Growth and Photocatalytic Properties of NiO Nanostructures Prepared

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Growth and Photocatalytic Properties of NiO Nanostructures Prepared in Acidic and Alkaline Solution with Same Reagents Feng Tian, Song Liu, Hua Tian, Ruixue Dong, Yuhui Zhang, Dongdong Wei, Luyang Ye, and Lawrence Whitmore J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09384 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 23, 2018

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The Journal of Physical Chemistry

Growth and Photocatalytic Properties of NiO Nanostructures Prepared in Acidic and Alkaline Solution with Same Reagents

Feng Tian,a,

†, *

Song Liu,

a, ‡

Hua Tian,a,

‖

Ruixue Dong,† Yuhui Zhang,† Dongdong Wei,† Luyang Ye§ and L.

Whitmore§§

†School

of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai,

200093, China ‡Institute

of Chemical Biology and Nanomedicine, State Key Laboratory of Chemo/Biosensing and Chemometrics,

College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, China ‖Sinosteel

§School

Zhengzhou Research Institute of Steel Wire, 450001, Zhengzhou, China

of Chemistry, Pingdingshan University, 467300, Pingdingshan, China

§§ Department

of Chemistry and Physics of Materials, Paris-Lodron University, 5020, Salzburg, Austria

*Corresponding

author: F. Tian. Email address: [email protected].

1

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ABSTRACT: Different Ni compound nanomaterials have been prepared successfully by simple hydrothermal synthesis, in which only the dosages of reactants and pH values of the solutions are controlled. Under acidic conditions NiC2O4 · 2H2O nanotubes are prepared, while under alkaline conditions single crystalline Ni(OH)2 nanosheets can be made. For the first time, the crystalline structure of NiC2O4 · 2H2O is given based on X-ray diffraction. After annealing, the as-prepared nanostructures become NiO nanotubes and nanosheets, respectively. However, their crystalline structures are different from the original counterparts for the loss of H2O molecules. The photocatalytic properties of the final NiO products are measured. The nanotubes show much stronger performance in degradation of methylene blue than the nanosheets. The mechanisms for growth and photocatalysis are also discussed.

KEYWORDS: NiO, TEM, XRD, nanostructure, hydrothermal, precursor

2


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Organic compound is a main source of pollution in water, but it is not easy to remove them completely with procedures based on using chemical reagents.1-4 Photocatalysis is a promising new method for treating contaminated water.5-7 Compared to the traditional chemical process, the organic pollutants can be decomposed by photocatalysis into small harmless molecules. At the same time, no new chemicals are introduced. TiO2 is the most frequently used semiconductor-type photocatalyst, which has been applied in photocatalysis to destroy pollutants.8-11 However, TiO2 can only work in ultra violet range, which limits its applications in many occasions.12-14 Herein, study of materials which can work in visible light range is very attractive.15-18 Nickel monoxide (NiO) is another important transition-metal oxide that has been employed in various fields including catalysis, lithium-ion batteries and supercapacitors because of its high thermal stability and environmental friendliness.19-21 NiO is classified as a Mott-Hubbard insulator with bandgap of 4.3 eV. For such kind of wide-bandgap oxides, it is generally believed that the activation energy arising from the polaronic hopping of charge possibly makes them work in visible light range for photocatalytic applications.22 Furthermore, nanosizing can improve the photocatalytic performance of materials dramatically and probably change the light interaction energy.23-25 So, NiO nanomaterials are selected for study. Different nanostructures of NiO, such as nanospheres, nanoflakes and nanotubes, have been prepared successfully by hydrothermal or electrodeposition methods and the photocatalytic performance has improved dramatically.26-30 However, these reactions are always dedicated, which means in fabrication processes, only one kind of products can be made. In this paper, we report a new synthesis strategy, in which two different NiO nanostructures can be made just by changing the ratio of reagents. Ni2+ can react with C2O42- at acidic or alkaline conditions in different ways. Utilizing this character, by changing the chemical ratio of NaOH and 3



oxalic acid in reaction, different intermediate products and final NiO nanostructures can be prepared on Ni foils.

a

b

2µm

1µm

e

c

f

d

g

(-1100)

h

(10-10) [0001]

° 120

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1µm

0.4µm

120

°

i

4


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j

Figure 1. Characterization and structural models of intermediate products prepared at pH=2.00 (a, c, e, g, i) and pH=10.00 (b, d, f, h, j) respectively. a) and b) are SEM images. c) and d) are experimental and simulated XRD spectra of both samples showing the compounds are NiC2O4·2H2O (pH=2.00) and Ni(OH)2 (pH=10.00). e) and f) are the top (upper) and side view (lower) of the unit cell of NiC2O4·2H2O and Ni(OH)2 retrieved from the XRD spectra. The blue spheres are Ni atoms and white are H atoms. Red are O in e), while they are OH- in f). g) and h) are TEM images of intermediate products with their electron diffraction patterns shown in insets. i) and j) STEM images of both samples and their corresponding EDX elemental mapping shown at the right side. Fabrication and Characterization of Intermediate Products. The scanning electron microscopy (SEM) morphology of the intermediate products is shown in Figure 1a and b. For the solution with pH=2.00, the products are quadrangular, as shown in Figure 1a. Most of the pillars’ cross-sections are approximately parallelogram with diagonal length from 50 nm to 0.5 µm. The surface of the pillars is smooth. The SEM image of the products prepared in solutions of pH=10.00 is shown in Figure 1b. Hexagonal nanosheets grown vertically to the Ni foil substrate can be found. The fully grown nanosheets are hexagonal, as shown in the inset of Figure 1b, with side length of 0.5 ~ 1 µm and thickness 50 ~ 100 nm. Some are not fully formed but their shapes are still clearly part hexagonal. Most of the nanosheets are independent, but some are found to cross each other in growth. From the SEM image, the surface of the nanosheets is also smooth. X-ray diffraction (XRD) spectra of the intermediate products are shown in Figure. 1c and d respectively. In both spectra, the strong (111) and (220) peaks are from the Ni substrate, which 5



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suggests that the X-rays penetrate the intermediate products. According to the experimental spectrum in Figure 1c, the intermediate products made from the solution of pH=2.00 can be identified as monoclinic nickel oxalate dihydrate (NiC2O4·2H2O, JCPDS No. 25-0581, C2/c). The proposed crystal structure of the as-made NiC2O4·2H2O is shown in Figure 1e. In the unit cell of the crystal structure, the C2O42- units connect with Ni2+ to form a chain. The chains are bonded by hydrogen bonds between H2O molecules which are bonded to the Ni ions. Unit cell parameters are a=11.707 Å, b=5.4487 Å, c=9.6477 Å, α=90o, β=126.155o and γ=90o. The atomic positions in primitive cell are: Ni (0, 0.07229, 1/4), O1oxalate (0.0873, 0.3672, 0.4232), O2oxalate (0.4122, 0.2782, 0.0769), O3water (0.1751, 0.0646, 0.2428), C (0.4487, 0.0748, 0.1492), H1 (0.265, 0.111, 0.35), H2 (0.161, 0.136, 0.141). Based on this structure, the corresponding powder diffraction spectrum is calculated by Materials Studio software, with the peak shape modeled by a pseudo-Voigt function. From Figure 1c, the calculated diffraction peaks show good agreement with the experimental ones. Since in the calculation bulk crystal is adopted, while in experiments the materials are nanopillars, the intensity ratios between different peaks are not strictly the same as the experimental ones. However, the model of unit cell can still be verified. To our knowledge, this is the first crystalline model of NiC2O4·2H2O, although the crystalline parameters have been decided. Compared to the intermediate products made under acidic conditions, those prepared in the pH=10.00 solution can be identified as hexagonal nickel hydroxide (Ni(OH)2, JCPDS No. 14-0117, P-3m1) from the experimental XRD spectrum shown in Figure 1d. The proposed crystalline structure is shown in Figure 1f. In the unit cell, a Ni atom occupies the (0, 0, 0) position. Two different O atoms occupy positions of (1/3, 2/3, 0.226) and (2/3, 1/3, 0.774), respectively. The H atoms are at (1/3, 2/3, 0.47) and (2/3, 1/3, 0.53). The crystal structure matches the peaks of experimental XRD spectrum 6


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well. But the nanostructure makes the intensity ratios of peaks deviate slightly from the powder diffraction peaks, as shown in the calculated spectrum in Figure 1d. TEM images of both intermediate products are shown in Figure 1g and h. For the products prepared from the solution with pH=2.00, most of the pillars observed are shown to be tubes with length about 2 µm and wall thickness from 10 to 200nm, which suggest the successful preparation of NiC2O4·2H2O nanotubes. The corresponding SAED pattern (inset) shows that the diffraction is diffuse, only faint polycrystalline rings can be identified. In the crystalline structure model shown in Figure 1e, NiC2O42- chains bonded by H2O molecules. The layers created by these chains are probably responsible for the formation of nanotubes. Furthermore, in XRD experiments, the nanopillars stand vertically on the substrate and preferred orientation may contribute to the diffraction intensity. While in TEM experimental no preferred orientation exists anymore. High water content and no preferred orientation explain why the rings are diffuse in electron diffraction while peaks can be found in XRD spectra. For the nanomaterials prepared in the solution with pH=10.00, the TEM image of an as-made nanosheet is shown in Figure 1h. The nanosheet demonstrates a clear hexagonal shape albeit it not perfectly manifest with small deviation of side length and vertex angle. The corresponding SAED pattern (inset) shows that the nanosheet is single crystalline Ni(OH)2 with zone axis [0001] out of plane. Scanning TEM (STEM) images of both samples are shown in Figure 1 i and j. The Z-contrast images shows most of the nanopillars are hollow (Figure 1i). These nanotubes are dispersed on thin continuous carbon support film. However, stronger C kα signal from the sample than the underneath carbon film can be found, which suggests the sample consists of C element. While for the Ni(OH)2 nanosheet (Figure 1j) intruding on the pore of carbon film, weak C kα signal can be read out, 7



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especially on the impending part of the nanosheet. The comparison of both element maps verifies the different gradients of the nanotubes and nanosheets.

a

1µm

b

10µm

5µm

c

e

1µm

d

f

Figure 2. Characterization and structural model of NiO prepared by annealing intermediate products made at pH=2.00 (a, c, d, e) and pH=10.00 (b, c, d, f) respectively. a) and b) are SEM images. Inset is the local magnified area of the samples. c) XRD spectra of NiO prepared by annealing the products made from the solution with pH=2.00 (black) and pH=10.00 (red). d) unit cell of NiO (upper top view and lower front view). e) and f) TEM images of NiO nanostructures showing, e) nanotubes obtained from products prepared at pH=2.00 with corresponding SAED pattern, and f) a nanosheet obtained from intermediate products prepared at pH=10.00 together with the corresponding SAED pattern and 8


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high resolution TEM image.

Fabrication and Characterization of NiO Nanomaterials. Figure 2a and b show SEM images of intermediate products after annealing at 600 °C for 2 h. From Figure 2a, after annealing, the surfaces of the pillars made from solution with pH=2.00 are no longer smooth, but in many cases, are pitted and contain holes. During annealing the pillars have lost molecules and been transformed into more porous structures. Additionally, the pillars have generally assumed a flatter position with respect to the substrate after annealing. For the sample prepared in solution with pH=10.00, after annealing, the nanosheets still stand vertically on the surface of the Ni substrate, as shown in Figure 2b. However, the nanosheets are slightly deformed after annealing, having become bent and thinner than before and in many cases tapered rather than parallel faced as they were before annealing. XRD spectra of the annealed samples are shown in Figure 2c. Except of the peaks from the Ni substrate, other peaks correspond to cubic NiO (JCPDS No. 47-1049, Fm-3m), which shows both samples are pure cubic NiO with face centered cubic (fcc) crystalline structure. Therefore, it is clear that the intermediate products have been transformed into NiO by annealing. Figure 2e and f show TEM images of the two distinct NiO nanostructures. The annealed nanotubes prepared at pH=2.00, are shown in Figure 2e. After annealing, the wall of the tubes is composed of nanograins. The nanotubes turn into nano-grain tubes. While for some thin tubes they have transformed to be nanowires made of nanograins. From Figure 2e, the size of the nanograins is about 10 ~ 110 nm. The length and diameter of the tubes are similar to their intermediate counterparts, which suggest the skeleton of nanotube keeps in the process of water loss. From SAED pattern shown in Figure 2e, the nanograins are fcc NiO. The rings of the pattern are sharp, and no 9



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other phases are found, which shows the intermediate products have transformed fully. The TEM images of annealed nanosheets from intermediate products prepared in solution with pH=10.00, are shown in Figure 2f. The nanosheet in Figure 2f is not fully grown, but the exposed part shows the shape of half hexagon. From Figure 2f, after annealing, the nanosheet still keeps its shape. The SAED pattern in upper right shows the nanosheet is fcc one. The zone axis of the nanosheet is [111]. A high magnification image of the nanosheet is shown at the lower right part of Figure 2f. Grain boundaries can be found, which shows that after annealing the internal structure of the nanosheet changes and a lot of small flaky grains come out, each maintaining the preferred orientation. Growth Mechanism. Based on more than 10 times of serial pH-value dependent experiments, it is found that the experiments show good repeatability. For the NiC2O4·2H2O nanotubes prepared from the solution with pH=2, acidic conditions, metallic Ni reacts with minor O2 and Na2C2O4 in the autoclave to create the coordinated ion [Ni(C2O4)2]2 − , which is then ionized into Ni2+ and 2C2O42-. Ni2+ reacts with C2O42- and H2O to produce NiC2O4·2H2O. After annealing NiO is prepared. The reaction formulae are as follows: Ni + 2C2O42- + O2 + 4H+ ⇌ 2[Ni(C2O4)2]2− + 2H2O

(1)

[Ni(C2O4)2]2- ⇌ Ni2++ 2C2O42−

(2)

Ni2++ C2O42− + 2H2O → NiC2O4····2H2O

(3)

NiC2O4·2H2O → NiC2O4 + 2H2O

(4)

2NiC2O4 → NiO + 4CO2

(5)

For the Ni(OH)2 nanosheets prepared from the solution with pH=10.00, alkaline environment, Ni reacts with the minor O2 and Na2C2O4 in the autoclave to create complex ions, which are ionized to be Ni2+ and 2C2O42-. Ni2+ reacts with OH- to produce Ni(OH)2. After annealing, NiO is prepared. The 10


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reaction formulae are as follows: 2Ni + 4C2O42− + 2H2O + O2 ⇌ 2[Ni(C2O4)2]2− + 4OH−

(6)

[Ni(C2O4)2]2− ⇌ Ni2+ + 2C2O42−

(7)

Ni2+ + 2OH− → Ni(OH)2

(8)

Ni(OH)2 → NiO + H2O

(9)

In both reactions, Na2C2O4 is important. The complex reaction between Ni and Na2C2O4 can happen under acidic or alkaline conditions just with getting the OH- or H2O. After the complex decomposes, the overdose of OH- will suppress the combination of Ni2+ and 2C2O42 − to produce NiC2O4·2H2O, and Ni(OH)2 is generated instead. For the reaction, the acid and alkaline reactants are the same, but the contents are different, which causes the pH values of the solution to be different and further different intermediate products. In the annealing process, for NiC2O4·2H2O nanotubes, not only crystalline water molecules are lost but CO2 is emitted. So, the walls of the nanotubes become porous and granular tubes are created. It should be noted that after annealing the skeleton of the nanotube persists although it cannot support the standing of nanotubes, which also verifies that crystalline water molecules are in the middle of NiC2O4 skeleton. For Ni(OH)2, during annealing, fewer water molecules are lost than that of NiC2O4·2H2O nanotubes. So, the nanosheets can keep their hexagonal shape even after annealing. The effect of pH value of the solution on the nanomaterials are also studied by SEM with density of the products observed. In acidic conditions, nanotubes can be always made. But pH value in 2.00 is best for the growth of nanotubes. While at alkaline conditions, pH value in 10.00 is best for the growth of nanosheets. At pH=7.00, few nanostructures can be found. Further, it is also found that 11



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the nanomaterials cannot grow well if the pH value of the solution is less than 1.00 or higher than 11.00. In the growth of intermediate products, the complex reaction between C2O42 − and Ni2+ is pivotal. Too much H+ or OH- may impede the reaction and no nanomaterials can grow anymore. In the TEM and SEM images of Ni(OH)2, not fully grown nanosheets can be found. Probably the highdensity nanostructures cover the surface of the Ni foil and the complex reaction cannot continue anymore. Photocatalytic Properties. The photocatalytic properties of the final NiO nanomaterials are also studied. In the experiments, methylene blue is adopted as the indicator, which has two absorbance peaks at 612 and 662 nm in visible light range. Since the peak at the 662 nm is much stronger than the other, so the absorbance of methylene blue at 662 nm is adopted as the zero reference. a

b

Figure 3. Methylene blue degraded by as-prepared NiO nanomaterials with Ni foil as reference. a) relative degradation rate of NiO nanosheets and nanotubes at 0, 30 and 60 min respectively. b) The absorbance curve measured at 60 min.

Ni foil, NiO nanosheets and nanotubes with their substrates are cut to be the same size and put into the methylene blue solution respectively to measure their absorbance. The moment before the Xeon lamp irradiation is set to be 0 and every 30 mins the absorbance of the solution is measured. In the experiments after the absorbance measurement the sampling solution (3 mL) is returned to 12


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the beaker used in photocatalysis. This may lose a small amount of methylene blue and cause small error. But it can minimize the error caused by the system since the interval between two measurements is long. From Figure 3a, after the samples are put into the solution absorbance occurs. For Ni, nanosheets and nanotubes the degradation rates are 8.4%, 11.1% and 17.4%, respectively. Since at time 0 min, there is no light irritation, so the absorbance is mainly from absorbing of the materials. For nanotubes, the specific surface is largest, and they can absorb more methylene blue. So, the absorbance is the largest. For nanosheets, the absorbance is not as high as nanotubes, but it is still larger than Ni foil. Therefore, nano-sizing the surface improves the absorbance. After 30 mins of light irradiation, the degradation rates are measured again, and they are 50.1%, 61.2% and 78.5% for Ni foil, nanosheets and grain-tubes respectively, as shown in Figure 3a. After 60 mins, the degradation rates are 66.6%, 80.9% and 97.7% for Ni foil, nanosheets and grain-tubes respectively, as shown in Figure 3a. The methylene blue is almost fully degraded by the NiO nanotubes, which suggests the nanotubes are good photocatalytic materials. NiO nanosheets also show good performance. In the experiments, Ni foil also show photocatalytic properties, even though it is not strong as its counterparts covered by nanosheets and nanotubes. All of the absorbance curves measured at 60th min are shown in Figure 3b. In photocatalytic process, Ni foil does not show evident blue shift as others, which may be related to the photocatalytic essence of nanostructures. We noted some experiments reporting fabrication of NiO nanosheets with similar method to ours. Making the experimental conditions comparable, 5 hours are needed to reach 95% of degradation of all the methyl hexanoate.31 The NiO nanosheets in our experiments only need 1.5 hours to reach that value for methylene blue, not to the NiO nanotubes prepared by us which can degrade 97.7% of methylene blue in 60 mins. The larger 13



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specific surface area and higher quality than reported may be the reason why the materials prepared by us show higher performance in photocatalysis. In these experiments, simply by controlling the content of reactants different NiO nanostructures can be made based on different growth mechanism. The final NiO nanostructures still show high active photocatalytic properties and can work in visible light range, which can pave a new way to improve the photocatalytic performance of Ni foil by surface treatment according to requirements in different applications. The high activity of the nanostructure may find applications in batteries and supercapacitors. In this paper, a simple hydrothermal method to prepare different nanostructures on the surface of Ni foil is reported. By simply changing the chemical ratio of reactants, or pH value of the solutions, different nanostructures, such as NiC2O4·2H2O nanotubes, Ni(OH)2 nanosheets, NiO nanosheets and NiO grain-tubes, can be made. Photocatalytic experiments show that as-made NiO nanostructures are highly active. The surface treatment technology of Ni foils can be adopted in devices to improve the performance further.

Growth procedure. High purity (99.99%) Ni foils (10 mm × 10 mm × 0.3 mm) are ground and polished using silicon carbide papers to remove the surface oxidation layer. Then the foils are washed with deionized water and ethanol alternately in an ultrasonic cleaner for 15 minutes to remove any residue. After drying in an inert nitrogen air stream, the foils are kept under vacuum. For the first group of samples, 15 ml aqueous solution of 0.75 mmol oxalic acid and 1.65 mmol NaOH are added into a 25 mL Teflon autoclave. The pH value of the solution is 10±0.05. A Ni foil is immersed into the solution and the autoclave is sealed and placed in an oven at 150 °C. After 24 h, the autoclave is 14


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removed and the foil taken out and washed three times with deionized water. For the second group of samples, 15 mL aqueous solution of 1.24 mmol oxalic acid and 1 mmol NaOH are added into a 25 mL Teflon autoclave. The pH value of the solution is 2±0.05. A Ni foil is immersed into the solution and the autoclave is sealed and placed in an oven at 150 °C. After 24 h the foil is taken out and washed three times with deionized water, just as the first sample. After drying in air, both samples are annealed in a Muffle oven at 600 °C for 2 h. Structural Characterization. The morphology of all samples is observed and characterized using a FEI Quanta 200 SEM. The crystalline phases of the samples are determined using an Bruker D8 Advanced diffractometer with Cu Kα radiation. For these measurements, the treated foils are mounted on a clay holder. Some of the remaining foils are then sectioned into smaller pieces and ultrasonically cleaned in ethanol for 3 min, after which the suspensions are dropped onto holy carbon grids for investigation by JEOL JEM 2010F and ARM 200F (STEM and EDS) microscopes. Photocatalytic Measurement. The photocatalytic properties are measured using a Perkin Elmer Lambda 750 UV/VIS/IR spectrophotometer. In experiments, the samples are cut into 2 mm × 2 mm × 0.3 mm pieces and immersed in 20 ml of 10 mg/l methylene blue solution. The solution is stirred using a magnetic stirrer at 30 rpm. In photocatalysis measurement, the solution is first kept in the dark for 30 mins to eliminate the effects of ambient light absorption. Then it is irradiated by a 50-W CEL-HXBF300 Xenon lamp. The temperature of the reactant solution was maintained at 20 °C by a flow of cooling water during the test. Every 30 mins 3 mL methane blue solution is taken out and centrifuged. The concentration of supernatant is measured with the spectrophotometer. After rapid concentration measurement, the sampled methylene blue solution is returned to the beaker for photocatalytic experiment. The degradation rate of methylene blue is calculated by: 15



V = [(𝐶0 ― 𝐶) 𝐶0] × 100%

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(1)

where C0 and C are the concentrations of methylene blue solution before and after degradation.

AUTHOR INFORMATION a

These authors contribute equally.

NOTES The authors declare no competing financial interests.

ACKNOWLEDGEMENT F. Tian, H. Tian and R. X. Dong contribute equally to this work. F. Tian thanks the financial support from National Natural Science Foundation of China (51271124) and S. Liu thanks National Natural Science Foundation of China (21705036) and Fundamental Research Funds for the Central Universities from Hunan University.

REFERENCES 1.

Schultz, D. M.; Yoon, T. P., Solar Synthesis: Prospects in Visible Light Photocatalysis. Science 2014,

343, 1239176. 2.

Nebel, C. E., Photocatalysis a Source of Energetic Electrons. Nat. Mater. 2013, 12, 780-781.

3.

Linic, S.; Aslam, U.; Boerigter, C.; Morabito, M., Photochemical Transformations on Plasmonic Metal

Nanoparticles. Nat. Mater. 2015, 14, 567-576. 4.

Yu, H. X.; Li, R. Y.; Liu, B.; Chen, X.; Yu, H. L., Printing and Dyeing Wastwater Processing Method. 16


Page 17 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


J. Univ. Shanghai Sci. Tech. 2016, 39, 320-322. 5.

Hussain, H., et al., Structure of a Model Tio2 Photocatalytic Interface. Nat. Mater. 2017, 16, 461-+.

6.

Karkas, M. D.; Matsuura, B. S.; Stephenson, C. R. J., Enchained by Visible Light-Mediated

Photoredox Catalysis. Science 2015, 349, 1285-1286. 7.

Selcuk, S.; Selloni, A., Facet-Dependent Trapping and Dynamics of Excess Electrons at Anatase

Tio2 Surfaces and Aqueous Interfaces. Nat. Mater. 2016, 15, 1107-+. 8.

Canlas, C. P.; Lu, J. L.; Ray, N. A.; Grosso-Giordano, N. A.; Lee, S.; Elam, J. W.; Winans, R. E.; Van

Duyne, R. P.; Stair, P. C.; Notestein, J. M., Shape-Selective Sieving Layers on an Oxide Catalyst Surface.

Nat. Chem. 2012, 4, 1030-1036. 9.

Schrauben, J. N.; Hayoun, R.; Valdez, C. N.; Braten, M.; Fridley, L.; Mayer, J. M., Titanium and Zinc

Oxide Nanoparticles Are Proton-Coupled Electron Transfer Agents. Science 2012, 336, 1298-301. 10. Setvin, M.; Aschauer, U.; Scheiber, P.; Li, Y. F.; Hou, W. Y.; Schmid, M.; Selloni, A.; Diebold, U., Reaction of O2 with Subsurface Oxygen Vacancies on Tio2 Anatase (101). Science 2013, 341, 988-991. 11. Tao, J.; Luttrell, T.; Batzill, M., A Two-Dimensional Phase of Tio2 with a Reduced Bandgap. Nat Chem 2011, 3, 296-300. 12. Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S., Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746-50. 13. Dulub, O.; Batzilln, M.; Solovev, S.; Loginova, E.; Alchagirov, A.; Madey, T. E.; Diebold, U., ElectronInduced Oxygen Desorption from the Tio2(011)-2x1 Surface Leads to Self-Organized Vacancies. Science 2007, 317, 1052-6. 14. Tada, H.; Mitsui, T.; Kiyonaga, T.; Akita, T.; Tanaka, K., All-Solid-State Z-Scheme in Cds-Au-Tio2 Three-Component Nanojunction System. Nat Mater 2006, 5, 782-6. 17



Page 18 of 20

15. Guo, W.; Zhang, F.; Lin, C.; Wang, Z. L., Direct Growth of Tio2 Nanosheet Arrays on Carbon Fibers for Highly Efficient Photocatalytic Degradation of Methyl Orange. Adv. Mater. 2012, 24, 4761-4. 16. Liu, X. Y., et al., 0d-2d Quantum Dot: Metal Dichalcogenide Nanocomposite Photocatalyst Achieves Efficient Hydrogen Generation. Adv. Mater. 2017, 29. 17. Zhang, J.; Yu, J. G.; Zhang, Y. M.; Li, Q.; Gong, J. R., Visible Light Photocatalytic H2-Production Activity of Cus/Zns Porous Nanosheets Based on Photoinduced Interfacial Charge Transfer. Nano Lett. 2011, 11, 4774-4779. 18. Ma, J.; Ji, T. T.; Jian, K. F.; Hou, Y. Y., Hyfrothermal and Thermo-Decomposition Synthesis of SelfAssembled Mco2o4 Nnaomaterials. J. Univ. Shanghai Sci. Tech. 2015, 37, 520-526. 19. Lee, D. U.; Fu, J.; Park, M. G.; Liu, H.; Kashkooli, A. G.; Chen, Z. W., Self-Assembled Nio/Ni(Oh)2 Nanoflakes as Active Material for High-Power and High-Energy Hybrid Rechargeable Battery. Nano Lett. 2016, 16, 1794-1802. 20. Liu, A.; Zhu, H. H.; Guo, Z. D.; Meng, Y.; Liu, G. X.; Fortunato, E.; Martins, R.; Shan, F. K., Solution Combustion Synthesis: Low-Temperature Processing for P-Type Cu:Nio Thin Films for Transparent Electronics. Adv. Mater. 2017, 29, 1701599. 21. Xu, X. B.; Liu, Z. H.; Zuo, Z. X.; Zhang, M.; Zhao, Z. X.; Shen, Y.; Zhou, H. P.; Chen, Q.; Yang, Y.; Wang, M. K., Hole Selective Nio Contact for Efficient Perovskite Solar Cells with Carbon Electrode. Nano

Lett. 2015, 15, 2402-2408. 22. Yu, J. G.; Rosso, K. M.; Bruemmer, S. M., Charge and Ion Transport in Nio and Aspects of Ni Oxidation from First Principles. J. Phys. Chem. C 2012, 116, 1948-1954. 23. Zeng, Y. X.; Meng, Y.; Lai, Z. Z.; Zhang, X. Y.; Yu, M. H.; Fang, P. P.; Wu, M. M.; Tong, Y. X.; Lu, X. H., An Ultrastable and High-Performance Flexible Fiber-Shaped Ni-Zn Battery Based on a Ni-Nio 18


Page 19 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Heterostructured Nanosheet Cathode. Adv. Mater. 2017, 29. 24. Kim, J. H.; Zhu, K.; Yan, Y. F.; Perkins, C. L.; Frank, A. J., Microstructure and Pseudocapacitive Properties of Electrodes Constructed of Oriented Nio-Tio2 Nanotube Arrays (Vol 10, Pg 4099, 2010).

Nano Lett. 2011, 11, 5098-5098. 25. Li, L.; Gibson, E. A.; Qin, P.; Boschloo, G.; Gorlov, M.; Hagfeldt, A.; Sun, L. C., Double-Layered Nio Photocathodes for P-Type Dsscs with Record Ipce. Adv. Mater. 2010, 22, 1759-+. 26. Zhu, T.; Chen, J. S.; Lou, X. W., Highly Efficient Removal of Organic Dyes from Waste Water Using Hierarchical Nio Spheres with High Surface Area. J. Phys. Chem. C 2012, 116, 6873-6878. 27. Song, X. F.; Gao, L.; Mathur, S., Synthesis, Characterization, and Gas Sensing Properties of Porous Nickel Oxide Nanotubes. J. Phys. Chem. C 2011, 115, 21730-21735. 28. Lu, L.; Yan, X.; Wang, J. B.; Zheng, H.; Hu, X. Y.; Tang, Y. W.; Jia, Z. Y., Oriented Nio Nanosheets with Regular Hexagonal Nanopores. J. Phys. Chem. C 2012, 116, 14638-14643. 29. D'Amario, L.; Boschloo, G.; Hagfeldt, A.; Hammarstrom, L., Tuning of Conductivity and Density of States of Nio Mesoporous Films Used in P-Type Dsscs. J. Phys. Chem. C 2014, 118, 19556-19564. 30. Castillo, C. E., et al., Visible Light-Driven Electron Transfer from a Dye-Sensitized P-Type Nio Photocathode to a Molecular Catalyst in Solution: Toward Nio-Based Photoelectrochemical Devices for Solar Hydrogen Production. J. Phys. Chem. C 2015, 119, 5806-5818. 31. Zhang, J.; Yang, H.; Yang, R.; Yang, W.; Song, Y.; Jiao, H.; Li, L.; Ma, J.; Wang, M., Fabrication and Photocatalytic Properties of Vertical Standing Nio Nanosheets on Metallic Ni Substate Chin. Sci. Bull. 2007, 52, 2015-2020.

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Table of Contents Graphic. Schematic of the formation processes of NiO nanostructures in solutions

with different pH values. Different Ni compound nanomaterials have been prepared successfully by simple hydrothermal synthesis, in which only the dosages of reactants and pH values of the solutions are controlled.

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Growth and Photocatalytic Properties of NiO Nanostructures Prepared

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