Tunable synthesis of novel 3D CuI hierarchical architectures and their excellent Cr(VI) removal capabilities

Abstract

Two novel 3D CuI hierarchical architectures were prepared by a simple hydrothermal method in the presence of poly(vinyl pyrrolidone) (PVP) and triethanolamine (TEA). The observation from scanning electron microscopy showed that both CuI microflowers and microspheres were composed of numerous nanoplates. Interestingly, the morphology of the final products could be controlled by varying the amount of PVP and TEA. A possible formation mechanism was proposed based on the experimental results. When the as prepared CuI microflowers and CuI microspheres were tested as absorbents for Cr(VI) removal, both of them showed excellent adsorption properties. The effect of pH on adsorption, adsorption rate and the Cr(VI) removal capacity was discussed. Results showed that both of the CuI adsorbents exhibited excellent removal capacity and fast adsorption rate for Cr(VI) at a pH value of 3.0. The Cr(VI) removal capacity of CuI microflowers and microspheres could reach to 88.2 mg g⁻¹ and 98.7 mg g⁻¹, respectively.

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Introduction

In recent years, inorganic three-dimensional (3D) hierarchical superstructures constructed of nanosized building blocks are of great interest due to potential applications in catalysis,¹ photocatalysis,² energy conversion,³ adsorption,⁴ storage,⁵ sensing,⁶ optoelectronics⁷ and drug delivery.⁸ Therefore, various strategies have been developed successfully to elaborately organize the low dimensional (0D, 1D and 2D) primary building blocks into controllable 3D superstructures based on different driving mechanisms.^9,10 Although great progress has been made on the synthesis approaches for hierarchical architectures, there still remains a great challenge to design a facile and economic approach for the fabrication of hierarchically self-assembled architectures with novel and interesting morphologies. In the solution-based self-assembly process, the introduction of small chelating agents such as poly(vinyl pyrrolidone) (PVP) or triethanolamine (TEA) which act as shape modifiers can be rationally directed to obtain products with desirable morphologies and hierarchical structures.^11,12 In this study, two novel 3D CuI hierarchical architectures were prepared by a simple hydrothermal method in the presence of PVP and TEA.

CuI is a water-insoluble solid with three crystallographic phases, α, β and γ. The low temperature γ-CuI is a p-type semiconductor and has a large direct band gap (3.1 eV).^13–15 As one of the most important I–VII semiconductors, γ-CuI has attracted considerable attention in the past decades owing to its potential applications in fields such as solid-state dye-sensitized solar cells, organic catalysis, optics, adsorption and superhydrophobicity.^16–20 In recent years, dramatic efforts have been dedicated to developing various methods for the fabrication of γ-CuI nano/microstructures with well-defined morphology. CuI microcrystals, nanosheets, nanowires, nanorods, nanotetrahedrons, nanospheres, and cauliflower-like nanostructures have been synthesized by using various strategies, including the hydrothermal method, solvothermal process, sonochemical approach, and normal deposition method.^21–24 However, to the best our knowledge, there have been fewer reports about fabricating uniform CuI three-dimensional (3D) hierarchical superstructures constructed by two-dimensional (2D) nanoplates.

Chromium (Cr), a common toxic heavy metal, exists in the effluents of various industries such as leather tanning, electroplating, pigments and metal finishing.^25,26 It has two stable valence states in the environment, trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)]. Chromium(VI) is 500 times more toxic than Cr(III) and is highly mobile. It causes lung cancer, gastrointestinal disorders, dermatitis and kidney damage in humans.^27–29 Therefore, the removal of Cr(VI) from a contaminated aqueous solution is of great importance. Various methods have been developed for removing Cr(VI) from wastewater, such as chemical precipitation, solvent extraction, ion exchange, membrane filtration, reduction, and adsorption.^30–34 Among these, adsorption has proved to be a convenient and effective method due to its low cost, simplicity and effectiveness.³⁵ Although various adsorbents such as activated carbon, nanoadsorbent, mesoporous material, hybrid polymeric adsorbent and hierarchical materials, etc., have shown excellent adsorption capacities for Cr(VI) ions in water,^36–39 adsorption suffers from some drawbacks such as subsequent difficult solid–liquid separation, low adsorption efficiency, and slow kinetics. Hence, it is highly desirable to prepare a more efficient adsorbent to overcome these defects.

In the present study, we demonstrated a simple hydrothermal route for controllable synthesis of novel hierarchical CuI microcrystals with a high yield and good uniformity in the presence of PVP, TEA and ascorbic acid (AA) using CuSO₄·5H₂O and KI as precursors. It was found that the morphology of the final CuI samples was influenced by the concentration of PVP and TEA introduced into the reaction system. The possible formation mechanism of the CuI hierarchical architectures was carefully investigated. The CuI samples possessed 3D hierarchical architectures and the total size of CuI was in micrometers. When tested as adsorbents for removal of Cr(VI), CuI microflowers and microspheres showed good adsorption properties and they were easy to separate from the liquid after absorption. The adsorption rates on both adsorbents were very fast and the Cr(VI) removal capacity of CuI microflowers and microspheres could reach to 88.2 mg g⁻¹ and 98.7 mg g⁻¹, respectively.

Experimental section

Synthesis of materials

All reagents were of analytical grade and used as received without further purification. Distilled water was used throughout. In a typical procedure, 0.75 mmol of CuSO₄·5H₂O and a varying amount of PVP K30 (MW 10000) were dissolved into 20 mL distilled water. Then, a given amount of TEA, 7 mL 0.01 M aqueous ascorbic acid (AA), 10 mL aqueous 0.15 M KI were added sequentially into the above solution with continuous stirring. After being further stirred for 10 min, the solution was transferred into a 50 mL Teflon-lined autoclave, and maintained at 120 °C for 4 h. Then the autoclave cooled down to room temperature naturally. The resulting precipitate was collected by centrifugation, washed with distilled water and absolute alcohol several times, and finally dried in a vacuum at 60 °C for 6 h. The detailed experimental parameters are listed in Table 1.

Table 1 Experimental conditions for the preparation of CuI hierarchical structures

Sample	PVP (g)	TEA (mL)	Temp. (°C)	Time (h)
S1	0.2	0.5	120	4
S2	0.2	1	120	4
S3	0.2	1.5	120	4
S4	0.4	1	120	4
S5	0.6	1	120	4
S6	0.2	0	120	4
S7	0	1	120	4

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Characterization of materials

The phase purity and crystallinity of the products were identified by X-ray powder diffraction on a Bruker D8-advance power diffractometer equipped with a Cu-Kα radiation source (λ = 0. 15406 nm). The product morphology and microstructures were investigated by scanning electron microscopy (SEM JEOL JSM-6390LV) and transmission electron microscopy (TEM JEOL JEM-2100). Elemental analysis of the samples was conducted using an energy-dispersive X-ray (EDX) detector (DX4, EDAX, USA) attached to the SEM.

Cr(VI) adsorption batch experiments

In these batch experiments, Cr(VI) solutions with different concentration by diluting 1000 mg L⁻¹ K₂Cr₂O₇ standard solution in distilled water were used as the Cr(VI) source. In order to investigate the effect of pH on adsorption, the pH values of Cr(VI) solutions (50 mg L⁻¹) were adjusted to 2.0–9.0 by 0.1 M HCl or NaOH. Then, 15 mg of adsorbent was added to 20 mL of the above solutions under stirring for 2 h and placed for 12 h at room temperature. For the adsorption isothermal study, 15 mg of adsorbent was added to 20 mL solution with the concentration of Cr(VI) from 25 to 200 mg L⁻¹ at a pH value of 3.0. The liquids were stirred for 2 h and placed for 12 h at room temperature. For the adsorption kinetic study, 15 mg of adsorbent was added to 20 mL Cr(VI) solutions (50 mg L⁻¹, pH = 3.0) under stirring. After a specified time, the solid and liquid was separated immediately. The adsorbent was separated from the mixture by centrifugation and the concentration of Cr(VI) was determined using an atomic absorption spectrometer (PerkinElmer PE700). The amount of Cr(VI) adsorbed per unit mass of the adsorbent was evaluated by using the following mass balance equation.

q_e = (C₀ − C_e)V/m (1)

Where q_e (mg g⁻¹) is the amount adsorbed per gram of adsorbent, C₀ (mg L⁻¹) is the initial concentration of Cr(VI) in the solution, C_e (mg L⁻¹) is the equilibrium concentration of Cr(VI) ions, V (L) is the volume of the solution and m is the mass of the adsorbent used (g).

Results and discussion

Structural study

The chemical composition and phase purity of the as-obtained products were characterized by XRD and EDX.

Fig. 1a shows the XRD patterns of CuI microflowers (S3) and CuI microspheres (S5) obtained in the presence of 0.2 g PVP, 1.5 mL TEA and 0.6 g PVP, 1 mL TEA, respectively. All the diffraction peaks of the two products can be readily indexed to cubic γ-CuI [marshite, JCPDS No. 06-0246, F3m (216)]. No other impurity peaks could be detected, suggesting the high purity of the as obtained products. The broadened diffraction peaks imply that the obtained two samples are constructed by nanoscale substructures. The EDX spectrum given in Fig. 1b shows the stoichiometry of the products. The molar ratios of Cu : I in CuI microspheres and microflowers are 1.11 : 1, and 1.01 : 1, respectively, which are in agreement with the stoichiometries proportion of CuI.

Fig. 1 (a) XRD patterns of as-synthesized CuI samples S3 and S5; (b) EDX patterns of as-synthesized CuI samples S3 and S5.

Morphology

In this present work, two typical CuI microcrystals with 3D hierarchical architectures self-assembled from 2D nanosheets were controllably synthesized using a facile hydrothermal method mediated by the PVP and TEA introduced to the reaction system. Flower-like CuI hierarchical structures were prepared when the amount of PVP was 0.2 g and the volume of TEA was 1.5 mL at 120 °C for 4 h. Interestingly, when the amount of PVP increased to 0.6 g and the volume of TEA was 1 mL, the morphology of the final products changed into 3D microspheres constructed by numerous nanosheets. The typical SEM images of the two different morphologies are shown in Fig. 2. Fig. 2a shows the overall SEM image of the flower-like CuI sample. Obviously, the as-obtained product is almost entirely composed of numerous monodispersed flower-like microstructures with a good uniformity and an average diameter of 10 μm. The magnified image (Fig. 2b) shows that the CuI microflowers are constructed of a lot of hyperbranched dendritic structures, which radiate out in all directions from the centre. The length of the dendritic structure is about 3–5 μm and the diameter is about 500 nm. Fig. 2c shows more detailed structural studies of the dendrites. It is found that each of dendrites consisted of many highly aligned 2D nanoplates with an average thickness of 40 nm. The numerous tiny nanoplates are tightly interwoven with each other along a central axis to form a dendrite through self-assembly. Fig. 2d shows the overall SEM image of the 3D CuI microspheres. It can be seen that the microspheres have good mono-dispersion and the average diameter of the product is about 5 μm. No other morphologies could be detected, indicating a high yield of such 3D microspheres. The higher magnification image, Fig. 2e shows that the outer surface of spheres is particularly coarse, suggesting that the microspheres may possess a hierarchical structure. As can be seen from Fig. 2f, that these 3D microspheres are actually built from plenty of 2D nanoplates with a thickness of about 40 nm. The nanoplates are closely aligned layer by layer and intercrossed with each other to form hierarchical microspheres.

Fig. 2 SEM images of CuI hierarchical structures: (a) the overall image of flower-like CuI hierarchical structures S3, (b, c) the magnified images of flower-like CuI hierarchical structures S3; (d) the overall image of CuI microspheres S5, (e, f) the magnified images of CuI microspheres S5.

More detailed structural studies on the primary nanoplates were carried out by transmission electron microscopy (TEM) and selected area electron diffraction (SAED). After ultrasonic treatment during the TEM preparation process, some broken branches of flowers constructed by many nanoplates were observed, as illustrated in Fig. 3a. The HRTEM image and SAED pattern taken from the selected red circular area in (a) are shown in Fig. 3b. The interplanar distance is determined to be 0.349 nm, which coincides well with the d-spacing value of the (111) plane of the cubic CuI crystal, demonstrating the high crystallinity of the product. Meanwhile, the selected area electron diffraction (SAED) (inset in Fig. 3b) reveals that the nanoplate is a single crystal structure. This indicates that the CuI hierarchical structures are composed of numerous single crystal nanoplates.

Fig. 3 (a) TEM image of a typical branch from the flower-like CuI, (b) HRTEM image and SAED pattern taken from the selected red circular area in (a).

Morphological evolution and growth mechanisms of CuI hierarchical structures

In the synthetic process, the amount of PVP and TEA introduced to the system played a crucial role in determining the morphology of the final products. Fig. 4 shows the different morphologies of the as-obtained products due to the different amounts of PVP and TEA added into the system at 120 °C for 4 h. The red arrows among the figures represent the direction of the increasing volume of TEA while keeping the amount of PVP at 0.2 g. The blue arrows represent the direction of the increasing amount of PVP with the volume of TEA at 1 mL.

Fig. 4 SEM images of CuI samples obtained under different experiment conditions: (a) S1, (b) S2, (c) S4, (d) S5, (e) S3, (f) S6, and (g) S7.

Fig. 4a shows the typical SEM image of the as-obtained CuI sample prepared when the amount of PVP was 0.2 g and the volume of TEA was 0.5 mL. It's clear that the sample is composed of a large scale of uniform microspheres with a diameter of about 5 μm. The surface of the microspheres is dramatically rough, which indicates the microspheres possess hierarchical structures. As the volume of TEA increased to 1 mL and the amount of PVP remained unchanged, a large proportion of flower-like CuI microcrystals emerge, coexisting with microspheres in a minority, as shown in Fig. 4b. At a further increased volume of TEA of 1.5 mL, the product was totally composed of flower-like CuI hierarchical structures with the diameter of about 10 μm (Fig. 4e). Fig. 4c shows the typical SEM of CuI microstructures obtained in the presence of 0.4 g PVP and 1 mL TEA. Obviously, the sample also presents morphologies of microflowers and microspheres. However, it could be seen that the number of spheres increased and the branches of microflowers became shorter and thicker. This may indicate an evolutionary tendency from flowers to spheres. When the amount of PVP increased to 0.6 g with the volume of TEA kept at 1 mL, the product is totally composed of a large quantity of uniform microspheres with 3D hierarchical architectures, as illustrated in Fig. 4d.

From the SEM observation, it's not difficult to find the laws of morphological evolution from microspheres to microflowers or vice versa. Firstly, as the amount of PVP increases, the morphology of the CuI microcrystals gradually evolved into uniform microspheres from microflowers. This phenomenon illuminates that PVP is an indispensable factor for formation of microspheres in this reaction system. Secondly, with the increasing volume of TEA in the presence of 0.2 g PVP, the CuI microspheres evolved into microflowers gradually. This clearly indicates that the increasing TEA contributes to the formation of flower-like CuI hierarchical architectures. In order to further investigate the influences of PVP and TEA on the final morphology, another two parallel experiments were conducted. Fig. 4f shows the SEM image of the CuI products obtained in the presence of 0.2 g PVP without adding TEA to the system. The products are also composed of numerous microspheres with the diameter of 5 μm. The microspheres possess 3D hierarchical structures and are constructed of many nanoplates. This result more fully proves that PVP plays an irreplaceable role in the formation of CuI microspheres. When no PVP was added, as shown in Fig. 4g, some broken polyhedrons without a fixed size were obtained in the presence of 1 mL TEA. This indicated that flower-like hierarchical structures couldn't be obtained when only TEA existed in the reaction system. TEA and the moderate amount of PVP are of significance to form CuI microflowers.

According to the product morphologies obtained under different amounts of PVP and TEA, a possible formation mechanism of 3D CuI hierarchical structures can be proposed, as shown in Scheme 1. Acting as excellent chelating reagents, both PVP and TEA complex with Cu²⁺ effectively, which controlled the reaction rate and thus facilitated the growth of high-quality crystals in view of the kinetic process. Upon the introduction of ascorbic acid, the Cu²⁺ complex could be reduced to the Cu⁺ complex. Under hydrothermal conditions, chelation of the Cu⁺ complex would be weakened and the TEA and PVP would be replaced by I⁻ to form CuI nuclei (nanoparticles). PVP, a great surfactant, may serve in two other important roles. First, it might be adsorbed on certain crystallographic facets of CuI nuclei preferentially to prevent the growth of these facets and thus direct the formation of nanoplates. Second, it could be seen as an “organic coating”⁴⁰ which absorbed on the nanoplates, making them aggregate layer by layer and interweave with each other to form homogeneous microspheres. At the same time, the TEA introduced to this system was also a good structure-directing agent, which helped in the formation of the 3D flower-like architecture. With the increasing amount of TEA, the nanoplates aligned and interconnected each other to form many branches on the microspheres. The branches arranged in a radialized pattern with the microspheres as the core and finally evolved into 3D flower-like hierarchical structures.

Scheme 1 Illustration of the possible formation mechanism of different hierarchical architectures obtained in the presence of different amounts of PVP and TEA.

Cr(VI) ion adsorption property

In these adsorption batch experiments, the effects of pH, contact time and initial Cr(VI) concentration on the adsorption of Cr(VI) were investigated using CuI microflowers and CuI microspheres as absorbents. The results indicated that both adsorbents showed good adsorption properties of removing Cr(VI) at pH 3.0.

Solution pH is an important controlling parameter affecting the removal of Cr(VI) because it determines the charge density of the adsorbent and states of the Cr(VI) in solution. The effect of pH on adsorption of Cr(VI) was studied at room temperature by varying the pH of the Cr(VI) solution from 2.0 to 9.0. Fig. 5 shows that the optimum Cr(VI) adsorption is at pH 3.0.

Fig. 5 The effect of pH on the Cr(VI) ions removal in the presence of CuI microflowers (S3) and CuI microspheres (S5).

When the pH of Cr(VI) solution was 2.0, the removal efficiency of Cr(VI) ion was less than 50%. When the pH of Cr(VI) solution rose from 5 to 7 and 9, the Cr(VI) removal efficiency dropped sharply to less than 9%. This phenomenon may be tentatively explained by considering the surface charge of the adsorbents and the degree of ionization of the sorbates. Cr(VI) ions existed in aqueous solution in HCrO₄⁻, Cr₂O₇²⁻, and CrO₄²⁻ forms, which are negatively charged species. With a high concentration of H⁺, the CuI microcrystals were positively charged. Therefore, the electrostatic attraction between positively charged CuI samples and negatively charged Cr(VI) species was the initial driving force to bind the anions onto the surface. However, CuI samples would be dissolved in acid solution when the concentration of H⁺ was too high. So the Cr(VI) removal efficiency decreased at pH 2.0. With the increase of the solution pH, the high concentration of OH⁻ which competed with HCrO₄⁻ suppressed the adsorption reaction, thus accounting for the decrease in the adsorption percentage of Cr(VI) ions at high pH.

Fig. 6a shows the effect of contact time on the adsorption of Cr(VI) ions with an initial concentration of 50 mg L⁻¹ using CuI microflowers and CuI microspheres as adsorbents, respectively, when the pH of the solution was 3.0. The results indicated that both the adsorption rates were very fast during the first 20 min and this phenomenon may be explained by a large number of adsorption sites on CuI samples. With the time increasing, the adsorption rates became slow and the equilibrium was established after 60 min. The removal efficiency of Cr(VI) ions could reach 87.8% on both microspheres and microflowers.

Fig. 6 (a) Adsorption rates of Cr(VI) with Cr(VI) concentrations of 50 mg L⁻¹ using flower-like CuI (S3) and CuI microspheres (S5) as absorbents; (b) Adsorption isotherms of Cr(VI) with different Cr(VI) initial concentrations using flower-like CuI (S3) and CuI microspheres (S5) as absorbents.

To further investigate the effect of initial Cr(VI) concentration on the adsorption, adsorption isotherms were obtained with different Cr(VI) initial concentrations ranging from 25 to 200 mg L⁻¹, as shown in Fig. 6b. The adsorption capacities for Cr(VI) increased with the increase of its equilibrium concentrations in the low concentration region. Two empirical equations, the Langmuir⁴¹ and Freundlich⁴² isotherm models, were used to analyze the adsorption data.

1/q_e = 1/q_m + (1/q_mb) (1/C_e) (2)

q_e = kC_e^1/n (3)

Where C_e is the equilibrium concentration of Cr(VI) ion (mg L⁻¹), q_e is the equilibrium adsorption capacity (mg g⁻¹), q_m (mg g⁻¹) and b (mg L⁻¹) are constants related to the maximum adsorption capacity and energy of adsorption, respectively, k and n are the constants of Freundlich adsorption.

The represented parameters fitted using Langmuir and Freundlich adsorption models (Table 2) indicated that the experimental data were fitted better with the Langmuir adsorption model isotherm compared to the Freundlich isotherm. From which the Cr(VI) removal capacities of CuI microflowers and microspheres were calculated to be 88.2 mg g⁻¹ and 98.7 mg g⁻¹, respectively. These values were much higher than the reported values for ZnO hollow spheres (8.26 mg g⁻¹), flowerlike α-Fe₂O₃ (30 mg g⁻¹), magnetic Fe₃O₄@graphene (33.7 mg g⁻¹) and flowerlike BiOBr (56.2 mg g⁻¹).^39,43–45 It is indicated that the as prepared CuI samples showed a removal capacity comparable to the reported excellent inorganic compound adsorbents for Cr(VI) ions, suggesting that they would be significant potential absorbents for heavy metal ion removal from water.

Table 2 Freundlich–Langmuir adsorption isotherm parameters for the removal of Cr(VI) by CuI microflowers and microspheres

Isotherms	Parameters	Adsorbents
		S3	S5
Langmuir	q m (mg g^−1)	88.2	98.7
	b (L mg^−1)	0.08	0.71
	R ^2	0.991	0.991
Freundlich	k (mg g^−1 (L mg^−1)^1/n)	18.7	46.6
	n	3.04	4.94
	R ^2	0.961	0.900

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Conclusions

In summary, the 3D CuI hierarchical structures self-assembled by nanoplates were successfully synthesized by a hydrothermal method in the presence of PVP and TEA. It was found that the product morphology was varied remarkably with the different amounts of PVP and TEA introduced into the reaction system. A possible growth mechanism was proposed to explain the formation of the CuI hierarchical structures self-assembled with nanoplates. The as prepared CuI microflowers and microspheres were used as adsorbents for removal of Cr(VI) for the first time and exhibited an excellent ability for removal of Cr(VI) in this study. The present work provides a new strategy to prepare novel 3D CuI hierarchical structures and the CuI products would be significant potential absorbents for heavy metal ion removal from water.

Acknowledgements

This study was supported by the Henan Fundamental and Frontier Technology Program (No. 082300420200), China, and the Foundation of Henan Educational Committee (2010B150015), China.

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