Preparation of a hierarchical flower-like γ-Al₂O₃@C composite exhibiting enhanced adsorption performance for congo red by high temperature transformation of γ-AlOOH@C precursors

Abstract

A hierarchical flower-like γ-Al₂O₃@C composite was synthesized via a hydrothermal method combined with a heat treatment process using glucose and Al nanopowders as reactants. Herein, glucose plays key roles in the formation of the flower-like structure and carbon shell. Due to the high specific surface area of the flower-like γ-Al₂O₃@C composite (346.2 m² g⁻¹), its adsorption ability was investigated. The organic dye Congo Red (CR) was selected as a subject in the study. Over 90% of CR could be removed from aqueous solution within 5 min, and the final adsorption capacity reaches 244.5 mg g⁻¹ which is much higher than most γ-AlOOH and γ-Al₂O₃ materials reported before, indicating it would be a promising material used in the water treatment industry.

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Nowadays, water pollution is one of the biggest and the most alarming problems that demands formidable and effective solutions.^1,2 Congo Red (CR), is well known as a carcinogenic azo dye pollution source.³ The treatment of CR in wastewater is difficult, because the dye is generally present in its sodium salt form giving it very good water solubility. So far, an adsorption technique is widely regarded as a promising method for removing hazardous and environmentally undesirable chemicals such as azo dyes.⁴ Some nano-adsorbents such as carbons, metal oxides, transition metal oxides, and composites etc. have been used for the removal of CR from aqueous solutions.^5–8 However, simple preparation method, low-cost and nontoxic raw materials, and good adsorption efficiency are still challenges for using the above-mentioned nanostructures.

Combining multiple components into one material is useful for sewage treatment.^9–11 To overcome low adsorption efficiency, toxic, and complex preparation procedures of the current adsorbents, it would be very meaningful to design a composite consisting of common materials, such as alumina and carbon. In previous reports, both alumina and carbon are the most frequently used as water treatment material.^12,13 Although lots of alumina and carbon materials have been applied in water treatment, as far as we know, there is rare report about the investigation on CR removal and adsorption mechanism using composite of alumina and carbon.

Herein, hierarchical flower-like γ-Al₂O₃@C composite was synthesized via a hydrothermal method combined with a heat treatment process using glucose and Al nanopowders as reactants (detailed experimental methods and characterizations seen in ESI†). Significantly, the hierarchical flower-like γ-Al₂O₃@C composite showed high specific surface area and exhibited excellent efficient absorption ability for CR.

A typical X-ray diffraction (XRD) pattern of the γ-Al₂O₃@C composite is shown in Fig. 1a, and the two wide angle diffraction peaks can be perfectly indexed to the cubic γ-Al₂O₃ (JCPDS card no. 10-0425). However, no obvious diffraction signals of C phase have been detected, which might be attributed to thin carbon layer and amorphous structure. Compared the XRD pattern of γ-Al₂O₃@C composite with the XRD pattern of corresponding precursor (seen in Fig. S1, ESI†), it can be found that γ-AlOOH is completely transformed to γ-Al₂O₃ after calcinations. SEM and TEM images of the flower-like γ-Al₂O₃@C composite are shown in Fig. 1b to d. Compared with the SEM and TEM images of corresponding precursor (seen in Fig. S2, ESI†), it can be observed that the morphology of γ-Al₂O₃@C composite still presents the uniform and well-distributed hierarchical flower-like structures even after 600 °C of calcinations, and the thickness of nanoflakes is about 3 nm measured from SEM image. The size distribution from the TEM image exhibits that the particles are in the size range of 280–480 nm, and the average size is about 360 nm. The size and morphology of γ-Al₂O₃@C composite are extremely similar to those of the γ-AlOOH@C precursor, indicating that the size and morphology can be well preserved by carbon during the phase-transformation. From STEM image in Fig. 1e and the corresponding elemental mappings in Fig. 1f to i, Al, O and C atoms are uniformly distributed in the whole particle, which further indicates that flower-like γ-Al₂O₃@C composite is synthesized.

Fig. 1 Flower-like γ-Al₂O₃@C composite: (a) XRD pattern; (b) SEM image; (c and d) TEM images with different magnifications; (e) STEM images; (f) mixed elemental mappings image; corresponding elemental mappings for (g) Al, (h) O, and (i) C.

Raman spectroscopy was used to further characterize the presence of carbon coating on the surface of γ-Al₂O₃@C composite, and the result is shown in Fig. 2a. The peak at about 1600 cm⁻¹ (G band) corresponds to the vibration of sp²-bonded carbon atoms, and the D band at about 1370 cm⁻¹ is ascribed to defects like edges and disordered carbon.¹⁴ The calculated I_D/I_G ratio is around 0.76, indicating the predominantly amorphous/disordered carbon nature, which is totally consistent of the XRD results and further proves the existence of carbon coating. To investigate the specific surface area and porosity of the as-prepared γ-Al₂O₃@C composite, N₂ adsorption analysis was carried out. As shown in Fig. 2b, the isotherm of the sample is type IV, which is characteristic of mesoporous materials.¹⁵ The BET surface area was calculated to be 346.2 m² g⁻¹, showing that the sample has a relatively high surface-to-volume ratio. The corresponding BJH pore size distribution plot is shown in the inset of Fig. 2b, which reveals a quite narrow distribution for the γ-Al₂O₃@C composite centered at 3–4 nm. Furthermore, the pore volume is estimated to be 0.68 cm³ g⁻¹. The flower-like γ-Al₂O₃@C composite possesses extremely high BET surface area and large pore volume which may be due to its nanoflakes-aggregated structure.

Fig. 2 (a) Raman spectra and (b) nitrogen adsorption/desorption isotherm and corresponding Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset) of flower-like γ-Al₂O₃@C composite.

To investigate the role of glucose, γ-AlOOH product was prepared in the absence of glucose. Only lamellar structures were formed (seen in Fig. S3 and S4, ESI†). It is reasonable to consider that glucose is a very important factor on the growth process of the flower-like γ-Al₂O₃@C, not only as carbon resources but also deciding the formation of flower-like morphology. To obtain the proper flower-like γ-AlOOH@C composite, the best reaction temperatures and time were investigated (Fig. S5 and S6, ESI†). The well-defined γ-AlOOH@C precursor can be obtained at 180 °C for 3 h. During the hydrothermal process at 180 °C, Al powders reacted with H₂O, then Al(OH)₃ was present. Because the newly formed Al(OH)₃ was active and unstable, it further dehydrated and converted to γ-AlOOH. After 0.5 h of hydrothermal treatment, the surface of Al microsphere became coarse and small γ-AlOOH nanoflakes were formed. Meanwhile, amorphous carbonaceous layer with large number of oxygen-containing functional groups uniformly covered on the surface of γ-AlOOH nanoflakes due to the dehydration and aromatization of glucose.^16,17 Hydrogen bonds were formed between the carboxyl of carbonaceous layer and the hydroxyl ions in adjacent planes of AlO₆ octahedral of γ-AlOOH, which restrained the further nucleation and growth of these new formed nanoflakes to large area of lamellar structures.^18,19 With the increasing of carbonaceous amount coated on the surface of γ-AlOOH nanoflakes, which favors the self-assembly of nanoflakes by van der Waals forces and the hydrogen bonding into 3D architectures.²⁰ Since the raw Al powders have a certain degree of aggregation, large sized ellipsoidal and subsphaeroidal γ-AlOOH@C assemblies were obtained. (Fig. S6a, ESI†). An increase of the reaction time initiated the epitaxial growth of the newly formed nanoflakes and the further dissolution process of those ellipsoidal and subsphaeroidal structures to achieve a minimum total surface free energy,²¹ and thus uniformly distributed flower-like γ-AlOOH@C composite with thicker carbon coating were fabricated. Finally, the γ-AlOOH@C precursor was calcined at 600 °C to achieve the topotactic phase transition from flower-like γ-AlOOH to flower-like γ-Al₂O₃@C composite. The above-mentioned process was exhibited in Scheme 1. When there was no glucose participating in the hydrothermal process, without the function of hydrogen bonds, the γ-AlOOH crystallites prefer to grow on the edges of the already existing nanoflakes along the main crystallographic [001] axis to form nanoflakes with higher thermodynamic stability under appropriate basic hydrothermal conditions.¹⁵ Thus, a large area of lamellar γ-AlOOH was formed (seen in Fig. S4, ESI†). From the above discussions, it is noting that glucose is not only as carbon source, but also controls the formation of flower-like structure. The flower-like structure for the γ-Al₂O₃@C composite have given it the high BET surface area and large pore volume which make it a promising candidate for application in dyes removal from water.

Scheme 1 Formation process of flower-like γ-Al₂O₃@C composite and CR adsorption diagram.

TG/DTA analyses of γ-AlOOH were performed under Ar atmosphere to investigate the transformation of γ-AlOOH to γ-Al₂O₃ (Fig. S7, ESI†). It can be found that the weight loss occurred in three steps with a total weight loss of approximately 44.2%. The three endothermic DTA peaks of the first stage (below 120 °C) were caused by desorption of physically adsorbed water, and it corresponded to 17% of weight loss. The second stage ranged from 120–400 °C, and the TG curve shows a continuous weight loss of 13.9% due to the evaporation of residual water. At even higher temperatures (400–600 °C), the strong endothermic DTA peak around 450 °C can be contributed to the dehydroxylation of γ-AlOOH and its transformation into γ-Al₂O₃, and the corresponding weight loss is 13.3%.

To further confirm the flower-like γ-Al₂O₃@C composite possessing good adsorption abilities, the CR adsorption experiment was carried out at room temperature (adsorption experiments for CR seen in ESI†). Fig. 3a shows the calculated adsorption capacity of the γ-Al₂O₃@C which possesses high adsorption rate. Over 90% of CR can be removed from aqueous solution within 5 min. With the prolonging of adsorption time, the final adsorption capacity of the γ-Al₂O₃@C can reach as high as 244.5 mg g⁻¹. Results show that the as-prepared γ-Al₂O₃@C powders exhibit excellent adsorption efficiency than most of γ-AlOOH and γ-Al₂O₃ materials reported before (Table S1, ESI†). From the corresponding successive UV-visible spectra (Fig. 3b), the quick decrease of the CR concentration can also be visually observe. Furthermore, the γ-Al₂O₃@C samples containing CR could be regenerated by a simple thermal treatment under Ar atmosphere at 280 °C for 4 h,²² and the results are shown in Fig. S8, ESI.† It can be found that their performance kept quiet well after the first regeneration (curves b). With the increase of cycle index, though the adsorption performance weakly decreased, the final removal ability of the regenerated material after four cycles can kept 92.8% of the original.

Fig. 3 (a) Time-dependent CR adsorption capacities on γ-Al₂O₃@C and (b) corresponding successive UV-visible spectra of CR adsorption at different time intervals.

For comparison, the adsorption experiments of pure γ-Al₂O₃ (the corresponding XRD pattern is shown in Fig. S11, ESI†) and pure carbon sample for CR were also performed, as shown in Fig. S9, ESI.† Their adsorption capacities and adsorption rate were listed in Table S2, ESI.† It's obvious that the γ-Al₂O₃@C composite exhibits much better adsorption efficiency than the other two samples.

N₂ adsorption analysis of pure γ-Al₂O₃ and carbon samples was also carried out (Fig. S10, ESI†). The surface areas of γ-Al₂O₃ and carbon were calculated to be 282.2 m² g⁻¹ and 426.5 m² g⁻¹, respectively (Table S3, ESI†). The pore volume of γ-Al₂O₃ and carbon were separately estimated to be 1.16 cm³ g⁻¹ and 0.25 cm³ g⁻¹. Compared with the values of γ-Al₂O₃@C composite, though the carbon sample possessed the highest surface area, it still displayed far lower adsorption performance than those of the other two samples due to its quiet low pore volume and almost negligible mesoporous. The γ-Al₂O₃ has larger pore volume while lower BET surface area than those of γ-Al₂O₃@C, so its adsorption capacities are lower than γ-Al₂O₃@C. In general, the remarkable adsorption ability of γ-Al₂O₃@C can be contributed to the combined effect of high specific surface area, large pore volume and narrow pore size distribution centered at 3–4 nm.

The kinetic curves of the CR adsorption on γ-Al₂O₃@C were investigated using different kinetic models, and pseudo-first-order (eqn (1)) and pseudo-second-order (eqn (2)) are given below.

In the above equations, the corresponding rate constants are denoted by k₁ (min⁻¹) and k₂ (g mg⁻¹ min⁻¹), and q_e and q_t indicate the amount of CR adsorbed (mg g⁻¹) at equilibrium and at time t, respectively.

Fig. 4 shows the two time-dependent plots, and the corresponding calculated kinetic parameters are listed in Table 1. The comparison of correlation coefficients (R²) of both models suggests that the adsorption mechanism of γ-Al₂O₃@C can be better explained by pseudo-second-order (0.99992) than pseudo-first-order (0.6614). Furthermore, the calculated adsorption capacity 244.49 mg g⁻¹ from the pseudo-second-order kinetics is extremely consistent with the experimental adsorption capacity 244.5 mg g⁻¹. However, the calculated adsorption capacity 40.7 mg g⁻¹ from the pseudo-first-order is much smaller than the experimental value. All these above data shows that the adsorption process present an ideal fit to the pseudo-second-order kinetics, indicating the adsorption mechanism depends on the adsorbate and adsorbent.

Fig. 4 Pseudo-first-order kinetics (a) and pseudo-second-order kinetics (b) for the adsorption of CR on γ-Al₂O₃@C.

Table 1 Adsorption parameters got from the two kinetic models for the adsorption of CR on γ-Al₂O₃@C

Kinetic models	qe (mg g^−1)	k	R2
Pseudo-first-order	40.7	3.36 × 10^−2 (min^−1)	0.6614
Pseudo-second-order	244.49	9.14 × 10^−3 (g mg^−1 min^−1)	0.99992

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Based on the adsorption kinetics and experimental results, the main adsorption mechanism may be explained as electrostatic interaction (Scheme 1). CR molecules possess sulfonic acid group and amine group (–NH₂) which can be completely dissolved and protonated in water, thus become negative charged and positive charged in water, respectively.²³ The carbon coating with negative charges^24,25 of γ-Al₂O₃@C composites are prone to adsorb the protonated and positively charged –NH₂, accordingly the CR adsorbed on the surface of composites. At last, the clear solution was obtained after the electrostatic interaction between the γ-Al₂O₃@C and CR.

Conclusions

In summary, hierarchical flower-like γ-Al₂O₃@C composite was successfully synthesized by a facile and environmentally friendly one-step hydrothermal route followed by a heat treatment process. During the reaction system, glucose not only as carbon source, but also controlled the formation of the flower-like products. Moreover, a possible glucose promoted formation mechanism was proposed. Further adsorption investigation indicates that the as-obtained γ-Al₂O₃@C composite possesses high adsorption efficiency for CR removal in aqueous solutions, and the adsorption capacity can be as high as 244.5 mg g⁻¹ which is much higher than most of γ-AlOOH and γ-Al₂O₃ materials reported before. Hence, the novel hierarchical flower-like γ-Al₂O₃@C composite may be a promising material for dyes sewage treatment.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51501068) and 2013 Open Foundation of the Key Lab of Automobile Materials, Jilin University, from Natural Scientific Basic Research Fund for Platform and Base Construction (Grant No. 13-450060491571).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18339a

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