CrystEngComm

PAPER

Cite this: CrystEngComm, 2017, 19,

129

Received 25th October 2016,Accepted 16th November 2016

DOI: 10.1039/c6ce02241c

www.rsc.org/crystengcomm

One-step synthesis of ultrathin nanobelts-assembled urchin-like anatase TiO2

nanostructures for highly efficient photocatalysis†

Xin Yu,‡a Zhenhuan Zhao,‡b Jian Zhang,c Weibo Guo,d Linlin Li,*a

Hong Liu*ae and Zhong Lin Wang*a

Nanostructured TiO2 materials with a controlled morphology and structure have drawn considerable atten-

tion to both fundamental research and practical applications owing to their unique characteristics. Herein,

a novel, facile, and one-step hydrothermal approach was developed to synthesize urchin-like anatase TiO2

hierarchical nanostructures assembled from ultrathin nanobelts using urea as the morphology-directing

agent. The effects of the urea concentration in the preparation process were discussed intensively. Photo-

catalytic experiments showed that the urchin-like anatase TiO2 nanostructures possessed a much higher

degradation rate of methyl orange and phenol than the most successful commercial semiconductor

photocatalyst P25. The reasons for the highly efficient photocatalytic activity was ascribed to the high spe-

cific surface area (171 m2 g−1) and ultrathin 1D nanobelts of anatase TiO2 self-assembled into the urchin-

like hollow spheres. The urchin-like anatase TiO2 nanostructures as photocatalysts have potential applica-

tions in environmental and energy fields for photocatalytic degradation, hydrogen production, Li-ion batte-

ries, and dye-sensitized solar cells. In addition, new hydrothermal method can be developed for synthesis

of other hierarchical nanostructures.

Introduction

Semiconductor-based photocatalysis for pollutant degradationand hydrogen generation by water splitting has been consid-ered as one of the most crucial approaches to solve theworld's energy and environmental crisis.1–3 Titanium dioxide(TiO2) has been investigated extensively in several applica-tions owing to its unique characteristics, including environ-mental benignancy, safety and stability.4,5 TiO2 possesses awide bandgap of about 3.2 eV and thus possesses excellentphotocatalytic activity under UV light illumination.6–8 It hasbeen commercialized in many industrial fields and daily life,

e.g., it is used as a white pigment in paints and as a UV ab-sorber in sunscreens.9–11

It has been generally accepted that the geometric mor-phology has tremendous effects on the physicochemicalproperties of TiO2.

12,13 Controlling the morphology and sizeof TiO2 nanostructures has proven to be crucial for obtainingsuperior photocatalytic, photovoltaic, and electrochemicalperformance.14–18 Substantial research on the preparation,characterization, and fundamental understanding hasboosted the utilization of TiO2 nanomaterials with outstand-ing performance in the past few decades.19–22 Synthesismethods for nanostructured TiO2 materials from zero tothree-dimensional (3D) structures have been reported.23–27 Inparticular, one-dimensional (1D) nanostructures are of greatimportance owing to the particular exposed facets and aniso-tropic structures.28,29 A multitude of 1D TiO2 nanostructures,such as nanowires,30,31 nanorods,32,33 nanotubes34–36 andnanobelts,37 have been widely reported with enhanced perfor-mance in photocatalysis, dye-sensitized solar cells, andlithium-ion batteries.38–40

Several studies have reported the synthesis and propertiesof nanostructured anatase TiO2. For example, Lou's group de-veloped a facile one-pot solvothermal method to synthesizeanatase TiO2 nanowires with high productivity and yield.41

Wu's group developed W, N co-doped and Ni doped TiO2

nanobelts with high photocatalytic activity.42,43 A recent study

CrystEngComm, 2017, 19, 129–136 | 129This journal is © The Royal Society of Chemistry 2017

a Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences,

National Center for Nanoscience and Technology (NCNST), Beijing, 100083, P. R.

China. E-mail: lilinlin@binn.cas.cn, zlwang@gatech.edub Institute of Fundamental and Frontier Sciences, University of Electronics Science

and Technology of China, Chengdu, 610054, P. R. Chinac Universite de lyon, ECL, INSA-Lyon, UCBL, CPE, CNRS, INL, UMR 5270, 36

Avenue Guy de Collongue, 69134 Ecully Cedex, Franced Shandong Provincial Key Laboratory of Detection Technology for Tumor

Markers, College of Chemistry and Chemical Engineering, Linyi University, Linyi

276005, P. R. Chinae State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100,

P. R. China. E-mail: hongliu@sdu.edu.cn

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ce02241c‡ Xin Yu and Zhenhuan Zhao are contributed equally to this work.

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by Zhao's group synthesized uniform 3D open macro/meso-porous TiO2 hollow microspheres with highly crystalline ana-tase thin shells by a simple solvent evaporation-driven con-fined self-assembly method.44 Chen and co-workerssynthesized ordered TiO2 mesophyll cell-like microsphereswith a prolonged electron lifetime for high performancephotocatalytic water reduction and oxidation.45 These studiesshowed that high specific surface areas and 1D nanostruc-tures with anatase crystals can improve the photocatalytic ac-tivity. Recently, remarkable efforts have been devoted to thesynthesis of hollow structured TiO2,

2,4,24 particularly yolk–shell structures,5,46 through non-templating hydrothermal ap-proaches. However, it is still difficult to fabricate TiO2 nano-materials with all these properties via a facile method.

To date, a few synthetic methods on the formation of TiO2

nanostructures have been developed via the hydrolysis ofK2TiO(C2O4)2 under various conditions. For example, hierar-chical TiO2 nanowires with anatase nanorod branches wereobtained via hydrothermal treatment of K2TiO(C2O4)2 in di-ethylene glycol and H2O solution at 180 °C.47 Rutile TiO2

mesocrystals were achieved by a controlled dissolution andprecipitation procedure in a HNO3, C3H6N6 and H2O2 aque-ous solution at a temperature of 80 °C.21 Hierarchicalnanosheet-assembled yolk–shell anatase TiO2 microsphereswere synthesized by the slow hydrolysis of K2TiO(C2O4)2 in amixed solution of H2O2 and HNO3, at temperatures as low as40 °C.46

In this study, we propose a one-step strategic hydrother-mal method for the synthesis of 3D biomimic urchin-like an-atase TiO2 nanostructure, which is assembled from 1D TiO2

ultrathin nanobelts. Enhanced photodegradation perfor-mance was obtained due to the large surface area andultrathin 1D nanobelt-assembled 3D urchin-like structures.Moreover, the urchin-like TiO2 structure is advantageous forfacilitating electrolyte infiltration and promoting electrontransportation.

Results and discussion

The morphology and microstructure of the 3D urchin-likeTiO2 nanostructures were characterized by scanning electronmicroscopy (SEM) and high resolution transmission electronmicroscopy (HRTEM). With 1 g urea in the reaction system,the as-synthesized TiO2 nanostructures had a typical bionicurchin-like morphology assembled from several ultrathinnanobelts (Fig. 1a). From a broken sphere in a Fig. 1a arrow,it was found that the TiO2 nanostructures had hierarchicalhollow structures. The diameter of the hollow sphere wasabout 1 μm. From the high magnification image (Fig. 1b),the hollow sphere was composed of radially assembledultrathin nanobelts with a width of about 4–8 nm and lengthof about 200–300 nm. The inner wall of the hollow spherewas very compact, forming an urchin-like hollow structure.The TEM image in Fig. 1c shows that the hollow interior ofthe urchin-like TiO2 structure was about 300–400 nm. More-over, the selected area electronic diffraction (SAED) pattern

of the urchin-like TiO2 shows the appearance of a series ofrings corresponding to the diffraction from the (101), (004),(200), (105), (204), (220), (215) and (224) planes, indicatingthe formation of anatase TiO2.

48 The enlarged TEM images inFig. 1e and S1† also confirmed that the shell of the urchin-like TiO2 nanostructures was composed of ultrathin nano-belts. In Fig. 1f, the high-resolution TEM (HRTEM) imagesshowed the perfect continuous two-dimensional atomic lat-tices with a spacing of 0.35 nm, corresponding to the (101)planes of anatase TiO2. The urchin-like TiO2 structures as-sembled with numerous ultrathin nanobelts exposed moresurface area, and the particular exposed facets and reactivesites may be advantageous for facilitating electrolyte infiltra-tion and promoting electron transportation.

The crystallographic structure of the urchin-like TiO2

nanostructures was confirmed by powder X-ray diffraction(XRD). As shown in Fig. 2a, all diffraction peaks wereassigned unambiguously to anatase TiO2 (JCPDS card no. 73-1764).49 This result was consistent with the SAED patterns inFig. 1d. The chemical composition and valence state ofurchin-like TiO2 nanostructures were characterized by X-rayphotoelectron spectroscopy (XPS). The full scanned spectrumof the urchin-like anatase TiO2 in the range of 0–900 eV isshown in Fig. S2.† This overview spectrum demonstrates that

Fig. 1 (a and b) SEM images of urchin-like anatase TiO2 nanostruc-tures, (c) TEM image of the urchin-like TiO2 nanostructure, (d) corre-sponding SAED pattern, (e and f) HRTEM image of TiO2

nanostructures.

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C, Ti, O, and N exist in the nanostructures. C was from thesystem, and can be used to normalize the peaks. No otherimpurity peaks other than carbon's peak were found in thespectra. The X-ray photoelectron spectrum of Ti 2p can bedeconvoluted into two peaks centered at 463.75 eV (Ti 2p 1/2)and 458.05 eV (Ti 2p 3/2), which were both assigned to theTi4+ oxidation state.29,50 The O 1s XPS spectrum (Fig. 2c)showed two chemical states of oxygen. The sharp peak at529.25 eV was assigned to the Ti–O bond, indicating themain lattice oxygen atoms. The other peak at 531.15 eV wasassigned to O–H bonds from the interaction of the urchin-like TiO2 nanostructures shells with an ambient environ-ment.51 Furthermore, from the XPS spectra, weak N 1s peaksat 399.65 eV and 395.05 eV related to N–H were found(Fig. 2d).52 This may be caused by urea during the surface ad-sorption in the reactants. However, in the XRD pattern, nopeaks belonging to nitrogen containing compounds or othertitanic polymorphs can be found. Furthermore, there was noevident shift of the XRD peaks indexed to the anatase phase.Thus, the crystallinity of the anatase TiO2 is not affected bythe presence of nitrogen.

To understand the formation mechanism of such a hollowsphere with a hierarchical structure, experiments under dif-ferent urea amounts were conducted, which indicated thatthe amount of urea was crucial in controlling the morphologyof the products. Fig. 3 presents SEM images of the TiO2

nanostructures obtained without and with different amountsof urea. As shown in Fig. 3a, TiO2 nanobelt crystals wereobtained without urea in the synthetic system. The length ofthe nanobelts is up to several micrometers. The width andthickness of the nanobelts are in the range 50–100 nm and20–30 nm, respectively. When the urea amount was 0.1 g, aspinous rod-like structure (several micrometers in length andabout 200 nm in diameter) was formed (Fig. 3b). Enlargedimages in Fig. S3† reveal that the ultrathin nanobelts were at-tached to each other at the end. When the synthesis wasperformed with 0.5 g of urea, the ultrathin nanobelts assem-

bled together to form a ball with the nanobelts reaching outto the outer surface (Fig. 3c). The TiO2 sphere possessed asolid core (Fig. S4†). When the amount of urea further in-creases to 1.0 g, as shown in Fig. 3d, well-developed urchin-like nanostructures with a hollow interior were formed byself-assembly from the ultrathin nanobelts.

To investigate the growth kinetics of urchin-like TiO2

nanostructures, the time-dependent morphology evolutionwas monitored. In a short time of 30 min, nanoparticles withan irregular morphology and broad size distribution from 50nm to 200 nm were observed (Fig. 4a). A hollow structure wasfound from a broken sphere, which consisted of several small

Fig. 2 (a) XRD pattern and XPS spectra of (b) Ti 2p (c) O 1s (d) N 1s forthe urchin-like anatase TiO2 nanostructures.

Fig. 3 SEM images of TiO2 sample with different synthesis conditions.(a) Without urea, (b) with 0.1 g urea, (c) with 0.5 g urea and (d) with 1 gurea. The insets in the SEM images show the correspondingmorphology sketches.

Fig. 4 SEM images of urchin-like TiO2 nanostructures prepared at dif-ferent reaction time intervals (a) 30 min, (b) 40 min, (c) 1 h, (d) 2 h at180 °C with 1.0 g urea. (e) Schematic of the formation mechanism ofurchin-like microspheres.

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nanoparticles (Fig. 4a insert). At 40 min (Fig. 4b), the nano-spheres grew up into larger particles with a diameter of about200 nm, still with a hollow structure. As the reaction timewas prolonged further to 1 h, two types of spheres were co-existent in the reaction; the smaller one, similar to thatobtained in 40 min, had a relatively smooth surface with aparticle size of 100 nm. The larger urchin-like sphere beganto appear with a diameter of about 500 nm. A few ultrathinnanobelts, 200–300 nm in length, formed starting from thesurface of the urchin-like nanospheres, forming a hierarchi-cal structure (Fig. 4c). When the hydrothermal reaction timewas prolonged to 2 h, well-developed urchin-like TiO2 nano-structures were uniformly formed (Fig. 4d).

According to the abovementioned results, urea played akey role in the formation of the urchin-like microsphere. Theunderlying mechanism for the formation of urchin-like TiO2

nanostructures was proposed, as shown in Fig. 4e. Urea wasunstable and could decompose to NH3 and CO2 at tempera-tures higher than 90 °C.53 At a high hydrothermal reaction of180 °C, urea that was introduced into the reaction rapidlydecomposed. The decomposition product of NH3 acted as abasic catalyst to accelerate the formation of TiO2 seeds withburst-nucleation. In addition, Fig. S6† shows that when NH4-OH was used instead of urea only TiO2 nanobelts formed,but the nanobelts agglomerated into blocks. Simultaneously,the generated gas bubbles in the aqueous solution acted assoft templates for providing aggregation centers of TiO2 seedsto form hollow structures (Fig. 4a). As time progressed, TiO2

grew further along the TiO2 seed and gradually assembledinto TiO2 nanobelts. Finally, urchin-like hierarchical nano-structures were generated (Fig. 4d). In the process, diethyleneglycol also assisted in nanobelt growth as “cosurfactant” and“cosolvent”.47,54 Without or with less diethylene glycol, theTiO2 nanoparticle agglomerated into blocks (Fig. S5†). At lowurea concentrations (0.1 g), the gas bubble was not enoughto support the formation of the hollow spherical structures.The co-influence of basic condition and diethylene glycolcontrolled the morphology of TiO2 nanorods with spinousrod-like structure (Fig. 3b). In conventional hydrothermal re-action without urea, it tended to form a nanobelt structureaccording to the crystal habit of TiO2 (Fig. 3a). In this study,TiO2 possessed an anatase structure. According to the sym-metry of the crystal structure, the [010] crystal orientationgrows fast and [101] crystal orientation grows slow. Therefore,it is easy to form the nanobelt structure. In the urea-assistednew hydrothermal method, the morphology of TiO2 could becontrolled easily by varying the urea concentration and hy-drothermal reaction time. An increase in the concentration ofurea can improve the product yield and reduce the reactiontime. From Table S1,† the TiO2 yield with 1.0 g urea and a 4h hydrothermal reaction is 5 times of that without urea.

1D semiconductor ultrathin nanobelts have superiorelectron transport and light scattering ability compared tonanoparticles.55 In addition, the photocatalytic performanceof anatase TiO2 is better than that of the rutile ones. Herein,we demonstrated the potential application of the hierarchical

urchin-like anatase TiO2 nanostructures composed of nano-belts as an excellent photocatalyst for the degradation ofmethyl orange (MO). The result in Fig. S7† indicates that thedegradation effect can be ruled out for the system containingphotocatalyst without light irradiation, as well as the systemwith light irradiation without a photocatalyst. Fig. 5a showsthe photocatalytic activity of TiO2 nanobelts synthesized with0.1 g urea, urchin-like anatase TiO2 nanostructures synthe-sized with 1.0 g urea, commercial anatase TiO2 (A-TiO2) andDegussa P25 nanoparticles for comparison (Fig. S10b andc†). Under the 365 nm UV irradiation in a short period of 30min, the urchin-like TiO2 could degrade over 95% of the orig-inal organic dye, whereas the degradation rate for the TiO2

nanobelt, A-TiO2 and P25 over the same time period was onlyabout 35%, 50%, and 75%, respectively. When they were ap-plied to degrade phenol, 10% phenol was retained in waterafter 120 min of UV illumination for the urchin-like TiO2,whereas the phenol remaining for the TiO2 nanobelt, A-TiO2

and P25 was about 70%, 50%, and 20%, respectively(Fig. 5b). P25 is thought as the most successful commercialsemiconductor photocatalyst because of its high photocata-lytic performance derived from its large specific surface areaand anatase–rutile bi-phase structure. It was encouragingthat the photocatalytic activity of urchin-like TiO2 nanostruc-tures was much higher than that of P25. Furthermore, com-pared to other TiO2 samples, with different morphologiessynthesized with different amounts of urea, the urchin-likeTiO2 was still the best, as depicted in the Fig. S8.† To exam-ine the photocatalytic stability and recyclability of the cata-lysts, the efficiency of the MO photodegradation was assessedwith a repetitive mode by collecting the photocatalyst in thereaction solution and repeating the photocatalysis assess-ment. As shown in Fig. 5c and d, the MO dye and phenol wasquickly decomposed after each injection of the MO or phenolsolution and the photodecomposition rate was nearly con-stant after six repeated experiments. In addition, the urchin-

Fig. 5 Photocatalytic degradation of (a) MO and (b) phenol with TiO2

nanobelt, commercial A-TiO2, P25 and urchin-like TiO2. Photocatalyticdegradation of (c) MO and (d) phenol by the TiO2 hierarchical hybridnanostructure in repeated experiments.

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like TiO2 can be easily separated from the aqueous solutionby sedimentation for reuse, probably due to the large length-to-diameter ratio of the one-dimensional ultrathin nanobelton the shell of the urchin-like TiO2 (Fig. S9†). In contrast, theDegussa P25 was extremely difficult to be collected and couldonly be separated by high-speed centrifugation, a highenergy-assumption collection process. Thus, the urchin-likeTiO2 was an effective and stable photocatalyst.

The high photocatalytic performance of the urchin-like an-atase TiO2 might arise for the following reasons. First, ana-tase itself has higher photocatalytic activity than other crys-talline phases. Second, urchin-like anatase TiO2 has uniquenanostructure composed of several single crystal ultrathinnanobelts. The single crystalline structure indicates fewer de-fects with fast charge transportation. In addition, the unique1D nanostructure promotes charge separation in the nano-crystals. Third, the urchin-like anatase TiO2 has a small grainsize because of the ultrathin nanobelts. The average crystal-lite size of the urchin-like anatase TiO2 was calculated to be17 nm from 2θ = 25.3° in the XRD pattern, whereas TiO2

nanobelt, A-TiO2 and P25 samples showed a relatively largerhalf peak width with a diameter of 92 nm, 35 nm, and 39nm, respectively. In addition, the hierarchical and hollowstructure endows a large surface area to the material. Gener-ally, when the catalyst size is smaller, there is a more per unitmass of catalyst particles. In addition, higher specific surfacearea induces more efficient surface contact to the pollutant,promoting the efficiency of photocatalytic degradation of theorganic matter. According to the SEM images of the differentsamples (Fig. S10†) and the Debye–Scherrer formula (Fig.S11†), it can be concluded that the urchin-like TiO2 has thesmallest catalyst size.

Optical properties of the TiO2 samples were investigatedby diffuse reflectance ultraviolet-visible spectroscopy (UV-vis

spectra) and the results are shown in Fig. 6a. Urchin-like ana-tase TiO2 showed a band-edge absorption around 395 nm,typical for anatase TiO2 with a band-gap of 3.15 eV. In con-trast, P25 showed evident light absorption up to 410 nm,which should have originated from the heterogeneous junc-tion between anatase and rutile, significantly reducing theabsorption in visible light region.56

The Kubelka–Munk function based on the diffuse reflec-tance spectra is employed to determine the band gap.57,58 Forthe direct band gap semiconductor, the relationship betweenthe absorption coefficient (A) and photon energy (hν) can bewritten as

where K is the absorption constant for direct transitions, A isthe diffuse reflectance UV absorbance, hν = 1240/λ. The plotsof transformed Kubelka–Munk function versus the energy oflight (Fig. 6b) gives band-gaps of 3.26, 3.17 and 3.02 eV for A-TiO2, TiO2 nanobelt and P25, respectively.

The excellent photocatalytic degradation of MO withurchin-like anatase TiO2 can also be attributed partially totheir large specific area. Nitrogen adsorption–desorptionmeasurements were performed to obtain the Brunauer–Emmett–Teller (BET) specific surface area. The result isshown in Fig. 6c, and all the samples' nitrogen adsorption–desorption curve and BET specific surface area are shown inFig. S12 and Table S2.† The BET surface area of the urchin-like anatase TiO2 was the highest approximately 171 m2 g−1,which is 3.4 times of the P25.

Photoluminescence spectra are usually used to explore theefficiency of the charge carrier trapping, immigration, andtransfer. In addition, it is quite helpful to understand the fateof the electron–hole pairs in semiconductor particles becausephotoluminescence emission is thought to have arisen fromthe recombination of photo-induced carriers.59 Whenelectron and hole recombination occurs, it emits fluores-cence. Therefore, the low fluorescent emission intensity indi-cates a low electron–hole recombination rate, which causeshigh photocatalytic activity of the semiconductor.60,61 Fig. 6dshows the time-resolved fluorescence decay. The derived twocomponents of the lifetime and their relative amplitudes aregiven in Table 1. An evident decrease in the photo-luminescence lifetime of the urchin-like TiO2 (33.75 ns) wasobserved compared to that of TiO2 nanobelt (40.74 ns), P25(43.76 ns) and the commercial A-TiO2 (43.72 ns), indicatingthat the urchin-like anatase TiO2 effectively diminished therecombination of photoinduced electron–hole pairs, whichwas beneficial to the photocatalytic activity.

ExperimentalMaterials

All the reagents were of analytic grade and commerciallyavailable. Diethylene glycol (DEG), titanium oxalate

Fig. 6 (a) UV-vis diffuse reflectance spectra of the TiO2 nanobelt,commercial A-TiO2, P25, and urchin-like anatase TiO2. (b) Kubelka–Munk plot derived from diffuse reflectance UV-vis spectroscopy exper-iments. (c) Nitrogen adsorption–desorption isotherms of the urchin-like anatase TiO2. (d) Time-resolved fluorescence decay of the samplesat 450 nm of the samples exited by a 375 nm laser.

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K2TiO(C2O4)2, urea (H2NCONH2), and methyl orange werepurchased from China National Medicines Corporation Ltd.The control titania P25 (TiO2; ca. 80% anatase, and 20% ru-tile) and anatase TiO2 (A-TiO2) were purchased from Sigma-Aldrich. All the chemicals were used as received without fur-ther purification.

Synthesis

In a typical experiment for the synthesis of urchin-like TiO2,0.35 g K2TiO(C2O4)2 and 1 g urea were added to a mixture of10 mL of deionized water and 30 mL of diethylene glycol. Af-ter stirring vigorously for 1 h, the solution was transferred toa 50 mL Teflon-lined stainless steel autoclave, which washeated to 180 °C and maintained for 12 h. The autoclave wasthen cooled to room temperature. The obtained productswere washed with deionized water and ethanol to removeionic residue and then dried in an oven at 80 °C for 4 h. Byvarying the amount of urea, TiO2 with other morphologieswere fabricated.

Characterization

The XRD patterns of the samples were obtained by X-ray dif-fraction spectrometer (D8-advance, BrukerAXS, Germany)using Cu Kα radiation. The accelerating voltage and appliedcurrent were 40 kV and 40 mA, respectively. X-ray photoelec-tron spectroscopy (XPS) was conducted on an ESCALAB 250photoelectron spectrometer (Thermo Fisher Scientific) at 2.4× 10−10 mbar using a monochromatic Al Kα X-ray beam(1486.60 eV). All binding energies were referenced to the C 1speak (284.60 eV) arising from adventitious hydrocarbons. Themorphology and microstructure of the samples were exam-ined by SEM (HITACHI S-8020). The TEM images were ac-quired on a JEOL JEM 2100 microscope with an operatingvoltage of 200 kV. The UV-vis diffuse reflectance spectra ofthe samples were recorded on a UV-vis spectrophotometer(UV-3600, Shimadzu) with an integrating sphere attachmentwithin the range from 250 to 600 nm and with BaSO4 as thereflectance standard. The specific surface area was measuredon Micromeritics, ASAP2020 and calculated using theBrunauer–Emmett–Teller (BET) method. The time-resolvedfluorescence decay curve was taken out with combined steadystate and time resolved fluorescence spectrometer (FLS980,Edinburgh).

Photocatalysis

The photocatalytic activity was assessed by the photo-degradation of methyl orange (MO) and phenol on a photo-chemical reaction apparatus. In a typical experiment, 20 mLaqueous suspensions of MO (20 mg L−1) or phenol (20 mgL−1) and 20 mg of the TiO2 sample were placed in a 50 mLbeaker. Prior to irradiation, the suspensions were magneti-cally stirred in the dark for 30 min at room temperature toestablish adsorption/desorption equilibrium between the dyeand the surface of the catalyst. A 300 W mercury lamp with amaximum emission of 365 nm was used as the UV lightsource. For comparison, the photodegradation abilities of theother TiO2 control samples were evaluated under the sameexperimental conditions. At pre-determined irradiation timeintervals, the residual MO or phenol concentration in thesupernatant was analyzed by UV-vis spectroscopy (ShimadzuUV-3600). The normalized concentration of the solutionequalled the normalized maximum absorbance, and hencewe used C0/C in place of A0/A, where C0 and C are the initialand actual concentration of MO or phenol, respectively.

Conclusions

We successfully synthesized the 3D urchin-like anatase TiO2

nanostructures by a one-step urea-assisted hydrothermal ap-proach with urea as the morphology directing agent. Theurea concentration plays a crucial role in determining themorphology and crystallite size. Compared to commercial an-atase TiO2 and Degussa P25, the as-prepared urchin-like ana-tase TiO2 show the largest specific surface area, which en-hances the photocatalytic activity for degradation of MO andphenol. The present study motivates us to use the new facilehydrothermal method for the preparation of new nano-materials with a special morphology, which could beemployed not only in photocatalysis, but also in solar watersplitting systems and solar energy conversion systems, suchas dye-sensitized solar cells with high efficiency.

Acknowledgements

This study was supported by the National Natural ScienceFoundation of China (No. 51372142, No. 81471784, No.31270022, and No. 51402063), the Youth Innovation Promo-tion Association of the Chinese Academy of Sciences

Table 1 Summary of the photoluminescence decay time (τ) and their relative amplitude ( f) in the samples, which are derived from the time-resolved PLspectra in Fig. 6d by biexponential decays

Sample

Decay time [ns] Relative amplitude [%] Average lifetime [ns]

τ1 τ2 f1 f1 τa

P25 2.32 44.02 10.80 89.20 43.76A-TiO2 2.76 44.51 22.33 77.67 43.72TiO2 nanobelt 1.63 40.92 10.90 89.1 40.74Urchin-like TiO2 1.17 28.28 55.21 44.79 33.75

a The average lifetime was calculated using equation: <τ> = ( f1τ12 + f2τ2

2)/( f1τ1 + f2τ2).

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(2015023), and the “Thousands Talents” program for pioneerresearcher and his innovation team, China.

Notes and references

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