TiO2 Nanotube Arrays for

Subscriber access provided by Purdue University Libraries

Article

Synthesis of #-Bi2Mo3O12/TiO2 Nanotube Arrays for Photoelectrochemical COD Detection Application Yajun Pang, Guangqing Xu, Qiang Feng, Jiaqin Liu, Jun Lv, Yong Zhang, and Yucheng Wu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01826 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

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

Langmuir

Synthesis of α-Bi2Mo3O12/TiO2 Nanotube Arrays for Photoelectrochemical COD Detection Application Yajun Pang1, Guangqing Xu*1, 2, Qiang Feng1, Jiaqin Liu2, 3, Jun Lv1, 2, Yong Zhang1, 2, Yucheng Wu*1, 2, 3 1 School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China 2 Anhui Provincial Key Laboratory of Advanced Functional Materials and Devices, Hefei University of Technology, Hefei 230009, China. 3

Laboratory of Non-Ferrous Metals and Processing Engineering of Anhui Province, Hefei 230009, China

Abstract

ACS Paragon Plus Environment

Langmuir

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

Page 2 of 23

One-dimensional anodic TiO2 nanotube arrays hold great potential as photoelectrochemical sensor for the determination of chemical oxygen demand (COD). In this work, we report a warm synthesis of modified TiO2 nanotube arrays with enhanced photoelectrochemical determination performance. Herein, a bismuth-based semiconductor (α-Bi2Mo3O12) was introduced to TiO2 nanotube arrays by sequential chemical bath deposition (CBD) at room temperature. The field-emission scanning electron microscopy, X-ray diffraction, Raman and X-ray photoelectron spectroscopy were used to investigate the morphologies, structures and elemental analysis of the products, respectively. The photoelectrochemical properties of TiO2 and α-Bi2Mo3O12/TiO2 NTAs were measured by amperometry and cyclic votammetry methods. The α-Bi2Mo3O12/TiO2 nanotube arrays decrease background photocurrent and increase current response to organics at the same time, both of which are

benefit

for

enhancing

photoelectrochemical

detection

performance.

The

optimized

α-Bi2Mo3O12/TiO2 NTAs with enhanced photoelectrochemical detection performance can achieve detection sensitivity of 2.05 µA·cm-2/(mg·L-1) and COD detection range of 0.366 -208.9 mg/L respectively. With the α-Bi2Mo3O12 modification, the surface electrochemical reactions of TiO2 NTAs were regulated, and the mechanisms of which were also further studied. Keywords:

TiO2

nanotube

arrays,

α-Bi2Mo3O12,

surface

photoelectrochemical COD detection

2


electrochemical

reactions,

Page 3 of 23

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

Langmuir

1 Introduction After the single-crystal TiO2 electrode for photocatalytic water-splitting were first discovered by Fujishima and Honda,1 extensive researches has proved that TiO2 is an intriguing material for photocatalytic environmental purification under ultraviolet (UV) illumination.2-5 TiO2 based materials with different types and structures have shown great application potential as one extensively used photocatalysts for various significant reactions due to their low cost, chemical stability, nontoxicity, and high reactivity.6 In comparison with other TiO2 films prepared by other methods, such as sol-gel process, liquid deposition, and magnetron sputtering, the electrochemical anodic oxidation method is a relatively simple technique for the synthesis of TiO2 nanotube arrays (NTAs) with a large specific surface area. Furthermore, the high ordered nanotube arrays possess many peculiar advantages, such as high mechanical stability, integrity and unique shape with fewer interfacial grain boundaries, which promote charge transport and electron/hole pairs separation.7-9 These remarkable properties of TiO2 NTAs have attracted great research interest and been shown to be promising in catalysis, photoelectrolysis, nanotemplating and sensing application.10 Environmental monitoring and control has become a global concern in view of the increase in the amount of pollutants discharged into the water body. How to achieve an effective determination method of the pollutants in water still remains a formidable challenge. Chemical oxygen demand (COD) is one of the most used methods in the water-quality analysis field. Up till now, during the process of COD water determination, there still exist many problems waiting to be solved, which requires a long reflow to achieve sufficient oxidation and also leads to several expensive corrosive and toxic chemicals consumption. In view of the remarkable properties of TiO2 NTAs and the present situation of COD water determination, the COD photoelectrochemical sensors based on TiO2 nanotube arrays has attracted widespread attention, which provide an accurate, fast and environmentally friendly approach for the detection of COD.10, 11-13 In our previous works,14, 15 we have found that the surface electrochemical reactions during its photoelectrochemical processes play a key role on the detection performances. In concern with the band gap of TiO2 NTAs, modification of TiO2 NTAs with appropriate semiconductor will regulate the charge transfer and change the surface reactions, which will enhance photoelectrochemical detection properties of TiO2 NTAs. Recently, Bi-based component semiconductors have been regarded as an excellent photocatalyst due to its intrinsic chemical and physical properties.16, 17 Especially, ternary metal oxides containing 3


Langmuir

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

Bi(III) have exhibited interesting properties for photocatalytic application.18-20 Among them, bismuth molybdate (α-Bi2Mo3O12, β-Bi2Mo2O9 and γ-Bi2MoO6) have become the promising photocatalysts due to their narrow band-gap, layered structure, and fast-moving carriers.21-23 In addition to that, our previous work found that the photo-generated holes of Bi-containing nanostructures possess low productivity of ·OH radicals than that of TiO2 nanomaterials, indicating that the direct oxidation to organics by holes is the main manner in photocatalytic process of Bi-containing nanostructures.14 It will be a key aspect to achieve the regulation of surface electrochemical reactions of TiO2 for enhancing photoelectrochemical COD detection performance. Also, in consideration with the band gap of TiO2 NTAs, we believe that the charge transfer and the regulated surface electrochemical reactions between bismuth molybdate and TiO2 NTAs will further improve the photoelectrochemical determination performance of TiO2 NTAs. Hence, α-Bi2Mo3O12, an important photocatalyst in bismuth-based semiconductor, were modified on TiO2 NTAs by a simple assembly method at room temperature here. Various methods have been reported for preparing α-Bi2Mo3O12, such as hydrothermal method and mechanical-mixed calcination.24, 25 In comparison with those methods, this work provide a simple, low cost and highly effective method to obtain α-Bi2Mo3O12 modified TiO2 NTAs with enhanced photoelectrochemical detection properties. The mechanisms were further discussed deeply by analyzing the surface electrochemical reactions before and after modification. 2 Experimental section 2.1 Sample preparation 2.1.1 Preparation of TiO2 NTAs Highly orderly TiO2 NTAs were prepared by anodization method similar to previous work.14 Briefly, the Ti foil as working electrode was put into a electrolytic cell with two-electrode containing a graphite counter electrode. The titanium foils were carried out in a glycol ethylene electrolyte containing 0.15 M NH4F (5 vol% water) under 60 V for 6 hours. After that, TiO2 NTAs were obtained and then washed several times to remove the residual solution. The samples were ultrasonic vibrated in ethylene glycol for 1 min to remove the debris covering on the upper surface of TiO2 NTAs. Finally, the dried TiO2 NTAs were carried put in in a muffle furnace annealed at 500℃ for 2 hours, from which the anatase TiO2 were obtained. 2.1.2 Preparation of α-Bi2Mo3O12/TiO2 NTAs The modification of α-Bi2Mo3O12 was achieved by a sequential chemical bath deposition (CBD) 4


Page 4 of 23

Page 5 of 23

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

Langmuir

method. In a typical procedure, TiO2 NTAs were sequentially immersed in two ethylene glycol solutions containing Bi(NO3)3 with different concentrations and (NH4)7Mo7O24 with relative concentrations for each 2 min to complete a deposition at room temperature.

Scheme 1. Schematic diagram of the procedures for preparation of α-Bi2Mo3O12/TiO2 NTAs. 2.2 Characterizations X-ray diffraction patterns of the samples were obtained on an X-ray diffractometer (X'Pert PRO MPD, Netherland) using Cu Kα radiation and operating at 40 kV/40 mA. The Raman spectra were recorded by a LabRam HR Evolution (Horiba Jobin Yvon, France) instrument, which was performed with the excitation wavelength of 532 nm. Morphologies of the samples were observed by field-emission scanning electron microscope (FE-SEM) (Hitachi SU8020, Japan). The chemical compositions of α-Bi2Mo3O12/TiO2 NTAs were investigated by X-ray photoelectron spectroscopy (Thermo ESCALAB250Xi, America) using Al Kα monochromatized radiation. The binding energies of the spectra were calibrated to C1s line at 284.7 eV. The UV-vis light absorption were recorded with a Hitachi UV-3600 spectrophotometer (Japan) using BaSO4 as a reference to measure the samples. 2.3 Photoelectrochemical measurement Photoelectrochemical

measurements

were

carried

out

in

a

self-made

photoelectrochemical cell based on nanotube arrays, as described before.

13

flow-injection

In a typical procedure,

α-Bi2Mo3O12/TiO2 NTAs were put on a copper plate as working electrode. The photocurrent in buffer solution were measured in amperometric mode. The bias potential was set as 0.2 V. 0.1 mM glucose was added in buffer solution to get the current response. In addition to that, cyclic voltammograms were measured with potentials ranging from 0 to 1 V and a sweeping rate of 10 mV/s. All experiments were carried out under ambient conditions. 5


Langmuir

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

Organics determination performance, including determination sensitivity, range linear and detection limit, were also studied by the amperometric method. Phosphate buffer solution was pumped into the thin cell to get a stable background photocurrent under UV light illumination. Glucose solution with fixed concentration was injected into the thin cell to get an increment of photocurrent, and this step was repeated as many times as possible in purpose to get the relationship between current increment and glucose concentration. 3 Results and discussion 3.1 Characterization

Figure 1. Typical SEM morphologies of TiO2 and α-Bi2Mo3O12/TiO2 NTAs, (a) top images of TiO2 NTAs, (b) and (c) top images of α-Bi2Mo3O12/TiO2 NTAs, (d) EDS spectrum of α-Bi2Mo3O12/TiO2 NTAs Figure 1 shows the SEM morphologies of TiO2 and the α-Bi2Mo3O12/TiO2 NTAs. As shown in Figure 1a, highly ordered, vertically aligned TiO2 NTAs were formed on Ti substrate, with tube diameter ranging from 130 to 145 nm and walls thicknesses ranging from 10 to 14 nm. After a simple sequential chemical bath deposition at room temperature, α-Bi2Mo3O12/TiO2 NTAs were 6


Page 6 of 23

Page 7 of 23

obtained with α-Bi2Mo3O12 distributed on the top surface of the nanotubes, as shown in Figure 1b and 1c. The α-Bi2Mo3O12/TiO2 NTAs still remain well-ordered and uniform tubular structure, confirming that this simple assembly method has no influence to the basic structure of TiO2 NTAs. The composition of α-Bi2Mo3O12/TiO2 NTAs were determined by EDX spectroscopy, as shown in Figure 1d, mainly containing the elements Ti, O, Bi and Mo, respectively. (i) TiO2 NTAs

a

(004)

α -Bi2Mo3O12/TiO2 NTAs

Eg

TiO2 NTAs α -Bi2Mo3O12/TiO2 NTAs

(101)

(ii) Ti (101)

(i)

Intensity / (a.u.)

Intensity / (a.u.)

(iii) α -Bi2Mo3O12

Relative intensity

TiO2 NTAs

b

(ii) α -Bi2Mo3O12/TiO2 NTAs

(032)

(iii)

B1g

JCPDS no.23-1033

10

20

30

40

50

60

70

80

100

200

300

400

2 Theta / degree

B1g+A1g

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

Langmuir

500

954

600

700

Eg

600

700

800

Raman shift / cm-1

800 900 -1

900

1000

1000 1100 1200

Raman shift / cm

Figure 2. XRD patterns (a) and Raman spectra (b) of TiO2 NTAs and α-Bi2Mo3O12/TiO2 NTAs. XRD analysis were employed to characterize the crystal phase of the α-Bi2Mo3O12/TiO2 NTAs, as shown in Figure 2a. For comparison, the XRD patterns of pure TiO2 NTAs and the products of the direct reaction between two solutions of Bi(NO3)3 and (NH4)7Mo7O24 were also characterized. Curve (i) in Figure 2a shows the XRD pattern of TiO2 NTAs after being annealed at 500℃. All diffraction peaks can be well-indexed by anatase TiO2 phase (JCPDS file no. 21-1272) and the metallic Ti phase (JCPDS file no. 44-1294), which are originated from the TiO2 NTAs and Ti substrate respectively. However, compared to pure TiO2 NTAs, no obvious peak for α-Bi2Mo3O12 can be observed in the composite nanotube arrays, as shown in curve (ii), which can be ascribed to the small amount and low crystallinity of assembled α-Bi2Mo3O12. In consideration that α-Bi2Mo3O12 were synthesized by sequential chemical bath method, the XRD pattern of the products of the direct reaction was shown in curve (iii). It is very clear that there is only an appreciably broad diffraction peak at 2θ from 26 to 30° in the XRD pattern of the products, indicating the poor crystallinity, which is consisted with the results of α-Bi2Mo3O12 prepared as the references.26, 27 Raman scattering, as a local probe, is very sensitive to crystallinity and the microstructures of materials. Raman spectroscopy was employed to confirm the formation of a TiO2 anatase phase and the existence of α-Bi2Mo3O12 on the surface of the TiO2 NTAs. The typical Raman spectra of TiO2 7


Langmuir

and α-Bi2Mo3O12/TiO2 NTAs are shown in Figure 2b. The Raman peaks at approximately 145, 396, 516, 638 cm-1 observed in both samples can be assigned to the Eg, B1g, B1g or A1g, Eg modes of the anatase phase, respectively.28, 29 In addition to that, it’s clearly that the Raman spectra show much different in the region from 800 cm-1 to 1000 cm-1 between TiO2 and α-Bi2Mo3O12/TiO2 NTAs. Evidently, the specific band appearing at 954 cm-1 can be assigned to the crystalline phase of α-Bi2Mo3O12.30, 31 Hence, the results of XRD and Raman spectra show that following the synthesis route proposed in this work, it is possible to prepare the molybdate α-Bi2Mo3O12. O 1s

a

1200000

α -Bi2Mo3O12/TiO2 NTAs

1000000

Counts / s

800000

Ti 2p

600000

Mo 3d Bi 4f 400000

C 1s 200000

0 600

400

200

0

Binding Energy / eV 70000 60000

45000

b Bi 4f

c Mo 3d 232.7 eV

40000

159.5 eV

50000 40000

35000

164.8 eV

235.8 eV

30000

Counts / s

Counts / s

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

Page 8 of 23

30000 20000

25000 20000 15000

10000 10000 0 168

5000 166

164

162

160

158

156

154

240

238

Binding Energy / eV

236

234

232

230

228

226

224

Binding Energy / eV

Figure 3. XPS spectra of α-Bi2Mo3O12/TiO2 NTAs, (a) survey patterns, high resolution patterns of Bi 4f (b) and Mo 3d (c) electrons. The elemental compositions and chemical valence status of the α-Bi2Mo3O12/TiO2 NTAs were further analyzed by XPS. Figure 3a shows the XPS survey patterns of the α-Bi2Mo3O12/TiO2 NTAs, confirming the existence of Ti, O, Bi, Mo and C elements. The binding energy peaks of Bi and Mo were also analyzed, and the corresponding high-resolution spectra are shown in Figure 3b and 3c. In Figure 3b, the peaks at 159.5 and 164.8 eV can be ascribed to Bi 4f7/2 and Bi 4f5/2 respectively, 8


Page 9 of 23

confirming that bismuth species in α-Bi2Mo3O12/TiO2 NTAs are Bi3+ cations. Figure 3c shows the Mo 3d peaks at binding energies of 232.7 eV (Mo 3d5/2) and 235.8 eV (Mo 3d3/2), which can be ascribed to the photoelectron peaks of Mo6+ in α-Bi2Mo3O12.32, 33 In addition, the atomic percentages of Bi, Mo and O in the products of the direct reaction between the two solutions is 7.95%, 11.04% and 53.85%, respectively, which was also consisted with the atomic percentages of α-Bi2Mo3O12. Therefore, the observed XPS results confirm the successful fabrication of the α-Bi2Mo3O12/TiO2 NTAs. 3.2 Photoelectrochemical measurement 200

with glucose addition

-2

Current density / µA⋅cm

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

Langmuir

150

a 100

b c

50

a TiO2 NTAs

d

b 10 mM c 20 mM d 30 mM

0 0

200

400

600

800

1000

Time / s Figure 4 Photocurrents and current responses of TiO2 and α-Bi2Mo3O12/TiO2 NTAs obtained with different Bi3+ concentrations. Figure 4 shows the photocurrents and current responses to glucose addition (0.1 mM) of TiO2 and α-Bi2Mo3O12/TiO2 NTAs obtained with different Bi3+ concentrations, which were measured by the amperometric method under an applied potential of 0.2 V in buffer solution using a UV light with optical powers of 3%. With α-Bi2Mo3O12 modification, the photocurrent decreases evidently from 143.0 µA·cm-2 to 64.95, 42.95 and 24.74 µA·cm-2 of α-Bi2Mo3O12/TiO2 NTAs obtained with Bi3+ concentrations of 10 mM, 20 mM and 30 mM, respectively. Meanwhile, the current responses to glucose of TiO2 NTAs increase from 10.83 to 22.34, 28.75 and 27.17 µA·cm-2 with the optimized current response at concentration of 20 mM. Both the lower photocurrents and higher current 9


Langmuir

response are key issues for enhancing the photoeletrcochemical COD detection performance of TiO2 NTAs. 100

a

(iv)

-2

∆I=10.31 µAcm

160

-2

∆I=6.49 µAcm

(iii) 140 120

(i) in dark in buffer (ii) in dark in glucose (iii) under illumination in buffer (iv) under illumination in glucose

100 80 60 40 20

(i) 0 0.0

b

-2

180

Current intensity / µA⋅cm

-2

200

Current intensity / µA⋅cm

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

Page 10 of 23

(iv) 2

80

∆I=29.70 µAcm

2

∆I=25.85 µAcm

(iii)

60

(i) in dark in buffer (ii) in dark in glucose (iii) under illumination in buffer (iv)under illumination in glucose (i)

40

20

0

(ii)

(ii) 0.2

0.4

0.6

0.8

1.0

0.0

0.2

Potential / V

0.4

0.6

0.8

1.0

Potential / V

Figure 5. CVs of TiO2 (a) and α-Bi2Mo3O12/TiO2 (b) NTAs measured under different conditions. In order to investigate and compare the electrochemical activities of TiO2 and α-Bi2Mo3O12/TiO2 NTAs, the cyclic voltammograms were measured, as shown in Figure 5. Figure 5a shows the CVs of TiO2 NTAs in buffer solution in absence and presence of 0.1 mM glucose in dark and under illumination, respectively. Curve (i) and (ii) compare CVs of TiO2 NTAs without and with addition of 0.1 mM glucose in dark in the buffer solution, which shows simple electrochemical behavior in the two solutions. With addition of glucose in buffer solution, the currents decrease a little, indicating that almost no electrochemical oxidation of glucose on the surface of TiO2 NTAs in dark. The electrochemical oxygen evolution of TiO2 were restricted in dark with the addition of glucose. Curve (i) and (iii) show the photocurrent of TiO2 NTAs at different potentials. Current increases evidently when UV light turns on. The comparison of curve (iii) and (iv) indicate the current response of TiO2 NTAs. Current responses of TiO2 NTAs are 10.31 µA·cm-2 (0.2 V) and 6.49 µA·cm-2 (1.0 V) respectively. High potential shows no effect on enhancing the current response to organics. Figure 5b shows the CVs of α-Bi2Mo3O12/TiO2 NTAs under different conditions. Decrease of current in the whole potentials with glucose addition indicate that α-Bi2Mo3O12/TiO2 NTAs show no electroactivity to glucose in dark, as shown in curve (i) and (ii). The photocurrents in whole potentials obtained from curve (i) and (iii) decrease evidently, as compared with that of TiO2 NTAs. Comparison of curve (iii) and (iv) shows the current response of α-Bi2Mo3O12/TiO2 NTAs at different potentials. Evidently, the current response of α-Bi2Mo3O12/TiO2 NTAs are much higher 10


Page 11 of 23

than that of unmodified TiO2 NTAs, such as increase from 10.31 µA·cm-2 to 29.70µA·cm-2. Also, high potential cannot enhance the photocatalytic oxidation of glucose on α-Bi2Mo3O12/TiO2 NTAs any more. 500

Current increment / µA·cm-2

-2

a 400

3% 6% 9%

300

54

C urrent intensity / µ Acm -2

Current intensity / µA·cm

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

Langmuir

200

100

3% 6% 9%

52

50 -2

0.25 µAcm 48

0.24 µAcm

-2

46 -2

0.17 µAcm 44 250

300

350

500

1000

1500

2000

2500

300

3% 6% 9%

200 Slope

100

400

Time / s

0

b 400

3%

1.16789

Standard Error 0.03376

0~95.11

Range

Adj. R-Squar e 0.992

6%

0.96742

0.02364

0~320.95

0.988

9%

2.04616

0.02845

0~208.90

0.997

0 3000

0

100

Time / s

200

300 -1

400

COD Range / mgL

Figure 6. Photoelectrochemical COD determination performance of α-Bi2Mo3O12/TiO2 NTAs to organics: (a) curve of current intensity-time curve, (b) plots of current increments vs. COD range. Figure 6 shows the photoelectrochemical COD determination performances of α-Bi2Mo3O12/TiO2 NTAs to organics, which gives the detection parameters of sensitivity, linear COD range and detection limit. From the previous literatures,13, 14 the determination performances of TiO2 NTAs have been investigated deeply, hence, only the amperometric response of modified TiO2 NTAs were measured in this work. Figure 6a shows the current change with successive glucose addition under UV light illumination with different optical powers (3%, 6% and 9%). The start point of the current-time curves were shifted together for convenient comparison. The samples show excellent current response to glucose addition, and the currents increase rapidly with glucose addition. The plots of current increment vs. COD range are shown in Figure 6b, from which determination sensitivity, COD linear range and detection limit (dl) of α-Bi2Mo3O12/TiO2 NTAs can be obtained. The determination sensitivity (slop) of α-Bi2Mo3O12/TiO2 NTAs (3%) is 1.17 µA·cm-2/(mg·L-1). The determination sensitivity changes from 1.17 µA·cm-2/(mg·L-1) to 0.98 µA·cm-2/(mg·L-1), and 2.05 µA·cm-2/(mg·L-1) with the enhancement of optical power, respectively. Beside, wider COD linear range from 0 to 320 mg·L-1 (200 mg·L-1) can be achieved with optical power of 6% (9%), while at the optical power of 3% the linear range is only from 0 to 95.11 mg·L-1. confirming that COD determination range can be regulated by the optical power. The optimized detection limit (dl) can be calculated based on the sensitivity of 2.05 µA·cm-2/(mg·L-1) and current noise of 0.25 µA·cm-2 to be 0.366 mg·L-1 at optical power output of 9%. Of course, in comparison with the detection 11


Langmuir

performances of unmodified TiO2 NTAs, the detection sensitivity and detection limit of α-Bi2Mo3O12/TiO2 NTAs were much higher than that of TiO2 NTAs for photoelectrochemical COD detection application. 3.3 Mechanism Discussion Herein, we report a warm assembly method to obtain α-Bi2Mo3O12/TiO2 NTAs with enhancing photoelectrochemical determination performance. And the mechanism will be discussed through whole the photoelectrochemical processes, such as optical absorption, charges transfer and surface reactions between TiO2 and α-Bi2Mo3O12/TiO2 NTAs. 3.3.1 Optical absorption 1.8


1.6

Intensity / (a.u.)

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

Page 12 of 23

365 nm

1.4

1.2

∆ = 0.034 1.0

0.8

0.6 200

300

400

500

600

700

800

Wavelength / nm Figure 7. Optical absorptions of TiO2 and α-Bi2Mo3O12/TiO2 NTAs. The optical absorptions of TiO2 and α-Bi2Mo3O12/TiO2 NTAs were shown in Figure 7. The α-Bi2Mo3O12 modification does not significantly affect the optical absorption of TiO2 NTAs at wavelength of 365 nm, the fixed wavelength of UV LED spot source used in photoelectrochemical determination in this work. Hence, optical absorption is not the factor enhancing the photoelectrochemical performances of α-Bi2Mo3O12/TiO2 NTAs. 3.3.2 Charges transfer

12


Page 13 of 23

300

a

TiO2 NTA

b (ahv)2 / (a.u.)

Intensity / (a.u.)

α -Bi2Mo3O12

200

TiO2 NTAs 100

3.23 200

300

400

500

600

700

0 2.5

800

3.0

Wavelength / nm

3.5

Photo energy / eV

150

c

d Intensity / (a.u.)

100

(ahv)2 / (a.u.)

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

Langmuir

α -Bi2Mo3O12 50

2.21 TiO2 NTAs

2.89

3.02

α -Bi2Mo3O12 0 2.0

2.5

3.0

-2

3.5

0

2

4

6

8

10

12

Binding energy / eV

Photo energy / eV

Figure 8. UV-Vis absorption spectra (a), transformed Kubelka-Munk function vs. photon energy curves (b) and (c) and XPS valance band edge (d) of TiO2 NTAs and α-Bi2Mo3O12 In order to clarify the charge transfer between TiO2 and α-Bi2Mo3O12, the band gaps and valence bands were studied at first, as shown in Figure 8. Figure 8a shows the optical absorption of TiO2 NTAs and α-Bi2Mo3O12, from which the band gaps of TiO2 NTAs and α-Bi2Mo3O12 can be estimated from the plot of (ahv)2 (For direct band gap) verus photo energy (hv), as shown in Figure 8b and 8c.34-37 The intercepts of the tangent to the x-axis give good approximation of the band gap of TiO2 NTAs and α-Bi2Mo3O12 to be approximate 3.23 eV and 3.02 eV respectively. The XPS valence band spectra of TiO2 NTAs and α-Bi2Mo3O12 were presented in Figure 8d. The VBs of TiO2 NTAs and α-Bi2Mo3O12 are 2.89 and 2.21 eV respectively. Hence, combined the results of the band gaps with the XPS valence band, the CBs of TiO2 NTAs and α-Bi2Mo3O12 can be calculated to be -0.34 and -0.81 eV respectively.

13


Langmuir

50

a

30000


b

TiO2 NTAs - dark α -Bi2Mo3O12/TiO2 NTAs - dark

TiO2 NTAs - light

25000

40

α -Bi2Mo3O12/TiO2 NTAs - light 2500

20000 30

20

2000

TiO2 NTAs - dark α -Bi2Mo3O12/TiO2 NTAs - dark

15000

TiO2 NTAs - light α -Bi2Mo3O12/TiO2 NTAs - light

1500

-Z'' (Ω)

-Z'' (Ω)

Intensity / (a.u.)

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

Page 14 of 23

10000 10

1000

500

5000 0 0

0

200

400

0 400

450

500

0

10000

Wavelength / nm

20000

600

800

Z' (Ω)

1000

1200

1400

30000

40000

Z' (Ω)

Figure 9. PL emission spectra (a) and Nyquist plots (b) of TiO2 and α-Bi2Mo3O12/TiO2 NTAs Photoluminescence measurement was widely used to assess the charge carrier recombination behaviors.38, 39 The PL spectra of TiO2 and α-Bi2Mo3O12/TiO2 NTAs samples are shown in Figure 9a. The α-Bi2Mo3O12/TiO2 NTAs exhibited much lower emission intensity than that of TiO2 NTAs, indicating that the introduction of α-Bi2Mo3O12 can restrict the recombination of photo-generated electron-hole pairs of TiO2 NTAs. For further investigation, electrochemical impedance spectroscopy (EIS) is an effective method to study the charge transfer process in the photoelectrochemical process.40, 41 Figure 9b exhibits EIS plots of different electrodes in the frequency range from 10-1 to 105 at a potential of 0.1 V. The Nyquist plot of the α-Bi2Mo3O12/TiO2 NTAs electrode exhibits similar semicircle with that of TiO2 NTAs in dark, indicating α-Bi2Mo3O12 modification cannot enhance the electric conductivity of TiO2 nanotubes. However, the semicircle of α-Bi2Mo3O12/TiO2 NTAs under illumination is much small than that of TiO2 NTAs, implying fast transfer of photogenerated charge carriers under illumination. The efficient charge separation and fast charge transfer increase the lifetime of the charge carriers and enhance the photoelectrochemical efficiency, which are the basis for photoelectrochemical determination.42 However, they still cannot explain the low background photocurrents in buffer solution and high current response with organics addition. 3.3.3 Photoelectrochemical processes and surface reactions

14


Page 15 of 23

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

Langmuir

Scheme 2. Proposed schematic diagram of photoelectrochemical processes and surface reactions of α-Bi2Mo3O12/TiO2 NTAs. Under UV light illumination, both α-Bi2Mo3O12 and TiO2 NTAs will be excited and generate photo-generated

holes and

electrons

pairs.

And

the

proposed

schematic

diagram

of

photoelectrcochemical processes of α-Bi2Mo3O12/TiO2 NTAs is shown in Scheme.2. From the energy band structure diagram of α-Bi2Mo3O12/TiO2 NTAs, it can be found that the photo-generated electrons transfer from the CB of α-Bi2Mo3O12 to the CB of TiO2. Conversely, the photo-generated holes transfer from the VB of TiO2 to the VB of α-Bi2Mo3O12, indicating the efficient separation of photo-generated electron-hole pairs and different electrochemical surface reactions. Based on our previous studies, the surface reactions play key roles during the photoelectrochemical determination of TiO2 NTAs, especially, the direct organics oxidation by holes and the indirect organics oxidation by ·OH radicals formed by holes.15 Concerned reactions on α-Bi2Mo3O12/TiO2 NTAs are listed as follows: TiO2 + hv → TiO2 (e- + h+)

(1)

TiO2 (h+) + OH- → ·OH

(2)

TiO2 (h+) + organics →······→ CO2 + H2O + ······

(3)

·OH+ organics →······→ CO2 + H2O + ······

(4)

TiO2 (h+) → α-Bi2Mo3O12 (h+)

(5)

α-Bi2Mo3O12 (h+) + OH- → ·OH

(6) 15


Langmuir

α-Bi2Mo3O12 (h+) + organics →······→ CO2 + H2O + ······

(7)

Hence, it’s necessary to investigate the influence between the direct oxidation (reaction (3)) and the indirect oxidation (reaction (2) and (4)) during the surface reaction of TiO2 NTAs and α-Bi2Mo3O12/TiO2 NTAs respectively. (a) Blank solution (b) TiO2 NTAs (c) a -Bi2Mo3O12/TiO2 NTAs

Intensity / (a.u.)

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

Page 16 of 23

(b)

(c) (a)

400

450

500

550

600

Wavelength / nm Figure 10. Photoluminescence patterns of terephthalic acid after being illuminated under UV light with TiO2 and α-Bi2Mo3O12/TiO2 NTAs. Firstly, in order to compare ·OH concentrations of TiO2 and α-Bi2Mo3O12/TiO2 NTAs in the solution during their photoelectrochemical processes, the terephthalic acid oxidation method were used.43, 44 Terephthalic acid can react with ·OH formed by the holes radicals of photocatalyst under relative excitation light illumination, and the product of dihydroxy terephthalic acid shows a emission peak at around 420 nm when excited by UV light at 315 nm. The ·OH concentration in terephthalic acid solution will be presented by the intensity of emission peak. Figure 10 shows the photoluminescence patterns of terephthalic acid after being illuminated under UV light with TiO2 and α-Bi2Mo3O12/TiO2 NTAs. The terephthalic acid after being illuminated without photocatalyst shows no evidently emission, as shown in curve (a). Comparison with the blank solution, the emission peak at 420 nm appears when terephthalic acid treated with TiO2 or α-Bi2Mo3O12/TiO2 NTAs, the emission intensity of terephthalic acid treated with α-Bi2Mo3O12/TiO2 NTAs (curve (c)) is lower than that treated with TiO2 NTAs (curve (b)), which indicates that the surface reaction (2) during photoelectrochemical process will be restricted with the modification of α-Bi2Mo3O12. In other way, this results means that the photo-generated holes of α-Bi2Mo3O12/TiO2 16


Page 17 of 23

NTAs possess low productivity of ·OH radicals than that of TiO2 NTAs. Based on the results, we can explain the low background photocurrent of α-Bi2Mo3O12/TiO2 NTAs. When being measured in buffer solution, the background photocurrent depends on the consumption of holes through the oxygen evolution reaction. The oxygen evolution reaction on α-Bi2Mo3O12/TiO2 NTAs is restrained due to low reaction activity between holes and H2O on α-Bi2Mo3O12 (corresponding to reaction (6)). Hence, low consumption rate of holes on α-Bi2Mo3O12/TiO2 NTAs results in low background photocurrent. And in order to further confirm those results, the active species (·OH and holes) during the photoelectrochemical processes of TiO2 and α-Bi2Mo3O12/TiO2 NTAs were investigated by the addition of trapping agents.

a

Current density / µA·cm

-2

with 10 mM IPA

250

239.62

with 10 mM AO

200

without addition

155.34

150

b

350

295.77

-2

300

Current density / µA·cm

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

Langmuir

100

50

338.05

with 10 mM AO

245.23

with 10 mM IPA

50.24

without addition

300 250 200 150 100 50 0

0 0

100

200

300

400

500

0

600

100

200

300

400

500

600

Time / s

Time / s

Figure 11. The photocurrents of TiO2 and α-Bi2Mo3O12/TiO2 NTAs in buffer solution with additions of AO and IPA respectively. Trapping agents for h+ (ammonium oxalatein (AO)) and ·OH (isopropyl alcohol (IPA)) were added in the electrolyte for clarifying the surface reaction during the photoelectrochemical process. Figure 11 shows the photocurrent of unmodified TiO2 and α-Bi2Mo3O12/TiO2 NTAs with addition of trapping agents. The photocurrent of TiO2 NTAs increase from 155.34bµA·cm-2 to 239.62 µA·cm-2 and 295.77 µA·cm-2 with 10 mM AO and IPA addition respectively, as shown in Figure 11a, indicating that ·OH radicals are the main active species during the photoelectrochemical COD detection process. However, it’s much different for α-Bi2Mo3O12/TiO2 NTAs, as shown in Figure 11b. The photocurrent increase from 50.24 µA·cm-2 to 338.05µA·cm-2 and 245.23 µA·cm-2 with AO and IPA addition respectively. Direct oxidation by holes play key roles on organics oxidation during 17


Langmuir

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

photoelectrochemical COD detection process. Small amount of ·OH radicals was produced on α-Bi2Mo3O12/TiO2 NTAs during photoelectrochemical process, which is consist with the results of ·OH concentration measurement by photoluminescence methods in Figure 10. On the basis of the above results, the significant enhancement in photoelectrochemical determination performance of TiO2 with the introduction of α-Bi2Mo3O12 depends on the reduction of recombination between the photo-generated electrons and holes by the combination of α-Bi2Mo3O12 and TiO2 NTAs. And besides, both the surface electrochemical reaction of TiO2 NTAs regulated by the introduction of α-Bi2Mo3O12 and the direct oxidation of organics by the holes radicals play much key roles for enhancing the determination performances. 4 Conclusions In summary, we have synthesized α-Bi2Mo3O12 modified TiO2 nanotube arrays with enhanced photoelectrochemical determination performance successfully by sequential chemical bath deposition at room temperature. Herein, a series of characterization techniques, including field-emission scanning electron microscopy, X-ray diffraction, Raman and X-ray photoelectron spectroscopy were used to study the morphologies, structures and elemental analysis of the products respectively. The photoelectrochemical properties of TiO2 and α-Bi2Mo3O12/TiO2 NTAs were measured, and α-Bi2Mo3O12/TiO2 nanotube arrays possess lower background photocurrent in buffer solution and enhanced current response to organics at the same time, both of which are benefit for photoelectrochemical COD detection of organics with high determination sensitivities and effective detection COD range. The optimized α-Bi2Mo3O12/TiO2 NTAs photoelectrochemical detection sensor can achieve the determination sensitivity of 2.05 µA·cm-2/(mg·L-1) and detection COD range of 0.366~208.9 mg/L. Direct oxidation of organics by holes on α-Bi2Mo3O12 play key role on enhancing the photoelectrochemical determination performances. ACKNOWLEDGEMENTS This work was supported by National Basic Research Program of China (973 Project, 2014CB660815), Nature Science Foundation of China (51102071, 51172059, 51272063 and 51402078), Fundamental Research Funds for the Central Universities (2013HGQC0005, JZ2016HGTB0711), Nature Science Foundation of Anhui Province (1408085QE86, 1608085QE105) and Laboratory of Non-Ferrous Metals and Processing Engineering of Anhui Province (15czs0803). AUTHOR INFORMATION Corresponding Authors 18


Page 18 of 23

Page 19 of 23

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

Langmuir

*Tel: +86 551 62901372. Email: [email protected] *Tel: +86 551 62901372. Email: [email protected] Notes The authors declare no competing financial interest. References [1] Fujishima, A. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. [2] Khan, S. U.; Alshahry, M.; Jr, I. W. Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2. Science 2002, 297 (5590), 2243-2245. [3] Meng, N. C.; Bo J.; Chow, C. W. K.; Saint, C. Recent Developments in Photocatalytic Water Treatment Technology: A Review. Water Res. 2010, 44 (10), 2997-3027. [4] Ge, M.; Cao, C.; Huang, J.; Li, S.; Chen, Z.; Zhang, K. Q.; Al-Deyab S. S.; Lai, Y. A Review of One-dimensional TiO2 Nanostructured Materials for Environmental and Energy Applications. J. Mater. Chem. A 2016, 4 (18), 6772-6801. [5] Georgieva, J.; Valova, E.; Armyanov, S.; Philippidis, N.; Poulios, I.; Sotiropoulos, S. Bi-component semiconductor oxide photoanodes for the photoelectrocatalytic oxidation of organic solutes and vapours: a short review with emphasis to TiO2-WO3 photoanodes. J. Hazard. Mater. 2012, 211, 30-46. [6] Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Understanding TiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014, 114 (19), 9919-9986. [7] Liu, Z.; Hou, W.; Pavaskar, P.; Aykol, M.; Cronin, S. B. Plasmon Resonant Enhancement of Photocatalytic Water Splitting Under Visible Illumination. Nano lett. 2011, 11 (3), 1111-1116. [8] Roy, P.; Das, C.; Lee, K.; Hahn, R.; Ruff, T.; Moll, M.; Schmuki, P. Oxide Nanotubes on Ti-Ru Alloys: Strongly Enhanced and Stable Photoelectrochemical Activity for Water Splitting. J. Am. Chem. Soc. 2011, 133 (15), 5629-5631. [9] Li, Y.; Yu, H.; Zhang, C.; Song, W.; Li, G.; Shao, Z.; Yi, B. Effect of Water and Annealing Temperature of Anodized TiO2 Nanotubes on Hydrogen Production in Photoelectrochemical Cell. Electrochim. Acta 2013, 107, 313-319. 19


Langmuir

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

[10] Zheng, Q.; Zhou, B.; Bai, J.; Li, L.; Jin, Z.; Zhang, J.; Li, J.; Liu, Y.; Cai, W.; Zhu, X. Self-Organized TiO2 Nanotube Array Sensor for the Determination of Chemical Oxygen Demand. Adv. Mater. 2010, 20 (5), 1044-1049. [11] Natarajan, T. S.; Natarajan, K.; Bajaj, H. C.; Tayade, R. J. Energy Efficient UV-LED Source and TiO2 Nanotube Array-based Reactor for Photocatalytic Application. Ind. Eng. Chem. Res. 2011, 50 (13), 7753-7762. [12] Li, S.; Qiu, J.; Ling, M.; Peng, F.; Wood, B.; Zhang, S. Photoelectrochemical Characterization of Hydrogenated TiO2 Nanotubes as Photoanodes for Sensing Applications. ACS Appl. Mater. Inter. 2013, 5 (21), 11129-11135. [13] Feng, C.; Xu, G.; Liu, H.; Lv, J.; Zheng, Z.; Wu, Y. A Flow-Injection Photoelectrochemical Sensor Based on TiO2 Nanotube Arrays for Organic Compound Detection. J. Electrochem. Soc. 2014, 161 (3), H57-H61. [14] Pang, Y.; Xu, G.; Zhang, X.; Lv, J.; Shi, K.; Zhai, P.; Xue, Q.; Wang, X.; Wu, Y. Photoelectrochemical Properties and the Detection Mechanism of Bi2WO6 Nanosheet Modified TiO2 Nanotube Arrays. Dalton Trans. 2015, 44 (40), 17784-17794. [15] Pang, Y.; Xu, G.; Fan, C.; Lv, J.; Qin, Y.; Zhang, Y.; Wu, Y. TiO2 Nanotube Arrays: A Study on the Surface Electrochemical Reactions During the Photoelectrochemical Process. J Solid State Electrochem. 2017, 21 (4), 987-993. [16] Zhou, Y.; Wang, H.; Sheng, M.; Zhang, Q.; Zhao, Z.; Lin, Y.; Liu, H.; Patzke, G. R. Environmentally friendly room temperature synthesis and humidity sensing applications of nanostructured Bi2O2CO3. Sensor. Actuat. B: Chem. 2013, 188, 1312-1318. [17] Zhao, Z.; Zhou, Y.; Wang, F.; Zhang, K.; Yu, S.; Cao, K. Polyaniline-decorated {001} facets of Bi2O2CO3 nanosheets: In situ oxygen vacancy formation and enhanced visible light photocatalytic activity. ACS Appl. Mater. & Inter. 2014, 7(1), 730-737. [18] Zhou, Y.; Zhang, X.; Zhang, Q.; Dong, F.; Wang, F.; Xiong, Z. Role of graphene on the band structure and interfacial interaction of Bi2WO6/graphene composites with enhanced photocatalytic oxidation of NO. J. Mater. Chem. A, 2014, 2(39), 16623-16631. [19] Monfort, O.; Pop, L. C.; Sfaelou, S.; Plecenik, T.; Roch, T.; Dracopoulos, V.; Stathatos, E.; Plesch, G.; Lianos, P. Photoelectrocatalytic hydrogen production by water splitting using BiVO4 photoanodes. Chem. Eng. J. 2016, 286, 91-97. 20


Page 20 of 23

Page 21 of 23

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

Langmuir

[20] Monfort, O.; Sfaelou, S.; Satrapinskyy, L.; Plecenik, T.; Roch, T.; Plesch, G.; Lianos, P. Comparative

study

between

pristine

and

Nb-modified

BiVO4

films

employed

for

photoelectrocatalytic production of H2 by water splitting and for photocatalytic degradation of organic pollutants under simulated solar light. Catal. Today 2017, 280, 51-57. [21] Li, H.; Li, K.; Wang, H. Hydrothermal Synthesis and Photocatalytic Properties of Bismuth Molybdate Materials. Mater. Chem. Phys. 2009, 116 (1), 134-142. [22] LuÉVano-HipÓLito, E.; MartÍNez-De La Cruz, A.; Yu, Q. L.; Brouwers, H. J. H. Photocatalytic Removal of Nitric Oxide by Bi2Mo3O12 Prepared by Co-Precipitation Method. Appl. Catal. A: Gen. 2013, 468, 322-326. [23] Dai, Z.; Qin, F.; Zhao, H.; Ding, J.; Liu, Y.; Chen, R. Crystal Defect Engineering of Aurivillius Bi2MoO6 by Ce Doping for Increased Reactive Species Production in Photocatalysis. ACS Catal. 2016, 6 (5), 3180-3192. [24] Gruar, R.; Tighe, C. J.; Reilly, L. M.; Sankar, G.; Darr, J. A. Tunable and Rapid Crystallisation of Phase Pure Bi2MoO6 (Koechlinite) and Bi2Mo3O12 Via Continuous Hydrothermal Synthesis. Solid State Sci. 2010, 12 (9), 1683-1686. [25] Zhou, D.; Li, W. B.; Pang, L. X.; Guo, J.; Qi, Z. M.; Shao, T.; Randall, C. A. Phase Evolution and Microwave Dielectric Properties of xBi2/3MoO4-(1- x)BiVO4 (00≤x≤10) Low Temperature Firing Ceramics. Dalton Trans. 2014, 43 (19), 7290-7297. [26] Cao, S. Y.; Chen, C. S.; Liu, T. G.; Tsang, Y. H.; Chen, W. W. Polymer-Assisted UV Excitation Method to Synthesize Reduced Graphene Oxide/α-Bi2Mo3O12 Nanoplates with High Activity. J. Environ. Chem. Eng. 2014, 2 (4), 2103-2110. [27] Cao, S. Y.; Chen, C. S.; Liu, T. G.; Tsang, Y. H.; Zhang, X. M.; Chen, W. W. Synthesis of Reduced Graphene Oxide/α-Bi2Mo3O12@β-Bi2O3 Heterojunctions by Organic Electrolytes Assisted UV-Excited Method. Chem. Eng. J. 2014, 257, 309-316. [28] Zhang, W. F.; He, Y. L.; Zhang, M. S.; Yin, Z.; Chen, Q. Raman Scattering Study on Anatase TiO2 Nanocrystals. J Phys. D: Appl. Phys. 2000, 33 (8), 912. [29] Song, P.; Zhang, X.; Sun, M.; Cui, X.; Lin, Y. Graphene Oxide Modified TiO2 Nanotube Arrays: Enhanced Visible Light Photoelectrochemical Properties. Nanoscale 2012, 4 (5), 1800-1804. [30] Hardcastle, F. D.; Wachs, I. E. Molecular Structure of Molybdenum Oxide in Bismuth Molybdates by Raman Spectroscopy. J. Phys. Chem. 1991, 95 (26), 10763-10772. 21


Langmuir

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

Page 22 of 23

[31] Ghule, A. V.; Ghule, K. A.; Tzing, S. H.; Chang, J. Y.; Chang, H.; Ling, Y. C. Pyridine Intercalative Sonochemical Synthesis and Characterization of α-Bi2Mo3O12 Phase Nanorods. Chem. Phys. Lett. 2004, 383 (1), 208-213. [32] Uchida, K.; Ayame, A. Dynamic XPS Measurements on Bismuth Molybdate Surfaces. Surf. Sci. 1996, 357, 170-175. [33] Devillers, M.; Tirions, O.; Cadus, L.; Ruiz, P.; Delmon, B. Bismuth Carboxylates As Precursors For The Incorporation Of Bismuth In Oxide-Based Materials. J. Solid State Chem. 1996, 126 (2), 152-160. [34] Butler, M. A. Photoelectrolysis and Physical Properties of the Semiconducting Electrode WO2. J. Appl. Phys. 1944, 48 (5), 1914-1920. [35] Park, J. H.; Kim, S.; Bard, A. J. Novel Carbon-Doped TiO2 Nanotube Arrays with High Aspect Ratios for Efficient Solar Water Splitting. Nano Lett. 2006, 6 (1), 24-28. [36] Wang, F.; Qian, X.; Li, X.; Ye, J.; Han, Z.; Chen, Y.; Liu, G.; Li, J. Optical-Thermal Properties of Reduced TiO2 Microspheres Prepared by Flame Spraying. Mater. Lett. 2015, 151, 82-84. [37] Kang, D.; Park, Y,; Hill J. C.; Choi, K. S. Preparation of Bi-Based Ternary Oxide Photoanodes BiVO4, Bi2WO6, and Bi2Mo3O12 Using Dendritic Bi Metal Electrodes. J. Phys. Chem. Lett. 2014, 5 (17), 2994-2999. [38] Gong, Y.; Wang, D.; Wu, R.; Gazi, S.; Soo, H. S.; Sritharan, T.; Chen, Z. New Insights into the Photocatalytic Activity of 3-D Core-Shell P25@Silica Nanocomposites: Impact of Mesoporous Coating. Dalton Trans. 2017, 46 (15), 4994-5002. [39] Sheng, P.; Li, W.; Cai, J.; Wang, X.; Tong, X.; Cai, Q.; Grimes, C. A. A Novel Method for the Preparation of a Photocorrosion Stable Core/Shell Cdte/Cds Quantum Dot TiO2 Nanotube Array Photoelectrode Demonstrating an AM 1.5 G Photoconversion Efficiency of 6.12%. J. Mater. Chem. A 2013, 1 (26), 7806-7815. [40] Leng, W. H.; Zhang, Z.; Zhang, J. Q.; Cao, C. N. Investigation of the Kinetics of a TiO2 Photoelectrocatalytic Reaction Involving Charge Transfer and Recombination Through Surface States by Electrochemical Impedance Spectroscopy. J. Phys. Chem. B 2005, 109 (31), 15008-1502. [41] Sun, M.; Fang, Y.; Kong, Y.; Sun, S.; Yu, Z.; Umar, A. Graphitic Carbon Nitride (g-C3N4) Coated

Titanium

Oxide

Nanotube

Arrays

with

Enhanced

Performance. Dalton Trans. 2016, 45 (32), 12702-12709. 22


Photo-Electrochemical

Page 23 of 23

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

Langmuir

[42] Zhang, M.; Shao, C.; Mu, J.; Zhang, Z.; Guo, Z.; Zhang, P.; Liu, Y. One-Dimensional Bi2MoO6/TiO2

Hierarchical

Heterostructures

with

Enhanced

Photocatalytic

Activity. CrystEngComm 2012, 14 (2), 605-612. [43] Yu, J.; Wang, W.; Cheng B.; Su, B. L. Enhancement of Photocatalytic Activity of Mesporous TiO2 Powders by Hydrothermal Surface Fluorination Treatment. J. Phys. Chem. C 2009, 113 (19), 6743-6750. [44] Li, H.; Xia, Z.; Chen, J.; Lei L.; Xing, J. Constructing Ternary CdS/Reduced Graphene Oxide/TiO2 Nanotube Arrays Hybrids for Enhanced Visible-Light-Driven Photoelectrochemical and Photocatalytic Activity. Appl. Catal. B Environ. 2015, 168, 105-113.

Table of Contents Graphic

23


TiO2 Nanotube Arrays for

Recommend Documents