Improved photocatalytic activity of TiO2 produced by an alcohothermal approach through in-situ decomposition of NH4HCO3

Accepted Manuscript

Improved photocatalytic activity of TiO2 produced by an alcohothermal approach through in-situ decomposition of NH4 HCO3

http://www.journals.elsevier.com/ journal-of-energy-chemistry/

Changyuan Hu , Chengjiang Lian , Shizheng Zheng , Xiaoyu Li , Tiewen Lu , Quanhong Hu , Shuwang Duo , Rongbin Zhang , Yingying Sun , Fei Chen PII: DOI: Reference:

S2095-4956(16)00038-3 10.1016/j.jechem.2016.02.003 JECHEM 114

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

19 September 2015 22 October 2015 2 November 2015

Please cite this article as: Changyuan Hu , Chengjiang Lian , Shizheng Zheng , Xiaoyu Li , Tiewen Lu , Quanhong Hu , Shuwang Duo , Rongbin Zhang , Yingying Sun , Fei Chen , Improved photocatalytic activity of TiO2 produced by an alcohothermal approach through in-situ decomposition of NH4 HCO3 , Journal of Energy Chemistry (2016), doi: 10.1016/j.jechem.2016.02.003

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Improved photocatalytic activity of TiO2 produced by an alcohothermal approach through in-situ decomposition of NH4HCO3 Changyuan Hua,b,*, Chengjiang Liana,c, Shizheng Zhenga,b, Xiaoyu Lia,b, Tiewen Lua,d, Quanhong Hua,b, Shuwang Duoa,b, Rongbin Zhangd, Yingying Suna,b, Fei Chena,b a

CR IP T

Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science and Technology Normal University, Nanchang 330013, Jiangxi, China

b

School of Materials and Mechanical & Electrical Engineering, Jiangxi Science and Technology

c

AN US

Normal University, Nanchang 330013, Jiangxi, China

Chemistry and Chemical Engineering College, Jiangxi Science and Technology Normal University, Nanchang 330013, Jiangxi, China d

M

College of Chemistry, Nanchang University, Nanchang 330013, Jiangxi, China

*Corresponding author. Tel/Fax: +86-791-83831266. E-mail: [email protected] (C. Hu).

CE

PT

ED

This work was supported by the National Natural Science Foundation of China (No. 21163008, No. 21366020), Jiangxi Collaborative Innovation Center for in vitro diagnostic reagents and instruments, the Natural Science Foundation of Jiangxi Province (No. 20114BAB203009), Scientific & Technological Project of Jiangxi Science and Technology Normal University (No. 2013ZDPYJD01), and Graduate Innovation Foundation of Jiangxi Science and Technology Normal University (No. YC2014-X2). Article history:

AC

Received 19 September 2015 Revised 22 October 2015 Accepted 2 November 2015 Available online

Abstract High crystallinity of TiO2 was prepared by a modified alcohothermal method, in which

1

ACCEPTED MANUSCRIPT

titanium isopropoxide was used as the titania precursor, absolute ethanol as the reaction medium, and NH4HCO3 as the raw materials for release of water, ammonia and carbon dioxides via in-situ decomposition. The X-ray powder diffraction (XRD) and transmission electron microscope (TEM) measurements showed that water and ammonia from the in-situ decomposition of NH4HCO3

CR IP T

played an important role in conducting the size, shape, crystallinity and microstructure of TiO2. The photoluminescence spectroscopy and photocurrent measurements indicated that enhanced crystallinity could hinder the recombination and promote the separation of electron-hole pairs in

AN US

TiO2, which contribute to the improvement of photocatalytic activity. Methyl orange photodegradation under UV light confirmed that high crystallinity of TiO2 did present a high photocatalytic activity due to the effective separation of photoinduced charges.

M

Keywords: Titanium dioxide; Solvothermal method; NH4HCO3; Photocatalysis 1. Introduction

ED

Environmental pollution is a serious problem that can cause illness and even death to human

PT

beings [1]. Heterogeneous photocatalysis appears to be one of the most efficient and economic techniques for the remediation of a contaminated environment [2–6]. Among various

CE

photocatalysts, TiO2 is regarded to be the ideal one and has been extensively investigated by

AC

taking advantage of its excellent redox power, excellent photostability, ecofriendliness, and low cost [7–9]. How to improve the photocatalytic activity of TiO2 has been an interesting research topic. Previous studies have reported that the photocatalytic activity of TiO2 is greatly influenced by several physicochemical parameters, such as crystal structure, crystallinity, morphology, particle size distribution and specific surface area [10,11], in which the crystallinity of TiO2 plays an important role in the excited carrier (electron or hole) transport, and hence affects its

2

ACCEPTED MANUSCRIPT

photocatalytic activity. Much effort has been devoted to increase the crystallinity of sol-gel synthesized TiO2 via a high temperature thermal treatment, generally calculation at above 500 oC [12]. However, such high temperature calculation would lead to the aggregation of particles, the decrease of specific surface area and the loss of surface functional groups [13,14].

CR IP T

Alternatively, solvothermal technique has been proved to be a promising route for constructing TiO2 with small grain sizes, high specific surface areas, and high crystallinity [15]. Furthermore, the inclusion of water during solvothermal process could promote the growth of

AN US

particle size, improve the crystallinity and form high-energy {001} facet [16–20], and thus enhance its photocatalytic activity. For instance, Wu et al. reported that TiO2 produced by nonaqueous method through inclusion of a trace amount of water in benzyl alcohol and

M

oleylamine medium exhibited conspicuous activity in the photocatalytic degradation of organic contaminants due to the large percentage of exposed high-energy facets [19]. Do et al. [20] found

ED

that the presence of an appropriate amount of water vapor along with the desired molar ratio of

PT

oleic acid/oleylamine played a crucial role in controlling size and shape of TiO2 nanocrystals. In addition, studies of Liu et al. [15] and our group [17] certified the significant role of water in the

CE

synthesis of TiO2 by alcohothermal method, which could tune the particle size, crystallinity,

AC

explosion high-energy facets and ultimately affect the photocatalytic activity of TiO2. However, most reported works about the influence of water on the physicochemical performance of TiO2 [17,19,21] were carried out with the introduction of special organic agents such as oleylamine, oleic acid, linoleic acid, triethylamine and cyclohexane. Additionally, addition water to the raw materials during solvothermal process has some drawbacks. First of all, it is difficult to control the vigorous reaction between titania precursors and water since most of titania precursors are easy to

3

ACCEPTED MANUSCRIPT

hydrolyze. On the other hand, the addition of water into alcohol medium could lead to homogeneous nucleation occurrence preferentially rather than heterogeneous nucleation, whereas heterogeneous nucleation is the key step for the preparation of TiO2-based composites. Therefore, it is desirable to investigate the influence of in situ water on the physicochemical and

CR IP T

photocatalyst performance of TiO2. It is well known that NH4HCO3 will decompose into water, ammonia and carbon dioxide with the increase of temperature. Moreover, the decomposition products of NH4HCO3 have two key influences on the growth of TiO2 particles. On one hand, the

AN US

in-situ released water can accelerate the hydrolytic process to form TiO2 with large particle sizes and high crystallinity. On the other hand, fresh formed ammonia renders TiO2 with rhombus morphology [22].

M

Herein, TiO2 was prepared by a modified alcohothermal method, using alcohol as the reaction medium, titanium isopropoxide as the precursor of TiO2, and NH4HCO3 as the raw

ED

materials for release of water, ammonia and carbon dioxides via in-situ decomposition. As a

PT

control, TiO2 was also synthesized by alcohothermal method in a similar manner without the addition of NH4HCO3. As expected, TiO2 prepared by the former method exhibited much larger

CE

particle sizes and higher crystallinity than TiO2 synthesized by the latter method, implying the

AC

important role of the decomposition products of NH4HCO3 in conducting the size, shape and crystallinity of TiO2. Importantly, the synthesized TiO2 by the modified alcohothermal method presented high photocatalytic activity under UV light irradiation, probably due to effective separation of photoinduced charges based on the enhanced crystallinity. 2. Experimental 2.1. Materials

4

ACCEPTED MANUSCRIPT

Titanium isopropoxide (TIP, 97%) was purchased from Sigma-Aldrich. Absolute ethanol and NH4HCO3 were obtained from Xilong Chemical Co., Ltd, China. The chemicals were used as received and all aqueous solutions were prepared with deionized water (resistivity = 18.2 M cm).

CR IP T

2.2. Synthesis of TiO2 TiO2 were synthesized according to our previous work with some modification, using absolute ethanol, titanium isopropoxide (TIP, 97%, Sigma-Aldrich) and NH4HCO3 as the starting

AN US

materials [17]. In a typical experiment, the desired amount of NH4HCO3 was mixed with absolute ethanol and then determinate TIP was added into this mixture drop by drop under strongly stirring. After sonication for 1 h, the mixture was transferred to a 100 mL Teflon-lined stainless steel

M

autoclave, sealed tightly, and then heated at 180 oC for 24 h. After cooled to room temperature naturally, the resulting TiO2-Nx (x means the weight of NH4HCO3) were collected by

ED

centrifugation, rinsed by ethanol and dried under vacuum.

PT

2.3. Characterization of samples

The X-ray powder diffraction (XRD) patterns were obtained on a Rigaku X-ray

CE

diffractometer to determine the crystallite identity of TiO2 samples. The microscopic structures

AC

and particle sizes of TiO2 samples were investigated with a JEM 2100 transmission electron microscope (TEM). BET specific surface area was carried out using Quantachrome ASIQM000-200-6 automated gas sorption analyzer. Room temperature photoluminescence (PL) spectroscopy measurement at 325 nm excitations was performed using F-4600. UV-visible absorbance spectra were obtained for the dry-pressed disk samples with a UV-visible spectrophotometer (UV-2550, Shimadzu, Japan). BaSO4 was used as a reflectance standard in a

5

ACCEPTED MANUSCRIPT

UV-visible diffuse reflectance experiment. The X-ray photoelec-tron spectroscopy (XPS) measurement was made on a KRATOS Analytical AXISHSi spectrometer with a monochromatized Al K X-ray source (1486.6 eV photons). The binding energy scale was calibrated by Au 4f7/2 peak at 83.9 eV as well as Cu 2p3/2 peak at 76.5 and 932.5 eV.

CR IP T

2.4. Photoelectrochemical measurement Photocurrent measurement was performed in a three-electrode quartz cells with 0.1 M Na2SO4 electrolyte solution. Platinum wire was used as counter and saturated calomel electrode

AN US

(SCE) used as reference electrodes, respectively, and TiO2-0 and TiO2-N3 films electrodes (1×1 cm2) on FTO served as the working electrode. The photoelectrochemical experiment results were recorded with an electrochemical system (CHI-760C Instruments). Potentials were given with

off were measured at 0.0 V.

M

reference to the SCE. The photoresponses of the photocatalysts as UV light (λ = 254 nm) on and

ED

2.5. Photocatalytic activity measurement

PT

Photocatalytic performance of TiO2 was studied by degradation of methyl orange (MO) under UV light irradiation. In a typical process, 30 mg photocatalyst was added to 150 mL MO (15 mg/L)

CE

solution. Adsorption was then conducted in the dark for 30 min under magnetical agitation in

AC

order to make TiO2 to reach adsorption equilibrium. Subsequently, the mixture was exposed to UV irradiation by two germicidal lamp (λ=254 nm, 8 W) at room temperature. Aeration was performed by an air pump to ensure a constant supply of oxygen and promoted complete mixing of solution and photocatalysts during photoreaction. The sample was collected by centrifugation at given time intervals and MO concentration was measured by UV-vis spectroscopy at 464.2 nm. 3. Results and discussion

6

ACCEPTED MANUSCRIPT

3.1. Phase structures and morphology XRD was conducted to characterize the influence of the in-situ water on the particle size and crystalline of TiO2, and the results were displayed in Figure 1. It is clear that the peaks of TiO2-0 sample at 2θ values of 25.3°, 37.8°, 48.0°, 53.9°, 55.1°, 62.7°, 68.8°, 70.3° and 75.0° can be

CR IP T

indexed to (101), (004), (200), (105), (211), (204), (116), (220) and (215) facets, the typical reflections of anatase phase structure of TiO2 [10]. After introduction of different amount of NH4HCO3 during the alcohothermal process, only anatase phase of TiO2 is detected,

AN US

demonstrating that the in-situ water decomposed from NH4HCO3 does not change the phase of TiO2. However, it is worth noting that with the addition of NH4HCO3, the intensity of TiO2 characteristic peaks increases obviously and their shapes become sharper, which means the higher

M

crystallinity and larger particle size of TiO2. Similar phenomenon was also observed in our previous work [17,18]. The reason is that a trace amount of water from the in situ decomposition

AC

215

204

TiO2-N4

220 116

105 211

200

004

Intensity(a.u.)

CE

PT

101

ED

of NH4HCO3 promotes the growth of crystallites and enhances the crystallinity of TiO2.

TiO2-N3 TiO2-N2 TiO2-N1 TiO2-0

10

20

30

40

50

60

70

80

2 theta/degree

Figure 1. XRD patterns of TiO2-Nx (x=0–4) samples. To evaluate the role of water and ammonia from the in-situ decomposition of NH4HCO3 in the growth of TiO2 particles, TEM was carried out to characterize the microstructure and

7

ACCEPTED MANUSCRIPT

morphology of TiO2. As shown in Figure 2(a), without the addition of NH4HCO3, TiO2 particles generally appear to show small size and with a narrow size distribution in the range of 8–10 nm. Moreover, the profile of single TiO2 particle is hard to be well distinguished since TiO2 particles are aggregated each other and poor crystallized. After introduction of NH4HCO3, the in-situ water

CR IP T

and fresh ammonia play a crucial role in promoting the growth of TiO2 and controlling the morphology and microstructure of TiO2, as shown in Figure 2(b)-2(d). In detail, in the presence of in-situ water and fresh ammonia, the orhomb-shaped large particles appear and the maximum size

AN US

is appropriately as large as 100 nm. The formation of orhomb-shaped large TiO2 particles is mainly ascribed to the following two aspects. On one hand, the in-situ water can accelerate the hydrolysis reaction of TIP, resulting in the formation of large particles. On the other hand, the

M

newly formed ammonia renders the TiO2 with rhombus morphology [22]. Furthermore, with the increase of added NH4HCO3, the density of small TiO2 particles is found to be decreased, whereas

ED

the density of orhomb-shaped large TiO2 particles increased. BET specific surface area was also

PT

carried out to confirm the variation of particle size, and the results are shown in Table 1. It is demonstrated that with the increase of added NH4HCO3, the BET specific surface area of TiO2 is

CE

trend to decrease. For example, the BET specific surface area of TiO2-0 sample is 168 m2/g, and

AC

decreased to 132 m2/g after introduction of 3 g NH4HCO3, demonstrating the increase of particle size. Therefore, both of the TEM images and BET results clearly reveal the role of in-situ water and ammonia decomposed from NH4HCO3 in promoting the growth of TiO2, and tuning the shape and crystallinity of TiO2, which agrees well with the XRD results.

8

AC

CE

PT

ED

M

AN US

CR IP T

ACCEPTED MANUSCRIPT

9

CR IP T

ACCEPTED MANUSCRIPT

Figure 2. TEM images of TiO2-Nx (x=0–3). (a) TiO2-0, (b) TiO2-N1, (c) TiO2-N2, (d) TiO2-N3 Table 1. BET surface area characteristics of TiO2 and TiO2-Nx (x=1–3) samples.

SBET (m2/g)

TiO2-0 168

TiO2-N1

TiO2-N2

TiO2-N3

AN US

Samples

176

116

132

X-ray photoelectron spectroscopy (XPS) is an effective method to analyze the surface element

M

states and provide the information of chemical binding. Since ammonia and carbon dioxides are

ED

released from NH4HCO3 during the alcohothermal process, XPS measurement was performed to evaluate whether C and/or N-doping happened in TiO2. XPS survey spectra (Figure 3a) exhibit

PT

that no nitrogen element exists in TiO2. The observed carbon element should be ascribed to the

CE

impurities of adventitious hydrocarbon from XPS instrument itself because no core level XPS spectra of C 1s are found [23]. Therefore the in-situ released ammonia and carbon dioxides from

AC

NH4HCO3 during the alcohothermal process don’t lead to the formation of C and/or N-doped TiO2. The Ti and O core level XPS spectra of TiO2-0 and TiO2-N3 samples are shown and compared in Figure 3(b) and 3(c), respectively. The Ti 2p3/2 and 2p1/2 doublet peaks locate at 458.2 and 463.9 eV, respectively, which are typical values for Ti4+ species in TiO2-0 (Figure 3b) [24]. When compared with TiO2-0, the Ti 2p3/2 and 2p1/2 peaks of TiO2-N3 sample shift to higher binding energy, which is probably due to the change of morphology [25]. As shown in Figure 3(c), the O

10

ACCEPTED MANUSCRIPT

1s spectra of samples can be fitted to three peaks. The main peak of TiO2-0 at 529.7 eV is assigned to Ti-O-Ti (lattice O). The peak with higher binding energy located at 530.9 eV is attributed to the Ti-OH. The peak centered at 532.6 eV is probably ascribed to the absorbed water [23]. Similarly, the positive shift of O 1s peaks is also observed for TiO2-N3, as shown in Figure 3(c). Therefore,

CR IP T

water and ammonia from the in-situ decomposition of NH4HCO3 can affect the surface chemical states of TiO2. 12000 (a) TiO2-N3

O (KLL)

10000

O 1s

AN US

CPS (a.u.)

8000 Ti 2p

Ti 2s

6000 4000 2000

C 1s

Ti 3p Ti 3s

0 200

400

600

M

0

800

1000

Binding energy (eV) 2p3/2

ED

(b) Ti 2p

AC

CE

PT

Intensity (a.u.)

2p1/2

TiO2-N3

TiO2-0

466

464

462

460

Binding energy (eV)

11

458

456

ACCEPTED MANUSCRIPT

Ti-O-Ti

TiO2-N3

Ti-O-H

TiO2-0

534

532

530

Binding energy (eV)

528

CR IP T

Intensity (a.u.)

(c) O 1s

526

Figure 3. XPS survey spectra of TiO2-N3 (a) and high-resolution XPS spectra of Ti 2p (b) and O

AN US

1s (c) 3.2. Photoelectrochemical properties

The optical absorption of TiO2 with various addition of NH4HCO3 was measured with diffuse

M

reflectance spectra (DRS) and compared in Figure 4. It can be determined from Figure 4 that the band gap of TiO2-0 is about 3.1 eV, which agrees well with the reported band gap of TiO2 [10].

ED

With the introduction of NH4HCO3 during the alcohothermal process, the red shift of absorption

PT

edge, a typical characteristic of C and/or N-doped TiO2, is not observed, which confirms further that in-situ released ammonia and carbon dioxides do not give rise to the C and/or N-doped TiO2.

CE

On the contrary, the absorption edge of TiO2 is slightly blue shifted, which may be due to the

AC

appearance of orhomb-shaped large particles and the change of physicochemical properties [21]. The negligible blue-shift of the absorption edge (less than 5nm) should have insignificant influence on the photocatalytic performance of TiO2. Thus, it is not the change of optical property that causes the enhancement of photocatalytic activity. Further characterizations such as the photoluminescence spectra and photocurrent response should be conducted to understand the reason that is responsible for the improvement of photocatalytic activity of TiO2.

12

ACCEPTED MANUSCRIPT

1.0

Intensity (a.u.)

0.8

TiO2-0 TiO2-N1 TiO2-N2

Blue shift

TiO2-N3 TiO2-N4

0.6 0.4 0.2 397 nm

250

300

350

400 nm

400

450

Wavelength (nm)

500

Figure 4. Diffuse reflectance spectra of TiO2-Nx (x=0–4) samples

CR IP T

0.0 200

550

600

It is well known that the photoluminescence spectrum (PL) is an effective method to reflect

AN US

the charge behaviors (migration or recombination) in photocatalyst [26]. PL spectrum was applied to reveal the reason for the enhancement of photocatalytic performance of TiO2 produced by alcohothermal approach through the in-situ decomposition of NH4HCO3. Figure 5 shows the

M

comparison of PL spectra of TiO2-0 and TiO2-N3 in the wavelength ranged from 375 nm to 600

ED

nm with the excitation wavelength at 325 nm. As can be seen from Figure 5, the wide emission peak appeared at about 397 nm is attributed to the direct band-to-band radiative recombination of

PT

photoexcited electrons and holes. Some other peaks, whose central wavelength located at 451, 468

CE

and 483 nm, probably result from the structural defects of the TiO2 [27,28]. It is notably that when NH4HCO3 is introduced to alcohothermal system, the as-synthesized TiO2 exhibits relatively low

AC

PL intensity. This result indicates that the recombination of photoinduced electrons and holes is partially suppressed in TiO2-N3 ascribed to the higher crystallnity. This will also be reflected by the photocurrent results later. The reason may be that the highly crystallized TiO2 can promote the charge transfer from particle center to surface and thus suppress the recombination of charge carriers [10,17,29–32].

13

ACCEPTED MANUSCRIPT

4000

397

TiO2-0 TiO2-N3

Intensity (a.u.)

3000

468 451 483

2000

400

450

500

Wavelength (nm)

550

CR IP T

1000

600

Figure 5. Photoluminescence spectra of TiO2-0 and TiO2-N3 samples.

AN US

To further investigate the separation efficiency of photogenerated charge carriers, photocurrent response of TiO2-0 and TiO2-N3 samples was conducted under UV light irradiation (Figure 6). It is obvious that TiO2-0 exhibits weaker photocurrent response, when compared to TiO2-N3. After 3 g

M

NH4HCO3 is introduced to the alcohothermal system, the photocurrent density of TiO2-N3 is

ED

increased to two times than that of TiO2-0, demonstrating that the separation efficiency of photoinduced electron-hole pairs is enhanced in TiO2-N3. Based on the results of PL spectra and

PT

photocurrent, it is no doubt that the improved crystallnity can facilitate the separation of charge

0.6

2

Current density (mA/cm )

AC

CE

carriers in TiO2, which is beneficial to the enhancement of photocatalytic activity.

on

off TiO2-N3

0.4

0.2

0.0

TiO2-0

90

120

150

180

Time (s)

210

Figure 6. Photocurrent response of TiO2-0 and TiO2-N3 samples.

14

240

ACCEPTED MANUSCRIPT

3.3. Enhancement of photocatalytic performance The photoactivities of TiO2 samples are evaluated by photodecomposition of MO solution under UV light irradiation. Before irradiation, the sample suspension is stirred for 30 min in the dark to reach the adsorption-desorption equilibrium. The normalized maximum absorbance

CR IP T

changes A/A0 (A0 is the initial absorbance of MO.) of MO during the photodegradation are proportional to the normalized temporal concentration changes C/C0 and derive from the changes in the dye’s absorption profile (464.2 nm) at a given time interval. As shown in Figure 7(a), after

AN US

adsorption equilibrium in the dark for 0.5 h, negligible MO molecules are absorbed on the surface of TiO2. In terms of the photocatalytic performance, TiO2-0 exhibits low activity for degradation of MO. However, after the introduction of NH4HCO3 during the alcohothermal process, all of the

M

TiO2-Nx (x=1–4) samples present a considerable increase in the photodegradation of MO in comparison to TiO2-0, indicating that in-situ released water and ammonia from NH4HCO3 during

ED

the alcohothermal reaction is helpful for improving photocatalytic activity of TiO2.

PT

For a better comparison of the photocatalytic efficiency between TiO2-0 and TiO2-Nx, we apply the pseudo-first order model as expressed by equation ln(C0/C)=kt, which is generally used

CE

for photocatalytic degradation process if the initial concentration of pollutant is low [33]: where

AC

C0 and C are the concentrations of dye in solution at time 0 and t, respectively, and k is the pseudo-first-order rate constant. The kinetic linear simulation curves over the above catalysts suggest that the degradation reaction follows a Langmuir-Hinshelwood apparent first-order kinetics model due to the low initial concentrations of MO. The reaction constant k follows the order TiO2-N3>TiO2-N2>TiO2-N4>TiO2-N1>TiO2-0, from 1.86×10-2 min-1 to 1.2×10-3 min-1, as demonstrated in Figure 7(b). Namely, TiO2-N3 has the best photocatalytic performance toward the

15

ACCEPTED MANUSCRIPT

degradation of MO. The MO degradation activity of TiO2-N3 is increased to 15.5 times than that of TiO2-0. Therefore, the introduction of suitable amount of NH4HCO3 is vital for achieving an optimal photocatalytic performance for TiO2 during alcohothermal process. The above results verify that the addition of NH4HCO3 during the alcohothermal process remarkably improves the

CR IP T

photocatalytic efficiency of TiO2. Based on the above characterizations, the improvement of photocatalytic activity of TiO2 induced by the in-situ decomposition of NH4HCO3 is mainly due to the increased crystallinity. It

AN US

was reported that crystallinity had a vital effect on the charge carriers migration and separation. [10,17,29–32]. In general, TiO2 with high crystallinity can promote the charge transfer from particle center to surface. Furthermore, the highly crystallized TiO2 can eliminate the crystal defects, such as impurities, dangling bonds, and microvoids, which behave as recombination

M

centers for the e-/h+ pairs, thus the surface recombination is greatly suppressed. Above two

ED

important effects of higher crystallinity on the transport and separation of charges are verified by

PT

the PL and photocurrent results.

(a)

Light off

1.0

Light on

0.9 0.8 0.7

C/C0

AC

CE

1.1

0.6

TiO2-0 TiO2-N1

0.5

TiO2-N2

0.4

TiO2-N3

0.3 -30

TiO2-N4

-20

-10

0

10

20

30

Time (min)

16

40

50

60

70

ACCEPTED MANUSCRIPT

1.2

(b) Catalyst k (10-2/min) R2 TiO2-0

1.0 0.8

ln(C0/C)

0.12

TiO2-N1 0.75 TiO2-N2 0.94

TiO2-N3 1.86 TiO2-N4 0.80

0.6

0.9948 0.9931 0.9969 0.9910 0.9969

0.4 0.2

0

10

20

30

40

Time (min)

50

60

70

CR IP T

0.0

Figure 7. (a) Adsorption and photocatalysis of MO under UV light irradiation over TiO2-Nx (x = 0–4) samples. (b) Photocatalytic reaction kinetics of MO decomposition over TiO2-Nx (x = 0–4)

AN US

samples. 4. Conclusions

In summary, high crystallinity of TiO2 was synthesized by a modified alcohothermal method,

M

in which titanium isopropoxide was used as the titania precursor, absolute ethanol as the reaction

ED

medium, and NH4HCO3 as the raw materials for release of water, ammonia and carbon dioxides via in-situ decomposition. As-prepared TiO2 exhibited high photocatalytic activity toward MO

PT

degradation under UV light irradiation. Based on the XRD, TEM, PL and photocurrent data

CE

analyses, the high photocatalytic activity of TiO2 is mainly due to the high charges separation and translation rate based on the high crystallinity. This work provides a facile approach to synthesize

AC

TiO2-based photocatalyst with high photocatalytic activity. Acknowledgments The authors would like to express their thanks for the support of the National Natural Science Foundation of China (No. 21163008, No. 21366020), Jiangxi Collaborative Innovation Center for in vitro diagnostic reagents and instruments, the Natural Science Foundation of Jiangxi Province (No. 20114BAB203009), Scientific & Technological Project of Jiangxi Science and Technology

17

ACCEPTED MANUSCRIPT

Normal University (No. 2013ZDPYJD01), and Graduate Innovation Foundation of Jiangxi Science and Technology Normal University (No. YC2014-X2). References [1] Sun S M, Wang W Z, Zhang L, J Phys Chem C, 2013,117 (18): 9113

CR IP T

[2] Fujishima A, Honda K, Nature, 1972, 238 (5358): 37 [3] Xu Z J, Qi B, Di L B, Zhang X L, J Energy Chem, 2014, 23(6):679

[4] Chen X B, Shen S H, Guo L J, Mao S S, Chem Rev, 2010,110 (11): 6503

AN US

[5] Xiang Q J, Yu J G, Jaroniec M, J Am Chem Soc, 2012, 134 (15): 6575

[6] Wang X C, Kazuhiko M, Arne T, Kazuhiro T, Markus A, Nat Mater, 2009, 8 (1): 76 [7] Hashimoto K, Irie H, Fujishima A, Jpn J Appl Phys, 2005, 44 (12): 8269

M

[8]Farhadian M, Sangpour P, Hosseinzadeh G, J Energy Chem, 2015, 24(2):171 [9] Wetchakun N, Chainet S, Phanichphant S, Wetchakun K, Ceram Int, 2015, 41 (4):5999

ED

[10] Chen X B, Mao S S, Chem Rev, 2007,107 (7): 2891

PT

[11] Yang H G, Liu G, Qiao S Z, Sun C H, Jin Y G, Smith S C, Zou J, Cheng H M, Lu G Q, J Am Chem Soc, 2009,131 (11): 4078

AC

220

CE

[12] Sheng Q R, Cong Y, Yuan S, Zhang J L, Anpo M, Micropor Mesopo Mater, 2006, 95 (1-3):

[13] Chen Y G, Dionysiou D D, J Mol Catal A, 2006, 244 (1-2): 73 [14] Hu C Y, Duo S W, Liu T Z, Xiang J H, Li M S, Appl Surf Sci, 2011, 257 (8): 3697 [15] Wahi R K, Liu Y P, Falkner J C, Colvin V L, J Colloid Interface Sci, 2006, 302 (2): 530 [16] Akawat S, Ekkachai K, Piyasan P, Catal Comm, 2008, 9 (9): 1810 [17] Hu C Y, Lu T W, Chen F, Zhang R B, Lian C J, Zheng S Z, Hu Q H, Duo S W, Mater Res

18

ACCEPTED MANUSCRIPT

Bull, 2014, 53 (12): 42 [18] Lu T W, Zhang R B, Hu C Y, Chen F, Duo S W, Hu Q H, Phys Chem Chem Phys, 2013,15 (31): 12963 [19] Wu B H, Guo C Y, Zheng N F, Xie Z X, Stucky G D, J Am Chem Soc, 2008,130 (54):17563

CR IP T

[20] Dinh C T, Nguyen T D, Kleitz F, Do T O, Can J Chem Eng, 2012, 90 (1): 8 [21] Li X L, Peng Q, Yi J X, Wang X, Li Y D, Chem Eur J, 2006,12 (8): 2383

[22] Li W, Yang J P, Wu Z X, Wang J X, Li B, Feng S S, Deng Y H, Zhang F, Zhao D Y, J Am

AN US

Chem Soc, 2012,134 (29): 11864

[23] Lu M X, Shao C L, Wang K X, Lu N, Zhang X, Zhang P, Zhang M Y, Li X H, Liu Y C, ACS Appl Mater Interfaces, 2014, 6 (12): 9004

M

[24] Diebold U, Madey T, Surf Sci Spectra, 1996, 4 (3): 227

[25] Tong H, Ouyang S X, Bi Y P, Naoto U, Mitsutake O, Ye J H, Adv Mater, 2012, 24 (2): 229

ED

[26] Weng B, Yang M Q, Zhang N, Xu Y J, J Mater Chem A, 2014, 2 (24): 9380

PT

[27] Xiang Q J, Lv K L, Yu J G, Appl Catal B, 2010, 96 (3-4): 557

234

CE

[28] Wu J J, Lü X J, Zhang L L, Xia Y J, Huang F Q, Xu F F, J Alloys Compd, 2010, 496 (1-2):

AC

[29] Wang C C, Ying J Y, Chem Mater, 1999, 11 (11): 3113 [30] Bakardjieva S, Stengl V, Jirkovsky J, Petrova N, J Mater Chem 2006, 16 (18): 1709 [31] Tryba B, Toyoda M, Morawski A W, Inagaki M, Appl Catal B, 2007, 71 (3-4): 163 [32] Linsebigler A L, Lu G Q, Yates J T, Chem Rev, 1995,95 (3): 735 [33] Herrmann J M, Ait-Ichou Y, Lassaletta G, Fernandez A, Appl Catal B, 1997, 13 (3-4): 219

19

ACCEPTED MANUSCRIPT

Graphical abstract

-2 Catalyst k (10 /min) R2

0.8

ln(C0/C)

0.12

0.9948

TiO2-N1 0.75 TiO2-N2 0.94

0.9969

TiO2-0

1.0

TiO2-N3 1.86 TiO2-N4 0.80

0.6

0.9931 0.9910 0.9969

0.4 0.2

0

10

20

30

40

50

60

70

AN US

0.0

CR IP T

1.2

Time (min)

With the introduction of NH4HCO3 during alcohothermal process, photocatalytic

M

performance of TiO2 is significantly enhanced due to the increased crystallinity. When

AC

CE

PT

ED

the added NH4HCO3 is up to 3 g, TiO2-N3 exhibits the best photocatalytic activity.

20