Tuning of Electro-optical Properties of Nano-structured SnO₂:Ga Powders in a Micro Drop Fluidized Reactor

Abstract

Tuning of electro-optical properties of nano-structured $SnO_2:Ga$ powders in a micro drop fluidized reactor (MDFR) was highly effective to enhance the activities of powders to be used as sensor materials. The tuning was conducted continuously in a facile one-step process during the formation of powders. The microscopic hydrodynamic forces affected the band gap structure and charge transfer of $SnO_2:Ga$ powders through the oxygen and interfacial tin vacancies by providing plausible pyro-hydraulic conditions, which resulted in the decrease in the electrical resistance of the materials. The analyses of room-temperature photoluminescence (PL) spectra and FT-IR exhibited that the tuning could improve the surface activities of $SnO_2:Ga$ powders by adjusting the excitation as well as separation of electrons and holes, thus maximizing the oxygen vacancies at the surface of the powders. The scheme of photocatalytic mechanism of $SnO_2:Ga$ powders was also discussed.

1. Introduction

Tin oxides (SnO₂) have been regarded as materials to be developed as chemical sensing materials, due to their inherent characteristics including high electron mobility and transparent conductance, wide band gap structure and chemical stability. Those advantages of SnO₂ allow them to be applied in the fields of various kinds of light cells, gas sensors, batteries, electrodes, magnetic semiconductors, etc [1-13].

Since the doping of foreign components into the host materials has been simple but impactive to improve the qualities to reveal the unique functions of host materials, various kinds of ions have been doped into SnO₂ to develop and explore the specific functions of SnO₂ based materials [14-39]. Several methods, including hydrothermal, sol-gel, precipitation, chemical vapor deposition, spray pyrolysis and electrospinning method, have been utilized for the doping of foreign components into SnO₂ and preparation of SnO₂ based materials [25-41]. It has been understood that the optical, electronic, chemical and physical characteristics of the final products are directly related to the methods of doping as well as preparation, since the surface facet, crystallite structure, chemical bonding and electron distribution have been adjusted and determined by means of atomic and electronic behaviors of components during the formation and doping.

For practical applications, however, the doping and preparation method should be continuous in order to make a compromise with the batch modes, which are taking a long time, requiring additional steps such as calcination, annealing, washing or drying and preparing only small amount of product. Spray pyrolysis has been successfully utilized to prepare various kinds of advanced functional powder materials with uniform size and composition continuously [42-46]. However, a more effective method has been required to meet the needs of creation for the more specific and special functions of SnO₂ based advanced materials, which could be possible by controlling the operation conditions during the very short powder formation reaction. In addition, the method should be economic in order to be a process, which can be facilely employed in many fields of industries. To solve those practical demands, a micro drop fluidized reactor (MDFR) was developed, since it can realize the continuous preparation of nanostructured powders with uniform size and composition by means of a one-step process with reasonable production efficiency in a short time [47-52].

A micro drop fluidized reactor was employed in the present study to prepare the Ga doped SnO₂ (SnO₂:Ga) nanostructured powders in a one-step and continuous process with plausible production efficiency. The doping of Ga³⁺ ions was conducted to modify the SnO₂ surface to enhance the gas sensing abilities. Because the Ga³⁺ ion can be a better choice, since its radius (0.062 nm) is more similar to that of Sn⁴⁺ (0.069 nm) than those of other Group III elements [53], which would compromise the reduction of lattice deformation. The mechanism of sensing reaction of ethanol at the active sites of SnO₂:Ga was examined by considering the excitation and separation of electronhole pairs at the surface of the powders.

2. Experiments

Ga³⁺ doped SnO₂ nano-structured powders were prepared continuously with reasonable production efficiency by using an MDFR system. The reactor system was composed of micro drop generating, powder formation reaction, micro bubble generating and powder collection parts, as shown in Fig. 1 [48-51]. The micro drops of precursor solutions, which were generated in an atomizing chamber with four oscillators (1.7 MHz, Htech Green Tech.), were transported to the top of the vertical reaction zone by a carrier gas continuously. The reaction zone, which was made of quartz tube (0.03 m ID × 1.2 m in height), was heated by electric furnace to be maintained at 800 ^oC. The micro drops were fluidized during the reaction by means of micro bubbles generated by a micro bubble generator with a micro flow controller (MFC) and injected into the bottom of the reactor. The flow rates of micro drops and bubbles were 4.0 L/min and 0.4 L/ min, respectively, to adjust the favorable reaction conditions [48-51]. As-prepared powders of SnO₂ and SnO₂:Ga were collected by using a thimble filter (ADVANTEC, grade 84).

Fig. 1. Schematic diagram of experimental apparatus.

1. Reactor 2. Furnace 3. Ultrasonic atomizer 4. Precursor solution 5. Filtered & compressed air 6. Flow meter 7. Regulator & controller 8. Micro Bubble Generator 9. Microbubble port 10. Filter & collector 11. Gas scrubber 12. Distilled water reservoir 13. Liquid foam generator 14. Calming section

Diluted solution of SnCl₄·5H₂O (Sigma Aldrich 98%) in ethyl alcohol was used as the Sn source and diluted GaCl₃ (Alfa Aesar 99.9%) in de-ionized water as the Ga source, respectively, which were mixed sufficiently before atomizing. The atomic ratio of Ga/Sn was in the range of 0~3.0 at.%. The crystalline structure of as-prepared powders was confirmed by analyzing the X-ray diffraction patterns (XRD, MAX-2200 Ultima). The light absorption ability of each sample was analyzed by diffuse reflectance spectra (DRS, Solidspec-3700) using an UV-VIS-NIR spectrophotometer (Shimadzu, UV-3101PC). The shapes and surface morphologies of samples were characterized by using a field-emission scanning electron microscope (FE-SEM, JSM7000F). To analyze the room-temperature photoluminescence (PL) of samples, a Fluorolog 3 photoluminescence spectrometer (HJY, Japan) was used. The BET surface area and pore size distribution of each sample were determined from the nitrogen adsorption - desorption isotherms (ASAP2010, Micromeritics Ins. Corp.). The chemical compositions of the samples were checked by Fourier transform infrared (FT-IR) spectroscope (Nicolet 6700). The electrical resistivity of each sample, which was fabricated on the alumina substrate equipped with a pair of Pt electrodes, was measured by using an electrometer system. The sensing response was determined by means of the ratio of Ra and Re, which are the resistances in the air and in the ethanol vapor, respectively. The adsorption and recovery times were determined as the times taken by the sensing system to achieve 90% of the total resistance change.

3. Results and Discussion

Fig. 2 shows the crystalline phases of as-prepared SnO₂ and SnO₂:Ga powders determined by XRD with varying the amount of Ga³⁺ (C_Ga). In XRD patterns, all the diffraction peaks could be well indexed to the tetragonal structure of SnO₂ based on the standard data file (JCPDS file no. 41-1445), confirming that single crystal phases of SnO₂ and SnO₂:Ga are mainly formed in a one-step process without any other additional process such as calcination, annealing, washing, and drying. Any other considerable characteristic peaks of impurities were not detected. The magnifications of the main peaks around 26.7° for (110) plane were shifted toward lower angles with varying C_Ga (Fig. 3), indicating that the Ga³⁺ ions were doped into the crystal lattice of SnO₂. Since the ionic radius of Ga³⁺ (0.062 nm) is similar to that of Sn⁴⁺ (0.069 nm), some parts of Sn⁴⁺ in the crystal lattice of host material were easily substituted by Ga³⁺[53]. The substitution of Ga³⁺ for Sn⁴⁺ resulted in a slight distortion of the lattice structure of SnO₂ [18], representing a slight shift of the main peaks toward a lower angle. Note that the shift increased with increasing C_Ga up to 2.0 at.%; however, the shift tended to decrease with a further increase in C_Ga from 2.0 to 3.0 at%. The inset of Fig. 3(A) shows that the crystallite size of SnO₂:Ga Powders decreased with increasing C_Ga, because of the corresponding increase in the number of defects in the crystal lattice [42,54,55]. The crystallite size was calculated by using the Scherrer equation from XRD data.

Fig. 2. X-ray diffraction pattern of SnO₂ and SnO₂:Ga powders prepared in the micro drop fluidized reactor (C_Ga [at.%]: (a) 0, (b) 1.0, (c) 1.5, (d) 2.0, (e) 3.0).

Fig. 3. Magnification of the diffraction peak around 26.7°. The insets of (A) and (B) show the crystallite size and micro strain of SnO₂:Ga with varying C_Ga, respectively (C_Ga [at.%]: (a) 0, (b) 1.0, (c) 1.5, (d) 2.0, (e) 3.0).

Fig. 4 shows the diffuse reflectance spectra (DRS) of SnO₂ and SnO₂:Ga at different Ga³⁺ content doped into the SnO₂ lattice. The spectra were transformed to absorbance intensity through the KubelaMunk method. The doping of Ga³⁺ into SnO₂ extended the absorption band to the longer wavelength, indicating the modification of the band gap structure of SnO₂. In the inset of Fig. 4, the band gap energy of SnO₂:Ga powders, which was estimated from the onset of the absorption of DRS analysis, decreased with increasing Ga³⁺ content up to 2.0 at.% but it increased slightly with a further increase in C_Ga from 2.0 to 3.0 at.%, as listed in Table 1. The shifts of spectra pattern to the visible region are associated with the change of electron coordination around the Sn⁴⁺ ions, resulting in the change of band gap structure. It can be ascribed to the formation of a new energy level of Ga-O between the conduction and valance bands of SnO₂ [42,48-50, 56]. That is, the bandgap energy of SnO₂ powders prepared in the MDFR was 3.09 eV, which is composed of Sn 5s5p conduction band and O 2p valance band. However, the substitution of Ga³⁺ ions for Sn⁴⁺ ions could lead to the formation of an acceptor level (Ga³⁺ 4s4p) below the conduction band of Sn, which results in the decrease in the bandgap energy.

Fig. 4. Diffuse reflectance spectra of SnO₂ and SnO₂:Ga powders prepared in the micro drop fluidized reactor. The inset shows the change of band gap energy with varying C_Ga (C_Ga [at.%]: (a) 0, (b) 1.0, (c) 1.5, (d) 2.0, (e) 3.0).

Table 1. Crystallite size, bandgap energy and BET surface area of SnO₂:Ga powders

Fig. 5 shows the field-emission SEM images of as-prepared SnO₂ and SnO₂:Ga powders. The powders were spherical and porous; however, the surface became more wrinkled and furrowed with increasing C_Ga. The size of aggregate was in the range of 500 - 600 nm. The change of surface morphology of SnO₂:Ga powders is ascribed to the doping of Ga³⁺ into SnO₂ which induces the formation of partitioned parts at the surface of the powders. The unique partitioned parts can be due to the change of ionic strength and electro-negativity in the lattice of SnO₂ by doping Ga³⁺ ions. The effects of regional breaking of charge homogeneity at the surface of the powders could increase with increasing dopant content [39].

Fig. 5. Field-emission SEM images of SnO₂ and SnO₂:Ga powders prepared in the micro drop fluidized reactor (C_Ga [at.%]: (a) 0, (b) 1.0, (c) 1.5, (d)2.0, (e) 3.0).

Fig. 6 shows the room - temperature photoluminescence (PL) spectra of as-prepared powders. In Fig. 6, all the samples have a broad visible emission centered at 530~600 nm, due to the vacancies of oxygen and interstitial tin [7,16,17,22,31,32,38]. The intensity of PL spectra becomes weaker with an increase in the content of Ga³⁺. Similar trends were also observed by doping of Zn, Fe or Ag into SnO₂ lattice prepared by hydrothermal method [31,38]. Since the PL spectra reflect the charge transfer at the surface of the material, the doping of Ga³⁺ into SnO₂ lattice can induce the charge transfer due to vacancies of the oxygen and interfacial tin. The vacancies could lead to the formation of trapped states by forming metastable energy level in the band gap structure, which consequently results in the induction of charge transfer at the surface of host material. The vacancies of oxygen and interfacial tin could increase with increasing the content of Ga³⁺ doped into SnO₂ lattice. Thus, the intensity of PL spectra became weaker with increasing C_Ga up to 2.0 at.%. However, the PL intensity tended to increase slightly with a further increase in C_Ga from 2.0 to 3.0 at.%. Because, the modified band gap structure of SnO₂ by doping Ga³⁺ ions could be less effective with a further increase in C_Ga owing to the recombination of charge carriers [32,38].

Fig. 6. Room temperature photoluminescence spectra of SnO₂ and SnO₂:Ga powders prepared in the micro drop fluidized reactor (C_Ga [at.%]: (a) 0, (b) 1.0, (c) 1.5, (d) 2.0, (e) 3.0).

Fig. 7 shows the nitrogen adsorption isotherms (Fig. 7A) and pore size distribution (Fig. 7B) of SnO₂:Ga powders. All of the isotherms show Type IV hysteresis loops according to the IUPAC classification. The inset of Fig. 7(A) shows that the BET surface area of SnO₂:Ga powders increases with increasing C_Ga up to 2.0 at.%, but it decreases slightly at 3.0 at.%, indicating a maximum at 2.0 at.%. The main peaks of pore size distribution are in the range of 8~15 nm, and the distributions tend to shift to the smaller size range with increasing C_Ga, showing the smallest pore size distribution at the content of Ga³⁺ is 2.0 at.% (Fig. 7(B)).

Fig. 7. (A) Nitrogen adsorption isotherms and (B) BJH pore size distribution of SnO₂ and SnO₂:Ga powders prepared in the micro drop fluidized reactor. The inset of (A) shows the BET surface area with varying C_Ga (C_Ga [at.%]: (a) 0, (b) 1.0, (c) 1.5, (d) 2.0, (e) 3.0).

Fig. 8 shows the effects of C_Ga on the response of SnO₂:Ga samples to ethyl alcohol at 150 ^oC. The Ra and Re are the resistances in air and ethyl alcohol, respectively. Compared to the commercial powders, the response of SnO₂ powders prepared by MDFR shows higher relative resistance (Ra/Re). In addition, the doping of Ga³⁺ enhances the response intensity, exhibiting a maximum at C_Ga is 2.0 at.%, due to the increase in the number as well as strength of active sites [8,14,15,30].

Fig. 8. Effects of C_Ga on the response of SnO₂:Ga powders to ethyl alcohol (100 ppm) at 150 ℃.

Fig. 9 shows the transient relative response of SnO₂:Ga samples to ethyl alcohol (100 ppm) at 150 C with varying C_Ga. The response and recovery times were 5~8 s and 7~10 s, respectively. The response behavior becomes sensitive with increasing C_Ga up to 2.0 at.%. Since the electrical resistance stems basically from the charge transfer at the surface of the sensor, the doping of Ga³⁺ ions into SnO₂ lattice resulted in the loss of oxygen and generation of free electrons and holes, so that the extrinsic semiconducting properties could take place at the surface of host materials. The electric resistivity could decrease with increasing oxygen vacancies as well as densities of free electrons and holes at the active sites of the surface, since they are highly sensitive to the external sensing gas [3,17,57- 60]. However, the resistivity could increase with a further increase in C_Ga, showing a minimum value at C_Ga is 2.0 at.%. This can be because the charge carriers tend to interact with the ionic lattice in SnO₂ with increasing the amount of Ga³⁺ ions. That is, the extra free electrons and oxygen vacancies generated by the doping of Ga³⁺ ions could interact for recombination, which prevents the active sites of SnO₂:Ga from the surface reaction with exterior gas (ethyl alcohol) and thus from the gas sensing [17,38,61-64].

Fig. 9. Transient response of SnO₂:Ga powders to ethyl alcohol (100 ppm) at 150 ℃ with varying C_Ga (C_Ga [at.%]: (a) commercial (Acros Organics), (b) 0, (c) 1.5, (d) 2.0, (e) 3.0).

Fig. 10 shows the FT-IR spectrum of as-prepared SnO₂ and SnO₂:Ga powders at different content of Ga³⁺ ions, by which the chemical compositions and bonds were analyzed. Sn-O vibration modes such as Sn-O-Sn and O-Sn-O were observed between 600 and 800 cm^-1, indicating that some of metal oxides were bounded more than one oxygen atom [3,15,64-67]. The Sn-OH vibration peak was observed at around 1110 cm^-1. The O-H bending vibration peak due to the adsorbed water molecule was observed at 1610 cm^-1. Broad band peaks of hydrogen bounded OH group on the surface of SnO₂:Ga were observed between 3300~3650 cm^-1 [3,7,15,64,68-70]. Note that the distinctive peak indicating the presence of OH group on the surface of SnO₂:Ga powders was most noticeable at C_Ga is 2.0 at.%.

Fig. 10. FT-IR spectra of SnO₂ and SnO₂:Ga powders prepared in the micro drop fluidized reactor (C_Ga [at.%]: (a) 0, (b) 1.0, (c) 1.5, (d) 2.0, (e) 3.0).

Fig. 11 shows the scheme of excitation and separation of electronhole pairs and possible reaction mechanism of SnO₂:Ga powders with varying C_Ga. Since the new formed Fermi energy level of SnO₂:Ga hetero structure is lower than that of the bottom of the conduction band of the SnO₂, the excited and separated electrons could be easily transferred to the acceptor level of Ga 4s4p, with remaining holes at the surface of SnO₂ [38,61,70-72]. The adsorbed oxygen associated with the electrons on Ga³⁺ions could be formed O^2-, and the holes could decompose H₂O into OH- at the surface of SnO₂. Both radicals could capture organic component such as ethyl alcohol, resulting in the surface reaction and thus the sensing. However, the excessive Ga³⁺ ions over a certain value (C_Ga is 2.0 at.%) could act as recombination centers of separated electrons and holes owing to the electrostatic attraction between the Ga³⁺ions and holes, which consequently results in the decrease in the surface activities of SnO₂:Ga hetero structure [28,38,61-63,70-72].

Fig. 11. Scheme of excitation and separation of electrons-holes pairs in the band gap structure and possible reaction mechanism of SnO₂:Ga powders (C_Ga [at.%]: (a) 0, (b) 1.0, (c) 1.5, (d) 2.0,(e) 3.0).

4. Conclusion

Effective tuning of electro-optical properties of nano-structured SnO₂:Ga powders was conducted successfully by employing a micro drop fluidized reactor. The bend gap structure was modified easily and the surface activity of SnO₂:Ga powders was enhanced controllably in a facile one-step process. The amount of dopant (Ga³⁺ ions) to minimize the band gap energy and to maximize the BET surface area as well as charge transfer at the surface of the material was 2.0 at.% within this experimental conditions. The response of as-prepared SnO₂ powders to ethanol vapor was somewhat higher that of commercial powder, and the response became more sensitive with increasing the amount of Ga³⁺ ions doped into SnO₂ up to 2.0 at.%. A possible reaction mechanism was obtained to explain the surface activity of as-prepared SnO₂:Ga powders.