Impact of Li/Nb Ratio on Hydrothermal Synthesis and Photocatalytic Performance of Li‑Nb‑O Compounds

Abstract

We investigated how the lithium‑to‑niobium (Li/Nb) molar ratio influences the hydrothermal synthesis of Li‑Nb‑O compounds and their subsequent photocatalytic activity. A Li/Nb ratio below 3:1 promotes the formation of LiNbO₃, whereas ratios above 3:1 suppress LiNbO₃ formation and instead modify the Nb₂O₅ precursor through lithium incorporation. Excess LiOH drives the formation of Li₃NbO₄ rather than LiNbO₃, and any locally formed LiNbO₃ dissolves in the strongly alkaline medium. Remarkably, pure LiNb₃O₈ powders were synthesized using two opposite Li/Nb ratios (8:1 and 1:3). The 8:1 sample displays a porous, hollow morphology that confers a 4.2‑fold higher photocatalytic activity for methylene blue (MB) degradation compared to the 1:3 counterpart, attributable to its high surface area and layered crystal structure, which enhance charge separation.

Background

Niobium‐based materials, including oxides, alkali niobates, and columbite niobates, are prized for their versatile physical properties and wide range of applications—from catalysis [1,2,3] and memristors [4] to dye‑sensitized solar cells [5] and optical devices [6,7]. LiNbO₃, a prototypical alkali niobate, exhibits remarkable electro‑optical, nonlinear optical, pyroelectric, and piezoelectric behavior, making it a cornerstone of optical modulators, waveguides, and acoustic transducers.

For environmental remediation and energy conversion, niobates such as (Na,K)NbO₃ [8], BiNbO₄ [9], LiNbO₃ [10], and LiNb₃O₈ [11] have attracted intense scrutiny. Their distorted NbO₆ octahedra facilitate charge carrier delocalization [12], while Nb⁴⁺ 4d orbitals lower the redox potential of H⁺/H₂, promoting efficient separation and transfer of photogenerated carriers [13]. LiNb₃O₈ is especially noteworthy: as a lithium‑ion battery anode, it offers a theoretical capacity of 389 mAh g⁻¹ (two‑electron transfer) [14,15]; as a supercapacitor, its nanoflakes maintain negligible capacitance loss after 15,000 cycles [16]; and as a photocatalyst, it produces 83.87 µmol h⁻¹ of hydrogen under UV irradiation while remaining inert to visible light due to its 3.9 eV band gap [17,18].

LiNb₃O₈ often emerges as an impurity during LiNbO₃ synthesis, especially in high‑temperature or unevenly distributed precursor systems [19,20]. Traditional LiNbO₃ production methods include sol‑gel [19], hydrothermal [21], and laser‑irradiation [22]. Hydrothermal synthesis offers low‑temperature, environmentally friendly processing with homogeneous particle size distribution, but the effect of Li/Nb ratios exceeding 1:1 has not been thoroughly explored. This study addresses that gap.

Methods

The Li‑Nb‑O powders were prepared hydrothermally using lithium hydroxide monohydrate (LiOH·H₂O; ≥ 98 %) and niobium pentoxide (Nb₂O₅; 99.9 %). 3.5 mmol of Nb₂O₅ was dispersed in 35 mL deionized water with LiOH·H₂O to achieve Li:Nb molar ratios of 1:3, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, and 8:1. Samples with similar outcomes (4:1, 5:1, 6:1, 7:1) were grouped; only 4:1 and 7:1 are reported in detail. The suspensions were sealed in 50‑mL Teflon‑lined autoclaves and heated to 260 °C for 24 h, then cooled naturally. Powders were washed with water and ethanol, dried at 60 °C, and finally calcined at 500–800 °C (2 h, 5 °C min⁻¹ ramp).

X‑ray diffraction (XRD) employed a Bruker D8 Discover diffractometer (Cu Kα, 40 kV, 40 mA). Morphology was examined by FESEM (JSM‑6700F). Chemical bonds were probed via FTIR (2000–650 cm⁻¹), and surface composition by XPS (Thermo‑Fisher Escalab 250Xi). Specific surface area was measured by BET (Micromeritics ASAP 2460). Photoluminescence (PL) spectra used an F‑280 fluorescence spectrophotometer (excitation 320 nm).

Photocatalytic activity was evaluated by degrading 5 mg L⁻¹ methylene blue (MB) under a 500 W Hg lamp. 50 mg of catalyst was dispersed in 50 mL MB solution, equilibrated in the dark for 1 h, then irradiated. Residual MB was quantified at 665 nm every 30 min with a UV‑vis‑NIR spectrophotometer.

Results and Discussion

Figure 1 shows XRD patterns of products from different Li/Nb ratios. Pure LiNbO₃ (JCPDS 20‑0631) appears at Li:Nb = 2:1. Ratios below 2:1 (1:1, 1:3) still yield LiNbO₃ but with residual Nb₂O₅ (JCPDS 37‑1468), indicating insufficient Li for complete conversion. When Li is in excess (≥ 4:1), LiNbO₃ disappears entirely; only Nb₂O₅ remains in the XRD patterns. This suggests that excess LiOH promotes the formation of Li₃NbO₄, and any nascent LiNbO₃ is dissolved by the alkaline medium.

XRD patterns of Li‑Nb‑O powders after hydrothermal reaction with different Li/Nb mole ratios.

To elucidate the phase evolution at high Li content, Li/Nb = 8:1 samples were calcined at 500–800 °C (Figure 2). At 500 and 600 °C, a LiNbO₃ peak emerges, confirming the presence of Li in the post‑hydrothermal product. A diffraction at 30.26° (monoclinic LiNb₃O₈, (410) plane) appears at 600 °C, following the reaction: LiNbO₃ + Nb₂O₅ → LiNb₃O₈ [24]. At 700 °C, monoclinic LiNb₃O₈ dominates; at 800 °C, pure LiNb₃O₈ (JCPDS 36‑0307, space group P21/a) is obtained, providing a new, low‑temperature route to this phase.

XRD patterns of Li‑Nb‑O powders (Li:Nb = 8:1) after calcination at different temperatures for 2 h.

FTIR spectra (Figure 3) corroborate these findings. The Nb=O stretch at 962 cm⁻¹ persists up to 700 °C, confirming the presence of Nb₂O₅. At 500 and 600 °C, a new band at 891 cm⁻¹ appears and vanishes by 700 °C, matching the LiNbO₃ formation and consumption. At 700 and 800 °C, bands at 908 and 828 cm⁻¹ indicate LiNb₃O₈ formation, aligning with XRD data.

FTIR spectra of Nb₂O₅ raw material and Li‑Nb‑O powders (Li:Nb = 8:1) calcined at different temperatures.

Li/Nb ratios critically influence LiNbO₃ formation: ratios < 3:1 favor LiNbO₃, whereas > 3:1 suppress it, with excess Li forming Li₃NbO₄ or LiNb₃O₈ [28]. Excess LiOH dissolves any nascent LiNbO₃ due to its high alkalinity [29].

XPS analysis (Figure 4) shows both raw Nb₂O₅ and post‑hydrothermal products contain Nb⁵⁺ (Nb 3d_3/2/3d_5/2 splitting of 2.7 eV). The binding energy shifts ~0.5 eV to lower values after hydrothermal treatment, indicating a changed chemical environment caused by Li⁺ proximity.

XPS spectra of Nb₂O₅ raw material and products (Li:Nb = 8:1) after hydrothermal treatment.

SEM images (Figure 5) reveal that hydrothermal processing transforms large, irregular Nb₂O₅ crystals into smaller (~200 nm) particles that still aggregate. This morphological refinement is attributed to the high LiOH concentration during hydrothermal synthesis.

SEM images of a Nb₂O₅ raw material and b Li:Nb = 8:1 hydrothermal product.

Calcination at 800 °C of Li/Nb = 1:3, 2:1, and 8:1 yielded distinct phases (Figure 6). Li:Nb = 2:1 produced pure LiNbO₃, while 1:3 and 8:1 each yielded pure LiNb₃O₈. Other ratios produced mixed LiNbO₃/LiNb₃O₈ phases. SEM images (Figure 7) highlight morphological differences: LiNb₃O₈-8:1 forms a porous, honeycomb‑like network of micrometer‑scale nanoparticles, whereas LiNb₃O₈-1:3 aggregates into dense particles. BET measurements confirm the higher surface area of LiNb₃O₈-8:1 (4.46 m² g⁻¹) versus LiNb₃O₈-1:3 (0.96 m² g⁻¹). The porous morphology arises from lithium volatilization during calcination, creating new LiNb₃O₈ particles and interparticle networks [11]. LiNbO₃ calcined at 800 °C shows ~200 nm grains with irregular shapes and a BET area of 3.91 m² g⁻¹.

SEM images of Li‑Nb‑O powders (a) Li:Nb = 2:1 at 500 °C, (b) Li:Nb = 1:3, (c) Li:Nb = 8:1, and (d) Li:Nb = 2:1 at 800 °C.

Photocatalytic performance was evaluated by MB degradation (Figure 8). LiNb₃O₈-8:1 achieved ~85 % degradation after 30 min under UV, outperforming LiNb₃O₈-1:3, LiNbO₃, and the catalyst‑free control. Rate constants (k) derived from pseudo‑first‑order kinetics were 0.71 × 10⁻², 1.61 × 10⁻², 4.18 × 10⁻², and 6.73 × 10⁻² min⁻¹ for the control, LiNb₃O₈-1:3, LiNbO₃, and LiNb₃O₈-8:1, respectively. LiNb₃O₈-8:1’s k is 9.5× the control, 4.2× LiNb₃O₈-1:3, and 1.6× LiNbO₃. The superior activity stems from the high surface area and porous structure, providing abundant active sites.

UV‑vis spectra of MB degradation: (a) control, (b) LiNb₃O₈-1:3, (c) LiNbO₃, (d) LiNb₃O₈-8:1; (e) degradation curves; (f) kinetic fits.

Compared to LiNbO₃, LiNb₃O₈-8:1’s enhanced photocatalytic performance is also attributed to its layered crystal structure, which reduces symmetry and promotes charge separation, as evidenced by PL spectra (Figure 9). LiNb₃O₈ shows weaker emission at ~470 nm, indicating longer carrier lifetimes and more efficient interfacial transfer. Its slightly smaller band gap (~3.9 eV vs. 4.14 eV) allows broader light absorption.

Room‑temperature PL spectra of LiNb₃O₈-1:3, LiNbO₃, and LiNb₃O₈-8:1.

PL data confirm that LiNb₃O₈ achieves superior charge carrier separation, directly translating to higher photocatalytic activity.

Conclusions

The Li/Nb molar ratio is a decisive factor in the hydrothermal synthesis of Li‑Nb‑O compounds. Ratios below 3:1 favor LiNbO₃ formation, whereas ratios above 3:1 suppress it and instead yield LiNb₃O₈ (when Li/Nb = 8:1) or retain Nb₂O₅ (when Li/Nb = 1:3). Excess LiOH promotes Li₃NbO₄ formation and can dissolve nascent LiNbO₃ due to high alkalinity. Pure LiNb₃O₈ powders were synthesized at both 8:1 and 1:3 Li/Nb ratios; the 8:1 sample’s porous, hollow morphology yields a 4.2‑fold higher photocatalytic activity for MB degradation compared to the 1:3 counterpart. Compared to LiNbO₃, LiNb₃O₈-8:1’s layered structure and reduced symmetry enhance electron–hole separation, explaining its superior photocatalytic performance.