragmentation. However, in case of ethanol, the dielectric force
cr
exerted on the Cu2O particle is quite small which slows down the fragmentation process.
us
Ethanol’s refractive index is higher than that of water and therefore, the intensity of laser decreases more in the case of ethanol as compared to water. Besides, the density and boiling
an
point of ethanol is equal to 0.78 of that of water. All in all, in the first step, the laser fragments small portion from the large particle into nanowires and in the next step, under continuous laser
M
irradiation, the nanowires is fragmented to ultra-fine particles. Figure 5(c) shows the production
d
of nanowires along with small percentage of nanoparticles. The dominating % of nanowires in
te
the case of ethanol as the medium determines that the fragmentation process in this case needs more time to produce nanoparticles.
Ac ce p
4. Conclusions
ps laser was used to fragment coarse Cu2O particles in water and ethanol media into nanosized particles. In the case of water as the medium, the fragmented particles have retained the original FCC crystal structure. On the other hand, in the case of ethanol as the medium, CuO phase has also formed along with Cu2O. The fragmentation is found to involve two steps. In the first step, nanowires form, which are subsequently fragmented under continuous laser irradiation. References [1] A. Lossin, F.J. Westhoff, The production and application of cuprous oxide and cupric hydroxide, JOM-J. MIN. MET. MATS. 49 (1997) 38–39.
12 Page 12 of 15
[2] L. Hanke, D. Fröhlich, A.L. Ivanov, P.B. Littlewood, H. Stolz, LA phonoritons in Cu2O, Phys. Rev. Lett. 83 (1999) 4365–4368. [3] J. Brandt, D. Fröhlich, C. Sandfort, M. Bayer, H. Stolz, N. Naka, Ultranarrow optical
ip t
absorption and two-phonon excitation spectroscopy of Cu2O paraexcitons in a high magnetic field, Phys. Rev. Lett. 99 (2007) 217403.
cr
[4] O. Roslyak, U. Aparajita, J.L. Birman, S. Mukamel, Coherent manipulation of quadrupole
us
biexcitons in cuprous oxide by 2D femtosecond spectroscopy, Phys. Status Solidi B 249 (2012) 435–447.
an
[5] Y. Wang, F. Yang, H.X. Zhang, X.Y. Zi, X.H. Pan, F. Chen, W.-D. Luo, J.-X. Li, H.-Y. Zhu, Y.-P. Hu, Cuprous oxide nanoparticles inhibit the growth and metastasis of melanoma by
M
targeting mitochondria, Cell Death Dis. 4 (2013) e783; doi: 10.1038/cddis.2013.314.
d
[6] Mokhtar Ali, N.K. Rotte, V.V.S.S. Srikanth, Honey aided solution synthesis of
te
polycrystalline Cu2O particles, Mater. Lett. 128 (2014) 253–255. [7] A. Nath, A. Khare, Size induced structural modifications in copper oxide nanoparticles
Ac ce p
synthesized via laser ablation in liquids, J. Appl. Phys. 110 (2011) 043111. [8] J.M.J. Santillán, F.A. Videla, M.B. Fernández van Raap, D.C. Schinca, L.B. Scaffardi, Analysis of the structure, configuration, and sizing of Cu and Cu oxide nanoparticles generated by fs laser ablation of solid target in liquids, J. Appl. Phys. 113 (2013) 134305. [9] M.A. Gondal, T.F. Qahtan, M.A. Dastageer, Y.W. Maganda, D.H. Anjum, Synthesis of Cu/Cu2O nanoparticles by laser ablation in deionized water and their annealing transformation into CuO nanoparticles, J. Nanosci. Nanotechnol. 13 (2013) 5759–5766. [10] T. Ungár, G. Tichy, The Effect of dislocation contrast on x-ray line profiles in untextured polycrystals, Phys. Status Solidi A 171 (1999) 425–434.
13 Page 13 of 15
[11] T. Ungár, A. Revesz, A. Borbely, Dislocations and grain size in electrodeposited nanocrystalline Ni determined by the modified Williamson-Hall and Warren-Averbach procedures, J. Appl. Cryst. 31 (1998) 554–558.
ip t
[12] T. Ungár, I. Dragomir, A. Revesz, A. Borbely, The contrast factors of dislocations in cubic crystals: the dislocation model of strain anisotropy in practice, J. Appl. Cryst. 32 (1999) 992–
cr
1002.
us
[13] T. Ungár, J. Gubicza, G. Ribarik, A. Borbely, Crystallite size distribution and dislocation structure determined by diffraction profile analysis: principles and practical application to cubic
an
and hexagonal crystals, J. Appl. Cryst. 34 (2001) 298–310.
[14] M.H. Mahdieh, B. Fattahi, Size properties of colloidal nanoparticles produced by
M
nanosecond pulsed laser ablation and studying the effects of liquid medium and laser fluence,
d
Appl. Surf. Sci. 329 (2015) 47–57.
te
[15] V.S. Burakov, N.V. Tarasenko, A.V. Butsen, Laser-induced plasmas in liquids for nanoparticle synthesis, J. Appl. Spectrosc. 77 (2010) 386–393.
Ac ce p
[16] A.M. Morales, C.M. Lieber, A laser ablation method for the synthesis of crystalline semiconductor nanowires, Science 279 (1998) 208–211. [17] X.M. Meng, J.Q. Hu, Y. Jiang, C.S. Lee, S.T. Lee, Boron nanowires synthesized by laser ablation at high temperature, Chem. Phys. Lett. 370 (2003) 825–828. [18] C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Inorganic nanowires, Prog. Solid State Ch. 31 (2003) 5–147. [19] Y. Zhang, H. Ago, M. Yumura, S. Ohshima, K. Uchida, T. Komatsu, S. Iijima, Study of the growth of boron nanowires synthesized by laser ablation, Chem. Phys. Lett. 385 (2004) 177–183.
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[20] Kenry, C.T. Lim, Synthesis, optical properties, and chemical–biological sensing applications of one-dimensional inorganic semiconductor nanowires, Prog. Mater. Sci. 58 (2013)
Ac ce p
te
d
M
an
us
cr
ip t
705–748.
15 Page 15 of 15
S0169-4332(15)02439-3 http://dx.doi.org/doi:10.1016/j.apsusc.2015.10.041 APSUSC 31522
To appear in:
APSUSC
Received date: Revised date: Accepted date:
6-6-2015 1-10-2015 6-10-2015
Please cite this article as: M. Ali, N. Remalli, F. Yehya, A.K. Chaudhary, V.V.S.S. Srikanth, Picosecond laser induced fragmentation of coarse Cu2 O particles into nanoparticles in liquid media, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.10.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
HIGHLIGHTS Coarse Cu2O particles are fragmented into nanoparticles in liquid media using
ip t
picosecond laser. Fragmented particles retained the crystal structure.
cr
Lattice strain in fragmented particles in ethanol is more than in those fragmented in
us
water.
d
M
an
Laser fragmentation in liquid media allows continuous production of oxide nanoparticles.
te
Picosecond laser induced fragmentation of coarse Cu2O particles into nanoparticles in
Ac ce p
liquid media
Mokhtar Alia, Nagarjuna Remallia, Fahem Yehyab,c, Anil Kumar Chaudharyb,§, Vadali V. S. S. Srikantha,* a
School of Engineering Sciences and Technology (SEST), University of Hyderabad, Hyderabad
500046, India
§
Author
to
whom
correspondence
should
be
addressed.
Electronic
mail:
[email protected]; Phone No.: +91 40 23138807 *
Author to whom correspondence should be addressed. Electronic mail: [email protected];
Phone No.: +91 40 23134453 1 Page 1 of 15
b
Advanced Centre of Research in High Energy Materials (ACRHEM), University of Hyderabad,
Hyderabad 500046, India c
Department of Mathematics and Physics, Faculty of Education, Al-Baida University, Al-Baida
ip t
38018, Yemen Abstract
cr
Micron sized cuprous oxide (Cu2O) particles are easily fragmented into nanosized (5-10 nm)
us
particles using picosecond (ps) laser (wavelength = 532 nm) pulses. The coarse Cu2O particles are first synthesized by reducing copper chloride with the aid of honey. These particles are then
an
dispersed in liquid media (double distilled water or ethanol) and exposed to ps laser pulses to obtain well-dispersed nanosized Cu2O particles. By using this method of fragmentation,
M
morphology of the particles can be altered while retaining their crystal structure. The innate
d
nature of this method allows continuous production of nanoparticles from coarser particles.
1. Introduction
te
Keywords: picosecond laser; fragmentation; cuprous oxide; nanoparticles.
Ac ce p
Cuprous oxide (Cu2O) is considered as an important material system owing to its usefulness in diverse applications [1-5]. Similar to other nanomaterials, nanosized Cu2O is expected to have unique chemical and physical properties and therefore outperform its bulk counterpart in applications. However, synthesis of Cu2O and especially nanosized Cu2O is not straight forward [1,6]. In this context, Cu2O nanoparticles are often produced by laser induced breakdown at solid Cu-liquid (such as water) interface [7-9]. However, the final product is often not Cu2O alone but a composite of Cu, CuO and Cu2O phases. The oxide phases in the product could be controlled by varying the laser intensity (oxidation rate increases with laser intensity), adjusting the laser focus (defocused laser beam at the interface results in Cu2O whilst the focused beam results in
2 Page 2 of 15
CuO) and controlling the pH of the liquid medium (greater the basic nature of the medium greater is the oxidation of Cu). In order to overcome the difficulties in obtaining bulk amounts of phase pure Cu2O nanoparticles, in this work, easy fragmentation of micron sized Cu2O particles
ip t
into nanosized Cu2O particles using picosecond (ps) laser pulses in water and ethanol media is elucidated. The discussion includes the role of medium in the final outcome, the influence of
cr
irradiation time and the structural evolution of the fragmented particles. This work paves a way
us
to use laser fragmentation as an inexpensive method for continuous production of large volumes of other useful nanomaterials, especially oxides.
an
2. Experimental work
Synthesis procedure used to obtain coarse polycrystalline Cu2O particles is similar to our
M
previously reported work [6]. The synthesis procedure is briefly discussed here. Three separate
d
solutions were prepared by dissolving ~5 ml of edible honey, ~3.41 g of copper chloride
te
dehydrate (CuCl2·2H2O) and ~4 g of sodium hydroxide (NaOH) pellets in 100 ml of distilled water each. The three solutions were then mixed together and vigorously stirred for 1 h under
Ac ce p
room conditions until a homogeneous resultant reddish brown solution as shown in Fig. 1(a) was obtained. The solution was then allowed to reflux for 1 h leading to precipitation. The resultant precipitate was collected, washed using distilled water and then ethanol to remove any supernatant and finally dried at 120 °C for 2 h to obtain Cu2O particles. 1 g of these particles was then uniformly dispersed in 1 l of double distilled water using ultrasonication for 20 min. 80 ml of this solution was used in each laser fragmentation experiment. Picosecond laser (model EKSPLA- PL-2250; 532 nm, 10 Hz, 30 ps) was used for the fragmentation experiments. Laser energy of ~30.9 mJ (90% of the total power) was used for laser irradiation which was carried out for 1 h. The choice of 532 nm laser was owing to the
3 Page 3 of 15
anticipation that the material under consideration would exhibit a very strong absorption in the visible region. On the other hand, the use of ultrashort pulses (pico or femto seconds) helps in avoiding rapid heating and the associated violent surface material evaporation when the laser
ip t
interacts with a solid material. The use of ultrashort pulses minimizes the process of thermal conduction to the surroundings and in turn helps in the production of small sized particles
cr
through fragmentation. The maximum fluence of the laser used in this work (for the laser energy
us
of 38.8 mJ) was ~146.6x103 J/cm2. However, a lower fluence would be more desirable because it can minimize thermal effects and material damage, if any. On the other hand, too low fluence
an
may not aid in any local melting of the material when the laser interacts with the material. The point of focus of the laser was adjusted slightly below the surface of water in order to
M
avoid any refraction effect which reduces the penetration depth of the laser in the solution and
d
normally decreases the fragmentation efficiency. Additionally, during the laser irradiation, the
te
solution was under stirring to maximize the movement of the particles, which is necessary to enhance the efficiency of fragmentation. Figure 1(b) shows the photograph of the solution
Ac ce p
dispersed with fragmented (i.e., after laser interaction) Cu2O particles. A change in color of the solution is the first indication of fragmentation. Ethanol was also used as the medium in place of distilled water and the above mentioned laser fragmentation experiment was carried out.
4 Page 4 of 15
Figure 1. Photographs of Cu2O solution (a) before and (b) after its exposure to ps laser pulses. Morphological study of the samples was carried out using transmission electron microscope
ip t
(TEM) (model FEI Technai G2 S-Twin). Crystal structure of the samples was studied by x-ray diffraction (XRD) and electron diffraction techniques. The diffraction patterns were recorded
cr
using a Cu Kα line (λ=1.54 Å) in 2θ range of 20-80° (where θ is the angle of diffraction).
us
Bruker’s AXS Model D8 Advance System was used to carry out the XRD experiments. 3. Results and discussion
an
Figure 2 shows the x-ray diffractogram of as-synthesized coarse Cu2O particles. The diffraction peak positions at 29.75, 36.74, 42.63, 61.89, 73.82 and 78.48° are indexed to (110),
M
(111), (200), (220), (311), and (222) crystal planes, respectively of face centered cubic (FCC) Cu2O. The inset of Fig. 2 shows the plane view secondary electron micrograph of the coarse
d
Cu2O particles. It can be clearly observed from the micrograph that the Cu2O particles have
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spherical morphology. The Cu2O particles are observed to have sizes in the micron and sub-
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micron range which is expected in the case of chemical synthesis methods. Figure 3 shows the x-ray diffractogram of fragmented Cu2O particles. The diffraction peaks are indexed to FCC Cu2O like in the case of as-synthesized particles. This clearly indicates that the fragmented particles have retained the original crystal structure. However, in this case the diffraction peaks are broarder (in comparison to as-synthesized Cu2O particles’ case) indicating a decrease in the average crystallite size.
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Figure 2. X-ray diffractogram of as-prepared coarse Cu2O particles. Inset shows the secondary
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electron micrograph of the Cu2O particles.
Figure 3. X-ray diffractogram of Cu2O particles obtained after 1 h of laser (energy of ~30.9 mJ) fragmentation in water.
Transmission electron micrograph and the corresponding electron diffraction pattern of the fragmented Cu2O particles are shown in Fig. 4. A continuous network of uniformly sized (~7 nm) nanoparticles can be clearly observed from Fig. 4(a). The electron diffraction pattern (Fig. 4(b)), which is a ring pattern indicates polycrystalline nature of the fragmented particles. The diffraction rings in the pattern are indexed to the crystal planes of FCC Cu2O. However, the most important part of this work is concerning the morphology and crystallinity of the fragmented 6 Page 6 of 15
Cu2O particles (in distilled water and ethanol) when the laser irradiation was for 30 min for the same laser energy of 30.9 mJ. These experiments have also helped in understanding the fragmentation mechanism. TEM micrographs and the corresponding electron diffraction patterns
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of the fragmented Cu2O particles obtained in these experiments are shown in Fig. 5.
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Figure 4. (a) Transmission electron micrograph and (b) the corresponding electron diffraction
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pattern of the Cu2O particles obtained after 1 h of ps laser (energy of ~30.9 mJ) fragmentation in water.
The micrographs (Figs. 5(a) and (c)) show that there is a mixed morphology of fragmented spherical particles and nanowires in both distilled water and ethanol cases. In the case of water as the medium (i.e., in Fig. 5(a)), it can be clearly observed that the fiber-like features are constituted by smaller features which indicates that the fragmentation of the fibers has already begun in 30 min. In the case of water as the medium, all the diffraction rings (Fig. 5(b)) are indexed to the crystal planes of FCC Cu2O. This result is similar to the one discussed in the case of the fragmentation in water for 1 h. However, in the present case, the intensity of diffraction
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rings is faint (in comparison to those observed in Fig. 4(b)) indicating a difference in nature of crystallites diffracting the electrons. This is definitely supported by the mixed morphology (Fig. 5(a)) observed in this case. In the case of fragmentation in ethanol, three diffraction rings (Fig.
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5(d)) are indexed to monoclinic CuO whilst rest are indexed to FCC Cu2O. However, x-ray diffraction peaks (Fig. 6) corresponding to CuO could not be identified most probably owing to
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the presence of only very small volume percentage of CuO in the sample.
Figure 5. (a) Transmission electron micrograph and (b) the corresponding electron diffraction pattern of the fragmented Cu2O particles in water; (c) Transmission electron micrograph and (d) the corresponding electron diffraction pattern of the fragmented Cu2O particles in ethanol; The 8 Page 8 of 15
time of ps laser (energy of ~30.9 mJ) irradiation is 30 min. Figure 6 shows the x-ray diffractograms of fragmented Cu2O particles in distilled water and ethanol. The diffraction peaks in both water and ethanol cases are indexed to FCC Cu2O.
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Surprisingly, the diffraction peaks are sharper and intense in the case of fragmented Cu2O particles in both water and ethanol media. In order to analyze this aspect, the highest intensity
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(111) diffraction peak (Fig. 7(a)) of as-synthesized Cu2O particles is compared with that of
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fragmented Cu2O particles (Figs. 7(b) and (c)). It is evident from Fig. 6 and Fig. 7 that the (111) diffraction peaks of the fragmented Cu2O particles are sharper and intense in comparison that of
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as-synthesized Cu2O particles. This clearly shows that the crystallinity of the fragmented particles is better in comparison to the as-synthesized particles. In Fig. 7(a) symmetric peak
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broadening is observed, whereas in Fig. 7(b) and Fig. 7(c) asymmetric peak broadening is
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observed. All the diffraction peaks in Figs. 7(b) and (c) are shifted towards left and right with
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respect to the peaks in Fig. 7(a), respectively. This is due to the tensile and compressive residual
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stresses induced in fragmented particles in water and ethanol, respectively.
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Figure 6. XRD patterns of Cu2O nanoparticles produced by ps laser fragmentation in (a) water
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and (b) ethanol. The time of ps laser (energy of ~30.9 mJ) irradiation is 30 min.
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Figure 7. (111) diffraction peaks of (a) as-synthesized Cu2O particles (b) Cu2O particles fragmented in distilled water and (c) Cu2O particles fragmented in ethanol. The time of ps laser
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(energy of ~30.9 mJ) irradiation is 30 min.
Modified Williamson-Hall (W-H) method [10-13] was applied on the x-ray diffraction data
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(Fig. 6) to further understand the crystallinity of the samples after fragmentation. Modified W-H method takes into account the contribution of lattice strain in broadening the x-ray diffraction peaks. The average crystallite sizes as determined by modified W-H method are 9 and 12 nm for Cu2O particles fragmented in water and ethanol (30 min laser irradiation), respectively. However, it should be noted that in the case of fragmentation in ethanol, the peak broadening is more which is a clear indication that the strain in the fragmented particles in ethanol is more in comparison to the strain in particles fragmented in water. Please see Supplementary Data for more details. The electron micrographs of Cu2O particles fragmented at different experimental conditions
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clearly indicate a two-step fragmentation mechanism. It is apparent that nanowires are first formed followed by their fragmentation into small particles as the laser irradiation continued. The results also show that fragmentation process is faster in water than in ethanol. This is due to
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the difference in physical, optical and chemical properties of ethanol and water [14,15]. Based on the above presented morphological and crystallinity data of the fragmented Cu2O particles and
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the available literature about the fragmentation mechanism of semiconducting particles [16-20],
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the following fragmentation mechanism for Cu2O is proposed. Under the influence of focused ultra-speed laser pulses like ps laser pulses, the dynamic semiconducting particle’s surface in a
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liquid medium experiences (when the focused laser spot is almost equal to the particle size) a great increase in temperature and pressure. At the beginning, the randomly moving (due to
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stirring) micron sized Cu2O particles dispersed in distilled water are exposed to the laser
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irradiation pulses for 30 min. The repetition rate of the ps laser pulses was maintained at 10 Hz
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which means that the time between two pulses is 100 ms which is smaller than the particle rotation time. Therefore, when ps laser pulses irradiate a particle in liquid medium, there is a
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high probability to expose the same particle to several laser pulses. As a result, the rotation of the particles allows the laser pulses to partially remove longer and uniform part of the irradiated coarser particle which form nanowires in the initial step. The length of nanowires depends on the dynamic motion of particles in the solution. i.e., the time consumed to fragment a particle is considered when the particle crosses the focused laser spot. However, the diameter of the nanowires depends on the laser energy and pulse duration. The electron micrographs clearly reveal that some part of particles are totally fragmented while some are either partially fragmented or remain intact with their initial morphologies. Figs. 5(a) and (c) clearly show the presence of mixed morphology. In the next step, the nanowires are subdivided to nanoparticles
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due to long irradiation time. The results of two different types of nanoparticles in water and ethanol can be explained using dielectric nature of the solvent. The dielectric constant of water is high (~80.4) whereas the dielectric constant of ethanol is low (~24.3). Therefore, the
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nanoparticles produced by laser pulses experience some sort of a strong force due to dielectric nature of water and follow faster fragmentation. However, in case of ethanol, the dielectric force
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exerted on the Cu2O particle is quite small which slows down the fragmentation process.
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Ethanol’s refractive index is higher than that of water and therefore, the intensity of laser decreases more in the case of ethanol as compared to water. Besides, the density and boiling
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point of ethanol is equal to 0.78 of that of water. All in all, in the first step, the laser fragments small portion from the large particle into nanowires and in the next step, under continuous laser
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irradiation, the nanowires is fragmented to ultra-fine particles. Figure 5(c) shows the production
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of nanowires along with small percentage of nanoparticles. The dominating % of nanowires in
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the case of ethanol as the medium determines that the fragmentation process in this case needs more time to produce nanoparticles.
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4. Conclusions
ps laser was used to fragment coarse Cu2O particles in water and ethanol media into nanosized particles. In the case of water as the medium, the fragmented particles have retained the original FCC crystal structure. On the other hand, in the case of ethanol as the medium, CuO phase has also formed along with Cu2O. The fragmentation is found to involve two steps. In the first step, nanowires form, which are subsequently fragmented under continuous laser irradiation. References [1] A. Lossin, F.J. Westhoff, The production and application of cuprous oxide and cupric hydroxide, JOM-J. MIN. MET. MATS. 49 (1997) 38–39.
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[2] L. Hanke, D. Fröhlich, A.L. Ivanov, P.B. Littlewood, H. Stolz, LA phonoritons in Cu2O, Phys. Rev. Lett. 83 (1999) 4365–4368. [3] J. Brandt, D. Fröhlich, C. Sandfort, M. Bayer, H. Stolz, N. Naka, Ultranarrow optical
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absorption and two-phonon excitation spectroscopy of Cu2O paraexcitons in a high magnetic field, Phys. Rev. Lett. 99 (2007) 217403.
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[4] O. Roslyak, U. Aparajita, J.L. Birman, S. Mukamel, Coherent manipulation of quadrupole
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biexcitons in cuprous oxide by 2D femtosecond spectroscopy, Phys. Status Solidi B 249 (2012) 435–447.
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[5] Y. Wang, F. Yang, H.X. Zhang, X.Y. Zi, X.H. Pan, F. Chen, W.-D. Luo, J.-X. Li, H.-Y. Zhu, Y.-P. Hu, Cuprous oxide nanoparticles inhibit the growth and metastasis of melanoma by
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targeting mitochondria, Cell Death Dis. 4 (2013) e783; doi: 10.1038/cddis.2013.314.
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[6] Mokhtar Ali, N.K. Rotte, V.V.S.S. Srikanth, Honey aided solution synthesis of
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polycrystalline Cu2O particles, Mater. Lett. 128 (2014) 253–255. [7] A. Nath, A. Khare, Size induced structural modifications in copper oxide nanoparticles
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synthesized via laser ablation in liquids, J. Appl. Phys. 110 (2011) 043111. [8] J.M.J. Santillán, F.A. Videla, M.B. Fernández van Raap, D.C. Schinca, L.B. Scaffardi, Analysis of the structure, configuration, and sizing of Cu and Cu oxide nanoparticles generated by fs laser ablation of solid target in liquids, J. Appl. Phys. 113 (2013) 134305. [9] M.A. Gondal, T.F. Qahtan, M.A. Dastageer, Y.W. Maganda, D.H. Anjum, Synthesis of Cu/Cu2O nanoparticles by laser ablation in deionized water and their annealing transformation into CuO nanoparticles, J. Nanosci. Nanotechnol. 13 (2013) 5759–5766. [10] T. Ungár, G. Tichy, The Effect of dislocation contrast on x-ray line profiles in untextured polycrystals, Phys. Status Solidi A 171 (1999) 425–434.
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[11] T. Ungár, A. Revesz, A. Borbely, Dislocations and grain size in electrodeposited nanocrystalline Ni determined by the modified Williamson-Hall and Warren-Averbach procedures, J. Appl. Cryst. 31 (1998) 554–558.
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[12] T. Ungár, I. Dragomir, A. Revesz, A. Borbely, The contrast factors of dislocations in cubic crystals: the dislocation model of strain anisotropy in practice, J. Appl. Cryst. 32 (1999) 992–
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[13] T. Ungár, J. Gubicza, G. Ribarik, A. Borbely, Crystallite size distribution and dislocation structure determined by diffraction profile analysis: principles and practical application to cubic
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and hexagonal crystals, J. Appl. Cryst. 34 (2001) 298–310.
[14] M.H. Mahdieh, B. Fattahi, Size properties of colloidal nanoparticles produced by
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nanosecond pulsed laser ablation and studying the effects of liquid medium and laser fluence,
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Appl. Surf. Sci. 329 (2015) 47–57.
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[15] V.S. Burakov, N.V. Tarasenko, A.V. Butsen, Laser-induced plasmas in liquids for nanoparticle synthesis, J. Appl. Spectrosc. 77 (2010) 386–393.
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[16] A.M. Morales, C.M. Lieber, A laser ablation method for the synthesis of crystalline semiconductor nanowires, Science 279 (1998) 208–211. [17] X.M. Meng, J.Q. Hu, Y. Jiang, C.S. Lee, S.T. Lee, Boron nanowires synthesized by laser ablation at high temperature, Chem. Phys. Lett. 370 (2003) 825–828. [18] C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Inorganic nanowires, Prog. Solid State Ch. 31 (2003) 5–147. [19] Y. Zhang, H. Ago, M. Yumura, S. Ohshima, K. Uchida, T. Komatsu, S. Iijima, Study of the growth of boron nanowires synthesized by laser ablation, Chem. Phys. Lett. 385 (2004) 177–183.
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[20] Kenry, C.T. Lim, Synthesis, optical properties, and chemical–biological sensing applications of one-dimensional inorganic semiconductor nanowires, Prog. Mater. Sci. 58 (2013)
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