Shape-Dependent Charge Transfers in Crystalline ZnO Photocatalysts

Aug 28, 2014 - ZnO particles with rod and plate configurations were synthesized using a solvothermal method using zinc acetate and zinc chloride, ...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Shape-Dependent Charge Transfers in Crystalline ZnO Photocatalysts: Rods versus Plates Hye Won Jeong,† Seung-Yo Choi,† Seong Hui Hong,‡ Sang Kyoo Lim,*,‡ Dong Suk Han,§ Ahmed Abdel-Wahab,§ and Hyunwoong Park*,† †

School of Energy Engineering, Kyungpook National University, Daegu 702-701, Korea Division of Nano and Bio Technology, Daegu Gyeongbuk Institute of Science and Technology, Daegu 711-873, Korea § Chemical Engineering Program, Texas A&M University at Qatar, P.O. Box 23874, Education City, Doha, Qatar J. Phys. Chem. C 2014.118:21331-21338. Downloaded from pubs.acs.org by UNIVERSITE DE SHERBROOKE on 01/13/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: ZnO particles with rod and plate configurations were synthesized using a solvothermal method using zinc acetate and zinc chloride, respectively. The surface of the as-synthesized ZnO rods and plates were characterized using various analysis tools (XRD, XPS, photoluminescence, FE-SEM, HR-TEM, BET, and UV− vis) and their photocatalytic activities were examined for six different redox reactions. The surface areas and bandgaps of the two ZnO samples were nearly identical; however, XPS and photoluminescence (PL) studies showed that the rods and the plates have relatively pronounced oxygen vacancy and oxygen interstitial contributions, respectively. ZnO rods were found to be active for the decomposition of methylene blue and phenol, the production of OH radicals, and the generation of photocurrents, all of which are associated with single-electron transfer reactions. On the other hand, ZnO plates were more effective for the production of molecular hydrogen and hydrogen peroxide, both of which are initiated by two-electron transfer reactions. These single versus multiple charge transfers are discussed with regard to the roles of oxygen vacancies and oxygen interstitials, which are located near the conduction and the valence bands, respectively.

■

INTRODUCTION

causing skepticism about synthetic routes and the materials themselves. In the case of ZnO, the reported shape effects on photocatalytic activity vary significantly and are even contradictory. ZnO with an optical bandgap of ∼3.4 eV, can be synthesized with various dimensional structures, including spheres,15,21−23 rods/fibers, 10−15,21 sheets/disks,10,11 urchins,12,13 and pyramids.16 These structures display varying photocatalytic activities, and the differences have been attributed mostly to changes in surface area, crystallinity, oxygen vacancies, pore-size distribution, and so on. Surprisingly, the comparison of photocatalytic activity among ZnO samples with different shapes has been made only for a single kind of substrate 10,12−16,21,24 (mostly light-absorbing dyes,10,13,14,16,21,23,24 which have complicated photochemical redox reactions and accordingly are not recommended as model substrates in photocatalysis25−27), although in general, photocatalytic activities significantly vary depending on the substrate tested.28 This situation suggests that most evaluations of shape-dependent photocatalytic activities for ZnO lack reliability and need to be revisited. With this in mind, two crystalline ZnO samples with different shapes (rods and plates) were synthesized using a solvothermal method and examined for their optical and physicochemical

Oxide semiconductors have received wide attention as multifunctional materials with great application potential, particularly in the fields of sensors, energy conversion, and environmental cleanup technologies.1−4 The physicochemical properties of the oxide semiconductors are significantly influenced by various factors such as crystallinity, surface area, porosity, shape, and so on, all of which critically affect the overall performance of the semiconductors.1−4 When utilizing oxide semiconductors, particularly as photocatalysts, the effect of shape needs to be considered carefully in tailoring photocatalytic performance.4 For example, one-dimensional TiO2 nanofibers aligned with discrete spherical particles via physical interconnection exhibited far superior photocatalytic activities compared to randomly dispersed TiO2 particles, although the optical and physicochemical properties of the two were very similar.5,6 Such shape-dependent differences in the photocatalytic activity have been often observed,7−9 particularly with ZnO.10−19 Accurate assessment of such shape-dependent photocatalytic activity remains a challenge. Most studies have focused primarily on the surface properties of materials, and limited evaluations of the photocatalytic activity have been carried out.10,12−14,16,20 In many cases, well-tailored materials look very attractive but have limited applicability because of the under- or overestimated photocatalytic activity. This imprecision raises questions about reported photocatalytic activities, ultimately © 2014 American Chemical Society

Received: June 17, 2014 Revised: August 26, 2014 Published: August 28, 2014 21331

dx.doi.org/10.1021/jp506032f | J. Phys. Chem. C 2014, 118, 21331−21338

The Journal of Physical Chemistry C

Article

glass reactor (40 mL). To facilitate photocatalytic H 2 production, platinum ions (25 μM, H2PtCl6, Aldrich) were added to the suspension to photodeposit Pt nanoparticles in situ on ZnO particles.6 Prior to irradiation, the suspension was purged with argon gas for 30 min. A solar simulator equipped with an AM 1.5 G filter (LS-150 Xe, ABET Technologies) was used as a light source (100 mW·cm2). During irradiation, the headspace gas (15 mL) of the reactor was intermittently sampled (200 μL) and analyzed for H2 using a gas chromatograph (ACME 6100, Young Lin) equipped with a thermal conductivity detector and a molecular sieve 5 Å column. For the production of hydrogen peroxide (H2O2), ZnO (0.5 g·L−1) was suspended in an aqueous acetate solution (2 mM). Oxygen was purged through the suspension during the reaction and the produced H2O2 was analyzed using a colorimetric method.31 Photogenerated electrons in ZnO suspensions were collected in a three-electrode configuration following a conventional literature method.32−34 Briefly, a ZnO sample (1 g·L−1) was suspended in aqueous NaOH solution (0.95 M) with methyl viologen (MV2+, 0.5 mM) as an electron mediator. A Pt wire (1.5 mm diameter, 250 mm length), a saturated calomel electrode (SCE), and a graphite rod were immersed in the aqueous suspension as the working, reference, and counter electrodes, respectively. N2 gas was continuously purged through the suspension before and during irradiation (AM 1.5 G, 100 mW·cm−2). Time profiles of the photocurrents were collected by applying a constant potential (+0.5 or −0.4 V vs SCE) to the working Pt electrode using a potentiostat (Ivium Compact Stat). Surface Characterization. The X-ray diffraction (XRD, Rigaku D/max-2500) patterns of the samples were obtained using an X-ray diffractometer employing Cu Kα radiation (γ = 1.5405 Å) operating at 40 kV and 100 mA, with a scan rate of 0.02°·s−1. To identify the shape and structure of the samples, field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HR-TEM, JEM-2200FS) were employed. X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XPS spectrometer, VG Scientifics) was used to examine the binding energies of Zn and O atoms. Photoluminescence (PL) emission spectroscopy (SPEX 1403) was employed to study charge recombination and charge trapping pathways using a He−Cd laser excitation source (325 nm).

properties. In addition, the photocatalytic activities of these two samples were systematically evaluated and compared for various redox reactions, including oxidation reactions (e.g., decompositions of methylene blue and phenol, and quantification of OH radicals), reduction reactions (e.g., evolution of molecular hydrogen and production of hydrogen peroxide), and photocurrent generation.

■

EXPERIMENTAL SECTION Material Synthesis. A solvothermal method was employed to synthesize the crystalline ZnO rods and plates. For ZnO rods, 0.1 M zinc acetate dihydrate (Zn(CH3COO)2·2H2O, Junsei) was dissolved in pure ethanol (200 mL) and stirred for 10 min. Subsequently, 1 M NaOH was added to the Zn precursor solution and stirred for 30 min. This mixed solution was transferred to a Teflon-lined stainless steel autoclave and was thermally treated at 150 °C for 2 h. After cooling to room temperature, the precipitate was washed with deionized water several times and dried at 70 °C in a vacuum oven to remove the solvent. For ZnO plates, zinc chloride (ZnCl2, Daejung) was used as a precursor and the synthetic procedure was identical to that for the ZnO rods. The Brunauer−Emmet− Teller (BET) surface areas of the rods and plates were 10.2 and 11.3 m2·g−1, respectively. Photocatalytic and Photoelectrochemical Activity Tests. Methylene blue (MB, Daejung) and phenol (PhOH, Aldrich) were used as model substrates owing to their widespread use in photocatalytic reaction tests. An aqueous suspension of as-prepared ZnO (0.5 g·L−1) with MB (0.1 mM) or phenol (0.5 mM) was placed in a glass reactor equipped with a quartz disc for light penetration, dispersed ultrasonically for 10 min, and stirred vigorously for 30 min prior to irradiation. A dynamic light scattering study showed that the hydrodynamic radii of rods and plates are similar to ∼1.3 μm (Figure S1). This suggests that the geometric reaction areas of suspended particle aggregates are similar during photocatalysis although their aggregation states (e.g., the kinds and degrees of exposed facets) may be different. Assuming the uniformity of rods (60 nm × 500 nm) and plates (500 nm in diameter; see Figure 2), approximately 180 rods and 30 plates behave as aggregated particles. Accordingly, some specific facets with different photocatalytic activities can be attached to each other and become inactive. Quantitative evaluation for such photoactive or inactive surface is experimentally very difficult and beyond the scope of this study. Ultrasonic dispersion and vigorous stirring can minimize topotactic attachment and expose all facets with the same probability due to dynamic equilibrium during photocatalysis. For quantification of the OH radicals that are generated from irradiation of ZnO, the N,N-dimethyl-p-nitrosoaniline (RNO, Aldrich, 0.1 mM) bleaching test was carried out following the same photocatalysis procedure. MB and RNO were quantified using a UV−vis spectrometer (PerkinElmer, Lambda 950) by monitoring the absorbance changes at 66529 and 440 nm,30,31 respectively. Phenol and its intermediates were quantified using high-performance liquid chromatography (HPLC, YL9100) equipped with a UV detector (λ = 213 nm) and a C18 column (Thermo).30,31 The eluent was composed of 55 vol % distilled water (including 0.1 vol % phosphoric acid) and 45 vol % acetonitrile at a flow rate of 1 mL·min−1. For hydrogen (H2) evolution reactions, an aqueous suspension (25 mL) containing ZnO (0.5 g·L−1) and electron donors (0.1 M Na2S and 0.1 M Na2SO3) was stirred in a Pyrex-

■

RESULTS AND DISCUSSION Surface Characterization. Figure 1 shows the XRD patterns of ZnO samples solvothermally synthesized using zinc acetate and zinc chloride precursors. All the diffraction peaks observed were indexed to the hexagonal ZnO wurtzite structure (JCPDS card no. 36−1451) and no trace of other phases was observed, indicating high phase purity.35,36 The ZnO sample synthesized from zinc acetate exhibited a strong (101) peak and a relatively weak (002) peak, suggesting preferential growth orientation (i.e., rod type). The ZnO sample synthesized from zinc chloride displayed the same peak distribution, yet the peak intensities (e.g., (101) peak) were weak. Particularly, the relative intensity of the (002) peak with respect to the (101) peak increased from ∼0.47 (zinc acetate) to ∼0.64 (zinc chloride), indicating a decrease in the aspect ratio and shape change from a rod type to a plate type. This shape change with different precursors might be attributed to the different degrees of basicity of the precursors. Acetate is a 21332

dx.doi.org/10.1021/jp506032f | J. Phys. Chem. C 2014, 118, 21331−21338

The Journal of Physical Chemistry C

Article

crystallized lattice structure of the same (001) plane (Figure 2c,d). XPS analysis showed that the Zn/O atomic ratio of the ZnO plates is close to stoichiometric value (41.8:41.6), whereas that of the rods was greater than unity (42.8:38.8). This indicates that the latter has a greater degree of oxygen defects. Deconvolution of the O 1s spectra further indicated the coexistence of several oxygen sub-bands (Figure 3). For the rod sample, a strong band (α) centered at 530.5 eV and two subbands at 531.4 eV (β) and 532.1 eV (γ) were identified. The former band is associated with the lattice oxygen (Ol) in ZnO, whereas the latter bands are related to chemisorbed oxygen (Oa) originating from >Zn−OH (“>” refers to the surface).21 The plate sample exhibited a similar distribution pattern, but had an additional sub-band (δ: 532.8 eV). It is noteworthy that the fraction of chemisorbed oxygen (β, γ, and δ) is greater in the plates (45.6%) compared with the rods (32.7%), suggesting the presence of different kinds of oxygen atoms and the degrees of oxygen vacancies in the ZnO plates. The energy difference between Zn 2p3/2 and Zn 2p1/2 is approximately 23.0 eV, which is close to the standard value of 22.97 eV (Figure S2).36 However, the binding energy of Zn 2p3/2 was 0.2 eV greater for the plates (1023.0 eV) compared with the rods (1022.8 eV), indicating that in the plates there is a stronger interaction between zinc and oxygen likely due to >Zn−OH or Zn(OH)2.37 These differences in surface oxygen states affect the photoluminescence (PL) emission spectra of ZnO rods and plates. Upon excitation at λ = 325 nm at room temperature, a broad emission band centered at 550−560 nm and a relatively weak band at ∼390 nm were obtained with both samples (Figure S3). To understand the behavior of the charge transfer, the PL spectra were resolved into several subemission bands (Figure 4). For the rod samples, nine emission bands (R1−R9)

Figure 1. XRD patterns for ZnO particles synthesized from zinc acetate (ZnAc) and zinc chloride (ZnCl2).

much stronger conjugate base (pKb ∼ 9.2) than chloride, and the concentration of OH− in solution is increased when acetate is used. Accordingly, the molar ratio of [OH−]/[Zn2+] is higher in zinc acetate solution compared with zinc chloride solution. It is typical that ZnO grows more anisotropically as this molar ratio increases21,35,36 because of the large grain sizes obtained via the Ostwald ripening process. The different ZnO shapes were further confirmed by SEM and TEM analyses (Figure 2). The ZnO sample synthesized from zinc acetate was composed of a number of uniformly sized rods with an average diameter of ∼60 nm and a length of ∼500 nm (Figure 2a). A HR-TEM image for a single ZnO rod further showed a uniform lattice structure (Figure 2b). The interlattice fringe distance was 0.52 nm, which is close to the d spacing of the (001) plane. On the other hand, the ZnO sample synthesized from zinc chloride displayed a disc- or plate-like configuration with widths up to submicrons and a well-

Figure 2. FE-SEM (a, c) and HR-TEM (b, d) images of synthesized ZnO rods (a, b) and plates (c, d). 21333

dx.doi.org/10.1021/jp506032f | J. Phys. Chem. C 2014, 118, 21331−21338

The Journal of Physical Chemistry C

Article

Figure 3. XPS analysis of the O 1s spectra for the synthesized ZnO (a) rods and (b) plates. The original spectra (black, solid lines) were resolved into several sub-bands (dotted lines), the sums of which are shown as fitted curves (red, solid lines). The numbers in the legend refer to fractions.

Figure 4. Room temperature photoluminescence (PL) emission spectra of ZnO (a) rods and (b) plates. The original spectra (black, solid lines) were resolved into several sub-bands (color, dotted lines), the sums of which are shown as fitted curves (black, dotted lines). The wavelengths in the legend indicate the peak emission wavelengths, whereas the numbers refer to fractions.

were obtained (Figure 4a). The near-UV emission (387 nm) is associated with the band-edge emission of ZnO,38 the optical bandgap of which is approximately 3.26 eV (yet the sample is capable of absorbing light of λ ∼ 390 nm, see Figure S4). The long-wavelength emission bands (405−700 nm) are attributed to defect emissions.38 Among these emission bands, green emission bands (500−550 nm) are often attributed to singly ionized oxygen vacancies (VO•)35,38 and neutral oxygen vacancies (VO×)21,38 located at the surface, although this assignment is controversial.38 Yellow emission bands (550−600 nm) and orange-red emission bands (600−700 nm) are typically attributed to oxygen interstitials (Oi) located at the subsurface.38,39 ZnO plates exhibited eight emission bands (P1−P8) with a similar band pattern to that of the rods. Accordingly, both ZnO samples appear to have similar charge trapping/detrapping pathways. However, it is noteworthy that the contribution of each emission band to the overall emission spectra was different for the two samples. The fraction of green emission and yellow-red emission were estimated to be 50 and 39%, respectively, for rods, and 45 and 54%, respectively, for plates. In addition, the emission at 449 nm, whose contribution is quite high (6.7%) in the rods, was not observed in the plates. It is of note that the plate sample is oxygen-rich and has a greater fraction of the yellow-orange-red emission compared with the rod sample. Accordingly, the rod and plate samples have more pronounced oxygen vacancy and oxygen interstitial contributions, respectively. Comparison of Photocatalytic Activities. Figure 5a shows the time profile of the change in methylene blue (MB) concentration in ZnO suspensions under AM 1.5 light. MB is the most commonly used model substrate in assessing photocatalytic activity.25 It is apparent that the degradation of

Figure 5. Time profile of the changes in concentration of (a) methylene blue (MB, 100 μM) and RNO (100 μM), and (b) phenol (PhOH, 500 μM) in the suspensions of ZnO rods and plates under irradiation. Hydroquinone (HQ) and catechol (CC) are shown as intermediates in the decomposition of phenol. Experimental conditions: 0.5 g·L−1 ZnO; air-equilibrated; AM 1.5 (100 mW·cm−2).

21334

dx.doi.org/10.1021/jp506032f | J. Phys. Chem. C 2014, 118, 21331−21338

The Journal of Physical Chemistry C

Article

MB in rod suspensions is much faster than in plate suspensions. Assuming pseudo-first order kinetics (Ct = C0·exp(−k·t)), the reaction rate constant (k) with rods was 2.8-fold greater than that with plates. In both suspensions, the magnitude of the main MB absorption band (λ = 664 nm) was gradually decreased, along with a slight blue shift of the band with irradiation time (Figure S5). This indicates that MB was decomposed primarily via chromophore cleavage and that the N-demethylation process was the minor decomposition process.29,33 The primary reactive species for the degradation of MB is speculated to be photogenerated hydroxyl radicals (>Zn−OH•+ or simply OH•; reactions 1−3). ZnO + hν → e− + h+ −

e + O2 → O2

−

h+ + >Zn − OH → >Zn − OH•+

(1) Figure 6. Time profile of photocurrent generation in the suspensions of ZnO rods and plates under irradiation. For collection of photogenerated electrons, MV2+ was used as an electron mediator, while a Pt wire (electron collector) was held at −0.4 and +0.5 V vs SCE in the suspensions. Experimental conditions: 1 g·L−1 ZnO; 0.5 mM MV2+; 0.95 M NaOH; N2-purged prior to and during the reaction; AM 1.5 (100 mW·cm−2).

(2) (3)

To quantify the hydroxyl radicals produced during irradiation of ZnO, a photocatalytic bleaching test of RNO (a well-known OH• quencher) was carried out.31,40 Similar to the case of MB, the bleaching profile of RNO followed pseudo-first order kinetics and the rate constant for RNO bleaching with rods was ∼2.6-fold greater than that with plates (Figures 5a and S6). This result suggests that the primary reactive species is hydroxyl radicals, which are generated more effectively in the suspension of rods. The photocatalytic decomposition of phenol (C0 = 500 μM) further verified the superior performance of ZnO rods (Figure 5b). The pseudo-first order rate constant for phenol decomposition with rods was ∼3.2-fold greater than that with plates. During the phenol decomposition, hydroquinone (HQ) and catechol (CC) were found to be the primary intermediates.30,31,40 The presence of OH groups in aromatic compounds favors addition at the ortho and para positions, whereas the absence of meta-substituted intermediates (e.g., resorcinol) is typical.41 These intermediates are generated predominantly via the reaction of phenol with hydroxyl radicals, which is supported by that of RNO test. In addition to these hole transfer reactions, electron transfer reactions were also investigated. For this, the photogenerated electrons were collected at a Pt electrode held at −0.4 and +0.5 V in irradiated ZnO suspensions with methyl viologen (MV2+) as an electron mediator (E°(MV2+/+) = −0.44 V vs NHE; see reaction 4 and Figure S7).5,32−34,42−44 N2 was continuously purged through the suspension during the reaction to inhibit the reaction between electrons and O2 (reaction 2). e− + MV2 + → MV +

(4a)

MV + → MV2 + + e−(Pt)

(4b)

2e− + 2H+ + O2 → H 2O2

The H2 production reaction was performed in the presence of electron donors (S2− and SO32−).43,44 As shown in Figure 7, H2

Figure 7. Time profile of hydrogen evolution in the suspensions of ZnO rods and plates (bare and Pt deposited) under irradiation. The inset shows the production of hydrogen peroxide. Experimental conditions: 0.5 g·L−1 ZnO; 0.1 M Na2S and 0.1 M Na2SO3 (for H2); AM 1.5 (100 mW·cm−2). For H2 and H2O2 production, the solution was purged with N2 and O2, respectively, for 30 min prior to and continuously during the reactions.

was produced at constant rates in both ZnO suspensions, and the H2 production rate in plate suspensions (0.79 μmol·h−1) was 2.3-fold higher than that with rod suspensions (0.34 μmol· h−1). This enhancement is in contrast to the above results for oxidation (hole transfer) reactions (Figures 5 and 6). When ZnO was coupled with Pt through the in situ photodeposition,6 the H2 production rate was significantly enhanced due to the catalytic effect of Pt.5 However, the H2 production rate was still higher in plate suspensions (3.0 μmol·h−1) by a factor of 3 compared with rod suspensions. The superior activity of plates was also observed for the production of hydrogen peroxide via the two-electron reduction of O2 (reaction 6 and Figure 7, inset). Although the activity difference between the two ZnO samples was not as marked as that for H2 production, this result still supports the lower photocatalytic activity of the rods

Upon irradiation, the photocurrent generation at −0.4 V increased initially and then reached a plateau (Figure 6) due to the low collection efficiency (reaction 4b). At +0.5 V, the photocurrent generation plateau was elevated because of the enhanced collection efficiency. This trend was observed in both rod and plate suspensions. It is of note that the plateaus in the rod suspensions were always higher than those in the plate suspensions. This result is strong evidence that the rods are very effective for generation and interfacial transfer of electrons. We have further compared rods and plates for the productions of H2 (reaction 5) and H2O2 (reaction 6).

2e− + 2H+ → H 2

(6)

(5) 21335

dx.doi.org/10.1021/jp506032f | J. Phys. Chem. C 2014, 118, 21331−21338

The Journal of Physical Chemistry C

Article

the other hand, oxygen interstitials appear to be associated with hole transfer. The energy level of the oxygen interstitial is 2−3 eV21,38,45 and, hence, holes can be efficiently transferred to this trap site (reaction 8). An effective hole trap should facilitate CB electron transfers, while the surface may be rather acidic due to low [OH−]/[Zn2+] ratio during the synthesis, thus, increasing the availability of photogenerated (trapped) electrons and protons (Scheme 1b). Under such conditions, proton-coupled multielectron transfer would be possible (reactions 5 and 6).

compared with the plates for reduction (electron transfer) reactions. Reaction Mechanism: Rods versus Plates. ZnO rods were found to be more active for the photocatalytic decomposition of methylene blue and phenol. The rods were further shown to generate hydroxyl radicals and MV2+mediated photocurrents more efficiently compared to ZnO plates. On the other hand, ZnO plates were superior to the rods for the production of H2 and H2O2. We note that all the photocatalytic reactions for which rods are superior to plates are initiated via a single-electron transfer, whereas those for which plates are superior to rods are associated with a twoelectron transfer. Although planar ZnO with (0001) facet has been reported to be active for specific reactions, the reasons have not been disclosed clearly.10,17−19 Such different charge transfer behavior should be attributed to different charge trapping/detrapping processes. As the XPS and PL studies revealed, the rods and plates have more pronounced oxygen vacancy (e.g., VO×, VO•, and VO••) and oxygen interstitial (e.g., Oi, Oi′, and Oi″) contributions, respectively. The energy levels of the former are close to conduction band (CB), whereas the energy levels of the latter are close to the valence band (VB; Scheme 1), although the exact positions of these energy bands are still controversial.1−3,21,38,45−47

h+ + Oi ″ → Oi ′

Reaction 8 occurs in rods as well, yet is more efficient in plates.

■

CONCLUSIONS ZnO particles with rod and plate configurations were synthesized simply by varying the precursor. Surface analyses with XRD, SEM, and TEM showed a decrease in aspect ratio when zinc acetate was replaced with zinc chloride due to the different degree of basicity. Although the surface area and the bandgap of the ZnO rods and plates were nearly identical, their photocatalytic activities were significantly different. The rods were found to have higher activities for one-electron transfer reactions, whereas the plates were more active for two-electron transfer reactions. This different charge transfer behavior is attributed to different charge trapping/detrapping processes for rods and plates. XPS and PL studies revealed that the rods have more pronounced oxygen vacancies (VO×, VO•, and VO••) close to the conduction band energy level, which function as electron trap sites. Accordingly, the hole transfer reactions were enhanced in the rods. On the other hand, the plates have relatively pronounced oxygen interstitial (Oi, Oi′, and Oi″) contributions, which are close to the valence band energy level. Therefore, hole trapping was facilitated, increasing the availability of photogenerated electrons, resulting in more efficient multielectron transfer reactions. This study suggests that, although the apparent physicochemical and optical properties are similar for ZnO particles with different shapes, the photocatalytic activity can be markedly affected by subtle changes in the oxygen defects.

Scheme 1. Illustration of Photogenerated Charge Transfer Pathways Occurring in ZnO Rods and Platesa

VO•/× and Oi′/″ refer to oxygen vacancies and oxygen interstitials (Kröger-Vink notation), respectively, and the energy values (V vs NHE) are shown on the left axes.

a

■

With ZnO rods, the electron trapping/detrapping is speculated to predominantly occur at the oxygen vacancy sites (reaction 7a).2,38,45 The trapped electron can be transferred to an interfacial electron acceptor (A: O2 or MV2+; reaction 7b). e− + VO• → VO×

(7a)

VO× + A → VO• + A−

(7b)

(8)

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S7. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82-53-785-3510. E-mail: [email protected]. *Tel.: +82-53-950-8973. E-mail: [email protected].

This trapping/detrapping-mediated electron transfer should inhibit charge recombination and enhance charge separation efficiency. As a result, hole transfer occurs more effectively (reaction 3), and the photocatalytic degradations of methylene blue and phenol are enhanced in rods (Scheme 1a). The oxygen vacancy sites (VO•/VO and VO••/VO•) are reported to be located ∼0.05 eV below the CB2,38,46 and 1.2−1.6 eV above the VB,2,38,45−47 respectively. A deep oxygen vacancy site (VO••/VO•) can trap the photogenerated electrons, yet would also function as a charge recombination center because of its location close to the VB. Accordingly, effective electron transfer appears to occur primarily at the former vacancy (shallow trap). This trap site should be considered because the relatively strong emission at 449 nm (2.76 eV) was observed only with rods. On

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was financially supported by the Basic Science Research Program (NRF-2012R1A2A2A01004517), Framework of International Cooperation Program (NRF2013K2A1A2052901), Space Core Technology Development Program (NRF-2014M1A3A3A02034875) through the National Research Foundation (NRF), the Korea CCS R&D Center (KCRC; No. 2014M1A8A1049354) funded by the Ministry of Science, ICT and Future Planning, and the DGIST R&D Program of the MEST (14-NB-03), Korea. In addition, 21336

dx.doi.org/10.1021/jp506032f | J. Phys. Chem. C 2014, 118, 21331−21338

The Journal of Physical Chemistry C

Article

(20) Xu, F.; Zhang, P.; Navrotsky, A.; Yuan, Z.-Y.; Ren, T.-Z.; Halasa, M.; Su, B.-L. Hierarchically Assembled Porous ZnO Nanoparticles: Synthesis, Surface energy, and Photocatalytic Activity. Chem. Mater. 2007, 19, 5680−5686. (21) Zheng, Y. H.; Chen, C. Q.; Zhan, Y. Y.; Lin, X. Y.; Zheng, Q.; Wei, K. M.; Zhu, J. F.; Zhu, Y. J. Luminescence and Photocatalytic Activity of ZnO Nanocrystals: Correlation Between Structure and Property. Inorg. Chem. 2007, 46, 6675−6682. (22) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. Mechanisms Behind Green Photoluminescence in ZnO Phosphor Powders. J. Appl. Phys. 1996, 79, 7983−7990. (23) Deng, Z. W.; Chen, M.; Gu, G. X.; Wu, L. M. A Facile Method to Fabricate ZnO Hollow Spheres and Their Photocatalytic Property. J. Phys. Chem. B 2008, 112, 16−22. (24) Lu, Y. C.; Wang, L. L.; Wang, D. J.; Xie, T. F.; Chen, L. P.; Lin, Y. H. A Comparative Study on Plate-Like and Flower-Like ZnO Nanocrystals Surface Photovoltage Property and Photocatalytic Activity. Mater. Chem. Phys. 2011, 129, 281−287. (25) Bae, S.; Kim, S.; Lee, S.; Choi, W. Dye Decolorization Test For the Activity Assessment of Visible Light Photocatalysts: Realities and Limitations. Catal. Today 2014, 224, 21−28. (26) Ohtani, B. Preparing Articles on Photocatalysis - Beyond the Illusions, Misconceptions, and Speculation. Chem. Lett. 2008, 37, 217− 229. (27) Millis, A. An Overview of the Methylene Blue ISO Test for Assessing the Activities of Photocatalytic Films. Appl. Catal., B 2012, 128, 144−149. (28) Ryu, J.; Choi, W. Substrate-Specific Photocatalytic Activities of TiO2 and Multiactivity Test for Water Treatment Application. Environ. Sci. Technol. 2008, 42, 294−300. (29) Kim, J.; Choi, W.; Park, H. Effects of TiO2 Surface Fluorination on Photocatalytic Degradation of Methylene Blue and Humic Acid. Res. Chem. Intermed. 2010, 36, 127−140. (30) Yang, S. Y.; Choo, Y. S.; Kim, S.; Lim, S. K.; Lee, J.; Park, H. Boosting the Electrocatalytic Activities of SnO2 Electrodes for Remediation of Aqueous Pollutants by Doping with Various Metals. Appl. Catal., B 2012, 111, 317−325. (31) Yang, S. Y.; Kim, D.; Park, H. Shift of the Reactive Species in the Sb-SnO2-Electrocatalyzed Inactivation of E. coli and Degradation of Phenol: Effects of Nickel Doping and Electrolytes. Environ. Sci. Technol. 2014, 48, 2877−2884. (32) Park, H.; Choi, W. Photoelectrochemical Investigation on Electron Transfer Mediating Behaviors of Polyoxometalate in UVIlluminated Suspensions of TiO2 and Pt/TiO2. J. Phys. Chem. B 2003, 107, 3885−3890. (33) Park, H.; Choi, W. Effects of TiO2 Surface Fluorination on Photocatalytic Reactions and Photoelectrochemical Behaviors. J. Phys. Chem. B 2004, 108, 4086−4093. (34) Park, H.; Lee, J.; Choi, W. Study of Special Cases Where the Enhanced Photocatalytic Activities of Pt/TiO2 Vanish under Low Light Intensity. Catal. Today 2006, 111, 259−265. (35) Lim, S. K.; Hwang, S.-H.; Kim, S.; Park, H. Preparation of ZnO Nanorods by Microemulsion Synthesis and Their Application as a CO Gas Sensor. Sens. Actuators, B 2011, 160, 94−98. (36) Lim, S. K.; Hong, S. H.; Hwang, S.-H.; Kim, S.; Park, H. Characterization of Ga-Doped ZnO Nanorods Synthesized via Microemulsion Method. J. Mater. Sci. Technol. 2013, 29, 39−43. (37) NIST X-ray Photoelectron Spectroscopy Database; http:// srdata.nist.gov/xps/default.aspx. (38) Djurisic, A. B.; Leung, Y. H. Optical Properties of ZnO Nanostructures. Small 2006, 2, 944−961. (39) Li, D.; Leung, Y. H.; Djurisic, A. B. Different Origins of Visible Luminescence in ZnO Nanostructures Fabricated by the Chemical and Evaporation Methods. Appl. Phys. Lett. 2004, 85, 1601−1603. (40) Kang, U.; Park, H. Lithium Ion-Inserted TiO2 Nanotube Array Photoelectrocatalysts. Appl. Catal., B 2013, 140−141, 233−240.

this publication was made possible by a grant from the Qatar National Research Fund under its National Priorities Research Program (Award No. NPRP 7-865-2-320).

■

REFERENCES

(1) Janotti, A.; Van de Walle, C. G. Fundamentals of Zinc Oxide as a Semiconductor. Rep. Prog. Phys. 2009, 72, 126501. (2) Schmidt-Mende, L.; MacManus-Driscoll, J. L. ZnO - Nanostructures, Defects, and Devices. Mater. Today 2007, 10, 40−48. (3) Woll, C. The Chemistry and Physics of Zinc Oxide Surfaces. Prog. Surf. Sci. 2007, 82, 55−120. (4) Park, H.; Kim, W.; Park, Y.; Choi, W. Surface Modification of TiO2 Photocatalyst for Environmental Applications. J. Photochem. Photobiol. C 2013, 15, 1−20. (5) Choi, S. K.; Kim, S.; Lim, S. K.; Park, H. Photocatalytic Comparison of TiO2 Nanoparticles and Electrospun TiO2 Nanofibers: Effects of Mesoporosity and Interparticle Charge Transfer. J. Phys. Chem. C 2010, 114, 16475−16480. (6) Choi, S. K.; Kim, S.; Ryu, J.; Lim, S. K.; Park, H. Titania Nanofibers as a Photo-Antenna for Dye-Sensitized Solar Hydrogen. Photochem. Photobiol. Sci. 2012, 11, 1437−1444. (7) Bae, E.; Ohno, T. Exposed Crystal Surface-Controlled Rutile TiO2 Nanorods Prepared by Hydrothermal Treatment in the Presence of Poly(Vinyl Pyrrolidone). Appl. Catal., B 2009, 91, 634−639. (8) Murakami, N.; Kurihara, Y.; Tsubota, T.; Ohno, T. ShapeControlled Anatase Titanium(IV) Oxide Particles Prepared by Hydrothermal Treatment of Peroxo Titanic Acid in the Presence of Polyvinyl Alcohol. J. Phys. Chem. C 2009, 113, 3062−3069. (9) Tachikawa, T.; Yamashita, S.; Majima, T. Evidence for CrystalFace-Dependent TiO2 Photocatalysis from Single-Molecule Imaging and Kinetic Analysis. J. Am. Chem. Soc. 2011, 133, 7197−7204. (10) Mclaren, A.; Valdes-Solis, T.; Li, G.; Tsang, S. C. Shape and Size Effects of ZnO Nanocrystals on Photocatalytic Activity. J. Am. Chem. Soc. 2009, 131, 12540−12541. (11) Li, G. R.; Hu, T.; Pan, G. L.; Yan, T. Y.; Gao, X. P.; Zhu, H. Y. Morphology-Function Relationship of ZnO: Polar Planes, Oxygen Vacancies, and Activity. J. Phys. Chem. C 2008, 112, 11859−11864. (12) Wang, Y. X.; Li, X. Y.; Wang, N.; Quan, X.; Chen, Y. Y. Controllable Synthesis of ZnO Nanoflowers and Their MorphologyDependent Photocatalytic Activities. Sep. Purif. Technol. 2008, 62, 727−732. (13) Ali, A. M.; Emanuelsson, E. A. C.; Patterson, D. A. Photocatalysis with Nanostructured Zinc Oxide Thin Films: The Relationship Between Morphology and Photocatalytic Activity Under Oxygen Limited and Oxygen Rich Conditions and Evidence for a Mars Van Krevelen Mechanism. Appl. Catal., B 2010, 97, 168−181. (14) Shinde, V. R.; Gujar, T. P.; Noda, T.; Fujita, D.; Vinu, A.; Grandcolas, M.; Ye, J. H. Growth of Shape- and Size-Selective Zinc Oxide Nanorods by a Microwave-Assisted Chemical Bath Deposition Method: Effect on Photocatalysis Properties. Chem.Eur. J. 2010, 16, 10569−10575. (15) Xu, L.; Hu, Y.-L.; Pelligra, C.; Chen, C.-H.; Jin, L.; Huang, H.; Sithambaram, S.; Aindow, M.; Joesten, R.; Suib, S. L. ZnO with Different Morphologies Synthesized by Solvothermal Methods for Enhanced Photocatalytic Activity. Chem. Mater. 2009, 21, 2875−2885. (16) Lu, F.; Cai, W. P.; Zhang, Y. G. ZnO Hierarchical Micro/ Nanoarchitectures: Solvothermal Synthesis and Structurally Enhanced Photocatalytic Performance. Adv. Funct. Mater. 2008, 18, 1047−1056. (17) Jang, E. S.; Won, J.-H.; Hwang, S.-J.; Choy, J.-H. Fine Tuning of the Face Orientation of ZnO Crystals to Optimize Their Photocatalytic Activity. Adv. Mater. 2006, 18, 3309−3312. (18) Kim, S.; Kim, M.; Kim, T.; Baik, H.; Lee, K. Evolution of SpaceEfficient and Facet-Specific ZnO 3-D Nanostructures and Their Application in Photocataylsis. CrystEngComm 2013, 15, 2601−2607. (19) Zeng, J. H.; Jin, B. B.; Wang, Y. F. Facet Enhanced Photocatalytic Effect with Uniform Single-Crystalline Zinc Oxide Nanodisks. Chem. Phys. Lett. 2009, 472, 90−95. 21337

dx.doi.org/10.1021/jp506032f | J. Phys. Chem. C 2014, 118, 21331−21338

The Journal of Physical Chemistry C

Article

(41) Park, H.; Choi, W. Photocatalytic Conversion of Benzene to Phenol Using Pure and Modified TiO2. Catal. Today 2005, 101, 291− 297. (42) Park, H.; Choi, W.; Hoffmann, M. R. Effects of the Preparation Method of the Ternary CdS/TiO2/Pt Hybrid Photocatalysts on Visible Light-Induced Hydrogen Production. J. Mater. Chem. 2008, 18, 2379−2385. (43) Kim, Y. K.; Park, H. Light-Harvesting Multi-Walled Carbon Nanotubes and CdS Hybrids: Application to Photocatalytic Hydrogen Production from Water. Energy Environ. Sci. 2011, 4, 685−694. (44) Khan, G.; Choi, S. K.; Kim, S.; Lim, S. K.; Jang, J. S.; Park, H. Carbon Nanotubes as an Auxiliary Catalyst in Heterojunction Photocatalysis for Solar Hydrogen. Appl. Catal., B 2013, 142−143, 647−653. (45) Lima, S. A. M.; Sigoli, F. A.; Jafelicci, M., Jr; Davolos, M. R. Luminescent Properties and Lattice Defects Correlation on Zinc Oxide. Int. J. Inorg. Mater. 2001, 3, 749−754. (46) Xu, P. S.; Sun, Y. M.; Shi, C. S.; Xu, F. Q.; Pan, H. B. The Electronic Structure and Spectral Properties of ZnO and Its Defects. Nucl. Instrum. Methods B 2003, 199, 286−290. (47) Lin, B.; Fu, Z.; Jia, Y. Green Luminescent Center in Undoped Zinc Oxide Films Deposited on Silicon Substrates. Appl. Phys. Lett. 2001, 79, 943−945.

21338

dx.doi.org/10.1021/jp506032f | J. Phys. Chem. C 2014, 118, 21331−21338