- Nano Express
- Open Access
The Effects of Li/Nb Ratio on the Preparation and Photocatalytic Performance of Li-Nb-O Compounds
© The Author(s). 2017
- Received: 3 June 2017
- Accepted: 11 August 2017
- Published: 15 August 2017
The effects of Li/Nb ratio on the preparation of Li-Nb-O compounds by a hydrothermal method were studied deeply. Li/Nb ratio has a great impact on the formation of LiNbO3; the ratio smaller than 3:1 is beneficial to the formation of LiNbO3, while larger than 3:1, forms no LiNbO3 at all and the morphology and chemical bond of Nb2O5 raw material are totally modified by Li ions. The reason can be attributed to the large content of LiOH, which is beneficial to form Li3NbO4 not LiNbO3, and also, even if LiNbO3 particle locally forms, it is easily dissolved in LiOH solution with strong alkalinity. Pure LiNb3O8 powders are obtained with two absolutely opposite Li/Nb ratios: 8:1 and 1:3; the former shows a unique porous and hollow structure, quite different from the particle aggregation (the latter shows). Compared with Li/Nb = 1:3, the 4.2 times higher photocatalytic performance of LiNb3O8 (Li/Nb = 8:1) are observed and it can be attributed to the unique porous and hollow structure, which provides a high density of active sites for the degradation of MB. Compared to LiNbO3, the improved photocatalytic performance of LiNb3O8 can be attributed to its layered structure type with the reduced symmetry enhancing the separation of electrons and holes.
- Lithium Niobate
- Porous Materials
Niobium compounds, a very versatile group of materials, including niobium oxides, alkali niobates, and columbite niobates, exhibit many interesting physical properties and have been widely studied in many fields, such as catalysis [1–3], memristors , dye-sensitized solar cells , optical devices, and others [6, 7]. LiNbO3, as one of the most famous alkali niobates, presents prominent properties such as electro-optical and nonlinear optical behaviors, pyroelectricity, and piezoelectricity, and it is mainly used as optical modulators, waveguides, acoustic wave transducers, et al. in optical devices.
For environmental remediation and clean energy applications, niobates, such as (Na, K)NbO3 , BiNbO4 , LiNbO3 , and LiNb3O8 , have been deeply investigated, owing to their special distorted [NbO6] octahedral structures which favor a possible delocalization of charge carriers . Secondly, the conduction bands consisting of Nb4d orbitals located at a more negative state of redox potential of H+/H2 promote the separation and transfer of photo-induced charge carriers and result in high photocatalytic activities . Among these materials, LiNb3O8 displays unique performances. As a novel lithium-ion battery (LIB) anode material, the theoretical capacity of LiNb3O8 is 389 mAh/g assuming two-electron transfers (Nb5+ → Nb3+), larger than many other anode materials, such as Li4Ti5O12 [14, 15]. Used for supercapacitor devices, LiNb3O8 nanoflakes show excellent cycle stability with negligible specific capacitance decrease even after 15,000 cycles . Also, it is used as an efficient photocatalyst in the applications of hydrogen generation and degradation of organic pollutants. Pure LiNb3O8 is a highly active UV-photocatalyst for water reduction producing 83.87 μmol of hydrogen in 1 h, and it does not produce hydrogen under visible-light irradiation due to its large band gap (i.e., 3.9 eV) and inability to absorb visible light [17, 18]. LiNb3O8 nanoflakes show fast decolorization of toluidine blue O (TBO) dye under UV light compared to commercial TiO2 powders .
At most time, the appearance of LiNb3O8 is recognized as an impurity phase during the preparation of LiNbO3, especially in film samples, owing to high annealing temperature or inhomogeneous distribution of Li element in precursors [19, 20]. Due to the difficulty to prepare a pure phase, LiNb3O8 has been rarely studied, while for LiNbO3 powders, the preparation technologies are various, such as sol-gel , hydrothermal , and laser irradiation methods . Hydrothermal method is widely used to synthesize nanomaterials with advantages such as low temperature, environmental friendliness, and homogenous particle-size distribution, which can efficiently avoid the variation of Li/Nb molar ratio without going through high temperatures. As for hydrothermal method, the parameters of reaction temperature, raw material ratio, and holding time play important roles in determining the as-obtained materials, while the research of Li/Nb ratio much larger than 1:1 in the preparation of Li-Nb-O compounds has not been reported before.
In this paper, the effects of Li/Nb ratio on the preparation of Li-Nb-O compounds by a hydrothermal method were studied deeply. A series of analytical techniques were used to characterize the crystallinity, morphology, and chemical composition of the Li-Nb-O samples, especially the changes before and after the hydrothermal reaction. Pure LiNb3O8 and LiNbO3 photocatalysts were prepared, and the photocatalytic performance was studied with the effect of Li/Nb ratio in raw materials.
The preparation of Li-Nb-O compounds was carried out by the hydrothermal method using lithium hydroxide monohydrate (LiOH·H2O; Aladdin, ACS, ≥ 98.0%) and niobium pentaoxide (Nb2O5; Aladdin, AR, 99.9%) as starting materials. Firstly, 3.5 mmol of Nb2O5 was dispersed into 35 ml deionized water with a certain amount of LiOH·H2O under magnetic stirring. The mole ratios of Li:Nb are 1:3, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, and 8:1; as the results of the samples prepared with ratios of 4:1, 5:1, 6:1, and 7:1 are similar, only the ratios of Li:Nb = 4:1 and 7:1 are shown below. The suspension solutions were put into 50-mL Teflon-lined hydrothermal synthesis autoclave reactors and maintained at 260 °C for 24 h, then cooled down naturally to room temperature. The as-obtained powders were then washed with deionized water and ethanol for several times and dried at 60 °C. Finally, the products were calcined at various temperatures from 500 to 800 °C for 2 h with a ramp rate of 5 °C/min.
The X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Discover diffractometer with Cu Kα radiation (40 kV, 40 mA). The morphologies of the samples were characterized by field emission scanning electron microscope (FESEM; JSM-6700F). Chemical bonds were analyzed by Fourier-transformed infrared spectroscopy (FTIR) in the range of 2000–650 cm−1. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo-Fisher Escalab 250Xi instrument to characterize the chemical component of Li-Nb-O compounds. The specific surface area was measured on a surface area apparatus (Micromeritics ASAP 2460) at 77 K by N2 adsorption/desorption method (BET method). The photoluminescence (PL) spectra were detected using an F-280 fluorescence spectrophotometer with an excitation wavelength of 320 nm.
To evaluate the photocatalytic performance of Li-Nb-O compounds, the degradation of methylene blue (MB) aqueous solution (5 mg/L) was carried out under irradiation of a 500 W Hg lamp at a natural pH value. Fifty milligrams of powders were dispersed into 50 mL of MB aqueous solution. Before the irradiation, the suspension was stirred in dark for 1 h to achieve adsorption equilibrium. Then, the suspension was irradiated by the Hg lamp. The concentration of residual MB was analyzed with an interval of 30 min using an ultraviolet-visible near-infrared (UV-vis-NIR) spectrophotometer at 665 nm.
At 700 °C, the monoclinic LiNb3O8 is the predominant phase with almost negligible impurity. The pure phase of LiNb3O8 is obtained at 800 °C with all the diffraction peaks indexed to the monoclinic phase (JCPDF, No. 36-0307), a space group of P21/a, which provides a new way to prepare LiNb3O8 compounds.
Based on the results, we can conclude that Li/Nb ratio has a great impact on the formation of LiNbO3; the ratio smaller than 3:1 is beneficial to the formation of LiNbO3, while larger than 3:1, no LiNbO3 forms at all. Based on the diagram, the congruent Li content is 97.2 mol% of the Nb content for the preparation of perfect single-phase LiNbO3, and the excess or deficiency of the Li content is compensated by the formation of Li3NbO4 or LiNb3O8 phase . The large excess of LiOH is beneficial to form Li3NbO4 not LiNbO3, while no Li3NbO4 phase is observed after hydrothermal reaction due to the insufficient reaction condition; even if the LiNbO3 particle locally formed, it is easily dissolved in LiOH solution with strong alkalinity .
The separation efficiency of photogenerated carries of Li-Nb-O catalyst are investigated by PL spectra, as shown in Fig. 9. As we know, PL emission spectra mainly result from the recombination of free carriers. As seen in Fig. 9, LiNb3O8 shows smaller emitting peaks around 470 nm than LiNbO3. It means that LiNb3O8 has longer charge carrier lifetime and improved efficiency of interfacial charge transfer, which can be attributed to its layered structure with the reduced symmetry enhancing the separation of electrons and holes.
From the results above, we can conclude that Li/Nb ratio has a great impact on the formation of LiNbO3; the ratio smaller than 3:1 is beneficial to the formation of LiNbO3, while larger than 3:1, forms no LiNbO3 at all and the morphology and chemical bond of Nb2O5 raw material are totally modified by Li ions. The reason can be attributed to the large content of LiOH, which is beneficial to form Li3NbO4 not LiNbO3, and also, even if the LiNbO3 particle locally forms, it is easily dissolved in LiOH solution with strong alkalinity. Pure LiNb3O8 powders are obtained with two absolutely opposite Li/Nb ratios: 8:1 and 1:3; the former shows a unique porous and hollow structure, quite different with the particle aggregation (the latter shows). Compared with Li/Nb = 1:3, higher photocatalytic performance of LiNb3O8 (Li/Nb = 8:1) are observed and it can be attributed to the unique porous and hollow structure, which provides a high density of active sites for the degradation of MB. Compared to LiNbO3, the improved photocatalytic performance of LiNb3O8 can be attributed to its layered structure type with the reduced symmetry enhancing the separation of electrons and holes.
This work was financially supported by the National Natural Science Foundation of China (No. 51202107) and the Foundation of Henan Educational Committee (No. 16A140028).
HZ and HL conceived and designed the experiments; HL and LZ prepared the samples; CH and ZW performed the XRD and SEM measurements; JQ performed the XPS; JY participated in the photocatalytic test; HZ wrote the paper. All of the authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- Nico C, Monteiro T, Graça MPF (2016) Niobium oxides and niobates physical properties: review and prospects. Prog Mater Sci 80:1–37View ArticleGoogle Scholar
- Zhai HF, Li AD, Kong JZ, Li XF, Zhao J, Guo BL, Yin J, Li ZS, Wu D (2013) Preparation and visible-light photocatalytic properties of BiNbO4 and BiTaO4 by a citrate method. J Solid State Chem 202:6–14View ArticleGoogle Scholar
- Zhai HF, Shang SY, Zheng LY, Li PP, Li HQ, Luo HY, Kong JZ (2016) Efficient visible-light photocatalytic properties in low-temperature Bi-Nb-O system photocatalysts. Nanoscale Res Lett 11:383View ArticleGoogle Scholar
- Kumar S, Davila N, Wang ZW, Huang XP, Strachan JP, Vine D, Kilcoyne ALD, Nishi Y, Williams RS (2017) Spatially uniform resistance switching of low current, high endurance titanium-niobium-oxide memristors. Nano 9:1793–1798Google Scholar
- Abe R, Shinmei K, Koumura N, Hara K, Ohtani B (2013) Visible-light-induced water splitting based on two-step photoexcitation between dye-sensitized layered niobate and tungsten oxide photocatalysts in the presence of a triiodide/iodide shuttle redox mediator. J Am Chem Soc 135:16872–16884View ArticleGoogle Scholar
- Pullar RC (2009) The synthesis, properties, and applications of columbite niobates (M2+Nb2O6): a critical review. J Am Ceram Soc 92:563–577View ArticleGoogle Scholar
- Janner D, Tulli D, Garcia-Granda M, Belmonte M, Pruneri V (2009) Micro-structured integrated electro-optic LiNbO3 modulators. Laser Photonics Rev 3:301–313View ArticleGoogle Scholar
- Liu JW, Chen G, Li ZH, Zhang ZG (2007) Hydrothermal synthesis and photocatalytic properties of ATaO3 and ANbO3 (A = Na and K). Int J Hydrogen Energ 32:2269–2272View ArticleGoogle Scholar
- Zhai HF, Kong JZ, Wang AZ, Li HJ, Zhang TT, Li AD, Wu D (2015) The polymerization effect on synthesis and visible-light photocatalytic properties of low-temperature β-BiNbO4 using Nb-citrate precursor. Nanoscale Res Lett 10:457View ArticleGoogle Scholar
- Wang X, Yan WB, Zhang LX, Shi LH, Chen HJ, Zhang YW, Wu M, Zhang PJ (2015) Tunable photocatalytic activity of photochromic Fe-Mn-codoped LiNbO3 nanocrystals. Opt Mater Express 5:2240–2245View ArticleGoogle Scholar
- Zhai HF, Liu HR, Li HJ, Zheng LY, Hu CJ, Zhang X, Li QL, Yang JE (2017) Hydrothermal-assisted sintering strategy towards porous- and hollow-structured LiNb3O8 anode material. Nanoscale Res Lett 12:463View ArticleGoogle Scholar
- Yin J, Zou ZG, Ye JH (2003) Photophysical and photocatalytic properties of MIn0.5Nb0.5O3 (M = Ca, Sr, and Ba). J Phys Chem B 107:61–65View ArticleGoogle Scholar
- Liu JL, Shakir I, Kang DJ (2014) Single crystalline LiNb3O8 nanoflakes for efficient photocatalytic degradation of organic pollutants. RSC Adv 4:4917–4920View ArticleGoogle Scholar
- Jian ZL, Lu X, Fang Z, Hu YS, Zhou J, Chen W, Chen LQ (2011) LiNb3O8 as a novel anode material for lithium-ion batteries. Electrochem Commun 13:1127–1130View ArticleGoogle Scholar
- Xu HH, Shu J, Hu XL, Sun YM, Luo W, Huang YH (2013) Electrospun porous LiNb3O8 nanofibers with enhanced lithium-storage properties. J Mater Chem A 1:15053–15059View ArticleGoogle Scholar
- Liu JL, Shakir I, Kang DJ (2014) Lithium niobate nanoflakes as electrodes for highly stable electrochemical supercapacitor devices. Mater Lett 119:84–87View ArticleGoogle Scholar
- Sahoo PP, Maggard PA (2013) Crystal chemistry, band engineering, and photocatalytic activity of the LiNb3O8–CuNb3O8 solid solution. Inorg Chem 52:4443–4450View ArticleGoogle Scholar
- Zielińska B, Borowiak-Palen E, Kalenzuk RJ (2008) Preparation and characterization of lithium niobate as a novel photocatalyst in hydrogen generation. J Phys Chem Solids 69:236–242View ArticleGoogle Scholar
- Debnath C, Kar S, Verma S, Bartwal KS (2015) Synthesis of LiNbO3 nanoparticles by citrate gel method. J Nanosci Nanotechnol 15:3757–3763View ArticleGoogle Scholar
- Sumets M, Kostyuchenko A, Ievlev V, Kannykin S, Dybov V (2015) Influence of thermal annealing on structural properties and oxide charge of LiNbO3 films. J Mater Sci Mater Electron 26:7853–7859View ArticleGoogle Scholar
- Yu J, Liu XQ (2007) Hydrothermal synthesis and characterization of LiNbO3 crystal. Mater Lett 61:355–358View ArticleGoogle Scholar
- Ferreira NM, Ferro MC, Graca MPF, Costa FM (2017) Effect of laser irradiation on lithium niobate powders. Ceram Int 43:2504–2510View ArticleGoogle Scholar
- Ning HX, Liao QL, Xiong J (2009) Study on LiNbO3 ultrafine powders synthesized by hydrothermal method. Piezoelectrics & Acoustooptics 31:103–105Google Scholar
- Akazawaa H, Shimada M (2007) Mechanism for LiNb3O8 phase formation during thermal annealing of crystalline and amorphous LiNbO3 thin films. J Mater Res 22:1726–1736View ArticleGoogle Scholar
- Prado AGS, Bolzon LB, Pedroso CP, Moura AO, Costa LL (2008) Nb2O5 as efficient and recyclable photocatalyst for indigo carmine degradation. Appl Catal B-Environ 82:219–224View ArticleGoogle Scholar
- Katovic V, Djordjevic C (1970) Coordination complexes of niobium and tantalum. VII. Preparation and infrared spectra of oxygen-18 labeled terminal and bridging monoxoniobium(V) complexes and the course of coordinated alkoxo group hydrolysis in mixed-ligand niobium complexes. Inorg Chem 9:1720–1723View ArticleGoogle Scholar
- Braga VS, Garcia FAC, Dias JA, Dias SCL (2008) Phase transition in niobium pentoxide supported on silica-alumina. J Therm Anal Calorim 92:851–855View ArticleGoogle Scholar
- Bartasyte A, Plausinaitiene V, Abrutis A, Stanionyte S, Margueron S, Boulet P, Kobata T, Uesu Y, Gleize J (2013) Identification of LiNbO3, LiNb3O8 and Li3NbO4 phases in thin films synthesized with different deposition techniques by means of XRD and Raman spectroscopy. J Phys Condens Matter 25:205901View ArticleGoogle Scholar
- Cochez M, Ferriol M, Pöppl L, Polgár K, Péter Á (2005) Ternary system Li2O–K2O–Nb2O5 part I: phase equilibria around the lithium niobate existence field. J Alloy Compd 386:238–245View ArticleGoogle Scholar
- Al-Ekabi H, Serpone N (1988) Kinetics studies in heterogeneous photocatalysis. I. Photocatalytic degradation of chlorinated phenols in aerated aqueous solutions over titania supported on a glass matrix. J Phys Chem 92:5726–5731View ArticleGoogle Scholar
- Liu J, Xue DF (2008) Thermal oxidation strategy towards porous metal oxide hollow architectures. Adv Mater 20:2622–2627View ArticleGoogle Scholar
- Sabio EM, Chi MF, Browning ND, Osterloh FE (2010) Charge separation in a niobate nanosheet photocatalyst studied with photochemical labeling. Langmuir 26:7254–7261View ArticleGoogle Scholar
- Zlotnik S, Tobaldi DM, Seabra P, Labrincha JA, Vilarinho PM (2016) Alkali niobate and tantalate perovskites as alternative photocatalysts. ChemPhysChem 17:3570–3575View ArticleGoogle Scholar