- Email: [email protected]
Titania nanowires growing from P25 nuclei: Facile synthesis and the improved photocatalytic activity
Accepted Manuscript Titania nanowires growing from P25 nuclei: Facile synthesis and the improved photocatalytic activity Qing-Er Zhao, Wei Wen, Yu Xia, Jin-Ming Wu PII:
S0022-3697(18)31848-1
DOI:
10.1016/j.jpcs.2018.09.016
Reference:
PCS 8729
To appear in:
Journal of Physics and Chemistry of Solids
Received Date: 12 July 2018 Revised Date:
27 August 2018
Accepted Date: 11 September 2018
Please cite this article as: Q.-E. Zhao, W. Wen, Y. Xia, J.-M. Wu, Titania nanowires growing from P25 nuclei: Facile synthesis and the improved photocatalytic activity, Journal of Physics and Chemistry of Solids (2018), doi: 10.1016/j.jpcs.2018.09.016. 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
M AN U
SC
RI PT
Graphic Abstract
Urchin-like TiO2 powders with excellent photocatalytic performances were synthesized utilizing the Ti-H2O2 interactions, with the additive of Degussa P25 TiO2
AC C
EP
TE D
nanoparticles serving as nuclei.
ACCEPTED MANUSCRIPT
Titania nanowires growing from P25 nuclei: facile synthesis and the improved photocatalytic activity
a
RI PT
Qing-Er Zhao a, Wei Wen a,b, Yu Xia a, Jin-Ming Wu a* State Key Laboratory of Silicon Materials and School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310037, P. R. China.
College of Mechanical and Electrical Engineering, Hainan University, Haikou
M AN U
b
SC
[email protected]
570228, P. R. China.
Abstract
TE D
Titania powders with hierarchical nanostructures find potential applications in wastewater treatments because of their excellent photocatalytic performances and the advantage of easily recovering from the slurry. Commercial Degussa P25 titania
EP
nanoparticles are widely adopted photocatalyst with high photocatalytic activity. In
AC C
this study, we used P25 as nuclei to synthesize a hierarchical titania. With the additive of P25 nuclei in the Ti-H2O2 reactant maintained at a low temperature of 80 °C, hydrogen titanate nanowires radially grew from the P25 cores to form sea-urchin nanowire aggregates. A subsequent calcination in air at 450 °C decomposed the titanate nanowires to anatase, keeping the nanowire morphology unchanged. The weight ratio of titania nanowires to P25 nanoparticles can be simply tuned by varying the P25 concentration in the reactant. When utilized to assist photodegradations of 1
ACCEPTED MANUSCRIPT phenol in water under UV light illumination, the hierarchical titania consisting of 86.3 wt. % nanowires and 13.7 wt. % P25 exhibited a reaction rate constant of 0.79 h-1, which is 3.2 times that of titania nanowires (0.25 h-1), and even slightly higher than
RI PT
that of P25 nanoparticles (0.73 h-1). The reaction rate constant is also 1.5 times that of the mixture of titania nanowires and P25 nanoparticles, suggesting a synergetic effect probably arising from the phase junctions.
SC
Keywords: photocatalysis; heterogeneous nucleation; nanowire; titania; phenol
M AN U
1. Introduction
Nanostructured titania (TiO2) has been widely applied in various fields of photocatalysis [1], energy conversion and storage [2,3], gas sensing [4], etc., because of its merits of low cost, non-toxicity, chemical stability as well as unique optical and
TE D
electrical properties. One-dimensional (1D) nanostructures like nanowires, nanorods, and nanotubes, provide titania with high specific surface area, fast charge transfer rate, and low charge carrier recombination [5]. Among them, TiO2 nanowires, in various
EP
phases of anatase [6], rutile [7], and brookite [8], have been demonstrated as an
AC C
attractive photocatalyst [9]. The template thermolysis method, which uses hard or soft templates as the structure-directing agents, is a traditional technique to synthesize 1D TiO2 [10,11]. The templated route is also combined with a sol-gel process, which involves the hydrolysis of titanium alkoxide, to obtain 1D TiO2 [12]. However, the resultant TiO2 nanostructures are susceptible to harsh chemical reactions or high temperature calcinations, which are applied to remove the templates. To avoid the utilization
of
complicated
templates,
acid/alkali-hydrothermal 2
[13-16]
and
ACCEPTED MANUSCRIPT solvothermal [17] methods have also been developed to achieve 1D TiO2. Regretfully, the reactants usually involve titanium organics or inorganics in HCl, concentrated NaOH, or organic solvents, which is far from a green chemistry. To achieve 1D TiO2
RI PT
via reactants free of organics and strong acid/alkaline is of interesting [18-21]. In practice, the recovery of nanostructured photocatalysts from a slurry system is a challenge, which generally demands an ultrafiltration procedure. Hierarchical
SC
nanostructures such as urchin-like microspheres facilitate the catalysts recovery whilst
M AN U
maintaining the merits of the nanostructured constructing units [13,16,18,21]. For example, Yu et al. developed a one-step hydrothermal method for the synthesis of urchin-like anatase TiO2 assembled with ultrathin nanobelts, which exhibited excellent photocatalytic activity towards degradations of organic pollutants in water
TE D
[21]. There are relatively few reports on the template-free and green synthesis of 1D TiO2 nanostructures, or their assemblies, under a low temperature and in an open atmosphere [18]. In this paper, we report a green, template-free route to synthesize
EP
TiO2 nanowire aggregates. A facile reaction between metallic Ti sponge and H2O2
AC C
solution was adopted to provide supersaturated Ti(IV) ions and the commercial P25 TiO2 nanoparticles were used as nuclei to initiate the heterogeneous nucleation and the subsequent growth of the radially aligned nanowires. We expect that the utilization of P25, which possesses exceptional photocatalytic activity, would contribute to the photocatalytic performance of the resultants. The effects of the P25 additives in the Ti-H2O2 reactants on the morphology, optical property and efficiency to assist photodegradations of phenol in water under UV light illumination were studied. 3
ACCEPTED MANUSCRIPT 2. Experiment 2.1 Synthesis In a beaker, Degussa P25 TiO2 powders (2 mg, 5 mg, or 10 mg) were dispersed
RI PT
into 50 mL H2O2 solution with additives of 2.4 M melamine, 0.29 M HNO3, and 0.25 g metallic Ti sponge. The reactants were maintained at 80 °C for 48 h in an oven. The precipitates were collected and washed in sequence using distilled water and ethyl
SC
alcohol. After air drying at 60 °C, the powders were subjected to a final calcination in
M AN U
air at 450 °C for 1 h. The resultants were designated as NW-2, NW-5, and NW-10, respectively. It is noted that, in absence of P25, no nanowires can be collected from the solution. To achieve P25-free TiO2 nanowires (designated as NW), metallic Ti powders (200 mesh) were utilized instead of the Ti sponge. The details are provided
TE D
in the online Supporting Information. 2.2 Characterizations
The powder morphology was observed by a field-emission scanning electron
EP
microscopy (FESEM, Hitachi, S-4800) and a JEM-2100 microscopy (Jeol, Tokyo,
AC C
Japan). The X-ray diffraction (XRD) tests were carried out using a Rigaku D/max-3B diffractometer with a CuKα radiation, which is operated at 40 kV and 36 mA (λ = 0.15406 nm). The low-temperature N2 adsorption-desorption measurement was conducted at 77 K, using Autosorb-1-C (Quantachrome Instruments), and the sample was degassed at 80 °C for 12 h to remove physisorbed gases. The Brunauer–Emmett– Teller (BET) approach using adsorption data was followed to evaluate the specific surface area and the Barrett–Joyner–Halenda (BJH) method using the desorption 4
ACCEPTED MANUSCRIPT curve was applied to determine the pore size distribution and pore volume. The X-ray photoelectron spectra (XPS) were acquired on an EscalabMKII spectrometer equipped with a monochromatic Al Ka X-ray source (1486.8 eV). The binding energy
RI PT
was normalized to the C 1s (284.5 eV) for adventitious hydrocarbons as an indirect standard. The UV-Vis diffuse reflectance spectra were collected from a UV–Vis near-infrared spectrometer (UV-3150, Shimadzu). For the measurements, the same
SC
amounts of powders were piled on a layer of BaSO4 powder and then spread into a
M AN U
thin uniform layer using a glass rod. A DSCQ1000 instrument (Waters) was used to carry out the thermogravimetric (TG) and differential scanning calorimetry (DSC) investigations of as-precipitated nanowires in an air atmosphere, at a heating rate of 10 K min-1 from room temperature to 900 °C.
TE D
2.3 Photocatalytic evaluation
The photocatalytic performance was evaluated via photodegradations of phenol in water. In each run, 25 mg of powders were dispersed into 50 mL phenol solution
EP
(10 ppm). The UV light was provided by an 18 W UV lamp (PHILPS Lighting
AC C
Investment Company Limited, China) and the average intensity of UV irradiance reaching the liquid level was ca. 6.5 mW/cm2, which is measured in the wavelength range of 320-400 nm with a peak wavelength of 365 nm (Model: UV-A, Beijing Normal University, China). The slurry was stirred in the dark for 60 min to establish an absorption-desorption equilibrium before the photocatalytic reaction. The change in phenol concentration was monitored with a liquid chromatography apparatus (Wufeng LC100, WondaCract ODS-2 column, China), using a mixed solution of 5
ACCEPTED MANUSCRIPT methanol and acetic acid (methanol: water: acetic acid = 700: 300: 1 in volume) as a mobile phase. At an interval of 1 h, 0.2 mL phenol solution was sampled. 3. Results and Discussions
RI PT
The interactions of metallic Ti and H2O2 at 80 °C resulted in hydrogen titanate nanowires on certain substrates like Ti foils [19], Ti meshes [22], or seeded organic fibers [23,24]. In the current investigation, in absence of P25, only nanoparticles were
SC
collected from the solution (Figure S1a). It is observed that nanowires nucleated and
M AN U
grew preferably on granular Ti sponge (Figure S1b), which inhibited their homogeneous precipitations from the solution. In case that fine metallic Ti powders were utilized instead of the Ti sponge to provide the Ti source [18], the heterogeneous nucleation was inhibited and the homogeneous precipitation of nanowires was favored,
AC C
EP
TE D
which achieved P25-free TiO2 nanowire aggregates from the solution (Figure 1a).
Figure 1. FESEM images of TiO2 nanowires synthesized using various amounts of P25 nanoparticles as nuclei: (a) NW, (b) NW-2, (c) NW-5, and (d) NW-10. The insets are the corresponding low-magnification FESEM images. 6
ACCEPTED MANUSCRIPT When P25 nanoparticles were introduced into the reactants as nuclei for the nanowire precipitations, it is not necessary to use fine Ti particles to provide the Ti source; rather, much cheaper Ti sponge granules can be used, which also facilitate the
RI PT
products separations. In this case, the heterogeneous nucleation of nanowires on P25 single particles or their aggregates occurred, which led to sea-urchin nanowire aggregates in the solution (Figure 1b-d). The hierarchical nanostructures are
SC
assembles of P25 nanoparticles in the core and the surrounding nanowires with a
M AN U
diameter of ca. 15-25 nm and an average length of 1.5 µm. With the increasing P25 amounts in the reactant, the aggregates changed from the nearly monodispersed microspheres to larger irregular shapes. The increasing P25 concentration led to more P25 nanoparticulate aggregates, which as a whole served as nuclei and in turn resulted
TE D
in the larger irregular nanowire aggregates. However, too many P25 nanoparticles in the solution disrupted the sea-urchin nanowire aggregates, which results in randomly distributing nanowires incorporated with P25 nanoparticulate aggregates (Figure S2).
EP
Figure 2 shows XRD patterns of P25 nanoparticles and nanowire aggregates. The
AC C
standard data for anatase TiO2, which is derived from the Joint Committee on Powder Diffraction Standards (JCPDS) card No. 21-1272, is also presented. The P25 nanoparticles consisted of 80% anatase and 20% rutile [25]; therefore, most XRD peaks can be attributed to anatase and rutile TiO2 for all the samples. Except those XRD peaks from the P25 nuclei, one acute XRD peak located at ca. 8.5 ° in 2θ, which can be attributed to (200) plane of H2Ti5O11·3H2O (JCPDS card No. 44-0130), can be clearly seen for the sample NW-10 before the final calcination, which is labeled 7
ACCEPTED MANUSCRIPT as_NW-10 in Figure 2. Comparing the samples as_NW-10 and NW-10, it can be discerned that H2Ti5O11·3H2O nanowires decomposed to mainly anatase after heating in air at 450 °C for 1 h, which overshadowed the XRD peaks corresponding to rutile
RI PT
from P25. One XRD peak located at 31.5 ° in 2θ can also be found, which can be contributed to srilankite TiO2 [18]. The samples NW, NW-2 and NW-5 exhibit XRD patterns similar to that of NW-10.
SC
∆ rutile ◊ Srilankite ∇ H2Ti5O11⋅3H2O
NW-10
◊
NW-5
M AN U
Intensity (arb. units)
◊
◊
NW-2
◊
∇
NW
as_NW-10
∆
TE D
∆
10
20
30
∆
P25
∆
JCPDS Card No. 21-1272
40
50
60
70
80
2 theta (degree)
EP
Figure 2. XRD patterns of P25 and TiO2 nanowires synthesized using various amounts of P25 nanoparticles as nuclei: NW, NW-2, NW-5, and NW-10. The XRD
AC C
pattern of NW-10 just before the final calcination is also shown (as_NW-10). All the unmarked peaks correspond to anatase TiO2. Figure 3 illustrates the TEM observations of the sample NW-10. Radially aligned
nanowires can be clearly seen (Figure 3a). The corresponding selected area electron diffraction (SAED) pattern shows clear diffraction rings corresponding to anatase and srilankite TiO2, suggesting that the nanowires are polycrystalline (Figure 3b). A closer 8
ACCEPTED MANUSCRIPT observation of the nanowire demonstrates the staking of crystallites, which leaves several mesopores within the nanowires (Figure 3c). The high magnification TEM (HRTEM) image shows lattice fringes with an adjacent distance of 0.35 nm (Figure
RI PT
3d), which correspond to the (101) planes of anatase TiO2. The low temperature nitrogen adsorption–desorption isotherms of the samples NW-2, NW-5 and NW-10 are demonstrated in Figure 4. The corresponding pore size distribution curves exhibit
SC
similar dual pore structure of the titania nanowire aggregates. The maxima at a
M AN U
smaller size (ca. 5 nm) can be attributed to the voids arising from the alignment of anatase grains within a single nanowire (Figure 3c); whilst the other peak at a larger size (ca. 160 nm) can be contributed to the intra-aggregate pores of the nanowires [26].
AC C
EP
TE D
The calculated BET specific surface area and total pore volume are listed in Table 1.
Figure 3. TEM (a, c) and HRTEM (d) images of the sample NW-10. (b) The corresponding SAED pattern collected from (a).
9
ACCEPTED MANUSCRIPT
0.0000
100
1
10
100
1000
Pore diameter (nm) Adsorption Desorption
50 0
200
1
10
100
1000
Pore diameter (nm)
100 Adsorption Desorption
50
0.4
0.6
0.8
1.0
0.0010
0.0005
0.0000
100
1
10
100
1000
Pore diameter (nm) Adsorption Desorption
50 0
0.0
Relative pressure (p/p0)
0.2
0.4
0.6
0.8
0.0
1.0
0.2
0.4
0.6
0.8
1.0
RI PT
0.2
0.0015
3
150
0 0.0
-1
-1
0.0005
0.0000
150
dV(d) (cm ⋅g ⋅nm )
-1
0.0010
200
3
-1 -1
0.0015
Volume adsorbed (cm ⋅g )
0.0005
250
0.0020
250
3
-1
300
3
−1
-1
Volume adsorbed (cm ⋅g )
150
0.0010
(c)
0.0020
400 350
0.0015
3
200
dV(d) (cm ⋅g ⋅nm )
3
−1
Volume adsorbed (cm ⋅g )
250
(b)
0.0020
dV(d) (cm ⋅g ⋅nm )
(a) 300
Relative pressure (p/p0)
Relative pressure (p/p0)
Figure 4. The nitrogen adsorption-desorption isotherms of (a) NW-2, (b) NW-5 and (c)
SC
NW-10 at 77 K. The insets are the corresponding pore size distribution curve calculated by the BJH model.
M AN U
Table 1. The P25 content, BET specific surface area, total pore volume, band gap, and reaction rate constant (k) to assist photodegradations of phenol in water under UV light illumination for the various TiO2 powders Specific surface
Total pore
Band
k
content
area
volume
gap
h-1
m2/g
cm3/g
eV
46.3
0.427
2.65
0.30
wt. % 3.0 10.6
52.9
0.545
2.75
0.44
NW-10
13.7
52.8
0.371
2.88
0.79
NW
0
/
/
2.48
0.25
P25
100
50.0
/
2.93
0.73
AC C
NW-5
EP
NW-2
TE D
P25
The chemical bonding states of the sample NW-10 were determined by XPS measurements. Elements of Ti, O and N were detected in the XPS survey spectra (Figure S3). Figure 5a shows that, the binding energy of Ti 2p located at 464.8 eV and 10
ACCEPTED MANUSCRIPT 459.0 eV, which can be assigned to the spectrum of Ti 2p1/2 and Ti 2p3/2, with a separation of 5.8 eV that is typical of Ti4+ in the TiO2 lattice [27]. In Figure 5b, the O 1s spectrum is asymmetry and can be devolved into two contributions. The main peak
RI PT
of O 1s at 530.3 eV is attributed to the lattice oxygen (Ti-O-Ti), and the contribution at around 531.7 eV belongs to Ti-OH. The ratio of Ti-OH to the lattice oxygen is determined to be ca. 18%. The binding energy of N 1s is around 399.6 eV (Figure 5c),
SC
which can be attributed to N-O, N-N, or N-C bonds, because of the melamine
atom ratio of N/Ti is 0.03.
M AN U
decomposition during the precipitations of hydrogen titanate nanowires [27]. The
TG and DSC measurements were carried out to evaluate the P25 content in NW-2, NW-5 and NW-10. The results are demonstrated in Figure 6 for NW and
TE D
NW-10, and Figure S4 for NW-2 and NW-5. All TG curves exhibit three stages judging from the weight loss, which is similar to those observed from the thermal decomposition procedure of H2Ti5O11·3H2O nanowires [18]. The first stage was
EP
accompanied by a gradual weight loss with an endothermic peak from room
AC C
temperature to around 200 °C, which can be contributed to the loss of absorbed water and slight coordinated water from hydrogen titanate. The second stage presented a rapid weight loss accompanied with an exothermic peak in the range of 200-350 °C, resulting from the thermal decomposition of hydrogen titanate. Further heating beyond 350 °C, there was a small weight loss accompanying a broad exothermic peak which may be attributed to the phase transformation of TiO2 from anatase and/or srilankite to rutile. 11
ACCEPTED MANUSCRIPT (b)
(c)
O 1s
468
466
464
462
460
458
456
454
N 1s
534
533
532
531
530
529
528
Binding energy (eV)
Binding energy (eV)
408 406 404 402 400 398 396 394
RI PT
Intensity (arb. units)
Intensity (arb. units)
Ti 2p
Intensity (arb. units)
(a)
Binding energy (eV)
Figure 5. High-resolution XPS spectra of (a) Ti 2p, (b) O 1s, and (c) N 1s for NW-10.
-0.05 20.51 %
80
-0.10
2.778 %
-0.15
70
-0.20
60 50
0
200
400
600
800
Temperature (°C)
5.910 %
100 90 80
-0.10 1.57 %
-0.15
70
-0.20
60 50
-0.25 1000
-0.05
16.66 %
M AN U
Weight (%)
90
0.00
110
0
200
400
600
800
Tempereture difference (°C/mg)
5.658 %
100
(b)
SC
0.00
Weight (%)
110
Tempereture difference (°C/mg)
(a)
-0.25 1000
Temperature (°C)
TE D
Figure 6. TG and DSC curves of (a) NW and (b) NW-10 powders recorded in air atmosphere at a heating rate of 10 K/min.
EP
The first two stages in the weight loss correspond to the transformation from hydrogen titanates to mainly anatase TiO2. The total weight loss is 26.2 wt. % and
AC C
22.6 wt. % for NW (Figure 6a) and NW-10 (Figure 6b), respectively. Assuming that P25 nanoparticles are stable for up to 350 °C, during which region the decomposition of hydrogen titanate occurred, one can evaluate the proportion of P25 nanoparticles in NW-10, simply by calculating 1-22.6/26.2 = 0.137. In this way, the proportions of P25 nanoparticles in NW-2, NW-5 and NW-10 are estimated to be 3.0 wt. %, 10.6 wt. % and 13.7 wt. %. The proportion does not increase linearly with the increasing amounts of P25 nanoparticles in the reactants. The interactions between metallic Ti and H2O2 12
ACCEPTED MANUSCRIPT are quite complicated and not yet fully clarified. According to Tengvall et al., the Ti-H2O2 interaction resulted in complicated amorphous titania gel in abundance of superoxide and hydroxyl radicals [28]. In the current investigation, it is assumed that
ions into the solution, according to Eq. (1). Ti + 4
=
(
) +2
+
Eq. (1)
RI PT
oxidation of metallic Ti by H2O2 releases gaseous oxygen and at the same time Ti(IV)
SC
When the Ti(IV) ions reach a saturated value, hydrogen titanate (Figure 2)
M AN U
nucleates on P25 nanoparticles and then grows according to Eq. (2). Because of the well-defined, layered structure with a relatively large interlayer distance along the (100) direction [29], nanowires are commonly observed for hydrogen titanates, growing along the [010] direction [30].
⋅3
) =
+6
Eq. (2)
TE D
5 (
The melamine hydrolyzed gradually to urea and finally to ammonia [31], which, together with the nitric acid, provides an appropriate environment (pH value) for the
EP
formation of the hydrogen titanate nanowires. The proportion of P25 nanoparticles
AC C
seems to saturate upon the additive of 5 mg P25 in the reactant, which suggests that, in the current investigation, 5 mg P25 nanoparticles provided enough nuclei for the formations of hydrogen titanate nanowires. Figure 7a demonstrates the UV-Vis diffuse reflectance spectra of P25, NW,
NW-2, NW-5 and NW-10. When compared with P25, remarkable absorbance in visible light (400-500 nm) for samples containing nanowires can be discerned, which could be contributed to the nitrogen incorporation in TiO2 nanowires (Figure 5c). The 13
ACCEPTED MANUSCRIPT band gaps were evaluated and listed in Table 1, based on the Kubelka-Munk formula (Figure 7b). The band gaps increased with increasing P25 contents, suggesting a mixing effect in light adsorption. (b)
(a) 1.6
2.8
1.6
300
400
500
600
0.8
0.0 2.0
700
NW-10 NW NW-5 NW-2 P25
1.2
0.4
0.0
RI PT
0.4
2.0
SC
(eV⋅cm )
1/2
0.8
-1 1/2
NW-10 NW NW-5 NW-2 P25
1.2
(αhυ)
Absorbance (arb. units)
2.4
2.5
3.0
3.5
4.0
4.5
hυ (eV)
M AN U
Wavelength (nm)
Figure 7. (a) UV-Vis diffuse reflectance spectra of P25, NW, NW-2, NW-5 and NW-10. (b) Replotting of (a) in the (αhν)1/2~hν coordinate to evaluate the corresponding band gap.
TE D
Previous studies established superior electron transport capability along the longitudinal dimension and low charge carrier recombination rate for 1D nanostructures [32-34]. Herein, we explore the capacity of P25-incoporated nanowires
EP
as photocatalysts to assist photodegradations of phenol in water under UV light
AC C
illumination. Figure 8a shows the degradation curves for NW-2, NW-5, and NW-10. Those obtained from NW, P25 and NW+P25 (a simple mixture of 86.3 wt. % NW and 13.7 wt. % P25) are also included for comparisons. In absence of any photocatalysts, the sole UV light illumination of phenol in water induced no degradations [22]. The dark adsorption curves suggest that all the powders induced insignificant phenol adsorption. Figure 8b shows that, all the degradation curves can be well fitted using a pseudo first order reaction kinetics. The reaction rate constants for P25, NW, NW-2, 14
ACCEPTED MANUSCRIPT NW-5, and NW-10 are determined to be 0.73, 0.25, 0.30, 0.44, and 0.79 h-1. It can be seen that, with the increasing P25 contents in the nanowire aggregates, the photocatalytic activity increased dramatically. This is not surprising because P25
RI PT
exhibits an efficiency significantly higher than that of NW. However, the reaction rate constant of the sample NW-10 even exceeds slightly that of P25, which hints that a simple mixing rule cannot explain solely the enhanced photocatalytic activity for the
Dark
0.0
Illumination
0 -1
0
1
2
3
4
5
0
P2 5
1
0.4
0.2
2
3
Illuminating time (h)
Time (h)
0.6
c/c0
2
P2 5 NW +
NW NW-2 NW-5 NW-10 P25 NW + P25
1
0.2
M AN U
c/c0 0.4
0.8
W
3
ln(c0/c)
0.6
4
(c) 1.0
1.0 0.8 0.6 0.4 0.2 0.0
N
0.8
5
N W -2 N W -5 NW -1 0
NW NW-2 NW-5 NW-10 P25 NW + P25
-1
(b)
1.0
k (h )
(a)
SC
P25-incorporated nanowire aggregates.
4
5
0.0
0 1 2 30 1 2 30 1 2 30 1 2 30 1 2 30 1 2 3
Illuminating time (h)
TE D
Figure 8. (a) Photodegradation curves of phenol in water in the presence of NW, NW-2, NW-5, NW-10, P25, as well as a powder mixture of 86.3 wt. % NW and 13.7
EP
wt. % P25 (NW + P25), under UV light illumination; (b) the corresponding fitting results assuming a pseudo-first order reaction; (c) cycling performance of the sample
AC C
NW-10. The inset in (b) shows the reaction rate constant for each sample. For the simple mixture of 86.3 wt. % NW and 13.7 wt. % P25 in the slurry as a
photocatalyst, the reaction rate constant is 0.55 h-1, which is not surprisingly inferior to that of P25, and also smaller than that of NW-10 with the same composition. We therefore believe that direct nucleation and growth of nanowires from P25 nanoparticles led to phase junctions after the final calcination, which resulted in a 15
ACCEPTED MANUSCRIPT synergetic effect contributing to the photocatalytic activity [35,36]. As reported in literatures, TiO2 with mixed phases establishes junctions along the anatase/rutile [7] or anatase/srilankite [37] interfaces, which results in certain charge transfers, favoring
to be involved in the photocatalytic reactions.
RI PT
the charge separations and hence fast diffusions of charge carriers to the very surface
Figure 8c shows the cycling performance of NW-10. The photocatalytic
SC
efficiency remained stable by repetitively testing for up to 6 cycles. For repetitive
M AN U
catalyst utilizations, it is important to recover catalysts from the slurry. The excellent performance of commercial P25 nanoparticles relies on their well dispersions in the slurry system; however, the well dispersion usually means a boring recovery procedure, which hinders its wide application in the wastewater remediation. On the
TE D
contrary, the present TiO2 nanowire aggregates are tens of micrometers in size, which deposit readily when stop stirring (Figure S5). The facile synthetic route, together with the excellent performance, makes the P25-incorporated sea-urchin-like TiO2
EP
nanowires competitive for industrial applications.
AC C
4. Conclusions
In summary, we have developed a green and template-free synthesis route of
TiO2 nanowires by a facile reaction between metallic titanium sponge and H2O2 solution, using P25 nanoparticles as nuclei. The sea-urchin-like nanowire aggregates consisted of P25 cores and radially aligned nanowires ca. 15-25 nm in diameter and 1.5 µm in length. For photodegradations of phenol in water under UV light illumination, the aggregates exhibited increasing reaction rate constants when the P25 16
ACCEPTED MANUSCRIPT content increased. The nanowire aggregates containing 13.7 wt. % P25 exhibited an efficiency slightly higher than that of P25 nanoparticles, which makes it a potential photocatalyst for wastewater remediation.
RI PT
Acknowledgments This study was supported by the National Natural Science Foundation of China (No. 51502065) and State Key Laboratory of Silicon Materials (No. SKL2016-12).
SC
References
M AN U
[1] J. Schneider, M. Matsuoka, M. Takeuchi, J. Schneider, M. Matsuoka, M. Takeuchi, J. L. Zhang, Y. Horiuchi, M. Anpo, D. W. Bahnemann, Chem. Rev. 114 (2014) 9919−9986.
[2] X. Y. Wang, Z. Li, W. J. Xu, S. A. Kulkarni, S. K. Batabyal, S. Zhang, A. Y. Cao,
TE D
L. H. Wong, Nano Energy 11 (2015) 728–735.
[3] Z. Y. Weng, H. Guo, X. M. Liu, S. L. Wu, K. W. K. Yeung, P. K. Chu, RSC Adv. 3 (2013) 24758−24775.
EP
[4] L. S. Hu, C. C. Fong, X. M. Zhang, L. L. Chan, P. K. S. Lam, P. K. Chu, K. Y.
AC C
Wong, M. S. Yang, Environ. Sci. Technol. 50 (2016) 4430−4438. [5] M. Z. Ge, C. Y. Cao, J. Y. Huang, S. H. Li, Z. Chen, K. Q.
Zhang, S. S.
Al-Deyab, Y. K. Lai, J. Mater. Chem. A 4 (2016) 6772−6801.
[6] Q. J. Li, B. Y. Cheng, X. Yang, R. Liu, B. Liu, J. Liu, Z. Q. Chen, B Zou, T. Cui, B. B. Liu, J. Phys. Chem. C 117 (2013) 8516−8521. [7] D. Wang, X. T. Zhang, P. P. Sun, S. Lu, L. L. Wang, C. H. Wang, Y. C. Liu, Electrochimica Acta 130 (2014) 290–295. 17
ACCEPTED MANUSCRIPT [8] R. Devan, Y. R. Ma, R. Patilc, S. M. Lukas, RSC Adv. 6 (2016) 62218−62225. [9] I. S. Grover, R. C. Prajapat, S. Singh, B. Pal, Solar Energy 144 (2017) 612–618. [10] L. X. Yuan, S. Q. Meng, Y. Y. Zhou, Z. X. Yue, J. Mater. Chem. A 1 (2013)
RI PT
2552−2557. [11] W. Wang, M. Tian, A. Abdulagatov, S. M. George, Y. C. Lee, R. G. Yang, Nano
[12] X. L. Wang, J. Mater. Sci. 47 (2012) 739–745.
M AN U
[13] A. M. Youssef, RSC Adv. 4 (2014) 6811−6820.
SC
Lett. 12 (2012) 655−660.
[14] W. Y. Zhou, Y. C. He, Chem. Eng. J. 179 (2012) 412–416. [15] C. Liu, L. Q. Zhang, R. Liu, Z. F. Gao, X. P. Yang, Z. Q. Tu, F. Yang, Z. Z. Ye, L. S. Cui, C M. Xu, Y. F. Li, J. Alloy. Compd. 656 (2016) 24−32.
TE D
[16] Y. B. Mao, M. Kanungo, T. H. Benny, S. S. Wong, J. Phys. Chem. B 110 (2006) 702−710.
[17] H. B. Wu, H. H. Hong, X. W. (David) Lou, Adv. Mater. 24 (2012) 2567–2571.
EP
[18] L. L. Lai, J. M. Wu, RSC Adv. 4 (2014) 36212−36217.
AC C
[19] B. Li, J. M. Wu, T. T. Guo, M. Z. Tang, W. Wen, Nanoscale 6 (2014) 3046−3050. [20] X. Yu, N. Ren, J. Qiu, D. Sun, L. Li, H. Liu, Solar Energ. Mater. Solar Cells 183 (2018) 41–47.
[21] X. Yu, Z. Zhao, J. Zhang, W. Guo, L. Li, H. Liu, Z. L. Wang, CrystEngComm 19 (2017) 129–136. [22] R. Jiang, W. Wen, Y. Luo, J. M. Wu, J. Environ. Chem. Eng. 5 (2017) 4676–4683. [23] Y. Xu, W. Wen, J. M. Wu, J. Hazard. Mater. 343 (2018) 285–297. 18
ACCEPTED MANUSCRIPT [24] R. Jiang, W. Wen, J. M. Wu, New J. Chem. 42 (2018) 3020–3027. [25] J. M. Wu, B. Huang, J. Am. Ceram. Soc. 90 (2007) 283–286. [26] J. M. Wu, X. M. Song, M. Yan, J. Hazard. Mater. 194 (2011) 338–344.
RI PT
[27] J. M. Wu, J. X. Yin, RSC Adv. 5 (2015) 3465−3469. [28] P. Tengvall, H. Elwing, L. Sjöqvist, I. Lundström, L. M. Bjursten, Biomaterials 10 (1989) 118−120.
SC
[29] D. V. Bavykin, J. M. Friedrich, F. C. Walsh, Adv. Mater. 18 (2006) 2807–2824.
M AN U
[30] L.L. Lai, W. Wen, B. Fu, X.Y. Qian, J.M. Wu, Mater. Des. 108 (2016) 581–589. [31] J. M. Wu, H. X. Xue, J. Am. Ceram. Soc. 92 (2009) 2139–2143. [32] N. Q. Wu, J. Wang, D. N. Tafen, H. Wang, J. G.
Zheng, J. P. Lewis, X. G. Liu, S.
S. Leonard, A. Manivannan, J. Am. Chem. Soc. 132 (2010) 6679–6685.
TE D
[33] G. Lui, J. Y. Liao, A. Duan, Z. S. Zhang, M. Fowler, A. P. Yu, J. Mater. Chem. A 1 (2013) 12255−12262.
[34] K. Shankar, G. K. Mor, H. E. Prakasam, S. Yoriya, M. Paulose, O. K. Varghese, C.
EP
A. Grimes, Nanotechnology 18 (2007) 065707.
AC C
[35] S. G. Kumar, L. G. Devi, J. Phys. Chem. A 115 (2011) 13211–13241. [36] Q. E. Zhao, W. Wen, Y. Luo, J. M. Wu, Thin Solid Films 648 (2018) 103–107. [37] K. Saitow, T. Wakamiya, Appl. Phys. Lett. 103 (2013) 031916.
19
ACCEPTED MANUSCRIPT Table Table 1. The P25 content, BET specific surface area, total pore volume, band gap, and reaction rate constant (k) to assist photodegradations of phenol in water under UV
RI PT
light illumination for the various TiO2 powders
Figure captions
SC
Figure 1 FESEM images of TiO2 nanowires synthesized using various amounts of
M AN U
P25 nanoparticles as nuclei: (a) NW, (b) NW-2, (c) NW-5, and (d) NW-10. The insets are the corresponding low-magnification FESEM images.
Figure 2 XRD patterns of P25 and TiO2 nanowires synthesized using various amounts of P25 nanoparticles as nuclei: NW, NW-2, NW-5, and NW-10. The XRD pattern of
TE D
NW-10 just before the final calcination is also shown (as_NW-10). All the unmarked peaks correspond to anatase TiO2.
Figure 3 TEM (a, c) and HRTEM (d) images of the sample NW-10. (b) The
EP
corresponding SAED pattern collected from (a).
AC C
Figure 4. The nitrogen adsorption-desorption isotherms of (a) NW-2, (b) NW-5 and (c) NW-10 at 77 K. The insets are the corresponding pore size distribution curve calculated by the BJH model. Figure 5 High-resolution XPS spectra of (a) Ti 2p, (b) O 1s, and (c) N 1s for NW-10. Figure 6 TG and DSC curves of (a) NW and (b) NW-10 powders recorded in air atmosphere at a heating rate of 10 K/min. Figure 7. (a) UV-Vis diffuse reflectance spectra of P25, NW, NW-2, NW-5 and 20
ACCEPTED MANUSCRIPT NW-10. (b) Replotting of (a) in the (αhν)1/2~hν coordinate to evaluate the corresponding band gap. Figure 8 (a) Photodegradation curves of phenol in water in the presence of NW,
RI PT
NW-2, NW-5, NW-10, P25, as well as a powder mixture of 86.4 wt. % NW and 13.6 wt. % P25 (NW + P25), under UV light illumination; (b) the corresponding fitting results assuming a pseudo-first order reaction; (c) cycling performance of the sample
AC C
EP
TE D
M AN U
SC
NW-10. The inset in (b) shows the reaction rate constant for each sample.
21
ACCEPTED MANUSCRIPT Highlights Urchin-like TiO2 powders were synthesized utilizing the Ti-H2O2 interactions. Degussa P25 TiO2 nanoparticles served as nuclei for the nanowire growth.
RI PT
The photocatalytic activity of the composite powders is superior to the simple
AC C
EP
TE D
M AN U
SC
mixture of nanowires and P25.
S0022-3697(18)31848-1
DOI:
10.1016/j.jpcs.2018.09.016
Reference:
PCS 8729
To appear in:
Journal of Physics and Chemistry of Solids
Received Date: 12 July 2018 Revised Date:
27 August 2018
Accepted Date: 11 September 2018
Please cite this article as: Q.-E. Zhao, W. Wen, Y. Xia, J.-M. Wu, Titania nanowires growing from P25 nuclei: Facile synthesis and the improved photocatalytic activity, Journal of Physics and Chemistry of Solids (2018), doi: 10.1016/j.jpcs.2018.09.016. 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
M AN U
SC
RI PT
Graphic Abstract
Urchin-like TiO2 powders with excellent photocatalytic performances were synthesized utilizing the Ti-H2O2 interactions, with the additive of Degussa P25 TiO2
AC C
EP
TE D
nanoparticles serving as nuclei.
ACCEPTED MANUSCRIPT
Titania nanowires growing from P25 nuclei: facile synthesis and the improved photocatalytic activity
a
RI PT
Qing-Er Zhao a, Wei Wen a,b, Yu Xia a, Jin-Ming Wu a* State Key Laboratory of Silicon Materials and School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310037, P. R. China.
College of Mechanical and Electrical Engineering, Hainan University, Haikou
M AN U
b
SC
[email protected]
570228, P. R. China.
Abstract
TE D
Titania powders with hierarchical nanostructures find potential applications in wastewater treatments because of their excellent photocatalytic performances and the advantage of easily recovering from the slurry. Commercial Degussa P25 titania
EP
nanoparticles are widely adopted photocatalyst with high photocatalytic activity. In
AC C
this study, we used P25 as nuclei to synthesize a hierarchical titania. With the additive of P25 nuclei in the Ti-H2O2 reactant maintained at a low temperature of 80 °C, hydrogen titanate nanowires radially grew from the P25 cores to form sea-urchin nanowire aggregates. A subsequent calcination in air at 450 °C decomposed the titanate nanowires to anatase, keeping the nanowire morphology unchanged. The weight ratio of titania nanowires to P25 nanoparticles can be simply tuned by varying the P25 concentration in the reactant. When utilized to assist photodegradations of 1
ACCEPTED MANUSCRIPT phenol in water under UV light illumination, the hierarchical titania consisting of 86.3 wt. % nanowires and 13.7 wt. % P25 exhibited a reaction rate constant of 0.79 h-1, which is 3.2 times that of titania nanowires (0.25 h-1), and even slightly higher than
RI PT
that of P25 nanoparticles (0.73 h-1). The reaction rate constant is also 1.5 times that of the mixture of titania nanowires and P25 nanoparticles, suggesting a synergetic effect probably arising from the phase junctions.
SC
Keywords: photocatalysis; heterogeneous nucleation; nanowire; titania; phenol
M AN U
1. Introduction
Nanostructured titania (TiO2) has been widely applied in various fields of photocatalysis [1], energy conversion and storage [2,3], gas sensing [4], etc., because of its merits of low cost, non-toxicity, chemical stability as well as unique optical and
TE D
electrical properties. One-dimensional (1D) nanostructures like nanowires, nanorods, and nanotubes, provide titania with high specific surface area, fast charge transfer rate, and low charge carrier recombination [5]. Among them, TiO2 nanowires, in various
EP
phases of anatase [6], rutile [7], and brookite [8], have been demonstrated as an
AC C
attractive photocatalyst [9]. The template thermolysis method, which uses hard or soft templates as the structure-directing agents, is a traditional technique to synthesize 1D TiO2 [10,11]. The templated route is also combined with a sol-gel process, which involves the hydrolysis of titanium alkoxide, to obtain 1D TiO2 [12]. However, the resultant TiO2 nanostructures are susceptible to harsh chemical reactions or high temperature calcinations, which are applied to remove the templates. To avoid the utilization
of
complicated
templates,
acid/alkali-hydrothermal 2
[13-16]
and
ACCEPTED MANUSCRIPT solvothermal [17] methods have also been developed to achieve 1D TiO2. Regretfully, the reactants usually involve titanium organics or inorganics in HCl, concentrated NaOH, or organic solvents, which is far from a green chemistry. To achieve 1D TiO2
RI PT
via reactants free of organics and strong acid/alkaline is of interesting [18-21]. In practice, the recovery of nanostructured photocatalysts from a slurry system is a challenge, which generally demands an ultrafiltration procedure. Hierarchical
SC
nanostructures such as urchin-like microspheres facilitate the catalysts recovery whilst
M AN U
maintaining the merits of the nanostructured constructing units [13,16,18,21]. For example, Yu et al. developed a one-step hydrothermal method for the synthesis of urchin-like anatase TiO2 assembled with ultrathin nanobelts, which exhibited excellent photocatalytic activity towards degradations of organic pollutants in water
TE D
[21]. There are relatively few reports on the template-free and green synthesis of 1D TiO2 nanostructures, or their assemblies, under a low temperature and in an open atmosphere [18]. In this paper, we report a green, template-free route to synthesize
EP
TiO2 nanowire aggregates. A facile reaction between metallic Ti sponge and H2O2
AC C
solution was adopted to provide supersaturated Ti(IV) ions and the commercial P25 TiO2 nanoparticles were used as nuclei to initiate the heterogeneous nucleation and the subsequent growth of the radially aligned nanowires. We expect that the utilization of P25, which possesses exceptional photocatalytic activity, would contribute to the photocatalytic performance of the resultants. The effects of the P25 additives in the Ti-H2O2 reactants on the morphology, optical property and efficiency to assist photodegradations of phenol in water under UV light illumination were studied. 3
ACCEPTED MANUSCRIPT 2. Experiment 2.1 Synthesis In a beaker, Degussa P25 TiO2 powders (2 mg, 5 mg, or 10 mg) were dispersed
RI PT
into 50 mL H2O2 solution with additives of 2.4 M melamine, 0.29 M HNO3, and 0.25 g metallic Ti sponge. The reactants were maintained at 80 °C for 48 h in an oven. The precipitates were collected and washed in sequence using distilled water and ethyl
SC
alcohol. After air drying at 60 °C, the powders were subjected to a final calcination in
M AN U
air at 450 °C for 1 h. The resultants were designated as NW-2, NW-5, and NW-10, respectively. It is noted that, in absence of P25, no nanowires can be collected from the solution. To achieve P25-free TiO2 nanowires (designated as NW), metallic Ti powders (200 mesh) were utilized instead of the Ti sponge. The details are provided
TE D
in the online Supporting Information. 2.2 Characterizations
The powder morphology was observed by a field-emission scanning electron
EP
microscopy (FESEM, Hitachi, S-4800) and a JEM-2100 microscopy (Jeol, Tokyo,
AC C
Japan). The X-ray diffraction (XRD) tests were carried out using a Rigaku D/max-3B diffractometer with a CuKα radiation, which is operated at 40 kV and 36 mA (λ = 0.15406 nm). The low-temperature N2 adsorption-desorption measurement was conducted at 77 K, using Autosorb-1-C (Quantachrome Instruments), and the sample was degassed at 80 °C for 12 h to remove physisorbed gases. The Brunauer–Emmett– Teller (BET) approach using adsorption data was followed to evaluate the specific surface area and the Barrett–Joyner–Halenda (BJH) method using the desorption 4
ACCEPTED MANUSCRIPT curve was applied to determine the pore size distribution and pore volume. The X-ray photoelectron spectra (XPS) were acquired on an EscalabMKII spectrometer equipped with a monochromatic Al Ka X-ray source (1486.8 eV). The binding energy
RI PT
was normalized to the C 1s (284.5 eV) for adventitious hydrocarbons as an indirect standard. The UV-Vis diffuse reflectance spectra were collected from a UV–Vis near-infrared spectrometer (UV-3150, Shimadzu). For the measurements, the same
SC
amounts of powders were piled on a layer of BaSO4 powder and then spread into a
M AN U
thin uniform layer using a glass rod. A DSCQ1000 instrument (Waters) was used to carry out the thermogravimetric (TG) and differential scanning calorimetry (DSC) investigations of as-precipitated nanowires in an air atmosphere, at a heating rate of 10 K min-1 from room temperature to 900 °C.
TE D
2.3 Photocatalytic evaluation
The photocatalytic performance was evaluated via photodegradations of phenol in water. In each run, 25 mg of powders were dispersed into 50 mL phenol solution
EP
(10 ppm). The UV light was provided by an 18 W UV lamp (PHILPS Lighting
AC C
Investment Company Limited, China) and the average intensity of UV irradiance reaching the liquid level was ca. 6.5 mW/cm2, which is measured in the wavelength range of 320-400 nm with a peak wavelength of 365 nm (Model: UV-A, Beijing Normal University, China). The slurry was stirred in the dark for 60 min to establish an absorption-desorption equilibrium before the photocatalytic reaction. The change in phenol concentration was monitored with a liquid chromatography apparatus (Wufeng LC100, WondaCract ODS-2 column, China), using a mixed solution of 5
ACCEPTED MANUSCRIPT methanol and acetic acid (methanol: water: acetic acid = 700: 300: 1 in volume) as a mobile phase. At an interval of 1 h, 0.2 mL phenol solution was sampled. 3. Results and Discussions
RI PT
The interactions of metallic Ti and H2O2 at 80 °C resulted in hydrogen titanate nanowires on certain substrates like Ti foils [19], Ti meshes [22], or seeded organic fibers [23,24]. In the current investigation, in absence of P25, only nanoparticles were
SC
collected from the solution (Figure S1a). It is observed that nanowires nucleated and
M AN U
grew preferably on granular Ti sponge (Figure S1b), which inhibited their homogeneous precipitations from the solution. In case that fine metallic Ti powders were utilized instead of the Ti sponge to provide the Ti source [18], the heterogeneous nucleation was inhibited and the homogeneous precipitation of nanowires was favored,
AC C
EP
TE D
which achieved P25-free TiO2 nanowire aggregates from the solution (Figure 1a).
Figure 1. FESEM images of TiO2 nanowires synthesized using various amounts of P25 nanoparticles as nuclei: (a) NW, (b) NW-2, (c) NW-5, and (d) NW-10. The insets are the corresponding low-magnification FESEM images. 6
ACCEPTED MANUSCRIPT When P25 nanoparticles were introduced into the reactants as nuclei for the nanowire precipitations, it is not necessary to use fine Ti particles to provide the Ti source; rather, much cheaper Ti sponge granules can be used, which also facilitate the
RI PT
products separations. In this case, the heterogeneous nucleation of nanowires on P25 single particles or their aggregates occurred, which led to sea-urchin nanowire aggregates in the solution (Figure 1b-d). The hierarchical nanostructures are
SC
assembles of P25 nanoparticles in the core and the surrounding nanowires with a
M AN U
diameter of ca. 15-25 nm and an average length of 1.5 µm. With the increasing P25 amounts in the reactant, the aggregates changed from the nearly monodispersed microspheres to larger irregular shapes. The increasing P25 concentration led to more P25 nanoparticulate aggregates, which as a whole served as nuclei and in turn resulted
TE D
in the larger irregular nanowire aggregates. However, too many P25 nanoparticles in the solution disrupted the sea-urchin nanowire aggregates, which results in randomly distributing nanowires incorporated with P25 nanoparticulate aggregates (Figure S2).
EP
Figure 2 shows XRD patterns of P25 nanoparticles and nanowire aggregates. The
AC C
standard data for anatase TiO2, which is derived from the Joint Committee on Powder Diffraction Standards (JCPDS) card No. 21-1272, is also presented. The P25 nanoparticles consisted of 80% anatase and 20% rutile [25]; therefore, most XRD peaks can be attributed to anatase and rutile TiO2 for all the samples. Except those XRD peaks from the P25 nuclei, one acute XRD peak located at ca. 8.5 ° in 2θ, which can be attributed to (200) plane of H2Ti5O11·3H2O (JCPDS card No. 44-0130), can be clearly seen for the sample NW-10 before the final calcination, which is labeled 7
ACCEPTED MANUSCRIPT as_NW-10 in Figure 2. Comparing the samples as_NW-10 and NW-10, it can be discerned that H2Ti5O11·3H2O nanowires decomposed to mainly anatase after heating in air at 450 °C for 1 h, which overshadowed the XRD peaks corresponding to rutile
RI PT
from P25. One XRD peak located at 31.5 ° in 2θ can also be found, which can be contributed to srilankite TiO2 [18]. The samples NW, NW-2 and NW-5 exhibit XRD patterns similar to that of NW-10.
SC
∆ rutile ◊ Srilankite ∇ H2Ti5O11⋅3H2O
NW-10
◊
NW-5
M AN U
Intensity (arb. units)
◊
◊
NW-2
◊
∇
NW
as_NW-10
∆
TE D
∆
10
20
30
∆
P25
∆
JCPDS Card No. 21-1272
40
50
60
70
80
2 theta (degree)
EP
Figure 2. XRD patterns of P25 and TiO2 nanowires synthesized using various amounts of P25 nanoparticles as nuclei: NW, NW-2, NW-5, and NW-10. The XRD
AC C
pattern of NW-10 just before the final calcination is also shown (as_NW-10). All the unmarked peaks correspond to anatase TiO2. Figure 3 illustrates the TEM observations of the sample NW-10. Radially aligned
nanowires can be clearly seen (Figure 3a). The corresponding selected area electron diffraction (SAED) pattern shows clear diffraction rings corresponding to anatase and srilankite TiO2, suggesting that the nanowires are polycrystalline (Figure 3b). A closer 8
ACCEPTED MANUSCRIPT observation of the nanowire demonstrates the staking of crystallites, which leaves several mesopores within the nanowires (Figure 3c). The high magnification TEM (HRTEM) image shows lattice fringes with an adjacent distance of 0.35 nm (Figure
RI PT
3d), which correspond to the (101) planes of anatase TiO2. The low temperature nitrogen adsorption–desorption isotherms of the samples NW-2, NW-5 and NW-10 are demonstrated in Figure 4. The corresponding pore size distribution curves exhibit
SC
similar dual pore structure of the titania nanowire aggregates. The maxima at a
M AN U
smaller size (ca. 5 nm) can be attributed to the voids arising from the alignment of anatase grains within a single nanowire (Figure 3c); whilst the other peak at a larger size (ca. 160 nm) can be contributed to the intra-aggregate pores of the nanowires [26].
AC C
EP
TE D
The calculated BET specific surface area and total pore volume are listed in Table 1.
Figure 3. TEM (a, c) and HRTEM (d) images of the sample NW-10. (b) The corresponding SAED pattern collected from (a).
9
ACCEPTED MANUSCRIPT
0.0000
100
1
10
100
1000
Pore diameter (nm) Adsorption Desorption
50 0
200
1
10
100
1000
Pore diameter (nm)
100 Adsorption Desorption
50
0.4
0.6
0.8
1.0
0.0010
0.0005
0.0000
100
1
10
100
1000
Pore diameter (nm) Adsorption Desorption
50 0
0.0
Relative pressure (p/p0)
0.2
0.4
0.6
0.8
0.0
1.0
0.2
0.4
0.6
0.8
1.0
RI PT
0.2
0.0015
3
150
0 0.0
-1
-1
0.0005
0.0000
150
dV(d) (cm ⋅g ⋅nm )
-1
0.0010
200
3
-1 -1
0.0015
Volume adsorbed (cm ⋅g )
0.0005
250
0.0020
250
3
-1
300
3
−1
-1
Volume adsorbed (cm ⋅g )
150
0.0010
(c)
0.0020
400 350
0.0015
3
200
dV(d) (cm ⋅g ⋅nm )
3
−1
Volume adsorbed (cm ⋅g )
250
(b)
0.0020
dV(d) (cm ⋅g ⋅nm )
(a) 300
Relative pressure (p/p0)
Relative pressure (p/p0)
Figure 4. The nitrogen adsorption-desorption isotherms of (a) NW-2, (b) NW-5 and (c)
SC
NW-10 at 77 K. The insets are the corresponding pore size distribution curve calculated by the BJH model.
M AN U
Table 1. The P25 content, BET specific surface area, total pore volume, band gap, and reaction rate constant (k) to assist photodegradations of phenol in water under UV light illumination for the various TiO2 powders Specific surface
Total pore
Band
k
content
area
volume
gap
h-1
m2/g
cm3/g
eV
46.3
0.427
2.65
0.30
wt. % 3.0 10.6
52.9
0.545
2.75
0.44
NW-10
13.7
52.8
0.371
2.88
0.79
NW
0
/
/
2.48
0.25
P25
100
50.0
/
2.93
0.73
AC C
NW-5
EP
NW-2
TE D
P25
The chemical bonding states of the sample NW-10 were determined by XPS measurements. Elements of Ti, O and N were detected in the XPS survey spectra (Figure S3). Figure 5a shows that, the binding energy of Ti 2p located at 464.8 eV and 10
ACCEPTED MANUSCRIPT 459.0 eV, which can be assigned to the spectrum of Ti 2p1/2 and Ti 2p3/2, with a separation of 5.8 eV that is typical of Ti4+ in the TiO2 lattice [27]. In Figure 5b, the O 1s spectrum is asymmetry and can be devolved into two contributions. The main peak
RI PT
of O 1s at 530.3 eV is attributed to the lattice oxygen (Ti-O-Ti), and the contribution at around 531.7 eV belongs to Ti-OH. The ratio of Ti-OH to the lattice oxygen is determined to be ca. 18%. The binding energy of N 1s is around 399.6 eV (Figure 5c),
SC
which can be attributed to N-O, N-N, or N-C bonds, because of the melamine
atom ratio of N/Ti is 0.03.
M AN U
decomposition during the precipitations of hydrogen titanate nanowires [27]. The
TG and DSC measurements were carried out to evaluate the P25 content in NW-2, NW-5 and NW-10. The results are demonstrated in Figure 6 for NW and
TE D
NW-10, and Figure S4 for NW-2 and NW-5. All TG curves exhibit three stages judging from the weight loss, which is similar to those observed from the thermal decomposition procedure of H2Ti5O11·3H2O nanowires [18]. The first stage was
EP
accompanied by a gradual weight loss with an endothermic peak from room
AC C
temperature to around 200 °C, which can be contributed to the loss of absorbed water and slight coordinated water from hydrogen titanate. The second stage presented a rapid weight loss accompanied with an exothermic peak in the range of 200-350 °C, resulting from the thermal decomposition of hydrogen titanate. Further heating beyond 350 °C, there was a small weight loss accompanying a broad exothermic peak which may be attributed to the phase transformation of TiO2 from anatase and/or srilankite to rutile. 11
ACCEPTED MANUSCRIPT (b)
(c)
O 1s
468
466
464
462
460
458
456
454
N 1s
534
533
532
531
530
529
528
Binding energy (eV)
Binding energy (eV)
408 406 404 402 400 398 396 394
RI PT
Intensity (arb. units)
Intensity (arb. units)
Ti 2p
Intensity (arb. units)
(a)
Binding energy (eV)
Figure 5. High-resolution XPS spectra of (a) Ti 2p, (b) O 1s, and (c) N 1s for NW-10.
-0.05 20.51 %
80
-0.10
2.778 %
-0.15
70
-0.20
60 50
0
200
400
600
800
Temperature (°C)
5.910 %
100 90 80
-0.10 1.57 %
-0.15
70
-0.20
60 50
-0.25 1000
-0.05
16.66 %
M AN U
Weight (%)
90
0.00
110
0
200
400
600
800
Tempereture difference (°C/mg)
5.658 %
100
(b)
SC
0.00
Weight (%)
110
Tempereture difference (°C/mg)
(a)
-0.25 1000
Temperature (°C)
TE D
Figure 6. TG and DSC curves of (a) NW and (b) NW-10 powders recorded in air atmosphere at a heating rate of 10 K/min.
EP
The first two stages in the weight loss correspond to the transformation from hydrogen titanates to mainly anatase TiO2. The total weight loss is 26.2 wt. % and
AC C
22.6 wt. % for NW (Figure 6a) and NW-10 (Figure 6b), respectively. Assuming that P25 nanoparticles are stable for up to 350 °C, during which region the decomposition of hydrogen titanate occurred, one can evaluate the proportion of P25 nanoparticles in NW-10, simply by calculating 1-22.6/26.2 = 0.137. In this way, the proportions of P25 nanoparticles in NW-2, NW-5 and NW-10 are estimated to be 3.0 wt. %, 10.6 wt. % and 13.7 wt. %. The proportion does not increase linearly with the increasing amounts of P25 nanoparticles in the reactants. The interactions between metallic Ti and H2O2 12
ACCEPTED MANUSCRIPT are quite complicated and not yet fully clarified. According to Tengvall et al., the Ti-H2O2 interaction resulted in complicated amorphous titania gel in abundance of superoxide and hydroxyl radicals [28]. In the current investigation, it is assumed that
ions into the solution, according to Eq. (1). Ti + 4
=
(
) +2
+
Eq. (1)
RI PT
oxidation of metallic Ti by H2O2 releases gaseous oxygen and at the same time Ti(IV)
SC
When the Ti(IV) ions reach a saturated value, hydrogen titanate (Figure 2)
M AN U
nucleates on P25 nanoparticles and then grows according to Eq. (2). Because of the well-defined, layered structure with a relatively large interlayer distance along the (100) direction [29], nanowires are commonly observed for hydrogen titanates, growing along the [010] direction [30].
⋅3
) =
+6
Eq. (2)
TE D
5 (
The melamine hydrolyzed gradually to urea and finally to ammonia [31], which, together with the nitric acid, provides an appropriate environment (pH value) for the
EP
formation of the hydrogen titanate nanowires. The proportion of P25 nanoparticles
AC C
seems to saturate upon the additive of 5 mg P25 in the reactant, which suggests that, in the current investigation, 5 mg P25 nanoparticles provided enough nuclei for the formations of hydrogen titanate nanowires. Figure 7a demonstrates the UV-Vis diffuse reflectance spectra of P25, NW,
NW-2, NW-5 and NW-10. When compared with P25, remarkable absorbance in visible light (400-500 nm) for samples containing nanowires can be discerned, which could be contributed to the nitrogen incorporation in TiO2 nanowires (Figure 5c). The 13
ACCEPTED MANUSCRIPT band gaps were evaluated and listed in Table 1, based on the Kubelka-Munk formula (Figure 7b). The band gaps increased with increasing P25 contents, suggesting a mixing effect in light adsorption. (b)
(a) 1.6
2.8
1.6
300
400
500
600
0.8
0.0 2.0
700
NW-10 NW NW-5 NW-2 P25
1.2
0.4
0.0
RI PT
0.4
2.0
SC
(eV⋅cm )
1/2
0.8
-1 1/2
NW-10 NW NW-5 NW-2 P25
1.2
(αhυ)
Absorbance (arb. units)
2.4
2.5
3.0
3.5
4.0
4.5
hυ (eV)
M AN U
Wavelength (nm)
Figure 7. (a) UV-Vis diffuse reflectance spectra of P25, NW, NW-2, NW-5 and NW-10. (b) Replotting of (a) in the (αhν)1/2~hν coordinate to evaluate the corresponding band gap.
TE D
Previous studies established superior electron transport capability along the longitudinal dimension and low charge carrier recombination rate for 1D nanostructures [32-34]. Herein, we explore the capacity of P25-incoporated nanowires
EP
as photocatalysts to assist photodegradations of phenol in water under UV light
AC C
illumination. Figure 8a shows the degradation curves for NW-2, NW-5, and NW-10. Those obtained from NW, P25 and NW+P25 (a simple mixture of 86.3 wt. % NW and 13.7 wt. % P25) are also included for comparisons. In absence of any photocatalysts, the sole UV light illumination of phenol in water induced no degradations [22]. The dark adsorption curves suggest that all the powders induced insignificant phenol adsorption. Figure 8b shows that, all the degradation curves can be well fitted using a pseudo first order reaction kinetics. The reaction rate constants for P25, NW, NW-2, 14
ACCEPTED MANUSCRIPT NW-5, and NW-10 are determined to be 0.73, 0.25, 0.30, 0.44, and 0.79 h-1. It can be seen that, with the increasing P25 contents in the nanowire aggregates, the photocatalytic activity increased dramatically. This is not surprising because P25
RI PT
exhibits an efficiency significantly higher than that of NW. However, the reaction rate constant of the sample NW-10 even exceeds slightly that of P25, which hints that a simple mixing rule cannot explain solely the enhanced photocatalytic activity for the
Dark
0.0
Illumination
0 -1
0
1
2
3
4
5
0
P2 5
1
0.4
0.2
2
3
Illuminating time (h)
Time (h)
0.6
c/c0
2
P2 5 NW +
NW NW-2 NW-5 NW-10 P25 NW + P25
1
0.2
M AN U
c/c0 0.4
0.8
W
3
ln(c0/c)
0.6
4
(c) 1.0
1.0 0.8 0.6 0.4 0.2 0.0
N
0.8
5
N W -2 N W -5 NW -1 0
NW NW-2 NW-5 NW-10 P25 NW + P25
-1
(b)
1.0
k (h )
(a)
SC
P25-incorporated nanowire aggregates.
4
5
0.0
0 1 2 30 1 2 30 1 2 30 1 2 30 1 2 30 1 2 3
Illuminating time (h)
TE D
Figure 8. (a) Photodegradation curves of phenol in water in the presence of NW, NW-2, NW-5, NW-10, P25, as well as a powder mixture of 86.3 wt. % NW and 13.7
EP
wt. % P25 (NW + P25), under UV light illumination; (b) the corresponding fitting results assuming a pseudo-first order reaction; (c) cycling performance of the sample
AC C
NW-10. The inset in (b) shows the reaction rate constant for each sample. For the simple mixture of 86.3 wt. % NW and 13.7 wt. % P25 in the slurry as a
photocatalyst, the reaction rate constant is 0.55 h-1, which is not surprisingly inferior to that of P25, and also smaller than that of NW-10 with the same composition. We therefore believe that direct nucleation and growth of nanowires from P25 nanoparticles led to phase junctions after the final calcination, which resulted in a 15
ACCEPTED MANUSCRIPT synergetic effect contributing to the photocatalytic activity [35,36]. As reported in literatures, TiO2 with mixed phases establishes junctions along the anatase/rutile [7] or anatase/srilankite [37] interfaces, which results in certain charge transfers, favoring
to be involved in the photocatalytic reactions.
RI PT
the charge separations and hence fast diffusions of charge carriers to the very surface
Figure 8c shows the cycling performance of NW-10. The photocatalytic
SC
efficiency remained stable by repetitively testing for up to 6 cycles. For repetitive
M AN U
catalyst utilizations, it is important to recover catalysts from the slurry. The excellent performance of commercial P25 nanoparticles relies on their well dispersions in the slurry system; however, the well dispersion usually means a boring recovery procedure, which hinders its wide application in the wastewater remediation. On the
TE D
contrary, the present TiO2 nanowire aggregates are tens of micrometers in size, which deposit readily when stop stirring (Figure S5). The facile synthetic route, together with the excellent performance, makes the P25-incorporated sea-urchin-like TiO2
EP
nanowires competitive for industrial applications.
AC C
4. Conclusions
In summary, we have developed a green and template-free synthesis route of
TiO2 nanowires by a facile reaction between metallic titanium sponge and H2O2 solution, using P25 nanoparticles as nuclei. The sea-urchin-like nanowire aggregates consisted of P25 cores and radially aligned nanowires ca. 15-25 nm in diameter and 1.5 µm in length. For photodegradations of phenol in water under UV light illumination, the aggregates exhibited increasing reaction rate constants when the P25 16
ACCEPTED MANUSCRIPT content increased. The nanowire aggregates containing 13.7 wt. % P25 exhibited an efficiency slightly higher than that of P25 nanoparticles, which makes it a potential photocatalyst for wastewater remediation.
RI PT
Acknowledgments This study was supported by the National Natural Science Foundation of China (No. 51502065) and State Key Laboratory of Silicon Materials (No. SKL2016-12).
SC
References
M AN U
[1] J. Schneider, M. Matsuoka, M. Takeuchi, J. Schneider, M. Matsuoka, M. Takeuchi, J. L. Zhang, Y. Horiuchi, M. Anpo, D. W. Bahnemann, Chem. Rev. 114 (2014) 9919−9986.
[2] X. Y. Wang, Z. Li, W. J. Xu, S. A. Kulkarni, S. K. Batabyal, S. Zhang, A. Y. Cao,
TE D
L. H. Wong, Nano Energy 11 (2015) 728–735.
[3] Z. Y. Weng, H. Guo, X. M. Liu, S. L. Wu, K. W. K. Yeung, P. K. Chu, RSC Adv. 3 (2013) 24758−24775.
EP
[4] L. S. Hu, C. C. Fong, X. M. Zhang, L. L. Chan, P. K. S. Lam, P. K. Chu, K. Y.
AC C
Wong, M. S. Yang, Environ. Sci. Technol. 50 (2016) 4430−4438. [5] M. Z. Ge, C. Y. Cao, J. Y. Huang, S. H. Li, Z. Chen, K. Q.
Zhang, S. S.
Al-Deyab, Y. K. Lai, J. Mater. Chem. A 4 (2016) 6772−6801.
[6] Q. J. Li, B. Y. Cheng, X. Yang, R. Liu, B. Liu, J. Liu, Z. Q. Chen, B Zou, T. Cui, B. B. Liu, J. Phys. Chem. C 117 (2013) 8516−8521. [7] D. Wang, X. T. Zhang, P. P. Sun, S. Lu, L. L. Wang, C. H. Wang, Y. C. Liu, Electrochimica Acta 130 (2014) 290–295. 17
ACCEPTED MANUSCRIPT [8] R. Devan, Y. R. Ma, R. Patilc, S. M. Lukas, RSC Adv. 6 (2016) 62218−62225. [9] I. S. Grover, R. C. Prajapat, S. Singh, B. Pal, Solar Energy 144 (2017) 612–618. [10] L. X. Yuan, S. Q. Meng, Y. Y. Zhou, Z. X. Yue, J. Mater. Chem. A 1 (2013)
RI PT
2552−2557. [11] W. Wang, M. Tian, A. Abdulagatov, S. M. George, Y. C. Lee, R. G. Yang, Nano
[12] X. L. Wang, J. Mater. Sci. 47 (2012) 739–745.
M AN U
[13] A. M. Youssef, RSC Adv. 4 (2014) 6811−6820.
SC
Lett. 12 (2012) 655−660.
[14] W. Y. Zhou, Y. C. He, Chem. Eng. J. 179 (2012) 412–416. [15] C. Liu, L. Q. Zhang, R. Liu, Z. F. Gao, X. P. Yang, Z. Q. Tu, F. Yang, Z. Z. Ye, L. S. Cui, C M. Xu, Y. F. Li, J. Alloy. Compd. 656 (2016) 24−32.
TE D
[16] Y. B. Mao, M. Kanungo, T. H. Benny, S. S. Wong, J. Phys. Chem. B 110 (2006) 702−710.
[17] H. B. Wu, H. H. Hong, X. W. (David) Lou, Adv. Mater. 24 (2012) 2567–2571.
EP
[18] L. L. Lai, J. M. Wu, RSC Adv. 4 (2014) 36212−36217.
AC C
[19] B. Li, J. M. Wu, T. T. Guo, M. Z. Tang, W. Wen, Nanoscale 6 (2014) 3046−3050. [20] X. Yu, N. Ren, J. Qiu, D. Sun, L. Li, H. Liu, Solar Energ. Mater. Solar Cells 183 (2018) 41–47.
[21] X. Yu, Z. Zhao, J. Zhang, W. Guo, L. Li, H. Liu, Z. L. Wang, CrystEngComm 19 (2017) 129–136. [22] R. Jiang, W. Wen, Y. Luo, J. M. Wu, J. Environ. Chem. Eng. 5 (2017) 4676–4683. [23] Y. Xu, W. Wen, J. M. Wu, J. Hazard. Mater. 343 (2018) 285–297. 18
ACCEPTED MANUSCRIPT [24] R. Jiang, W. Wen, J. M. Wu, New J. Chem. 42 (2018) 3020–3027. [25] J. M. Wu, B. Huang, J. Am. Ceram. Soc. 90 (2007) 283–286. [26] J. M. Wu, X. M. Song, M. Yan, J. Hazard. Mater. 194 (2011) 338–344.
RI PT
[27] J. M. Wu, J. X. Yin, RSC Adv. 5 (2015) 3465−3469. [28] P. Tengvall, H. Elwing, L. Sjöqvist, I. Lundström, L. M. Bjursten, Biomaterials 10 (1989) 118−120.
SC
[29] D. V. Bavykin, J. M. Friedrich, F. C. Walsh, Adv. Mater. 18 (2006) 2807–2824.
M AN U
[30] L.L. Lai, W. Wen, B. Fu, X.Y. Qian, J.M. Wu, Mater. Des. 108 (2016) 581–589. [31] J. M. Wu, H. X. Xue, J. Am. Ceram. Soc. 92 (2009) 2139–2143. [32] N. Q. Wu, J. Wang, D. N. Tafen, H. Wang, J. G.
Zheng, J. P. Lewis, X. G. Liu, S.
S. Leonard, A. Manivannan, J. Am. Chem. Soc. 132 (2010) 6679–6685.
TE D
[33] G. Lui, J. Y. Liao, A. Duan, Z. S. Zhang, M. Fowler, A. P. Yu, J. Mater. Chem. A 1 (2013) 12255−12262.
[34] K. Shankar, G. K. Mor, H. E. Prakasam, S. Yoriya, M. Paulose, O. K. Varghese, C.
EP
A. Grimes, Nanotechnology 18 (2007) 065707.
AC C
[35] S. G. Kumar, L. G. Devi, J. Phys. Chem. A 115 (2011) 13211–13241. [36] Q. E. Zhao, W. Wen, Y. Luo, J. M. Wu, Thin Solid Films 648 (2018) 103–107. [37] K. Saitow, T. Wakamiya, Appl. Phys. Lett. 103 (2013) 031916.
19
ACCEPTED MANUSCRIPT Table Table 1. The P25 content, BET specific surface area, total pore volume, band gap, and reaction rate constant (k) to assist photodegradations of phenol in water under UV
RI PT
light illumination for the various TiO2 powders
Figure captions
SC
Figure 1 FESEM images of TiO2 nanowires synthesized using various amounts of
M AN U
P25 nanoparticles as nuclei: (a) NW, (b) NW-2, (c) NW-5, and (d) NW-10. The insets are the corresponding low-magnification FESEM images.
Figure 2 XRD patterns of P25 and TiO2 nanowires synthesized using various amounts of P25 nanoparticles as nuclei: NW, NW-2, NW-5, and NW-10. The XRD pattern of
TE D
NW-10 just before the final calcination is also shown (as_NW-10). All the unmarked peaks correspond to anatase TiO2.
Figure 3 TEM (a, c) and HRTEM (d) images of the sample NW-10. (b) The
EP
corresponding SAED pattern collected from (a).
AC C
Figure 4. The nitrogen adsorption-desorption isotherms of (a) NW-2, (b) NW-5 and (c) NW-10 at 77 K. The insets are the corresponding pore size distribution curve calculated by the BJH model. Figure 5 High-resolution XPS spectra of (a) Ti 2p, (b) O 1s, and (c) N 1s for NW-10. Figure 6 TG and DSC curves of (a) NW and (b) NW-10 powders recorded in air atmosphere at a heating rate of 10 K/min. Figure 7. (a) UV-Vis diffuse reflectance spectra of P25, NW, NW-2, NW-5 and 20
ACCEPTED MANUSCRIPT NW-10. (b) Replotting of (a) in the (αhν)1/2~hν coordinate to evaluate the corresponding band gap. Figure 8 (a) Photodegradation curves of phenol in water in the presence of NW,
RI PT
NW-2, NW-5, NW-10, P25, as well as a powder mixture of 86.4 wt. % NW and 13.6 wt. % P25 (NW + P25), under UV light illumination; (b) the corresponding fitting results assuming a pseudo-first order reaction; (c) cycling performance of the sample
AC C
EP
TE D
M AN U
SC
NW-10. The inset in (b) shows the reaction rate constant for each sample.
21
ACCEPTED MANUSCRIPT Highlights Urchin-like TiO2 powders were synthesized utilizing the Ti-H2O2 interactions. Degussa P25 TiO2 nanoparticles served as nuclei for the nanowire growth.
RI PT
The photocatalytic activity of the composite powders is superior to the simple
AC C
EP
TE D
M AN U
SC
mixture of nanowires and P25.