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Cite this: DOI: 10.1039/c5nr00271k
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Oxygen deficient ZnO1−x nanosheets with high visible light photocatalytic activity† Hong-Li Guo,‡ Qing Zhu,‡ Xi-Lin Wu, Yi-Fan Jiang, Xiao Xie and An-Wu Xu* Zinc oxide is one of the most important wide-band-gap (3.2 eV) materials with versatile properties, however, it can not be excited by visible light. In this work, we have developed an exquisite and simple way to prepare oxygen-deficient ZnO1−x nanosheets with a gray-colored appearance and excellent visible light photocatalytic activity. Detailed analysis based on UV-Vis absorption spectra, X-band electron paramagnetic resonance (EPR) spectra, and photoluminescence (PL) spectra confirms the existence of oxygen vacancies in ZnO1−x. The incorporation of oxygen defects could effectively extend the light absorption of ZnO1−x into the visible-light region due to the fact that the energy of the localized state is located in the forbidden gap. Thus, our obtained ZnO1−x shows a higher photodegradation of methyl orange (MO) compared to defect-free ZnO under visible light illumination. Additionally, the high content of •OH radicals
Received 13th January 2015, Accepted 14th March 2015 DOI: 10.1039/c5nr00271k www.rsc.org/nanoscale
with a strong photo-oxidation capability over the ZnO1−x nanosheets significantly contributes to the improvement in the photocatalytic performance. Our oxygen deficient ZnO1−x sample shows a very high photocatalytic activity for the degradation of MO even after 5 cycles without any obvious decline. The results demonstrate that defect engineering is a powerful tool to enhance the optoelectronic and photocatalytic performances of nanomaterials.
Introduction Photocatalysis technology has attracted enormous interest in organic wastewater treatment for environmental redemption because it is an inexpensive and convenient method that can totally decompose organic pollutants into small molecules (H2O, CO2, etc.).1 It is particularly difficult to find a stable semiconductor system with a suitable band gap for visible light absorption and driving the subsequent redox reactions. Zinc oxide (ZnO) is one of the most important wide-band-gap materials (3.2 eV) with versatile properties, such as high transparency in the visible wavelength region, near-UV emission, transparent conductivity, a high piezoelectric constant and a large excitation binding energy (∼60 meV) at room temperature. Using photocatalysis, ZnO nanomaterials find potential use in organic wastewater treatment.2,3 However, the relatively wide band gap significantly limits their photocatalytic applications using solar light because UV light only accounts for about 4% of the total solar spectrum.4 Thus, the exploration of facile, mild, and effective routes for the rational synthesis of
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, P. R. China. E-mail: [email protected]; Fax: (+86) 551-6360 2346 † Electronic supplementary information (ESI) available: Additional figures. See DOI: 10.1039/c5nr00271k ‡ These authors contributed equally to this work.
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visible-light responsive catalysts with superior photocatalytic activity is highly desired. Tremendous efforts have been made to regulate the band structure of ZnO, making it sensitive to visible light. Several studies have been reported where the absorption edge is shifted towards the visible light region by doping with metal ions or non-metal or rare earth elements.5 However, recent studies have both theoretically and experimentally demonstrated that doping does not inevitably translate into a higher photocatalytic activity since the impurity or defect states can trap electrons and facilitate non-radiative recombination, which will reduce the photocatalytic performance.6–10 Bearing these facts in mind, our group has recently developed a series of non-stoichiometric semiconductor-based photocatalysts, including oxygen-deficient TiO2−x, SnO2−x and ZnO nanoparticles, which display excellent visible light photocatalytic activities.11–13 The distortion of the crystal lattice provides an ideal platform for reducing the recombination of the photogenerated electrons and holes to improve the photoconversion efficiency, thus showing excellent catalytic activity and stability under visible-light irradiation. Therefore, it is highly desirable to design and tune the surface chemistry and oxygen vacancies of wide-band-gap semiconductors to trigger a response to visible light, which accounts for 43% of the spectrum of sunlight. Several methods have been employed to produce semiconductors containing oxygen vacancy species such as UV irradiation,14 plasma treatment,15 laser,16 or γ-ray17 bombardment.
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Although these routes have their own advantages, they are limited by high costs and complicated procedures, and they are not suitable for large-scale production. Moreover, there have been few reports paying close attention to the relationship between oxygen vacancies and photocatalytic activity. The understanding of the defect-dependent visible-light-driven photocatalytic mechanism still remains elusive. Herein, we report a facile hydrothermal approach for the mass-production of ZnO1−x nanosheets with abundant oxygen vacancies. Benefiting from the introduction of oxygen vacancies, the visible light photocatalytic activity and photocurrent of ZnO1−x have been significantly improved compared with oxygen vacancyfree ZnO. The mechanism of visible light photocatalysis and the charge transfer process is also proposed. This work has developed a novel approach to enhance the photocatalytic performance and utilization of ZnO as a visible light photocatalyst for potential use in highly efficient solar energy conversion devices with a low cost.
Experimental section Photocatalyst preparation All reagents were analytical grade and were used without any further purification. For the synthesis of the ZnO1−x nanosheets (NSs), 2 mmol of zinc chloride (ZnCl2) was dissolved in distilled water (15 mL) with vigorous stirring for 5 min, and then metal zinc powder (1.6 mmol) was added into the zinc chloride solution. After stirring for another 5 min, 2 mmol of ammonium bicarbonate (NH4HCO3) was added (NH4HCO3 was used as a CO2 source for simplicity). The mixed solution was transferred into a 25 mL Teflon-lined stainless steel autoclave, and then heated at 180 °C for 15 h in an oven. After the reaction was complete, the sample was thoroughly washed with distilled water and ethanol three times until the pH = 7 and dried at 60 °C in a vacuum oven. Finally, the gray-colored ZnO1−x product was harvested. It is worth mentioning that ZnO1−x can be fabricated on a large scale by using a large autoclave (1000 mL) for the hydrothermal treatment, and gray ZnO1−x can be obtained on the gram level (see photograph in Fig. S1†). A stoichiometric white ZnO sample was prepared by calcining the ZnO1−x NSs at 500 °C for 2 h in air. Photocatalytic activity measurements Under exterior irradiation for a certain period of time, the photocatalytic activities of the samples were evaluated from the photocatalytic decomposition of methyl orange (MO) at room temperature. The light source was a 300 W Xe lamp (PLS-SXE300/300UV, Trusttech Co., Ltd., Beijing) equipped with an ultraviolet cutoff filter to block the light in the ultraviolet (UV) region. Under normal conditions, 50 mg of the photocatalyst was added to 50 mL of 10 mg L−1 methylene orange (MO) aqueous solution in a reactor with a double layer so that the reaction could be cooled by circulating water to keep the temperature constant. Before irradiation, the liquid
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was dispersed with an ultrasound machine for 15 min and magnetically stirred for 15 min in the dark. Thus, the establishment of an adsorption/desorption equilibrium between the photocatalyst and MO could be guaranteed. After that, the suspension was illuminated by the Xe lamp combined with a UV autofilter under magnetic stirring at room temperature. A small aliquot of approximately 2 mL was sampled at given time intervals, centrifuged and filtered through a 0.22 μm inorganic membrane filter to remove the remaining ZnO1−x or ZnO nanoparticles. The content of MO at different time intervals could be monitored by the absorption peak at λ = 464 nm using a U-3900/3900H UV-Vis spectrophotometer (Hitachi). The initial concentration (C0) was considered to be the MO concentration after adsorption equilibrium. To establish the adsorption/desorption equilibrium of MO on the catalysts, blank experiments were also carried out using the same method by magnetic stirring in the dark for 30 minutes. The recycling experiments were performed for five consecutive cycles in order to test the durability. After each cycle, the catalyst was centrifuged and washed thoroughly with distilled water several times to remove residual dye impurities, and then dried at 60 °C ready for the next test. The photoelectrochemical test system was composed of a CHI 660D electrochemistry potentiostat (Shanghai Chenhua Limited, China), a 300 W xenon lamp with cutoff filters (λ ≥ 400 nm), and a home-made three-electrode cell with Pt wire as the counter electrode, Ag/AgCl as the reference electrode, and 1 M Na2SO4 as the electrolyte. ZnO1−x and ZnO electrodes were prepared by depositing suspensions (the concentration of ZnO1−x and ZnO dissolved by absolute distilled water was 10 mg mL−1) onto Ti foil using the drop casting method. The electrodes were annealed at 60 °C for 12 h. During the measurements, the electrodes were pressed against a □-shape of an electrochemical cell with a working area of 4.0 cm2. The detection of active hydroxyl radicals (·OH) upon visiblelight irradiation was also investigated in order to detect the active species during the photocatalytic reaction. A 50 mL aqueous solution containing 0.15 g of NaOH and 3 mM terephthalic acid was dispersed with an ultrasound machine, and then 10 mg of the sample was added. After irradiation for 10 min, 2 mL of the solution was collected and centrifuged. Excitation light wavelength of 320 nm was used for recording the fluorescence spectra. During the photoreactions, no oxygen was allowed to be bubbled into the suspension. Characterization Field emission scanning electron microscopy (FE-SEM) images were carried out using a JEOL JSM-6330F scanning electron microscope operating at a beam energy of 15.0 kV. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) were carried out using a JEOL JEM-ARF200F atomic resolution analytical microscope with an accelerating voltage of 200 kV. The phase structure of the product was characterized by X-ray powder diffraction (XRD) patterns using a Philips X’Pert Pro Super diffractometer with
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Cu Kα radiation (1.54178 Å) at 40 kV and 200 mA. UV-Vis absorption spectra were acquired with the use of a Shimadzu SOLID3700 spectrophotometer. The Fourier transform infrared (FT-IR) spectra were measured on a MAGNA-IR 750 (Nicolet Instrument Co., USA). Raman spectra were measured on a SPEX 1403 spectrometer equipped with an Ar+ laser at an excitation wavelength of 514.5 nm. X-ray photoelectron spectroscopy (XPS) measurements were obtained from the Photoemission Endstation in the National Synchrotron Radiation Laboratory (NSRL, Hefei, China). The electron paramagnetic resonance (EPR) spectra were recorded on a JEOL JES-FA200 EPR spectrometer (140 K, 9064 MHz, 0.998 mW, X-band).
Results and discussion In this study, we report the one-step synthesis of oxygendeficient ZnO1−x nanosheets (denoted as ZnO1−x NSs) through the oxidation of metal Zn under simple hydrothermal conditions. The crystal structure of the as-prepared gray-colored oxygen-deficient ZnO1−x NSs was investigated using the X-ray diffraction (XRD) pattern. As shown in Fig. 1, all the diffraction peaks can be readily indexed to the hexagonal wurtzite structure of ZnO crystals with calculated lattice parameters of a = b = 3.258 Å and c = 5.210 Å, according to Bragg’s law, and consistent with the standard diffraction data (JCPDS no. 36-1451). No characteristic peaks of any other phases or impurities are observed, indicating the high purity of the products. Under hydrothermal conditions, NH4HCO3 decomposes to CO2, which oxidizes metal Zn into ZnO, and the CO2 molecules are reduced to HCOOH.18 After calcination at 500 °C in air, the gray ZnO1−x NSs were oxidized to white ZnO with an irregular morphology (see the SEM image of ZnO in Fig. S2†). The microstructure of the obtained sample was examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction
Fig. 1 XRD pattern of the as-prepared ZnO1−x nanosheets. The standard card for hexagonal ZnO is shown at the bottom.
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Fig. 2 SEM images (a, b), TEM image (c), and HRTEM image (d) of the ZnO1−x nanosheets. Inset in (d) shows the corresponding SAED pattern.
(SAED). As shown in the SEM images in Fig. 2a and 2b, it is apparent that the obtained ZnO1−x displays a hierarchical nanostructure that is composed of numerous nanosheets with an average thickness of 30 nm and a side length in the range of 50–100 nm. TEM measurements were carried out to get further confirmation. As shown in Fig. 2c, the TEM image clearly shows the sheet-like morphology, which agrees well with the SEM observation. The HRTEM image (Fig. 2d) shows the high degree of [001] orientation, which was further confirmed by the corresponding SAED pattern image (inset in Fig. 2d). It is noted that the SAED pattern displays a spot pattern, indicating the single-crystalline characteristics of the obtained oxygen-deficient ZnO1−x nanosheets. The FT-IR spectra and Raman spectra of the ZnO1−x and ZnO samples (see Fig. S3, ESI†) give further details. Fig. S3a† shows strong absorption peaks at 1390 and 1505 cm−1. These signals are ascribed to the COO− group, suggesting that the product formic acid is adsorbed on the ZnO1−x surface.18 Moreover, no elemental C was detected in the Raman spectra (elemental C is very sensitive with typical peaks at 1360 and 1580 cm−1) (see Fig. S3b†). We also used dilute HCl to completely dissolve ZnO1−x, and we did not find elemental C residues. All these results demonstrate that no elemental C exists in the sample. Fig. 3a shows the UV-Vis absorption spectra of the asprepared ZnO1−x and ZnO in order to explore the optical response of the obtained samples. It can be clearly seen that the oxygendeficient ZnO1−x sample displays a gray color and a wide strong absorption in the visible light region. The considerably large absorption tail occurring in the visible to NIR regions supplies indisputable evidence that the ZnO1−x NSs contain a large number of oxygen vacancies.11 Electronic transitions from the valence band to the localized states, and from these isolated states to the conduction band, are responsible for the visible–NIR absorption (λ ≥ 400 nm) of the gray-colored ZnO1−x NSs. Theoretical studies have also demonstrated that a high
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Fig. 3 The UV-Vis diffuse reflectance spectra (a) and band-gap evaluation (b) from the plots of (αEphoton)2 vs. the energy of the absorbed light for the ZnO and ZnO1−x samples. The insets in (a) show the corresponding photographs.
oxygen vacancy concentration could form an electronic state vacancy band below the conduction band.11 In contrast, the oxygen defect-free ZnO sample obtained from the calcination of ZnO1−x at 500 °C in air shows a white color and only responds to ultraviolet light owing to its intrinsic wide band gap.19 It can be seen that the absorption edge of the ZnO1−x NSs exhibits a little red shift compared with defect-free ZnO, which indicates a slight decrease in the band gap energy. As zinc oxide is a direct semiconductor, the plots of (αhν)2 versus the energy of the absorbed light would give the band gap of the ZnO1−x NSs (Fig. 3b). This reveals that the band gap of the ZnO1−x is 3.15 eV, slightly smaller than that of the defect-free ZnO sample (3.25 eV), which can be attributed to the existence of predominating oxygen vacancies in the as-obtained ZnO1−x nanosheets. Combined with the results from the UV-Vis measurements, it can be speculated that oxygen vacancies induced visible-light absorption occurs due to isolated states in the forbidden gaps in the ZnO1−x NSs.20 Such an extension in the absorption to the visible light region is beneficial for solar light photocatalytic applications. Moreover, it is worth mentioning that the hierarchically structured ZnO1−x NSs are very stable in air because there was no significant color change after keeping the gray-colored ZnO1−x samples in air for at least one year. To examine the paramagnetic characteristics of the ZnO1−x NSs, EPR spectroscopy was performed. The defect-free ZnO is
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Fig. 4 The X-band EPR spectra of the ZnO1−x and ZnO samples recorded at T = 130 K (a). PL spectrum of the ZnO1−x nanosheets excited at 325 nm (b).
antiferromagnetic and no signal can be seen in the EPR spectrum, as shown in Fig. 4a. The gray ZnO1−x sample gives rise to a very strong EPR signal at g = 2.01, which has previously been identified as electrons trapped on surface O-vacancies.21 A surface oxygen vacancy is prone to adsorb atmospheric O2 molecules, which would be reduced to ·O2−, thus generating an EPR signal at g = 2.01.22 The EPR spectrum for the ZnO1−x sample recorded after the irradiation reaction and compared with that of the fresh ZnO1−x sample is shown in Fig. S4.† It can be obviously seen that the signal is almost identical after the reaction without distinct change, further confirming that our ZnO1−x sample is stable enough during the photocatalytic process. The ZnO1−x NSs had a large amount of oxygen vacancy defects in contrast to annealing the ZnO sample in air. Fig. 4b shows the solid-state room temperature photoluminescence (PL) spectrum of the as-prepared ZnO1−x. A sharp emission at 388 nm is observed, which represents the intrinsic emission from zinc oxide. Meanwhile, a strong wide band defect related emission from 450 nm to 600 nm, centred at 494 nm, is also detected. This yellow-green emission can be ascribed to a singly ionized surface oxygen vacancy,23 which is in good agreement with the UV-Vis and EPR measurements. X-ray photoelectron spectroscopy (XPS) of the ZnO1−x and ZnO samples was investigated to determine the elemental compositions and chemical states. The survey scan spectra
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(Fig. 5a) show obvious peaks for zinc and oxygen, and suggest that there are no impurities in the ZnO1−x and ZnO samples. Fig. 5b presents the high-resolution Zn 2p XPS spectrum of the ZnO1−x sample with one peak at 1020.86 eV for 2p3/2. The typical O 1s peak shows two peaks centered at 531.02 and 529.84 eV by fitting the experimental dots with two Gaussian curves (Fig. 5c). The component on the low binding energy side, accounting for almost 45% of the O 1s spectrum, can be assigned to O2− ions in the wurtzite structure with hexagonal Zn2+ ions arranged periodically. The high binding energy component accounting for the remaining 55% is attributed to oxygen deficient regions within the matrix of our ZnO1−x NSs sample.24 The Zn content (54.1%) is more than the O content (45.9%) in the ZnO1−x (x = 0.15) sample, calculated from the XPS analysis, which further demonstrates the deficiency of oxygen atoms in the ZnO1−x NSs. This is in good agreement with the energy dispersive spectroscopy (EDS) measurements (Fig. S5, ESI†). To investigate the energy band structure of ZnO1−x and ZnO, the valence band XPS spectra (VB-XPS) near the Fermi level were measured, as shown in Fig. 5d. The valence band edge is scaled to be 2.22 eV for ZnO and 2.90 eV for ZnO1−x. Combined with the results from the optical measurements, the ZnO sample displays a valence band (VB) with the edge of the maximum energy at about 2.22 eV and the conduction band (CB) minimum would occur at about −0.96 eV. Meanwhile, for the ZnO1−x NSs sample, the VB minimum of the ZnO1−x NSs down-shifts by 0.68 eV and occurs at 2.90 eV compared to that of ZnO. That is to say, the VB of the ZnO1−x NSs has a stronger water oxidation capability for generating active hydroxyl radical species, such as •OH, by reacting with
Fig. 5 The survey XPS spectra (a) of the ZnO1−x and ZnO samples. The Zn 2p spectrum of ZnO1−x (b) and O 1s spectrum of ZnO1−x (c). The VB-XPS spectra (d) of the ZnO1−x and ZnO samples.
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surface adsorbed water molecules and hydroxyl groups.25 It is the oxygen vacancy defects that give rise to the VB down-shift, and at the same time the isolated intermediate states formed by the introduction of oxygen vacancies can extend the visible light absorption range of the ZnO1−x NSs. Methyl orange (MO) was used as a model compound to evaluate the photocatalytic activity of ZnO1−x and ZnO under visible light illumination. A blank experiment shows that no obvious photo bleaching of MO was observed within 1 h of irradiation without a catalyst. Before the photocatalytic reactions, the suspensions were left in the dark with magnetic stirring for 30 min, in order to reach an adsorption/desorption equilibrium of MO on the catalysts. Then the photodegradation of the MO dye under visible-light irradiation was started. C/C0 was used to describe the degradation (C and C0 are the MO concentrations at time t and 0, respectively). As shown in Fig. 6a, it can be seen that the ZnO1−x NSs decompose 92.5% MO within 60 min, while only 3.4% was decomposed with the ZnO sample. There is no doubt that the ZnO1−x NSs exhibit superior photocatalytic activity over the ZnO sample. Total concentrations of MO were determined from the maximum absorption (λ = 464 nm) measurements in the UV-Vis spectra. The characteristic absorption peak of MO at 464 nm decreased
Fig. 6 Photodegradation of MO (a) over the ZnO1−x and ZnO samples under visible light irradiation and a blank experiment. C and C0 are the MO concentrations at time t and 0, respectively. UV-Vis spectra (b) of MO in aqueous ZnO1−x dispersions as a function of irradiation time with visible light irradiation (λ ≥ 400 nm).
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quickly under visible light irradiation, as clearly shown in Fig. 6b. The characteristic absorption peak at different times form a downward trend as time goes by. A photograph inset in Fig. 6b clearly displays the color changes of the MO solution as the reaction proceeds; it changes quickly from orange to almost clear and transparent within 60 min. The decomposition of MO could be assigned to a pseudo-first-order kinetic reaction with a simplified Langmuir–Hinshelwood model.26 As shown in Fig. S6,† the kinetic constant for the ZnO1−x NSs is 68 times higher than that of the defect-free ZnO sample. The significant photocatalytic enhancement is due to the considerable increase in the visible light absorption and the efficient separation of electron–hole pairs, which is exclusively achieved by oxygen vacancy defects.27 To further study the enrichment of the photocatalytic activity, we used several on–off cycles of intermittent visible light irradiation to test the transient photocurrent responses of the ZnO1−x and ZnO samples. The photocurrent response experiments were carried out on a photoelectrochemical test device fabricated by drop casting ZnO1−x and ZnO dispersed alcohol solutions onto Ti foil. A series of I–T curves of the two electrodes were compared. Fig. 7 shows the photocurrent response supporting the result of photocatalytic activity very well. The photocurrent response of stoichiometric ZnO reveals a low efficiency of photon-to-photocurrent conversion under several on/off irradiation cycles, which is in agreement with a low MO degradation efficiency. The photocurrent conversion efficiency of the ZnO1−x NSs is about 12 times higher than that of ZnO. This much larger photocurrent for the ZnO1−x sample means a higher photoelectron transfer efficiency owing to enhanced visible light absorption and lower recombination of photogenerated electrons and holes, and these factors eventually contribute to the outstanding photocatalytic activity.28 The above experimental results have clearly demonstrated the advantages of the oxygen deficient ZnO1−x NSs for highly improved photocatalytic activity. Upon photoexcitation, holes with sufficient oxidation power in the VB, or in localized states within the band gap, can
either be directly involved in the photocatalytic degradation reactions or, alternatively, generate active species, such as hydroxyl radicals (·OH), by reacting with surface adsorbed water and hydroxyl groups.12 We chose the reaction between terephthalic acid (TA) and ·OH species in basic solution to generate 2-hydroxy terephthalic acid (TAOH), which emits a strong fluorescence band with a maximum intensity at 425 nm. Fig. 8a shows that significant fluorescence signals associated with TAOH were generated upon visible-light irradiation of the ZnO1−x NSs suspended in a TA solution for 10 min. It was found that the fluorescence intensity increases linearly with irradiation time for the ZnO1−x sample. Nevertheless, the slope of the reaction with the ZnO1−x NSs is much higher than that with the ZnO sample. Obviously, the formation rate and content of ·OH on the ZnO1−x NSs is significantly higher than on the defect-free ZnO sample. Such a result is largely correlated with the optical absorption properties mentioned above. More photo-generated holes and ·OH are produced for ZnO1−x because more visible light is harvested. This result clearly demonstrates that the excellent visible-light-driven photocatalytic activity for the ZnO1−x NSs could be considered to be due to accompanying oxygen vacancies, which result in the high photocatalytic activity efficiency.29
Fig. 7 Current density response vs. time for the ZnO1−x and ZnO samples under intermittent irradiation at a bias potential of 0.5 V vs. Ag/ AgCl (λ ≥ 400 nm).
Fig. 8 Time dependence of the fluorescence intensity at 425 nm (a) and fluorescence spectra (b) for the visible-light irradiation of ZnO1−x and ZnO in terephthalic acid (3 mM) solution.
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In order to verify the stability and reusability of ZnO1−x, cycling experiments were done by evaluating the decreased concentration of MO under the same reaction conditions during cycling tests. These experiments for five cycles show that the degradation rate was almost constant, confirming that the ZnO1−x photocatalyst is inherently stable during prolonged photocatalytic reactions. As shown in Fig. 9, it can be clearly seen that our recovered ZnO1−x sample exhibits a high activity and good recyclability for the degradation of MO. The removal efficiency of MO was found to be 92.5% and 82% for the first and final cycle, respectively, demonstrating that our ZnO1−x NSs photocatalyst is stable throughout repeated photocatalytic reactions. Taken together, several factors have been considered for the improved photocatalytic activity of the ZnO1−x NSs to clarify the photocatalysis mechanism. As shown in Scheme 1, the influence of the oxygen vacancies on the band structures of the as-prepared ZnO1−x sample can be comprehended as follows. While the band gap does not change significantly with
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the existence of oxygen vacancies, self-doping and the accompanying oxygen vacancies can generate isolated states in the forbidden gap.11 These defects are unoccupied states and usually act as an electron acceptor. The electrons in the VB can be excited to these unoccupied sites. Meanwhile, electrons in these isolated sites could be excited to the CB by visible light illumination. Electronic transitions from the VB to the localized states, and from these isolated states to the CB, are responsible for the visible–NIR absorption (λ ≥ 400 nm) of the gray-colored ZnO1−x NSs. Additionally, the down-shifted VB is beneficial for the separation of the charge carriers, since the VB width intrinsically controls the mobility of the holes. The down shift of the VB maximum not only makes the photogenerated holes more positive to react with molecular water to generate ·OH radicals, but also promotes the transfer of photoexcited holes to reactants, resulting in the inhibition of the electron–hole pair recombination.30 The down-shifted VB results in the higher mobility of the holes generated, which leads to a better photo-oxidation of the holes. The photo excited holes can trap surface water or hydroxide to form reactive species, such as ·OH, to attack the organic pollutants until the dye molecules are completely degraded due to its high aggressiveness, which could be considered as a key point in the mechanism of the improved photocatalytic performance.14,31 As a result, ultrathin ZnO1−x nanosheets with abundant oxygen vacancies show much superior photocatalytic activity over the defect-free ZnO sample.
Conclusions
Fig. 9 Cycling runs for the photodegradation of MO in the presence of ZnO1−x under visible light irradiation.
Scheme 1 Schematic photocatalytic reaction processes and charge transfer of the ZnO1−x and ZnO photocatalyst under visible-light illumination.
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In summary, a new kind of oxygen ZnO1−x photocatalyst with high visible light photocatalytic activity has been successfully fabricated via a facile hydrothermal reaction. Our novel synthetic route for the mass production of oxygen-deficient ZnO1−x is of great significance, and may be applied to other nonstoichiometric semiconductors for versatile use. The ZnO1−x nanosheets with yellow-green photoluminescence show superior visible-light-driven photocatalytic degradation of methyl orange compared to stoichiometric ZnO, which is only sensitive to UV light owing to its intrinsic large band gap. The incorporation of oxygen vacancy defects can be an effective way to extend the light absorption of ZnO1−x into the visiblelight region due to the fact that the energy of the localized state is located in the forbidden gap. Moreover, the ZnO1−x NSs exhibit a much higher catalytic activity and stability than ZnO, and no obvious decrease in the photocatalytic activity was observed even after 5 cycles. We attribute this superior photocatalytic performance to a high content of generated hydroxyl radicals with a strong photo-oxidation capability, which reflects the enhanced separation and migration of the photogenerated electrons and holes in these oxygen-deficient ZnO1−x NSs. It can be expected that our synthetic strategy to form oxygen deficient ZnO1−x NSs could be extended to other non-stoichiometric metal oxide materials with enhanced properties for other advanced applications.
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Acknowledgements
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A. W. Xu acknowledgements the special funding support from the Scientific Research Grant of Hefei Science Center of CAS (2015SRG-HSC048), the National Basic Research Program of China (2011CB933700) and the National Natural Science Foundation of China (21271165).
Notes and references 1 (a) A. Fujishima and K. Honda, Nature, 1972, 238, 37–38; (b) G. M. Wang, Y. C. Ling and Y. Li, Nanoscale, 2012, 4, 6682–6691. 2 M. Fernández-García, A. Martínez-Arias, J. C. Hanson and J. A. Rodriguez, Chem. Rev., 2004, 104, 4063–4104. 3 D. V. Talapin, J. S. Lee, M. V. Kovalenko and E. V. Shevchenko, Chem. Rev., 2010, 110, 389–458. 4 J. H. Lim, C. K. Kang, K. K. Kim, I. K. Park, D. K. Hwang and S. J. Park, Adv. Mater., 2006, 18, 2720–2724. 5 A. W. Xu, Y. Gao and H. Q. Liu, J. Catal., 2002, 207, 151– 157. 6 (a) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269–271; (b) T. L. Thompson and J. T. Yates, Chem. Rev., 2006, 106, 4428–4453. 7 (a) T. R. Gordon, M. Cargnello, T. Paik, F. Mangolini, R. T. Weber, P. Fornasiero and C. B. Murray, J. Am. Chem. Soc., 2012, 134, 6751–6761; (b) X. Liu, S. M. Gao, H. Xu, Z. Z. Lou, W. J. Wang, B. B. Huang and Y. Dai, Nanoscale, 2013, 5, 1870–1875. 8 G. M. Wang, H. Y. Wang, Y. C. Ling, Y. C. Tang, X. Y. Yang, R. C. Fitzmorris, C. C. Wang, J. Z. Zhang and Y. Li, Nano Lett., 2011, 11, 3026–3033. 9 Z. K. Zheng, B. B. Huang, X. D. Meng, J. P. Wang, S. Y. Wang, Z. Z. Lou, Z. Y. Wang, X. Y. Qin, X. Y. Zhang and Y. Dai, Chem. Commun., 2013, 49, 868–870. 10 Z. Wang, C. Y. Yang, T. Q. Lin, H. Yin, P. Chen, D. Y. Wan, F. F. Xu, F. Q. Huang, J. H. Lin, X. M. Xie and M. H. Jiang, Energy Environ. Sci., 2013, 6, 3007–3014. 11 Q. Zhu, Y. Peng, L. Lin, C. M. Fan, G. Q. Gao, R. X. Wang and A. W. Xu, J. Mater. Chem. A, 2014, 2, 4429–4437. 12 C. M. Fan, Y. Peng, Q. Zhu, L. Lin, R. X. Wang and A. W. Xu, J. Phys. Chem. C, 2013, 117, 24157–24166. 13 Y. Peng, Y. Wang, Q. G. Chen, Q. Zhu and A. W. Xu, CrystEngComm, 2014, 16, 7906–7913.
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14 Z. K. Zheng, B. B. Huang, X. Y. Qin, X. Y. Zhang, Y. Dai and M. H. Whangbo, J. Mater. Chem., 2011, 21, 9079– 9087. 15 I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara and K. Takeuchi, J. Mol. Catal. A: Chem., 2000, 161, 205– 212. 16 T. L. Mercier, J. M. Mariot, P. Parent, M. F. Fontaine, C. F. Hague and M. Quarton, Appl. Surf. Sci., 1995, 86, 382– 386. 17 J. Jun, M. Dhayal, J. H. Shin, J. C. Kim and N. Getff, Radiat. Phys. Chem., 2006, 75, 583–589. 18 F. M. Jin, X. Zeng, J. K. Liu, Y. J. Jin, L. Y. Wang, H. Zhong, G. D. Yao and Z. B. Huo, Sci. Rep., 2014, 4, 4503. 19 A. M. Ali, E. A. C. Emanuelsson and D. A. Patterson, Appl. Catal., B, 2010, 97, 168–181. 20 (a) L. Liu and X. B. Chen, Chem. Rev., 2014, 114, 9890– 9918; (b) T. Xia, P. Wallenmeyer, A. Anderson, J. Murowchick, L. Liu and X. B. Chen, RSC Adv., 2014, 4, 41654–41658. 21 I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara and K. Takeuchi, J. Mol. Catal. A: Chem., 2000, 161, 205– 212. 22 X. X. Zou, J. K. Liu, J. Su, F. Zuo, J. S. Chen and P. Y. Feng, Chem. – Eur. J., 2013, 19, 2866–2873. 23 R. Q. Song, A. W. Xu, B. Deng, Q. Li and G. Y. Chen, Adv. Funct. Mater., 2007, 17, 296–306. 24 S. M. Park, T. Ikegami and K. Ebihara, Thin Solid Films, 2006, 513, 90–94. 25 Q. Zhu, W. S. Wang, L. Lin, G. Q. Gao, H. L. Guo, H. Du and A. W. Xu, J. Phys. Chem. C, 2013, 117, 5894–5900. 26 Y. H. Lv, W. Q. Yao, X. G. Ma, C. S. Pan, R. L. Zong and Y. F. Zhu, Catal. Sci. Technol., 2013, 3, 3136–3146. 27 F. Zuo, K. Bozhilov, R. J. Dillon, L. Wang, P. Smith, X. Zhao, C. Bardeen and P. Y. Feng, Angew. Chem., 2012, 124, 6327– 6330. 28 L. Jia, D. H. Wang, Y. X. Huang, A. W. Xu and H. Q. Yu, J. Phys. Chem. C, 2011, 115, 11466–11473. 29 Z. K. Zheng, B. B. Huang, X. D. Meng, J. P. Wang, S. Y. Wang, Z. Z. Lou, Z. Y. Wang, X. Y. Qin, X. Y. Zhang and Y. Dai, Chem. Commun., 2013, 49, 868–870. 30 M. L. Guan, C. Xiao, J. Zhang, S. J. Fan, R. An, Q. M. Cheng, J. F. Xie, M. Zhou, B. J. Ye and Y. Xie, J. Am. Chem. Soc., 2013, 135, 10411–10417. 31 A. Dodd, A. McKinley, T. Tsuzuki and M. Saunders, Mater. Chem. Phys., 2009, 114, 382–386.
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Cite this: DOI: 10.1039/c5nr00271k
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Oxygen deficient ZnO1−x nanosheets with high visible light photocatalytic activity† Hong-Li Guo,‡ Qing Zhu,‡ Xi-Lin Wu, Yi-Fan Jiang, Xiao Xie and An-Wu Xu* Zinc oxide is one of the most important wide-band-gap (3.2 eV) materials with versatile properties, however, it can not be excited by visible light. In this work, we have developed an exquisite and simple way to prepare oxygen-deficient ZnO1−x nanosheets with a gray-colored appearance and excellent visible light photocatalytic activity. Detailed analysis based on UV-Vis absorption spectra, X-band electron paramagnetic resonance (EPR) spectra, and photoluminescence (PL) spectra confirms the existence of oxygen vacancies in ZnO1−x. The incorporation of oxygen defects could effectively extend the light absorption of ZnO1−x into the visible-light region due to the fact that the energy of the localized state is located in the forbidden gap. Thus, our obtained ZnO1−x shows a higher photodegradation of methyl orange (MO) compared to defect-free ZnO under visible light illumination. Additionally, the high content of •OH radicals
Received 13th January 2015, Accepted 14th March 2015 DOI: 10.1039/c5nr00271k www.rsc.org/nanoscale
with a strong photo-oxidation capability over the ZnO1−x nanosheets significantly contributes to the improvement in the photocatalytic performance. Our oxygen deficient ZnO1−x sample shows a very high photocatalytic activity for the degradation of MO even after 5 cycles without any obvious decline. The results demonstrate that defect engineering is a powerful tool to enhance the optoelectronic and photocatalytic performances of nanomaterials.
Introduction Photocatalysis technology has attracted enormous interest in organic wastewater treatment for environmental redemption because it is an inexpensive and convenient method that can totally decompose organic pollutants into small molecules (H2O, CO2, etc.).1 It is particularly difficult to find a stable semiconductor system with a suitable band gap for visible light absorption and driving the subsequent redox reactions. Zinc oxide (ZnO) is one of the most important wide-band-gap materials (3.2 eV) with versatile properties, such as high transparency in the visible wavelength region, near-UV emission, transparent conductivity, a high piezoelectric constant and a large excitation binding energy (∼60 meV) at room temperature. Using photocatalysis, ZnO nanomaterials find potential use in organic wastewater treatment.2,3 However, the relatively wide band gap significantly limits their photocatalytic applications using solar light because UV light only accounts for about 4% of the total solar spectrum.4 Thus, the exploration of facile, mild, and effective routes for the rational synthesis of
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, P. R. China. E-mail: [email protected]; Fax: (+86) 551-6360 2346 † Electronic supplementary information (ESI) available: Additional figures. See DOI: 10.1039/c5nr00271k ‡ These authors contributed equally to this work.
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visible-light responsive catalysts with superior photocatalytic activity is highly desired. Tremendous efforts have been made to regulate the band structure of ZnO, making it sensitive to visible light. Several studies have been reported where the absorption edge is shifted towards the visible light region by doping with metal ions or non-metal or rare earth elements.5 However, recent studies have both theoretically and experimentally demonstrated that doping does not inevitably translate into a higher photocatalytic activity since the impurity or defect states can trap electrons and facilitate non-radiative recombination, which will reduce the photocatalytic performance.6–10 Bearing these facts in mind, our group has recently developed a series of non-stoichiometric semiconductor-based photocatalysts, including oxygen-deficient TiO2−x, SnO2−x and ZnO nanoparticles, which display excellent visible light photocatalytic activities.11–13 The distortion of the crystal lattice provides an ideal platform for reducing the recombination of the photogenerated electrons and holes to improve the photoconversion efficiency, thus showing excellent catalytic activity and stability under visible-light irradiation. Therefore, it is highly desirable to design and tune the surface chemistry and oxygen vacancies of wide-band-gap semiconductors to trigger a response to visible light, which accounts for 43% of the spectrum of sunlight. Several methods have been employed to produce semiconductors containing oxygen vacancy species such as UV irradiation,14 plasma treatment,15 laser,16 or γ-ray17 bombardment.
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Although these routes have their own advantages, they are limited by high costs and complicated procedures, and they are not suitable for large-scale production. Moreover, there have been few reports paying close attention to the relationship between oxygen vacancies and photocatalytic activity. The understanding of the defect-dependent visible-light-driven photocatalytic mechanism still remains elusive. Herein, we report a facile hydrothermal approach for the mass-production of ZnO1−x nanosheets with abundant oxygen vacancies. Benefiting from the introduction of oxygen vacancies, the visible light photocatalytic activity and photocurrent of ZnO1−x have been significantly improved compared with oxygen vacancyfree ZnO. The mechanism of visible light photocatalysis and the charge transfer process is also proposed. This work has developed a novel approach to enhance the photocatalytic performance and utilization of ZnO as a visible light photocatalyst for potential use in highly efficient solar energy conversion devices with a low cost.
Experimental section Photocatalyst preparation All reagents were analytical grade and were used without any further purification. For the synthesis of the ZnO1−x nanosheets (NSs), 2 mmol of zinc chloride (ZnCl2) was dissolved in distilled water (15 mL) with vigorous stirring for 5 min, and then metal zinc powder (1.6 mmol) was added into the zinc chloride solution. After stirring for another 5 min, 2 mmol of ammonium bicarbonate (NH4HCO3) was added (NH4HCO3 was used as a CO2 source for simplicity). The mixed solution was transferred into a 25 mL Teflon-lined stainless steel autoclave, and then heated at 180 °C for 15 h in an oven. After the reaction was complete, the sample was thoroughly washed with distilled water and ethanol three times until the pH = 7 and dried at 60 °C in a vacuum oven. Finally, the gray-colored ZnO1−x product was harvested. It is worth mentioning that ZnO1−x can be fabricated on a large scale by using a large autoclave (1000 mL) for the hydrothermal treatment, and gray ZnO1−x can be obtained on the gram level (see photograph in Fig. S1†). A stoichiometric white ZnO sample was prepared by calcining the ZnO1−x NSs at 500 °C for 2 h in air. Photocatalytic activity measurements Under exterior irradiation for a certain period of time, the photocatalytic activities of the samples were evaluated from the photocatalytic decomposition of methyl orange (MO) at room temperature. The light source was a 300 W Xe lamp (PLS-SXE300/300UV, Trusttech Co., Ltd., Beijing) equipped with an ultraviolet cutoff filter to block the light in the ultraviolet (UV) region. Under normal conditions, 50 mg of the photocatalyst was added to 50 mL of 10 mg L−1 methylene orange (MO) aqueous solution in a reactor with a double layer so that the reaction could be cooled by circulating water to keep the temperature constant. Before irradiation, the liquid
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was dispersed with an ultrasound machine for 15 min and magnetically stirred for 15 min in the dark. Thus, the establishment of an adsorption/desorption equilibrium between the photocatalyst and MO could be guaranteed. After that, the suspension was illuminated by the Xe lamp combined with a UV autofilter under magnetic stirring at room temperature. A small aliquot of approximately 2 mL was sampled at given time intervals, centrifuged and filtered through a 0.22 μm inorganic membrane filter to remove the remaining ZnO1−x or ZnO nanoparticles. The content of MO at different time intervals could be monitored by the absorption peak at λ = 464 nm using a U-3900/3900H UV-Vis spectrophotometer (Hitachi). The initial concentration (C0) was considered to be the MO concentration after adsorption equilibrium. To establish the adsorption/desorption equilibrium of MO on the catalysts, blank experiments were also carried out using the same method by magnetic stirring in the dark for 30 minutes. The recycling experiments were performed for five consecutive cycles in order to test the durability. After each cycle, the catalyst was centrifuged and washed thoroughly with distilled water several times to remove residual dye impurities, and then dried at 60 °C ready for the next test. The photoelectrochemical test system was composed of a CHI 660D electrochemistry potentiostat (Shanghai Chenhua Limited, China), a 300 W xenon lamp with cutoff filters (λ ≥ 400 nm), and a home-made three-electrode cell with Pt wire as the counter electrode, Ag/AgCl as the reference electrode, and 1 M Na2SO4 as the electrolyte. ZnO1−x and ZnO electrodes were prepared by depositing suspensions (the concentration of ZnO1−x and ZnO dissolved by absolute distilled water was 10 mg mL−1) onto Ti foil using the drop casting method. The electrodes were annealed at 60 °C for 12 h. During the measurements, the electrodes were pressed against a □-shape of an electrochemical cell with a working area of 4.0 cm2. The detection of active hydroxyl radicals (·OH) upon visiblelight irradiation was also investigated in order to detect the active species during the photocatalytic reaction. A 50 mL aqueous solution containing 0.15 g of NaOH and 3 mM terephthalic acid was dispersed with an ultrasound machine, and then 10 mg of the sample was added. After irradiation for 10 min, 2 mL of the solution was collected and centrifuged. Excitation light wavelength of 320 nm was used for recording the fluorescence spectra. During the photoreactions, no oxygen was allowed to be bubbled into the suspension. Characterization Field emission scanning electron microscopy (FE-SEM) images were carried out using a JEOL JSM-6330F scanning electron microscope operating at a beam energy of 15.0 kV. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) were carried out using a JEOL JEM-ARF200F atomic resolution analytical microscope with an accelerating voltage of 200 kV. The phase structure of the product was characterized by X-ray powder diffraction (XRD) patterns using a Philips X’Pert Pro Super diffractometer with
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Cu Kα radiation (1.54178 Å) at 40 kV and 200 mA. UV-Vis absorption spectra were acquired with the use of a Shimadzu SOLID3700 spectrophotometer. The Fourier transform infrared (FT-IR) spectra were measured on a MAGNA-IR 750 (Nicolet Instrument Co., USA). Raman spectra were measured on a SPEX 1403 spectrometer equipped with an Ar+ laser at an excitation wavelength of 514.5 nm. X-ray photoelectron spectroscopy (XPS) measurements were obtained from the Photoemission Endstation in the National Synchrotron Radiation Laboratory (NSRL, Hefei, China). The electron paramagnetic resonance (EPR) spectra were recorded on a JEOL JES-FA200 EPR spectrometer (140 K, 9064 MHz, 0.998 mW, X-band).
Results and discussion In this study, we report the one-step synthesis of oxygendeficient ZnO1−x nanosheets (denoted as ZnO1−x NSs) through the oxidation of metal Zn under simple hydrothermal conditions. The crystal structure of the as-prepared gray-colored oxygen-deficient ZnO1−x NSs was investigated using the X-ray diffraction (XRD) pattern. As shown in Fig. 1, all the diffraction peaks can be readily indexed to the hexagonal wurtzite structure of ZnO crystals with calculated lattice parameters of a = b = 3.258 Å and c = 5.210 Å, according to Bragg’s law, and consistent with the standard diffraction data (JCPDS no. 36-1451). No characteristic peaks of any other phases or impurities are observed, indicating the high purity of the products. Under hydrothermal conditions, NH4HCO3 decomposes to CO2, which oxidizes metal Zn into ZnO, and the CO2 molecules are reduced to HCOOH.18 After calcination at 500 °C in air, the gray ZnO1−x NSs were oxidized to white ZnO with an irregular morphology (see the SEM image of ZnO in Fig. S2†). The microstructure of the obtained sample was examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction
Fig. 1 XRD pattern of the as-prepared ZnO1−x nanosheets. The standard card for hexagonal ZnO is shown at the bottom.
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Fig. 2 SEM images (a, b), TEM image (c), and HRTEM image (d) of the ZnO1−x nanosheets. Inset in (d) shows the corresponding SAED pattern.
(SAED). As shown in the SEM images in Fig. 2a and 2b, it is apparent that the obtained ZnO1−x displays a hierarchical nanostructure that is composed of numerous nanosheets with an average thickness of 30 nm and a side length in the range of 50–100 nm. TEM measurements were carried out to get further confirmation. As shown in Fig. 2c, the TEM image clearly shows the sheet-like morphology, which agrees well with the SEM observation. The HRTEM image (Fig. 2d) shows the high degree of [001] orientation, which was further confirmed by the corresponding SAED pattern image (inset in Fig. 2d). It is noted that the SAED pattern displays a spot pattern, indicating the single-crystalline characteristics of the obtained oxygen-deficient ZnO1−x nanosheets. The FT-IR spectra and Raman spectra of the ZnO1−x and ZnO samples (see Fig. S3, ESI†) give further details. Fig. S3a† shows strong absorption peaks at 1390 and 1505 cm−1. These signals are ascribed to the COO− group, suggesting that the product formic acid is adsorbed on the ZnO1−x surface.18 Moreover, no elemental C was detected in the Raman spectra (elemental C is very sensitive with typical peaks at 1360 and 1580 cm−1) (see Fig. S3b†). We also used dilute HCl to completely dissolve ZnO1−x, and we did not find elemental C residues. All these results demonstrate that no elemental C exists in the sample. Fig. 3a shows the UV-Vis absorption spectra of the asprepared ZnO1−x and ZnO in order to explore the optical response of the obtained samples. It can be clearly seen that the oxygendeficient ZnO1−x sample displays a gray color and a wide strong absorption in the visible light region. The considerably large absorption tail occurring in the visible to NIR regions supplies indisputable evidence that the ZnO1−x NSs contain a large number of oxygen vacancies.11 Electronic transitions from the valence band to the localized states, and from these isolated states to the conduction band, are responsible for the visible–NIR absorption (λ ≥ 400 nm) of the gray-colored ZnO1−x NSs. Theoretical studies have also demonstrated that a high
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Fig. 3 The UV-Vis diffuse reflectance spectra (a) and band-gap evaluation (b) from the plots of (αEphoton)2 vs. the energy of the absorbed light for the ZnO and ZnO1−x samples. The insets in (a) show the corresponding photographs.
oxygen vacancy concentration could form an electronic state vacancy band below the conduction band.11 In contrast, the oxygen defect-free ZnO sample obtained from the calcination of ZnO1−x at 500 °C in air shows a white color and only responds to ultraviolet light owing to its intrinsic wide band gap.19 It can be seen that the absorption edge of the ZnO1−x NSs exhibits a little red shift compared with defect-free ZnO, which indicates a slight decrease in the band gap energy. As zinc oxide is a direct semiconductor, the plots of (αhν)2 versus the energy of the absorbed light would give the band gap of the ZnO1−x NSs (Fig. 3b). This reveals that the band gap of the ZnO1−x is 3.15 eV, slightly smaller than that of the defect-free ZnO sample (3.25 eV), which can be attributed to the existence of predominating oxygen vacancies in the as-obtained ZnO1−x nanosheets. Combined with the results from the UV-Vis measurements, it can be speculated that oxygen vacancies induced visible-light absorption occurs due to isolated states in the forbidden gaps in the ZnO1−x NSs.20 Such an extension in the absorption to the visible light region is beneficial for solar light photocatalytic applications. Moreover, it is worth mentioning that the hierarchically structured ZnO1−x NSs are very stable in air because there was no significant color change after keeping the gray-colored ZnO1−x samples in air for at least one year. To examine the paramagnetic characteristics of the ZnO1−x NSs, EPR spectroscopy was performed. The defect-free ZnO is
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Fig. 4 The X-band EPR spectra of the ZnO1−x and ZnO samples recorded at T = 130 K (a). PL spectrum of the ZnO1−x nanosheets excited at 325 nm (b).
antiferromagnetic and no signal can be seen in the EPR spectrum, as shown in Fig. 4a. The gray ZnO1−x sample gives rise to a very strong EPR signal at g = 2.01, which has previously been identified as electrons trapped on surface O-vacancies.21 A surface oxygen vacancy is prone to adsorb atmospheric O2 molecules, which would be reduced to ·O2−, thus generating an EPR signal at g = 2.01.22 The EPR spectrum for the ZnO1−x sample recorded after the irradiation reaction and compared with that of the fresh ZnO1−x sample is shown in Fig. S4.† It can be obviously seen that the signal is almost identical after the reaction without distinct change, further confirming that our ZnO1−x sample is stable enough during the photocatalytic process. The ZnO1−x NSs had a large amount of oxygen vacancy defects in contrast to annealing the ZnO sample in air. Fig. 4b shows the solid-state room temperature photoluminescence (PL) spectrum of the as-prepared ZnO1−x. A sharp emission at 388 nm is observed, which represents the intrinsic emission from zinc oxide. Meanwhile, a strong wide band defect related emission from 450 nm to 600 nm, centred at 494 nm, is also detected. This yellow-green emission can be ascribed to a singly ionized surface oxygen vacancy,23 which is in good agreement with the UV-Vis and EPR measurements. X-ray photoelectron spectroscopy (XPS) of the ZnO1−x and ZnO samples was investigated to determine the elemental compositions and chemical states. The survey scan spectra
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(Fig. 5a) show obvious peaks for zinc and oxygen, and suggest that there are no impurities in the ZnO1−x and ZnO samples. Fig. 5b presents the high-resolution Zn 2p XPS spectrum of the ZnO1−x sample with one peak at 1020.86 eV for 2p3/2. The typical O 1s peak shows two peaks centered at 531.02 and 529.84 eV by fitting the experimental dots with two Gaussian curves (Fig. 5c). The component on the low binding energy side, accounting for almost 45% of the O 1s spectrum, can be assigned to O2− ions in the wurtzite structure with hexagonal Zn2+ ions arranged periodically. The high binding energy component accounting for the remaining 55% is attributed to oxygen deficient regions within the matrix of our ZnO1−x NSs sample.24 The Zn content (54.1%) is more than the O content (45.9%) in the ZnO1−x (x = 0.15) sample, calculated from the XPS analysis, which further demonstrates the deficiency of oxygen atoms in the ZnO1−x NSs. This is in good agreement with the energy dispersive spectroscopy (EDS) measurements (Fig. S5, ESI†). To investigate the energy band structure of ZnO1−x and ZnO, the valence band XPS spectra (VB-XPS) near the Fermi level were measured, as shown in Fig. 5d. The valence band edge is scaled to be 2.22 eV for ZnO and 2.90 eV for ZnO1−x. Combined with the results from the optical measurements, the ZnO sample displays a valence band (VB) with the edge of the maximum energy at about 2.22 eV and the conduction band (CB) minimum would occur at about −0.96 eV. Meanwhile, for the ZnO1−x NSs sample, the VB minimum of the ZnO1−x NSs down-shifts by 0.68 eV and occurs at 2.90 eV compared to that of ZnO. That is to say, the VB of the ZnO1−x NSs has a stronger water oxidation capability for generating active hydroxyl radical species, such as •OH, by reacting with
Fig. 5 The survey XPS spectra (a) of the ZnO1−x and ZnO samples. The Zn 2p spectrum of ZnO1−x (b) and O 1s spectrum of ZnO1−x (c). The VB-XPS spectra (d) of the ZnO1−x and ZnO samples.
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surface adsorbed water molecules and hydroxyl groups.25 It is the oxygen vacancy defects that give rise to the VB down-shift, and at the same time the isolated intermediate states formed by the introduction of oxygen vacancies can extend the visible light absorption range of the ZnO1−x NSs. Methyl orange (MO) was used as a model compound to evaluate the photocatalytic activity of ZnO1−x and ZnO under visible light illumination. A blank experiment shows that no obvious photo bleaching of MO was observed within 1 h of irradiation without a catalyst. Before the photocatalytic reactions, the suspensions were left in the dark with magnetic stirring for 30 min, in order to reach an adsorption/desorption equilibrium of MO on the catalysts. Then the photodegradation of the MO dye under visible-light irradiation was started. C/C0 was used to describe the degradation (C and C0 are the MO concentrations at time t and 0, respectively). As shown in Fig. 6a, it can be seen that the ZnO1−x NSs decompose 92.5% MO within 60 min, while only 3.4% was decomposed with the ZnO sample. There is no doubt that the ZnO1−x NSs exhibit superior photocatalytic activity over the ZnO sample. Total concentrations of MO were determined from the maximum absorption (λ = 464 nm) measurements in the UV-Vis spectra. The characteristic absorption peak of MO at 464 nm decreased
Fig. 6 Photodegradation of MO (a) over the ZnO1−x and ZnO samples under visible light irradiation and a blank experiment. C and C0 are the MO concentrations at time t and 0, respectively. UV-Vis spectra (b) of MO in aqueous ZnO1−x dispersions as a function of irradiation time with visible light irradiation (λ ≥ 400 nm).
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quickly under visible light irradiation, as clearly shown in Fig. 6b. The characteristic absorption peak at different times form a downward trend as time goes by. A photograph inset in Fig. 6b clearly displays the color changes of the MO solution as the reaction proceeds; it changes quickly from orange to almost clear and transparent within 60 min. The decomposition of MO could be assigned to a pseudo-first-order kinetic reaction with a simplified Langmuir–Hinshelwood model.26 As shown in Fig. S6,† the kinetic constant for the ZnO1−x NSs is 68 times higher than that of the defect-free ZnO sample. The significant photocatalytic enhancement is due to the considerable increase in the visible light absorption and the efficient separation of electron–hole pairs, which is exclusively achieved by oxygen vacancy defects.27 To further study the enrichment of the photocatalytic activity, we used several on–off cycles of intermittent visible light irradiation to test the transient photocurrent responses of the ZnO1−x and ZnO samples. The photocurrent response experiments were carried out on a photoelectrochemical test device fabricated by drop casting ZnO1−x and ZnO dispersed alcohol solutions onto Ti foil. A series of I–T curves of the two electrodes were compared. Fig. 7 shows the photocurrent response supporting the result of photocatalytic activity very well. The photocurrent response of stoichiometric ZnO reveals a low efficiency of photon-to-photocurrent conversion under several on/off irradiation cycles, which is in agreement with a low MO degradation efficiency. The photocurrent conversion efficiency of the ZnO1−x NSs is about 12 times higher than that of ZnO. This much larger photocurrent for the ZnO1−x sample means a higher photoelectron transfer efficiency owing to enhanced visible light absorption and lower recombination of photogenerated electrons and holes, and these factors eventually contribute to the outstanding photocatalytic activity.28 The above experimental results have clearly demonstrated the advantages of the oxygen deficient ZnO1−x NSs for highly improved photocatalytic activity. Upon photoexcitation, holes with sufficient oxidation power in the VB, or in localized states within the band gap, can
either be directly involved in the photocatalytic degradation reactions or, alternatively, generate active species, such as hydroxyl radicals (·OH), by reacting with surface adsorbed water and hydroxyl groups.12 We chose the reaction between terephthalic acid (TA) and ·OH species in basic solution to generate 2-hydroxy terephthalic acid (TAOH), which emits a strong fluorescence band with a maximum intensity at 425 nm. Fig. 8a shows that significant fluorescence signals associated with TAOH were generated upon visible-light irradiation of the ZnO1−x NSs suspended in a TA solution for 10 min. It was found that the fluorescence intensity increases linearly with irradiation time for the ZnO1−x sample. Nevertheless, the slope of the reaction with the ZnO1−x NSs is much higher than that with the ZnO sample. Obviously, the formation rate and content of ·OH on the ZnO1−x NSs is significantly higher than on the defect-free ZnO sample. Such a result is largely correlated with the optical absorption properties mentioned above. More photo-generated holes and ·OH are produced for ZnO1−x because more visible light is harvested. This result clearly demonstrates that the excellent visible-light-driven photocatalytic activity for the ZnO1−x NSs could be considered to be due to accompanying oxygen vacancies, which result in the high photocatalytic activity efficiency.29
Fig. 7 Current density response vs. time for the ZnO1−x and ZnO samples under intermittent irradiation at a bias potential of 0.5 V vs. Ag/ AgCl (λ ≥ 400 nm).
Fig. 8 Time dependence of the fluorescence intensity at 425 nm (a) and fluorescence spectra (b) for the visible-light irradiation of ZnO1−x and ZnO in terephthalic acid (3 mM) solution.
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In order to verify the stability and reusability of ZnO1−x, cycling experiments were done by evaluating the decreased concentration of MO under the same reaction conditions during cycling tests. These experiments for five cycles show that the degradation rate was almost constant, confirming that the ZnO1−x photocatalyst is inherently stable during prolonged photocatalytic reactions. As shown in Fig. 9, it can be clearly seen that our recovered ZnO1−x sample exhibits a high activity and good recyclability for the degradation of MO. The removal efficiency of MO was found to be 92.5% and 82% for the first and final cycle, respectively, demonstrating that our ZnO1−x NSs photocatalyst is stable throughout repeated photocatalytic reactions. Taken together, several factors have been considered for the improved photocatalytic activity of the ZnO1−x NSs to clarify the photocatalysis mechanism. As shown in Scheme 1, the influence of the oxygen vacancies on the band structures of the as-prepared ZnO1−x sample can be comprehended as follows. While the band gap does not change significantly with
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the existence of oxygen vacancies, self-doping and the accompanying oxygen vacancies can generate isolated states in the forbidden gap.11 These defects are unoccupied states and usually act as an electron acceptor. The electrons in the VB can be excited to these unoccupied sites. Meanwhile, electrons in these isolated sites could be excited to the CB by visible light illumination. Electronic transitions from the VB to the localized states, and from these isolated states to the CB, are responsible for the visible–NIR absorption (λ ≥ 400 nm) of the gray-colored ZnO1−x NSs. Additionally, the down-shifted VB is beneficial for the separation of the charge carriers, since the VB width intrinsically controls the mobility of the holes. The down shift of the VB maximum not only makes the photogenerated holes more positive to react with molecular water to generate ·OH radicals, but also promotes the transfer of photoexcited holes to reactants, resulting in the inhibition of the electron–hole pair recombination.30 The down-shifted VB results in the higher mobility of the holes generated, which leads to a better photo-oxidation of the holes. The photo excited holes can trap surface water or hydroxide to form reactive species, such as ·OH, to attack the organic pollutants until the dye molecules are completely degraded due to its high aggressiveness, which could be considered as a key point in the mechanism of the improved photocatalytic performance.14,31 As a result, ultrathin ZnO1−x nanosheets with abundant oxygen vacancies show much superior photocatalytic activity over the defect-free ZnO sample.
Conclusions
Fig. 9 Cycling runs for the photodegradation of MO in the presence of ZnO1−x under visible light irradiation.
Scheme 1 Schematic photocatalytic reaction processes and charge transfer of the ZnO1−x and ZnO photocatalyst under visible-light illumination.
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In summary, a new kind of oxygen ZnO1−x photocatalyst with high visible light photocatalytic activity has been successfully fabricated via a facile hydrothermal reaction. Our novel synthetic route for the mass production of oxygen-deficient ZnO1−x is of great significance, and may be applied to other nonstoichiometric semiconductors for versatile use. The ZnO1−x nanosheets with yellow-green photoluminescence show superior visible-light-driven photocatalytic degradation of methyl orange compared to stoichiometric ZnO, which is only sensitive to UV light owing to its intrinsic large band gap. The incorporation of oxygen vacancy defects can be an effective way to extend the light absorption of ZnO1−x into the visiblelight region due to the fact that the energy of the localized state is located in the forbidden gap. Moreover, the ZnO1−x NSs exhibit a much higher catalytic activity and stability than ZnO, and no obvious decrease in the photocatalytic activity was observed even after 5 cycles. We attribute this superior photocatalytic performance to a high content of generated hydroxyl radicals with a strong photo-oxidation capability, which reflects the enhanced separation and migration of the photogenerated electrons and holes in these oxygen-deficient ZnO1−x NSs. It can be expected that our synthetic strategy to form oxygen deficient ZnO1−x NSs could be extended to other non-stoichiometric metal oxide materials with enhanced properties for other advanced applications.
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Acknowledgements
Published on 26 March 2015. Downloaded by University of Sussex on 26/03/2015 18:13:37.
A. W. Xu acknowledgements the special funding support from the Scientific Research Grant of Hefei Science Center of CAS (2015SRG-HSC048), the National Basic Research Program of China (2011CB933700) and the National Natural Science Foundation of China (21271165).
Notes and references 1 (a) A. Fujishima and K. Honda, Nature, 1972, 238, 37–38; (b) G. M. Wang, Y. C. Ling and Y. Li, Nanoscale, 2012, 4, 6682–6691. 2 M. Fernández-García, A. Martínez-Arias, J. C. Hanson and J. A. Rodriguez, Chem. Rev., 2004, 104, 4063–4104. 3 D. V. Talapin, J. S. Lee, M. V. Kovalenko and E. V. Shevchenko, Chem. Rev., 2010, 110, 389–458. 4 J. H. Lim, C. K. Kang, K. K. Kim, I. K. Park, D. K. Hwang and S. J. Park, Adv. Mater., 2006, 18, 2720–2724. 5 A. W. Xu, Y. Gao and H. Q. Liu, J. Catal., 2002, 207, 151– 157. 6 (a) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269–271; (b) T. L. Thompson and J. T. Yates, Chem. Rev., 2006, 106, 4428–4453. 7 (a) T. R. Gordon, M. Cargnello, T. Paik, F. Mangolini, R. T. Weber, P. Fornasiero and C. B. Murray, J. Am. Chem. Soc., 2012, 134, 6751–6761; (b) X. Liu, S. M. Gao, H. Xu, Z. Z. Lou, W. J. Wang, B. B. Huang and Y. Dai, Nanoscale, 2013, 5, 1870–1875. 8 G. M. Wang, H. Y. Wang, Y. C. Ling, Y. C. Tang, X. Y. Yang, R. C. Fitzmorris, C. C. Wang, J. Z. Zhang and Y. Li, Nano Lett., 2011, 11, 3026–3033. 9 Z. K. Zheng, B. B. Huang, X. D. Meng, J. P. Wang, S. Y. Wang, Z. Z. Lou, Z. Y. Wang, X. Y. Qin, X. Y. Zhang and Y. Dai, Chem. Commun., 2013, 49, 868–870. 10 Z. Wang, C. Y. Yang, T. Q. Lin, H. Yin, P. Chen, D. Y. Wan, F. F. Xu, F. Q. Huang, J. H. Lin, X. M. Xie and M. H. Jiang, Energy Environ. Sci., 2013, 6, 3007–3014. 11 Q. Zhu, Y. Peng, L. Lin, C. M. Fan, G. Q. Gao, R. X. Wang and A. W. Xu, J. Mater. Chem. A, 2014, 2, 4429–4437. 12 C. M. Fan, Y. Peng, Q. Zhu, L. Lin, R. X. Wang and A. W. Xu, J. Phys. Chem. C, 2013, 117, 24157–24166. 13 Y. Peng, Y. Wang, Q. G. Chen, Q. Zhu and A. W. Xu, CrystEngComm, 2014, 16, 7906–7913.
Nanoscale
Nanoscale
14 Z. K. Zheng, B. B. Huang, X. Y. Qin, X. Y. Zhang, Y. Dai and M. H. Whangbo, J. Mater. Chem., 2011, 21, 9079– 9087. 15 I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara and K. Takeuchi, J. Mol. Catal. A: Chem., 2000, 161, 205– 212. 16 T. L. Mercier, J. M. Mariot, P. Parent, M. F. Fontaine, C. F. Hague and M. Quarton, Appl. Surf. Sci., 1995, 86, 382– 386. 17 J. Jun, M. Dhayal, J. H. Shin, J. C. Kim and N. Getff, Radiat. Phys. Chem., 2006, 75, 583–589. 18 F. M. Jin, X. Zeng, J. K. Liu, Y. J. Jin, L. Y. Wang, H. Zhong, G. D. Yao and Z. B. Huo, Sci. Rep., 2014, 4, 4503. 19 A. M. Ali, E. A. C. Emanuelsson and D. A. Patterson, Appl. Catal., B, 2010, 97, 168–181. 20 (a) L. Liu and X. B. Chen, Chem. Rev., 2014, 114, 9890– 9918; (b) T. Xia, P. Wallenmeyer, A. Anderson, J. Murowchick, L. Liu and X. B. Chen, RSC Adv., 2014, 4, 41654–41658. 21 I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara and K. Takeuchi, J. Mol. Catal. A: Chem., 2000, 161, 205– 212. 22 X. X. Zou, J. K. Liu, J. Su, F. Zuo, J. S. Chen and P. Y. Feng, Chem. – Eur. J., 2013, 19, 2866–2873. 23 R. Q. Song, A. W. Xu, B. Deng, Q. Li and G. Y. Chen, Adv. Funct. Mater., 2007, 17, 296–306. 24 S. M. Park, T. Ikegami and K. Ebihara, Thin Solid Films, 2006, 513, 90–94. 25 Q. Zhu, W. S. Wang, L. Lin, G. Q. Gao, H. L. Guo, H. Du and A. W. Xu, J. Phys. Chem. C, 2013, 117, 5894–5900. 26 Y. H. Lv, W. Q. Yao, X. G. Ma, C. S. Pan, R. L. Zong and Y. F. Zhu, Catal. Sci. Technol., 2013, 3, 3136–3146. 27 F. Zuo, K. Bozhilov, R. J. Dillon, L. Wang, P. Smith, X. Zhao, C. Bardeen and P. Y. Feng, Angew. Chem., 2012, 124, 6327– 6330. 28 L. Jia, D. H. Wang, Y. X. Huang, A. W. Xu and H. Q. Yu, J. Phys. Chem. C, 2011, 115, 11466–11473. 29 Z. K. Zheng, B. B. Huang, X. D. Meng, J. P. Wang, S. Y. Wang, Z. Z. Lou, Z. Y. Wang, X. Y. Qin, X. Y. Zhang and Y. Dai, Chem. Commun., 2013, 49, 868–870. 30 M. L. Guan, C. Xiao, J. Zhang, S. J. Fan, R. An, Q. M. Cheng, J. F. Xie, M. Zhou, B. J. Ye and Y. Xie, J. Am. Chem. Soc., 2013, 135, 10411–10417. 31 A. Dodd, A. McKinley, T. Tsuzuki and M. Saunders, Mater. Chem. Phys., 2009, 114, 382–386.
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