The Effects of Li/Nb Ratio on the Preparation and Photocatalytic Performance of Li-Nb-O Compounds

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.


Background
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][2][3], memristors [4], dye-sensitized solar cells [5], optical devices, and others [6,7]. LiNbO 3 , 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)NbO 3 [8], BiNbO 4 [9], LiNbO 3 [10], and LiNb 3 O 8 [11], have been deeply investigated, owing to their special distorted [NbO 6 ] octahedral structures which favor a possible delocalization of charge carriers [12]. Secondly, the conduction bands consisting of Nb4d orbitals located at a more negative state of redox potential of H+/H 2 promote the separation and transfer of photo-induced charge carriers and result in high photocatalytic activities [13]. Among these materials, LiNb 3 O 8 displays unique performances. As a novel lithium-ion battery (LIB) anode material, the theoretical capacity of LiNb 3 O 8 is 389 mAh/g assuming two-electron transfers (Nb 5+ → Nb 3+ ), larger than many other anode materials, such as Li 4 Ti 5 O 12 [14,15]. Used for supercapacitor devices, LiNb 3 O 8 nanoflakes show excellent cycle stability with negligible specific capacitance decrease even after 15,000 cycles [16]. Also, it is used as an efficient photocatalyst in the applications of hydrogen generation and degradation of organic pollutants. Pure LiNb 3 O 8 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]. LiNb 3 O 8 nanoflakes show fast decolorization of toluidine blue O (TBO) dye under UV light compared to commercial TiO 2 powders [13].
At most time, the appearance of LiNb 3 O 8 is recognized as an impurity phase during the preparation of LiNbO 3 , 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, LiNb 3 O 8 has been rarely studied, while for LiNbO 3 powders, the preparation technologies are various, such as sol-gel [19], hydrothermal [21], and laser irradiation methods [22]. 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 LiNb 3 O 8 and LiNbO 3 photocatalysts were prepared, and the photocatalytic performance was studied with the effect of Li/Nb ratio in raw materials.

Methods
The preparation of Li-Nb-O compounds was carried out by the hydrothermal method using lithium hydroxide monohydrate (LiOH·H 2 O; Aladdin, ACS, ≥ 98.0%) and niobium pentaoxide (Nb 2 O 5 ; Aladdin, AR, 99.9%) as starting materials. Firstly, 3.5 mmol of Nb 2 O 5 was dispersed into 35 ml deionized water with a certain amount of LiOH·H 2 O 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 . Xray 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 N 2 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 nearinfrared (UV-vis-NIR) spectrophotometer at 665 nm.

Results and Discussion
The XRD patterns of the products obtained after hydrothermal reaction with different Li/Nb mole ratios are shown in Fig. 1. It is obvious that pure LiNbO 3 phase (JCPDF, No. 20-0631) is obtained with Li:Nb = 2:1. For the ratio of Li/Nb smaller than 2:1, such as 1:1 or 1:3, the main phase is still LiNbO 3 , accompanied with the residual of Nb 2 O 5 (JCPDF, No. 37-1468), which means that the Li content is not sufficient to fully react with Nb 2 O 5 to form LiNbO 3 . When we increase the Li content largely, an amazing phenomenon occurs: there is no LiNbO 3 formed at all after the hydrothermal reaction, as  Fig. 1. When the ratio of Li/Nb is 4:1 or larger, only Nb 2 O 5 phase exists in XRD patterns, no other impurities detected. Is the Li ion washed away during the washing process? Just like the former literature reported [23].
To illustrate the phase evolution when the Li/Nb ratio is large enough, the products obtained by the hydrothermal method, using Li/Nb = 8:1 as an example, are calcined at different temperatures and the XRD patterns are shown in Fig. 2. When the products are calcined at 500 and 600°C, a new phase LiNbO 3 appears which proves that a Li element truly exists in the products obtained just after the hydrothermal reaction, though not detected by XRD. Also, a diffraction peak at 30.26°appears at 600°C, which can be indexed as (410) plane of monoclinic LiNb 3 O 8 . The reaction can be described by the following Eq. (1) [24]: At 700°C, the monoclinic LiNb 3 O 8 is the predominant phase with almost negligible impurity. The pure phase of LiNb 3 O 8 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 LiNb 3 O 8 compounds.
FTIR test is also performed to study the phase evolution of the products with Li:Nb = 8:1, as shown in Fig. 3. The raw material Nb 2 O 5 is tested as a reference. In Fig.  3, the band at 962 cm −1 due to the stretching vibrations of Nb = O in Nb 2 O 5 is existent until 700°C [25]. After hydrothermal reaction, no other bands detected at this range means the only niobate is still Nb 2 O 5 . When the calcination temperature is 500 and 600°C, a new band at 891 cm −1 appears, while disappears at 700°C, consistent with the XRD results of the formation and reaction of LiNbO 3 . At 700 and 800°C, the bands at 908 and 828 cm −1 correspond to the formation of LiNb 3 O 8 compounds [26,27]. The FTIR results are well consistent with the XRD results of Fig. 2.
Based on the results, we can conclude that Li/Nb ratio has a great impact on the formation of LiNbO 3 ; the ratio smaller than 3:1 is beneficial to the formation of LiNbO 3 , while larger than 3:1, no LiNbO 3 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 LiNbO 3 , and the excess or deficiency of the Li content is compensated by the formation of Li 3 NbO 4 or LiNb 3 O 8 phase [28]. The large excess of LiOH is beneficial to form Li 3 NbO 4 not LiNbO 3 , while no Li 3 NbO 4 phase is observed after hydrothermal reaction due to the insufficient reaction condition; even if the LiNbO 3 particle locally formed, it is easily dissolved in LiOH solution with strong alkalinity [29].
As discussed above, the Li element is not detected after the hydrothermal reaction without further calcination, while it truly exists in the products with Li:Nb = 8:1. For Nb 2 O 5 , is it still the same as the raw material after the hydrothermal reaction? The XPS test is carried out to characterize the chemical component of Nb 2 O 5 raw material and the products obtained after hydrothermal reaction, as shown in Fig. 4. The difference of Nb 3d 3/2 and 3d 5/2 is the 2.7 eV for both samples, indicating the Nb 5+ state in both samples without other reduced Nb oxides species [3]. The binding energies of Nb 3d shift towards the low binding-energy state after the hydrothermal reaction, about 0.5 eV difference. It means that the chemical environment of Nb changes, while no other compounds are formed. The change may  The influence of Li ions on Nb 2 O 5 is also observed in SEM images, as shown in Fig. 5. Figure 5a is the image of Nb 2 O 5 raw material, with irregular shape, dense structure, and length of several micrometers. After the hydrothermal reaction, the large crystal particle is di-     former results from the porous and hollow structure. The morphology difference can be attributed to the different morphologies of the reactants: for LiNb 3 O 8 -8:1, the reactant of LiNbO 3 is formed based on the calcinations of the products after hydrothermal reaction, the morphology of the products is shown in Fig. 5b, while for LiNb 3 O 8 -1:3, the morphology of LiNbO 3 obtained directly after the hydrothermal reaction is hexahedronlike, as shown in Fig. 7a [21]. The formation of the porous and hollow structure for LiNb 3 O 8 -8:1 can be attributed to the lithium volatilization during the calcinations process, which is beneficial to the formation of new LiNb 3 O 8 particles and networks between the particles [11]. For LiNbO 3 calcined at 800°C (i.e., Li/Nb = 2:1), its grain size is about 200 nm and the shape seems irregular, as shown in Fig. 7d; the BET area is about 3.91 m 2 /g.
The photocatalytic performances of LiNb 3 O 8 and LiNbO 3 are shown in Fig. 8. From the UV-vis absorption spectra of MB at the varied irradiation time (Fig. 8a-d), it is observed that the maximum absorption band (665 nm) becomes weak with the increase of the irradiation time. The degradation rate of MB is largely improved with the catalysts of LiNb 3 O 8 and LiNbO 3 under UV light, especially for LiNb 3 O 8 -8:1, about 85% of MB degraded after 30 min irradiation, as shown in Fig. 8e. As the photo-degradation of MB using Li-Nb-O catalysts obeys the pseudo-first-order kinetics, described by the modified Langmuir-Hinshelwood kinetics mode [30], the constants of the pseudo-first-order rate (k) are calculated, displayed in Fig. 8f. The obtained first-order rate constants of MB without catalysts, LiNb 3 O 8 -1:3, LiNbO 3 , and LiNb 3 O 8 -8:1 are 0.71 × 10 −2 , 1.61 × 10 −2 , 4.18 × 10 −2 , and 6.73 × 10 −2 min −1 , respectively. The higher the first-order rate constant is, the more outstanding the photocatalytic performance is. The k of LiNb 3 O 8 -8:1 is 9.5 times of MB without catalysts, 4.2 times of LiNb 3 O 8 -1:3, and 1.6 times of LiNbO 3 . Compared with LiNb 3 O 8 -1:3, the higher photocatalytic performance of LiNb 3 O 8 -8:1 can be attributed to the unique porous and hollow structure, which provides a high density of active sites for the degradation of MB [31].
Compared to LiNbO 3 , the improved photocatalytic performance of LiNb 3 O 8 -8:1, which has almost the same absorption ability of MB as that of LiNbO 3 , can be attributed to its layered structure type with the reduce symmetry. The layered structure can enhance the separation of electrons and holes [32], consistent with the PL spectra, as shown in Fig. 9. At the same time, the LiNb 3 O 8 framework is constructed by three different niobate octahedrons and Li atoms share partial octahedral sites; the higher niobate octahedral site is expected to provide more active sites for photocatalysis. Finally, the smaller energy band gap of LiNb 3 O 8 (about 3.9 eV) than that of LiNbO 3 (4.14 eV) means that it can utilize more incident light to participate in the photocatalytic process [33].
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, LiNb 3 O 8 shows smaller emitting peaks around 470 nm than LiNbO 3 . It means that LiNb 3 O 8 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.

Conclusions
From the results above, we can conclude that Li/Nb ratio has a great impact on the formation of LiNbO 3 ; the ratio smaller than 3:1 is beneficial to the formation of LiNbO 3 , while larger than 3:1, forms no LiNbO 3 at all and the morphology and chemical bond of Nb 2 O 5 raw material are totally modified by Li ions. The reason can be attributed to the large content of LiOH, which is beneficial to form Li 3 NbO 4 not LiNbO 3 , and also, even if the LiNbO 3 particle locally forms, it is easily dissolved in LiOH solution with strong alkalinity. Pure LiNb 3 O 8 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 LiNb 3 O 8 (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 LiNbO 3 , the improved photocatalytic performance of LiNb 3 O 8 can be attributed to its layered structure type with the reduced symmetry enhancing the separation of electrons and holes.