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Simple but effective way to enhance photoelectrochemical solar water splitting performance of ZnO nanorod arrays: charge-trapping Zn(OH)2 annihilation and oxygen vacancy generation by vacuum annealing Minki Baek, Donghyung Kim, and Kijung Yong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12555 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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ACS Applied Materials & Interfaces

Simple but Effective Way to Enhance Photoelectrochemical Solar Water Splitting Performance of ZnO Nanorod Arrays: Charge-Trapping Zn(OH)2 Annihilation and Oxygen Vacancy Generation by Vacuum Annealing Minki Baek, Donghyung Kim and Kijung Yong* Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, P ohang University of Science and Technology (POSTECH), Pohang 790-784, Korea

Abstract This study presents an effective and the simplest method to substantially improve the photoelectrochemical water splitting ability of hydrothermally grown ZnO nanorod arrays (NRAs). In the hydrothermal growth of ZnO NRAs, unwanted Zn(OH)2 species are formed, which act as trapping sites of photo-excited charges. We found that those inherent chargetrapping sites could be annihilated by the desorption of the hydroxyl groups upon vacuum annealing above 200 °C, which resulted in an enhancement of the charge separation efficiency and photocurrent density. Another drastic increase in the photocurrent density occurred when ZnO NRAs were treated with annealing at higher temperature (700 °C), which can be attributed to the introduced oxygen vacancies acting as shallow donors in the ZnO crystal lattice. The removal of the charge-trapping Zn(OH)2 and the generation of oxygen vacancies were confirmed by photoluminescence (PL) and XPS analyses. The ZnO NRAs treated by this simple method yield a photocurrent density of 600 µA/𝑐𝑚2 at 1.23 𝑉𝑅𝐻𝐸 under 1 sun 1

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illumination, which is 20 times higher than that obtained from as-grown ZnO NRAs. This study presents a highly efficient way of increasing the bulk electric conductivity and photoelectrochemical activity of metal oxide nanorods without requiring the introduction of any extrinsic dopants.

Keywords ZnO photoanode, photoelectrochemical cell, water splitting, vacuum annealing, oxygen vacancy in metal oxide

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Introduction Solar water splitting is a simple and eco-friendly energy harvesting method for using solar energy to produce hydrogen and oxygen. Among various methods for solar water splitting, the photoelectrochemical (PEC) cell is one of the most efficient ways because it uses a lowcost system and has a high solar energy conversion efficiency with a small external bias.1-2 Many n-type metal oxide materials have been studied as candidates for the photoanodes3 in the PEC cell because they generally have suitable band bending for water oxidation at the surface when they are immersed in electrolytes, with good stability in aqueous solution. ZnO is one of the most widely studied n-type metal oxide photoanodes because it is earth-abundant and has a direct bandgap with favorable band edges for the water-splitting redox levels.4-5 However, ZnO has also some drawbacks that limit efficient water splitting. It has a wide bandgap energy (approximately 3.37 eV), so it can only absorb UV light, which accounts for only 4 % of the whole solar spectrum. Additionally, particularly concerning the hydrothermal growth of ZnO, it has a rapid recombination rate due to the formation of unwanted species that act as trapping sites, which results in a low solar-to-hydrogen (STH) conversion efficiency.6 Thus, it is necessary to develop a method to annihilate these charge-trapping sites of hydrothermally grown ZnO to enhance its charge transfer. In this study, we found that the vacuum annealing of ZnO effectively removes charge-trapping Zn(OH)2, which is induced by thermal desorption of hydroxyl groups from the surface. To improve water splitting performance, utilizing visible light is the most commonly used strategy by fabricating the ZnO heterostructure with other small band gap materials such as quantum dots or plasmonic metal nanoparticles.7-11 For example, Seol et al constructed a ZnO photoanode sensitized with CdSe/CdS quantum dots and their photoanode produced 3



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highly enhanced photocurrent than the bare ZnO photoanode due to increasing visible light absorption.8 Also, Zhang et al fabricated 3D ZnO nanowire arrays (NRAs) decorated with plasmonic Au nanoparticles for utilizing visible light.10 Doping other elements and introducing vacancy defects into the ZnO crystal lattice are alternative approaches for enhancing the water-splitting performance.12-14 Dopants can work as donors or acceptors; thus, they increase the charge carrier density, improve the electrical bulk conductivity and reduce the recombination rate.15 The other method, introducing vacancy defects, can also modulate the electrical properties. In this study, we focus on enhancing the electrical properties and PEC performance of the ZnO photoanode by introducing oxygen vacancies. Oxygen vacancies are inherent donors with a low formation energy in ZnO.13,

16

Theoretically, oxygen loss can be balanced with two electrons and a doubly ionized oxygen 1

vacancy when it escapes from the ZnO crystal lattice (𝑂𝑂 → 2 𝑂2 (𝑔) + 𝑉𝑂•• + 2𝑒 ′ ). This basic principle increases the charge carrier density and improves the electrical conductivity as n-type dopants do. Many studies have proven this relation between the oxygen vacancy density and charge carrier density.17-19 In general, the hydrogenation method is preferred to introduce oxygen vacancies into the metal oxide.20-25 Hydrogen, which has a high chemical reactivity, not only introduces oxygen vacancies but also forms bonds with anions in the metal oxide and then inject electrons into the lattice.15 Hence, the effect of introducing oxygen vacancies alone cannot be evaluated for hydrogenation. Furthermore, the usage of pure hydrogen in the hydrogenation method is highly dangerous due to its combustibility. In this study, we found that a vacuum annealing of ZnO NRAs at 700 oC generates oxygen vacancies in the ZnO crystal lattice, which play a role as shallow donors, thus 4


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increasing the free charge carrier density. Due to the effect of vacuum annealing, the photoanode of 700 oC-annealed ZnO NRAs produces a drastically increased photocurrent density at 1.23 𝑉𝑅𝐻𝐸 (standard water splitting potential) by a factor of approximately 20 compared to that of as-grown samples. This unusual enhancement of the PEC performance is due to charge-trapping site annihilation and oxygen vacancy generation by vacuum annealing, which results in a reduction in charge recombination and an increased electrical conductivity of the photoanode. Also, our vacuum-annealed ZnO NRAs show a fast hole transfer rate at surface, which was verified by comparing onset potential and photocurrent density of the ZnO NRAs with and without hole scavenger. Photocurrent density of the ZnO NRAs with and without oxygen evolution reaction (OER) catalyst (Co-Pi) were also compared to confirm the fast hole transfer property. The simple but effective vacuum annealing method developed in current study is applicable to other metal oxide nanostructures for solar water splitting.

Experimental Section Preparation of sample: Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98 %) and ammonium hydroxide (28 wt% NH3 in water, 99.99 %) were purchased from Aldrich and used as received. ZnO NRAs were grown using a hydrothermal method developed by our research group.26 First, a 50-nm ZnO film was deposited on a FTO glass as a seed layer for hydrothermal growth by radio-frequency magnetron sputtering under an Ar atmosphere at room temperature. Then, ZnO NRAs were grown on the seed layer as follows. The ZnO seed layer was immersed in a 10 mM Zn(NO3)2·6H2O (98 %, Aldrich) aqueous solution (100 mL). Then, 3.7 mL of ammonium 5



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hydroxide (28 wt%) was added to raise the pH of the solution above 10, and the sample was kept in an oven at 95 °C for 12 hr. The as-grown ZnO NRAs were annealed in vacuum (below 1 mTorr) at various temperatures (every 100 °C from 100 to 700 °C, with a ramping rate of 15 °C/min) for 1 hr. After annealing, the ZnO NRAs were cooled to room temperature. OER catalyst, Co-Pi, was deposited onto surface of the ZnO NRAs by photo-assited electrodeposition

reported

elsewhere.27-29

Three-electrode

potentiostat

system

(potentiostat/galvanostat, model 263A, EG&G Princeton Applied Research) was used with the ZnO photoanode as the working electrode, a Pt rod as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode to deposit Co-Pi. 1.05 𝑉𝑆𝐶𝐸 was applied to the ZnO NRA photoanode in a solution of 0.5mM Co(NO3)2·6H2O (buffered with 0.1 M KPi at pH 7) with a solar-simulated light source (AM 1.5 G filtered, 100 mW/cm2, 91160, Oriel) for 5 minutes. Characterization: Images of the ZnO NRAs were obtained using FE-SEM (XL30S, Philips) and HRTEM (JEM-2100FS with Image Cs-corrector, JEOL). The diffraction patterns of the ZnO NRAs were obtained by HR-TEM (JEM-2100FS with Image Cs-corrector, JEOL) and X-ray diffraction (XRD, Max-2500V, RIGAKU). The chemical binding energy of O 1s was investigated by X-ray photoelectron spectroscopy (XPS, LAB250/ VG scientific with Monochromatic Al X-ray source at 15kV). The photoluminescence (PL) was characterized at room temperature using a 325-nm Xe lamp as an excitation source.

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Photoelectrochemical measurement: A linear sweep voltammetry (LSV) of the ZnO NRAs was studied using a threeelectrode potentiostat system, with the same electrodes mentioned above in a 0.5 M Na2SO4 aqueous solution (buffered with 0.1 M KPi at pH 7) as an electrolyte under a solar-simulated light. The incident photon to current efficiency (IPCE) of the ZnO photoanode was measured in the same solution as above using a photomodulation spectroscopic setup (model Merlin, Oriel).

Results and Discussion Structural Characterization of Vacuum-annealed ZnO NRAs: Compared to nanoparticle-structured photoanodes, which have wide interfaces with a number of grain boundaries facilitating electron-hole recombination, nanorod-structured photoanodes have several advantages, such as enhancing light absorption through scattering, preventing easy electron-hole recombination by fast charge transfer via the 1-D structure, and the short diffusion length of the holes to an electrolyte. In the current study, vertically aligned ZnO NRAs were hydrothermally grown on the FTO glass substrate for application as a photoanode. The grown ZnO nanorods have an average diameter of 150 nm and length of 3.2 µm (figure 1(a)). ZnO have same nanorods structure after annealing (figure S1(a), (b)). The HR-TEM image and SAED pattern of the as-grown and 700 °C -annealed ZnO NRAs are shown in figure 1(b) and figure 1(c), respectively. In the HRTEM image of a single nanorod, the ZnO fringes are separated by 0.52 nm, corresponding to the d-spacing of the (001) plane, confirming a preferential growth along the c-axis toward Zn7



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terminated surface. After heat treatments (400 °C and 700 °C annealing), ZnO NRAs show much clearer TEM images, which suggests the annealed ZnO NRAs have enhanced crystallinity than as-grown ones (figure 1(c) and figure S1(c)). The crystallinity of the ZnO NRAs was further confirmed by X-ray diffraction. The XRD patterns of the as-grown and 700 °C -annealed samples are shown in figure 1(d). Both samples show the wurtzite diffraction pattern of the ZnO crystal structure (JCPDS 36-1451) with a strong intensity of the (002) peak, indicating that the hexagonal wurtzite phase of the ZnO NRAs grew well with a preferred (002) orientation by hydrothermal growth. Photoelectrochemical (PEC) Properties of Vacuum-annealed ZnO NRAs: The PEC perfomance of the ZnO NRAs were studied by measuring a LSV using a three-electrode system with chopped light illumination. Figure 2 shows the photocurrent density from the ZnO NRAs prepared at various annealing temperatures from 100 to 700 °C. The as-grown ZnO NRAs exhibit a very low photocurrent density compared with the annealed samples. This low photocurrent density is attributed to the high concentration of chargetrapping sites in the ZnO NRAs and poor crystallinity. In a hydrothermal growth environment, 𝑍𝑛(𝑂𝐻)2 , which is known as a charge-trapping site at the surface,30 is easily formed by competing with the ZnO formation reaction.31-32 ZnO formation reaction 𝑍𝑛2+ (aq) + 4𝑁𝐻3 (𝑎𝑞) → 𝑍𝑛(𝑁𝐻3 )4 𝑍𝑛(𝑁𝐻3 )4

2+

2+

(𝑎𝑞)

(𝑎𝑞) + 2𝑂𝐻 − (𝑎𝑞) → 𝑍𝑛𝑂(𝑠) + 4𝑁𝐻3 (𝑎𝑞) + 𝐻2 𝑂(𝑙) Zn(OH)2 formation reaction 8


(1) (2)

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Zn2+ (𝑎𝑞) + 2𝑁𝐻3 (𝑎𝑞) + 2𝐻2 𝑂(𝑙) → 𝑍𝑛(𝑂𝐻)2 (𝑠) + 2𝑁𝐻4 + (𝑎𝑞)

(3)

Photoexcited holes in the poor crystalline ZnO NRAs are trapped at these sites and cannot be transferred rapidly for the water oxidation. They recombine easily with photoelectrons, which results in a low PEC performance.30, 33 The low-temperature annealing of NRAs at 100 °C is not different in the photocurrent generation compared to the as-grown sample (not included in figure 2). When the ZnO NRAs are annealed at 200 °C, the photocurrent density begins to increase all over the measured potentials. This result implies that 𝑍𝑛(𝑂𝐻)2 begins to decompose at over 200 °C by the thermal desorption of hydroxyl groups (𝑍𝑛(𝑂𝐻)2 (𝑠) → 𝑍𝑛𝑂(𝑠) + 𝐻2 𝑂(𝑔)), thus reduces recombination and allows the detrapped holes to migrate into the electrolyte solution.31, 34-35 Upon 300 °C annealing, the photocurrent density is enhanced more substantially. The photocurrent densities of the ZnO NRAs annealed at 400 and 500°C are almost same as those of the ZnO NRAs annealed at 300 °C. These results imply that the Zn(OH) 2 is fully removed and the crystallinity of the ZnO NRAs is enhanced after thermal annealing over 300 °C. The presence and removal of Zn(OH)2 will be discussed further by the photoluminescence (PL) analysis results. Another drastic change in the photocurrent density is observed when the ZnO NRAs were treated with vacuum annealing at a higher temperature of 700 °C. It yields a value of approximately 600 µA/𝑐𝑚2 at a potential of 0.57 𝑉𝑆𝐶𝐸 (1.23 𝑉𝑅𝐻𝐸 , 𝑉𝑅𝐻𝐸 = 𝑉𝑆𝐶𝐸 + 0.0591  pH + 0.244). This leap in the photocurrent density can be attributed to oxygen vacancy generation in the ZnO NRAs. In the oxygen-deficient condition during high-temperature annealing (700 °C), the oxygen in the ZnO crystal lattice is removed from its original site. Several groups have argued that oxygen can easily escape from its original lattice site, migrate 9



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to interstitial position and oxygen vacancy is created by thermal annealing at temperatures over 600 °C in oxygen deficient conditions due to the low formation energy of oxygen vacancy in ZnO. Then, the formed interstitial oxygen atoms associate to evaporate from the surface.36-39 𝑍𝑛𝑂(𝑠) → 𝑍𝑛𝑍𝑛 + 𝑂𝑖 + 𝑉𝑜

(4)

2𝑂𝑖 → 𝑂2 (𝑔)

(5)

Generated oxygen vacancy can easily leave electrons behind, while it is converted to +1 or +2 charged state (𝑉𝑂 → 𝑉𝑂• + 𝑒 ′ → 𝑉𝑂•• + 2𝑒 ′ ). This substantially increases the charge carrier density of the ZnO NRAs, leading to a higher photocurrent density. The formation of oxygen vacancies will be confirmed by PL and XPS analyses. To investigate the photoconversion efficiency of the ZnO NRAs at every wavelength, IPCE measurements were performed. Figure 3(a) shows the IPCE spectra of the ZnO NRAs annealed at various annealing temperatures, measured at 0.57 𝑉𝑆𝐶𝐸 . The IPCE can be calculated by the equation IPCE (%) = (1240  J𝑝ℎ )/(λ  𝑃𝑖𝑛 ), in which J𝑝ℎ is the measured photocurrent density of the ZnO NRAs, λ is the wavelength and 𝑃𝑖𝑛 is the intensity of the incident light. Above a wavelength of 400 nm, all samples show negligible IPCE values. However, below 400 nm, the IPCE values of the annealed samples were enhanced at every wavelength as the annealing temperature increased from 200 to 700 °C. An especially drastic enhancement was observed for the 700 °C -annealed sample. This demonstrates that annealing in vacuum does not widen the light absorption region or band gap of the ZnO NRAs but does significantly enhance the photoconversion efficiency by increasing the charge carrier density and charge separation efficiency. When the photoanode is illuminated in a PEC cell, electron-hole pairs are generated 10


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by photo-excitation. To oxidize water, they should be separated, and the separated holes should reach the surface and be injected into the electrolyte solution. Zhou et al adapted this water oxidation process to a simply proposed equation that calculates the photocurrent density (𝐽𝑃𝐸𝐶 ),40 𝐽𝑃𝐸𝐶 = 𝐽𝑎𝑏𝑠  𝑃𝑠𝑒𝑝  𝑃𝑖𝑛𝑗

(6)

where 𝐽𝑎𝑏𝑠 is the photon absorption rate, expressed as a current density, 𝑃𝑠𝑒𝑝 is the fraction of photogenerated holes that do not recombine in the bulk and reach the surface, and 𝑃𝑖𝑛𝑗 is the fraction of those separated holes that are injected into the electrolyte solution. Water oxidation is kinetically difficult because four holes must be used quickly to make one mole of 𝑂2 by oxidizing water (2𝐻2 𝑂 + 4ℎ+ → 𝑂2 + 4𝐻 + ). Hence, fast hole transfer to the electrolyte solution is very crucial in the water oxidation of the photoanode. So, to evaluate the hole injection efficiency (𝑃𝑖𝑛𝑗 ), we measured the photocurrent density with and without a hole scavenger (sodium sulfite, Na2SO3), and the results are shown in figure 3(b). In the presence of the hole scavenger, the hole transfer to the electrolyte becomes extremely fast, so 𝑃𝑖𝑛𝑗 is thought to be 1. Thus, if a photoanode material that has a poor hole transfer property is in a solution with a hole scavenger, it will experience a large cathodic onset potential shift and photocurrent density increase because of the hole scavenging effect.41-42 However, figure 3(b) shows that both the onset potential shift and photocurrent density increase (under 0.3 𝑉𝑆𝐶𝐸 ) are almost negligible upon the application of the hole scavenger. This means that our 700 °C -annealed ZnO NRAs have an excellent hole transfer property at the surface. The photocurrent density difference in the high-bias region (over 0.3 𝑉𝑆𝐶𝐸 ) is mainly due to redox potential difference between water (1.23 V) and sulfite (1 V), not by the 11



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hole transfer property.43 The higher dark current in sulfite oxidation boosts the photocurrent density over 0.3 𝑉𝑆𝐶𝐸 . By equation (6), 𝐽𝑃𝐸𝐶 𝑁𝑎2 𝑆𝑂3 = 𝐽𝑎𝑏𝑠  𝑃𝑠𝑒𝑝 because 𝑃𝑖𝑛𝑗 is 1 in the hole scavenger. Hence, the hole injection efficiency (𝑃𝑖𝑛𝑗 ) of the ZnO NRAs can be calculated simply by the ratio of 𝐽𝑃𝐸𝐶 𝐻2 𝑂 /𝐽𝑃𝐸𝐶 𝑁𝑎2 𝑆𝑂3 . In the inset of figure 3(b), 𝑃𝑖𝑛𝑗 is over 50 % even at low bias and reaches nearly 90 % over 0.2 𝑉𝑆𝐶𝐸 due to the great hole transfer property of our sample. The fast hole transfer property was verified again by measuring the photocurrent density of the ZnO NRAs with Co-Pi. Co-Pi is well-known OER catalyst which improves water oxidation kinetics. The improved water oxidation lowers probability of surface recombination, shifts the onset potential cathodically and increases photocurrent density especially at low potential even without hole scavenger. However as shown in figure S2 (supporting information), deposition of Co-Pi on the 700 °C -annealed ZnO NRAs doesn’t yield cathodic onset potential shift or photocurrent increase. This can be another proof for explaining the fast hole transfer. Rather, at high potential, photocurrent density is reduced after Co-Pi deposition due to interference of light absorption by Co-Pi. But above 1.2 𝑉𝑆𝐶𝐸 , dark current of the Co-Pi deposited ZnO NRAs starts to increase by catalytic activity of Co-Pi and photocurrent density exceeds that of bare ZnO NRAs. Investigation of Charge Trapping Site Annihilation and Oxygen Vacancy Formation in Vacuum-annealed ZnO NRAs: To understand the mechanism for the enhanced PEC performance of the vacuumannealed ZnO NRAs, we analyzed the PL and XPS spectra of the samples annealed at various temperatures. The black curve in figure 4(a) is the PL spectrum of the as-grown ZnO NRAs 12


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hydrothermally grown at 95 °C. The ZnO NRAs have a large concentration of defects, 𝑍𝑛(𝑂𝐻)2 , when they are grown in a hydrothermal growth environment.31-32 𝑍𝑛(𝑂𝐻)2 is a charge-trapping site where a hole from the valence band is trapped in the mid-gap states and induces two unique PL characteristics, as shown in figure 4(a). First, they emit a yellow-green defect emission (560 nm, 2.21 eV) by radiative recombination between electrons and trapped holes. Second, they quench an excitonic UV emission (370 nm, 3.35 eV).30, 34 When ZnO NRAs are annealed at 200 °C, the number of the defects decreases by the decomposition reaction of Zn(OH)2 through the thermal desorption of hydroxyl groups. Thus, the Zn(OH)2-related yellow-green emission is gradually reduced, and the excitonic UV emission begins to increase, which is shown as a blue curve in figure 4(a).31, 34 Annealed above 400 °C, the defects are fully eliminated, so the UV to visible emission ratio is highly increased, shown as a red curve in figure 4(a). Also, the excitonic peak becomes much sharper, which indicates enhanced crystallinity. Thus upon 400 °C annealing, ZnO NRAs have a highly crystalline structure, with the effective quenching of Zn(OH)2.44 Both recombination at the trapping site and improved crystallinity of ZnO are critical factors in the efficient PEC reaction.45-46 These PL results well explain why the photocurrent density is enhanced drastically upon 400 °C vacuum annealing in Figure 2. When annealed at 700 °C, the ZnO NRAs exhibit a newly emerging green emission (530 nm, 2.34 eV), shown as a green curve in figure 4(b). An origin of this green emission is still controversial, but ionized oxygen vacancies are known to be the strongest candidate for explaining a green PL emission.17-19, 47-50 Two radiative recombination pathways were proposed for this green emission by oxygen vacancies: i) a hole is trapped at a deep trap site (𝑉𝑜 •• ) and emits a green emission by recombining with an electron in the conduction band;19, 47, 49 and ii) 13



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the recombination of an electron in a singly ionized oxygen vacancy (𝑉𝑜 • ) with a hole in the valence band emits a green emission.17-18 Although we cannot determine which of the two recombination paths is dominant in the current stage, it is certain that the new generation of this green PL emission is related to the newly generated oxygen vacancies in the crystal lattice of the ZnO NRAs prepared by annealing at 700 °C. The generated oxygen vacancies in the crystal lattice make free charge carriers compensated by ionized interstitial ions, and thus, they behave as donors and improve the electrical properties of the ZnO photoanode.16,17 The increased charge carrier density results in a nearly two-fold increase in the photocurrent density compared to the ZnO NRAs annealed at 400 °C along all measured voltages, as shown in figure 2. Another indication of generated oxygen vacancies will be discussed later in analysis of the XPS data. All radiative PL emission routes are summarized in the energy diagrams presented in the insets of figure 4. The chemical binding states of the annealed ZnO NRA samples were studied by XPS analysis. Figure 5(a) and 5(b) are normalized O 1s XPS spectra of the ZnO NRAs annealed at 400 and 700 °C, respectively. The O 1s spectrum of the ZnO NRAs shifts to a higher binding energy after annealing at 700 °C in comparison with those annealed at 400 °C, as shown in figure 5(a). To elucidate the cause of this shift, the O 1s spectra of annealed ZnO NRAs are deconvoluted into two sub-peaks by fitting Gaussian distribution curves. The left peak positioned at a lower binding energy (530.0 - 530.4 eV) is attributed to oxygen ions in the wurtzite Zn-O crystal structure. It is a characteristic peak of the normal ZnO structure. The right peak positioned at a higher binding energy (532.0 - 532.2 eV) is typically attributed to oxygen ions in oxygen-deficient regions.13, 31, 51-53 We could determine the presence of the 14


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oxygen vacancies by comparing the intensities of the higher binding state peaks. Clearly, the O 1s spectrum in figure 5(b) shows a drastic increase of the intensity of the higher binding energy peak (532.2 eV), which indicates that oxygen vacancies are substantially generated upon annealing at 700 °C. XPS spectra of other elements are in figure S3 (supporting information). We also analyzed EDS during TEM imaging analysis of ZnO NRAs to check the oxygen vacancies. Although EDS is not very powerful quantitative analysis, it provides us with rough atomic percentage values of each element. As seen in figure S4 (supporting information), atomic percentage of oxygen is clearly reduced after 700 °C annealing. This result well corresponds with the PL and XPS results obtained with ZnO NRAs after 700 °C annealing, proving oxygen vacancy generation. To directly compare the free charge carrier density of the ZnO NRAs, Mott-Schottky analysis was conducted in dark conditions at a frequency of 10 kHz. In the Mott-Schottky plot, a donor density can be calculated according to the equation54 𝑁𝑑 = (2/𝑒0 𝜀𝜀0 )[𝑑(1/𝐶 2 )/𝑑𝑉]−1

(7)

where 𝑁𝑑 is the donor density of the ZnO NRAs, 𝑒0 is the electron charge, ε is the dielectric constant of ZnO (ε = 8, typical value of bulk ZnO), 𝜀0 is the permittivity of vacuum, C is the capacitance, and V is the applied bias at the electrode. This equation indicates that the charge carrier density is inversely proportional to the slope of the Mott-Schottky plot. In figure 6(a), the as-grown ZnO NRAs have a much steeper slope than the two annealed ZnO NRAs. The charge trapping of Zn(OH)2 is thought to be a main factor for the low charge carrier density. When it is removed by vacuum annealing, the slope becomes much 15



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lower, as shown by the black and red curves. In an inset of figure 6(a), the slopes of the ZnO NRAs annealed at 400 °C and 700 °C are compared with each other. As expected, oxygen vacancies increase the amount of free charge carriers and make the slope lower when annealing at 700 °C. The calculated charge carrier densities of the as-grown, 400 °C -annealed, and 700 °C -annealed ZnO NRAs are 4.94 × 1017 , 5.17 × 1021 , and 8.43 × 1021 𝑐𝑚−3 , respectively. These enhanced charge carrier densities of the annealed ZnO NRAs well explain enhancement of photocurrent generation in our PEC system. Furthermore, we fabricated Ag/FTO/ZnO NRAs/Ag device, as shown in figure 6(b), to measure dark current-voltage (I-V) behaviors of the ZnO NRAs in the air. We made the device involving Ag/FTO/ZNO NRAs/Ag junction. The top and bottom Ag electrode were deposited by evaporating Ag on the ZnO NRAs/FTO electrode through a shadow mask. Au probe tip was placed on the top electrode while bottom electrode was grounded during I-V measurement. As indicated in I-V results of figure 6(c), 700 °C-annealed ZnO NRAs show a drastically increased dark current and as-grown ZnO NRAs exhibit very little current compared with the annealed samples. These I-V behaviors also correspond to the results of Mott-Schottky plot and again verify the improved conductivity of ZnO NRAs due to the increased charge carrier density caused by generation of oxygen vacancies.

Conclusions In this study, we demonstrated a drastically improved PEC water-splitting performance of hydrothermally grown ZnO photoanodes by simple vacuum-annealing treatments. Before annealing, as-grown ZnO NRAs have an abundance of charge-trapping sites that interrupt efficient charge transfer. Above 200 °C, those charge-trapping species (Zn(OH)2) begin to decompose, which causes an enhancement of the photocurrent density. Another abnormal leap 16


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in the photocurrent density is observed when ZnO NRAs were annealed at 700 °C. This hightemperature annealing generates oxygen vacancies in the ZnO crystal lattice, which play a role as shallow donors, thus increasing the free charge carrier density. We could obtain a superior water-splitting performance of the ZnO photoanode via this increased charge carrier density. The photocurrent density at 1.23 𝑉𝑅𝐻𝐸 is approximately 20 times higher after the simple vacuum annealing treatment of ZnO NRAs. Another promising point we want to raise from the current results is that our vacuumannealed ZnO NRAs would be excellent candidates for a frame material of a photoanode when they are combined with other small band gap semiconductors or quantum dots for solar water splitting due to their enhanced electrical conductivity.

■ ASSOCIATED CONTENT Supporting information. SEM, TEM images of samples, Linear sweep voltammetry of Co-Pi deposited ZnO NRAs, XPS spectra of wide scan area, Zn 2p and O 1s, EDS data of samples.

■ AUTHOR INFORMATION *Corresponding Author: E-mail. [email protected]; Phone. +82-54-279-2278.

■ ACKNOWLEDGMENT

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This work was supported by the National Research Foundation of Korea (NRF2016R1A4A1010735).

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Figure 1. (a) SEM cross-sectional view of as-grown ZnO NRAs. (Inset) top view of ZnO NRAs, (b) HRTEM image and corresponding SAED pattern of as-grown ZnO nanorod, (c) HRTEM image and corresponding SAED pattern of ZnO nanorod annealed in vacuum at 700 °C (vac 700), (d) XRD patterns of as-grown and 700 °C -annealed ZnO NRAs.

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Figure 2. Linear sweep voltammetry of ZnO NRAs annealed in vacuum at various temperature. All samples are illuminated with chopped 1 sun illumination (100 mW/cm2) in 0.5 M Na2SO4 solution (buffered at pH 7).

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Figure 3. (a) IPCE of ZnO NRAs annealed at various temperatures collected from 300 to 460 nm at a potential of 0.57 VSCE (1.23 VRHE) in 0.5 M Na2SO4 solution (buffered at pH 7). (Inset) magnified image of IPCE of black (as-grown) and green (vac 200) curve, (b) Linear sweep voltammetry of 700°C annealed ZnO NRAs in 0.5 M Na2SO4 (buffered at pH 7, black curve) and 0.5 M Na2SO3 solution (buffered at pH 7, red curve) under 1 sun illumination (solid) and in the dark (dotted). (Inset) hole injection efficiency of 700°C -annealed ZnO NRAs in 0.5 M Na2SO4 solution.

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Figure 4. Room-temperature PL spectra of ZnO NRAs annealed at various temperatures. (inset) Schematic energy diagram of PL emission routes. (a) as-grown, 200°C -annealed and 400°C-annealed ZnO NRAs, (b) 400°C -annealed and 700°C -annealed ZnO NRAs.

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Figure 5. Normalized O 1s XPS spectra of ZnO NRAs annealed at (a) 400 °C and (b) 700 °C. Both are deconvoluted into two fitted Gaussian distribution peaks near 530 and 532 eV.

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Figure 6. (a) Mott-Schottky plots of as-grown, 400°C -annealed and 700 °C -annealed ZnO NRAs collected at a frequency of 10 kHz in dark. (Inset) Magnified image of Mott-Schottky plot of 400 °C and 700 °C annealed ZnO NRAs. (b) Schematic image of Ag/FTO/ZnO NRAs/Ag device. (Inset) photograph of the device (c) Dark current – voltage (I-V) curve using the device 28


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