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Facile preparation of mesoporous titanium nitride microspheres as novel adsorbent for trace Cd2+ removal from aqueous solution
Journal of Physics and Chemistry of Solids 81 (2015) 20–26
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Facile preparation of mesoporous titanium nitride microspheres as novel adsorbent for trace Cd2 þ removal from aqueous solution Xianghua Li a,c, Ying Sun a, Yihe Zhang b,n, Minhua Cao a,n a Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, Beijing Institute of Technology, Beijing 100081, PR China b National Laboratory of Mineral Materials, School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, PR China c School of Pharmacy, Hubei University of Science and Technology, Xianning 437100, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 November 2014 Received in revised form 13 January 2015 Accepted 27 January 2015 Available online 28 January 2015
A simple and facile route is developed for the preparation of mesoporous titanium nitride (TiN) microspheres with a large surface area and a highly porous structure. This method involves the preparation of an amorphous precursor via a solvothermal reaction and subsequent short-time nitridation process to mesoporous TiN. X-ray diffraction and X-ray photoelectron spectroscopy analyses confirm the composition of the resultant sample. The mesoporous structure of the as-prepared TiN sample has been studied by nitrogen adsorption/desorption measurement. The surface area obtained by the Brunauer–Emmett– Teller method is 50.6 m2 g 1 and the pore sizes are in the range of 2.0–4.0 nm. In addition, the obtained sample is evaluated as a new sorbent for Cd2 þ removal. Experimental parameters such as solution pH, contact time and concentration of adsorbate are optimized. The maximum adsorption capacity for Cd2 þ removal is found to be 12.40 mg g 1 and it is a potentially attractive adsorbent for Cd2 þ removal from aqueous solution. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Inorganic compounds Nanostructures Chemical synthesis Surface properties
1. Introduction Transition metal nitrides, such as NbN, VN, TiN, and Ta3N5, have drawn much attention due to their unique properties and many practical applications [1,2]. Among transition metal nitrides, TiN has received particular attention owing to its high melting temperature, special hardness and strong corrosion resistance. TiN is often deposited on the surface of titanium, steel and aluminum components in order to protect their properties [3]. Recent researches show that TiN can be employed in a wide variety of fields including catalysis, sensors, electroanalysis, and energy storage both in Li-based batteries and supercapacitors [4–6]. In addition, TiN also shows promising photocatalytic property and good biocompatibility [7,8]. During the past decades, TiN nanoparticles (NPs) are usually prepared by high-temperature reaction of titanium oxide with different nitrogen sources, such as NH3, N2, cyanamide, urea, etc. [9–14], since high temperature heat treatment is beneficial to promote its transformation and re-crystallization. However, the products thus obtained generally suffer from the collapse of the nanopores and smaller surface areas. This is because that the high n
Corresponding authors. E-mail address: [email protected] (M. Cao).
http://dx.doi.org/10.1016/j.jpcs.2015.01.010 0022-3697/& 2015 Elsevier Ltd. All rights reserved.
temperature sintering process unavoidably increases particle size, which severely hinders the formation of porous structure. But it is worth noting that the high performances for mesoporous structures are often achieved among the reported TiN nanostructures. The improved properties of such mesoporous structures are largely associated with their high surface area, effective active sites or fast mass transfer rate [15]. So the preparation of mesoporous microstructures is particularly necessary for their emerging applications. General strategy towards synthesizing mesoporous TiN nanostructures tends to use templating approaches. In the past decades, soft templates such as surfactants or polymers and hard templates, for example, rigid silica, mineral, or carbon nanostructures, are widely employed as structure directing agents [16]. For example, mesoporous TiN/C composites recently have been prepared using nanostructured graphitic carbon nitride (g-CN) as a reactive template and nitrogen source [17]. Unfortunately, silica hard template needs to be prepared in advance for the hightemperature synthesis of the g-CN and the silica template removal has to use hydrofluoric acid, which is a hazardous chemical. Thereby this approach is very complicated, expensive and cumbersome. In addition, Yang and coworkers reported mesoporous TiN nanostructures, which was obtained by ammonolyzing CdTiO3 (here CdTiO3 also as a reactive template). The thermal
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volatilization of Cd component successfully brings a uniform mesoporous structure for final product [2]. But this reaction process is also time-consuming, which needs a long time in order to produce smaller pores and higher surface area. Despite many successes, templating methods face quite a few challenges, such as the tedious and time-consuming procedure for template-synthesis or the tedious process to get rid of the template after material synthesis. Therefore, it is a challenge to explore a facile synthesis procedure for preparing porous TiN. Recently, Dong et al. have reported a template-free strategy of the synthesis of mesoporous TiN spheres by using mesoporous TiO2 spheres as a precursor and using cyanamide as nitrogen source [4,18]. Mesoporous TiO2 spheres can be directly converted into mesoporous TiN spheres with well preserved morphology. Inspired by this synthetic strategy, herein, we demonstrate a simple, efficient and reproducible strategy for the synthesis of high-surface-area mesoporous TiN microspheres, which involves the preparation of porous TiO2 precursor via a solvothermal reaction and subsequent short-time nitridation process to mesoporous TiN. It is worth mentioning that the preparation of the TiO2 precursor is more convenient, and the pretreatment of TiO2 precursor and nitrogen source before calcination is avoided in our work. More importantly, the resultant mesoporous TiN microspheres obtained by this facile and reproducible way can be employed as a new adsorbent for trace Cd2 þ removal from aqueous solution. They show high adsorption performance towards the trace Cd2 þ , which has far higher drinking-water quality standards recommended by World Health Organization (WHO). The contact time and pH are monitored to optimize the adsorption process, and the kinetics and adsorption efficiency of Cd2 þ removal with TiN absorbent are also studied. All these studies have proved that the mesoporous TiN microspheres can be used as an effectively adsorbent for trace Cd2 þ removal. To the best of our knowledge, no reports exist concerning the exploitation of mesoporous TiN microspheres as a sorbent for trace Cd2 þ removal.
2. Experimental section 2.1. Synthesis of mesoporous TiN microspheres All chemicals used were of analytical grade and were employed without further purification. In a typical procedure, 14 mmol ascorbic acid was dissolved in 60 mL of absolute ethanol in a beaker with magnetic stirring, and then 2 mmol TiCl4 was added to form a clear transparent solution with brown color. Subsequently, the solution was transferred into 80 mL Teflon-line stainless steel autoclave and heated in an oven at 200 °C for 7 h. After the autoclave was cooled down to room temperature in air, the solid product was separated by centrifugation, washed with deionized water and absolute alcohol several times, and dried in a vacuum at 80 °C for 6 h. Finally, the sample was treated at 800 °C in NH3 gas (99.999%) with a flow rate of 100 mL min 1 for 4 h and eventually mesoporous TiN microspheres were obtained.
21
spectroscopy (XPS, ESCALAB250) using Al K irradiation. Inductively-coupled plasma spectrometer (ICP) was determined using a Jarrel-ASH (ICAP-9000). 2.3. Adsorption experiments The solutions containing different concentrations of Cd2 þ (1, 10, 20, 30, 50, and 100 mg L 1) were prepared by dissolving Cd(NO3)2 6H2O in deionized water. Typically, 40 mg of mesoporous TiN adsorbent were added into a series of 50 mL flasks, respectively, which contained 25 mL of aqueous Cd2 þ solution with the desired concentration. The initial pH values of the solution were adjusted ranging from 2.0 to 7.0 using 0.1 mol L 1 NaOH and HCl, respectively. Then the flasks were placed in a horizontal shaker and shaken at 180 rpm while keeping the temperature at 25 °C. At each given time interval, 10 mL of suspension was collected and the TiN sorbent was removed from the solution by centrifugation. The concentration of metal ion in the supernatant liquid was determined by Jarrel-ASH equipment.
3. Results and discussion 3.1. Characterization of mesoporous TiN microspheres The synthesis strategy for the mesoporous TiN microspheres includes two steps. First, amorphous TiO2 precursor is prepared by solvothermal reaction of the ethanol solution of TiCl4 in the presence of ascorbic acid. Second, TiN microspheres with a well-defined mesoporous structure can be easily obtained after nitriding the amorphous TiO2 precursor at 800 °C for 4 h. XRD is used to determine the phase composition of the assynthesized samples before and after ammonolysis (Fig. 1). The precursor obtained by the solvothermal reaction in the presence of ascorbic acid exhibits an amorphous structure and no diffraction peaks is found. After ammonolysis at 600 °C under NH3 atmosphere for 4 h, pure anatase TiO2 is formed. When the temperature is increased to 700 °C, cubic TiN phase starts to form, but the product is a mixture of TiN and TiO2. Compared to the TiO2, TiN gives rather weak signals. Nonetheless, it is obvious that one major diffraction peak can be observed at around 43 °C. Finally, a complete conversion from amorphous TiO2 to TiN phase is achieved at 800 °C. The result of XRD analyses suggests no oxynitride phase is
2.2. Characterizations X-ray diffraction (XRD) patterns were obtained on a Shimadzu XRD-6000 diffractometer (D/max 40 kV) with Cu Kα radiation (λ ¼ 1.54178 Å). The product morphology was observed by a fieldemission scanning electron microscope (FESEM, JEOL S-4800) and transmission electron microscopy (TEM, H-8100) operating at 200 kV accelerating voltage. Brunauer–Emmett–Teller (BET) surface area of the sample was measured by nitrogen adsorption using a Micromeritics ASAP 2010 system. The chemical states of the surface elements were recorded with X-ray photoelectron
Fig. 1. XRD patterns of the precursor (obtained by solvothermal reaction) before and after calcination in NH3 for 4 h.
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Fig. 2. Typical FESEM images of the amorphous TiO2 precursor (a,b) and TiN prepared after nitridation (c,d). (e) The selected FESEM area for the element mapping and (f,g) corresponding elemental mapping images for Ti and N, respectively.
detected during the ammonolysis process in the tested temperatures. The similar trend in the ammonolysis treatment has been reported in the literatures but the reactions generally require a higher temperature or a longer time [10,13]. The formation of the TiN phase reported here is easier, which is mainly due to the amorphous nature of the TiO2 precursor used in our experiment. The average crystallite size of TiN sample determined using the Scherrer equation is about 10 nm, which is consistent with the TEM observation below. The morphology of the as-synthesized amorphous precursor and the TiN prepared by ammonolysis treatment at 800 °C is investigated by FESEM. Fig. 2a shows a typical FESEM image of the amorphous precursor by the solvothermal method, which indicates that the sample is composed of nearly spherical microspheres and that some microspheres twin together. The average diameter of the microspheres is approximately 1 μm. A highmagnification FESEM image (Fig. 2b) clearly reveals the detailed structure of the spherical morphology. The microspheres actually are composed of many individual NPs, which may further give rise to a porous structure. After ammoniation, the TiN sample almost completely retains the spherical morphology of the precursor except that the high temperature treatment unavoidably brings disadvantage of aggregation (Fig. 2c). In addition, the TiN microspheres also maintain the size of the TiO2 precursor. The magnified FESEM image (Fig. 2d) reveals that there are many nanopores spreading on each TiN microsphere. Moreover, the distribution of Ti and N elements is relatively homogeneous, which is proved by
the corresponding elemental mapping images (Fig. 2e–g). In this process, the amorphous precursor has been successfully transformed into TiN without destroying the pores even after being ammonolyzed at 800 °C. The morphology and microstructure are further studied by TEM measurements. Fig. 3a shows a low-magnification TEM image, displaying that the sample mainly consists of twinned spherical particles, in well agreement with that from above FESEM image. But, well-defined spheres occasionally are observed (Fig. 3b). The high-magnification TEM image discloses that the twinned microspheres are assembled from irregular NPs (Fig. 3c). These NPs are highly crystalline (Fig. 3d) and the lattice fringe presented in the high-resolution TEM image (Fig. 3e) has an interplanar distance of 0.21 nm, which can be attributed to the (200) plane of TiN. The diffraction rings in the selected area electron diffraction (SAED) image indicate the polycrystalline nature of the TiN microspheres (Fig. 3f). To further investigate the surface electronic state and composition of the TiN microspheres, XPS is carried out. Fig. 4 shows the high resolution N 1s and Ti 2p spectra. The N 1s peak at 396 eV can be assigned to nitrogen species of metal nitrides in lattice (Fig. 4a), which is absent in TiO2 crystalline phase [19,20]. The Ti 2p3/2 peak has split into three major doublets (Fig. 4b). More specifically, the Ti 2p3/2 peak at 455.1 eV is typical for TiN, the Ti 2p3/2 peak at 456.8 eV for TiO or TiOxNy, and the one at 458.1 eV for TiO2 [10,21]. Furthermore, it can be clearly seen that the intensity of the Ti 2p3/2 XPS peak from TiN is far weaker than those of corresponding oxide
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Fig. 3. TEM images of the mesoporous TiN spheres (a–d). HRTEM image of the same TiN sphere figuring out (200) lattice fringes (e) and the corresponding SAED pattern (f).
or oxynitride, but no TiO2 or TiOxNy is detected from the XRD pattern. This case is often common in reported studies, and can be attributed to inevitable surface passivation with atmospheric oxygen before the XPS measurement [10,22]. Therefore, from the XPS spectrum it can be seen that TiN, together with oxynitride and amorphous TiO2 formed during air exposure all are present on the surface of the TiN sample. To get further insight into the porous nature of the TiN microspheres, the nitrogen adsorption-desorption measurements are performed. The nitrogen sorption–desorption isotherm and corresponding pore-size distribution of TiN microspheres are showed in Fig. 5. It can be seen that the nitrogen sorption isotherms (Fig. 5a) can be classified as type IV with a H2 hysteresis loop according to IUPAC classification, indicating that the TiN microspheres exhibit a mesoporous structure [23–25]. The pore size distribution curve discloses that the mesopore diameter mainly centers at 3.2 nm, which is obtained by the Barret–Joyner–Halenda (BJH) analysis from the adsorption branch (Fig. 5b). The mesoporous structure may result from the void space of TiN NPs. Moreover, BET surface area of the TiN microspheres is determined to be 50.6 m2 g 1. This surface area is very large in comparison to TiN prepared by traditional high temperature synthesis [17,26,27].
a
We expect that the as-obtained TiN microspheres can be useful in the water treatments due to their porous structure and high surface area. To elucidate the formation process of the mesoporous TiN microspheres, a brief illustration of the solvothermal-nitridation two-step synthetic method is proposed as shown in Scheme 1. First, the alcoholysis of Ti4 þ in the presence of ascorbic acid leads to the formation of smaller particles under solvothermal conditions, which subsequently self-assemble into spherical porous TiO2 particles. The porous nature of the TiO2 spheres has been confirmed by the nitrogen sorption-desorption measurement (Fig. S1). Then, TiN microspheres with a well-defined mesoporous structure can be easily obtained after nitriding the amorphous porous TiO2 precursor at 800 °C for 4 h, during which the porous structure and spherical morphology of the TiO2 precursor have been well maintained during the high temperature treatment. The mesopores of the TiN mainly result from the accumulation of TiN nanoparticles. This work also demonstrates that the present approach can mildly produce a mesoporous structure compared to conventional hard-template methods.
b
Fig. 4. XPS spectra for TiN microspheres of N 1s (a) and Ti 2p (b).
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a
b
Fig. 5. Nitrogen adsorption–desorption isotherms (a) and pore size distribution (b) of the mesoporous TiN microspheres.
Scheme 1. Schematic illusmation of the formation process of mesoporous TiN microspheres.
3.2. Cd2 þ removal test Due to good capability to adsorb a large amount of metal ions, mesoporous materials with high specific surface area have been widely in the field of waste water treatment [28–31]. As we all know, cadmium is a strong carcinogen and can be accumulated in human body, leading to destruction of the human bones and nerves. Therefore, we have investigated the sorption of the mesoporous TiN microspheres for Cd2 þ removal from aqueous solution. Fig. 6a shows the contact time of Cd2 þ removal at pH 6.16 with
a
b
different initial Cd2 þ concentrations of 1 and 11 mg L 1 by the mesoporous TiN microspheres. It can be observed that the adsorption process for the low Cd2 þ concentration is far faster than the one for the higher Cd2 þ concentration. It is noteworthy to mention that the adsorption equilibrium time is as short as 15 min with 0.6 g L 1 of the TiN microsphere adsorbent at the initial Cd2 þ concentration of 1 mg L 1. Cd2 þ concentration is decreased to 0.0015 mg L 1, which is far lower than the drinking water quality standards (0.005 mg L 1) set by WHO [32]. However, most reports basically have been focused on the removal of high concentration Cd2 þ from water and the final Cd2 þ concentration is still higher than 0.06 mg L 1 after treatment [33–36]. Furthermore, we also study the adsorption dynamics of the mesoporous TiN microspheres, as shown in Fig. 6b and c. The plots of t/qt against time t exhibits well linearity with correlation coefficient (r2) of above 0.99 (Fig. 6b), while plots of ln(qe qt) versus time t are not reasonable straight line (Fig. 6c). Here, qe and qt are the adsorption capacity at equilibrium and at time t, respectively. This observation suggests that the Cd2 þ removal by TiN adsorbent ought to follow the pseudo-second-order model. The correlation coefficient constants of two kinetics models by theoretical calculation are presented in Table1. It has been widely reported that solution pH is a crucial factor affecting the adsorption capacities towards Cd2 þ [37,38]. As shown in Fig. 7a, the effect of initial pH on removal efficiency of Cd2 þ is investigated. The adsorption capacity of Cd2 þ is found to increase with the increase of initial pH value in solution and this can be due to competitive behavior between H þ and Cd2 þ on the surface of the adsorbate [39]. The pH values corresponding to the optimal adsorption capacity are found in the range of 6–7, while the Cd2 þ sorption is quite weak at pH 2 (1.25 mg L 1) and 3 (1.85 mg L 1), respectively. In order to fully inhibit Cd2 þ hydrolysis and avoid low-soluble-OH species, such as Cd(OH) þ and Cd(OH)2, sorption experiments are carried out at pH 6.1. Moreover, we also investigate the pH changes before and after adsorption, which are showed in Fig. 7b. It is interesting to observe that all of
c
Fig. 6. The contact time for absorption of Cd2 þ (a), linear fit of pseudo-second-order model (b) and pseudo-first-order model (c).
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Table 1 The correlation coefficient constants of pseudo-first-order model and pseudo-second-order model. Cd2 þ concentration (mg L 1)
1 11
Theoretical qe (mg g 1)
0.565 6.77
Pseudo-first-order constants
Pseudo-second-order constants
qe (mg g 1)
k1 (h 1)
r2
qe (mg g 1)
k2 (mg g 1 h 1)
r2
6.717 10 3 2.3714
0.511 0.5944
0.5109 0.9063
0.565 6.931
1.35 103 5.546
1 0.999
Fig. 7. Effect of initial pH values on adsorption of 20 mg L 1 Cd2 þ solution (a), pH values on adsorption in various concentrations of Cd2 þ solution before and after absorption (b).
a
b
c
Fig. 8. Adsorption isotherm for the adsorption of Cd2 þ (a), linear fit of Langmuir model (b) and Freundlich model (c).
Table 2 The correlation coefficient constants of Langmuir model and Freundlich model. Metal ions
Cd2 þ
Langmuir constants
Freundlich constants
qm (mg g 1)
KL (L mg 1)
r2
KF (mg g 1) (L g 1)1/n
n
r2
12.4
1.795 102
0.999
0.884
1.465
0.957
the pH changes are within 0.8 for Cd2 þ concentrations in the range of 1.26–55.54 mg L 1. Therefore the pH stability provides further evidence for the selection of initial pH value. To well study the adsorption mechanism and kinetics, we continue to investigate the adsorption process of Cd2 þ by the pseudo-first-order and pseudo-second-order kinetics models. The pseudo-first-order equation for metal ion adsorption kinetics is below: ln(qe qt) ¼ln qe k1t, where qe (mg g 1) and qt (mg g 1) are the mass of ions adsorbed at equilibrium and time t, respectively. The reaction rate equilibrium constant k1 can be calculated by the linear of ln(qe qt) versus time t [38]. The pseudo-second-
order equation is also given: t/qt ¼ 1/k2qe þt/qe, thus k2 can be obtained from the plot of t/qt against time t and qe also can be obtained by gradient [40]. The adsorption isotherm studies disclose the interaction between the adsorbate with adsorbent. Fig. 8a shows adsorption isotherm on as-prepared TiN with Cd2 þ concentrations in the range of 1–150 mg L 1 at pH 6.1 and 25 °C. The Langmuir equation (ce/qe ¼ce/qm þ1/kLqm) and Freundlich equation (ln qe ¼ln kF þ1/n ln ce) are used to investigate the adsorption behavior of Cd2 þ by TiN (Fig. 8b and c). The correlation coefficient constants of two absorption models are calculated by fitting the absorption
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data to equations, which are listed in Table 2. It is more appropriate to use the Langmuir absorption model than the Freundlich absorption model, because the linear relationship between ce/qe and ce is perfect with the correlation coefficient (r2) of 0.999. The maximum adsorption capacity of Cd2 þ is 12.40 mg g 1 by Langmuir isotherm calculation. The Langmuir model basically refers to behavior of surface coverage between absorbate and surface site without interaction [41]. Therefore, the absorption of Cd2 þ , following the Langmuir absorption model, is likely based on the mesoporous structure feature of TiN.
4. Conclusions In summary, a simple and mild route for the synthesis of mesoporous TiN microspheres is demonstrated using the amorphous TiO2 as the precursor. The mesoporous TiN microspheres with a large surface area of 50.6 m2 g 1 and pore size distribution from 2.0 to 4.0 nm exhibit excellent water treatment performance with high removal behavior towards trace Cd2 þ . The kinetics of adsorption and adsorption equilibrium isotherm have been investigated and the saturated adsorption capacity of Cd2 þ is given to be 12.40 mg g 1. More importantly, the TiN has no any effect on the pH of the test solution, which is especially important for water treatment. This is the first report on mesoporous TiN as absorbent for trace Cd2 þ removal. Therefore, the application of TiN microspheres for heavy metal removal has a great potential in waste water engineering.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 21471016 and 21271023) and the 111 Project (B07012).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jpcs.2015.01.010.
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Facile preparation of mesoporous titanium nitride microspheres as novel adsorbent for trace Cd2 þ removal from aqueous solution Xianghua Li a,c, Ying Sun a, Yihe Zhang b,n, Minhua Cao a,n a Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, Beijing Institute of Technology, Beijing 100081, PR China b National Laboratory of Mineral Materials, School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, PR China c School of Pharmacy, Hubei University of Science and Technology, Xianning 437100, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 November 2014 Received in revised form 13 January 2015 Accepted 27 January 2015 Available online 28 January 2015
A simple and facile route is developed for the preparation of mesoporous titanium nitride (TiN) microspheres with a large surface area and a highly porous structure. This method involves the preparation of an amorphous precursor via a solvothermal reaction and subsequent short-time nitridation process to mesoporous TiN. X-ray diffraction and X-ray photoelectron spectroscopy analyses confirm the composition of the resultant sample. The mesoporous structure of the as-prepared TiN sample has been studied by nitrogen adsorption/desorption measurement. The surface area obtained by the Brunauer–Emmett– Teller method is 50.6 m2 g 1 and the pore sizes are in the range of 2.0–4.0 nm. In addition, the obtained sample is evaluated as a new sorbent for Cd2 þ removal. Experimental parameters such as solution pH, contact time and concentration of adsorbate are optimized. The maximum adsorption capacity for Cd2 þ removal is found to be 12.40 mg g 1 and it is a potentially attractive adsorbent for Cd2 þ removal from aqueous solution. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Inorganic compounds Nanostructures Chemical synthesis Surface properties
1. Introduction Transition metal nitrides, such as NbN, VN, TiN, and Ta3N5, have drawn much attention due to their unique properties and many practical applications [1,2]. Among transition metal nitrides, TiN has received particular attention owing to its high melting temperature, special hardness and strong corrosion resistance. TiN is often deposited on the surface of titanium, steel and aluminum components in order to protect their properties [3]. Recent researches show that TiN can be employed in a wide variety of fields including catalysis, sensors, electroanalysis, and energy storage both in Li-based batteries and supercapacitors [4–6]. In addition, TiN also shows promising photocatalytic property and good biocompatibility [7,8]. During the past decades, TiN nanoparticles (NPs) are usually prepared by high-temperature reaction of titanium oxide with different nitrogen sources, such as NH3, N2, cyanamide, urea, etc. [9–14], since high temperature heat treatment is beneficial to promote its transformation and re-crystallization. However, the products thus obtained generally suffer from the collapse of the nanopores and smaller surface areas. This is because that the high n
Corresponding authors. E-mail address: [email protected] (M. Cao).
http://dx.doi.org/10.1016/j.jpcs.2015.01.010 0022-3697/& 2015 Elsevier Ltd. All rights reserved.
temperature sintering process unavoidably increases particle size, which severely hinders the formation of porous structure. But it is worth noting that the high performances for mesoporous structures are often achieved among the reported TiN nanostructures. The improved properties of such mesoporous structures are largely associated with their high surface area, effective active sites or fast mass transfer rate [15]. So the preparation of mesoporous microstructures is particularly necessary for their emerging applications. General strategy towards synthesizing mesoporous TiN nanostructures tends to use templating approaches. In the past decades, soft templates such as surfactants or polymers and hard templates, for example, rigid silica, mineral, or carbon nanostructures, are widely employed as structure directing agents [16]. For example, mesoporous TiN/C composites recently have been prepared using nanostructured graphitic carbon nitride (g-CN) as a reactive template and nitrogen source [17]. Unfortunately, silica hard template needs to be prepared in advance for the hightemperature synthesis of the g-CN and the silica template removal has to use hydrofluoric acid, which is a hazardous chemical. Thereby this approach is very complicated, expensive and cumbersome. In addition, Yang and coworkers reported mesoporous TiN nanostructures, which was obtained by ammonolyzing CdTiO3 (here CdTiO3 also as a reactive template). The thermal
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volatilization of Cd component successfully brings a uniform mesoporous structure for final product [2]. But this reaction process is also time-consuming, which needs a long time in order to produce smaller pores and higher surface area. Despite many successes, templating methods face quite a few challenges, such as the tedious and time-consuming procedure for template-synthesis or the tedious process to get rid of the template after material synthesis. Therefore, it is a challenge to explore a facile synthesis procedure for preparing porous TiN. Recently, Dong et al. have reported a template-free strategy of the synthesis of mesoporous TiN spheres by using mesoporous TiO2 spheres as a precursor and using cyanamide as nitrogen source [4,18]. Mesoporous TiO2 spheres can be directly converted into mesoporous TiN spheres with well preserved morphology. Inspired by this synthetic strategy, herein, we demonstrate a simple, efficient and reproducible strategy for the synthesis of high-surface-area mesoporous TiN microspheres, which involves the preparation of porous TiO2 precursor via a solvothermal reaction and subsequent short-time nitridation process to mesoporous TiN. It is worth mentioning that the preparation of the TiO2 precursor is more convenient, and the pretreatment of TiO2 precursor and nitrogen source before calcination is avoided in our work. More importantly, the resultant mesoporous TiN microspheres obtained by this facile and reproducible way can be employed as a new adsorbent for trace Cd2 þ removal from aqueous solution. They show high adsorption performance towards the trace Cd2 þ , which has far higher drinking-water quality standards recommended by World Health Organization (WHO). The contact time and pH are monitored to optimize the adsorption process, and the kinetics and adsorption efficiency of Cd2 þ removal with TiN absorbent are also studied. All these studies have proved that the mesoporous TiN microspheres can be used as an effectively adsorbent for trace Cd2 þ removal. To the best of our knowledge, no reports exist concerning the exploitation of mesoporous TiN microspheres as a sorbent for trace Cd2 þ removal.
2. Experimental section 2.1. Synthesis of mesoporous TiN microspheres All chemicals used were of analytical grade and were employed without further purification. In a typical procedure, 14 mmol ascorbic acid was dissolved in 60 mL of absolute ethanol in a beaker with magnetic stirring, and then 2 mmol TiCl4 was added to form a clear transparent solution with brown color. Subsequently, the solution was transferred into 80 mL Teflon-line stainless steel autoclave and heated in an oven at 200 °C for 7 h. After the autoclave was cooled down to room temperature in air, the solid product was separated by centrifugation, washed with deionized water and absolute alcohol several times, and dried in a vacuum at 80 °C for 6 h. Finally, the sample was treated at 800 °C in NH3 gas (99.999%) with a flow rate of 100 mL min 1 for 4 h and eventually mesoporous TiN microspheres were obtained.
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spectroscopy (XPS, ESCALAB250) using Al K irradiation. Inductively-coupled plasma spectrometer (ICP) was determined using a Jarrel-ASH (ICAP-9000). 2.3. Adsorption experiments The solutions containing different concentrations of Cd2 þ (1, 10, 20, 30, 50, and 100 mg L 1) were prepared by dissolving Cd(NO3)2 6H2O in deionized water. Typically, 40 mg of mesoporous TiN adsorbent were added into a series of 50 mL flasks, respectively, which contained 25 mL of aqueous Cd2 þ solution with the desired concentration. The initial pH values of the solution were adjusted ranging from 2.0 to 7.0 using 0.1 mol L 1 NaOH and HCl, respectively. Then the flasks were placed in a horizontal shaker and shaken at 180 rpm while keeping the temperature at 25 °C. At each given time interval, 10 mL of suspension was collected and the TiN sorbent was removed from the solution by centrifugation. The concentration of metal ion in the supernatant liquid was determined by Jarrel-ASH equipment.
3. Results and discussion 3.1. Characterization of mesoporous TiN microspheres The synthesis strategy for the mesoporous TiN microspheres includes two steps. First, amorphous TiO2 precursor is prepared by solvothermal reaction of the ethanol solution of TiCl4 in the presence of ascorbic acid. Second, TiN microspheres with a well-defined mesoporous structure can be easily obtained after nitriding the amorphous TiO2 precursor at 800 °C for 4 h. XRD is used to determine the phase composition of the assynthesized samples before and after ammonolysis (Fig. 1). The precursor obtained by the solvothermal reaction in the presence of ascorbic acid exhibits an amorphous structure and no diffraction peaks is found. After ammonolysis at 600 °C under NH3 atmosphere for 4 h, pure anatase TiO2 is formed. When the temperature is increased to 700 °C, cubic TiN phase starts to form, but the product is a mixture of TiN and TiO2. Compared to the TiO2, TiN gives rather weak signals. Nonetheless, it is obvious that one major diffraction peak can be observed at around 43 °C. Finally, a complete conversion from amorphous TiO2 to TiN phase is achieved at 800 °C. The result of XRD analyses suggests no oxynitride phase is
2.2. Characterizations X-ray diffraction (XRD) patterns were obtained on a Shimadzu XRD-6000 diffractometer (D/max 40 kV) with Cu Kα radiation (λ ¼ 1.54178 Å). The product morphology was observed by a fieldemission scanning electron microscope (FESEM, JEOL S-4800) and transmission electron microscopy (TEM, H-8100) operating at 200 kV accelerating voltage. Brunauer–Emmett–Teller (BET) surface area of the sample was measured by nitrogen adsorption using a Micromeritics ASAP 2010 system. The chemical states of the surface elements were recorded with X-ray photoelectron
Fig. 1. XRD patterns of the precursor (obtained by solvothermal reaction) before and after calcination in NH3 for 4 h.
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Fig. 2. Typical FESEM images of the amorphous TiO2 precursor (a,b) and TiN prepared after nitridation (c,d). (e) The selected FESEM area for the element mapping and (f,g) corresponding elemental mapping images for Ti and N, respectively.
detected during the ammonolysis process in the tested temperatures. The similar trend in the ammonolysis treatment has been reported in the literatures but the reactions generally require a higher temperature or a longer time [10,13]. The formation of the TiN phase reported here is easier, which is mainly due to the amorphous nature of the TiO2 precursor used in our experiment. The average crystallite size of TiN sample determined using the Scherrer equation is about 10 nm, which is consistent with the TEM observation below. The morphology of the as-synthesized amorphous precursor and the TiN prepared by ammonolysis treatment at 800 °C is investigated by FESEM. Fig. 2a shows a typical FESEM image of the amorphous precursor by the solvothermal method, which indicates that the sample is composed of nearly spherical microspheres and that some microspheres twin together. The average diameter of the microspheres is approximately 1 μm. A highmagnification FESEM image (Fig. 2b) clearly reveals the detailed structure of the spherical morphology. The microspheres actually are composed of many individual NPs, which may further give rise to a porous structure. After ammoniation, the TiN sample almost completely retains the spherical morphology of the precursor except that the high temperature treatment unavoidably brings disadvantage of aggregation (Fig. 2c). In addition, the TiN microspheres also maintain the size of the TiO2 precursor. The magnified FESEM image (Fig. 2d) reveals that there are many nanopores spreading on each TiN microsphere. Moreover, the distribution of Ti and N elements is relatively homogeneous, which is proved by
the corresponding elemental mapping images (Fig. 2e–g). In this process, the amorphous precursor has been successfully transformed into TiN without destroying the pores even after being ammonolyzed at 800 °C. The morphology and microstructure are further studied by TEM measurements. Fig. 3a shows a low-magnification TEM image, displaying that the sample mainly consists of twinned spherical particles, in well agreement with that from above FESEM image. But, well-defined spheres occasionally are observed (Fig. 3b). The high-magnification TEM image discloses that the twinned microspheres are assembled from irregular NPs (Fig. 3c). These NPs are highly crystalline (Fig. 3d) and the lattice fringe presented in the high-resolution TEM image (Fig. 3e) has an interplanar distance of 0.21 nm, which can be attributed to the (200) plane of TiN. The diffraction rings in the selected area electron diffraction (SAED) image indicate the polycrystalline nature of the TiN microspheres (Fig. 3f). To further investigate the surface electronic state and composition of the TiN microspheres, XPS is carried out. Fig. 4 shows the high resolution N 1s and Ti 2p spectra. The N 1s peak at 396 eV can be assigned to nitrogen species of metal nitrides in lattice (Fig. 4a), which is absent in TiO2 crystalline phase [19,20]. The Ti 2p3/2 peak has split into three major doublets (Fig. 4b). More specifically, the Ti 2p3/2 peak at 455.1 eV is typical for TiN, the Ti 2p3/2 peak at 456.8 eV for TiO or TiOxNy, and the one at 458.1 eV for TiO2 [10,21]. Furthermore, it can be clearly seen that the intensity of the Ti 2p3/2 XPS peak from TiN is far weaker than those of corresponding oxide
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Fig. 3. TEM images of the mesoporous TiN spheres (a–d). HRTEM image of the same TiN sphere figuring out (200) lattice fringes (e) and the corresponding SAED pattern (f).
or oxynitride, but no TiO2 or TiOxNy is detected from the XRD pattern. This case is often common in reported studies, and can be attributed to inevitable surface passivation with atmospheric oxygen before the XPS measurement [10,22]. Therefore, from the XPS spectrum it can be seen that TiN, together with oxynitride and amorphous TiO2 formed during air exposure all are present on the surface of the TiN sample. To get further insight into the porous nature of the TiN microspheres, the nitrogen adsorption-desorption measurements are performed. The nitrogen sorption–desorption isotherm and corresponding pore-size distribution of TiN microspheres are showed in Fig. 5. It can be seen that the nitrogen sorption isotherms (Fig. 5a) can be classified as type IV with a H2 hysteresis loop according to IUPAC classification, indicating that the TiN microspheres exhibit a mesoporous structure [23–25]. The pore size distribution curve discloses that the mesopore diameter mainly centers at 3.2 nm, which is obtained by the Barret–Joyner–Halenda (BJH) analysis from the adsorption branch (Fig. 5b). The mesoporous structure may result from the void space of TiN NPs. Moreover, BET surface area of the TiN microspheres is determined to be 50.6 m2 g 1. This surface area is very large in comparison to TiN prepared by traditional high temperature synthesis [17,26,27].
a
We expect that the as-obtained TiN microspheres can be useful in the water treatments due to their porous structure and high surface area. To elucidate the formation process of the mesoporous TiN microspheres, a brief illustration of the solvothermal-nitridation two-step synthetic method is proposed as shown in Scheme 1. First, the alcoholysis of Ti4 þ in the presence of ascorbic acid leads to the formation of smaller particles under solvothermal conditions, which subsequently self-assemble into spherical porous TiO2 particles. The porous nature of the TiO2 spheres has been confirmed by the nitrogen sorption-desorption measurement (Fig. S1). Then, TiN microspheres with a well-defined mesoporous structure can be easily obtained after nitriding the amorphous porous TiO2 precursor at 800 °C for 4 h, during which the porous structure and spherical morphology of the TiO2 precursor have been well maintained during the high temperature treatment. The mesopores of the TiN mainly result from the accumulation of TiN nanoparticles. This work also demonstrates that the present approach can mildly produce a mesoporous structure compared to conventional hard-template methods.
b
Fig. 4. XPS spectra for TiN microspheres of N 1s (a) and Ti 2p (b).
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a
b
Fig. 5. Nitrogen adsorption–desorption isotherms (a) and pore size distribution (b) of the mesoporous TiN microspheres.
Scheme 1. Schematic illusmation of the formation process of mesoporous TiN microspheres.
3.2. Cd2 þ removal test Due to good capability to adsorb a large amount of metal ions, mesoporous materials with high specific surface area have been widely in the field of waste water treatment [28–31]. As we all know, cadmium is a strong carcinogen and can be accumulated in human body, leading to destruction of the human bones and nerves. Therefore, we have investigated the sorption of the mesoporous TiN microspheres for Cd2 þ removal from aqueous solution. Fig. 6a shows the contact time of Cd2 þ removal at pH 6.16 with
a
b
different initial Cd2 þ concentrations of 1 and 11 mg L 1 by the mesoporous TiN microspheres. It can be observed that the adsorption process for the low Cd2 þ concentration is far faster than the one for the higher Cd2 þ concentration. It is noteworthy to mention that the adsorption equilibrium time is as short as 15 min with 0.6 g L 1 of the TiN microsphere adsorbent at the initial Cd2 þ concentration of 1 mg L 1. Cd2 þ concentration is decreased to 0.0015 mg L 1, which is far lower than the drinking water quality standards (0.005 mg L 1) set by WHO [32]. However, most reports basically have been focused on the removal of high concentration Cd2 þ from water and the final Cd2 þ concentration is still higher than 0.06 mg L 1 after treatment [33–36]. Furthermore, we also study the adsorption dynamics of the mesoporous TiN microspheres, as shown in Fig. 6b and c. The plots of t/qt against time t exhibits well linearity with correlation coefficient (r2) of above 0.99 (Fig. 6b), while plots of ln(qe qt) versus time t are not reasonable straight line (Fig. 6c). Here, qe and qt are the adsorption capacity at equilibrium and at time t, respectively. This observation suggests that the Cd2 þ removal by TiN adsorbent ought to follow the pseudo-second-order model. The correlation coefficient constants of two kinetics models by theoretical calculation are presented in Table1. It has been widely reported that solution pH is a crucial factor affecting the adsorption capacities towards Cd2 þ [37,38]. As shown in Fig. 7a, the effect of initial pH on removal efficiency of Cd2 þ is investigated. The adsorption capacity of Cd2 þ is found to increase with the increase of initial pH value in solution and this can be due to competitive behavior between H þ and Cd2 þ on the surface of the adsorbate [39]. The pH values corresponding to the optimal adsorption capacity are found in the range of 6–7, while the Cd2 þ sorption is quite weak at pH 2 (1.25 mg L 1) and 3 (1.85 mg L 1), respectively. In order to fully inhibit Cd2 þ hydrolysis and avoid low-soluble-OH species, such as Cd(OH) þ and Cd(OH)2, sorption experiments are carried out at pH 6.1. Moreover, we also investigate the pH changes before and after adsorption, which are showed in Fig. 7b. It is interesting to observe that all of
c
Fig. 6. The contact time for absorption of Cd2 þ (a), linear fit of pseudo-second-order model (b) and pseudo-first-order model (c).
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Table 1 The correlation coefficient constants of pseudo-first-order model and pseudo-second-order model. Cd2 þ concentration (mg L 1)
1 11
Theoretical qe (mg g 1)
0.565 6.77
Pseudo-first-order constants
Pseudo-second-order constants
qe (mg g 1)
k1 (h 1)
r2
qe (mg g 1)
k2 (mg g 1 h 1)
r2
6.717 10 3 2.3714
0.511 0.5944
0.5109 0.9063
0.565 6.931
1.35 103 5.546
1 0.999
Fig. 7. Effect of initial pH values on adsorption of 20 mg L 1 Cd2 þ solution (a), pH values on adsorption in various concentrations of Cd2 þ solution before and after absorption (b).
a
b
c
Fig. 8. Adsorption isotherm for the adsorption of Cd2 þ (a), linear fit of Langmuir model (b) and Freundlich model (c).
Table 2 The correlation coefficient constants of Langmuir model and Freundlich model. Metal ions
Cd2 þ
Langmuir constants
Freundlich constants
qm (mg g 1)
KL (L mg 1)
r2
KF (mg g 1) (L g 1)1/n
n
r2
12.4
1.795 102
0.999
0.884
1.465
0.957
the pH changes are within 0.8 for Cd2 þ concentrations in the range of 1.26–55.54 mg L 1. Therefore the pH stability provides further evidence for the selection of initial pH value. To well study the adsorption mechanism and kinetics, we continue to investigate the adsorption process of Cd2 þ by the pseudo-first-order and pseudo-second-order kinetics models. The pseudo-first-order equation for metal ion adsorption kinetics is below: ln(qe qt) ¼ln qe k1t, where qe (mg g 1) and qt (mg g 1) are the mass of ions adsorbed at equilibrium and time t, respectively. The reaction rate equilibrium constant k1 can be calculated by the linear of ln(qe qt) versus time t [38]. The pseudo-second-
order equation is also given: t/qt ¼ 1/k2qe þt/qe, thus k2 can be obtained from the plot of t/qt against time t and qe also can be obtained by gradient [40]. The adsorption isotherm studies disclose the interaction between the adsorbate with adsorbent. Fig. 8a shows adsorption isotherm on as-prepared TiN with Cd2 þ concentrations in the range of 1–150 mg L 1 at pH 6.1 and 25 °C. The Langmuir equation (ce/qe ¼ce/qm þ1/kLqm) and Freundlich equation (ln qe ¼ln kF þ1/n ln ce) are used to investigate the adsorption behavior of Cd2 þ by TiN (Fig. 8b and c). The correlation coefficient constants of two absorption models are calculated by fitting the absorption
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data to equations, which are listed in Table 2. It is more appropriate to use the Langmuir absorption model than the Freundlich absorption model, because the linear relationship between ce/qe and ce is perfect with the correlation coefficient (r2) of 0.999. The maximum adsorption capacity of Cd2 þ is 12.40 mg g 1 by Langmuir isotherm calculation. The Langmuir model basically refers to behavior of surface coverage between absorbate and surface site without interaction [41]. Therefore, the absorption of Cd2 þ , following the Langmuir absorption model, is likely based on the mesoporous structure feature of TiN.
4. Conclusions In summary, a simple and mild route for the synthesis of mesoporous TiN microspheres is demonstrated using the amorphous TiO2 as the precursor. The mesoporous TiN microspheres with a large surface area of 50.6 m2 g 1 and pore size distribution from 2.0 to 4.0 nm exhibit excellent water treatment performance with high removal behavior towards trace Cd2 þ . The kinetics of adsorption and adsorption equilibrium isotherm have been investigated and the saturated adsorption capacity of Cd2 þ is given to be 12.40 mg g 1. More importantly, the TiN has no any effect on the pH of the test solution, which is especially important for water treatment. This is the first report on mesoporous TiN as absorbent for trace Cd2 þ removal. Therefore, the application of TiN microspheres for heavy metal removal has a great potential in waste water engineering.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 21471016 and 21271023) and the 111 Project (B07012).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jpcs.2015.01.010.
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