Enhanced photoelectric performance of (2Al, S) co-doped rutile SnO₂

Abstract

In this study, theoretical calculations and experiments have been carried out to investigate the photoelectric performance of (2Al, S) co-doped rutile SnO₂. The electronic structures are studied by density functional theory (DFT). It is found that the metal Al can assist the bonding of the incorporated S with the neighboring O in SnO₂, introducing new energy levels in the forbidden band of SnO₂, which enhance the photoelectric performance. Meanwhile, the experiments are conducted to verify this. The (2Al, S) co-doped SnO₂ with different doping ratios are prepared by a hydrothermal method. The samples are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Results show that all the samples have rutile structure without any extra phase, and the dopant S²⁻ ion was implanted into the crystalline lattice of (2Al, S) co-doped SnO₂ and Al dopants replaced Sn atoms. The photoelectric performance tests show Al and S co-doping can improve the photoelectric performance, especially with a doping ratio of 5%, when the photocurrent reaches maximum of 3.0 μA cm⁻² which is almost twice as much as pure SnO₂, and the impedance is the smallest. The experiments results are consistent with our theoretical calculations. These findings are expected to be helpful for the design of highly active tin oxide-based photoelectric materials.

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Introduction

SnO₂ is a kind of rutile semiconductor oxide with wide band gap of 3.6 eV.^1,2 It has advantages of good optical transmission to visible light, big ultraviolet absorption coefficient, low preparation temperature, stable chemical property and strong acid and alkali resistance at room temperature.^3–6 Therefore, it can been widely used in organic light emitting diodes (OLED), liquid crystal displays (LCD), solar cells, gas sensors and field effect transistors.^7–11 The preparation and performance of SnO₂ materials have attracted wide attention. However, the photoelectric performance of pure SnO₂ is poor. Doping can adjust the change of band gap and promote the separation of photo-induced electrons and holes, which improves the photoelectric performance. Therefore, research mainly focuses on the influence of doping on the photoelectric performance and stability of SnO₂. Huang et al.¹² prepared Zn-doped SnO₂ nanorods by a simple hydrothermal method. The results show that photocatalytic activity of the synthesized Zn-doped SnO₂ nanorods is much higher than pure SnO₂ nanorods and bulk SnO₂ powders. Lee et al.¹³ studied electrocatalytic activities and stabilities of Pt supported on Sb-doped SnO₂ (ATO) for methanol (MOR) and ethanol (EOR) oxidation reactions. The results show that the Pt/ATO exhibited much higher electrochemical stabilities than Pt supported on carbon (Pt/C). Ahmed et al.¹⁴ synthesized Al doped SnO₂ thin films by a sol–gel dip coating technique with different ratios of Al on glass and silicon substrates. The results show doping can enhance its photoelectric performance. With regard to SnO₂ doping, there are many researches on single doping. The reports on co-doping is few, co-doping is firstly proposed during the study on semiconductor materials with single polarity and wide band gap.¹⁵ Two or more elements doping can further improve the photoelectric performance of materials by theory.¹⁶

In our paper, to gain detailed insight into the effect of Al and S dopants on SnO₂ photoelectric performance, we used density function theory to investigate the electronic structures. Based on the theory calculation, we prepared (2Al, S) co-doping SnO₂ photoelectrode. X-ray diffraction (XRD), scanning electron microscopy (SEM). X-ray photoelectron spectra (XPS) were taken to characterize pure SnO₂ and different ratios of co-doping SnO₂. Meanwhile, photoelectric performance of (2Al, S) co-doped rutile SnO₂ are tested. As a result, the exceptionally high photoelectric performance observed for (2Al, S) co-doped SnO₂ is ascribed to metal Al assisting S–O bonding.

Results and discussion

Lattice parameters

Fig. 1 shows the optimized configurations of pure SnO₂ and (2Al, S) co-doped SnO₂. Compared with pure SnO₂, the positions of Al atoms and S atom move to a certain degree accompanying the distortion of bonds. The distance of co-planar O and adjacent perpendicular O atom in pure SnO₂ is 2.902 Å. However, for the (2Al, S) co-doped SnO₂, the distance of S and O atoms is 1.641 Å, indicating the formation of S–O covalent bond. The definite lattice parameters are listed in Table 1. The lattice parameters are all bigger than that of pure one, this may because the radius of S²⁻ is bigger than that of O²⁻ radius (O²⁻: 1.22 Å, S²⁻: 1.70 Å).

Fig. 1 2 × 2 × 2 SnO₂ supercell (a) and (2Al, S) co-doped SnO₂ supercell (b).

Table 1 The calculated lattice parameters of pure and (2Al, S) co-doped 2 × 2 × 2 SnO₂

	a (Å)	b (Å)	c (Å)
Pure SnO2	9.475	9.475	6.373
(2Al, S) co-doped SnO2	10.065	9.761	6.519

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Band structure and density of states

The calculated band gap of pure SnO₂ is 1.07 eV, which is same with other theory calculations.¹⁷ However, the experiment value of SnO₂ band gap is 3.6 eV. The difference may be caused by the underestimation of density functional theory to band gap.¹⁸ The defect of density functional theory leads to smaller forbidden band comparing to experiment value. In this paper, the calculated band gaps are used to compare the changes. Hence, the error does not affect the qualitative analysis on calculation results. As shown in Fig. 2, the band gap of (2Al, S) co-doped SnO₂ is 0.92 eV, while the band gap of pure SnO₂ is 1.07 eV. The difference suggests that (2Al, S) co-doping can narrow the band gap. Further analysis shows that the co-doping introduce fully occupied energy levels, which forms new top of valance band. At the same time, the bottom of conduct band move down.

Fig. 2 Band structures of pure SnO₂ (a) and (2Al, S) co-doped SnO₂ (b).

The total density of states (TDOS) and partial density of states (PDOS) of pure and (2Al, S) co-doped SnO₂ shown in Fig. 3 are discussed to explain the formation mechanism of fully occupied energy levels. The valence band of pure SnO₂ is mainly composed of O 2p state mixing with small Sn 3d state, and the conduction band is constituted of O 2p and Sn 5p states. While for (2Al, S) co-doped SnO₂, the fully occupied energy levels of valence band are the hybridization of O 2p and S 3p states, which indicate the formation of S–O bond. Similar S–O bond can be found for TiO₂ doping.¹⁹ The bonding mechanism (Fig. 4) are as follows.

Fig. 3 TDOS (a) and PDOS (b) of pure SnO₂ and (2Al, S) co-doped SnO₂.

Fig. 4 Schematic plot of bonding mechanism for Al assisted S–O bonding in (2Al, S) co-doped SnO₂.

Two metal acceptors Al substitute for two Sn atoms of SnO₂ supercell, introducing two holes. Then, one S atom substitute for adjacent O atom, which make two electrons transfer from S atom to metal acceptors Al due to smaller electron negativity of S atom comparing with O atom. At this point, 3p_z orbital of S atom is empty, which make the formation of covalent bond with adjacent O atom possible. The hybridization of p_z orbitals of both S atom and O atom can form bonding orbital σ and antibonding orbital σ*, while their p_x and p_y orbitals can form bonding orbital π and antibonding orbital π*. The fully occupied energy levels are the antibonding orbital π*. The band gap is narrowed by the antibonding orbital π* and conductive band bottom. Therefore, the metal acceptors Al can assist S–O bonding by taking two electrons away from S atom.

SEM analysis

Fig. 5 shows SEM images of different ratios of (2Al, S) co-doped SnO₂. The pure SnO₂ is spherical particle and the surface is nonuniform. For (2Al, S) co-doped SnO₂, the particle sizes decrease. Small particle size can short spreading time of charge carrier from the body to the surface and reduce electron–hole recombination rate, which promote the photocatalytic activity. Different levels of reunion appear in all samples. When the doping ratio is 5%, the particles are more dispersed and the granularity is more uniform comparing with others. Hence, the photoelectric performance is the best.

Fig. 5 SEM images of different ratios of (2Al, S) co-doped SnO₂. (a) 0%, (b) 5%, (c) 10% and (d) 15%.

XRD analysis

Fig. 6 show the XRD patterns of different co-doping ratios of SnO₂ crystal. The peaks appeared at 2θ = 26.506°, 33.945° and 51.94°, which are the characteristic peaks of rutile SnO₂.^20,21 The corresponding crystal faces are (110), (101) and (211). No characteristic diffraction peaks of other SnO₂ crystal were found, indicating that they still keep the structure of rutile SnO₂ after co-doping. In addition, comparing with pure SnO₂, no significant shift in the peak positions was found.

Fig. 6 XRD patterns of pure SnO₂ and different ratios of (2Al, S) co-doped SnO₂.

XPS analysis

The XPS spectrum of 5% (2Al, S) co-doping SnO₂ are displayed in Fig. 7. Fig. 7a shows O 1s spectrum, it has two asymmetry peaks, which indicate the presence of two different types of oxygen states.²² The peaks at 530.88 eV and 531.84 eV belong to lattice oxygen (O²⁻) and adsorbed oxygen^23,24 respectively. The peak of O 1s at 530.88 eV belongs to SnO_2−x structural O atoms or oxygen inside non-stoichiometric oxides within the surface region. The peak at 531.84 eV suggests that oxygen from O₂ molecules of the ambient atmosphere adsorbed on the grains or surface of SnO₂. Compared to O 1s spectrum of pure SnO₂ (Fig. 7e), the peaks move to lower binding energy, which indicates that (2Al, S) co-doped SnO₂ contain more oxygen deficiency. The spectrum of Sn 3d in Fig. 7b have two binding energy peaks. The peaks of Sn 3d_5/2 and Sn 3d_3/2 respectively locate at 486.98 eV and 495.43 eV, suggesting the presence of Sn⁴⁺. Also, the different value between Sn 3d_5/2 and Sn 3d_3/2 is 8.45 eV, which is consistent with the standard spectrum of Sn as reported in the Handbook of X-ray photoelectron spectroscopy.²⁶ However, compared to Sn 3d spectrum of pure SnO₂ (Fig. 7f), the peaks moves towards high binding energy. The generation of chemical displacement is not only the potential energy change caused by valence electron transfer, but also the contribution of lattice field. For (2Al, S) co-doped SnO₂, due to the small difference between the electronegativity of Al and Sn, there is no chemical displacement caused by valence electron transfer. The doped Al dispersed in the lattice, causing SnO₂ lattice defects, which can lead to the change of the lattice field, resulting in the increase of binding energy. S 2p XPS spectrum is shown in Fig. 7c, the peaks at 160.95 eV and 162.81 eV indicate the presence of S²⁻. Therefore, we can conclude that S²⁻ ion replace the O²⁻ ion in the lattice of SnO₂. However, it also has a peak at 167.49 eV except the above two peaks, which suggests the existence of S(+6). This may be caused by SO₄²⁻ ions adsorbed on the surface.²⁵ As shown in Fig. 7d, the binding energy of Al 2p peak at 74.55 eV shows that the Al³⁺ ions replace the Sn⁴⁺ ions of SnO₂.

Fig. 7 XPS spectrum of 5% (2Al, S) co-doped SnO₂ (a) O 1s, (b) Sn 3d, (c) S 2p, (d) Al 2p and pure SnO₂ (e) O 1s, (f) Sn 3d.

Transient photocurrent response test

Fig. 8 display the transient photocurrent response of pure SnO₂ and different doping ratios of (2Al, S) co-doped SnO₂ photoelectrode under 0.5 V bias. The photocurrent value of pure SnO₂, 5%, 10%, and 15% (2Al, S) co-doped SnO₂ reached 1.7, 3.0, 2.4 and 1.2 μA cm⁻². When the co-doping amount is 5%, the photocurrent value reaches maximum (the photocurrent values of 0–10% are shown in Fig. S1 and S2†). The common photoelectric metal oxides materials are TiO₂ and ZnO, which show excellent photoelectric properties among metal oxides. Our photocurrent value of 5% (2Al, S) co-doped SnO₂ is almost equal to TiO₂ with etching depth of 387 nm ²⁷ and ZnO under UV light irradiation.²⁸ As the amount of co-doping continuously increase, the photocurrent value decreases. The results show that suitable amount of co-doping can improve the sunlight utilization rate of SnO₂. 5% (2Al, S) co-doped SnO₂ has small particles and large surface area, which can increase contact area with sunlight. This is consistent with above SEM analysis. Meanwhile, a proper amount of co-doping can inhibit the recombination of photoelectric charge by promoting the separation of photoelectron and hole pairs. In addition, the photocurrent decreases with illumination time, which may due to the slight corrosion of electrodes.

Fig. 8 Transient photocurrent response curve of pure SnO₂ and different ratios of (2Al, S) co-doped SnO₂.

Electrochemical impedance spectroscopy

Electrochemical impedance curve of pure SnO₂ and different doping amount of (2Al, S) co-doped SnO₂ electrodes are shown in Fig. 9. The curve is composed of teratogenic arc of high frequency area and straight line of low-frequency area. The arc diameter of 5% (2Al, S) co-doped SnO₂ is the smallest, suggesting minimal electrode reaction resistance and more active points of electrochemical reaction. Meanwhile, the slope is relatively higher than others, which shows faster diffusion of ions. In addition, 5% (2Al, S) co-doped SnO₂ has smaller solution resistance, indicating that moderate amount of co-doping can increase the effective contact area of the material and shorten the transmission path of electron and ion.

Fig. 9 Electrochemical impedance plots of pure SnO₂ and different ratios of (2Al, S) co-doped SnO₂.

Conclusions

In summary, DFT calculations have been carried out to detailedly study the effect of photoelectric performance of (2Al, S) co-doped SnO₂. Compared to pure SnO₂, the (2Al, S) co-doped SnO₂ has smaller band gap of 0.92 eV, introducing new energy levels in the forbidden band, which enhance the photoelectric performance in a certain degree. Furthermore, the S–O bonding mechanism assisting by metal Al was explored. The theory calculation can provide evidence supporting experiment conduct. Based on this, different ratios of (2Al, S) co-doped SnO₂ were synthesized by hydrothermal method. In order to study the photoelectric performance, we prepared (2Al, S) co-doped SnO₂ electrodes. The photocurrent of 5% (2Al, S) co-doped SnO₂ reached maximum value of 3.0 μA cm⁻². At this point, the impedance is the smallest. These findings can pave the way for developing electrodes with high photoelectric performance in photoelectric catalysis field.

Method

Calculation methods and calculation model

In this study, first-principles calculation was carried out using the Cambridge Serial Total Energy Package (CASTEP) code^29,30 based on density functional theory (DFT).³¹ The exchange correlation potential was described with generalised gradient approximation (GGA) in the scheme of Perdew–Burke–Ernzerhof function (PBE).^32,33 The Brillouin zone sampling point K values are 4 × 4 × 3. The cutoff energy for plane waves is set as 340 eV. All geometry structures are fully relaxed until the convergence criteria of energy and force are less than 2.0 × 10⁻⁵ eV per atom and 0.03 eV Å⁻¹ respectively. The maximum stress is 0.1 GPa.

In calculation, we choose a 2 × 2 × 2 SnO₂ supercell with 48 atoms as calculation model. It has two types of Sn atoms and oxygen atoms respectively. The Sn atoms respectively locate in the vertex and body position. The oxygen atoms can be divided into two classes: one is coplanar with Sn atom, the other is perpendicular to the surface. For (2Al, S) co-doped rutile SnO₂ supercell, two metal Al replace the body-centered Sn atoms, and S atom replace O atom perpendicular to the surface. For an investigation on the electronic structure and ground-state property, the valence electronic configurations are O-2s²2p⁴, S-3s²3p⁴, Sn-5s²5p²and Al-3s²3p¹ states. To obtain reliable results, structural optimization for bulk SnO₂ obtained the following parameters: a = 4.737 Å, c = 3.186 Å, in good agreement with experiment (a = 4.734 Å, c = 3.187 Å).³⁴

Synthesis of (2Al, S) co-doped SnO₂ electrodes

The synthesis of (2Al, S) co-doped SnO₂ used a one-step hydrothermal method.^35,36 The process is as follows: SnCl₄·5H₂O (10.058 g) was firstly dissolved in a beaker with 60 mL deionized water and stirred about 1 h. The sulfourea and AlCl₃·6H₂O, as S and Al sources, were added into the solution. After stirring for 4 h, the solution was transferred to a 100 mL stainless-steel autoclave with a Teflon liner. The autoclave was sealed and heated at 433 K for 12 h without shaking or stirring during this period. Black precipitate was collected after the autoclave cooled down to room temperature and washed with distilled water several times in order to remove any impurities. After that, the samples were obtained by high temperature calcinations at 573 K for 2 h. Three (2Al, S) co-doped SnO₂ samples with initial doping molar ratios of 5%, 10% and 15% were synthesized. In the preparation, the molar ratio of Al to S is 2 : 1. Pure SnO₂ was synthesized by the same procedure but without any additives.

The above samples were grinded and dispersed in glycerine forming 4 suspensions: 0%, 5%, 10% and 15%. The suspensions were ultrasonicated for 5 min. Then electrodes were prepared by dispensing the suspensions on glass substrates. Finally, the electrodes were burned for 2 h to reserve.

Characterization

The crystal structures of the samples were identified by X-ray diffraction (XRD, X'pert PRO). The morphologies of the samples were characterized by scanning electron microscopy (SEM) with a JSM-7001F (JEOL, Japan). Elemental compositions of the samples were detected by X-ray photoelectron spectroscopy (XPS, Thermo, ESCALAB 250Xi, USA).

Photoelectric performance tests

A trielectrode system was carried out to test the photoelectric performance of SnO₂. The pure SnO₂ and different ratios of (2Al, S) co-doped electrodes, Pt electrode and saturated calomel electrode (SCE) were respectively used as tested electrodes, auxiliary electrode and reference electrode. Na₂SO₄ solution with concentration of 1.0 mol L⁻¹ was used as electrolyte solution. The bias voltage is set as 0.5 V.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is jointly funded by the National Natural Science Foundation of China and Shenhua Group Corp. (Grant No. U1261103), the Natural Science Foundation of Shanxi Province of China. (Grant No. 201601D011023) and Graduate Science and Technology Innovation Fund Project of Shanxi (No. 2017BY053).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra07891a

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