Tin stearate organometallic precursor prepared SnO2 quantum dots nanopowder for aqueous- and non-aqueous medium photocatalytic hydrogen gas evolution

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Tin stearate organometallic precursor prepared SnO2 quantum dots nanopowder for aqueous- and non-aqueous medium photocatalytic hydrogen gas evolution Choonyian Haw a, Weesiong Chiu a,∗, Noor Hamizah Khanis a, Saadah Abdul Rahman a, Poisim Khiew b, Shahidan Radiman c, Roslan Abd-Shukor c, Muhammad Azmi Abdul Hamid c a

Low Dimensional Materials Research Centre, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Chemical Engineering, Faculty of Engineering, University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia c School of Applied Physics, Faculty of Science and Technology, National University of Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan, Malaysia b

a r t i c l e

i n f o

Article history: Received 26 December 2015 Revised 9 February 2016 Accepted 14 March 2016 Available online xxx Keywords: SnO2 Tinstearate Precursors Ligand exchange Photocatalyst Hydrogen gas production

a b s t r a c t Current study reports a rapid one-pot non-hydrolytic condition in the synthesis of SnO2 QDs nanopowder using tin (II) stearate (Sn(St)2 ) as environmentally-benign organometallic precursor, which is an unprecedentedly employed-compound in preceding SnO2 nanopowder productions. The as-synthesized SnO2 QDs that are hydrophobic can be easily transferred from organic solvent to aqueous solution through a robust ligand exchange method. The stearate-capping ligands on the surface of QDs can be replaced by beta-cyclodextrin (β -CD) and eventually render the QDs highly water soluble, which ultimately make it exhibit bi-functionality for different liquid medium applications. Structural characterizations reveal that the bi-functional QDs are indeed well-matched with the standard rutile SnO2 cassiterite phase without the presence of any impurities. The QDs can be interchangeably used as photocatalyst for both aqueous and non-aqueous phase, where it shows significant enhancement of hydrogen gas production as compared to that of commercial SnO2 nanopowder. © 2016 Science Press and Dalian Institute of Chemical Physics. All rights reserved.

1. Introduction The rapid growth in nanoscience and nanotechnology development in recent years assures a wealth of applications in a myriad of industries evoked on account amongst research community to continuously explore for a greener synthetic scheme of nanomaterials. Therefore, the ability to fabricate structures with nanometric precision is of fundamental importance to many nanomaterial scientists in order to cater many of the pressing needs for a better future of mankind. Recently, green chemistry principles have been widely adopted in developing synthetic scheme for producing nanostructures especially through pyrolysis approach due to environmental concern as well as its feasibility in producing high quality nanostructures [1]. The criteria that being taken into account in this so-called “green-synthesis scheme” includes types of precursors, solvents and all the parameters that are associated with the aspects of safety, clean yet economic.

∗

Corresponding author. Tel: +603-79674197; Fax: +60 3 7967 4146. E-mail address: [email protected] (W.S. Chiu).

Pyrolysis synthesis is one of the effective approaches in the production of nanostructural materials particularly in the case of semiconductors nanostructures [2]. As one of the relatively successful method in the preparation of nanostructures, this method has greatly promoted the bottom-up approach for the rational production of diverse nanomaterials with improved properties and excellent crystallinity, well-defined shape cum size. This synthetic scheme also has enables the investigation of truly distinguishable properties that are inherent to nanometer regime, such as photocatalytic- [3], optical- [4] and magnetic-properties [5]. For conventional pyrolysis approach [6], in general, the solvent that being adopted is phosphine-based solvent that is expensive, highly toxic, pyrophoric, explosive and unstable at room temperature and even undergoes explosion at elevated temperatures [7]. Therefore, the preparation is carried out in small scale, and very restricted apparatus and synthetic condition are required. Due to these reasons, control cost of chemical pollution would increase significantly, which hinders the large-scale production [8]. Yu et al. had reported the use of cadmium oxide as precursor in the shape-controlled synthesis of CdTe nanocrystals [9]. They showed that the shape of the CdTe can be varied from dot, rods up to tetrapods. By introducing ligands, the precursor reactivity

http://dx.doi.org/10.1016/j.jechem.2016.04.006 2095-4956/© 2016 Science Press and Dalian Institute of Chemical Physics. All rights reserved.

Please cite this article as: C. Haw et al., Tin stearate organometallic precursor prepared SnO2 quantum dots nanopowder for aqueous- and non-aqueous medium photocatalytic hydrogen gas evolution, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.04.006

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is being altered and subsequently allows the modulation of the monomer release rate onto active site of the preform nuclei for the ease of shape control. These nanocrystals with special morphology are found to exhibit better photoluminescence quantum yield than those of the preceding reports [10]. Meanwhile, Qu et al. also had showed that the types of precursors can render a controllable manner in producing CdSe nanocrystals [11]. The precursors being adopted include metal-acetate and -carbonate with lower toxicity. The use of these precursors had enabled the precise tuning of the size for CdSe nanocrystals. With this achievement, the investigation that precisely distinguishes the emission wavelength ranges from UV-up to near infrared-region that is correlated with the size effect is viable [12]. Peng has work actively in proposing green-chemistry approach in the large scale preparation of chalcogenide-based semiconductors nanocrystals through pyrolysis approach [13]. In their study, several major modifications especially on the aspect of raw materials had been implemented and the result shows that this alternative route are safe, versatile, reproducible, inexpensive, and producing nanostructures with well-defined size and shape distribution [7]. For example, cadmium-acetate and -carbonate [11] had been employed to replace dimethyl cadmium as environmental-benign precursor in the synthesis of CdSe nanocrystals and the results are found to prevail in contrast to that of using dimethyl cadmium scheme that is highly hazardous. Hence, this new synthetic scheme not only becomes an inspiring example and model system in the field, but also is an environmentally benign and user friendly advocation. As a complementary semiconductor material, tin oxide (SnO2 ) is another well-recognized group II–IV one that widely used in the optoelectronic applications. However, in comparison with those aforementioned chalcogenide-based semiconductors, the availability of similar colloidal dispersion that has desired shape and congruent properties had not yet been shown in the market. Therefore, it is crucial to design an efficient synthetic protocol that can produce SnO2 quantum dots (QDs) with desired size, morphology and novel optical properties [14]. Methods such as sol–gel [15], microwave technique [16], carbothermal reduction [17], wet chemical precipitation [18], laser ablation technique [19] and hydrothermal method [20] have been widely reported in preparing SnO2 QDs. In this work, thermal pyrolysis method are adopted to synthesize SnO2 QDs with the use of tin (II) stearate (Sn(St)2 ) as a precursor for the first time. The Sn(St)2 is an organometallic precursor that can be derived from natural based resources such as palm oil with relatively low toxicity [21]. Moreover, the effects of particle size, morphology, crystallinity and congruent properties of SnO2 nanoparticles prepared from Sn(St)2 have to be explored since the preparation of SnO2 QDs from this organometallic compound has never been reported yet. Present study is envisaged to test the viability of Sn(St)2 as a precursor for the synthesis of QDs nanopowder, which lies in the initiative to develop a synthesis scheme that used organometallic precursor which is green, cost-effective and facile one-pot pyrolysis. Surface-ligand exchange will be tested using β -cyclodextrin (β -CD) to make the as-synthesized SnO2 QDs nanopowder interchangeably applicable in both aqueous- and non-aqueous mediums [22]. Detailed structural- and optical-properties of the QDs will be investigated and its feasibility to be used as bifunctional photocatalyst material in both aqueous and non-aqueous phases for photocatalytic hydrogen gas evolution will be evaluated by comparing with the performance of the commercial SnO2 nanoparticles. It is anticipated that current study would serve as alternative route in the processing of nanopowder for the advancement of photocatalysis technology and its other relevant applications in energy related matters.

2. Experimental 2.1. Chemicals Tin (II) stearate (Sn(O2 C18 H35 )2 , Alfa Aesar with CAS no. 699459-8) was used as organometallic precursor, n-hexadecane (C16 H34 Merck KGaA with ≥99%) was used as the solvent for the synthesis and 1,2-dodecanediol (CH3 (CH2 )9 CH(OH)CH2 OH, 90%), obtained from Aldrich Chemistry) was used as reducing agent in the preparation of tin oxide quantum dots nanopowder. β -Cyclodextrin (with 98% of CAS no: 7585-39-9) was purchased from Sigma for surface modification. Identical mass of SnO2 commercial (99.9%, product no. 7012LC, Sky Spring Nanomaterials Inc.) with 50–70 nm was used for all characterization measurement as well as in the study of photocatalytic of hydrogen gas for comparison purpose. All other chemicals used in this study were of analytical grade and used as received without further purification. All solutions were prepared from deionized water using water purification system. 2.2. Preparation of SnO2 quantum dots (QDs) In a typical synthesis process, 1.5 mmol of tin (II) stearate (Sn(St)2 ), 0.75 mmol of 1, 2-dodecanediol and 30 mL of nhexadecane were loaded into a 250 mL four-neck round-bottom flask. Under constant magnetic stirring, the mixture was heated to 120 °C and saturated under nitrogen gas blanket for 1 h. The mixture was then raised to 287 °C at 4 °C/min of ramping rate and further refluxed for 5 h. Under vigorous stirring, a temporal change in the turbidity of solution from clear to pale yellow indicated the formation of SnO2 QDs. The system was then cooled to room temperature and the pale yellowish precipitate was purified by centrifugation technique with the used of pair of solvent (hexane) and non-solvents (acetone). The precipitate was dried at 80 °C in the oven and ready to be used either as stearate-capped hydrophobic QDs or further underwent ligand exchange reaction to transfer it into hydrophilic QDs that capped by β -cyclodextrin. 2.3. Surface ligand exchange Briefly, 5 mM of β -cyclodextrin (β -CD) was diluted in deionized water and was equivalently mixed with 5 mg/mL of hexanedispersed SnO2 QDs that obtained initially. The mixture is subjected to vigorous stirring for 24 h. After desired duration, it is observed that the SnO2 QDs can diffuse across the interface between hexane- and aqueous-phase, signifying that the surface ligand replacement is accomplished. The resultant precipitate was then washed several times with ethanol/DIW that coupled with centrifugation process and labeled as β -CD capped QDs. 2.4. Characterizations The crystalline structure of the as-synthesized SnO2 QDs nanopowders (both stearate-capped and β -CD-capped) is analyzed using a Phillips X’Pert PRO diffractometer with CuKα radiation of ˚ Data were collected in 2θ of scan range from wavelength 1.54 A. 20° to 80°. The particle size, structural and morphology of QDs were recorded by high resolution transmission electron microscope (HRTEM, model JEM 2100F) with operating voltage of 200 kV. Raman spectra were collected on a Renishawinvia Raman Microscope with UV laser of 325 nm and mounted with an InGaAs electron multiplied (EM) detector. The scan range was performed in the wavenumber of 100 cm−1 to 10 0 0 cm−1 . UV–visible absorption spectra were recorded using Perkin-Elmer Lambda 750 UV–Vis–NIR spectrophotometer. Photoluminescence (PL) spectrometer equipped with a 325 nm He–Cd laser was used to measure the PL intensity

Please cite this article as: C. Haw et al., Tin stearate organometallic precursor prepared SnO2 quantum dots nanopowder for aqueous- and non-aqueous medium photocatalytic hydrogen gas evolution, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.04.006

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Fig. 1. Schematic diagram of (a) thermal decomposition of tin stearate as precursor to synthesize stearate-capped SnO2 QDs that are highly-dispersible in hexane, (b) surface ligand exchange with β -cyclodextrin (β -CD) in phase transfer of stearate-capped QDs from non-aqueous phase into aqueous phase in water.

of the samples and correlate to the carrier-lifetimes to the photocatalytic performance in hydrogen gas production. 2.5. Photocatalytic hydrogen gas evaluation In the non-aqueous phase photocatalytic hydrogen gas evaluation, the photocatalyst was suspended in a mixture solution of glycerol/water (10 vol%, 4.2 mL) in the Quartz made reaction vessel. The vessel was then degassed for 10 min to remove dissolved oxygen followed by irradiation with a 300 W Xe arc lamp (Newport 66901) which is equipped with a dichroic mirror (Newport 81045) that mounted on dichroic mirror holder (Newport 66245) to simulate the UV light. The suspension was stirred under constant irradiation of UV light for 5 h at a fixed distance of 5 cm. For the aqueous phase, the experimental set up was remained exactly the same for the β -CD capped QDs photocatalysts. Prior to the test, the surface-modified photocatalysts were dispersed by a magnetic stirrer in an aqueous containing equimolar mixture of (0.35 M) Na2 S and Na2 SO3 as the sacrificial reagents. The cumulative hydrogen gas production rate for both samples was therefore calculated and compared. 3. Results and discussion 3.1. Mechanistic formation of SnO2 quantum dots (QDs) For the first time, the synthesis of SnO2 QDs using tin (II) stearate (Sn(St)2 ) as single precursor is tested in this study. Sn(St)2 is firstly decomposed in n-hexadecane (as a solvent) under the presence of 1,2-dodecanediol (as reducing agent). No additional capping agent (oleic acid and oleylamine) is required for the reduction of the Sn precursor during the synthesis. The formation kinetics of highly crystalline SnO2 QDs is described in Fig. 1, in which Sn(St)2 monomer is firstly dissolved in n-hexadecane and the ramp of the temperature persistently

promotes the thermal decomposition of this monomer toward the supersaturating state. Upon attaining the critical point of supersaturation, a burst of nucleation takes place immediately to release the build-in chemical potential and this has trigger the formation of SnO2 nuclei that are capped with stearate alkyl chain. This is well-complement with LaMer model in the crystallization of nanocrystals (1950), where there is a high energy barrier needed to be overcome prior for the nucleation process to take place. The higher reaction concentration promotes more monomers to be generated [23]. When the supersaturation of the monomer is high enough to prevail over to this energy barrier, burst nucleation will occur, resulting in the formation and accumulation of stable nuclei. The nuclei with the size more than the minimum radius will proceed to grow and develop into SnO2 QDs with periodical array in the form of well-order cassiterite crystal structure. If the size of the nuclei less than the critical radius, it will dissolve in the solution to form monomer. The 1,2-dodecanediol serves as reducing agent by coordinating to the particles surfaces and facilitating in reducing the Sn2+ ions. This diol appears to be an efficient reduction age because its electron-donating ability [24] enables the electrons to preferably bind with metal ions on the specific SnO2 facets in promoting the growth of SnO2 QDs. In order to test the interchangeability of the as-synthesized SnO2 QDs to be dispersed in both aqueous- and non-aqueousphase, the QDs had been subjected to surface ligand exchanges reaction, where the originally stearate-capped surface have been replaced using beta cyclodextrins (β -CDs) (Fig. 1(b)). β -CD is a cyclic oligosaccharide that consists of seven glucopyranose units. They have hydrophobic interior cavity and hydrophilic outer surface with the rims of hydroxyl groups (–OH) that can form complexes with various inorganic nanoparticles. The phase transfer of QDs made possible through the formation of inclusion complexes between β -CD and the stearate-stabilized QDs. By the virtue of –OH groups on the hydrophilic surface of β -CD capped

Please cite this article as: C. Haw et al., Tin stearate organometallic precursor prepared SnO2 quantum dots nanopowder for aqueous- and non-aqueous medium photocatalytic hydrogen gas evolution, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.04.006

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Fig. 2. XRD patterns of (1) stearate-capped SnO2 QDs, and (2) β -cyclodextrin (β -CD) modified SnO2 QDs. Insets show the respective dried powder of the as-obtained samples.

QDs, hydrogen bonding can easily be formed between the counterparts and water molecules that can render efficient wettability of β -CD capped QDs complexes which will effectively promote its dispersion and facilitate the photocatalytic hydrogen gas evolution in aqueous phase [25,26].

3.2. X-ray diffraction The XRD patterns of the as-synthesized SnO2 QDs and β -CDcapped SnO2 QDs are shown in Fig. 2. Three dominant peaks at 2θ values of 26.73°, 34.18°, and 52.04° (for SnO2 QDs) and 26.66°, 34.07°, and 51.90° (for β -CD capped QDs) are corresponding well to (110), (101) and (211) diffractions of standard tetragonal SnO2 with JCPDS number of 041-1445. Any spurious diffractions that source from any impurities are not observed in both spectra and this indicates that pure cassiterite SnO2 of rutile phase has successfully been obtained. The most intense diffraction peak of (211) plane is observable in both samples and this suggests that there are prefer orientation along this direction. In conjunction with this, numerous of studies have shown that (211) plane is one of the highest surface energy facet known to be highly reactive for photocatalysis reaction [27,28]. Noticeably, both diffraction peaks appear to be broad which imply that the crystallite sizes are small. By selecting (110), (101) and ¯ ) of the SnO2 (211) dominant peaks, the average crystallite size (D QDs and the β -CD capped QDs is calculated using Debye–Scherrer formula and the results are listed in Table 1,

D=

Kλ βhkl cosθ

(1)

where K is a correlation or shape factor (0.94), λ is the wavelength of the X-ray (0.154 nm), βhkl is the full-width half maximum of the selected peak of hkl miller indice and θ is Bragg’s diffraction angle in degree. The crystallite size of the respective (hkl) plane for three dominant peaks has been tabulated in Table 1 and the average crystallite size for both SnO2 QDs and the β -CD capped QDs samples is determined to be 6.43 nm and 8.02 nm, respectively.

3.3. TEM analysis Fig. 3(a) depicts the TEM images of the as-synthesized SnO2 which is quasi-spherical in shape. There are contrast differences observed occasionally within each domain of the QDs because the random scattering phenomenon arises from interaction of electron beam with the random-oriented lattice-fringes within the QDs which lying on the surface of copper grid. Meanwhile, there are slight agglomerations among the QDs due to its extremely high surface energy that have driven them to interact with each other [29–31]. This implies that QDs have high surface to volume ratio which will serve as an advantage for surface reaction since the ultrafine particle possesses better reactivity that will be beneficial to the photocatalytic activity. Fig. 3(b) represents the histogram for the as-synthesized SnO2 QDs and the size distribution is wellcommensurate with the trend of normal distribution, in which the QDs are fall within the range of 2.99 to 10.92 nm. The as-calculated average size is 5.58 ± 1.16 nm with polydispersity (PD) of 0.21. The bulk crystallinity of the QDs is analyzed by inverted selected area electron diffraction (SAED) (Fig. 3(c)), where it shows a series of visible homocentric rings extended from the center to the edge. Each of the rings can be well-indexed to the (110), (101), (200), (211), (220) and (310) planes of rutile cassiterite SnO2 that abides to the JCPDS pattern number of 041-1445. Also, a calculated intensity line profile corresponding to the respective d-spacing values of SnO2 QDs has been overlaid on the SAED patterns, which further confirms QDs are truly SnO2 without the presence of mixed-phase such as Sn(St)2 , Sn and SnO. The homogenous intensity of the rings reveals that the QDs exhibit a polycrystalline nature due to the random lattice orientations along the surface of the copper grid, which is well-agree with the contrast difference in Fig. 3(a). In order to get detailed insight about intrinsic crystalline structure of the QDs, a single QD has been chosen and examined by HRTEM. Fig. 4(a) indicates that the QD is indeed a highly crystalline nanodots and the corresponding intensity profile that covers the 110 plane lattice fringes is represented in Fig. 4(b), in which the fringe spacing of ∼3.075 A˚ (Fig. 4(b)) are clearly observed.

Please cite this article as: C. Haw et al., Tin stearate organometallic precursor prepared SnO2 quantum dots nanopowder for aqueous- and non-aqueous medium photocatalytic hydrogen gas evolution, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.04.006

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Table 1. Calculated average particle size of SnO2 QDs and β -CD/SnO2 QDs. Samples

2θ (°)

θ (°)

d (nm)

Diffraction, hkl

FWHM of prominent peak (β /radians)

Particle size, d (nm)

SnO2 standard (041-1445) SnO2 QDs

26.61 33.89 51.78 26.73 34.18 52.04 26.66 34.07 51.90

13.31 16.95 25.89 13.37 17.09 26.02 13.33 17.04 25.95

0.3347 0.2643 0.1764 0.3332 0.2620 0.1755 0.3340 0.2628 0.1760

(110) (101) (211) (110) (101) (211) (110) (101) (211)

– – – 0.0258 0.0215 0.0219 0.0170 0.0197 0.0185

– – – 5.53 6.73 7.04 8.41 7.34 8.31

β -CD/SnO2 QDs

Fig. 3. (a) Low magnification TEM image of QDs, (b) histogram of QDs within the range of 2–10 nm and average size of 5.58 ± 1.16 nm, (c) SAED with the corresponding intensity profile.

The corresponding FFT pattern in Fig. 4(c) indicates the singlecrystal nature of a square-shaped crystalline (selected from region in Fig. 4(a)) with the zone axis of [100]. The FFT pattern is further confirmed by matching with a simulated electron diffraction spots that is schematically depicted in Fig. 4(d). 3.4. Raman analysis Rutile SnO2 belongs to the point group of D4h and space group 14 of P42/mnm, as well as Z = 2 in the tetragonal structure. The SnO2 unit cell consists of two metal atoms and four oxygen atoms. Each Sn atom is situated amidst six oxygen atoms of which the normal lattice vibration at  points of the Brillouin zone is given on the basis of group theory [32,33]:

 = 1A1g +1A2g +1A2u +1B1g +1B2g +2B1u +1Eg +3Eu

(2)

Among them, the three non-degenerated Raman active modes are c1g at 630 cm−1 with the strongest Raman intensity, followed by

Average particle size, D¯ (nm)

–

6.43

8.02

the mode B2g at 771 cm−1 , and a doubly degenerated mode Eg at 473 cm−1 . The three bands represent the first-order Raman active modes of SnO2 QDs which are observable in Fig. 5. According to Abello et al., in Raman active modes, the oxygen atoms vibrate while the Sn atoms remain practically motionless. For both modes A1g and B2g , the oxygen atoms vibrate in the plane perpendicular to the c-axis, while for a doubly degenerated Eg mode oxygen atoms vibrate along the direction of c-axis, which are in good agreement with those for the rutile bulk SnO2 [34]. Hence, these peaks validate that the as-prepared QDs having the characteristics of the tetragonal rutile structure [35]. In the silent mode of A2g at 421 cm−1 (infrared, IR active), Sn and oxygen atoms vibrate in the c-axis direction, while the triply degenerate mode Eu observed at 354 cm−1 ascribed to both Sn and O atoms vibrate in the plane perpendicular to c-axis. The other inactive mode of B1u at 558 cm−1 corresponds to vibrations of Sn and O atoms in the direction of the c-axis. Additionally, there is a weak Raman band observed at 696 cm−1 that seems to be contributed by A2u LO mode (longitudinal optical phonon) which has been reported previously [36,37]. One can see that a broad peak can be observed (Eu at ∼354 cm−1 ) for both spectra. This broad peak was also reported in extra-fine nanoparticles in the range of size of 3–5 nm in the previous studies [38,39]. The studies showed that some silent modes in bulk material can be actively evoked as the particle decreasing in size, making it becomes salient in the spectra. Moreover, the A2u peak Raman modes become noticeable due to the fact that the Raman selection rule relaxed with the increase in oxygen vacancies contents. Those results imply that the oxygen vacancies play a crucial factor in Raman scattering. Hence, the appearance of these peaks might be considered as a consequence of the reduced particle size and defects on the surface of nanoparticles. The oxygen vacancies can be found not only on the superficial region of nanoparticles, but also in the interface between the particles, which in turn could lead to lattice distortion.

3.5. UV–Vis spectrophotometry Fig. 6 shows the UV–Vis absorption spectra for both of the SnO2 QDs and β -CD capped QDs. There is a strong onset absorptions in visible region that falls on 600 nm and a progressive increment of absorption extended to the UV range. Obviously, the absorption takes place intensely in the ultraviolet region and this pattern is very similar to the as-reported values previously [40]. Also, both of the spectra have a cut-off wavelength of ∼336 nm and the band gap for both samples is estimated to be ∼3.69 eV, which is derived from the below equation [27]:

Eg =

hc

λ

(3)

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˚ (c) FFT Fig. 4. (a) High resolution TEM images of the selected single SnO2 QD, (b) intensity profile along lattice fringes of (110) plane with the inter planar spacing of 3.39 A, pattern of the selected area of single QD in (a), (d) simulated diffraction patterns along the zone axis of ref. [100].

where h is the Planck constant, c is the light velocity and λ is the cut-off wavelength. It is noteworthy that the exchange process does not affect the size of the QDs, as determined from UV–Vis spectra where there is no obvious peak shift can be observed before and after exchanges. This result well-conformed with the band gap energy of SnO2 reported from previous literature [41]. 3.6. Photoluminescence spectroscopy Fig. 7 depicts the photoluminescence (PL) emission spectra of the QDs and commercial rutile SnO2 . Noticeably, there is a broad peak at wavelength of 594 nm for all the three specimens. It is also observed that the center wavelength of the emission is independent of the particle size (from Fig. 7(1)–(3)), which suggests that the luminescence is attributed by a defect level within band gap of valence band (VB) and conduction band (CB) most likely contributed by the oxygen vacancies [42,43] and dangling bonds [44] that present in the samples. The commercial SnO2 nanopowder (Fig. 7(1)) exhibits the highest PL intensity (∼70.6%) under identical excitation conditions that corresponds to high electronic density of states inside the band gap. This emission can be assigned to the direct recombination of CB’s electrons in Sn 4p band and a hole in the O 2p VB [45]. In metal oxide nanocrystallites, oxygen vacancies are known to be the most common defects and keeping in mind that Raman results have proved the as-synthesized QDs (Fig. 7(2)) having high con-

centration of these defects. Usually, the defects act as the radiative centers by forming the defect levels located inside the band gap and therefore can effectively trapping electrons from the valence band. As a result, there will be a delayed in the average decay time of electrons captured by the holes among the much shallower defects found in interstitial bands resulting lower PL intensity [46]. In fact, it has been reported by Longo et al. that the lower PL intensity is also of disorder in the periodic lattice of nanostructures [47]. As for β -CD capped QDs (Fig. 7(3)), due to the fact that surface of SnO2 has been functionalized with β -CD, there is a possibility of the uncoordinated atoms on the superficial region generating an excess of energy associated with the surface atoms that will significantly influence on the formation of band defect structure in the interstitial bands.

3.7. Photocatalytic hydrogen production The evolution of hydrogen (H2 ) gas in water (aqueous) and glycerol (non-aqueous) using the as-prepared SnO2 QDs and β -CD capped QDs was evaluated under constant UV irradiation for 5 h and the results are illustrated in Fig. 8. For the sake of comparison, identical amount of photocatalyst was used for all the studies using xenon lamp irradiation at a fixed distance and constant interval durations of hydrogen gas collection. According to Fig. 8, the graphs for the hydrogen gas evolution depict the trend of square

Please cite this article as: C. Haw et al., Tin stearate organometallic precursor prepared SnO2 quantum dots nanopowder for aqueous- and non-aqueous medium photocatalytic hydrogen gas evolution, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.04.006

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Fig. 5. (a) Raman spectroscopy of (1) stearate-capped QDs and (2) β -CD capped QDs. (b) Schematic drawing of the Raman active modes and inactive modes.

root, where the reaction rate proceed with swift rate and attain partially (but not) saturation at fifth-hours of irradiation. Under non-aqueous condition with the used of glycerol as splitting medium, 5.078 mmol/gcat of hydrogen gas was collected over commercial SnO2 nanopowder for 5 h of irradiation (Fig. 8a(1)) while a remarkable amount of hydrogen gas (31.12 mmol/gca ) was produced by as-synthesized stearate-capped QDs in glycerol (Fig. 8a(2)). The enhancement of hydrogen gas evolution is almost six times higher than that of the yield produced by commercial SnO2 nanopowder. This is attributed to the reasons where the SnO2 QDs are smaller in particle size as compared to that of commercial SnO2 , which ranges from 3–5 nm in size as depicted in Fig. 3(b). Numerous studies have proven that the photocatalytic activity of photocatalyst for hydrogen production is greatly governed by

the particle size [48–50], where for nanoparticles with size less than 10 nm, there is the abundance of active reaction sites due to large portion of atoms located on the surface. As a consequence, additional active reaction sites that render better surface exposure (or higher surface area-to-volume ratio) are available for redox reaction to take place by providing the particle greater contact area with glycerol molecules that facilitates the splitting of organic molecules more efficiently in generating hydrogen yield. In addition to contribution by improved surface area, the higher work function of SnO2 also is another factor that contributes to better hydrogen gas production rate. As reported in the literature [51], the work function (Ф) of particles strictly-dependent on the size of the particle size (d) and it can be expressed as Ф = Ф∞ (eV) + 1.08 (eV nm)/d (nm), where d is the average size of the particles (nm), Ф∞ is the work function of particles of infinitely large diameter

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Fig. 6. UV-Visible absorption for (1) stearate-capped SnO2 QDs and (2) β -CD capped SnO2 QDs.

[52]. Base on the equation, it is realized that smaller size of SnO2 QDs will render greater work function with higher Schottky barrier which subsequently facilitates the rectification effect and thus prolongs the lifespan of charge carrier. This explanation is wellconformed with the PL intensity recorded in Fig. 7(2) that reflects relatively lower photoemission intensity as compared to that of commercial SnO2 nanopowder. On the other hand, aqueous based hydrogen gas evolution has been conducted and the results are illustrated in Fig. 8a(3) and (4). The evolutions of hydrogen gas for commercial SnO2 and surfacemodified β -CD capped QDs are recorded to be 79.41 mmol/gcat and 104.3 mmol/gcat , respectively. In contrast, β -CD capped SnO2 QDs showed higher photocatalytic activity relative to commercial SnO2 nanoparticles with almost 31.3% increment in hydrogen yield. Aqueous solubilization of the SnO2 QDs using β -CD has proved to increase the surface moiety and hydrophilization of SnO2 particles

[53]. The hydrophilic –OH groups of β -CD make the QDs possess improved solubility in aqueous solution which in turn create better wettability and promote improved water diffusion rate onto the surface of QDs [54]. As mentioned earlier, smaller particles render higher surface areas and hence increase total active site densities for hydrogen gas production in β -CD capped QDs. However, another reason of better performance in hydrogen gas production in aqueous medium is lower rates of photogenerated e− /h+ recombination. This is again proven by the lower intensity of PL spectrum in Fig. 7(3), where the β -CD capped QDs render better chargecarriers separation efficiency (or improved life-span). In general, the absorption of incident photon energy has created bounded excitons (charge-carriers) in the form of electronhole pairs. Upon the photo-excitation process, the electrons are transfer from the valence band to the conduction band. This is followed by stabilization via photoemission, in which the photoexcited electrons in the conduction band decay back to the valance band and substantial amount of photon and the heat is released [55]. This so called PL has a specific wavelength and highly dependence on the lifetime of the photo-excited electron that stay in conduction band, while the lifetime is governed by the defects that present within the QDs, in which higher density of defects within the nanocrystals generally render lower photoemission intensity and vice-versa. For nanocrystals, it is repeatedly proven that the surface states and dangling bonds are the dominant defects rather than those bulk defects such as oxygen vacancies and stackingfaults [56]. Hence, the defect that prevails in β -CD capped QDs is believed to be source from both of the surface states and dangling bonds. Meanwhile, if the number of the photogenerated electrons resulting from the recombination between excited electrons and positively charged holes is increased, the PL intensity increases, and the photoactivity subsequently decreases [57]. Therefore, there is a strong connection between PL intensity and photoactivity. Due to the presence of β -CD on the surface of QDs, it is suggested that β -CD bound to the specific surface state of SnO2 QDs could effectively cause electron trap, thereby depressing the recombination process. Consequently, lowest PL intensity is observed (Fig. 7(3)), indicating highest photocatalytic activities are expected [58].

Fig. 7. Photoluminescence spectra for (1) SnO2 commercial nanopowder, (2) stearate-capped SnO2 QDs and (3) β -CD capped SnO2 QDs.

Please cite this article as: C. Haw et al., Tin stearate organometallic precursor prepared SnO2 quantum dots nanopowder for aqueous- and non-aqueous medium photocatalytic hydrogen gas evolution, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.04.006

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9

Fig. 8. (a) Photocatalytic hydrogen production curves obtained by (1) commercial SnO2 in glycerol, (2) as-synthesized SnO2 QDs in glycerol, (3) commercial SnO2 in aqueous and (4) β -CD capped SnO2 QDs in aqueous. All set up of experiments are carried out under constant irradiation of UV light. (b) Comparison of the hydrogen production rate for the first 5 h of as-prepared samples in two different mediums. Inset shows typical set up for photocatalytic performance to measure hydrogen gas evolution under UV irradiation.

Fig. 9. Mechanism of photocatalytic hydrogen gas generation from (a) glycerol medium (non-aqueous) and (b) aqueous phase with a mixture of S2 − /SO3 2 − as holes scavengers.

As an overview, the hydrogen gas evolution profile for all the samples can be summarized in Fig. 8(b), which reveals that the as-prepared SnO2 QDs are readily switchable to be used in both mediums. Indeed, hydrogen yield in aqueous phase is much greater than the glycerol. It should be noted that the non-aqueous medium containing mixture of glycerol/water (10 vol%) has relatively weak hydrophilization over SnO2 QDs that debilitates the photoacitivity, in turn lowering the overall hydrogen yield. Conversely, β -CD capped QDs has better dispersibility in water and even allow the penetration or diffusion of water molecules into the space between alkyl-chains of β -CD molecules. Thus, the ease in the accessibility by water molecules to the surface of QDs can be realized and further ease the redox reaction to take place for further boost up the hydrogen gas production rate.

A possible mechanistic explanation of hydrogen gas evolution for both a non-aqueous and aqueous mediums based on the literature are illustrated in Fig. 9 [59–62]. During the photo irradiation process, photon is bombarded on the surface of SnO2 QDs and hence electron-hole pairs are created in the conduction band (CB) and valence band (VB), respectively. The equation for the process is shown as below:

SnO2 + hv→ SnO2 (e− CB + h+ VB )

(4)

There is a possibility of recombination for these charge-carriers since the photo-excited electrons may experience de-excitation in turn reducing the photoactivity.

Please cite this article as: C. Haw et al., Tin stearate organometallic precursor prepared SnO2 quantum dots nanopowder for aqueous- and non-aqueous medium photocatalytic hydrogen gas evolution, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.04.006

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Glycerol (CH2 OHCHOHCH2 OH) adsorbed on the stearate-capped SnO2 QDs surfaces can react with the formed hydroxyl radical, as follows:

CH2 OHCHOHCH2 OH → CH2 OHCHOHCH2 + •OH

(5)

CH2 OHCHOHCH2 + •OH → CH2 OHCHOHCHOH + H2 O

(6)

The as-produced CH2 OHCHOHCHOH in Eq. (6) can further react with water molecule to form CH2 OHCHOHCH(OH)2 and •H radical which can further transform into hydrogen gas:

CH2 OHCHOHCHOH + H2 O →CH2 OHCHOHCH(OH)2 + H2

(7)

The CH2 OHCHOHCH(OH)2 is an unstable intermediate organic compound, which will transform into aldehyde thereafter:

CH2 OHCHOHCH(OH)2 → CH2 OHCHOHCHO + H2 O

(8)

The •OH radical can then react progressively with the resulted aldehyde and further forms carboxylic acid:

CH2 OHCHOHCHO + •OH → CH2 OHCHOCO + H2 O

(9)

CH2 OHCHOCO + H2 O → CH2 OHCHOHCOOH + H

(10)

The CH2 OHCHOHCOOH can react directly with photo-induced hole (h+ ) in VB and thus decarboxylation process takes place:

CH2 OHCHOHCOOH + h+ VB → CH2 OHCHOH + CO2 + H+

(11)

The proton ion will undergo reduction, whereby H+ is captured with photogenerated electron in CB and thus reduced to hydrogen gas.

H+ + e− CB → H2

(12)

The as-produced CH2 OHCHOH repeats the reactions from (7) to (12) and will continuously produces additional hydrogen gas. In addition, the hydrogen gas generation in aqueous phase that contains a mixture of S2− /SO3 2− as hole scavengers [63] is expressed as below. Upon photo-excitation, hole tends to react with water molecule in producing hydroxyl radicals and proton as below:

H2 O + e− CB →OH− + H+

(13)

At the same time, the electron in the conduction band also assisting in the reduction of proton to produce hydrogen gas as below:

2H+ + SnO2 (2e− CB ) → H2

(14)

As aforementioned, there is a high probability for the surface defects and dangling bonds form within the nanocrystal especially for those with diameter below 10 nm. The presence of dangling bond render an ideal compatibility for the incorporation of β -CD molecules onto the surface of the QDs since these defects generally serve as positive binding sites with external organic molecules through metal-carboxylic functional group binding [64,65]. As a result, interstitial bands are created and this could effectively trap the photoelectrons from being directly recombined with the holes in VB. These electrons are further captured by proton and subsequently it was reduced into hydrogen gas.

H+ + e−

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trapped in interstitial band →

H2

(15)

In order to further inhibit undesired electron-hole recombination, the role of hole-scavengers (Na2 S and Na2 SO3 ) is also vital, where there are numerous of path ways for hole scavenging reactions could take place through below routes:

SO3 2 − + H2 O + 2h+ VB → SO4 2− + 2H+

(16)

2S2 − + 2h+ VB → S2 2 −

(17)

S2 2 − + SO3 2− → S2 O3 2− + S2 −

(18)

S2 − + SO3 2 − + 2h+ VB → S2 O3 2−

(19)

S2 O3 2 − + H+ → HSO3 − + S

(20)

S + 2e− → S2 −

(21)

The intermediate ions such as S2 O3 2 − and S2 2 − can be regenerated during the course of reaction and this in turn has prolonged the carrier lifetime that will further benefit the redox reactions along the interface of QDs and water molecules. Nevertheless, the contribution of Na2 S and Na2 SO3 for scavenging the photo induced holes for improving the turnover rate of hydrogen gas, specifically for the case of nanocrystals still under debate [66,67]. However, current study depicts that the contribution of sacrificial agent is crucial for attaining higher yield of hydrogen gas. Furthermore, the photoactivity observed in aqueous phase is quite promising and thus it is prospected that the Sn(St)2 -prepared SnO2 QDs could potentially serve as a promising candidate for sustainable heterogeneous photocatalysis hydrogen gas production [68,69]. 4. Conclusions In summary, tin (II) stearate (Sn(St)2 ) has been adopted as a new precursor in synthesizing SnO2 QDs using a rapid one-pot non-hydrolytic method. Systematic characterization had been conducted to probe the structural- and optical-properties of the QDs. The QDs had been subjected to surface-ligand exchange for interchangeably application as photocatalyst for hydrogen gas generation with enhanced performance in both aqueous and non-aqueous mediums. Current study can serve as a potential pathway in the processing of SnO2 QDs for the advancement of photocatalysis technology as well as its relevant applications to commensurate the need of green- and sustainable-development. Acknowledgments This work was supported by e-Sciencefund (13-02-033093), Postgraduate Research Grant (PPP) (PG027-2013B), FRGS (FP038-2014B), and MOHE-ERGS (ER002-2013A). Additional sources of funding from UMRG (RP007B-13AFR), High Impact Research Program (UM.C/625/1/HIR/079) and HIR-MOHE (UM.C/625/1/HIR/MOHE/SC/06) are also highly appreciated. References [1] R.H. Crabtree, Organometallics 30 (2011) 17. [2] B. Kim, J. Kim, H. Baik, K. Lee, Cryst. Eng. Comm. 17 (2015) 4977. [3] T.R. Gordon, M. Cargnello, T. Paik, F. Mangolini, R.T. Weber, P. Fornasiero, C.B. Murray, J. Am. Chem. Soc. 134 (2012) 6751. [4] I. Mekis, D.V. Talapin, A. Kornowski, M. Haase, H. Weller, J. Phys. Chem. B 107 (2003) 7454. [5] W.S. Chiu, S. Radiman, M.H. Abdullah, P.S. Khiew, N.M. Huang, R. Abd-Shukor, Mater. Chem.Phys. 106 (2007) 231. [6] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am Chem. Soc. 115 (1993) 8706. [7] Z.A. Peng, X. Peng, J. Am Chem. Soc. 123 (2001) 183. [8] J.H. Liu, J.B. Fan, Z. Gu, J. Cui, X.B. Xu, Z.W. Liang, S.L. Luo, M.Q. Zhu, Langmuir 24 (2008) 5241. [9] W.W. Yu, Y.A. Wang, X. Peng, Chem. Mater. 15 (2003) 4300.

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