Phase‑Dependent Charge Dynamics and Photocatalytic Performance of Tin Niobate: Froodite versus Pyrochlore

Abstract

We synthesized tin niobate photocatalysts with two distinct crystal structures—froodite (SnNb₂O₆) and pyrochlore (Sn₂Nb₂O₇)—using a simple solvothermal route. By systematically varying the synthesis pH, we tuned particle size, band‑edge potentials, and surface chemistry, and correlated these changes with photocatalytic activity for methyl orange (MO) degradation and hydrogen (H₂) evolution under visible light. Electrochemical impedance spectroscopy (EIS), transient photocurrent, and transient absorption spectroscopy (TAS) revealed that the froodite phase promotes superior charge separation and faster electron‑hole recombination suppression. X‑ray photoelectron spectroscopy (XPS) showed that Sn⁴⁺ species in the pyrochlore phase act as electron traps, diminishing activity. Photocatalytic tests confirmed that SnNb₂O₆ outperforms Sn₂Nb₂O₇ by up to 11.4× in H₂ evolution and exhibits a maximum MO degradation rate constant of 0.112 min⁻¹. Spin‑resonance and trapping experiments identify photogenerated holes, superoxide (O₂^−•), and hydroxyl (OH^•) radicals as the dominant reactive species in MO photodegradation.

Background

Global energy scarcity and environmental pollution drive the pursuit of semiconductor‑based photocatalysis as a clean technology. The efficiency of such systems hinges on light absorption and the kinetics of photogenerated charge carriers—processes inherently governed by the electronic structure, which in turn is dictated by crystal symmetry. Previous studies on oxide semiconductors (e.g., NaTaO₃/Na₂Ta₂O₆, SrNb₂O₆/Sr₂Nb₂O₇) have highlighted the importance of phase control for optimizing photocatalytic performance.

Sn‑based niobates, specifically SnNb₂O₆ (froodite) and Sn₂Nb₂O₇ (pyrochlore), are attractive visible‑light photocatalysts. Froodite features 2‑D layered NbO₆ sheets and distorted SnO₈ octahedra, yielding a narrow band gap (~2.3 eV) that facilitates hydrogen evolution. Pyrochlore, with a 3‑D corner‑sharing tetrahedral framework, presents a similar band gap but different electronic band positions. Understanding how these structural differences influence charge dynamics is essential for rational catalyst design.

Experimental Methods

Synthesis

SnNb₂O₆ and Sn₂Nb₂O₇ were prepared by solvothermal reaction of K₇HNb₃O₁₉•13H₂O (0.360 g) and SnCl₂•2H₂O (0.225 g) in 10 mL aqueous medium. The mixture was adjusted to pH 1–11 using KOH, sealed, and heated at 180 °C for 24 h. Products were washed and dried at 80 °C for 12 h.

Characterization

XRD (Cu Kα, Rigaku DMAX2500) confirmed phase purity; SEM (Hitachi S‑4800) and TEM (FEI Tecnai G² F20) revealed morphology; UV–vis DRS (PerkinElmer Lambda 750) provided optical gaps; XPS (ESCALab 220i‑XL) determined surface chemistry; BET (Micromeritics ASAP 2020) measured specific surface area. Electrochemical impedance spectroscopy (EIS) and transient photocurrent were measured in a three‑electrode setup with FTO electrodes. Transient absorption spectroscopy (TAS) used a 532 nm laser to probe charge‑carrier dynamics.

Photocatalytic Tests

For hydrogen evolution, 0.1 g photocatalyst, 80 mL water, and 20 mL triethanolamine (TEOA) were irradiated (λ ≥ 420 nm) with a 300 W Xe lamp; H₂ was quantified by GC. For MO degradation, 25 mg photocatalyst in 50 mL 2 × 10⁻⁵ mol L⁻¹ MO solution was stirred 120 min in the dark, then irradiated under the same light source; absorbance was monitored at 464 nm.

Results and Discussion

Phase Evolution and Morphology

Figure 1a shows XRD patterns: pH 1–5 yielded pure SnNb₂O₆, pH 7 produced a mixed phase, and pH 9–11 gave pure Sn₂Nb₂O₇. Crystallite sizes, calculated via the Scherrer equation, increased from 7.6 nm to 24.7 nm for SnNb₂O₆ as pH rose, and decreased from 47.0 nm to 17.4 nm for Sn₂Nb₂O₇. SEM/TEM images (Figure 2) reveal irregular nanosheets for SnNb₂O₆ and uniform clumps for Sn₂Nb₂O₇; HRTEM confirms lattice spacings of 0.285 nm (600) for SnNb₂O₆ and 0.611 nm (111) for Sn₂Nb₂O₇.

Optical Properties

Both phases absorb visible light. Tauc plots (Figure 3b) give indirect band gaps of 2.12 eV (SnNb₂O₆) and 2.22 eV (Sn₂Nb₂O₇), consistent with literature values.

Surface Chemistry

XPS survey shows Sn, Nb, O, C. Sn 3d spectra: SnNb₂O₆ displays Sn²⁺ peaks; Sn₂Nb₂O₇ shows both Sn²⁺ and Sn⁴⁺, the latter arising from alkaline oxidation and acting as electron traps. Nb is exclusively Nb⁵⁺. O 1s deconvolution indicates lattice O, surface hydroxyl, and adsorbed O₂ species; SnNb₂O₆ has higher O_ad content (11.8 %) than Sn₂Nb₂O₇ (8.3 %), suggesting superior oxygen adsorption.

Photocatalytic Activity

MO degradation: SnNb₂O₆ (pH 1) achieved 99.6 % removal in 40 min, with a rate constant of 0.112 min⁻¹. Activity decreased with higher pH. Hydrogen evolution: SnNb₂O₆ produced 5.94 μmol g⁻¹ h⁻¹, 3.2× and 11.4× higher than mixed and pyrochlore phases, respectively.

Charge‑Carrier Dynamics

EIS Nyquist plots (Figure 6a) show lowest charge‑transfer resistance (R_ct = 16.1 kΩ) for SnNb₂O₆, indicating efficient interfacial charge transfer. Transient photocurrent (Figure 6b) confirms higher current density for SnNb₂O₆. TAS (Figure 6c,d) demonstrates longer effective lifetimes (τ = 0.273 ms) for SnNb₂O₆ relative to Sn₂Nb₂O₇ (0.264 ms), reflecting superior charge separation.

Surface Area and Active Sites

BET analysis (Figure 7) gives 44 m² g⁻¹ (SnNb₂O₆), 37 m² g⁻¹ (mixed), and 60 m² g⁻¹ (Sn₂Nb₂O₇). The lower surface area of SnNb₂O₆ does not limit activity; the nanosheet morphology and smaller crystallite size dominate.

Reactive Species Identification

Quenching studies (Figure 8a) show that hydroxyl (TBA), superoxide (BQ), and hole (AO) scavengers reduce MO degradation, while electron scavenger (CCl₄) enhances it, indicating holes, O₂^−•, and OH^• as key species. EPR with DMPO confirms the formation of DMPO–O₂^−• and DMPO–OH^• under visible light.

Band Alignment

Mott–Schottky plots (positive slope) confirm n‑type behavior. Flat band potentials: –0.685 eV (SnNb₂O₆), –0.67 eV (mixed), –0.626 eV (Sn₂Nb₂O₇). With band gaps of 2.12 eV and 2.22 eV, the valence band positions are 1.435 eV and 1.352 eV, respectively, enabling oxygen reduction and pollutant oxidation.

Proposed Mechanism

Under visible light, electrons in SnNb₂O₆ are excited to a CB at –0.685 eV, reducing O₂ to O₂^−• and eventually to OH^•, which oxidize MO. Holes in the VB (1.435 eV) directly oxidize MO. The superior charge separation, lower recombination, and favorable band alignment account for the enhanced photocatalytic performance of the froodite phase.

Conclusions

We demonstrated that tin niobate phase and electronic structure critically influence charge kinetics and photocatalytic efficiency. Froodite SnNb₂O₆ exhibits a narrower band gap, higher charge‑separation efficiency, and absence of Sn⁴⁺ trap states, resulting in an 11.4× improvement in H₂ evolution and the highest MO degradation rate. Active species are predominantly holes, O₂^−•, and OH^•. This work underscores the importance of crystal‑phase engineering in designing high‑performance photocatalysts for sustainable energy and environmental applications.