Electronic configuration modulation of tin dioxide by phosphorus dopant for pathway change in electrocatalytic water oxidation

Abstract

The oxygen evolution process is a crucial part determining the total reaction kinetics of electrocatalytic overall water splitting for clean hydrogen production. However, for some semiconductor materials, e.g. tin dioxide (SnO₂) and tungsten oxide (WO₃), the water oxidation process generally proceeds following the two-electron pathway with rather high driving potential for the generation of hydrogen peroxide (H₂O₂ or HO₂⁻). In this work, efficient electronic configuration modulation of SnO₂ nanoparticles was realized by non-metallic phosphorus (P) doping, giving rise to the considerable transformation into a four-electron transfer-governed water oxidation reaction with the electrocatalytic activity even comparable to the benchmark RuO₂ nanoparticles. These results validate the great potential in remarkably tuning the electrocatalytic performance of transition metal compound electrocatalysts with non-metallic element dopants.

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1. Introduction

With the ever-increasing energy crisis as well as the environmental issues, the development of alternative sustainable and clean energy resource candidates to conventional fossil fuels is urgently desired. Due to the clean combustion and relatively high energy density, hydrogen (H₂) fuel shows great potential in the future of renewable energy.^1,2 Among current industrial approaches for H₂ generation, electrochemical overall water splitting (2H₂O → 2H₂ + O₂) using a reproducible electricity resource is highly appreciated; however, it is held back by the kinetically sluggish water oxidation part.^3,4 As a matter of fact, water oxidation is categorized into three types.^5,6 (i) Four-electron-transfer water oxidation, i.e. oxygen evolution reaction (simplified as OER), is described as 2H₂O → O₂ + 4H⁺ + 4e with a theoretical potential (E°) of 1.23 V. (ii) Two-electron-transfer water oxidation for H₂O₂ generation proceeds as 2H₂O → H₂O₂ + 2H⁺ + 2e with an E° value of 1.76 V. (iii) One-electron-transfer pathway to form the hydroxyl radical (˙OH) follows the equation: H₂O → ˙OH + H⁺ + e (E° = 2.38 V). To realize the energetic overall water splitting, advanced electrochemical materials catalyzing the OER efficiently are imminently anticipated on account of the lowest applied potential driving water oxidation. Currently, the state-of-the-art electrocatalyst benchmarks are usually noble metal-based materials such as IrO₂ and RuO₂.⁷ Nevertheless, they suffer from severe scarcity as well as high cost. It is, therefore, significant to develop high-performance OER electrocatalysts based on cheap and Earth-abundant resources for large-scale H₂ production.

Tin dioxide (SnO₂), a typical semiconductor material of transition metal oxide compounds, has been widely used in sensors, electronic devices, Li-ion batteries, photo(electro)chemistry, etc.^8–12 In very recent years, materials based on SnO₂ have also been demonstrated as efficient electrocatalysts for CO₂ reduction.^13,14 When it comes to electrocatalytic water oxidation, SnO₂ materials generally induce the two-electron-transfer pathway although it is a promising approach for the synthesis of H₂O₂, a prominent chemical product.^5,15,16 On the other hand, composition optimization is regarded as a rational route modulating the electronic configuration and thus significantly regulating the electrocatalytic performance. Among the various composition tuning techniques, element doping through atomic substitution in host lattices by guest atoms/ions is one of the most effective methods.^17,18 To modify the function of SnO₂ materials in catalyzing water oxidation, numerous efforts have been made in previous studies. For instance, Abbou et al. proposed that Sb and/or Ta dopants could markedly enhance the activity of SnO₂ in water oxidation.¹⁹ Moreover, Ullah and co-workers demonstrated an evolution of water oxidation from a typical two-electron-transfer pathway to the conventional OER through W doping on SnO₂ nanoparticles.²⁰ These metal element dopants have verified the considerable influences on SnO₂ materials in tuning their water oxidation performance. On the other hand, doping with non-metallic elements has also been reported to be effective in modulating the physicochemical properties of transition metal oxides.^21–23 Early literature has predicted that P doping could dramatically regulate the electronic configuration of SnO₂ and improve the electroconductivity theoretically,²⁴ which might be favorable for the performance optimization in catalyzing the water oxidation reaction. However, few related experimental works have verified this assumption.

Herein, SnO₂ nanoparticles modified with non-metallic P dopant were prepared. As the merit of P doping, the electrocatalytic water oxidation proceeded following a four-electron transfer-governed OER process rather than the typical two-electron pathway for the doped SnO₂ product (denoted as P–SnO₂) with the activity even comparable to the RuO₂ benchmark. In addition, theoretical calculations also provided a piece of strong support for both the electronic structure modulation and reaction pathway change. The results may further verify the role of non-metallic dopants in the careful design of advanced electrocatalysts.

2. Experimental section

2.1. Synthesis of SnO₂ nanoparticles

First, a CoSn(OH)₆ precursor was synthesized by the co-precipitation of stoichiometric Co²⁺ and Sn⁴⁺ ions in the presence of OH⁻ at room temperature according to a previous work.²⁵ Then the precursor was calcined at 650 °C in an air environment for 1 h. After cooling down naturally, the as-calcined powder was dispersed in a concentrated nitric acid solution (2 M) and transferred into a Teflon-lined autoclave (40 cm³) and maintained at 180 °C for 2 h. A brown product was harvested via separating the hydrothermal precipitate and being washed with deionized water several times. Finally, after drying at 60 °C, SnO₂ nanoparticles were obtained.

2.2. Synthesis of P–SnO₂ nanoparticles

To realize the successful preparation of P–SnO₂ nanoparticles, the as-obtained SnO₂ counterparts were thermally treated by a phosphorus source at an elevated temperature. In a typical procedure, hydrated sodium hypophosphite (NaH₂PO₂·H₂O) and the as-obtained SnO₂ powder with a weight ratio of 1 : 10 were placed separately with the former positioned at the upstream side, and then heated to 350 °C and maintained for 2 h under an Ar flow. After cooling down naturally, a grey powder was obtained.

2.3. Synthesis of RuO₂ nanoparticles

RuO₂ nanoparticles were prepared following the previous report.²⁶ Typically, 1 mmol of hydrated ruthenium chloride (RuCl₃·xH₂O) was dissolved in 20 mL of deionized water. Then the mixture was heated to 100 °C and 1 M NaOH solution (0.1 mL) was added dropwise. After continuous agitation for 1 h, the precipitate was collected by high-speed centrifugation, washed several times and dried under vacuum. RuO₂ nanoparticles were attained by calcining the co-precipitate product at 200 °C for 3 h.

2.4. Material characterization

The crystal phases and composition information of the as-prepared samples were recorded using an X-ray diffractometer (XRD, λ = 1.54178 Å, D/max 2500VB+), energy dispersive X-ray spectroscope (EDX) and inductively coupled plasma optical emission spectrometer (ICP-OES, SPECTRO BLUE SOP), respectively. Morphological and structural data were investigated through a scanning electron microscope (SEM, FEI Helios Nanolab 600i) and a transmission electron microscope (TEM, JEOL 3100). The chemical bonding energy information was recorded using an X-ray photoelectron spectroscope (XPS, ESCALAB250Xi).

2.5. Electrochemical measurements

All the electrochemical measurements were conducted on a CH Instruments model 760E workstation coupled with a conventional three-electrode setup using a graphite rod as the counter electrode. Hg/HgO and Hg/Hg₂SO₄ electrodes acted as the reference in alkaline KOH (0.1 M) and acidic H₂SO₄ (1.0 M) electrolytes, respectively. A catalyst ink was prepared by dispersing the catalyst powder homogeneously in a mixture of 2-propanol, deionized water and Nafion solution (10 wt%) at a volume ratio of 25/75/1 under ultrasonic treatment. Then the ink was dripped onto the polished clean glassy carbon electrode and dried at 105 °C to ensure the catalyst loading as ∼0.20 mg cm⁻². A rotation rate of 1600 rpm and a scan rate of 10 mV s⁻¹ with 95% iR compensation were adopted for the polarization curve tests. All the measured potentials in the current work were calibrated to the reversible hydrogen electrode (RHE) scale according to the equation: E_(RHE) = E_{(vs. Ref.)} + E_st. + 0.059 × pH, where E_{(vs. Ref.)} represents the metric potential versus the reference electrode and E_st. was 0.098 V and 0.658 V for Hg/HgO and Hg/Hg₂SO₄ electrodes, respectively. Electrochemical impedance spectroscopy (EIS) data were investigated at a potential of 1.75 V vs. RHE in the frequency range of 1000–0.1 Hz with an AC voltage amplitude of 5 mV. Rotating ring-disk electrode (RRDE) voltammograms setting the ring potential as 1.50 V vs. RHE were used to determine the electron transfer number (N) according to the equation: N = 4 × I_d/(I_d + I_r/N_c),^27,28 where I_d represents the disk current, I_r represents the ring current and N_c represents the current collection efficiency. The electrochemical surface area (ECSA) was estimated from the cyclic voltammetry (CV) curves at incremental rates of 10, 20, 30, 40 and 50 mV s⁻¹.

2.6. Calculations

All calculations were performed using the Vienna Ab initio Simulation Package (VASP) based on the first principles calculations.^29,30 For the exchange–correlation functional, the Perdew–Burke–Ernzerhof functional of generalized gradient approximation was adopted. In the calculation, a kinetic energy cutoff of 450 eV for the plane wave expansion and 5 × 6 × 1 grid of k points were set for the heterostructures in this work. The energy and force convergence criteria are 10⁻⁵ eV and 10⁻² eV Å⁻¹ in the relaxation, respectively. Given the periodic boundary conditions, a vacuum layer of 15 Å was used to avoid the image interaction.

3. Results and discussion

SnO₂ nanoparticles were prepared by hydrothermal acid treatment of the calcined product of the CoSn(OH)₆ precursor which was synthesized via the co-precipitation of stoichiometric Co²⁺ and Sn⁴⁺ ions in the presence of OH⁻, as depicted in Fig. 1. For the P–SnO₂ counterparts, they were synthesized by treating SnO₂ nanoparticles with NaH₂PO₂·H₂O at an elevated temperature (e.g. 350 °C).

Fig. 1 Scheme for the preparation of SnO₂ and P–SnO₂ nanoparticles.

To verify the phase composition, all the samples were examined by XRD. Fig. S1a† shows the XRD trace of the pink co-precipitation product, which matched well with the cubic CoSn(OH)₆ simulation (space group: Pn3̄m, PDF file no. 13-0356). When the pink product was calcined at 650 °C, a mixture of cubic Co₃O₄ (space group: Fd3̄m, PDF file no. 42-1467) and tetragonal SnO₂ (space group: P42/mnm, PDF file no. 41-1445) phases was obtained, as evidenced by the XRD pattern in Fig. S1b.† After sequential acid treatment, a brown product was obtained. Fig. 2a shows the XRD plots of both the samples before and after hypophosphite treatment, which were readily indexed as a tetragonal SnO₂ phase (space group: P42/mnm, PDF file no. 41-1445). No other diffraction peaks of any possible impurity were detected, implying the high phase purity of the as-prepared samples. Moreover, magnified patterns at the 2theta range of 24°–36°, as can be seen in Fig. 2b, further showed an obvious shift to higher angles for both (110) and (101) peaks after the hypophosphite treatment. In addition, the EDX result, as shown in Fig. S2,† confirmed the existence of the P element in the P–SnO₂ product. Moreover, the atomic ratio of P/Sn in the P–SnO₂ product was determined to be 0.128 using ICP-OES. According to the diffraction data, lattice parameters of the samples were refined using the Appleman method based on the least-squares refinement.^31,32 As plotted in Fig. 2c and d, the lattice parameters decreased from initial a = 4.749(2) Å and c = 3.191(4) Å for SnO₂ to a = 4.695(3) Å and c = 3.167(1) Å for the P-containing counterpart, suggesting the presence of the P dopant in the SnO₂ lattice.

Fig. 2 XRD results. (a) XRD patterns of SnO₂ and P–SnO₂ products. (b) Magnification of (a) in the 2theta range of 24–36°. Lattice parameters (c) a and (d) c for the tin oxide samples.

SEM and TEM observations were carried out on the as-prepared products to investigate the morphological and nanostructural details. As can be seen from Fig. S3a and b,† the co-precipitated product, i.e. CoSn(OH)₆, consists of uniform nanocubes with sizes of 250–300 nm. After the harsh calcination and subsequent acid treatment, the cube-like morphology was almost maintained, respectively (Fig. S3c and d†). Fig. 3a displays the SEM image of the P–SnO₂ sample. It was worth noting that even after the further thermal treatment in the presence of hypophosphite, the cubic nanoarchitecture was still well maintained. Fig. 3b shows the corresponding TEM result. Consistent with the SEM observation, regular nanocubes were also observed. It should be noted that the nanocubes were porous, as indicated by the area labelled by the brown arrows, which should be ascribed to the removal of the Co₃O₄ phase in the as-calcined product during the acid treatment under hydrothermal conditions. Fig. 3c shows the HRTEM image, which was recorded at the marked circle in Fig. 3b. Defined lattice fringes with an interplanar distance of 0.264 nm were observed, which were readily indexed to the (101) planes. The SAED pattern, as presented in the inset of Fig. 3c, further revealed the polycrystalline nature of the P–SnO₂ product. The composition of the doped product was also investigated using a high-resolution scanning transmission electron microscope coupled with EDX mapping, as plotted in Fig. 3d. All of the Sn, O and P elements were homogeneously dispersed over an individual particle. It was notable that although P was in a relatively low content, the EDX signal was still sufficient to delineate a well-defined elemental map in accordance with the cubic shape, indicating a homogeneous doping result.

Fig. 3 Morphological and nanostructural characterization on P–SnO₂ nanoparticles. (a) SEM, (b) TEM and (c) HRTEM images and (d) elemental distribution map. The inset of (c) shows the corresponding SAED pattern and the scale bar in (d) is 200 nm.

To investigate the chemical states as well as the possible charge transfer originating from the doping behavior, XPS analyses were conducted on the as-prepared oxides. The survey spectrum, as shown in Fig. S4,† clearly indicated the co-existence of the Sn, O and P atoms in the P–SnO₂ product. Fig. 4a shows the high-resolution patterns of Sn 3d orbits for both pristine and doped SnO₂ samples. With respect to that of the undoped SnO₂, a clear shift to a higher binding energy was detected for the signal of the doped product, which might be ascribed to the introduction of P atoms into the lattice on account of the more negative chemical state of P. A similar shift phenomenon was also demonstrated by P-doped hematite materials.³³Fig. 4b displays the P 2p spectrum of the P–SnO₂ product. The peak located at the lower binding energy (BE) corresponded to typical P–metal bonds, i.e. P–Sn herein (while the one at higher BE might be attributed to the possible oxidation of P species),^34,35 further providing a piece of strong evidence for the doping behavior of P atoms in the SnO₂ lattice.

Fig. 4 XPS spectra of tin oxide products: (a) Sn 3d and (b) P 2p.

To gain a deep insight into the doping effects of non-metallic P on transition metal oxides, the density of states (DOS) calculation analyses were performed, as shown in Fig. 5a. Electron distribution originating from the interaction among Sn, P and O atoms was presented around the Fermi level for the P–SnO₂ phase compared to that of its pristine counterpart, implying the electroconductivity improvement after P doping. It was generally accepted that superior conductivity features would promote electron transfer during electrocatalysis. Thus, the P dopant might bring an advantageous effect on the SnO₂ phase in electrochemical applications. Furthermore, as can be seen in Fig. 5b, the calculated Bader charge of +2.45e for Sn in P–SnO₂, significantly larger than +1.98e for that in the pristine oxide phase, revealed the electron migration from Sn after the doping process, providing strong support for the XPS result in Fig. 4a.³⁶ This electron transfer might be of advantage to the improvement of OH⁻ affinity, favorable for the water oxidation process.

Fig. 5 (a) DOS of the SnO₂ and P–SnO₂ phases. (b) Bader charge distribution of the P–SnO₂ phase, wherein purple, red and blue balls stand for Sn, O and P atoms, respectively.

The electrocatalytic water oxidation performance of the as-prepared catalysts was evaluated in an alkaline system. Fig. 6a shows the linear sweep voltammetry (LSV) curves of the as-prepared catalysts. It could be seen that no obvious current response was detected for pristine SnO₂ nanoparticles in the potential range of 1.2–1.8 V vs. RHE, in accordance with the reported literature.^5,15 On the other hand, the P–SnO₂ product achieved current densities of 10 and 63.1 mA cm⁻² at 1.69 and 1.80 V vs. RHE, respectively, which were much superior to that of the CoSn(OH)₆ precursor and even comparable to that of RuO₂ nanoparticles prepared following the previous reports.^26,37 In view of the onset potential location as well as the high current response at such a potential region, the water oxidation process catalyzed by P–SnO₂ nanoparticles should be ascribed to the energetic OER. In contrast to the negligible current response by the pristine SnO₂ product, this high activity indicated a considerable transformation from the two-electron-transfer water oxidation for H₂O₂ generation into a four-electron-involving pathway originating from P doping. Fig. 6b shows a Tafel slope of ∼39.7 mV dec⁻¹ for P–SnO₂ nanoparticles in comparison with 60.9 mV dec⁻¹ for the CoSn(OH)₆ precursor, suggesting much faster electrocatalytic reaction kinetics of the P–SnO₂ product. Moreover, such a value, close to 2.3 × RT/2F (i.e. 30 mV dec⁻¹; herein R, T and F were the gas constant, temperature and faradaic constant, respectively), might imply the formation of *OOH (* represents the catalytic sites) as the rate determining step.^36–38 In addition, such favorable reaction kinetics could also be inferred by the EIS results, as displayed in Fig. 6c. On the basis of the EIS data, an equivalent circuit model shown in the inset of Fig. 6c was established. The intersections of the spectra with the real axis, i.e. R_s in the circuit with the value of ∼14.0 Ω, corresponded to the electrolyte resistance value. The charge-transfer resistance, represented by R_ct, was estimated to be ∼72.5 Ω, much smaller than 154.8 Ω for the pristine SnO₂, revealing significant electroconductivity improvement. Moreover, this resistance was also smaller compared to 106.3 Ω for the hydroxide precursor, indicating much more favorable reaction kinetics. According to the LSV curves, the turnover frequency (TOF) was also evaluated to be 0.125 s⁻¹ for the P–SnO₂ product, markedly surpassing 0.085 s⁻¹ for CoSn(OH)₆ nanoparticles at an overpotential of 0.57 V based on the assumption that all the catalytic sites were accessible. In addition, based on the double-layer capacitance (C_dl), as depicted in Fig. 6d and Fig. S5,† ECSA was estimated to be 2.27, 1.96 and 2.92 m² g⁻¹ for the hydroxide precursor, pristine and doped SnO₂ products, respectively,³⁹ suggesting that the doping treatment made the active sites of SnO₂ sample much more accessible. Through normalizing the performance by ECSA, the intrinsic activity of the three samples could be attained, as plotted in Fig. 7a. It could be seen that in the OER-active region, especially 1.60–1.80 V vs. RHE, the intrinsic activity of the P–SnO₂ product was still permanently superior to the CoSn(OH)₆ precursor. For instance, the normalized current response was more than twice that of the hydroxide sample at 1.8 V vs. RHE. The electron transfer number, as plotted in Fig. S6† and the inset of Fig. 7a, was rather close to 4, confirming the doped SnO₂ nanoparticle-catalyzed water oxidation process as the OER. The reaction process was also uncovered using theoretical simulations. Fig. 7b displays the free energy chart of a typical OER on the P–SnO₂ product. It should be noted that the Gibbs free energy for the formation of *O was 0.77 eV, far lower than the typical formation energy of H₂O₂ (3.5 eV).^40,41 This implied that a more favorable OER rather than H₂O₂ generation occurred on the P–SnO₂ catalyst, providing strong theoretical support for the transition from a two-electron pathway to the OER during the water oxidation process induced by P doping. Moreover, a wide energy barrier existed for the formation of *OOH, which indicated Step III to be the rate-determining step, consistent with the Tafel slope analysis. In addition, the chronopotentiometry measurement at 10 mA cm⁻² (Fig. S7†), the comparison between the LSV curves after the 1st and 50th CV scans (Fig. S8†) and XPS analysis after electrocatalysis (Fig. S9†) further confirmed that the P–SnO₂ product showed satisfactory durability and stability during the continuous electrocatalytic process. In addition, the reaction pathway evolution of water oxidation was found in the acid electrolyte (Fig. S10†), further confirming the role of the P dopant in regulating the reaction pathway.

Fig. 6 Electrocatalytic performance of tin oxide and CoSn(OH)₆ samples in 0.1 M KOH solution. (a) LSV curves at a scan rate of 5 mV s⁻¹, (b) Tafel slopes and (c) EIS plot. (d) The plots of differences between the anodic and cathodic current densities at 1.15 V vs. RHE as a function of scan rate. Based on the slopes of the plots, C_dl was estimated to be 27.3, 23.5 and 35.0 mF cm⁻² for the hydroxide precursor, pristine and doped SnO₂ products, respectively.

Fig. 7 Electrocatalytic activity for water oxidation. (a) Normalized LSV curves by ECSA, and the inset shows the estimated electron transfer number (n) of the reaction catalyzed by the P–SnO₂ sample. (b) Gibbs free energy change during the typical oxygen evolution process catalyzed by the P–SnO₂ product.

4. Conclusions

In summary, P–SnO₂ nanoparticles were prepared through the thermal treatment of a P-containing resource (NaH₂PO₂·H₂O) on its pristine SnO₂ counterpart. Theoretical calculations predicted both the electronic configuration modulation of the SnO₂ phase and its pathway regulation in electrocatalytic water oxidation originating from the P dopant. As a result, P–SnO₂ nanoparticles achieved a current density of 10 mA cm⁻² at an overpotential of 0.46 V with a low Tafel slope of 39.7 mV dec⁻¹ in 0.1 M KOH solution, realizing the reaction pathway change in electrocatalytic water oxidation and even comparable to the benchmark RuO₂ nanoparticles. This pathway evolution uncovered the significant role of the P dopant in tuning the physicochemical properties of SnO₂ materials, offering a promising route for developing advanced transition metal compound electrocatalysts through considerably modulating their electronic structures with non-metallic dopants.

Author contributions

H. W., X. L. and R. M. designed the experimental study. H. W. and X. X. carried out the materials preparation, characterization and electrocatalytic measurements. Y. L. completed the theoretical calculations. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51874357, 51872333, and U20A20123), the Hunan Provincial Natural Science Foundation of China (2019JJ10006), and the China Postdoctoral Science Foundation (2020M682352). H. W. is thankful for the support from the Youth Talent Program of Zhengzhou University and Henan Provincial Key Technology R&D Program (212102210597). R. M. acknowledges support from JSPS KAKENNHI (18H03869).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qi01169c

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