Introduction

TiO2 has been extensively considered as a potential anode candidate in rechargeable batteries and an efficient photocatalyst for water splitting, dye-sensitized solar cells and degradation of organic pollutants, in order to deal with the urgent issues of energy storage and conversion, as well as environmental crisis [1,2,3,4]. TiO2 is multifunctional material with the inherent advantages of low cost, non-toxicity, good chemical stability and environmental benignity. It has drawn great interests on photocatalytic hydrogen evolution due to its better carrier mobility, good electron-hole separation ability, higher conduction band edge energy and higher redox driving force [1, 5, 6]. Several micro-/nanostructured TiO2 such as nanotubes [7], nanobelts [8], nanosheets [9] and hollow microspheres [10, 11] have been used for H2 evolution.

TiO2 has also been studied as a promising anode material for lithium-ion batteries (LIBs) because of its high energy density, long cycling life and small volume expansion (< 4%) during Li+ insertion–extraction process [12,13,14,15,16,17,18,19,20,21,22,23,24]. However, there are still some intrinsic drawbacks of TiO2 such as poor electronic conductivity and low ionic diffusion efficiency, which limit its further application for lithium storage. To overcome above issues, various forms of structure including nanostructure [12], hierarchical structure [18], mesoporous structure [19] have been designed in order to enlarge the contact area between electrolyte and solid electrodes and shorten the transfer pathway of lithium ions [3, 4, 20, 21].

Combining the inherent defects and potential utilization of TiO2 both as a promising photocatalytic reagent and an alternative anode material for lithium ion batteries, it is essential to fabricate TiO2 with large specific surface area, high crystallinity, enhanced light adsorption and rapid lithium ions transfer channels [25,26,27].

It has been realized that hollow structure can enhance light absorption and the rapid transport of lithium ions [28,29,30]. While microspherical morphology has been reported to be beneficial to multiple reflections of the incident light, resulting in high photon absorption associated with light scattering [31, 32]. TiO2 hollow microspheres have attracted much attention due to the large specific surface area [33, 34].

Many strategies have been proposed to synthesize TiO2 hollow microspheres [35,36,37,38,39,40,41,42,43]. The hard-template method that employs silica spheres [37], carbon spheres [39], polystyrene beads [40], or other spherical particles as removable templates has been considered to be the most direct and effective way to fabricate TiO2 hollow microspheres. Recently, Liu et al. [37] prepared TiO2 hollow microspheres by sintering SiO2-TiO2 core–shell structural spheres at 500 °C for 5 h and then etching with 5% HF to remove SiO2. Wang et al. [38] synthesized TiO2 hollow microspheres through a sol-gel process by using carbon spheres as templates. Zhong et al. [40] obtained TiO2 hollow microspheres by templating the precursor solutions against crystalline arrays of monodisperse polystyrene beads. Although the hard-template method is a universal and straightforward method, some problems with this method remain, such as complex process, toxic reagent, residual impurity, time consuming or high cost.

In this work, we report a simple template-free method to synthesize anatase TiO2 hollow microspheres in one step. As expected, the resultant sample shows a superior rate performance in lithium-ion batteries, great adsorption ability of organic pollutants and excellent photocatalytic water splitting performance for H2 evolution.

Experimental

Synthesis of TiO2 hollow microspheres

Tetrabutyl titanate (chemically pure), anhydrous ethanol (guaranteed reagent) and NH4HCO3 (analytical reagent) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All chemicals were used without further purification. In a typical experiment, 1 mL TBOT and 7 mL anhydrous ethanol were dropped into a 25-mL beaker, followed by ultrasonication and constantly stirring. Then, 4 g NH4HCO3 was added into a 100-mL Teflon-lined autoclave. Subsequently, the beaker with mixed solution was transferred into the autoclave, as shown in Scheme 1. The autoclave was maintained at 250 °C for 3, 6, and 10 h. After cooling down to room temperature, the products were obtained by rinsing with deionized water and then drying at 60 °C overnight.

Scheme 1
scheme 1

Schematic illustration of the preparation process for TiO2 hollow spheres

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Characterizations

The morphology and microstructure of the samples were characterized using a scanning electron microscope (Philips, XL30FEG) and a transmission electron microscope (TECNAI, G2F20). The crystal structure of the samples was analyzed using an X-ray diffractometer (Bruker, D/8). The nitrogen adsorption-desorption isotherms were performed using a surface area and pore size analyzer (Quantachrome, Quadrasorb evo) at 77 K, the specific surface area of the samples was calculated according to the Brunauer–Emmett–Teller (BET) equation. UV-vis diffuse reflectance spectra were obtained with a UV-vis spectrophotometer (Shimadzu, UV 3600) using barium sulfate powder as a standard.

Adsorption property for MB

Adsorption test for methylene blue (MB) was carried out to evaluate the adsorption performance of the as-prepared samples. Typically, 50 mg sample was introduced into 50 mL MB solution (20 mg L−1) in a quartz container. The mixed suspension was stirred for 5 h at room temperature in dark to reach the adsorption equilibrium status. Then, the powders were removed by centrifugation, and the residual MB solution was determined by its absorption spectrum at 664 nm, which was recorded on a UV-2300 spectrophotometer. Thus, the equilibrium adsorption amount of MB was calculated according to Eq. (1) [42].

$$ {Q}_{\mathrm{e}}=\left({C}_0-{C}_{\mathrm{e}}\right)\cdot V/m $$
(1)

where Q e (mg g−1) is the equilibrium adsorption amount of MB, C 0 and C e (mg L−1) is the initial and equilibrium centration of MB, respectively. V (L) is the volume of MB solution, and m (g) is sample mass.

Photocatalytic hydrogen generation

1 wt.% platinum nanoparticles were loaded onto the hollow TiO2 samples as a kind of co-catalyst to facilitate photocatalytic hydrogen production by a photo-deposition method. In a typical experiment, 100 mg hollow TiO2 powders, 2 mL methanol, 50 mL deionized water and a certain amount H2PtCl6·6H2O were mixed to form a homogeneous suspension. The mixed solutions were irradiated by a mercury light source for 30 min at room temperature and then stirred 1 h under dark condition. The Pt loaded samples were gained after centrifugation and drying. The photocatalytic hydrogen generation tests were performed in a top-irradiation reactor connected to a gas-insulated circulation and evacuation system (Perfect Light Labsolar-III AG). Typically, 20 mg Pt loaded sample powders and 10 mL methanol were dispersed into 90 mL deionized water, which was then sealed and evacuated in the reactor. Before reaction, the whole system was pumped out to remove the air. The reaction solution was irradiated under stirring condition by using a 300 W Xe arc lamp (Perfect Light, PLS-SXE300, 300–2000 nm) as an external light source. The lamp was placed 4 cm away from the reaction vessel. The gas component was analyzed using an online gas chromatograph (Shiweipx, GC7806) equipped with a thermal conductivity detector (TCD) and molecular sieve (5 Å pore size). High purity nitrogen was used as the carrier gas.

Electrochemical measurements

The electrochemical properties of the TiO2 samples were also investigated as anode materials in CR2016-type coin cells. Prior to use, the as-prepared samples were dried at 100 °C under vacuum for 24 h to remove surface adsorbed water. The working electrode was composed of active component (TiO2 hollow microspheres), conductive agent (Super P) and polymer binder (polyvinylidene fluoride, PVDF), a mixture of these three components in a weight ratio of 8:1:1 was stirred and dispersed in N-methyl-2-pyrrolidone (NMP) solvent and then evenly pasted on a copper foil, followed by drying at 60 °C for 24 h under vacuum. Metal lithium was employed as the counter electrode, and a polymer film (Celgard-2300) served as the separator. The electrolyte was 1.0 M LiPF6 dissolved into a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in volume). The cells were assembled in Ar-filled glove box with a concentration of moisture and oxygen below 1.0 ppm. Cyclic voltammetry (CV) was recorded using an electrochemical workstation (CHI 601D, CH Instruments) at a scan rate of 0.2 mV s−1. Galvanostatic charge–discharge cycling was conducted on CT2001A battery tester system (Wuhan LAND Electronic Co., Ltd). Electrochemical impedance spectroscopy (EIS) was collected on PARSTAT 4000 (Princeton Applied Research) over a frequency range from 100 kHz to 0.01 Hz with a 5-mV amplitude. Both cells were discharged-charged for 30 cycles before EIS measurement in order to reach the stable states.

Results and discussion

Figure 1 illustrates XRD patterns of the TiO2 samples prepared for different solvothermal reaction time. All observed diffraction peaks of each sample can be well-indexed to the anatase phase of TiO2 according to JCPDS card No. 21-1272 [44]. The sharp peaks indicate that pure anatase TiO2 with high crystallization can be directly obtained after 3 h. No detectable effect on the formation of anatase phase was observed with extending of reaction time to 10 h. According to the full width at half maximum of (101) lattice plane, the grain size of each sample was approximately 10 nm based on the Scherrer equation [12].

Fig. 1
figure 1

XRD patterns of the hollow TiO2 samples prepared for 3 h (a), 6 h (b), and 10 h (c)

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Figure 2 displays SEM images of the samples prepared for 3 h (a, b), 6 h (c, d), and 10 h (e, f). The TiO2 microspheres with diameters in the range of 1.0–4.0 μm can be obtained after solvothermal process of 3 h, and no enlargement of particle size can be found even the reaction time lasting to 10 h. Broken microspheres can be found in Fig. 2d, f, which indicates the formation of hollow structure. The cross-section morphology of the samples can be clearly observed from the area highlighted by the red dots (Fig. 2d). The surface of hollow spheres is rough and the shell thickness is approximately 150 nm.

Fig. 2
figure 2

SEM images of the hollow TiO2 samples prepared for a, b 3 h; c, d 6 h; and e, f 10 h

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To confirm the hollow structure, the sample was further examined by TEM. The hollow structure can be clearly observed in Fig. 3a–c. The shell thickness is about 150 nm, which is consistent with the SEM results. The high-resolution image (Fig. 3d) reveals that the TiO2 hollow microsphere comprises fine nanocrystallites of approximately 10 nm, which is in good correspondence to the XRD result. The lattice spacing of 0.35 nm is associated with the (101) plane of anatase phase.

Fig. 3
figure 3

a-c TEM images of the hollow TiO2 sample prepared for 10 h at different magnification; d High-resolution TEM image of the hollow TiO2 sample

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To investigate the effect of NH4HCO3 on the formation of TiO2 hollow spheres, samples were prepared following the same synthesis procedure except for the replacement of 4 g NH4HCO3 by 1 and 2 g NH4HCO3 during the solvothermal process. Supplementary Fig. S1 shows the XRD results of the hollow TiO2 samples prepared with 1 g (a), 2 g (b), and 4 g (c) NH4HCO3. No detectable difference can be observed, indicating that the amount of NH4HCO3 has no influence on phase structure. The typical SEM images are shown in Fig. S2. Microspheres with diameters in the range of 1.0–4.0 μm can be observed for both samples, revealing that the morphology has no variation along with the change on NH4HCO3 amount.

The role of NH4HCO3 in this work is a matter of the precursor for producing intermediate H2O through its decomposition at high temperature (Eq. (2)). Then, TBOT hydrolyses with the generated H2O to form TiO2 particles, as illustrated in Eqs. (3–7) [45]. Furthermore, the formation of hollow TiO2 microspheres follows a typical inside-out Ostwald ripening, as shown in Scheme 2. To demonstrate this formation mechanism of hollow TiO2 microspheres, the samples prepared for different time (1 h, 3 h, 6 h, 10 h) were further examined by TEM. As shown in Fig. 4, the solid structure (Fig. 4a) turned to hollow structure (Fig. 4d) gradually with the time increasing, and thickness of the shell also became thinner and thinner. These results are consistent with the Ostwald ripening mechanism that the inside small TiO2 nanoparticles gradually dissolve and transfer to microspheres surface, and outer TiO2 nanoparticles trend to coarsen during the solvothermal treatment based on the Ostwald ripening. The mass transport from the center to outward, leading to the formation of hollow structure [46, 47].

$$ {\mathrm{NH}}_4{\mathrm{H}\mathrm{CO}}_3\overset{\Delta}{\to }{\mathrm{NH}}_3\uparrow +{\mathrm{CO}}_2\uparrow +{\mathrm{H}}_2\mathrm{O} $$
(2)
$$ \mathrm{Ti}{\left(\mathrm{OBu}\right)}_4+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ti}{\left(\mathrm{OBu}\right)}_3\left(\mathrm{OH}\right)+\mathrm{BuOH} $$
(3)
$$ \mathrm{Ti}{\left(\mathrm{OBu}\right)}_3\left(\mathrm{OH}\right)+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ti}{\left(\mathrm{OBu}\right)}_2{\left(\mathrm{OH}\right)}_2+\mathrm{BuOH} $$
(4)
$$ \mathrm{Ti}{\left(\mathrm{OBu}\right)}_2{\left(\mathrm{OH}\right)}_2+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ti}\left(\mathrm{OBu}\right){\left(\mathrm{OH}\right)}_3+\mathrm{BuOH} $$
(5)
$$ \mathrm{Ti}\left(\mathrm{OBu}\right){\left(\mathrm{OH}\right)}_3+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ti}{\left(\mathrm{OH}\right)}_4+\mathrm{BuOH} $$
(6)
$$ \mathrm{Ti}{\left(\mathrm{OH}\right)}_4\to {\mathrm{TiO}}_2+2{\mathrm{H}}_2\mathrm{O} $$
(7)
Scheme 2
scheme 2

Schematic illustration of the formation mechanism of TiO2 hollow spheres

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Fig. 4
figure 4

TEM images of the TiO2 samples prepared for a 1 h, b 3 h, c 6 h, and d 10 h

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The nitrogen adsorption-desorption isotherms and the pore size distribution curve of the samples synthesized for 3 h (a), 6 h (b), and 10 h (c) are exhibited in Fig. 5. The isotherms of each sample can be well indexed to type IV isotherms. The specific surface area of the TiO2 hollow microspheres for 3, 6, and 10 h are 158, 179, and 180 m2 g−1 respectively according to the BET equation. The inset images in Fig. 5a–c show that a same narrow pore size distribution at approximately 5–10 nm can be observed for all samples. The average pore size of the sample for 3, 6, and 10 h is 9.0, 7.8, and 9.6 nm, respectively, indicating spacing between the crystals [43]. The detailed textural properties including specific surface area, average pore size and pore volume of these three samples and the specific surface area data of P25 are summarized in Table 1. Compared to the specific surface area of P25 (50 m2 g−1) [48], much larger specific surface area was observed for all samples. The specific surface area increases when the reaction time extends to 6 h, the hollow structure began to be formed during this process, but a little enlargement is detected for further lasting to 10 h, indicating the complete formation of the hollow structure.

Fig. 5
figure 5

The nitrogen adsorption-desorption isotherms of the hollow TiO2 samples prepared for a 3 h, b 6 h, and c 10 h

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Table 1 Textural properties of the hollow TiO2 samples prepared for different reaction time
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Figure 6 shows the adsorption performance of the samples prepared under different reaction time. Figure 6a displays the adsorption spectra of a solution of MB in the presence of the samples. Obviously, the main adsorption peaks (about 664 nm) almost vanish for the sample prepared for 6 and 10 h, demonstrating the excellent adsorption performance of these two samples. As shown in Fig. 6b, it can be calculated that the equilibrium adsorption amount of P25 was 1.2 mg g−1, and 9.5, 16.7 and 17.6 mg g−1 for the samples prepared for 3, 6, and 10 h, respectively. The enhanced adsorption property is highly credited to the enlarged specific surface area.

Fig. 6
figure 6

a Absorption spectra of a solution of MB in the presence of P25 (a) and the samples prepared for 3 h (b), 6 h (c), and 10 h (d). b Equilibrium adsorption amount of MB on P25 (a) and the samples prepared for 3 h (b), 6 h (c), and 10 h (d)

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The photocatalytic performance of the TiO2 samples were evaluated by water splitting hydrogen evolution. Figure 7 displays the comparison of hydrogen productions and H2 evolution rates of the samples. For the sample prepared at 250 °C for 3 h, 2.1 mmol H2 has been evolved in 5 h and the H2 evolution rate is 0.43 mmol h−1. A slight increase of H2 evolution was observed for the sample prepared for 6 h, the H2 evolution rate is 0.45 mmol h−1. Apparently, the sample exhibits higher H2 evolution activity that 3.3 mmol H2 is generated in 5 h and the H2 evolution rate is about 0.66 mmol h−1 of the sample prepared for 10 h. The enhanced photocatalytic performance of the sample is attributed to the hollow structure and Pt as the co-catalyst. Besides, Fig. S3 displays the UV-vis diffusion spectra of samples before and after loading Pt, as well as commercial P25. A slight adsorption enhancement was observed in the visible region of the spectra (400–800 nm). This result indicates that the hollow structure could reflect and capture more light. Much stronger adsorption covers the whole visible light region for the Pt-loaded sample, suggesting the improved visible light harvesting behavior with the presence of Pt which is consistent with our former work [49].

Fig. 7
figure 7

Hydrogen productions and H2 evolution rates of the hollow TiO2 samples prepared for 3 h (a), 6 h (b), and 10 h (c) with Pt. The reaction system was 20 mg photocatalyst in 100 mL 10% (v/v) methanol-water solution

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To further demonstrate the utility of the as-prepared TiO2 hollow microspheres, the electrochemical performance as a potential anode material for LIBs was investigated. Figure 8a exhibits the CV curves for initial three cycles at a scan rate of 0.2 mV s−1 within a potential window of 1.0–3.0 V (vs. Li/Li+). In the first cycle, two well-defined peaks at 1.7 V (cathodic sweep) and 2.0 V (anodic sweep) correspond to the typical lithium insertion and extraction process, which follows the reaction mechanism illustrated in Eq. (8) [50]. And an additional weak, broad peak in Fig.8a (around 1.5 V), which only can be observed in the first cycle is ascribed to irreversible trapped lithium ions into LixTiO2 (x ≈ 0.03) phase [51,52,53]. Furthermore, the CV curves of the second and third cycles overlap well, indicating good cycling stability of the TiO2 hollow microspheres.

$$ {\mathrm{TiO}}_2+x{\mathrm{Li}}^{+}+x{\mathrm{e}}^{\hbox{-}}\leftrightarrow {\mathrm{Li}}_{\mathrm{x}}{\mathrm{TiO}}_2 $$
(8)
Fig. 8
figure 8

The electrochemical performance of the TiO2 sample prepared for 10 h. a Cyclic voltammograms curves at a scan rate of 0.2 mV s−1 within a potential window of 1.0–3.0 V (vs. Li/Li+). b Charge-discharge voltage profiles at 300 mA g−1. c Cycling performance at 300 mA g−1. d Rate capability at 1 C, 2 C, 5 C, 10 C, 15 C, 2 C, and 1 C (1 C = 168 mA g−1)

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The charge–discharge voltage profiles of the first three cycles are shown in Fig. 8b for the hollow TiO2 samples at a specific current of 300 mA g−1. The voltage plateaus arise at approximately 1.7 V in the discharge curve and 2.0 V in the charge curve, respectively, which is consistent with the result of the CVs (Fig. 8a). Additionally, the discharge and charge curves of the second and third cycles are nearly overlapped, indicating a stable capacity retention of the as-prepared TiO2 hollow microspheres.

Figure 8c depicts the cycling performance at a specific current of 300 mA g−1. The discharge and charge capacities in the first cycle were 193 and 172 mAh g−1, corresponding to a Coulombic efficiency of 89%. The irreversible capacity loss might be resulted by the formation of SEI film, Li+ inserts into the irreversible site and the decomposition of electrolyte, which consume some quantity of lithium ions. Moreover, a reversible capacity remained 143 mAh g−1 after 100 charge–discharge cycles, indicating outstanding capacity reversibility and good cycling stability of the TiO2 hollow microspheres. This electrochemical performance is comparable to the results reported in the literatures [17, 18, 35, 38, 39, 41, 43], as summarized in Table 2.

Table 2 Comparative cycling performance of representative TiO2 with hollow structure as anode for lithium-ion batteries
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The rate capability of the sample at various charge–discharge rates was measured as well. As shown in Fig. 8d, the average discharge capacities decreased to 176, 160, 134, 110, and 98 mAh g−1 with the rates increasing to 1 C, 2 C, 5 C, 10 C and 15 C (Here 1 C is equivalent to 168 mA g−1). When the current rate was reverted to 2 C, the capacity recovered to 157 mAh g−1, with only 2% capacity fading. Furthermore, when the current rate decreased to 1 C, a capacity of 165 mAh g−1 was delivered with a capacity retention ratio of 94%. As an example for detail, Han’s group [41] synthesized TiO2 hollow microspheres through a two-step hydrothermal process. The resultant sample exhibited specific capacities of 145, 111, 77, and 50 mAh g−1 at 1 C, 2 C, 5 C and 10 C, respectively.

In order to investigate the potential reasons for the excellent cycling performance and rate capability of hollow TiO2 microspheres, electrochemical impedance spectra (EIS) were implemented on the hollow TiO2 microspheres (a) and solid TiO2 microspheres (b) as shown in Fig. 9. A semicircle at high-medium frequency region is attributed to the charge transfer reaction, while an inclined line in low frequency region is correlated with the diffusion of lithium ions in the solid electrode. In the inserted equivalent circuit, Rct denotes the charge transfer resistance between electrolyte and electrode. The corresponding fitting data of circuit elements are listed in Table 3. As shown in Table 3, the R ct is 36.8 Ω for hollow TiO2 microspheres, while it is 50.6 Ω for solid TiO2 microspheres. The decreased R ct implies that better charge carrier transport than that of solid TiO2 owing to the presence of the hollow structure, which can reduce the interface resistance [30, 41].

Fig. 9
figure 9

Nyquist plots of (a) the hollow TiO2 microspheres (prepared for 10 h) and (b) the solid TiO2 microspheres (prepared for 3 h) after 30 discharge-charge cycles and the fitting line of experimental impedance spectra using the equivalent circuit modelling as inset

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Table 3 The fitting data of the equivalent circuit elements
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Moreover, the diffusion coefficient of lithium ions for the electrodes are evaluated based on the impedance spectra using Eqs. (9, 10). In the low frequency region of Nyquist plots, Z′ is proportional to ω−1/2 (angular frequency), and σ w is the Warburg factor, which can be calculated according to Eq. (9). As shown in Table 3, the σ w of hollow and solid TiO2 microspheres is linear fitted to be 17.8 and 43.2 Ω s−1/2, respectively. The Li+ diffusion coefficient (D) can be estimated by using Eq. (10) [51].

$$ {Z}^{'}={R}_s\kern0.5em +{R}_{ct}+{\sigma}_W{\omega}^{-1/2} $$
(9)
$$ D=\frac{R^2{T}^2}{2{A}^2{F}^4{\sigma}_W^2{C}^2} $$
(10)

In Eq. (10), R is the gas constant (8.3145 J mol−1 K−1), T is 298.15 K, A is the surface area of electrode, F is Faraday’s constant (96,485 C mol−1) and C is the molar concentration of lithium ions. The diffusion coefficient of lithium ions is 2.7 × 10−12 cm2 s−1 of hollow TiO2 microspheres, which is six times higher than that of the solid TiO2 microspheres (4.5 × 10−13 cm2 s−1). This improvement is mainly resulted from the shorter transfer pathway due to the hollow structure with larger specific surface area and thinner shell as well.

Considering the advantages of the facile one-step strategy to synthesize TiO2 hollow microspheres as well as their superior photocatalytic and excellent electrochemical performance, it should be meaningful for this work to facilitate the preparation and application of anatase TiO2.

Conclusions

In summary, we have developed a one-step template-free approach to fabricate TiO2 hollow microspheres through a modified solvothermal method with the assistance of NH4HCO3. As a kind of photocatalyst, the hollow TiO2 microspheres exhibit a high hydrogen evolution ability. The H2 evolution rate of the optimal sample is about 0.66 mmol h−1 after loaded with 1 wt.% Pt. As an anode material for LIBs, the initial discharge capacity was 193 mAh g−1 and remained at 143 mAh g−1 at 300 mA g−1 after 100 cycles. Furthermore, the specific capacity was maintained at 98 mAh g−1, even at the high rate of 15 C. The impressive photocatalytic performance and lithium storage capabilities can be attributed to the hollow structure of the sample. Considering the features of low cost and simple preparation, the proposed template-free method may transfer to synthesize other relevant oxide materials.