CC BY
7Химия твердых веществ и нанотехнология
УДК 666.22
Mohamed I. Abdelghany1, Elena V. Kolobkova2, Sergey V. Mjakin3, Maxim M. Sychov4
EFFECT OF Ag AND Cl IONS ADDITION UPON THE FORMATION OF CuCl NANOPARTICLES IN FLUOROPHOSPHATE GLASSES
St. Petersburg State Institute of Technology (Technical University), Moskovsky Pr., 26, St Petersburg, 190013, Russia e-mail: [email protected]
The addition of Ag ions into CuCl-containing fluorophosphate glasses is found to result in a change of crystallization mechanism and growth of CuCl nanocrystals via a heterogeneous nucle-ation. Depending on Cl ions addition either AgCl clusters or Agn. nanoclusters act as nucleating agents in the excess or reduced amount of chlorine correspondingly. The increase of silver concentration above 0.2 mol.% prevents from the formation of CuCl crystal phase. The appearance of surface plasmon resonance bands in the spectra of the studied glasses suggests the growth of Cu and Ag nanoparticles.
Keywords: nanocrystals,CuCl,excitons,fluorophosphateglass
M.I. Abdelghany, Е.В. Колобкова, С.В. Мякин, М.М. Сычев
ВЛИЯНИЕ ВВЕДЕНИЯ ИОНОВ СЕРЕБРА И ХЛОРА НА ФОРМИРОВАНИЕ НАНОЧАСТИЦ ^а ВО ФТОРФОСФАТНОМ СТЕКЛЕ
Санкт-Петербургский государственный технологический институт (технический университет), Московский пр., 26, Санкт-Петербург, 190013, Россия e-mail: [email protected]
Показано, что введение во фторфосфатное стекло ионов серебра изменяет механизм процесса кристаллизации и приводит к росту нанокристаллов CuCl по механизму гетерогенной кристаллизации стекле. При избытке ионов хлора нуклеаторами становятся кластеры AgCl а при недостатке молекулярные кластеры Agn. Обнаружено, что при увеличении концентрации серебра свыше 0.2 мол. % CuCl не выделяется в качестве кристаллической фазы. Появление в спектре поглощения полос поверхностного плазмонного резонанса свидетельствует о росте Cu и Ag металлических наночастиц
Ключевые слова: нанокристаллы, фторфосфатные стекла
CuCl, экситоны,
DOI 10.15217/issn1998984-9.2016.34.40
Introduction
For the recent years, the formation of semiconductor nanocrystalline phases in a glass matrix as well as the control of their properties are considered as important problems in the technology of nanostructured materials preparation. The most interesting feature of such structures relates to their size-dependent physical properties and significantly determines their applications [1]. The effect of quantum confinement on the optical properties of semiconductor materials and glasses containing NPs was studied in [2, 3]. CuCl NCs demonstrated strong exciton absorption in the near-UV spectral region that is useful for their application in optical filters with sharp absorption edge at the UV region boundary providing the cutoff of UV radiation and transparency in visible - near IR spectral ranges [4]. Glasses with CuCl NCs are characterized by high nonlinear optical response and photochromism [5, 6]. FP glass containing CuCl NCs do not possess any photochromic properties, in contrast to borosilicate glass [7, 8].
Nanoscale particles of transition metals like Ag, Au, and Cu embedded in glass as the dielectric matrix are of great interest because of their potential applications in the field of photonics owing to their unique optical characteristics origi-
nating from the strong interaction between incident light and metallic NPs. This interaction results in collective oscillations of electron clouds, called surface plasmon resonance (SPR), at the interface of the metallic NPs and the dielectric matrix. The resonance frequency of this interaction is strongly dependent on the metal, the surrounding dielectric medium, as well as the size and shape distribution of the NPs [9].
Ag nanoclusters are known to act as nuclei and promote the formation of quantum dots (QDs) in glass matrices [10]. This result suggests that Ag clusters in glasses may promote formation of QDs in glasses, and that adjusting Ag concentration may allow control of the number of nucleation sites.
The presence of Cu and Ag NPs in such glasses can be determined using spectroscopic measurements according to specific absorption bands of their surface plasmon resonance (SPR) at 560-570 nm [11] and around 410 nm [12], respectively.
This paper reports the effect of heat treatment and HCl/AgNO3 on the precipitation of CuCl NCs on the absorption coefficients and size of the such NCs, in addition to investigates the nature of the crystalline phase formed during H.T. Four glasses differing in the nature of additives (AgNO3 and/ or HCl) and Ag concentration were prepared and comparatively studied.
1 Mohamed I. Abdelghany, post-graduate student, Theory of Materials Science Department e-mail: [email protected] Mohamed I. Abdelghany, аспирант, каф. теоретических основ материаловедения
2 Elena V. Kolobkova, Dr. Sci. (Chem.), Professor, Department of Chemical Technology of refractory nonmetallic silicate materials, e-mail: kolobok106@ rambler.ru
Колобкова Елена Вячеславовна - д-р хим. наук, профессор, каф. химической технологии тугоплавких неметаллических и силикатных материалов
3 Sergey V. Myakin, PhD (Chem.), Associate Professor, Theory of Materials Science Department, е-mail: [email protected] Мякин Сергей Владимирович, канд. хим. наук, доцент, каф. теоретических основ материаловедения
4 Maxim M. Sychev Dr. Sci. (Eng.), Professor, Head of the Department of Theory of Materials Science, e-mail: [email protected] Сычев Максим Максимович, д-р техн. наук, профессор, зав. каф. теоретических основ материаловедения
Received April, 18 2016
Experimental
A conventional glass quenching procedure was applied to prepare FP glasses of the compositions NaPO3 -Ba(PO3)2 - AlF3. To obtain the NCs in these glasses, small amounts of CuCl, AgNO3 and HCl (40 % wt.) were added above 100 % as listed in the table 1. Then the appropriate quantities of NaPO3, Ba(PO3)2, AlF3 were also added to form the total batch of 30 g . All the glass components were mixed together and the synthesis was performed in a closed glassy carbon crucible using the crucible-in-crucible method at 900 °C for 30 min. The glassy liquid was maintained in the furnace until completely refined before quenching by pouring onto a glassy carbon surface at room temperature. The glass transition temperature (Tg) of the studied glasses was measured using STA 449F1 Jupiter differential scanning calorimeter (Netzsch) and found to be about 320 oC. After the synthesis the first three glass samples were subjected to H.T. at 400 oC (above Tg) for 1, 3, 6, 9, 12, 15 and 18 hours. The absorption spectra were measured at room temperature using a spectrophotometer SF-56 (LOMO, St. Petersburg) in the wavelength range 190-1100 nm with a step 1 nm.
Table. Chemical Composition of the studied glasses
Sample ID CuCl AgNOs HCl*
mol.% over 100 % mL
NB-1 - 7,4
NB-2 0,05 7,4
NB-3 1,7 0,05 -
NB-4 0,2 -
* HCl amount added to 30 g of the batch mixture.
Results and Discussion
All the prepared FP glasses are transparent and featured with a faint green color (while a glass without any additives is colorless) due to the presence of Cu2+ ions providing a broad absorption band at 600-1100 nm [13]. Figure 1 illustrates the absorption spectra at room temperature for FP glass (NB-3, for example) as prepared and after 1 hour of H.T. at 400 °C.
Figure 1. Absorption spectra of FP glass NB-3 before (1) and after heat treatment at 400 oC for 1 hour (2).
The absorption spectra of the initial FP glasses before H.T.(figure 1) exhibit a smooth falloff from wavelength 300 to 370 nm determined by the distribution of Cu+ ions in the glass matrix. The formation of CuCl-NCs was confirmed by the appearance of exciton absorption band in the near UV spectral region. Upon H.T. at 400 °C for 1 hour, Z12 and Z3 exciton absorption peaks appear in CuCl-NCs in the range 350380 nm. The excitons are formed by an electron in the lowest conduction band (symmetry re) and a hole either in the upper
most twofold-degenerate Ty valence band (Z3 exciton) or in fourfold-degenerate Te valence band (Z12 exciton) [13]. The figure 1 also shows the absence of both absorption bands at 560-570 and 410 nm relating to SPR of Cu and Ag-NPs, as considered above, and correspondingly no yellow coloration was observed. However, the color of the synthesized glass NB-1, 2 and 3 did not change after H.T., which suggests that no Ag NPs are formed during the preparation of glasses or H.T. Hence, it can be concluded that the addition of small Ag amounts into the FP glass provides a nucleating agent in the formation of NCs in the glass matrix [14] without the formation of metallic silver.
Figure 2 shows absorption spectra of the samples at room temperature in the region 320-420 nm (relating to exciton absorption bands of CuCl NCs) subjected to H.T. at 400 °C for different times. From these spectra, the absorption value at the band Z12 maximum was used to determine the amount of CuCl crystalline phase formed in glass. In addition, the change of the absorption band in the samples NB-1 and NB-3 is featured with a similar behavior depending on the growth of the nanocrystalline phase with increasing the time of H.T., while in the sample NB-2, this phase is mostly formed after 1 hour of H.T. and only slightly changes during the subsequent process time.
140
120
100
320
340
360
380
400
420
Wavelength, nm
Figure 2. Effect of heat treatment at 400 °C on the room temperature absorption spectra of NB-1 (a), NB-2(b) and NB-3(c) glasses for 0 (1), 1 (2), 6 (3), 12 (4) and 18 hours (5).
As shown in figure 3, the glass NB-1 absorption coefficient linearly increases after lhour H.T. to 18 hours from 4,2 to 66,8 cm-1 in two stages, the first one from 1 hours to 9 hours, the second stage with higher rate, probably due to the increase in the content of CuCl nanocrystalline phase. For the glass NB-2, the considered absorption band sharply grows after 1 h of H.T. followed by a small subsequent increase with H.T. time. Furthermore, the coefficient K is found to change linearly from 69 to 87 cm-1. For the third glass NB-3, the absorption coefficient increases linearly up to 9 hours of H.T. with the same rate as for NB-1 sample, while from 9 to 18 hours of H.T., the absorption coefficient increases from 40 to 140 cm-1 with the rate much higher compared with both NB-1 and NB-2. The comparison of data for these two glasses suggests that their crystallization proceeds according to different mechanisms, i.e. a conventional growth for NB-1 and a heterogeneous crystallization on Ag nuclei for NB-2. The comparison of samples NB-2 and NB-3 differing in chloride ions concentration shows that for the glass NB-3 featuring with a reduced Cl content no AgCl phase is formed and Agn clusters can act as crystallization nuclei. Thus, the growth of CuCl NCs can be stimulated or catalyzed by different Ag clusters. Noteworthy, the addition of both HCl and/or AgNO3 provides the increase in the amount of CuCl NCs formed in CuCl-con-taining FP glasses, with the introduction of only AgNO3 providing a higher growth compared with the addition of only HCl.
140 120 100 80 - 60 40
9 12 t, hours
15
18
Figure 3. The change of Zi,2 exciton peak absorption coefficient with heating time at 400 oC for the synthesized glasses.
The position of Zi,2 exciton peak absorption maximum can be used to derive the average radius of CuCl NCs as discuss in [13] according to the quantum confinement effect.
The data on the variation in the average CuCl nano-crystal radius at different H.T. times are presented in figure 4. there are small radius CuCl NCs precipitated in glass NB-1 (1,6-2,5 nm) comparing with (2.5-4,3 nm) and (3,4-4,5 nm) in glass NB-2 and NB-3, respectively. As follows from figures 3 and 4, in NB-1 glass this average radius changes by a factor of 0.93 alongside with the increase in the absorption coefficient (amount of CuCl NCs) by the factor of 16. For the glass NB-3 the corresponding factors are 1.33 and 5.9 and in the case of NB-2 sample 1.57 and 1.26, accordingly. These data indicate that the addition of only HCl facilitates the formation of numerous relatively small CuCl NCs, introduction of only AgNO3 promotes the growth of NC size while the charge of both additives affords the maximum rate of NC growth, highest nanocrystal size and the lowest amount of NCs.
4.5
4.0
3.5
<u 2.5 <
2.0
1.5
NB-3
NB-2
NB-1
9 12 t, hours
15
18
Figure 4. The average CuCl nanocrystal size in the studied glasses as a function of H.T. time.
The size of such NCs in FP glass is in agreement with earlier published data [15]. In the glass NB-1 the crystallization of CuCl in glass matrix depends on the critical size of the initial CuCl phase corresponding to the possibility of (CuCl)n molecular clusters formation at melting process and/or the initial stage of heat treatment at temperature below Tg as a result of thermal diffusion [15, 16]. A complex composition of the nanocrystalline phase formed in the glass matrix with CuCl phase containing NaCl, i.e. the presence of an eutectic system with congruently melting NaCl-CuCl crystals results in a reduced melting temperature of CuCl nanophase [17]. Therefore, in the FP glass, a similar system of eutectic crystals can be present due to a large amount of sodium as the glass component and addition of HCl (glass NB-1). The addition of small amount of monovalent silver to a glass can provide the incorporation of silver particles into CuCl clusters. A similar effect was studied in borosil-icate glass, where Cu had the same valence as Ag and some silver replaced copper in the CuCl nanophase to form a dilute solid solution CunAgmCl. The considered affect can account for the formation of large size NCs (glass NB-3) in the same glass matrix under the same heat treatment conditions. Both of two proposed CuCl rich phases can be applied on glass NB-2, with small linear growth of CuCl NCs during H.T, which provided this explanation.
The fourth glass samples, NB-4, was prepared with high content AgNO3 to investigate the nature of silver in glass matrix. Figure 5 shows the absorption spectra of glass NB-4 after H.T. at 400 °C for 3 hours. Three bands are clearly observed, at 383, 441, 562 nm. The bands at 383 and 441 nm are characteristic of metal silver nanoparticles. The glass NB-4 shows SPR peaks around 383 nm and 441 nm corresponding to the electric quadrupole resonance, in addition to the primary dipole resonance of silver NPs [18]. According to
0
3
6
0
3
6
[19], the appearance of these two bands could be explained by the formation of elongated shapes of silver NPs, such as nanorods. The absorption bands at 562 nm relate to Cu NPs.
2,5-i
2,0
1,5-
1,0
0,5-
0,0
Cu 843
200 300 400 500 600 700 800 900 1000 1100 Wavelength, nm
Figure 5. UV-VIS absorption spectra of FP glass NB-4 before (dashed line) and after H.T. at 400 °C for 3 hours (solid line)
In glass NB-4, H.T. at 400°C for 3 hours, results in the reduction of Ag+ and Cu+ to Ag and Cu NPs probably due to the reductive properties of the reaction determined by the presence of carbon from the crucible [20].
As reported before [21], The formation of bi-metallic Ag-Cu NPs in glass matrix was mostly by multi-step ion exchange and H.T., but in this work, these NPs were formed by addition of these particles in the melting process followed by H.T.. Bi-metallic Ag-Cu NPs can be considered in "core-shell" form. In ion-exchange process, the type and radius of core or shell depends on conditions of ion-exchange process. But in the studied glass (NB-4), there are difficult to differentiate the type of core and shell NPs.
Conclusion
Different size CuCl-NCs in FP glass were formed depends on the mechanism of crystalline phase and H.T. time. There are a conventional growth for glass with HCl and a heterogeneous crystallization on Ag nuclei. In addition to, the decrease of chloride ions concentration shows that no AgCl phase is formed and Agn clusters can act as crystallization nuclei. Thus, the growth of CuCl-NCs can be stimulated or catalyzed by different Ag clusters.
H.T. promotes the precipitation of crystalline phases with a complex composition depending on the additives (HCl/ AgNO3) that provides a control over the size and concentration of NCs. Two separated SPR bands are observed in the absorption spectra of silver-enriched FP glass due to formation bi-metallic Ag-Cu core-shell NPs, corresponding to the appearance of a brown-reddish color after H.T.
Generally, the effect of NC size and concentration adjustment is useful for such applications as optical limiters and band pass filters. Furthermore, the considered approach is generally promising for obtaining NCs/NPs of various compounds with desired performances.
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