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Influence of hot water treatment during laser ablation in liquid on the shape of PbO nanoparticles
Applied Surface Science 483 (2019) 835–839
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Influence of hot water treatment during laser ablation in liquid on the shape of PbO nanoparticles
T
V.Ya. Shur , E.V. Gunina, A.A. Esin, E.V. Shishkina, D.K. Kuznetsov, E.A. Linker, E.D. Greshnyakov, V.I. Pryakhina ⁎
School of Natural Sciences and Mathematics, Ural Federal University, Ekaterinburg, Russia
ARTICLE INFO
ABSTRACT
Keywords: Nanoparticles Laser ablation in water Hot water treatment Nonspherical particles Seed growth
The significant role of the unavoidable hot water treatment during laser ablation of lead in water in formation of the nonspherical 2D (plates) and 3D micro- and nanoparticles (octahedra and rods) in suspension and at the target surface has been shown experimentally. The formation of the nonspherical particles has been demonstrated at hot water treatment alone at temperature above 70 °C achieved frequently during laser ablation in water. The simple model was applied for explanation of the obtained formation of 3D particles. The 2D plates formed by seedless growth. The influence of the unavoidable hot water treatment should be considered as an additional mechanism for explanation of the formation of the nonspherical nanoparticles during laser ablation in water.
1. Introduction The shape variation of the nanoparticles (NPs) is important for several applications including creation of the antibacterial coatings and nanotoxicological research. Currently, the expansion of the study of nanoparticle toxicity needs production of the stable colloids of high concentration with model pure NPs of given composition, sizes and shapes [1,2]. The laser ablation in water (LAW) gives the unique ability to produce the colloids (stable water suspensions) of spherical nanoparticles of pure metals and metal oxides. Moreover, the nanoparticles can be transformed from octahedron to spherical shape by non-focused laser irradiation in a liquid in very short time [3]. However, other morphologies such as hollow particles, cubes, rods/spindles/tubes, disks/plates/sheets also have been fabricated with the assistance of external factors and directly by LAW [4–6]. The pulsed laser ablation in liquid medium (LAL), was utilized to produce precursor solutions, and leaf-like tungsten oxide (WO3) nanoplatelets [7] and nanoflakes [8] were synthesized after sequential aging treatment of precursors in temperature range from 30 to 90 °C. It was shown that hydrothermal treatment allowed to produce the well-dispersed and brick-like in shape nanostructured WO3 objects [9]. The mixture of cubes, pyramids, triangular plates, pentagonal rods, and bars has been developed by pulsed excimer laser ablation of bulk silver in water using polysorbate 80 as surfactant [10]. It was demonstrated recently that a wide variety of metals can form
⁎
metal oxide nanostructures (MONSTRs) on metal surfaces after Hot Water Treatment (HWT) representing “simple interaction with hot water” [11–15]. Appearance of the several experimentally observed shapes of MONSTRs including cubes, pyramids, plates, wires, spheres, and leaf-like (plate) nanostructures has been attributed to difference in crystal structures, surface diffusion, and surface energy minimization through formation of certain crystal facets [11,16]. It is necessary to point out that the surfaces with MONSTRs have gained considerable interest in various applications such as sensors, detectors, energy harvesting cells, and batteries [11]. It is necessary to point out that the change of the NPs shapes (ripening) produced by LAW during subsequent aging has been studied in various materials [7–9]. It was demonstrated that the process is accelerated at the elevated temperatures. Uncontrolled water heating unavoidably takes place during LAW. Thus, the HWT procedure should be considered as an additional attribute factor leading to formation of the nonspherical NPs just after LAW without subsequent long-term aging procedure. Moreover, this procedure modified the target surface morphology and composition. It was demonstrated that lead is one of the most reactive metals for HWT [11]. This is the reason for us to use Pb as a model metal for studying the role of unavoidable HWT in the process of nonspherical particles formation by LAW. In this paper we represent the results of experimental research of formation of the suspension with nonspherical PbO NPs and
Corresponding author. E-mail address: [email protected] (V.Y. Shur).
https://doi.org/10.1016/j.apsusc.2019.03.303 Received 21 September 2018; Received in revised form 20 March 2019; Accepted 27 March 2019 Available online 28 March 2019 0169-4332/ © 2019 Published by Elsevier B.V.
Applied Surface Science 483 (2019) 835–839
V.Y. Shur, et al.
Fig. 1. Formation of NPs by laser ablation in water. SEM images of NPs (a), (b), (c) in suspension and (d), (e), (f) at the target surface appeared after LAW for various target surface preparations: (a), (d) aged at ambient conditions, (b) cleaned by LAW, (c), (e) etched by plasma, (f) treated by hot water. (c) The spherical NPs sampled from suspension immediately after LAW.
of deionized water. The water volume was about 40 ml. The focused laser beam (fluence 80 J/cm2, spot diameter 40 μm) has been scanned over the target area about 16 mm2 with linear velocity 270 mm/s. The typical duration of the ablation process was about 4 min. The water temperature was measured near the target surface by thermocouple. Various target surface preparations have been used: (1) aging by long time exposure at ambient conditions, (2) cleaning by LAW and subsequent ultrasonification, (3) etching by CCP oxygen plasma, (4) treating by hot deionized water. The aging at ambient conditions was realized during above 30 days at the normal pressure and relative humidity about 30%. The uniform grey color of the target surface confirmed its complete oxidation. Cleaning by LAW has been done by one scanning of the target area using the same irradiation parameters as for laser ablation. Subsequent ultrasonic rinsing in acetone and isopropanol and deionized water with final purging by dry nitrogen allowed to remove the ablation products from the target surface. Plasma etching was realized by Plasmalab 80RIE, Oxford Instruments for 30 min at 300 W power at the pressure 20 mTorr with 20 sccm oxygen flow. Treatment by hot deionized water has been done at 75 °C during 16 min with subsequent careful removing of the water.
Table 1 Types of NPs at the target surfaces and in suspension appeared after LAW for various target surface preparations. Target preparation
NPs on target
NPs in suspension
Aged at ambient conditions Cleaned by LAW Etched by plasma Treated by hot water
Octahedra and rods Rods and plates Octahedra and rods Octahedra and rods
Plates Plates Spherical and plates Plates
modification of the Pb target surface morphology by LAW and HWT. The series of laser ablations of Pb targets after various surface preparations allowed us to attribute the formation of nonspherical NPs to ordered growth during unavoidable HWT. 1.1. Experiment The pulse Yb fiber laser (Fmark-20 RL, Laser technology center Ltd., Russia) with 1062 nm wavelength, 100 ns pulse duration and repetition rate about 21 kHz has been used for laser ablation. The Pb target of 99.99% purity with diameter 46 mm and thickness about 2 mm was placed on the bottom of the glass Petri dish and covered by 5 mm layer
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Applied Surface Science 483 (2019) 835–839
V.Y. Shur, et al.
The NPs morphology and electron diffraction patterns were obtained by transmission electron microscope JEM-2100, JEOL. The nanoparticles were deposited onto 200-mesh carbon-coated copper grids and were investigated with the accelerating voltage of 200 KV. The electron diffraction patterns were obtained under the same voltage with a camera length of 20 cm. The size distribution function of NPs and ζ - potential of water suspension were measured by particle size analyzer Zetasizer Nano ZS, Malvern. The shape and sizes of NPs were revealed by analysis of SEM images. The samples for SEM imaging have been prepared by vacuum freeze drying (lyophilization) of the water suspension drop on the Si/ SiO2 substrate by means of freeze drier Alpha 2–4 LSC, Martin Christ. 2. Results It was found that the laser ablation of the Pb targets aged at ambient conditions, cleaned by LAW and treated by hot water leads to appearance of nonspherical 2D and 3D NPs in suspension and at the target surface (Fig. 1). The obtained results cannot be explained by wellknown mechanisms of the laser ablation of metal target in water which predict formation of spherical NPs only [17,18]. The formation of the complicated particle shapes has been attributed to additional mechanisms, such as aging or precipitation taking place after laser ablation [4–10]. In suspension the hexagonal and circular plates (2D NPs) of micron and sub-micron diameter and thickness about 10 nm were obtained after laser ablation for targets aged in ambient conditions (Fig. 1a), cleaned by LAW (Fig. 1b) and treated by hot water. Moreover, the submicron sized octahedra and rods (3D NPs) were obtained at the target surfaces for the laser ablation of the Pb targets prepared by all used treatments (Fig. 1d, e, f). We were able to observe the NPs of the classical spherical shape in suspension immediately after LAW only (Fig. 1c). The spherical NPs shape changes rapidly in the water heated by LAW. The shapes obtained for all target surface preparations were summarized in Table 1. In our experiments the typical measured temperature of the target surface and water after finishing of the laser ablation process was above 70 °C and slow decreased in several minutes. It is clear, that the higher local temperatures existed during ablation process. This is the reason to attribute the formation of the nonspherical particles to influence of unavoidable HWT. Thus, we have studied separately the effect of HWT on the target surface morphology for various target surface preparations. The HWT at 75 °C has been done with duration ranged from 2 to 16 min of the target aged at ambient conditions. It was found that the octahedra and rods (3D NPs) at the target surfaces appeared even after HWT during 2 min and their concentration and sizes increased with duration (Fig. 2). Moreover the new surface morphology appeared after HWT representing the incoherent (spongy) layer consisted of PbO plates (ensembles of upright plates) (Fig. 2c,d). The similar surface nanostructuring was obtained after HWT for all target preparations.
Fig. 2. Formation of nanoparticles and nanostructure of the target by treatment in hot water. SEM images of target surfaces after aging at ambient conditions: (a) initial state, (b), (c) 3D NPs (octahedra and rods) and ensembles of upright plates appeared after HWT at 75 °C during 8 min.
3. Discussion
We measured the Raman spectrum to analyze the composition of the target after various preparations and used the electron-diffraction patterns to obtain the phase composition of the individual NPs. The target surface composition was analyzed by confocal Raman microscope Alpha 300 AR, WiTec. It was shown that all target treatments lead to formation of the oxidized surface layer of tetragonal PbO. The morphology and apparently thickness of the oxidized layers differed for various target treatments. The target surface morphology was imaged by means of scanning electron microscopy (SEM) using CrossBeam Workstation Auriga, Carl Zeiss.
The following model has been proposed for explanation of the formation of various experimentally obtained shapes of NPs considering the influence of unavoidable hot water treatment during laser ablation in water. It was shown by confocal Raman spectroscopy that the target is completely or partially covered by PbO layer and its crystallographic phase depends on the surface preparation (Fig. 3). After etching by CCP oxygen plasma or treating by hot deionized water the target is covered by tetragonal α-PbO (Fig. 3a,b), whereas after LAW cleaning the
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V.Y. Shur, et al.
Fig. 3. Raman spectra for target with various surface preparations: (a) etching by CCP oxygen plasma, (b) treating by hot deionized water, (c) LAW cleaning.
Fig. 4. Scheme of the growth of nonspherical nanoparticles: (a) laser ablation, (b) subsequent unavoidable HWT.
Fig. 5. The erosion of Pb surfaces as a result of oxidation in hot water. SEM images of pitted spherical Pb micro-size particles.
orthorhombic β-PbO phase appeared (Fig. 3c). The laser ablation in water removes the surface layer from the target covered by PbO. This process leads to formation of the target surface without oxide and to creation in water the spherical Pb and PbO nanoparticles and micro-size Pb particles (Fig. 4a). All Pb oxidize and the water heating during laser ablation accelerates this process. The PbO transfers from Pb surfaces to PbO nanoparticles floating in the water and lying on the target. The facetted growth of PbO nanoparticles (ripening) is observed. The anisotropy of the growth rate on the different crystal facets leads to formation of the nonspherical NPs with regular shape determined by oxide phase symmetry (Fig. 4b). This process is accompanied by erosion of the surface of the spherical Pb micro-size particles and target leading to appearance of pitted particles (Fig. 5a) and change of the target relief (Fig. 5b). The crystallographic phases of the various shape NPs were obtained by transmission electron microscopy. It was shown that the octahedra and rods NPs represent the single crystals of tetragonal α-PbO phase (Fig. 6a), whereas the plates composed of the single crystals of the orthorhombic β-PbO phase (Fig. 6b). The α-PbO NPs played the role of the nuclei (seeds) for 3D oxide particles (octahedrons and rods). It was shown earlier that the tetragonal phase can form octahedra [19,20]. The 2D NPs (plates) of orthorhombic phase β-PbO appeared by seedless growth (Fig. 4b). The similar plates have been synthesized by other methods earlier [21,22]. The qualitative change of the surface morphology by formation of the noncoherent (spongy) layer consisted of PbO plates after HWT can be attributed to seedless 2D growth close to the target surface. Their orientation normal to the target surface is caused by convectional flows.
4. Conclusion The formation of metal oxide nanoparticles and nanostructuring of the target surface by laser ablation in water and hot water treatment have been studied using Pb as a model metal. It was found that the laser ablation of the Pb targets aged at ambient conditions, cleaned by LAW and treated by hot water leads to appearance of nonspherical 2D (plates) and 3D NPs (octahedra and rods) in suspension and at the target surface. The NPs of the classical spherical shape appeared in suspension immediately after LAW and their shape changes rapidly in the heated water. It was found that the octahedra and rods appeared at the target surfaces immersed in the water with temperature about 70 °C after several minutes even without laser ablation. Thus, the noticeable role of the unavoidable hot water treatment during laser ablation in water in formation of the nonspherical micro- and nanoparticles was proved. The simple model based on growth of seed oxide nanoparticles was applied for explanation of the obtained formation of 3D particles. This process results in growth of PbO spherical nanoparticles and erosion of all Pb surfaces. The PbO NPs played the role of the seeds which phase depends on the target preparation procedure. The 2D particles (plates) appeared by seedless growth. The demonstrated influence of the unavoidable hot water treatment should be considered as an additional mechanism for explanation of the results of laser ablation in water especially related to formation of the nonspherical nanoparticles.
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Applied Surface Science 483 (2019) 835–839
V.Y. Shur, et al.
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Kulinich, S. Yang, J. He, W. Cai, Nanomaterials via laser ablation/irradiation in liquid: a review, Adv. Funct. Mater. 22 (2012) 1333–1353, https://doi.org/10.1002/adfm.201102295. [18] D. Zhang, B. Gökce, S. Barcikowski, Laser synthesis and processing of colloids: fundamentals and applications, Chem. Rev. 117 (2017) 3990–4103, https://doi. org/10.1021/acs.chemrev.6b00468. [19] X. Han, M. Jin, S. Xie, Q. Kuang, Z. Jiang, Y. Jiang, Z. Xie, L. Zheng, Synthesis of tin dioxide octahedral nanoparticles with exposed high-energy {221} facets and enhanced gas-sensing properties, Angew. Chem. Int. Ed. 48 (2009) 9180–9183, https://doi.org/10.1002/anie.200903926. [20] X. Hao, J. Zhao, Y. Li, Y. Zhao, D. Ma, L. Li, Mild aqueous synthesis of octahedral Mn3O4 nanocrystals with varied oxidation states, Colloids Surf. A Physicochem. Eng. Asp. 374 (2011) 42–47, https://doi.org/10.1016/j.colsurfa.2010.10.048. [21] L. Meng, J. Ustarroz, M.E. Newton, J.V. 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Fig. 6. The TEM images and corresponding selected area electron diffraction patterns for NPs of different shapes: (a) octahedra, (b) rod, (c) plate.
Acknowledgements The equipment of the Ural Center for Shared Use “Modern nanotechnology” UrFU was used. The work was supported by Government of the Russian Federation (Act 211, Agreement 02.A03.21.0006). VS is grateful for financial support to the Ministry of Education and Science of the Russian Federation (state task 3.4993.2017/6.7).
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Influence of hot water treatment during laser ablation in liquid on the shape of PbO nanoparticles
T
V.Ya. Shur , E.V. Gunina, A.A. Esin, E.V. Shishkina, D.K. Kuznetsov, E.A. Linker, E.D. Greshnyakov, V.I. Pryakhina ⁎
School of Natural Sciences and Mathematics, Ural Federal University, Ekaterinburg, Russia
ARTICLE INFO
ABSTRACT
Keywords: Nanoparticles Laser ablation in water Hot water treatment Nonspherical particles Seed growth
The significant role of the unavoidable hot water treatment during laser ablation of lead in water in formation of the nonspherical 2D (plates) and 3D micro- and nanoparticles (octahedra and rods) in suspension and at the target surface has been shown experimentally. The formation of the nonspherical particles has been demonstrated at hot water treatment alone at temperature above 70 °C achieved frequently during laser ablation in water. The simple model was applied for explanation of the obtained formation of 3D particles. The 2D plates formed by seedless growth. The influence of the unavoidable hot water treatment should be considered as an additional mechanism for explanation of the formation of the nonspherical nanoparticles during laser ablation in water.
1. Introduction The shape variation of the nanoparticles (NPs) is important for several applications including creation of the antibacterial coatings and nanotoxicological research. Currently, the expansion of the study of nanoparticle toxicity needs production of the stable colloids of high concentration with model pure NPs of given composition, sizes and shapes [1,2]. The laser ablation in water (LAW) gives the unique ability to produce the colloids (stable water suspensions) of spherical nanoparticles of pure metals and metal oxides. Moreover, the nanoparticles can be transformed from octahedron to spherical shape by non-focused laser irradiation in a liquid in very short time [3]. However, other morphologies such as hollow particles, cubes, rods/spindles/tubes, disks/plates/sheets also have been fabricated with the assistance of external factors and directly by LAW [4–6]. The pulsed laser ablation in liquid medium (LAL), was utilized to produce precursor solutions, and leaf-like tungsten oxide (WO3) nanoplatelets [7] and nanoflakes [8] were synthesized after sequential aging treatment of precursors in temperature range from 30 to 90 °C. It was shown that hydrothermal treatment allowed to produce the well-dispersed and brick-like in shape nanostructured WO3 objects [9]. The mixture of cubes, pyramids, triangular plates, pentagonal rods, and bars has been developed by pulsed excimer laser ablation of bulk silver in water using polysorbate 80 as surfactant [10]. It was demonstrated recently that a wide variety of metals can form
⁎
metal oxide nanostructures (MONSTRs) on metal surfaces after Hot Water Treatment (HWT) representing “simple interaction with hot water” [11–15]. Appearance of the several experimentally observed shapes of MONSTRs including cubes, pyramids, plates, wires, spheres, and leaf-like (plate) nanostructures has been attributed to difference in crystal structures, surface diffusion, and surface energy minimization through formation of certain crystal facets [11,16]. It is necessary to point out that the surfaces with MONSTRs have gained considerable interest in various applications such as sensors, detectors, energy harvesting cells, and batteries [11]. It is necessary to point out that the change of the NPs shapes (ripening) produced by LAW during subsequent aging has been studied in various materials [7–9]. It was demonstrated that the process is accelerated at the elevated temperatures. Uncontrolled water heating unavoidably takes place during LAW. Thus, the HWT procedure should be considered as an additional attribute factor leading to formation of the nonspherical NPs just after LAW without subsequent long-term aging procedure. Moreover, this procedure modified the target surface morphology and composition. It was demonstrated that lead is one of the most reactive metals for HWT [11]. This is the reason for us to use Pb as a model metal for studying the role of unavoidable HWT in the process of nonspherical particles formation by LAW. In this paper we represent the results of experimental research of formation of the suspension with nonspherical PbO NPs and
Corresponding author. E-mail address: [email protected] (V.Y. Shur).
https://doi.org/10.1016/j.apsusc.2019.03.303 Received 21 September 2018; Received in revised form 20 March 2019; Accepted 27 March 2019 Available online 28 March 2019 0169-4332/ © 2019 Published by Elsevier B.V.
Applied Surface Science 483 (2019) 835–839
V.Y. Shur, et al.
Fig. 1. Formation of NPs by laser ablation in water. SEM images of NPs (a), (b), (c) in suspension and (d), (e), (f) at the target surface appeared after LAW for various target surface preparations: (a), (d) aged at ambient conditions, (b) cleaned by LAW, (c), (e) etched by plasma, (f) treated by hot water. (c) The spherical NPs sampled from suspension immediately after LAW.
of deionized water. The water volume was about 40 ml. The focused laser beam (fluence 80 J/cm2, spot diameter 40 μm) has been scanned over the target area about 16 mm2 with linear velocity 270 mm/s. The typical duration of the ablation process was about 4 min. The water temperature was measured near the target surface by thermocouple. Various target surface preparations have been used: (1) aging by long time exposure at ambient conditions, (2) cleaning by LAW and subsequent ultrasonification, (3) etching by CCP oxygen plasma, (4) treating by hot deionized water. The aging at ambient conditions was realized during above 30 days at the normal pressure and relative humidity about 30%. The uniform grey color of the target surface confirmed its complete oxidation. Cleaning by LAW has been done by one scanning of the target area using the same irradiation parameters as for laser ablation. Subsequent ultrasonic rinsing in acetone and isopropanol and deionized water with final purging by dry nitrogen allowed to remove the ablation products from the target surface. Plasma etching was realized by Plasmalab 80RIE, Oxford Instruments for 30 min at 300 W power at the pressure 20 mTorr with 20 sccm oxygen flow. Treatment by hot deionized water has been done at 75 °C during 16 min with subsequent careful removing of the water.
Table 1 Types of NPs at the target surfaces and in suspension appeared after LAW for various target surface preparations. Target preparation
NPs on target
NPs in suspension
Aged at ambient conditions Cleaned by LAW Etched by plasma Treated by hot water
Octahedra and rods Rods and plates Octahedra and rods Octahedra and rods
Plates Plates Spherical and plates Plates
modification of the Pb target surface morphology by LAW and HWT. The series of laser ablations of Pb targets after various surface preparations allowed us to attribute the formation of nonspherical NPs to ordered growth during unavoidable HWT. 1.1. Experiment The pulse Yb fiber laser (Fmark-20 RL, Laser technology center Ltd., Russia) with 1062 nm wavelength, 100 ns pulse duration and repetition rate about 21 kHz has been used for laser ablation. The Pb target of 99.99% purity with diameter 46 mm and thickness about 2 mm was placed on the bottom of the glass Petri dish and covered by 5 mm layer
836
Applied Surface Science 483 (2019) 835–839
V.Y. Shur, et al.
The NPs morphology and electron diffraction patterns were obtained by transmission electron microscope JEM-2100, JEOL. The nanoparticles were deposited onto 200-mesh carbon-coated copper grids and were investigated with the accelerating voltage of 200 KV. The electron diffraction patterns were obtained under the same voltage with a camera length of 20 cm. The size distribution function of NPs and ζ - potential of water suspension were measured by particle size analyzer Zetasizer Nano ZS, Malvern. The shape and sizes of NPs were revealed by analysis of SEM images. The samples for SEM imaging have been prepared by vacuum freeze drying (lyophilization) of the water suspension drop on the Si/ SiO2 substrate by means of freeze drier Alpha 2–4 LSC, Martin Christ. 2. Results It was found that the laser ablation of the Pb targets aged at ambient conditions, cleaned by LAW and treated by hot water leads to appearance of nonspherical 2D and 3D NPs in suspension and at the target surface (Fig. 1). The obtained results cannot be explained by wellknown mechanisms of the laser ablation of metal target in water which predict formation of spherical NPs only [17,18]. The formation of the complicated particle shapes has been attributed to additional mechanisms, such as aging or precipitation taking place after laser ablation [4–10]. In suspension the hexagonal and circular plates (2D NPs) of micron and sub-micron diameter and thickness about 10 nm were obtained after laser ablation for targets aged in ambient conditions (Fig. 1a), cleaned by LAW (Fig. 1b) and treated by hot water. Moreover, the submicron sized octahedra and rods (3D NPs) were obtained at the target surfaces for the laser ablation of the Pb targets prepared by all used treatments (Fig. 1d, e, f). We were able to observe the NPs of the classical spherical shape in suspension immediately after LAW only (Fig. 1c). The spherical NPs shape changes rapidly in the water heated by LAW. The shapes obtained for all target surface preparations were summarized in Table 1. In our experiments the typical measured temperature of the target surface and water after finishing of the laser ablation process was above 70 °C and slow decreased in several minutes. It is clear, that the higher local temperatures existed during ablation process. This is the reason to attribute the formation of the nonspherical particles to influence of unavoidable HWT. Thus, we have studied separately the effect of HWT on the target surface morphology for various target surface preparations. The HWT at 75 °C has been done with duration ranged from 2 to 16 min of the target aged at ambient conditions. It was found that the octahedra and rods (3D NPs) at the target surfaces appeared even after HWT during 2 min and their concentration and sizes increased with duration (Fig. 2). Moreover the new surface morphology appeared after HWT representing the incoherent (spongy) layer consisted of PbO plates (ensembles of upright plates) (Fig. 2c,d). The similar surface nanostructuring was obtained after HWT for all target preparations.
Fig. 2. Formation of nanoparticles and nanostructure of the target by treatment in hot water. SEM images of target surfaces after aging at ambient conditions: (a) initial state, (b), (c) 3D NPs (octahedra and rods) and ensembles of upright plates appeared after HWT at 75 °C during 8 min.
3. Discussion
We measured the Raman spectrum to analyze the composition of the target after various preparations and used the electron-diffraction patterns to obtain the phase composition of the individual NPs. The target surface composition was analyzed by confocal Raman microscope Alpha 300 AR, WiTec. It was shown that all target treatments lead to formation of the oxidized surface layer of tetragonal PbO. The morphology and apparently thickness of the oxidized layers differed for various target treatments. The target surface morphology was imaged by means of scanning electron microscopy (SEM) using CrossBeam Workstation Auriga, Carl Zeiss.
The following model has been proposed for explanation of the formation of various experimentally obtained shapes of NPs considering the influence of unavoidable hot water treatment during laser ablation in water. It was shown by confocal Raman spectroscopy that the target is completely or partially covered by PbO layer and its crystallographic phase depends on the surface preparation (Fig. 3). After etching by CCP oxygen plasma or treating by hot deionized water the target is covered by tetragonal α-PbO (Fig. 3a,b), whereas after LAW cleaning the
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Fig. 3. Raman spectra for target with various surface preparations: (a) etching by CCP oxygen plasma, (b) treating by hot deionized water, (c) LAW cleaning.
Fig. 4. Scheme of the growth of nonspherical nanoparticles: (a) laser ablation, (b) subsequent unavoidable HWT.
Fig. 5. The erosion of Pb surfaces as a result of oxidation in hot water. SEM images of pitted spherical Pb micro-size particles.
orthorhombic β-PbO phase appeared (Fig. 3c). The laser ablation in water removes the surface layer from the target covered by PbO. This process leads to formation of the target surface without oxide and to creation in water the spherical Pb and PbO nanoparticles and micro-size Pb particles (Fig. 4a). All Pb oxidize and the water heating during laser ablation accelerates this process. The PbO transfers from Pb surfaces to PbO nanoparticles floating in the water and lying on the target. The facetted growth of PbO nanoparticles (ripening) is observed. The anisotropy of the growth rate on the different crystal facets leads to formation of the nonspherical NPs with regular shape determined by oxide phase symmetry (Fig. 4b). This process is accompanied by erosion of the surface of the spherical Pb micro-size particles and target leading to appearance of pitted particles (Fig. 5a) and change of the target relief (Fig. 5b). The crystallographic phases of the various shape NPs were obtained by transmission electron microscopy. It was shown that the octahedra and rods NPs represent the single crystals of tetragonal α-PbO phase (Fig. 6a), whereas the plates composed of the single crystals of the orthorhombic β-PbO phase (Fig. 6b). The α-PbO NPs played the role of the nuclei (seeds) for 3D oxide particles (octahedrons and rods). It was shown earlier that the tetragonal phase can form octahedra [19,20]. The 2D NPs (plates) of orthorhombic phase β-PbO appeared by seedless growth (Fig. 4b). The similar plates have been synthesized by other methods earlier [21,22]. The qualitative change of the surface morphology by formation of the noncoherent (spongy) layer consisted of PbO plates after HWT can be attributed to seedless 2D growth close to the target surface. Their orientation normal to the target surface is caused by convectional flows.
4. Conclusion The formation of metal oxide nanoparticles and nanostructuring of the target surface by laser ablation in water and hot water treatment have been studied using Pb as a model metal. It was found that the laser ablation of the Pb targets aged at ambient conditions, cleaned by LAW and treated by hot water leads to appearance of nonspherical 2D (plates) and 3D NPs (octahedra and rods) in suspension and at the target surface. The NPs of the classical spherical shape appeared in suspension immediately after LAW and their shape changes rapidly in the heated water. It was found that the octahedra and rods appeared at the target surfaces immersed in the water with temperature about 70 °C after several minutes even without laser ablation. Thus, the noticeable role of the unavoidable hot water treatment during laser ablation in water in formation of the nonspherical micro- and nanoparticles was proved. The simple model based on growth of seed oxide nanoparticles was applied for explanation of the obtained formation of 3D particles. This process results in growth of PbO spherical nanoparticles and erosion of all Pb surfaces. The PbO NPs played the role of the seeds which phase depends on the target preparation procedure. The 2D particles (plates) appeared by seedless growth. The demonstrated influence of the unavoidable hot water treatment should be considered as an additional mechanism for explanation of the results of laser ablation in water especially related to formation of the nonspherical nanoparticles.
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Fig. 6. The TEM images and corresponding selected area electron diffraction patterns for NPs of different shapes: (a) octahedra, (b) rod, (c) plate.
Acknowledgements The equipment of the Ural Center for Shared Use “Modern nanotechnology” UrFU was used. The work was supported by Government of the Russian Federation (Act 211, Agreement 02.A03.21.0006). VS is grateful for financial support to the Ministry of Education and Science of the Russian Federation (state task 3.4993.2017/6.7).
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