Substitutional Doping for Aluminosilicate Mineral and Superior Water Splitting Performance
- Yi Zhang†1Email authorView ORCID ID profile,
- Liangjie Fu†1, 2Email author,
- Zhan Shu1,
- Huaming Yang1, 3,
- Aidong Tang4 and
- Tao Jiang1
© The Author(s). 2017
Received: 21 March 2017
Accepted: 7 June 2017
Published: 14 July 2017
Substitutional doping is a strategy in which atomic impurities are optionally added to a host material to promote its properties, while the geometric and electronic structure evolution of natural nanoclay mineral upon substitutional metal doping is still ambiguous. This paper first designed an efficient lanthanum (La) doping strategy for nanotubular clay (halloysite nanotube, HNT) through the dynamic equilibrium of a substitutional atom in the presence of saturated AlCl3 solution, and systematic characterization of the samples was performed. Further density functional theory (DFT) calculations were carried out to reveal the geometric and electronic structure evolution upon metal doping, as well as to verify the atom-level effect of the La doping. The CdS loading and its corresponding water splitting performance could demonstrate the effect of La doping. CdS nanoparticles (11 wt.%) were uniformly deposited on the surface of La-doped halloysite nanotube (La-HNT) with the average size of 5 nm, and the notable photocatalytic hydrogen evolution rate of CdS/La-HNT reached up to 47.5 μmol/h. The results could provide a new strategy for metal ion doping and constructive insight into the substitutional doping mechanism.
KeywordsAluminosilicate mineral Halloysite nanotubes La doping Photocatalytic hydrogen evolution DFT calculations
Aluminosilicate minerals (e.g., kaolinite [1–3], zeolite [4, 5] montmorillonite [6, 7], and halloysite [8–13]) have been extensively investigated as catalyst support materials because they are non-toxic to the environment and abundantly available inexpensively from natural deposits. A number of techniques have been used to enhance the functionalities of support materials, such as polymer coating [14, 15], carbon coating , and atomic doping [16–19]. Doped aluminosilicate minerals can form in nature, but their synthesis in a laboratory allow for various properties with the specified dopants [20–23]. Incorporating metal ions into the aluminosilicate layer structure makes the corresponding nanomaterial attractive for various applications, including catalysis [24–26], controlled release of pharmaceuticals [27, 28] as well as lithium ion batteries [29, 30]. Recently, based on density functional theory (DFT) computations, the stability, electronic, and mechanical properties of the nanostructured aluminosilicates, like imogolite, halloysite, and chrysotile, have been revealed . However, the substitutional doping mechanism and the electronic structure evolution of a metal into the aluminosilicates are still ambiguous [32, 33].
Towards the goal of improving our understanding of this mechanism, we designed an efficient doping strategy for one of the representative aluminosilicate minerals (halloysite nanotube [34–36], HNT) through the dynamic equilibrium of a substitutional atom in the presence of saturated AlCl3 solution, which contained lanthanum salt. Then, a substitutional atomic doping strategy based on La doping into the HNT structure and the replaced part of the Al atom from the Al–O sheet is presented. Halloysite (HNTs, Al2Si2O5(OH)4∙nH2O),as a natural clay mineral, contains octahsedral gibbsite Al(OH)3 and tetrahedral SiO4 sheets, and it also consists of hollow cylinders formed by multiple rolled layers. CdS is an attractive semiconductor material that can convert solar energy to chemical energy under visible-light irradiation. Incorporating CdS nanoparticles into La-HNTs and its corresponding water splitting performance could demonstrate the effects of La doping. The diffusion process of Al3+ saturated solution and the alumina sheets from halloysite, the change of crystal shapes and surface structures, and the possibility of enhanced catalytic activity were investigated in detail. The microstructures and morphologies of samples were characterized, and the interfacial structure between CdS and La-HNT were investigated. The photocatalytic hydrogen activity was evaluated and the role of La-HNT for enhancing catalytic activity of CdS/La-HNT was also investigated.
Halloysite nanotubes (HNT) were obtained from Hunan, China. All chemicals were analytical grade and used without further purification. HNT were pretreated via emulsion dispersion, filtering, washing with distilled water, and drying for 8 h at 313 K. La-HNT were synthesized by a modified hydrothermal route. An amount of 34.3 g of AlCl3 was dissolved into 60 mL deionized water to form AlCl3 supersaturated solution, while 3 mmol HNT and 6 mmol La(NO3)3·6H2O were dissolved in deionized water (5 mL), respectively. Then La(NO3)3·6H2O solution and the HNT slurry were added to AlCl3 supersaturated solution to form a suspension. The resulting suspension was stirred for 10 min in a polypropylene beaker and sonicated for 10 min to break up aggregations of starting material. The volume was limited into 70 mL (L/S = 70–80). The mixture was transferred into a Teflon bottle(100 mL) and treated under auto-generated pressure without stirring at 373 K for 48 h. The autoclave was naturally cooled to room temperature, and the obtained precipitates were filtered and washed several times with deionized water, and finally dried at 353 K in vacuum (denoted as La-HNT). For comparision, acid-treated HNT synthesized by 1.00 g HNT were dissolved in 250 mL of 6 M HCl solution at 373 K in a water bath. The reaction was carried out in a conical flask for 4 h with constant stirring. The conical flask was naturally cooled to room temperature, and the obtained precipitates were filtered and washed several times with deionized water, and finally dried at 353 K in vacuum (denoted as acid-treated HNT).
CdS/La-HNT were synthesized by using the Successive Ionic Layer Adsorption and Reaction (SILAR) method, 3 mmol La-HNT was dissolved into 50 mL 0.5 M Cd(NO3)2 ethanol solution for 5 min, rinsed with ethanol, and then dissolved for another 5 min in a 50 mL 0.5 M Na2S methanol solution, and rinsed again with methanol. Such an immersion cycle was repeated several times until the desired deposition of CdS nanoparticles was achieved. Then, the obtained precipitates were filtered and washed several times with deionized water, and finally dried at 353 K in a vacuum (denoted as CdS/D-Lax-HNT).
X-ray photoelectron spectroscopy (XPS) analysis was performed on using a Thermo Fisher Scientific K-Alpha 1063 spectrometer equipped with an Al Ka monochromatic X-ray source. The test chamber pressure was maintained below 10−9 mbar during spectral acquisition. The XPS binding energy (BE) was internally referenced to the C 1s peak (BE = 284.1 eV). The crystalline phases were identified by XRD analysis using a RIGAKU D/max-2550VB1 18-kW powder diffractometer with Cu Ka radiation (λ = 1.5418 Å). The data were collected in the scanning range 2θ = 10–80°, with a scanning speed of 2°/min. FTIR spectra were recorded using a Nicolet 5700 spectrophotometer. The specific surface area was calculated from the nitrogen adsorption isotherms using the Brunauer–Emmet–Teller (BET) equation. Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-200CX instrument equipped with an energy dispersive X-ray spectroscopy (EDS) at an accelerating voltage of 200 kV. (PL) spectrum of the sample was detected on a Hitachi H-4500 fluorescence spectrometer using a Xe lamp as the light source. UV–vis spectra of samples in aqueous solution were obtained using a UV-2400 (Shimadzu Corp., Japan) spectrometer. Solid state 29Si and 27Al MAS NMR spectra were recorded using a Bruker AMX400 spectrometer in a static magnetic field of 9.4 T at a resonance frequency of 79.49 MHz. The electrochemical analysis was carried out in a conventional three-electrode cell using a platinum-black wire and saturated calomel electrode (SCE) as the counter electrode and reference electrode, respectively. The working electrode was prepared on FTO (fluorine tin oxide) conductor glass. In detail, 20 mg of sample was added into 10 mL ethanol and formed a uniform suspension. As in a standard spin-coating process, the ethanol suspension was spread onto FTO glass, whose side part was previously protected using Scotch tape. The spinning was at a high speed of 150 rps and then dried in an oven at 70 °C for 1 h. The transient photocurrent responses of different samples were measured in 0.1 M Na2S + 0.02 M Na2SO3 aqueous solution under visible-light irradiation (≥420 nm) at 0 V vs. SCE. The illuminated area of the working electrode is 2 cm2. The photoelectrochemical experiment was performed using a CHI-660A electrochemical workstation (ChenHua Instruments Co. Ltd., Shanghai, China). The electrochemical impedance spectroscopy (EIS) was measured with a CHI-660A electrochemical workstation (ChenHua Instruments Co. Ltd., Shanghai, China), and the electrolyte consisted of 0.01 mol/L potassium hexacyanoferrate (III), 0.01 mol/L potassium hexacyanoferrate (II), and 0.5 mol/L KCl. The applied potential was open circuit potential (OCP).
Water splitting reactions were carried out in a gas-closed circulation within a vacuum. A sample of 100 mg photocatalyst powder was dispersed in a 300-mL aqueous solution of 0.1 M Na2S and 0.1 M Na2SO3. The light source was a 300-W Xe lamp, and the light intensity reaching the surface of the reaction solution was 135 mW/cm2. The amount of H2 evolution was determined using a gas chromatograph (Agilent Technologies: 6890 N).
All calculations were performed with the CASTEP code, based on first-principle density functional theory (DFT). The local density approximation (LDA) potential was used for the calculations. The ultrasoft pseudo-potential plane-wave formalism and an energy cutoff of 400 eV were used. The Monkhorst-Pack grid with 3 × 3 × 1 k-points mesh was used for the accurate calculation of DOS results, while Gamma point was used during geometry relaxation. The self-consistent total energy in the ground state was effectively obtained by the density-mixing scheme. For geometry optimizations, the convergence threshold for self-consistent field (SCF) tolerance was set to 1.0 × 10−6 eV/atom, all forces on the atoms were converged to less than 0.03 eV/Å, the total stress tensor was reduced to the order of 0.05 GPa, and the maximum ionic displacement was within 0.001 Å. The cell parameters and atomic coordination of the structures were optimized using a Broyden–Fletcher–Goldfarb–Shanno (BFGS) minimization algorithm.
Results and Discussion
In order to examine the charge transfer and ion transport, the electrochemical impedance spectroscopy (EIS) was employed , and the impedance behavior of CdS/HNT and CdS/La-HNT were measured in Fig. 4d. The Nyquist plots showed a semicircle, which occurred due to the electrochemical process at a high frequency level, followed by a line that indicates the diffusive resistance of the electrolyte and active materials. The semicircle confined charge-transfer resistance, which is closely related to the reversibility of the electrochemical reactions. CdS/La-HNT shows a smaller arc than that of CdS/HNT, indicating that La doping led to more efficient charge transfer over CdS/La-HNT.
The smaller semicircle radius of CdS/La-HNTs compared with CdS/HNTs revealed that CdS/La-HNTs nanocomposite has smaller charge-transfer resistance and has good electrochemical resistance. The effects of La doping are also confirmed by the photoluminescence (PL) emission spectra (Fig. 4e), the lower the PL intensity, the higher the efficiency in photogenerated electron-hole separation.
In summary, natural halloysite nanotubes have been successfully doped by La atom. The La doping into the structure of HNT leads to crystal shapes and obvious surface structure changes, which brings continuous uniformity for CdS loading and changes the catalysis activity for nano catalytic composite materials, thus resulting in an enhanced photocatalytic hydrogen evolution rate. The contrast of the photocatalytic hydrogen evolution of CdS/La-HNT and CdS/HNT confirms the high efficiency of the La doping. This result is very encouraging and should be highly applicable for extending the doping technique to other aluminosilicate minerals and the corresponding design of functional materials.
The authors acknowledge the National Science Fund for Distinguished Young Scholars (51225403), the National Natural Science Foundation of China (41572036), the Postdoctoral Science Foundation of Central South University (155219), the China Postdoctoral Science Foundation (2015 M582346) and the State Key Laboratory of Powder Metallurgy, Central South University (2015-19) for their support given to this research. The authors thank Prof. Lianzhou Wang at Nanomaterials Centre of Chemical Engineering School, The University of Queensland for his helpful discussion.
The Strategic Priority Research Program of Central South University(ZLXD2017005).
Availability of Data and Materials
Supporting information is available from the Springer Online Library or from the author.
YZ and HMY developed the concept. YZ, HMY, LJF, and TJ conceived the project and designed the experiments. YZ wrote the final paper. YZ and LJF wrote the initial drafts of the work. YZ, LJF, ZS, and ADT performed the experiment and the data analysis. All authors discussed the results and commented on the manuscript.
The authors declare that they have no competing interests.
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