Abnormal Oxidation of Ag Films and Its Application to Fabrication of

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Abnormal Oxidation of Ag Films and Its Application to Fabrication of Photocatalytic Films With a-TiO/h-AgO Heterostructure 2

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Wei Xu, Shengqiang Wang, Qing-Yu Zhang, Chunyu Ma, Xiaona Li, Qing Wang, and Donghui Wen J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 25 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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The Journal of Physical Chemistry

Abnormal Oxidation of Ag Films and Its Application to Fabrication of Photocatalytic Films with a-TiO2/h-Ag2O Heterostructure W. Xu 1, S. Q. Wang 1, Q. Y. Zhang 1, a), C. Y. Ma 1, X. N. Li 2, Q. Wang 2, and D. H. Wen 2 Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Physics and Opto-electronic Technology,

2

1

School of

School of Materials Science and Engineering,

Dalian University of Technology, Dalian 116024, China Abstract: Metallic Ag films were observed to be layer-by-layer oxidized by reactive-sputtering deposition of TiO2, leaving a layer of Ag2O with hexagonal structure (h-Ag2O), which is rarely reported in literature. The oxidation of Ag film behaved rather abnormally because it was not until the Ag film was completely oxidized that amorphous TiO2 (a-TiO2) could form on the h-Ag2O layer, thus providing an effective method for fabrication of h-Ag2O films and a-TiO2/h-Ag2O heterostructures. O atoms in the plasma were proved to be responsible for the oxidation of Ag film, and the formation of TiO2 on the film that was not completely oxidized was disenabled, probably due to formation of a new kind of oxide, (TiAg)O2. The optical properties of h-Ag2O films were studied for the first time and the photocatalysis was found better than that of TiO2 films. The a-TiO2/h-Ag2O heterostructures exhibited excellent photocatalytic activity under ultraviolet and visible-light irradiation, much better than that of the a-TiO2 and the h-Ag2O single-layer films. The photocatalysis was discussed using the special electronic structure built in the a-TiO2/h-Ag2O heterostructures. In addition, decomposition of Ag2O was suggested playing an important role in the photocatalysis. a)

Author to whom correspondence should be addressed: [email protected] 1

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1. INTRODUCTION With development of industry all over the world, photocatalytic chemistry is receiving much attention, because there is an urgent need for the solution of many problems in the fields of energy resources, ecological environment, and pollution treatment. TiO2 is an intriguing material with good photocatalysis and has a study history of ~100 years 1. Since Fujishima and Honda’s discovery in the study on electrochemical photolysis of water under ultraviolet (UV) irradiation 2, TiO2 has been intensively investigated as a photocatalytic material and has been proven to be the most promising photocatalyst due to its chemical stability, nontoxicity, and low cost. Various forms of TiO2 with improved photocatalytic activity have been reported

3-8

and breakthroughs in the study of TiO2 photocatalysis have

led to many important applications, such as water treatment facilities utilizing solar photons for the photocatalytic depollution 9, even though the photocatalytic efficiency still needs improvement to meet practical requirements. Photocatalysis is ascribed to the photogenerated carriers (electrons and holes) created in semiconductors primarily by absorption of photons with energy equal to or larger than the band gap. The band-gap energies in the three crystalline phases of TiO2, namely, anatase, rutile, and brookite as well as the amorphous state, are larger than 3.0 eV

10

. Therefore,

various forms of TiO2 usually exhibit good UV photocatalysis, but the response to visible light needs to be further improved if utilizing the solar photons with high efficiency. For this purpose, many efforts have been made, including dye sensitization, impurity doping, and fabrication of TiO2-based heterostructures, etc. 9. Dye’s photosensitizing effect is widely 2


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used to improve the performance of TiO2 electrode for solar cells

11

, and the energy

conversion efficiency of dye sensitizing solar cells (DSSCs) is constantly increased year by year, but low-cost and stable DSSCs are still in development. Doped TiO2 has potential in the fields such as environmental depollution and splitting of water. Though improved photocatalytic activity is frequently reported, doped TiO2 still has problems in thermal instability, electron trapping by the dopants, and introduction of electron-hole recombination centers, etc.

12

. Similar to doped TiO2, TiO2-based heterostructures can be used in many

fields, including photocatalytic degradation of organic pollution, photolysis of water, and fuel synthesis. Using narrow band-gap semiconductors coupling with TiO2, TiO2-based heterostructures effectively avoid production of defects and impurities by doping, and thus have advantage over the doped TiO2 in high efficiency of utilizing photon energy. For example, Ag2O/TiO2 composites, such as Ag2O nanoparticles (NPs) decorated TiO2 nanobelts

13-16

and TiO2 microspheres

17, 18

, have been reported to have good UV

photocatalysis better than that of the TiO2 nanobelts or microspheres without Ag2O-NP decoration. Ag2O is a good candidate for fabrication of TiO2-based heterostructures because it is nontoxic and cost-effective. It is not very difficult to synthesize Ag2O NPs and films in the form of cubic phase (c-Ag2O) by routine methods, such as solution synthesis and vapor deposition including physical vapor deposition (PVD) and chemical vapor deposition (CVD). Differing from the usual c-Ag2O, Beesk et al.

19

had reported an unusual Ag2O with

hexagonal structure (h-Ag2O) grown by hydrothermal methods in silver tubes at moderate 3



temperature and high pressure (80°C, 4000 bar). Up to now, however, the report on successful synthesis of h-Ag2O NPs and thin films is still unavailable in literature. In this work, we report the synthesis of h-Ag2O films for the first time using abnormal oxidation of metallic Ag films by reactive-sputtering deposition of TiO2. The oxidation of Ag films was studied as a function of the length of time for deposition of TiO2 by morphology observation, structure characterization, and composition analysis. Furthermore, we studied the optical properties and photocatalysis of the h-Ag2O films. To improve the photocatalytic activity of h-Ag2O films, a layer of amorphous TiO2 (a-TiO2) was deposited on the h-Ag2O films, forming a-TiO2/h-Ag2O heterostructures. The a-TiO2/h-Ag2O heterostructures were found to have excellent photocatalytic activity under UV and visible-light irradiation, much better than that of the a-TiO2 and the h-Ag2O single-layer films. Based on the electronic structure in the a-TiO2/h-Ag2O heterostructures, the photocatalysis mechanisms were discussed. Besides, decomposition of Ag2O was suggested playing an important role in the photocatalysis.

2. EXPERIMENTAL SECTION 2.1 Preparation of TiO2/Ag2O Heterostructure The a-TiO2/h-Ag2O heterostructures were fabricated using a strategy named in this work as abnormal oxidation of Ag films, which was schematically shown in Figure 1. Briefly, metallic Ag films were first deposited on quartz or Si substrates at room temperature (Figure 1a). The deposition of Ag films was carried out on a JPG450 magnetron sputtering 4


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system using a radio-frequency (RF) power of ~30 W. Ag target with purity of 99.99% was used for deposition using pure argon (99.99%) as working gas. Before deposition, the vacuum chamber was pumped to a base pressure better than 1×10-3 Pa and the working pressure was controlled at ~0.5 Pa. Subsequently, the samples were turned to deposition of TiO2 on the metallic Ag films (Figure 1b). The working pressure for deposition of TiO2 was 0.3 Pa using 40 SCCM Ar and 2 SCCM O2 (SCCM denotes cubic centimeter per minute at standard pressure) and the deposition was conducted by applying ~120 W RF power to a metallic Ti target (99.99%) of 60 mm in diameter, without additional heating to the sample holder. By determining the rate of oxidation, h-Ag2O single-layer films and a-TiO2/h-Ag2O heterostructures could be fabricated by controlling the length of time for reactive-sputtering deposition of TiO2 (Figure 1c).

2.2 Film Characterization and Optical Measurement Structure evolution of the Ag films oxidized by reactive-sputtering deposition of TiO2 was studied using X-ray diffraction (XRD, Bruker D8 Focus), scanning electron microscopy (SEM, Hitachi S-4800), and transmission electron microscopy (TEM, Tecnai G220 S-Twin). Electron probe mass analysis (EPMA, EPMA-1600) was used to determine the compositions in the oxidized films. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) was used for examination of surface elements of a film that was not completely oxidized and the chemical states of elements were investigated by calibration of C1s peak at 284.6 eV. Optical properties of h-Ag2O were studied by ultraviolet-to-near-infrared (UV-to-NIR, Maya 5



2000-Pro) spectroscopy and spectroscopic ellipsometry (M-2000VI). The band-gap energies of h-Ag2O films were determined by Tauc plotting. Refractive index (n) and extinction coefficient (k) were determined as a function of wavelength by fitting Fresnel reflection coefficients in a spectral range of 370 to 1700 nm using Cauchy model with layered structures consisting of surface roughness, film layer, and substrate.

2.3 Evaluation of Photocatalytic Activity Photocatalytic activity was evaluated in a dark room by collecting time-dependent transmittance spectra of 10 mg/L methylene blue (MB) solution for a sample under UV or visible-light irradiation. The samples used for evaluation of photocatalytic activity were cut into a size of 10×10 mm and placed at the bottom of a quartz cell containing 0.8 mL MB aqueous solution. UV light came from a 12-cm long low-pressure mercury lamp (Philips TUV, 6 W) with the strongest optical emission at 254 nm. The UV lamp was placed at a distance of 50 mm right over the sample in solution. Visible light was provided by a broad-spectrum light source (PL-XQ500W) with a filter cutting off at 400 nm for simulating sunlight. The visible light delivered to the samples was ~40 mW/cm2. The time-dependent spectra of transmittance were in situ collected in a time interval of 5 min using a fiber spectroscope (Maya 2000-Pro). For comparison, a-TiO2 and crystalline TiO2 (c-TiO2) films with a thickness of ~100 nm were prepared. The a-TiO2 films were prepared on quartz substrates using reactive-sputtering deposition of TiO2 with the parameters same to that used for oxidation of Ag films. The c-TiO2 films were prepared by a sol-gel method 20, 21. Briefly, 6


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gel TiO2 films were first coated on quartz substrates, and then were crystallized into anatase polycrystals containing a few rutile phases by annealing the samples at 550 °C in air, because such a kind of TiO2 crystals was proved to have the best photocatalysis 22.

3. RESULTS AND DISCUSSION 3.1 Abnormal Oxidation of Ag Films Abnormal oxidation of Ag films was observed for the first time when attempting to grow TiO2 films on metallic Ag films by reactive RF magnetron sputtering method, and then, was systematically studied in this work. Figure 2 shows the XRD patterns of samples at different stages of oxidation. The original sample was polycrystalline Ag film, as shown in Figure 2a, with evidence of XRD peaks at 2θ = 38.2°, 44.4°, 64.6°, and 77.6° highly in accordance with XRD data of face-centered-cubic (fcc) Ag at (111)，(200)，(220), and (311) (JCPDS 04-0783). Reactive-sputtering deposition of TiO2 led to appearance of a new XRD peak at 2θ = 35.6°, as shown in Figure 2b. At the same time, the XRD peaks of Ag film were observed to be weakened. With the assistance of quantitative analysis by EPMA and examination by TEM, the XRD peak at 2θ = 35.6° was assigned to h-Ag2O (002) (JCPDS 72-2108). With the increase in the length of time for reactive-sputtering deposition of TiO2, the XRD peaks of Ag film were gradually weakened until disappearance, leaving the h-Ag2O (002) peak alone standing in the XRD pattern, as shown in Figure 2c. SEM and TEM observation revealed that the Ag films are oxidized from the top to the bottom and layer by layer. The length of time for completely oxidizing a ~600 nm Ag film into a 7



h-Ag2O film was ~120 min under the given conditions, with a rate of oxidation as high as ~5 nm/min. Despites no special texture in the polycrystalline Ag films, the h-Ag2O films were found to be highly (001)-texturized. On the other hand, oxidation of Ag films produced a stress rather large, thus leading to a shift of the h-Ag2O (002). The oxidation of Ag films by reactive-sputtering deposition of TiO2 was found rather abnormal. As shown in Figure 3a, no TiO2 film is visible in the sample that the Ag film is not completely oxidized. If using sufficient length of time for deposition of TiO2, a layer of a-TiO2 film is visible and Ag layer is no more observed (Figure 3b). Furthermore, the a-TiO2 film was found much thinner than that simultaneously deposited on a bare quartz substrate. EPMA showed that Ti atoms in the films that are completely oxidized are less than 2%, which is much less than the amount of Ti atoms that comes from reactive-sputtering deposition. The result strongly suggests that it is not until the Ag film is completely oxidized that the TiO2 film begins to form on the surface of h-Ag2O layer. To have more confident evidence, a series of Ag films within 600 nm were used for oxidation by reactive-sputtering deposition of TiO2 and the length of time for deposition of TiO2 was controlled at 150 min, which is long enough to completely oxidize a ~600 nm Ag film. The thicknesses of a-TiO2 and h-Ag2O layers were determined and plotted as a function of Ag film thickness, as shown in Figure 3c. The thickness of h-Ag2O layer is in proportion to that of Ag film used for oxidation, with a factor of ~1.33. The thickness of a-TiO2 layer is decreasing with the increase in the thickness of Ag film. For example, the thickness of a-TiO2 layer is larger than 100 nm on the bare quartz substrate, but is less than 40 nm on the Ag-coated substrates. To 8


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the best of our knowledge, such a phenomenon was not reported in the literature related to oxidation of Ag films. More importantly, the abnormal oxidation of Ag films provides an effective method for fabrication of h-Ag2O films, which were not reported to be successfully synthesized by routine methods including chemical solution synthesis, PVD, and CVD, though the growth of h-Ag2O single crystals was realized as early as 1974 19. To obtain more messages about the abnormal oxidation of Ag films, XPS was used to examine the surface elements of a sample that was not completely oxidized. Besides a few amount of C contaminants, the visible elements on the surface are O, Ag, and Ti. Using atomic sensitivity factors, O, Ag, and Ti on the surface were determined to be 52.4%, 46.5%, and 1.1%, respectively. As XPS signals are in a depth within 5 nm, we conclude that Ti atoms on the surface are insufficient for forming a monolayer in the form of TiO2. Taking into account O in combination with Ti, atomic ratio of O to Ag on the surface was found to be close to 1:1, which differs from that of O to Ag (1:2) in the film determined by EPMA; thus, we concluded that the surface phase is not same to Ag2O. After calibration of C 1s at 284.6 eV, the Ti 2p3/2 was found locating at 457.2 eV (Figure 4a), which was rarely reported in literature. The binding energy of Ti 2p3/2 is more than 1.0 eV lower than that in TiO2

23

,

but 2.2 eV higher than that in TiO 23, indicating that the valence of Ti is likely to be +3. The Ti+3 is probably from a new kind of oxide, (TiAg)O2, because a few Ti atoms and a large number of Ag atoms were detected. The conclusion was further confirmed by fitting the O 1s peak, which can be decomposed into two individual peaks, as shown in Figure 4b. The big peak at 530.7 eV is close to O 1s in M2O (M = metal) 23, and the small peak at 529.2 eV 9



is in accordance with O 1s in MO2 23. With consideration that the total ratio of Ag to O is 2:1 in the film, the former can be assigned to Ag2O in deep and the latter should be in association with the new oxides formed on the surface. As for the Ag 3d5/2, only is one peak observed at 367.8 eV, as shown in Figure 4c. Because Ag 3d5/2 is insensitive to the variation in valence

23

, we used Auger electrons (Ag M4VV) to determine the chemical state of Ag

because the kinetic energy of Auger electrons is more sensitive to the variation in valence for noble metals 23. The Ag M4VV peak is located at the kinetic energy of 356.1 eV (Figure 4d), in good agreement with Ag+ in Ag2O 23, indicating that both valences of Ag in the film and on the surface are +1, thus supporting the existence of (TiAg)O2. With evidence that TiO2 film cannot form on the film that is not completely oxidized, we suspected that (TiAg)O2 is likely to be volatile. As a result, a dense and sufficiently thick oxide layer cannot form to protect Ag layer in deep from oxidation because the incompact Ag2O layer is unable to arrest the diffusion of O atoms. Metallic Ag films could be oxidized by reactive-sputtering deposition of other oxides either, such as NiO, but leaving a layer of silver oxides different. Therefore, we are convinced that O atoms in the plasma play an important role in oxidizing Ag to Ag2O, as oxidation of metallic silver in air is rather slow at room temperature. In fact, Ag is a special metal much easy to be oxidized by atomic O. For instance, the rate of oxidation for metallic Ag in low-earth-orbit space was found as high as ~1.05 × 10−23 cm3 per O atom

24

, which is ~3 orders of magnitude higher than that for

oxidizing metallic Cu 24. Using a specially designed oxygen plasma beam, the role of atomic

10


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O in oxidizing Ag films was demonstrated, but the products are c-Ag2O containing a few amount of AgO phase.

3.2 Photocatalytic Activity of Ag2O and TiO2/Ag2O Films Except for the structure characterization by Beesk et al.

19

, other reports on h-Ag2O

(Ag2O II) are unavailable in literature. Therefore, the optical properties of h-Ag2O were studied for the first time using the samples synthesized in this work. Optically, the h-Ag2O films look transparent with yellow-brown color, which is much different from the brown-black color of c-Ag2O films. The optical properties were studied by spectroscopic ellipsometry and UV-to-NIR spectroscopy, as shown in Figure 5. The values of optical parameters n and k obtained by spectroscopic ellipsometry are in good agreement with that determined by UV-to-NIR spectroscopy (Figure 5a). The refractive index of the h-Ag2O films is rather large, n ≈ 2.37 at ~630 nm, close to the refractive index of rutile TiO2 (n⊥ ≈ 2.58) 25. In spectral range from ultraviolet to near-infrared band, the extinction coefficient of h-Ag2O films is increasing with the decrease in the wavelength of light. The optical band-gap energy was determined to be 2.45 ± 0.05 eV (Figure 5b), which is ~ 1 eV higher than that of c-Ag2O 13, indicating a probable utility in the spectral range of visible light, for example, photocatalysis. 10 mg/L MB solution was used to evaluate the photocatalytic activity for the samples under the UV or the visible-light irradiation. The photodegradation rate (γ) of MB solution for a sample was quantitatively determined using Langmuir−Hinshelwood model with the 11



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first-order reaction, dC = −γC ⇒ Ct = C0 e −γt dt

(1)

where C0 and Ct are the MB concentrations before and after irradiation for a specific length of time t, respectively. Figures 6a and 6b show the irradiation-time dependence of Ct for the given samples. The γ values were determined by linearly fitting the plots of ln(C0/Ct) as a function of irradiation time (Figures 6c and 6d) and the results were summarized in Figure 7. It is emphasized here that the γ value is a parameter depending on many factors including the intensity of irradiation light, the solution volume, the effective area of photocatalyst, and the purity of water, etc. Therefore, this parameter is just used for quantitative evaluation of the samples under the same conditions of measurement. On the other hand, MB solution under UV and visible-light irradiation is instable and will be photodegraded without assistance of photocatalyst, as shown in the curves of SiO2 sample in Figures 6a and 6b. In this work, a direct degradation rate (γ0) of MB solution was introduced and was estimated by the values for the SiO2 sample. Under the UV and the visible-light irradiation, the γ0 values were determined to be 0.29×10−2 and 0.43×10−2 min−1, respectively, and then a net degradation rate (γnet) of MB solution for a sample was then defined by γnet = γ − γ0 for quantitative comparison with others. As shown in Figures 6a and 6b, the h-Ag2O film, on which no TiO2 film was deposited, exhibited good photocatalytic activity in comparison with that of the a-TiO2 and c-TiO2 films. By linearly fitting the ln(C0/Ct) plots (Figures 6c and 6d), the γnet values for the a-TiO2 and c-TiO2 films under the UV irradiation were determined to be 0.13 and 0.39×10−2 min−1 12


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(Figure 7), respectively, indicating that crystallization produced an increase in the UV photocatalytic activity of TiO2 films. However, the γnet value for the h-Ag2O film under the UV irradiation was as high as 1.1×10−2 min−1, which is ~2.8 times that of MB solution for the c-TiO2 film. It is of interest that the photocatalytic activity of h-Ag2O films beyond 200 nm weakly depends on the film thickness. Under the visible-light irradiation, the a-TiO2 and c-TiO2 films exhibited poor photocatalytic activity and the γnet values were decreased to 0.08~0.09 ×10−2 min−1. Similarly, the γnet value for the h-Ag2O film was obviously reduced either (knet = 0.22×10−2 min−1), but still maintaining at ~2.8 times that of MB solution for the c-TiO2 film. The photocatalytic activity of h-Ag2O films is different from that of the c-Ag2O NPs reported by Zhou et al.

13

, because their data showed that the c-Ag2O NPs exhibited

good photocatalytic activity under visible-light irradiation, better than that of TiO2 nanobelts, but the UV photocatalysis was close to that of TiO2 nanobelts. Zhou et al.

13

found that the UV photocatalysis of TiO2 nanobelts can be improved by

decoration of c-Ag2O NPs. Similarly, Sarkar et al.

17

also reported the improvement in the

photocatalytic activity using c-Ag2O NPs/TiO2 microsphere composites. These authors ascribed the improvement to the formation of Ag2O/TiO2 heterostructures. Therefore, we further fabricated TiO2/Ag2O heterostructures by depositing a layer of a-TiO2 film on h-Ag2O films. Differing from the Ag2O/TiO2 composites reported by Zhou et al. Sarkar et al.

17

13

and

, the a-TiO2/h-Ag2O heterostructures have a layer of a-TiO2 film separating

the h-Ag2O layer from contact with solution. Such a heterostructure has advantages allowing

13



the photons with energy less than the band gap of TiO2 to enter the bottom layer of Ag2O for excitation; thus, high efficiency of utilizing photon energy is expected. Under UV and visible-light irradiation, the a-TiO2/h-Ag2O heterostructures exhibited excellent photocatalysis much better than that of the single-layer films, as shown in Figures 6 and 7. Under the UV irradiation, the γnet value for the a-TiO2/h-Ag2O heterostructure is as high as 2.7×10−2 min−1, which is ~20.8 and ~6.9 times that of MB solution for the a-TiO2 and the c-TiO2 films, respectively. In comparison with the h-Ag2O single-layer films, the additional a-TiO2 layer led the h-Ag2O film to be improved in photocatalysis by a factor of ~2.5. Under the visible-light irradiation, the γnet value for the a-TiO2/h-Ag2O heterostructure was found to be 0.42×10−2 min−1, which is ~5 and ~2 times that of MB solution for the TiO2 films and the h-Ag2O film, respectively. The above results are the evidence demonstrating that the heterostructure is capable of improving photocatalysis considerably. According to Zhou et al.’s data presented in their paper

13

, the Ag2O-NP-decorated TiO2 nanobelts have

good performance under UV irradiation, better than that of the Ag2O NPs and the TiO2 nanobelts. Under visible-light irradiation, however, the degradation curve for the Ag2O-NP-decorated TiO2 nanobelts was observed to be very close to that of MB solution for the Ag2O NPs despite of still having good photocatalysis better than that of the TiO2 nanobelts. In this work, the a-TiO2/h-Ag2O heterostructure exhibited an ability of improving both of UV and visible-light photocatalysis, thus having great potential in utilizing the solar photons over the c-Ag2O/TiO2 composites.

14


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3.3 Photocatalysis Mechanism in h-Ag2O and a-TiO2/h-Ag2O Films In Zhou et al.’s article

13

, they proposed two photocatalysis mechanisms for the

Ag2O/TiO2 composites under UV and visible-light irradiation, respectively. They suggested that the metallic Ag NPs, which is from decomposition of Ag2O NPs under UV irradiation, act as the electron traps for separation of electron-hole pairs created in the Ag2O/TiO2 composites, thus enhancing the production of hydroxyl radicals (OH•). Under irradiation of visible light, the electron-hole pairs were suggested to be created primarily in Ag2O NPs, and electron transfer from Ag2O to TiO2 was believed to be the mechanism responsible for the enhancement in photocatalysis, though their data showed that the Ag2O/TiO2 composites have the photocatalytic activity very close to that of the Ag2O NPs. Sarkar et al. 17 attributed the improved UV photocatalysis of Ag2O/TiO2 composites to the p-n junctions because TiO2 and Ag2O are the semiconductors usually exhibiting n-type and p-type conduction, respectively. They suggested that charge transfer (both electrons and holes) between TiO2 and Ag2O is the reason leading to enhancement in the UV photocatalysis. OH• radicals have been commonly accepted to be the primary species responsible for degradation of organic molecules in solution

26

. However, the reasons for OH• generation

are still in debate. Holes (h+) had been suggested to be the candidate responsible for − − formation of hydroxyl radicals through reaction of h + + OH ad is → OH • , where OH ad

OH− absorbed on the surface. The reaction is doubtful because the effective O 2p level was found far below the valence band maximum (VBM) of TiO2 9. Ag2O has a narrower band gap and a higher level of VBM in comparison with those of TiO2; thus, the holes created in 15



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Ag2O are unable to produce OH• either. Therefore, the photocatalysis mechanisms proposed by Zhou et al. have to be improved. Sarkar et al.

17

suggested that the p-n junctions play a

crucial role in separating the electron-hole pairs created in both of TiO2 microspheres and Ag2O NPs. The p-n junctions lead the electrons to transfer to TiO2 and holes to Ag2O, thus enhancing the photocatalysis for both of Ag2O NPs and TiO2 microspheres. As such, the holes have no contribution to the photocatalysis in our a-TiO2/h-Ag2O heterostructures, since the h-Ag2O layer is at the bottom beneath the a-TiO2 layer and has no probability contacting with solution. Therefore, the photocatalsis mechanisms in a-TiO2/h-Ag2O heterostructures need to be explored with paying attention to electrons in conduction band (eCB). Dissolved oxygen (O2)aq in water has been suggested as a candidate responsible for formation of OH• radical species when using TiO2 through following reactions 9, +

+

H H (O 2 ) aq + eCB → O •2- → HO •2 e,  → H 2O 2

(2)

H 2 O 2 + Ti 3+ → OH • + OH − + Ti 4+

(3)

H 2 O 2 + O •2− → OH • + OH − + O 2

(4)

H 2 O 2 → 2OH •

(5)

O solubility in water is ~9 mg/L at room temperature. Therefore, the dissolved O2 in this work was estimated to be ~0.45×10−6 mol, which is ~18 times the number of MB molecules in the solution, and thus sufficient for degradation of all the MB molecules, even if not taking into account the O2 that is constantly dissolved during degradation. Using the dissolved O2 mechanism, the excellent photocatalysis of the a-TiO2/h-Ag2O heterostructure 16


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can be ascribed to the improved photocatalytic activity of the a-TiO2 layer. As shown in Figure 8a, the a-TiO2/h-Ag2O heterostructure has a special electronic structure because the conduction band minimum (CBM) of Ag2O is located at a level above that of TiO2 in the one hand. On the other hand, Fermi-level alignment will lead the energy levels in Ag2O up-shift in respect to that in TiO2 because of the p-type Ag2O and the n-type TiO2. Therefore, electron transfer in conduction band from the h-Ag2O layer to the a-TiO2 layer is thermodynamically favorable for the a-TiO2/h-Ag2O heterostructure, thus enhancing the photocatalysis of the a-TiO2 layer. Under UV irradiation, both the a-TiO2 and the h-Ag2O layers contribute to the enhancement in photocatalysis, because many UV photons are able to penetrate through the ~30 nm a-TiO2 layer and to enter the bottom layer of Ag2O for excitation, thus enhancing the production of OH• radicals by the a-TiO2 layer. Figure 8b shows the photographs of pH test papers used for detection of OH• radicals produced by a-TiO2/h-Ag2O heterostructure and a bare sample of SiO2 in pure water. In comparison with SiO2, the a-TiO2/h-Ag2O heterostructure was capable of changing the pH test paper to be dark under UV irradiation for a specific length of time, proving that OH• radicals are the important species leading to degradation of MB solution. Under visible-light irradiation, the a-TiO2/h-Ag2O heterostructures also have the ability to enhance the photocatalysis of the a-TiO2 layer because the quantum efficiency of producing electron-hole pairs in the h-Ag2O layer still maintains at a level relatively high, but the photocatalytic activity will be reduced due to the low quantum-efficiency in the a-TiO2 layer.

17



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Though the conclusion deduced from the dissolved O2 mechanism is in accordance with the observation for a-TiO2/h-Ag2O heterostructures. However, the dissolved O2 mechanism cannot give a reason why the h-Ag2O films have good photocatalysis better than that of the a- and c-TiO2 films. Under UV and visible-light irradiation, we noticed that both of Ag2O in the single-layer films and in the a-TiO2/h-Ag2O heterostructures could be decomposed because metallic Ag was detected in the samples, as shown in Figure 9. Similar decomposition was observed by Zhou et al.

13

. As a large number of O atoms will be

produced with decomposition of Ag2O, we are convinced that the atomic O may also contribute to the photocatalysis of the h-Ag2O single-layer films by producing H2O2 with the following reactions, Ag 2 O + 2h + + 2e − → O + 2Ag

(6)

O + H 2O → H 2O 2

(7)

To have confident evidence, the number of O atoms in the h-Ag2O film was estimated and compared with that of MB molecules in the solution. As listed in Table 1, the decomposition of h-Ag2O film is able to provide sufficient O atoms for totally degrading the MB molecules in solution. By estimation of photocatalytic experiments relevant to Ag2O reported by others 13, 17, 18, 27

, as listed in Table 1, the same conclusion can be obtained. Therefore, the role of

atomic O in contributions to photocatalysis cannot be excluded. In the a-TiO2/h-Ag2O heterostructure, the a-TiO2 layer is very thin (~ 30 nm), and then the O atoms produced in the h-Ag2O layer are able to arrive the surface by diffusion, thus also make contributions to the enhancement in photocatalysis, as shown in Figure 8a. As evidence, the photocatalytic 18


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activity of a-TiO2/h-Ag2O heterostructures was found depending on both thicknesses of the a-TiO2 and the h-Ag2O layers.

4. CONCLUSIONS In conclusion, reactive-sputtering deposition of TiO2 was demonstrated to have ability leading metallic Ag films to be oxidized from the top to the bottom and layer by layer, with a ~5 nm/min rate of oxidation in this work. Atomic O in the plasma and (TiAg)O2 formed on the surface were proved to be the two crucial factors responsible for the oxidation of Ag films. The oxidation of Ag films by reactive-sputtering deposition of TiO2 behaved rather abnormally because it was not until Ag films were completely oxidized that TiO2 films could form on the h-Ag2O films. Using the abnormal oxidation of Ag films, h-Ag2O films were successfully prepared for the first time. The band-gap energy of h-Ag2O was determined to be ~2.45 ± 0.05 eV, ~1.0 eV larger than that of c-Ag2O. The h-Ag2O films exhibited good photocatalysis better than that of a- and c-TiO2 films. Based on the abnormal oxidation of Ag films, an effective method was established for fabrication of photocatalytic films with a-TiO2/h-Ag2O heterostructures, which exhibited excellent photocatalysis under UV and visible-light irradiation. The photocatalytic activities under the UV and the visible-light irradiation were estimated to be 20.8 and ~5 times that of the a-TiO2 films, respectively, and 2~2.5 times that of the h-Ag2O single-layer films, demonstrating its great potential in applications utilizing the solar photons. The improved photocatalysis was ascribed to the enhancement in producing OH• radicals due to the special electronic structure formed in the 19



a-TiO2/h-Ag2O heterostructure and the O atoms resulting from decomposition of Ag2O.

20


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ACKNOWLEDGEMENT The research is supported by the Basic Research Project for Key Laboratory of Liaoning Province of China (Grant No. LZ2014006) and the Fundamental Research Funds for the central Universities of China (Grant Nos. DUT16ZD207 and DUT16-LAB01). The authors gratefully acknowledge Dr. F. Q. Gong in Dalian Institute of Chemical and Physics, Chinese Academy of Sciences for his help in ellipsometry. Professor X. N. Li is thankful to her financial support from the National Natural Science Foundation of China under Grant No. 51271045. Professor Q. Wang is thankful to her financial support from the International Science & Technology Cooperation Program of China (Grant No. 2015DFR60370).

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Electronics Division: Eden Prairie, MN, 1979; pp 43–113. 24. Silverman, E. M. Space Environmental Effects on Spacecraft-LEO Material Selection Guide. NASA Contractor Report No. 4661. Langley Research Center, 1995, Part 2. 25. Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: New York, 1985. 26. Turci, C. S; Ollis, D. F. Photocatalytic Degradation of Organic Water Contaminants: Mechanisms Involving Hydroxyl Radical Attack. J. Catal. 1990, 122, 178-192. 27. Wang, X. F.; Li, S. F.; Yu, H. G.; Yu, J. G.; Liu, S. W. Ag2O as a New Visible-Light Photocatalyst: Self-Stability and High Photocatalytic Activity. Chem. Eur. J. 2011, 17, 7777 – 7780

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Tables Table 1 Numbers of O atoms in Ag2O and MB molecules in solution Sample Ag2O films Ag2O powder 3D Ag2O/TiO2 Ag2O/TiO2 nanobelt Ag2O/TiO2 sphere

MB (mol) −8

2.5×10 8.0×10−7 4.0×10−7 1.25×10−6 1.9×10−6

O in Ag2O (mol) −6

1.4×10 4.3×10−4 3.3-7.2×10−5 0.9-7.7×10−5 4.0×10−5

26


Refs. This work 27 17 13 18

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Captions of Figures Figure 1 Fabrication Processes of photocatalytic films with a-TiO2/h-Ag2O heterostructures, (a) deposition of Ag films on quartz substrates, (b) preparation of h-Ag2O films using abnormal oxidation of Ag films by reactive-sputtering deposition of TiO2, (c) fabrication of a-TiO2/h-Ag2O heterostructures by depositing a layer of TiO2 films on the h-Ag2O films. Figure 2 XRD patterns of metallic Ag film (a), the sample that is not completely oxidized (b), and the sample that is completely oxidized (c). The vertical bars are XRD data taken from JCPDS 04-0783 (Ag), 72-2108 (h-Ag2O), and 76-1393 (c-Ag2O). Figure 3 (a) and (b) TEM images taken from cross-sectional samples that are incompletely and completely oxidized, respectively. The inset in Figure b shows the high-resolution TEM image that was taken from the yellow box indicated in the figure. (c) Thicknesses of TiO2 and Ag2O layers determined from different thick samples oxidized by reactive-sputtering deposition of TiO2 for 150 min. Figure 4 XPS spectra of Ti 2p (a), O1s (b), Ag 3d (c), and Ag M4VV (d) on the surface of sample that is not completely oxidized. Figure 5 (a) optical constants (n and k) plotted as a function of wavelength determined by fitting the transmittance spectra and the ellipsometric data. (b) Determination of band-gap energy of h-Ag2O by Tauc plotting method using the samples with different thicknesses. Figure 6 (a) and (b) photodegradation curves of 10 mg/L MB solution for the given samples 27



under UV and visible-light irradiation, respectively. (c) and (d) Time-dependent ln (C0/Ct) plots calculated from (a) and (b), respectively. Figure 7 Photocatalytic degradation rates of MB solution for the given samples. Figure 8 (a) Schematic photocatalysis mechanisms of a-TiO2/h-Ag2O heterostructures. HO• radicals are suggested to be the primary species responsible for degradation of MB solution. The upper channel represents the dissolved O2 mechanism for producing HO• radicals. The lower channel represents the Ag2O decomposition mechanism for producing HO• radicals. (b) Photographs of pH test papers used for detection of HO• radicals produced by the a-TiO2/h-Ag2O heterostructure and a bare sample of SiO2 in solution under UV irradiation. Figure 9 XRD patterns of the a-TiO2/h-Ag2O heterostructure at the given lengths of time for UV irradiation. For comparison, the XRD pattern of a metallic Ag film is presented.

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TOC Graphic

29



Figure 1 Fabrication Processes of photocatalytic films with a-TiO2/h-Ag2O heterostructures, (a) deposition of Ag films on quartz substrates, (b) preparation of h-Ag2O films using abnormal oxidation of Ag films by reactive-sputtering deposition of TiO2, (c) fabrication of a-TiO2/h-Ag2O heterostructures by depositing a layer of TiO2 films on the h-Ag2O films. 180x39mm (300 x 300 DPI)


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Figure 2 XRD patterns of metallic Ag film (a), the sample that is not completely oxidized (b), and the sample that is completely oxidized (c). The vertical bars are XRD data taken from JCPDS 04-0783 (Ag), 72-2108 (hAg2O), and 76-1393 (c-Ag2O). 101x142mm (300 x 300 DPI)



Figure 3 (a) and (b) TEM images taken from cross-sectional samples that are incompletely and completely oxidized, respectively. The inset in Figure b shows the high-resolution TEM image that was taken from the yellow box indicated in the figure. (c) Thicknesses of TiO2 and Ag2O layers determined from different thick samples oxidized by reactive-sputtering deposition of TiO2 for 150 min. 203x81mm (300 x 300 DPI)


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Figure 4 XPS spectra of Ti 2p (a), O1s (b), Ag 3d (c), and Ag M4VV (d) on the surface of sample that is not completely oxidized. 203x146mm (300 x 300 DPI)



Figure 5 (a) optical constants (n and k) plotted as a function of wavelength determined by fitting the transmittance spectra and the ellipsometric data. (b) Determination of band-gap energy of h-Ag2O by Tauc plotting method using the samples with different thicknesses. 203x89mm (300 x 300 DPI)


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Figure 6 (a) and (b) photodegradation curves of 10 mg/L MB solution for the given samples under UV and visible-light irradiation, respectively. (c) and (d) Time-dependent ln (C0/Ct) plots calculated from (a) and (b), respectively. 203x155mm (300 x 300 DPI)



Figure 7 Photocatalytic degradation rates of MB solution for the given samples. 101x83mm (300 x 300 DPI)


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Figure 8 (a) Schematic photocatalysis mechanisms of a-TiO2/h-Ag2O heterostructures. HO• radicals are suggested to be the primary species responsible for degradation of MB solution. The upper channel represents the dissolved O2 mechanism for producing HO• radicals. The lower channel represents the Ag2O decomposition mechanism for producing HO• radicals. (b) Photographs of pH test papers used for detection of HO• radicals produced by the a-TiO2/h-Ag2O heterostructure and a bare sample of SiO2 in solution under UV irradiation. 175x76mm (300 x 300 DPI)



Figure 9 XRD patterns of the a-TiO2/h-Ag2O heterostructure at the given lengths of time for UV irradiation. For comparison, the XRD pattern of a metallic Ag film is presented. 101x77mm (300 x 300 DPI)


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Abnormal Oxidation of Ag Films and Its Application to Fabrication of

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