# 3D Cu(OH)2 Hierarchical Frameworks: Self-Assembly, Growth, and Application for the Removal of TSNAs

Authors: Hongwei Hou; You Zhu; Qingyuan Hu
Journal: Journal of Nanomaterials
 (2013)

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## Abstract

The 3D hierarchical Cu(OH)2 frameworks were successfully prepared via a simple and surfactant-free chemical self-assembled route. The frameworks were characterized by X-ray powder diffraction, field-emission scanning electron microscopy, and energy dispersive X-ray spectroscopy. The experimental investigations suggest that certain concentrations of NaOH and K2S2O8 are required for the self-assembly and growth of Cu(OH)2. In addition, the orthorhombic crystal structure of Cu(OH)2 may prove to be ideal for the structural development of the final 3D Cu(OH)2 hierarchical frameworks. The nitrogen adsorption and desorption measurements indicate that the Cu(OH)2 frameworks possess a Brunauer-Emmett-Teller surface area of approximately 163.76 m2 g−1. Barrett-Joyner-Halenda measurement of the pore size distribution, as derived from desorption data, presented a distribution centered at 3.05 nm. Additionally, 10 mg of the Cu(OH)2 framework can remove 47% of the N-nitrosonornicotine and 53% of the 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone in cigarette smoke. These results indicate that the Cu(OH)2 frameworks may be a potential adsorbent for removing tobacco-specific N-nitrosamines (TSNAs) from cigarette smoke.


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## Body

## 1. Introduction

Self-assembled materials are the building blocks of the 21st century just as alloys, plastics, and semiconductors are the building blocks of the 20th century. These materials are formed by interatomic and intermolecular interactions other than the traditional covalent, ionic, and metallic bonding forces [1]. Opportunities offered by self-assembled materials are becoming a significant factor in current research directions. Biological life forms demonstrate an intricate pattern of macroscopic structures and functions formed through a hierarchical series of forces. A consequence of this “intelligent self-assembly” is the functional utility of self-replication and self-repair.Until recently, the application of self-assembly to build inorganic structures with morphological diversity and complexity of natural minerals reflects a remarkable level of control over high-order organization of inorganic materials [2–4]. In the process, surfactant, as an important control factor, has been widely used [5–7]. This generally introduced heterogeneous impurities and limited the application and further research of inorganic materials due to trace surfactant absorption. Orthorhombic Cu(OH)2 with layered structure has opened a new challenging field in chemistry, biotechnology, and materials science due to its potential applications in sensors [8, 9]. Meanwhile, the magnetic property of Cu(OH)2, which is sensitive to the intercalation of molecular anions, has made the material a promising candidate for the preparation of copper-based biomineral, such as orthorhombic Cu2(OH)3Cl [10, 11]. Inspired by the layered structure and its accompanying fundamental and practical applications, Cu(OH)2 and the self-assembly synthesis of its elaborate morphologies have attracted considerable attention.There is growing interest in the synthesis and morphological control of theCu(OH)2 nanoribbons, nanowires, nanotubes, and nanorods [12–23]. Despite the excellent approaches on the synthesis routes of low-dimensional Cu(OH)2, only a few pieces of work on the 3D architectures of Cu(OH)2 have been reported. M. Y. Han’s group reported the 3D nanoarchitectures of dendritic Cu(OH)2 [23]. The oxygen from the atmosphere induced the spontaneous oxidation reaction of copper foil in the presence of formamide, and Cu2+ ions were released continuously from the copper foil into the formamide solution while oxygen was reduced. The released copper ions can be captured by coordinating with formamide molecules to form copper complexions (Cu+1/2O2+H2O+4HCONH2→[Cu(HCONH2)4]2++2OH-), which transformed into 3D dendritic copper hydroxide on substrate in 10 days.The removal of tobacco-specific nitrosamines (TSNAs), N-nitrosonornicotine (NNN), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), N-nitrosoanatabine (NAT), and N-nitrosoanabasine (NAB) from cigarette smoke has attracted growing interest over the past years, because of the potent carcinogens in animal studies of TSNAs [24–26]. The design and preparation of new materials are crucial for removing carcinogenic compounds from tobacco smoke. However, to the best of our knowledge, the reduction of TSNAs in cigarette smoke using the 3D hierarchical Cu(OH)2 frameworks has not been reported until now.In this work, a fast and template-free approach for the progressive production of 3D hierarchicalCu(OH)2 frameworks was developed from the oxidation of copper metal under a highly alkaline condition. The high-order, shell-ornamented, self-assembled structures keep the framework in the foreground of research in natural biominerals or synthetic materials. This unexplored Cu(OH)2 frameworks with interesting morphologies and high surface area are the extension of the work of 3D epitaxial scrolls on the Cu(OH)2 ribbons on the copper foil [14]. On the basis of a series of comparative experiments, the formation mechanism and the TSNAs reduction rate of the 3D Cu(OH)2 hierarchical frameworks in cigarette smoke were proposed and thoroughly investigated. It is expected that the impressive 3D frameworks of Cu(OH)2 might bring new ways to reduce carcinogenic compounds in cigarette smoke in the near future.


## 2. Experimental Methods

A typical synthesis of 3DCu(OH)2 hierarchical frameworks on copper foil was performed as follows: an aqueous solution was prepared in a 50 mL glass bottle by mixing 8.4 g of NaOH(7molL-1), 1.08 g of K2S2O8(0.13molL-1), and 30 mL of water that were analytically pure. A piece of 99.99% copper foil (5×2.5×0.25 mm3), which was cleaned in 30% nitric acid for 20 s, rinsed in deionized water and ultrasonically cleaned in acetone, was immersed in the solution. After 30 min, the copper foil was taken out of the solution and rinsed with distilled water.The phase identification of the samples was carried out using X-ray powder diffraction (XRD) patterns with a MAC Science Co. Ltd. MXP 18 AHF X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). The morphology of the products was in situ measured by scanning electron microscopy (FE-SEM JEOL JSM-6700F, FEI PHILIPS XL30 ESEM-TMP) and energy dispersive X-ray spectroscopy (EDS OXFORD INCA400). The nitrogen adsorption and desorption isotherms at 77 K were measured with a Micrometrics ASAP 2020 analyzer. Before measurement, the samples were degassed in vacuum at 140°C for at least 6 h.The content of TSNAs in cigarette smoke was analyzed by liquid chromatography with tandem mass spectrometry (LC-MS/MS, API 5500 triple quadruple mass spectrometer, Applied Biosystems, Foster City, CA, USA). For the wet cut tobacco, a certain amount of Cu(OH)2 hierarchical frameworks was dispersed in water to form an emulsion with a concentration of 50 g L−1. Subsequently, the cut tobacco was sprayed with different volumes of emulsion to obtain 10 mg cigarette−1. The wet cut tobacco samples were dried at 40°C until the moisture content was approximately 12%. The cut tobacco treated by this method was used to make cigarettes with an automated process, and the final weight was 850±30 mg. Before smoking, the cigarettes were conditioned at 295±1 K and 60±2% relative humidity for 48 h and then smoked by a Cerulean SM 450 smoking machine under the standard ISO regime (ISO 4387).


## 3. Results and Discussion

The composition of the sample was examined by X-ray diffraction (XRD). A typical XRD pattern of the sample is shown in Figure1. All of the diffraction peaks can be indexed to the orthorhombic phase Cu(OH)2 (space group Cmc21, JCPDS card No. 13-420) except those marked with an “*”, which were from the copper substrate.Figure 1
XRD pattern of the as-prepared sample with[NaOH]=7molL-1 and [K2S2O8]=0.13molL-1 for 30 min (peaks marked with “*” correspond to the copper foil).The panoramic morphology of the sample, as examined by scanning electron microscopy and shown in Figure2(a), indicated that the hierarchical framework with spherulitic texture grew onto the copper surface. Single particles had a round shape and a uniform size of 10 μm, and peanut- or dumbbell-like particles were formed when two particles merged together. Other shapes were also produced when three or more particles fused together. The structure and composition of the hierarchical framework was characterized by energy dispersive X-ray spectroscopy (EDS) as shown in Figure 2(b). The appropriate molar ratio of Cu : O = 1 : 2, which confirmed well to the XRD results.(a) FESEM image of the 3DCu(OH)2 hierarchical framework prepared with [NaOH]=7molL-1 and [K2S2O8]=0.13molL-1 for 30 min. (b) EDS analysis of the hierarchical framework.
(a)
(b)The images of a typical single framework are shown in Figures3(a) and 3(b), which indicated a flower-like morphology with lamellar and hierarchical structure. The magnified 3D spherical morphology confirmed that the highly porous structure with ultrathin lamellar structures of ~25 nm in thickness was identical throughout the hierarchical crystal. Figure 3(c) shows the submicrometer Cu(OH)2 structures prepared in a previous study with [KOH]=2.5molL-1and [K2S2O8]=0.13molL-1 for 15 min at 40°C   [14]. The products on the ribbons were Cu(OH)2 epitaxial scrolls. Furthermore, from the magnified images shown in Figure 3(d), these submicrometer structures were wool balls growing from the ribbons and formed from the lamellar structures. The difference in concentration of alkalinity and K2S2O8 was the sole contributing factor for the different morphologies in the as-prepared samples. This observation suggested that an appropriate concentration of alkalinity and K2S2O8 was essential to the unique and interesting morphology.((a), (b)) FESEM image of the 3DCu(OH)2 hierarchical framework with [NaOH]=7molL-1 and [K2S2O8]=0.13molL-1 for 30 min. ((c), (d)) FESEM image of the Cu(OH)2 scrolling structures on the Cu(OH)2 submicrometer ribbons with [KOH]=2.5molL-1 and [K2S2O8]=0.13molL-1 for 15 min at 40°C.
(a)
(b)
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(d)In the present approach, the formation ofCu(OH)2 on copper foil involved a simple oxidation process described as follows:
(1)Cu+2NaOH+K2S2O8→Cu(OH)2+Na2SO4+K2SO4It is well known that in order to attain the desired self-assembled structures, the effect of salts and ionic species in the system cannot be ignored [27]. This also suggests that a certain level of NaOH and K2S2O8 concentration was required for the growth of the self-assembled Cu(OH)2 hierarchical framework.To thoroughly investigate the formation mechanism of theCu(OH)2 hierarchical framework, the influence of the concentrations of NaOH and K2S2O8 to the Cu(OH)2 morphology was examined. The Cu(OH)2 samples produced under different concentrations of NaOH with [K2S2O8]=0.13molL-1 for 30 min were observed by SEM as shown in Figures 4(a)–4(d). At a lower concentration of [NaOH]=0.13molL-1, the oxidation process was slow and the final product was a few multiped crystals on the foil. When [NaOH]=1molL-1, many ribbons emerged. When [NaOH]=3molL-1, the products were some flower-like structures scrolled on the ribbons. After the concentration of NaOH was increased to 9molL-1, the bulk crystals appeared on the foil. Similarly, the Cu(OH)2 samples produced under different concentrations of K2S2O8 with [NaOH]=7molL-1 for 30 min were examined and the corresponding SEM images are shown in Figure 5. At a lower K2S2O8 concentration of 0.033molL-1, a mass of acicular crystals formed on the Cu foil as shown in Figures 5(a) and 5(b). When [K2S2O8]=0.17molL-1, some scrolling floccule structures appeared and scrolled on the ribbons, as shown in Figures 5(c) and 5(d).FESEM images ofCu(OH)2 structures prepared under different concentrations of NaOH with [K2S2O8]=0.13molL-1 for 30 min. (a) [NaOH]=0.13molL-1; (b) [NaOH]=1molL-1; (c) [NaOH]=3molL-1; (d) [NaOH]=9molL-1.
(a)
(b)
(c)
(d)FESEM images ofCu(OH)2 structures prepared under different concentrations of K2S2O8 with [NaOH]=7molL-1 for 30 min. ((a), (b)) [K2S2O8]=0.033molL-1; ((c), (d)) [K2S2O8]=0.17molL-1.
(a)
(b)
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(d)A comparison of the FESEM images of theCu(OH)2 structures prepared under different reaction times with [NaOH]=7molL-1 and [K2S2O8]=0.13molL-1 showed that the oxidation process was fast. Namely, the high alkalinity and the effect of the oxidant determined the rapid rate of growth. Through the careful analysis of the samples produced under different reaction times, a possible growth process may be proposed. Under the initial high alkalinity, the copper foil oxidized expeditiously and produced crystals with a prism-like morphology (Figure 6(a)). After 5 min, many spindle-like, multiped crystals were found, as shown in Figure 6(b), which were 500 nm in length on average and consisted of several nanoribbons. Spindle-like structures have been validated as one of the structures of hydroxide [28]. As the reaction proceeded, the coherent nanoribbons began curving as shown in Figure 6(c). After 15 min, the straight nanoribbons were edged out by curved ones (Figure 6(d)). After 20 min, the outline of the structures was finalized with a much smaller size (Figure 6(e)). Then at 30 min, the 3D hierarchical framework formed, as shown in Figure 6(f).FESEM images of theCu(OH)2 structures prepared with [NaOH]=7molL-1 and [K2S2O8]=0.13molL-1 for (a) 1 min, (b) 5 min, (c) 10 min, (d) 15 min, (e) 20 min, and (f) 30 min.
(a)
(b)
(c)
(d)
(e)
(f)Based on the above discussions ofCu(OH)2 morphology, the orthorhombic crystal structure of Cu(OH)2 may prove to be ideal for the development of the final Cu(OH)2 structure [29], as demonstrated by Huang [30–32]. In the process, the rapid rate of nucleation resulted in the small concentration of nucleus, which provided the possibility of anisotropic nuclear growth and determined the structure of Cu(OH)2. The schematic illustration of the proposed development of the Cu(OH)2 structure is shown in Figure 7. It is known that Cu2+  ions prefer square planar coordination by OH- (shown in Figure 7(a)), which leads to an extended chain (Figure 7(b)). The chains can be connected through the coordination of OH- to dz2 of Cu2+, forming two-dimensional (2D) ultrathin sheet-like structures (Figure 7(c)). Finally, the 2D layers are stacked through the relatively weak hydrogen bond interactions, and two or more adjacent hierarchical crystals can further expand and eventually self-organize/merge into continuous porous networks [33, 34] and an intricate three-dimensional (3D) crystal (Figure 7(d)).Figure 7
Schematic illustration of the proposed development of theCu(OH)2 structure. (a) The layered structure of orthorhombic Cu(OH)2, (b) further growing of layered structure, (c) formation of a two-dimensional (2D) ultrathin sheet-like structure; (d) the whole growth process of self-assembling and ripening of the 3D Cu(OH)2 hierarchical framework.The self-assembly process involves forces such as hydrogen bonding, dipolar forces, other van der Waals forces, hydrophilic or hydrophobic interactions (all frequently referred to as “supramolecular interactions”), chemisorption, surface tension, and gravity. Forces involving ions and ligands (the “coordinate-covalent bond”) have resulted in supramolecular structures [12]. Considering the related experimental results and the possible development of Cu(OH)2 structure, we predict that a certain level of concentration of NaOH and K2S2O8 is required for the self-assembly and growth of Cu(OH)2, and the lamellar structure of Cu(OH)2 is the determining factor for the final hierarchical framework. That is, the potential self-assembly forces in Cu(OH)2 will induce the formation of the 3D hierarchical framework under the appropriate concentration of NaOH and K2S2O8. Using experimental results, we propose the possible mechanism for the assembly and growth of the Cu(OH)2 hierarchical framework. After the initial nucleation, the newly formed Cu(OH)2 will grow into nanoparticles followed by the formation of primary, radical, and spindle-like multiped structures (as shown in Figure 6) as the result of the anisotropic nature of the Cu(OH)2 structure. Under the appropriate levels of NaOH and K2S2O8, the copper foil will be further eroded while making the Cu(OH)2 structure evolve rapidly from the lamellar structure to the 3D hierarchical framework. Namely, the spindle-like multiped structures will continue to fissure and grow, driven by the self-assembly forces as mentioned earlier [35, 36].Nitrogen adsorption/desorption measurements were conducted to characterize the Brunauer-Emmett-Teller (BET) surface area and internal pore structure. The recorded adsorption and desorption isotherms for the 3DCu(OH)2 hierarchical frameworks are shown in Figure 8. The BET specific surface area of the sample was calculated from N2 isotherms to be approximately 163.76 m2 g−1. Barrett-Joyner-Halenda (BJH) measurements for the pore-size distribution, as derived from desorption data, presented a distribution centered at 3.05 nm. The smaller pores presumably arose from the spaces within the hierarchical structure. The results showed that the obtained hierarchical frameworks were porous and confirmed well with the FESEM images.Figure 8
N
2 absorption and desorption isotherms and pore size distribution (inset) for the 3D Cu(OH)2 hierarchical framework.The as-prepared 3DCu(OH)2 hierarchical frameworks were wetted and put into the cut tobacco to study the TSNAs content in cigarette smoke. Table 1 shows the content of four TSNAs in cigarette smoke after adding 3D Cu(OH)2 hierarchical frameworks into cut tobacco, which was analyzed by LC-MS/MS. Compared with “blank” cigarettes with no 3D Cu(OH)2, the TSNAs contents of cigarettes with 10 mg cigarette−1  Cu(OH)2 were selectively reduced with no changes in tar content, which may influence the cigarette smoke quality. The adsorptive capacity of Cu(OH)2 for NNN, NAT, NAB, and NNK is 47%, 28%, 24%, and 53%, respectively. Because the Cu(OH)2 could block the N–NO group and hamper the formation of TSNAs [25], the 3D Cu(OH)2 hierarchical framework added into the cut tobacco not only influenced cigarette combustion, but also worked as adsorbents that reduce the amount of TSNAs. When Cu(OH)2 with sophisticated morphology was added into the wet tobacco, chemical transitions took place during burning and showed to be more effective than zeolite.Table 1
The amount of four TSNAs in cigarette smoke produced by cut tobacco with the 3D Cu(OH)2 hierarchical framework.
Compounds
Blank cigarette*
10 mg cigarette−1 3D Cu(OH)2
Reduction (%)
NNN
ng cig−1
49.56
26.25
47
NAT
28.22
20.35
28
NAB
15.21
11.56
24
NNK
18.20
8.62
53
Tar
mg cig−1
13.50
12.80
5
*The blank cigarette was identical to the one with no Cu(OH)2.A cigarette sample addedCu(OH)2 frameworks and a blank sample (with no Cu(OH)2 frameworks) were smoked under the standard ISO regime (ISO 4387). Toxicology tests including cytotoxic test, mutagenicity testing of salmonella typhimurium, and in vitro micronucleus test were used to detect the influence of health. The three smoke toxicology tests showed that there were no significant differences between the two samples.


## 4. Conclusions

A template-free, chemical route involving a simple oxidation process to grow self-assembled 3DCu(OH)2 hierarchical frameworks was developed. In comparison with other approaches, the alternative route is fast, safe, simple, and inexpensive and this effective and convenient method is suitable for modern chemical synthesis of 3D hierarchical frameworks. The experimental investigations suggest that certain concentrations of NaOH and K2S2O8 are required for the self-assembly and growth of Cu(OH)2. Furthermore, the orthorhombic crystal structure of Cu(OH)2 may prove to be ideal for the structural development of the final Cu(OH)2 hierarchical frameworks. The BET specific surface area of the sample was calculated from N2 isotherms to be approximately 163.76 m2 g−1. The BJH for the pore-size distribution presented a distribution centered at 3.05 nm. It was found that 10 mg of Cu(OH)2 framework can remove 47% of the NNN and 53% of the NNK in cigarette smoke. The successful fabrication of the hierarchical morphology provides new applications in reducing the carcinogenic compounds in cigarette smoke.



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*Source: 797082-2013-06-27.xml*