materials Article
Influence of Surface Defects and Size on Photochemical Properties of SnO2 Nanoparticles Mahdi Ilka 1 , Susanta Bera 1,2,3 and Se-Hun Kwon 1,2,3, * 1 2 3
*
School of Materials Science and Engineering, Pusan National University, Busan 46241, Korea;
[email protected] (M.I.);
[email protected] (S.B.) Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University, Busan 46241, Korea Institute of Materials Technology, Pusan National University, Busan 46241, Korea Correspondence:
[email protected]; Tel.: +82-051-510-3775
Received: 19 April 2018; Accepted: 24 May 2018; Published: 28 May 2018
Abstract: We report the successful synthesis of surface defective small size (SS) SnO2 nanoparticles (NPs) by adopting a low temperature surfactant free solution method. The structural properties of the NPs were analyzed using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The presence of surface defects, especially oxygen vacancies, in the sample were characterized using micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence emission. The Brunauer–Emmet–Teller (BET) nitrogen adsorption–desorption isotherms demonstrated the superior textural properties (high surface area and uniform pore size) of SS SnO2 compared to large size (LS) SnO2 . A comparable study was drawn between SS SnO2 and LS SnO2 NPs and a significant decrease in the concentration of surface defects was observed for the LS sample. The results showed that surface defects significantly depend upon the size of the NPs. The surface defects formed within the band gap energy level of SnO2 significantly participated in the recombination process of photogenerated charge carriers, improving photochemical properties. Moreover, the SS SnO2 showed superior photoelectrochemical (PEC) and photocatalytic activities compared to the LS SnO2 . The presence of a comparatively large number of surface defects due to its high surface area may enhance the photochemical activity by reducing the recombination rate of the photogenerated charges. Keywords: low temperature solution method; photoelectrochemical activity; photocatalytic activity
SnO2 nanoparticles;
surface defects;
1. Introduction The use of metal oxide semiconductors (MOS) in photochemical processes such as the photocatalytic degradation of water soluble organic pollutants and photocatalytic/ photoelectrochemical water splitting has received great attention [1–5]. MOS, such as ZnO, TiO2 , and SnO2 , are well-known photocatalysts that can show efficient photocatalytic/photoelectrochemical activity through the extraction of photoexcited electrons and holes [1,3–5]. The functional oxide, SnO2 , is a potential n-type semiconductor with outstanding optical, electrical, and electrochemical properties [6,7], and thus can exhibit efficient photocatalytic/photoelectrochemical activity [6–8]. However, this activity could be enhanced further by improving the separation of the photogenerated charges. Various extrinsic and intrinsic surface defects in semiconductors can act as active centers to trap photogenerated electrons and control the electron–hole recombination process [3–8]. Thus, surface defects can play a major role in the photogenerated charge separation process and enhance the photochemical process [3–8]. It is believed that the surface defects can vary with the size and Materials 2018, 11, 904; doi:10.3390/ma11060904
www.mdpi.com/journal/materials
Materials 2018, 11, 904
2 of 12
shape of nanostructures, and in this respect, it has been found that small size SnO2 nanoparticles (NPs) exhibit more surface defects, such as oxygen vacancies, compared to SnO2 nanorods and nanospheres [6,7,9]. Also, their favorable textural properties have attracted considerable attention due to distinctive features such as high surface areas, uniform pore sizes and large pore volumes which allow efficient electrochemical and photochemical activities [10]. These features can have a strong effect on the adsorption and diffusion efficiency of guest molecules within the pore network, especially in photocatalysis [11]. Moreover, the properties can provide a regularly aligned pathway for the diffusion of molecules as well as offering a large number of active sites in the interconnected mesopore wall for interaction with guest molecules. To date, little emphasis has been placed on the ability of the size, shape, and surface defects of SnO2 nanoparticles to improve their photocatalytic/photoelectrochemical activity. Thus, it is suggested that small SnO2 nanoparticles with a high specific surface area will contain many surface defects, improving photochemical activity via prolonging the photogenerated charge recombination process. Generally, SnO2 NPs with surface defects can be prepared by hydrothermal and microwave syntheses [5–8]. However, these synthesis techniques require the use of stringent conditions to grow the surface defective SnO2 NPs. Moreover, it is very difficult to understand the mechanism (for a rational synthesis strategy) involved in hydrothermal processing [12]. In this respect, low temperature solution methods in air at atmospheric pressure are more advantageous than other approaches for the synthesis of surface defective SnO2 NPs for photochemical applications. In this work, we synthesized surface defective small size (SS) SnO2 NPs from a surfactant free precursor solution using a simple low temperature (95 ◦ C) solution method in an air atmosphere. Structural (crystal phase, size, and morphology), surface defect, and textural (multi point BET surface area, porosity, and pore volume) analyses were used to characterize the material properties of the samples. Finally, the photochemical properties of the SS SnO2 NPs were systematically studied and compared with large size (LS) SnO2 NPs. The properties varied with NP size and greatly influenced the photoelectrochemical and photocatalytic activities of the NPs. 2. Materials and Methods 2.1. Synthesis of Small Size (SS) and Large Size (LS) SnO2 NPs Surface defective, small size (SS) SnO2 NPs were synthesized using a facile low temperature (95 ◦ C) solution method using tin (II) 2-ethylhexanoate (TEH) (Sigma-Aldrich, Saint Louis, MO, USA 95%) in dimethyl formamide (DMF) solvent. In this synthesis, 1 g of TEH was thoroughly mixed with 200 mL DMF by continuous stirring. Subsequently, the aliquot was placed in an air oven at 95 ◦ C for 24 h. The mixture was then centrifuged at 12,000 rpm for 10 min to separate the nanoparticles from the solution mixture. The product was washed several times using ethanol and deionized water separately. Finally, the samples were dried in an air oven (NEURONFIT, Seoul, Korea) at ~55 ◦ C for 3 h, and SS SnO2 NPs were obtained. The calculated yield of SS SnO2 NPs was ~84%. To make large size (LS) SnO2 NPs, the SS NPs were heated at 800 ◦ C for 2 h into a box furnace in air atmosphere. 2.2. Characterization of SnO2 NPs The surface features, including the cluster sizes of the SnO2 samples, were analyzed using a field emission scanning electron microscope (FESEM, S-4800, Hitachi, Chiyoda-Ku, Japan). The microstructural properties of the SS SnO2 NPs were further characterized using a transmission electron microscope (TEM, JEM-2100F(HR), JEOL, Akishima, Japan) at an operating power of 200 kV. For the measurement, the samples were dispersed in methanol for 2 h, and the dispersed material was drop cast onto a carbon coated Cu grid (300 mesh). Subsequently, the grid was dried in an air atmosphere. An X-ray diffractometer (XRD, D8 ADVANCE, Bruker, Hong Kong, China) using Cu-Kα1 at a wavelength of 1.5418 Å was used to analyze the crystallinity of the SnO2 samples. The surface areas of SS and LS SnO2 NPs were determined with Brunauer–Emmet–Teller (BET)
Materials 2018, 11, 904
3 of 12
nitrogen adsorption–desorption isotherms measurement at liquid nitrogen temperature (77 K) using a BET surface area and porosity analyzer (Autosorb-1, Quantachrome Instruments, Boynton Beach, FL, USA). Prior to the measurement, the samples were out-gassed in a vacuum at 90 ◦ C for ~3 h. The pore size distribution was estimated by the Barrett–Joyner–Halenda (BJH) method and pore volume was determined from the amount of adsorbed nitrogen at P/P0 = 0.994225. The Raman spectra of the SnO2 samples were measured using a micro-Raman spectrometer (Xper Ram 200, Nano Base). The X-ray photoelectron spectra (XPS) of the SnO2 samples were measured using an X-ray photoelectron spectrometer (K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA). Monochromated Al-Kα was used as the X-ray source. The position of the C 1s peak was taken as a reference (binding energy, 284.6 eV). The absorption spectra of the samples were measured using the diffuse reflectance method with a UV-Vis-NIR spectrophotometer (UV3600, Shimadzu, Kyoto, Japan) with an ISR attachment. The room temperature photoluminescence emission spectra of the samples were recorded using a fluorescence spectrometer (LS-55, Perkin-Elmer, Waltham, MA, USA) with an excitation wavelength of 325 nm. 2.3. Photoelectrochemical and Photocatalytic Activities Photoelectrochemical measurements of the SS and LS SnO2 samples were carried out using a modular high power potentiostat/galvanostat instrument (Autolab PGSTAT302N, Metrohm, Herisau, The Switzerland) with a standard three-electrode cell, under dark as well as visible light irradiation. A 50 W white LED lamp (FOCUS LED, AC 220 V, 6500 K, 4200 lm, Shanghai Haneup City Lighting Electric, Shanghai, China) was used as the visible light source. An Ag/AgCl (3 M KCl) electrode and a Pt foil were used as the reference electrode and counter electrode for the measurements, respectively. The working electrode was fabricated on cleaned FTO coated glass by the doctor-blading technique from a paste-like viscous material formed from the respective samples. The sample paste was produced by grinding the samples and mixing them with DMF solvent. The coated glass was then dried at 90 ◦ C for ~4 h in the air oven. An aqueous solution of 0.1 M Na2 SO4 was used as the electrolyte in this experiment. The photocurrent response was measured by manual control of on-off cycles. Electrochemical impedance spectra (EIS Nyquist plots) of the samples were measured in the frequency range of 0.01–100 KHz with an AC amplitude of 10 mV under visible light irradiation. Chronoamperometric study (photocurrent density vs. time, I–t curve) of the samples were performed at zero bias potential vs. Ag/AgCl (3 M KCl). The photocatalytic activity of the two samples during the degradation of an aqueous solution of methyl orange (MO) was measured under irradiation from the same visible light that was used in the photoelectrochemical measurements. Initially, 1.0 mg/mL of the photocatalyst was dispersed in 10−6 M of MO. The dispersed solutions were continuously stirred in the dark for 1 h to achieve an adsorption–desorption equilibrium. In this experiment, C0 and C indicate the initial concentration of the dye after achieving an adsorption–desorption equilibrium in the dark and the actual concentration of the MO dye solution after a specific light exposure time, respectively. The suspensions were centrifuged at different time intervals to measure their UV–Vis absorption spectra (UV1800, Shimadzu, Kyoto, Japan). The activity of the samples was determined from the plot of C/C0 against light irradiation time. 3. Results and Discussion 3.1. Material Properties In reaction medium, the majority of an Sn(II) precursor will form SnO2 NPs under an air atmosphere, while some of the Sn(II) ions will be introduced into the SnO2 lattice matrix during low temperature solution processing because of the comparable ionic radii of hexacoordinate Sn(IV) (0.69 Å) and Sn(II) (0.62 Å) [8]. Thus, the Sn(II) ions in the reaction medium can be doped into SnO2 , resulting in the formation of oxygen vacancies [13]. Figure 1 shows the XRD patterns of the SS and LS SnO2 samples. The patterns were fully consistent with tetragonal SnO2 (JCPDS No. 41-1445). However,
Materials 2018, 11, 904
4 of 12
the SS sample showed broader diffraction peaks due to the small sizes of the SnO2 crystallites [14]. In contrast, the pattern of the LS sample showed sharp XRD peaks, demonstrating the significant Materials 2018, 11, x FOR PEER REVIEW 4 of 12 crystalline growth of the SnO2 NPs, indicating the formation of a larger crystallite size. The average The average sizes of SS were and LS samples were to 25.5 be 2.8 nmrespectively, and 25.5 nm,using crystallite sizes ofcrystallite SS and LS samples estimated to be estimated 2.8 nm and nm, respectively, using A the Scherrer equation. A FESEM image (Figure 2a)the of formation the LS confirmed theSnO2 the Scherrer equation. FESEM image (Figure 2a) of the LS confirmed of larger formation of larger SnO 2 nanoclusters (20–50 nm). On the other hand, the FESEM image (Figure 2b) nanoclusters (20–50 nm). other hand, the FESEM image (Figure 2b) of the SS sample did not Materials 2018, 11, x FOR On PEERthe REVIEW 4 of 12 of the SS sample did not exhibit distinct grain boundaries, and the cluster/particle sizes of SnO2 could exhibit distinct grain boundaries, and the cluster/particle sizes of SnO2 could not be determined. Thus, not beThe determined. Thus, itsizes can be the particles of the SS sample would average crystallite of assumed and LSthat samples were estimated be 2.8 nm and be 25.5very nm,small. it can be assumed that the particles ofSSthe SS sample would be verytosmall. To determine the particle respectively, thesize, Scherrer equation. A FESEM image (Figure of the LSThe confirmed the To determine the using particle a TEM analysis of the SnO 2 NPs was 2a) performed. TEM images of size, athe TEM analysis of the SnO NPs was performed. The TEM images of the SS SnO sample are 2 2 of larger 2 nanoclusters the other hand, the FESEM image(Figure (Figure2e) 2b) SSformation SnO2 sample areSnO shown in Figure(20–50 2c,d,f.nm). TheOn particle size distribution curve of the shown in Figure 2c,d,f. The particle size distribution curve (Figure 2e) of the SnO NPs showed a of the SS sample did not exhibit distinct grain boundaries, and the cluster/particle sizes of SnO 2 could SnO2 NPs showed a narrow size range (2.3–3.8 nm) for the NPs. The HRTEM images of2 the SS SnO2 not be determined. Thus, it can be assumed that the particles of the SS sample would be very small. narrow size rangedistinct (2.3–3.8 nm)fringes for thewith NPs.inter-planar The HRTEM images thenm, SS SnO NPsand showed distinct NPs showed lattice distances of of 0.328 0.2622 nm 0.231 nm, To determine the particle size, a TEM analysis of the SnO2 NPs was performed. The TEM images of latticewhich fringes with inter-planar of 0.328 0.262 nm and 0.231 nm, which corresponded to corresponded to thedistances (110), (101) andnm, (200) planes of tetragonal SnO2, respectively, the SS SnO2 sample are shown in Figure 2c,d,f. The particle size distribution curve (Figure 2e) of the corroborating XRDplanes the (110), (101) andthe (200) tetragonal SnO the results. 2 , respectively, SnO2 NPs showed a results. narrowof size range (2.3–3.8 nm) for the NPs. Thecorroborating HRTEM images of theXRD SS SnO 2
20 20
30
40
50
2-theta (degree) 40 50
SS (202)
(202)
(112) (301)
(112) (310) (301)
(002) (310)
(002)
(211)
(220)
(211)
LS SS
(220)
(101) (101)
30
(200) (200) (111) (111)
(110)
LS
(110)
Intensity (a.u.)
Intensity (a.u.)
NPs showed distinct lattice fringes with inter-planar distances of 0.328 nm, 0.262 nm and 0.231 nm, which corresponded to the (110), (101) and (200) planes of tetragonal SnO2, respectively, corroborating the XRD results.
SnO2 (JCPDS 41-1445)
SnO2 (JCPDS 41-1445)
60
60
70
70
2-theta (degree)
Figure 1. XRD patterns thesmall smallsize size (SS) (SS) and SnO 2 nanoparticles. Figure 1. XRD patterns ofof the and large largesize size(LS) (LS) SnO 2 nanoparticles. Figure 1. XRD patterns of the small size (SS) and large size (LS) SnO2 nanoparticles.
Figure 2. FESEM images of the LS (a) and SS (b) samples. TEM images (c,d), and HRTEM image (f) of
Figure 2. FESEM images of the LS (a) and SS (b) samples. TEM images (c,d), and HRTEM image (f) of the SS SnO2 nanoparticles (NPs). Particle size distribution (e) measured from (c). the SSFigure SnO22.nanoparticles Particle distribution (e) measured from FESEM images(NPs). of the LS (a) andsize SS (b) samples. TEM images (c,d), and (c). HRTEM image (f) of the SS SnO2 nanoparticles (NPs). Particle size distribution (e) measured from (c).
Materials 2018, 11, 904
5 of 12
Materials 2018, 11, x FOR PEER REVIEW
5 of 12
Raman spectroscopy is an important tool for the study of surface defects on nanoparticles. The Raman spectra (Figure 3) SS andtool LS for SnO were measured detect the presence Raman spectroscopy is of an the important the study of surface defects on to nanoparticles. The 2 samples of surface defects, as 3) oxygen as well as thewere Raman activetomodes Raman spectrasuch (Figure of the vacancies, SS and LS SnO 2 samples measured detect of thetetragonal presence ofSnO2 . surface spectrum defects, such oxygen vacancies, as well as the Raman active of tetragonal 2. Thewere The Raman ofas the LS sample contained three peaks at 483,modes 630 and 772 cm−1SnO , which −1, which were Raman of and the LS contained peaks atand 483,were 630 and 772 cmwith attributed to spectrum the Eg , A1g B2gsample vibration modes,three respectively, consistent those of rutile attributed to the Eg, A1g and B2g vibration modes, respectively, and were consistent with those of rutile −1 for bulk SnO the SS sample; this peak was 2 [6,7]. However, an additional peak appeared at 575 cm bulk SnO2 [6,7]. However, an additional peak appeared at 575 cm−1 for the SS sample; this peak was attributed to surface defects on the SnO2 NPs [6]. This surface defect related peak was not detected in attributed to surface defects on the SnO2 NPs [6]. This surface defect related peak was not detected in the LS sample. To understand the rutile bands, along with the surface related defect band on the SS the LS sample. To understand the rutile bands, along with the surface related defect band on the SS sample, the broad peak was resolved peaks(inset (inset Figure Additionally, a small sample, the broad peak was resolvedinto intothree threeGaussian Gaussian peaks Figure 3). 3). Additionally, a small shifting in the position of the Raman bands was noticed with respect to the LS sample. Kar et shifting in the position of the Raman bands was noticed with respect to the LS sample. Kar et al. [7]al. [7] also found the same observation forfor SnO comparedtotobulk bulk SnO surface also found the same observation SnO nanoparticles compared SnO 2. Therefore, surface 2 2nanoparticles 2 . Therefore, defects, especially oxygen deficiencies, were SSSnO SnO2 2NPs, NPs, indicating the number defects, especially oxygen deficiencies, werefound found in in the SS indicating thatthat the number of of centers depends strongly thesize sizeofofthe thenanoparticles. nanoparticles. defectdefect centers depends strongly onon the
630
Intensity (a.u.)
Intensity (a.u)
575
487 639
400
450
500
550
600
650
700
-1
483
Raman shift (cm
)
575 772
LS
SS
300
400
500
600
700
800
900
1000
-1
Raman shift (cm ) Figure 3. Raman spectra andLS LSsamples; samples; inset part of SS Figure 3. Raman spectra of of thethe SSSSand insetshows showsthe themagnified magnified part ofsample. SS sample.
The presence of oxygen vacanciesininthe theSnO SnO2 samples was also confirmed by the XPS analysis. The presence of oxygen vacancies 2 samples was also confirmed by the XPS analysis. The XPS spectra of the SS and LS SnO 2 samples are shown in Figure 4. The O 1s binding energy The XPS spectra of the SS and LS SnO2 samples are shown in Figure 4. The O 1s binding energy (Figure 4a) of the SS SnO2 sample was resolved into two Gaussian peaks with binding energies (Figure 4a) of the SS SnO2 sample was resolved into two Gaussian peaks with binding energies centered at 530.4 eV (A1) and 531.4 eV (A2), corresponding to lattice oxygen (Olattice) and the oxygen centered at 530.4 eV (A1) and 531.4 eV (A2), corresponding to lattice oxygen (Olattice ) and the oxygen deficient region (Odefect) in a tetragonal SnO2 crystal, respectively [5,8,15]. However, for the LS sample, deficient region (O ) in a tetragonal SnOpeak respectively [5,8,15]. forcontents the LS (R, sample, 2 crystal, the intensity ofdefect the oxygen defect related decreased (Figure 4b) [16,17].However, The relative the intensity ofby thedividing oxygenthe defect related [16,17]. The relative calculated area of the A2peak peak decreased by the area (Figure of the A14b) peak) of oxygen defects incontents the SS (R, calculated dividing thewere area1.13 of the peak by the area of the A1clearly peak)indicates of oxygen defects in the and LSbySnO 2 samples andA2 0.37, respectively. This result a significant decrease in2the amountwere of oxygen the LS SnO2 NPs. decrease of oxygen vacancies in SS and LS SnO samples 1.13 vacancies and 0.37,inrespectively. ThisThe result clearly indicates a significant the LS may have been induced from the2 air atmosphere, and similar results were in decrease in sample the amount of oxygen vacancies inheating the LS in SnO NPs. The decrease of oxygen vacancies also foundmay for ZnO [18,19]. Sn binding peaks in the sample appeared at 486.4results eV andwere the LS sample have been The induced from energy heating in the air LS atmosphere, and similar 494.8 eV, and were assigned as the Sn 3d 5/2 and Sn 3d3/2 peaks of SnO2, respectively, as shown in also found for ZnO [18,19]. The Sn binding energy peaks in the LS sample appeared at 486.4 eV and Figure 4c [20]. However, the Sn3d peaks of the SS SnO2 sample were found at higher energy levels. 494.8 eV, and were assigned as the Sn 3d5/2 and Sn 3d3/2 peaks of SnO2 , respectively, as shown in As described above, a large number of oxygen vacancies acting as electron acceptors were present in Figure 4c [20]. However, the Sn3d peaks of the SS SnO2 sample were found at higher energy levels. the lattice of the SS SnO2. The presence of a greater number of electron acceptor centers near the Sn As described above, a large numberdensity of oxygen vacancies acting asinelectron acceptors were present in atoms may decrease the electron of these atoms, resulting an increase in the Sn3d binding the lattice of the SS SnO . The presence of a greater number of electron acceptor centers near the Sn 2 energy [8,21]. This result indirectly demonstrated the existence of a much greater number of oxygen atomsvacancies may decrease the electron density atoms, resulting in an increase in the Sn3d binding in the SS SnO 2 sample than in of thethese LS sample. energy [8,21]. This result indirectly demonstrated the existence of a much greater number of oxygen vacancies in the SS SnO2 sample than in the LS sample.
Materials 2018, 11, 904 Materials 2018, 11, x FOR PEER REVIEW
(a)
O 1s (SS)
6 of 12 6 of 12
(b)
Raw data Olattice (A1)
O 1s (LS)
Raw data Olattice (A1)
Odefect (A2)
528
530
532
534
Background Sum
Intensity (a.u.)
Intensity (a.u.) 526
Odefect (A2)
Background Sum
526
536
528
530
(c)
Sn 3d5/2 486.4
534
536
SS LS
486.7 Sn 3d3/2
Intensity (a.u.) 480
532
Binding energy (eV)
Binding energy (eV)
494.8
485
490
495.1
495
500
Binding energy (eV)
Figure Figure4.4.XPS XPSdata datafor forthe thebinding bindingenergies energiesofof(a) (a)OO1s 1sfor forthe theSS SSsample; sample;(b) (b)OO1s 1sfor forthe theLS LSsample sample and (c) core levels of Sn 3d for both samples. and (c) core levels of Sn 3d for both samples.
The (Figure 5), 5), i.e.,i.e., the the surface area,area, pore pore size, and of the SSofand Thetextural texturalproperties properties (Figure surface size,pore andvolume, pore volume, the LS samples were were studied usingusing multi-point BETBET nitrogen SS and LS samples studied multi-point nitrogenadsorption–desorption adsorption–desorption isotherm isotherm measurement. of of SSSS sample were type IV with an H4 hysteresis loop,loop, indicating the measurement.The Theisotherms isotherms sample were type IV with an H4 hysteresis indicating presence of narrow, slit-like mesopores in the sample matrix matrix [22] (Figure 5a,b). The measured average the presence of narrow, slit-like mesopores in the sample [22] (Figure 5a,b). The measured 3 /g respectively. specific area, porearea, volume and pore size m2/g,197.9 0.13 cm ~2.7 averagesurface specific surface pore volume andwere pore197.9 size were m23/g /g,and 0.13 cmnm, and ~2.7 nm, The pore sizeThe distribution of thecurves sample from the desorption branch showed a respectively. pore size curves distribution ofdetermined the sample determined from the desorption branch sharp peak at ~3.7peak nm (Figure the other theother LS sample IV withwas an H3 hysteresis showed a sharp at ~3.75b). nmOn (Figure 5b).hand, On the hand,was thetype LS sample type IV with loop, implying thatloop, it contained slit-shaped pores resulting from aggregates of plate-like particles [22] an H3 hysteresis implying that it contained slit-shaped pores resulting from aggregates of (Figure 5c,d). It was also noted that it exhibited bimodal wider pore size distribution. The specific plate-like particles [22] (Figure 5c,d). It was also noted that it exhibited bimodal wider pore size 2/g, 0.22 cm3/g and 8.8 surface area, pore average of theand LS sample distribution. Thevolume specificand surface area,pore poresize volume averagewere pore10.9 sizemof the LS sample were 2 3 nm, respectively. Therefore, the specific surface area of LS sample was found to be bysample about 10.9 m /g, 0.22 cm /g and 8.8 nm, respectively. Therefore, the specific surfacedecreased area of LS 18 times. Moreover, the hysteresis loop, and shape mesopores of LS sample were found was found to be decreased by about 18 pore times.size Moreover, theofhysteresis loop, pore size and shape of to be changed compared to the SS changed sample. The high specific surface of the SSThe sample mesopores of abruptly LS sample were found to be abruptly compared to thearea SS sample. high was comparatively those was of other surface defective SnO2those nanoparticles synthesized by specific surface areahigher of the than SS sample comparatively higher than of other surface defective various methods [5,7,23]. Thus, temperature solution synthesis in an airsolution atmosphere is SnO2 nanoparticles synthesized by low various methods [5,7,23]. Thus, low temperature synthesis advantageous for obtaining nanoparticles with a high specific surface uniform in an air atmosphere is advantageous for obtaining nanoparticles with a area high with specific surfacepore areasizes with which canpore playsizes an important improving photoelectrochemical and photocatalytic uniform which canrole playinan importantboth roletheir in improving both their photoelectrochemical activities. and photocatalytic activities.
Materials 2018, 11, 904 Materials 2018, 11, x FOR PEER REVIEW
7 of 12 7 of 12
(a)
(b)
0.08
80
dV/dD (cm3/g.nm)
3
Volume adsorbed (cm /g)
90
70 60 50 2
Surface area = 197.9 m /g 40 30 0.0
(c)
0.06
0.04
0.02
3
Pore volume = 0.13 cm /g Average pore diameter = 2.66 nm 0.00
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/po)
(d)
0
2
0
2
4
6
8
10
4
6
8
10
Pore diameter (nm)
0.020
dV/dD (cm3/g.nm)
3
Volume adsorbed (cm /g)
200
150 2
Surface area = 10.9 m /g 3
Pore volume = 0.22 cm /g
100
Average pore diameter = 8.8 nm 50
0 0.0
0.015
0.010
0.005
0.000 0.2
0.4
0.6
0.8
Relative pressure (p/po)
1.0
Pore diameter (nm)
Figure 5.5.Multipoint MultipointBET BET nitrogen adsorption–desorption isotherms of SS (a)and SS and (c)samples, LS samples, Figure nitrogen adsorption–desorption isotherms of (a) (c) LS and and pore size distribution curves ofSS (b)and SS and (d)samples, LS samples, calculated from desorption branch pore size distribution curves of (b) (d) LS calculated from the the desorption branch of of the isotherm. the isotherm.
3.2. Optical and Photochemical Properties 3.2. Optical and Photochemical Properties The UV–Vis absorbance spectra of the SS and LS SnO samples were measured by the diffuse The UV–Vis absorbance spectra of the SS and LS SnO22 samples were measured by the diffuse reflectance method and are shown in Figure 6. Compared to the SS sample, the absorption edge reflectance method and are shown in Figure 6. Compared to the SS sample, the absorption edge of of the LS sample showed a red shift. We estimated the optical direct band gap energy (inset of the LS sample showed a red shift. We estimated the optical direct band gap energy (inset of Figure 6) Figure 6) of SS sample to be ~3.75 eV, whereas the band gap energy of the LS sample was ~3.38 eV. of SS sample to be ~3.75 eV, whereas the band gap energy of the LS sample was ~3.38 eV. The size The size dependence of the band gap energies of the SnO2 NPs was due to the quantum confinement dependence of the band gap energies of the SnO2 NPs was due to the quantum confinement effect effect [6]. The band gap was found to decrease from ~3.75 to ~3.38 eV, indicating that the SS SnO2 [6]. The band gap was found to decrease from ~3.75 to ~3.38 eV, indicating that the SS SnO2 NPs NPs synthesized at low temperatures had more discrete properties than the LS SnO2 NPs produced synthesized at low temperatures had more discrete properties than the LS SnO2 NPs produced by by further heating. It is well-known that the band gap energy of a material significantly affects its further heating. It is well-known that the band gap energy of a material significantly affects its photochemical activity. In this work, although the SS SnO sample was found to have a higher band photochemical activity. In this work, although the SS SnO22 sample was found to have a higher band gap energy compared to the LS sample, it showed higher photochemical activity under visible light gap energy compared to the LS sample, it showed higher photochemical activity under visible light exposure than the LS sample (discussed later). A similar observation was reported by Anuchai et al. [5]; exposure than the LS sample (discussed later). A similar observation was reported by Anuchai et al. they concluded that the band gap energy has no direct influence on the efficiency of photocatalytic [5]; they concluded that the band gap energy has no direct influence on the efficiency of photocatalytic activity. In our study, the superior photochemical activity of SS sample could be due to the presence of activity. In our study, the superior photochemical activity of SS sample could be due to the presence a large number of surface defects and a high surface area, as confirmed from the Raman, XPS, and BET of a large number of surface defects and a high surface area, as confirmed from the Raman, XPS, and studies. The photoelectrochemical performances (Figure 7) of SS SnO and the LS sample were tested BET studies. The photoelectrochemical performances (Figure 7) of SS2 SnO2 and the LS sample were using a three-electrode system in a 0.1 M Na SO4 solution. The working electrode was prepared by the tested using a three-electrode system in a 0.12M Na 2SO4 solution. The working electrode was prepared doctor blade technique on FTO coated glass and acted as the photoanode for both the samples. The by the doctor blade technique on FTO coated glass and acted as the photoanode for both the samples. photocurrent responses of the samples were performed at 0 V vs. the Ag/AgCl (3 M KCl) reference The photocurrent responses of the samples were performed at 0 V vs. the Ag/AgCl (3 M KCl) electrode (Figure 7a). The SS sample showed enhanced photocurrent density compared to the LS reference electrode (Figure 7a). The SS sample showed enhanced photocurrent density compared to sample, indicating a significant improvement in photogenerated charge transport and separation in the the LS sample, indicating a significant improvement in photogenerated charge transport and SS SnO2 [8,24,25]. The electrochemical impedance spectra (EIS) further confirmed the improvement in separation in the SS SnO2 [8,24,25]. The electrochemical impedance spectra (EIS) further confirmed the improvement in charge separation efficiency (Figure 7b). The arc radius of the EIS Nyquist plots
Materials 2018, 11, 904
8 of 12
Materials 2018, 2018, 11, 11, xx FOR FOR PEER PEER REVIEW REVIEW Materials
of 12 12 88 of
charge separation efficiency (Figure 7b). The arc radius of the EIS Nyquist plots was smaller for the was smaller for the SSLSSnO SnO 2 than forresult the LS LSalso SnO 2. This result alsophotogenerated suggests that that the the photogenerated SS SnO forthe theSS SnO suggests that the charge separation was smaller 2 than for the SnO 2. This result also suggests photogenerated 2 thanfor 2 . This charge separation was more efficient in the SS sample [8,24] which could be due to the larger surface was more efficientwas in the SS efficient sample [8,24] could be due to could the larger surface ofsurface the SS charge separation more in the which SS sample [8,24] which be due to thearea larger area of of the the SSWe sample [26]. We We also tested the the photostability photostability of the theand SS sample, sample, anditfound found thatsignificant it showed showed sample [26]. also tested the photostability of the SS sample, found that showed area SS sample [26]. also tested of SS and that it significant PEC stability (Figure 7c)Ag/AgCl at 00 V V vs. vs.(3Ag/AgCl Ag/AgCl (3 M M KCl). KCl). PEC stability (Figure 7c) (Figure at 0 V vs. M KCl). (3 significant PEC stability 7c) at SS SS LS LS
[F(R)h [F(R)hνν] ]
22
Absorbance Absorbance(a.u.) (a.u.)
SS SS LS LS
~3.38 ~3.38
2.5 2.5
250 250
300 300
350 350
400 400
450 450
500 500
3.0 3.0
3.5 3.5
4.0 4.0
hνν (eV) (eV) h
550 550
600 600
Wavelength (nm) (nm) Wavelength
~3.75 ~3.75
650 650
700 700
Figure 6. 6. UV–Vis UV–Vis absorption absorption spectra spectra measured measured by by the the diffuse diffuse reflectance reflectance method. method. Inset Inset shows shows an an Figure 6. absorption measured by the diffuse reflectance method. estimation of of the the band band gap gap energy energy of of the the SS SS and and LS LS samples. samples. estimation gap energy of the SS and LS samples.
2 2
80 80
100 100
120 120
140 140
SS SS
0 04 4
(b) 30000 (b) 30000 25000 25000
LS LS
0
(c) (c)
20 20
20000 20000 15000 15000 10000 10000 5000 5000
Light offoff Light
2 2
Light onon Light
3 3
0 00
22
60 60
1 1
1 1
Current Currentdensity density(μ(A/cm μA/cm) )
Light onon Light
2 Current Currentdensity density(μ(A/cm μA/cm)2)
3 3
40 40
-Z" -Z"(ohm) (ohm)
20 20
4
Light offoff Light
(a) 00 (a) 4
40 40
60 60
80 80
Time (s) Time (s)
100 100
120 120
140 140
0 0
LS LS SS SS 0 0
1000 1000
2000 2000
3000 3000
Z' (ohm) Z' (ohm)
4000 4000
5000 5000
4 4 3 3
SS SS
2 2
Light on Light on
1 1 0 0
LS LS 0 0
50 50
100 100
150
150 (s) Time Time (s)
200 200
250 250
300 300
Figure 7. 7. Photoelectrochemical properties properties of the SS SS and LS LS SnO SnO22 samples: samples: (a) (a) photoresponse; photoresponse; (b) (b) Figure Figure 7. Photoelectrochemical Photoelectrochemical propertiesofofthe the SSand and LS SnO2 samples: (a) photoresponse; electrochemical impedance impedance (EIS; (EIS; Nyquist Nyquist plots) plots) of of the electrodes electrodes measured under under visible light light electrochemical (b) electrochemical impedance (EIS; Nyquist plots) ofthe the electrodesmeasured measured under visible visible light irradiation; and (c) change in the current density with time (I–t curves) recorded under visible light irradiation; and current density with time (I–t (I–t curves) recorded under visible light irradiation; and (c) (c)change changeininthe the current density with time curves) recorded under visible illumination. illumination. light illumination.
The improved improved photoelectrochemical photoelectrochemical properties properties of of the the SS SS SnO SnO2 can can be be attributed attributed to to its its enhanced enhanced The The improved photoelectrochemical properties of the SS SnO22 can be attributed to its enhanced charge transfer efficiency due to the influence of surface defects on the recombination process of the the charge transfer transfer efficiency efficiency due due to to the the influence influence of of surface surface defects defects on on the the recombination recombination process process of of charge the photoinduced electrons and holes. The photoluminescence (PL) spectra of the samples (Figure 8a) photoinduced electrons electrons and and holes. holes. The photoinduced The photoluminescence photoluminescence (PL) (PL) spectra spectra of of the the samples samples (Figure (Figure 8a) 8a) showed visible visible emissions emissions at at ~425 ~425 nm, nm, ~436 ~436 nm, nm, ~442 ~442 nm, nm, ~450 ~450 nm, nm, ~461 ~461 nm, nm, ~486 ~486 nm, nm, ~522 ~522 nm nm and and showed ~531 nm, along with UV emission at ~370 nm [5–8,13]. The UV PL peak at ~370 nm originated from ~531 nm, along with UV emission at ~370 nm [5–8,13]. The UV PL peak at ~370 nm originated from the band band edge edge emission emission of of SnO SnO22 [6]. [6]. The The visible visible emission emission from from the the SnO SnO22 nanocrystals nanocrystals was was associated associated the
Materials 2018, 11, x FOR PEER REVIEW
9 of 12
Materials 2018, 11, 904 9 of 12 with various intrinsic and extrinsic surface defects, such as oxygen vacancies and tin interstitials [5– 8,13]. The formation of such defect levels in the SnO2 bandgap depends upon several factors, such as the preparation method and doping [5,6,8,13,27]. The nm, UV and emissions of the areand in showed visible emissions at ~425 nm, ~436 nm, ~442 ~450visible nm, ~461 nm, ~486 nm,samples ~522 nm competition with each other. Large, perfectly crystalline SnO 2 nanocrystals show strong UV emission ~531 nm, along with UV emission at ~370 nm [5–8,13]. The UV PL peak at ~370 nm originated from the as well as aemission high ratio UV [6]. emission to visible emission In the present study, the LS band edge of of SnO The visible emission fromintensity the SnO2[6,7]. nanocrystals was associated with 2 SnO 2 NPs exhibited stronger UV emission than the SS SnO2 NPs. It is therefore probable that the band various intrinsic and extrinsic surface defects, such as oxygen vacancies and tin interstitials [5–8,13]. edge emission of SSdefect SnO2 levels was restricted, andbandgap that excited electrons onseveral the conduction levelasmay The formation of the such in the SnO depends upon factors, such the 2 have combined with the defect levels (oxygen vacancies and tin interstitials) within the bandgap preparation method and doping [5,6,8,13,27]. The UV and visible emissions of the samples are in energy, resulting a decreased relative intensity ratio ofSnO UV to visible emission. The PL spectrum of competition with in each other. Large, perfectly crystalline 2 nanocrystals show strong UV emission the SS SnO 2 also strongly justifies the above statement, as the visible emissions at ~436 nm and ~442 as well as a high ratio of UV emission to visible emission intensity [6,7]. In the present study, the LS nm more prominent than in the LS sample. Kasinathan et al. also found a prominent band SnOwere 2 NPs exhibited stronger UV emission than the SS SnO2 NPs. It is therefore probable that the centered at 442 nm inofCeO –TiO 2, and they proposed the peak related to oxygen defects [28]. Thus, it band edge emission the2SS SnO 2 was restricted, and that excited electrons on the conduction level can be surmised that the defect of the vacancies SS SnO2 and NPstincan strongly within participate in the may have combined with the defect centers levels (oxygen interstitials) the bandgap photoexcitation process and decrease the recombination rate. The Raman (Figure 3) and XPS (Figure energy, resulting in a decreased relative intensity ratio of UV to visible emission. The PL spectrum of 4) confirmed the presence surface defects, especially oxygen thenm SS thespectral SS SnO2studies also strongly justifies the aboveof statement, as the visible emissions at deficiencies, ~436 nm and in ~442 SnO sample. Thus, the surface defects can act as charge facilitating efficient were2 more prominent than in theoxygen LS sample. Kasinathan et al. also carrier found atraps, prominent band centered charge separation and improving the photoelectrochemical properties (PEC) of the SS SnO 2 sample at 442 nm in CeO2 –TiO2 , and they proposed the peak related to oxygen defects [28]. Thus, it can be [8,24]. A possible for theof separation of photogenerated electrons (e−) and holes (h+) is shown surmised that thepathway defect centers the SS SnO 2 NPs can strongly participate in the photoexcitation schematically in Figure where SnO2rate. absorbs of visible light to (Figure excite the electrons from process and decrease the8b, recombination The photons Raman (Figure 3) and XPS 4) spectral studies its valence to surface defect levels, and then promotes the electrons to the higher levels to induce confirmed the presence of surface defects, especially oxygen deficiencies, in the SS SnO2 sample. photochemical reactions Additionally, the textural of a material cancharge improve its PEC Thus, the surface oxygen [26]. defects can act as charge carrierproperties traps, facilitating efficient separation properties. In this respect, the high specific surface area, and uniform pore sizes of the SS sample and improving the photoelectrochemical properties (PEC) of the SS SnO2 sample [8,24]. A possible compared to the LS sample may allow faster diffusion of the electrolyte and thus, enhance redox pathway for the separation of photogenerated electrons (e− ) and holes (h+ ) is shown schematically kinetics the absorbs PEC performance efficiency in Figureand 8b, improve where SnO photons of[10,11]. visible The lightapplied to excitebias the photon-to-current electrons from its valence to 2 (ABPE) was calculated for the photoelectrochemical processes of SS and LS samples at 0 V vs. surface defect levels, and then promotes the electrons to the higher levels to induce photochemical Ag/AgCl (3 M Additionally, KCl) using Equation (1). The ABPE of and LS samples wereits 0.00352% and 0.00057%, reactions [26]. the textural properties ofSS a material can improve PEC properties. In this respectively. respect, the high specific surface area, and uniform pore sizes of the SS sample compared to the LS
sample may allow faster diffusion of(%) the =electrolyte and thus, enhance kinetics and improve(1) the ABPE [|Jphoto × (|1.23 − Vappl. |)]/Ptotalredox , PEC performance [10,11]. The applied bias photon-to-current efficiency (ABPE) was calculated for the where Jphoto is the photocurrent density (mA/cm2), Vappl. is the applied potential vs. RHE (reversible photoelectrochemical processes of SS and LS samples at 0 V vs. Ag/AgCl (3 M KCl) using Equation (1). hydrogen electrode) and Ptotal is power density of incident light (mW/cm2) The ABPE of SS and LS samples were 0.00352% and 0.00057%, respectively. The values were relatively low, but the SS sample exhibited an efficiency ~6 times higher than the LS sample. The enhanced photoelectrochemical efficiency of|)]/P the SS sample in comparison to LS ABPE (%) = [|Jphoto × (|1.23 − Vappl. (1) total , sample could be attributed to the efficient light absorption from the valence band to defect levels, 2 ),within efficient charge separation through defect centers the band energy and transfer through the where Jphoto is the photocurrent density (mA/cm Vappl. is the applied potential vs. RHE (reversible 2 intraparticle diffusionand path [29]. hydrogen electrode) Ptotal is power density of incident light (mW/cm ). (a)
(b) Conduction band - - -
436 442
PL intensity (a.u.)
e e- e e e e-
PL intensity (a.u.)
425 436 442 450 461
370
LS
SS
486 420
defect levels
e-
430
440
450
460
470
Light
Wavelength (nm)
522 531 SS
h+ h+ h+ h+
LS
350
400
450
500
Wavelength (nm)
550
600
oxygen vacancies
Valence band
Figure 8. 8. (a) (a) Photoluminescence Photoluminescence (PL) (PL) emission emission spectra spectra of of the the SS SS and and LS LS samples; samples; inset inset shows shows the the Figure magnified part of the spectra; (b) schematic representation of the photochemical process of surface magnified part of the spectra; (b) schematic representation of the photochemical process of surface defective SnO SnO22.. defective
Materials 2018, 11, 904
10 of 12
The values were relatively low, but the SS sample exhibited an efficiency ~6 times higher than the LS sample. The enhanced photoelectrochemical efficiency of the SS sample in comparison to LS sample could be attributed to the efficient light absorption from the valence band to defect levels, efficient charge separation through defect centers within the band energy and transfer through the intraparticle diffusion path [29]. Materials 2018, 11, x FOR PEER REVIEW 10 of 12 We also investigated the photocatalytic activity (Figure 9a) of the SnO2 samples towards the degradation an investigated aqueous solution of methyl orange under light irradiation. The SS We of also the photocatalytic activity(MO) (Figure 9a) visible of the SnO 2 samples towards theSnO2 sample showed higher photocatalytic than the LS SnOunder Tolight quantify the photocatalytic degradation of an aqueous solutionactivity of methyl orange (MO) visible irradiation. The SS 2 sample. SnO2 we sample showed photocatalytic than the LS SnO 2 sample. Tofirst quantify efficiency, calculated thehigher rate constants of theactivity degradation reaction using pseudo order the reaction photocatalytic we of calculated the rate constants of the degradation reaction using kinetics (Figure 9b).efficiency, The values the degradation rate constant, k, were determined to be pseudo 1.50 × 10−1 first × order (Figure 9b). samples, The values of the degradation ratethe constant, k, were and 0.26 10−1reaction h−1 for kinetics the SS and LS SnO respectively. Therefore, SS sample exhibited 2 determined to be 1.50 × 10−1 and 0.26 × 10−1 h−1 for the SS and LS SnO2 samples, respectively. Therefore, ~5.8 times greater photocatalytic activity than the LS sample. This could be attributed to the different the SS sample exhibited ~5.8 times greater photocatalytic activity than the LS sample. This could be textural properties and surface defect contents of the samples, as observed in the BET, Raman, XPS, attributed to the different textural properties and surface defect contents of the samples, as observed and PL studies. Moreover, the large surface area and uniform pore sizes of the SS sample compared to in the BET, Raman, XPS, and PL studies. Moreover, the large surface area and uniform pore sizes of the LS sample could offer a strong effect on could the adsorption andeffect diffusion of dye molecules the SS sample compared to the LS sample offer a strong on theefficiencies adsorption and diffusion within the pore network during photocatalysis. However, hybridization of noble nanometals efficiencies of dye molecules within the pore network the during photocatalysis. However, the or semiconductor nanoparticles with the or SnO would be with an efficient to further hybridization of noble nanometals semiconductor nanoparticles the SnOapproach 2 nanoparticles 2 nanoparticles would an efficient approach to further improve the photochemical activity of the material. improve thebephotochemical activity of the material. (a)
(b) 1.2
1.0
-1
-1
SS (k = 1.50 x 10 h ) -1 -1 LS (k = 0.26 x 10 h )
1.0
LS 0.8
C/C0
ln(C0/C)
0.8
0.6
0.6 0.4
SS 0.4
0.2 0.0
0.2 0
1
2
3
4
Irradiation time (h)
5
6
0
1
2
3
4
5
6
Irradiation time (h)
Figure 9. (a) Photocatalytic degradation of aqueous solutions of methyl orange dye in the presence of Figure 9. (a) Photocatalytic degradation of aqueous solutions of methyl orange dye in the presence SS and LS SnO2 under UV–Vis light; (b) estimation of the pseudo-first order rate constants of the of SS and LS SnO2 under UV–Vis light; (b) estimation of the pseudo-first order rate constants of the samples for the dye degradation; the rate constants for the degradation of the dye by the respective samples for the dye degradation; the rate constants for the degradation of the dye by the respective photocatalysts are embedded in the figure. photocatalysts are embedded in the figure.
4. Conclusions
4. Conclusions
A simple low temperature (95 °C) solution method for the synthesis of surface defective small
A simple low temperature (95 ◦ C) solution method for the synthesis of surface defective small size (SS) SnO 2 nanoparticles (NPs) from a surfactant-free precursor solution in an air atmosphere was size (SS) SnO2 Large nanoparticles fromwere a surfactant-free precursor solution an air atmosphere reported. size (LS) (NPs) SnO2 NPs prepared by further heating the SSinNPs. The structural was reported. Largetextural size (LS) SnO2 NPs prepared by further heating the oxygen SS NPs.vacancies, The structural properties, properties, and were the presence of surface defects, especially of the SS and LS SnO 2 NPs were analyzed. A significant decrease in theespecially concentration of surface defectsof the properties, textural properties, and the presence of surface defects, oxygen vacancies, wasLS observed for the LS analyzed. sample. TheAphotochemical properties upon changingofthe size and SS and SnO2 NPs were significant decrease invaried the concentration surface defects greatly influenced the photoelectrochemical and photocatalytic activities of the NPs. Moreover, the was observed for the LS sample. The photochemical properties varied upon changing the size and SS influenced SnO2 showed enhanced photoelectrochemical (PEC) and photocatalytic compared to thethe SS greatly the photoelectrochemical and photocatalytic activities activities of the NPs. Moreover, LS SnO2. The comparatively large number of surface defects in the SS sample formed within the band SnO2 showed enhanced photoelectrochemical (PEC) and photocatalytic activities compared to the LS gap energy level of SnO2 strongly participated in the photochemical process to reduce the SnO2 . The comparatively large number of surface defects in the SS sample formed within the band gap recombination rate of the photogenerated electrons and holes. energy level of SnO2 strongly participated in the photochemical process to reduce the recombination rate of the photogenerated electrons andM.I., holes. Author Contributions: Conceptualization, S.B. and S.-H.K.; Methodology, M.I. and S.B.; Validation, M.I. and S.B.; Formal Analysis, M.I. and S.B.; Investigation, M.I. and S.B.; Resources, S.-H.K.; Writing-Original Draft
Author Contributions: Conceptualization, M.I., S.B. andM.I., S.-H.K.; Methodology, M.I. andS.-H.K.; S.B.; Validation, Preparation, M.I. and S.B.; Writing-Review & Editing, S.B. and S.-H.K.; Supervision, Funding M.I. and S.B.; FormalS.-H.K. Analysis, M.I. and S.B.; Investigation, M.I. and S.B.; Resources, S.-H.K.; Writing-Original Acquisition, Draft Preparation, M.I. and S.B.; Writing-Review & Editing, M.I., S.B. and S.-H.K.; Supervision, S.-H.K.; Funding: This research Funding Acquisition, S.-H.K.was mainly supported by the Global Frontier R&D Program (2013M3A6B1078874) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning, Republic of Korea, and partially supported by the Lab to Convergence Program funded by Busan Institute of S&T Evaluation and Planning (BISTEP).
Materials 2018, 11, 904
11 of 12
Funding: This research was mainly supported by the Global Frontier R&D Program (2013M3A6B1078874) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning, Republic of Korea, and partially supported by the Lab to Convergence Program funded by Busan Institute of S&T Evaluation and Planning (BISTEP). Conflicts of Interest: The authors declare no conflict of interest.
References 1.
2. 3. 4.
5.
6. 7. 8. 9.
10.
11.
12. 13.
14.
15.
16. 17.
Dong, H.; Zeng, G.; Tang, L.; Fan, C.; Zhang, C.; He, X.; He, Y. An overview on limitations of TiO2 -based particles for photocatalytic degradation of organic pollutants and the corresponding countermeasures. Water Res. 2015, 79, 128–146. [CrossRef] [PubMed] Osterloh, F.E. Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting. Chem. Soc. Rev. 2013, 42, 2294–2320. [CrossRef] [PubMed] Bera, S.; Khan, H.; Biswas, I.; Jana, S. Polyaniline hybridized surface defective ZnO nanorods with long-term stable photoelectrochemical activity. Appl. Surf. Sci. 2016, 383, 165–176. [CrossRef] Rahman, M.A.; Bazargan, S.; Srivastava, S.; Wang, X.; Ellah, M.A.; Thomas, J.P.; Heinig, N.F.; Pradhan, D.; Leung, K.T. Defect-rich Decorated TiO2 Nanowires for super-efficient photoelectrochemical water splitting driven by visible light. Energy Environ. Sci. 2015, 8, 3363–3373. [CrossRef] Anuchai, S.; Phanichphant, S.; Tantraviwat, D.; Pluengphon, P.; Bovornratanaraks, T.; Inceesungvorn, B. Low temperature preparation of oxygen-deficient tin dioxide nanocrystals and a role of oxygen vacancy in photocatalytic activity improvement. J. Colloid Interface Sci. 2018, 512, 105–114. [CrossRef] [PubMed] Kar, A.; Kundu, S.; Patra, A. Surface defect-related luminescence properties of SnO2 nanorods and nanoparticles. J. Phys. Chem. C 2011, 115, 118–124. [CrossRef] Kar, A.; Sain, S.; Kundu, S.; Bhattacharyya, A.; Pradhan, S.K.; Patra, A. Influence of size and shape on the photocatalytic properties of SnO2 nanocrystals. ChemPhysChem 2015, 16, 1017–1025. [CrossRef] [PubMed] Yang, Y.; Wang, Y.; Yin, S. Oxygen vacancies confined in SnO2 nanoparticles for desirable electronic structure and enhanced visible light photocatalytic activity. Appl. Surf. Sci. 2017, 420, 399–406. [CrossRef] Sadhu, S.; Patra, A. A brief overview of some physical studies on the relaxation dynamics and förster resonance energy transfer of semiconductor quantum dots. ChemPhysChem 2013, 14, 2641–2653. [CrossRef] [PubMed] Zhao, T.; Ren, Y.; Yang, J.; Wang, L.; Jiang, W.; Elzatahry, A.A.; Alghamdi, A.; Deng, Y.; Zhao, D.; Luo, W. Hierarchical ordered macro/mesoporous titania with a highly interconnected porous structure for efficient photocatalysis. J. Mater. Chem. A 2016, 4, 16446–16453. [CrossRef] Dong, W.; Sun, Y.; Hua, W.; Yao, Y.; Zhuang, G.; Lv, X.; Ma, Q.; Zhao, D. Preparation of secondary mesopores in mesoporous anatase–silica nanocomposites with unprecedented-high photocatalytic degradation performances. Adv. Funct. Mater. 2016, 26, 964–976. [CrossRef] Niederberger, M. Nonaqueous sol–gel routes to metal oxide nanoparticles. Acc. Chem. Res. 2007, 40, 793–800. [CrossRef] [PubMed] Pramata, A.D.; Suematsu, S.; Quitain, A.T.; Sasaki, M.; Kida, T. Synthesis of highly luminescent sno2 nanocrystals: analysis of their defect-related photoluminescence using polyoxometalates as quenchers. Adv. Funct. Mater. 2018, 28, 1704620. [CrossRef] Wang, H.; Dou, K.; Teoh, W.Y.; Zhan, Y.; Hung, T.F.; Zhang, F.; Xu, J.; Zhang, R.; Rogach, A.L. Engineering of facets, band structure, and gas sensing properties of hierarchical Sn2+ doped sno2 nanostructures. Adv. Funct. Mater. 2013, 23, 4847–4853. Kumar, V.; Swart, H.C.; Gohain, M.; Bezuidenhoudt, B.C.B.; van Vuuren, A.J.; Lee, M.; Ntwaeaborwa, O.M. The role of neutral and ionized oxygen defects in the emission of tin oxide nanocrystals for near white light application. Nanotechnology 2015, 26, 295703. [CrossRef] [PubMed] Bera, S.; Pal, M.; Sarkar, S.; Jana, S. Hierarchically Structured Macro with Nested Mesoporous Zinc Indium Oxide Conducting Film. ACS Appl. Mater. Interfaces 2017, 9, 4420–4424. [CrossRef] [PubMed] Wang, J.; Wang, Z.; Huang, B.; Ma, Y.; Liu, Y.; Qin, X.; Zhang, X.; Dai, Y. Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO. ACS Appl. Mater. Interfaces 2012, 4, 4024–4030. [CrossRef] [PubMed]
Materials 2018, 11, 904
18.
19.
20. 21. 22. 23.
24.
25.
26.
27.
28.
29.
12 of 12
Kennedy, J.; Murmua, P.P.; Manikandan, E.; Lee, S.Y. Investigation of structural and photoluminescence properties of gas and metal ions doped zinc oxide single crystals. J. Alloys Compd. 2014, 616, 614–617. [CrossRef] Kennedy, J.; Murmua, P.P.; Leveneura, J.; Markwitz, A.; Futter, J. Controlling preferred orientation and electrical conductivity of zincoxide thin films by post growth annealing treatment. Appl. Surf. Sci. 2016, 367, 52–58. [CrossRef] Pal, M.; Bera, S.; Jana, S. Sol–gel based simonkolleite nanopetals with SnO2 nanoparticles in graphite-like amorphous carbon as an efficient and reusable photocatalyst. RSC Adv. 2015, 5, 75062–75074. [CrossRef] Xing, M.; Zhang, J.; Chen, F.; Tian, B. An economic method to prepare vacuum activated photocatalysts with high photo-activities and photosensitivities. Chem. Commun. 2011, 47, 4947–4949. [CrossRef] [PubMed] Sangwichien, C.; Aranovich, G.L.; Donohue, M.D. Density Functional theory predictions of adsorption isotherms with hysteresis loops. Colloids Surf. A 2002, 206, 313–320. [CrossRef] Li, P.; Lan, Y.; Zhang, Q.; Zhao, Z.; Pullerits, T.; Zheng, K.; Zhou, Y. Iodinated SnO2 Quantum Dots: A Facile and Efficient Approach to Increase Solar absorption for visible-light photocatalysis. J. Phys. Chem. C 2016, 120, 9253–9262. [CrossRef] Lei, F.; Sun, Y.; Liu, K.; Gao, S.; Liang, L.; Pan, B.; Xie, Y. Oxygen vacancies confined in ultrathin indium oxide porous sheets for promoted visible-light water splitting. J. Am. Chem. Soc. 2014, 136, 6826–6829. [CrossRef] [PubMed] Zhang, Z.; Gao, C.; Wu, Z.; Han, W.; Wang, Y.; Fua, W.; Li, X.; Xie, E. Toward efficient photoelectrochemical water-splitting by using screw-like SnO2 nostructures as photoanode after being decorated with CdS quantum dots. Nano Energy 2016, 19, 318–327. [CrossRef] Liu, J.; Shahid, M.; Ko, Y.-S.; Kim, E.; Ahn, T.K.; Park, J.H.; Kwon, Y.-U. Investigation of porosity and heterojunction effects of a mesoporous hematite electrode on photoelectrochemical water splitting. Phys. Chem. Chem. Phys. 2013, 15, 9775–9782. [CrossRef] [PubMed] Yanga, L.; Huang, J.; Shi, L.; Cao, L.; Zhou, W.; Chang, K.; Meng, X.; Liu, G.; Jie, Y.; Ye, J. Efficient hydrogen evolution over Sb doped SnO2 photocatalyst sensitized by Eosin Y under visible light irradiation. Nano Energy 2017, 36, 331–340. [CrossRef] Kasinathan, K.; Kennedy, J.; Elayaperumal, M.; Henini, M.; Malik, M. Photodegradation of organic pollutants RhB dye using UV simulated sunlight on ceria based TiO2 nanomaterials for antibacterial applications. Sci. Rep. 2016, 6, 38064. [CrossRef] [PubMed] Ding, C.; Shi, J.; Wang, Z.; Li, C. Photoelectrocatalytic water splitting: significance of cocatalysts, electrolyte, and interfaces. ACS Catal. 2017, 7, 675–688. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
