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Applied Physics A (2020) 126:561 https://doi.org/10.1007/s00339-020-03597-0

Enhanced photocatalytic activity of titania coatings fabricated at relatively low oxidation temperature with sulfate‑acid‑bath pretreatment

Sujun Guan1 · Liang Hao2 · Shota Kasuga3 · Hiroyuki Yoshida4 · Yanling Cheng5 · Yun Lu3

Received: 12 March 2020 / Accepted: 29 April 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020

AbstractWith the purpose of enhancing the photocatalytic activity of titania (TiO2) coatings with low cost, a method of sulfuric-acid-bath pretreatment followed by simple oxidation in air has been proposed to fabricate TiO2 coatings. The effect of oxidation temperature on the crystal structure, surface morphologies and photocatalytic activity of TiO2 coatings was investigated, to figure out the suitable oxidation temperature. XRD and Raman’s results show that the phase transformation of TiO2 starts at 773 K. The surface morphologies of TiO2 coatings clearly show the porous-like structure at lower than 873 K. With rais-ing the temperature, the photocatalytic activity of TiO2 coatings firstly increases, then decreases, and reaches the highest at relatively low oxidation temperature of 773 K.

Keywords TiO2 coatings · Sulfuric-acid-bath pretreatment · Low temperature · Phase transformation · Photocatalytic activity

1 Introduction

TiO2-based photocatalyst has been intensively used as the decomposition of contaminates from water and air envi-ronment, hydrogen generation, and production of carbona-ceous solar fuels due to its photocatalytic activity, excellent

stability, and eco-friendly nature [1–3]. It is well known that TiO2 absorbs enough energy from light, which would be accompanied by electron–hole pairs generation and the pro-duction of hydroxyl radicals through the reaction between the hole and hydroxyl groups from pollutants or adsorbed H2O molecules. Therefore, besides the energy absorption and recombination of electron–hole pairs, the surface mor-phology of TiO2 and its hydroxyl group density also play a significant role in the photocatalytic processes of decom-position of contaminates [4–6]. Considerable research has been dedicated to enhancing the photocatalytic efficiency of TiO2, focusing on narrowing band gap via doping element, suppressing recombination by charge transfer effect, and increased accessible surface area, as well as the improved adsorption capacity by additional hydroxyl groups on the surface [7–9]. The catalytic properties of TiO2 can be altered by the surface state, due to the surface absorption effect and photoabsorption. Numerous reports have shown that the sub-strates play an important role in the photocatalytic activity of TiO2, such as quartz tube, cenospheres, net, pebble, ball, and so on [10–14]. However, most current preparation methods, such as sol–gel method, hydrothermal method, direct oxi-dation method, chemical vapor deposition (CVD), physical vapor deposition (PVD) and electrodeposition [13–15], for TiO2 -based photocatalyst with increasing surface hydroxyl

Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s0033 9-020-03597 -0) contains supplementary material, which is available to authorized users.

* Yun Lu luyun@faculty.chiba-u.jp

1 Department of Physics, Tokyo University of Science, 1-3, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan

2 College of Mechanical Engineering, Tianjin University of Science and Technology, No. 1038, Dagu Nanlu, Hexi District, Tianjin 300222, People’s Republic of China

3 Faculty of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

4 Chiba Industrial Technology Research Institute, 6-13-1, Tendai, Inage-ku, Chiba 263-0016, Japan

5 Beijing Key Laboratory of Biomass Waste Resource Utilization, Beijing Union University, No. 18, Fatouxili 3 Area, Chaoyang District, Beijing 100023, People’s Republic of China

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groups, have complicated surface modification and usually cause a more complex system with some harsh reaction con-ditions, inevitably resulting in a high cost [15–17]. Accord-ingly, to enhance photocatalytic activity of TiO2 with low cost, it is an urgent need to develop an alternative method to prepare TiO2, with simple process, mild condition, and so on.

In this work, the enhanced photocatalytic activity of TiO2 coatings has been successfully fabricated at relatively low oxidation temperature for Ti coatings with SAP. The effect of the oxidation temperature on the crystal structure, surface morphologies and photocatalytic activity was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and the degradation of methylene blue (MB) solution. The relation-ship among the enhanced photocatalytic activity and mixed-phase of anatase and rutile, formed hydroxyl groups, as well as porous-like structure is discussed.

2 Experimental

2.1 Preparation of  TiO2 coatings

Ti coatings were formed on Al2O3 balls (average diame-ter: 1 mm) using Ti powder (average diameter: 30 μm), by mechanical coating operation [14]. The Ti coatings were firstly conducted by sulfuric-acid-bath pretreatment (SAP) in H2SO4 (1 M) at 80 °C for 0.5 h, and named as “SA0.5 h.” Then, the SA0.5 h sample was carried out by simple oxida-tion in air using an electric furnace at x K for 2 h to fabricate TiO2 coatings (average thickness: around 15 μm) and named as “SA-Ox”.

2.2 Characterization

An X-ray diffractometer (XRD, D8 Advance) equipped with Cu-Kα radiation at 40 kV and 40 mA and Raman spectros-copy (Horiba Scientific) with Ar laser radiation (514.5 nm) were used to determine the chemical composition and crystal structure of the samples. The surface morphology was observed by scanning electron microscopy (SEM, JSM-5300). Surface element valence states of the samples

were analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250Xi).

2.3 Photocatalytic activity

According to the international organization for standardiza-tion (ISO 10678), MB solution (10 μmol/L, 35 mL) was selected as the target pollutant to evaluate the photocatalytic activity of the samples, at room temperature with intervals of 20 min to 3 h [18]. Before photocatalytic activity evaluation, the samples were firstly dried under UV light (FL20S BLB, Toshiba) for 24 h. To avoid possible adsorption during the evaluation of photocatalytic activity, then the adsorption of MB solution (20 μmol/L, 35 mL) was carried out in a dark for 18 h. Two UV light lamps were set as the light source, for the irradiance of 1 mW/cm2. To clearly show the photocata-lytic activity of samples, the difference (R) of the degrada-tion rate constants between ksample and kMB solution was used.

3 Results and discussion

3.1 Appearance and crystal structure

Figure 1 shows the color changes of the SA-Ox samples by SAP followed by oxidation in air for Ti coatings. With extending the SAP time at 80 °C (Fig. S1), the color changes from silvery (Ti coatings) to brown (SA0.5 h), which clearly shows the reaction between Ti and H2SO4 solution. After oxidation in air for the SA0.5 sample, the color changes from black at 673 K, until slight white at 1073 K. Compared with those without SAP, the color of the SA-Ox samples is completely different. The difference also hints the reaction between Ti and H2SO4 solution.

To further investigate the reaction, Fig. 2 shows XRD patterns and Raman spectroscopy of the SA-Ox samples. For the SA-0.5 h sample, it is difficult to find any formed compounds, except the diffraction peaks from Ti coatings. It hints that the formed compounds such as the nucleation of TiO6 octahedra during SAP are too few to be detected [19–21]. With the formed nucleation during SAP, the dif-fraction peaks of anatase TiO2 could be found from the samples oxidized at relatively low temperature of 773 K, compared with that of without SAP [22]. With increasing

Fig. 1 The appearance photograph of the SA-Ox samples. The insets are the samples without SAP

Enhanced photocatalytic activity of titania coatings fabricated at relatively low oxidation…

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the oxidation temperature, it clearly shows the phase trans-formation from anatase to rutile at 873 K. The phase trans-formation has also been found from Raman spectroscopy of the SA-Ox samples (Fig. 2b). For comparison, the mass fraction of rutile (XR) was calculated from the respective peak intensities using the equation [23]:

where IR and IA are X-ray intensities of rutile (110) and anatase (004) peaks, respectively. The relationship between

XR(%) =

(

1 + 0.8 IA∕I

R

)−1× 100

XR and oxidation temperature is shown in Fig. 2c. It clearly shows phase transformation and the XR increases when the temperature is higher than 873 K.

3.2 Surface morphology evolution

After SAP, the influence of oxidation temperature in air on the surface morphology of the SA-Ox samples has been observed and is shown in Fig. 3. Compared with those with-out SAP in our previous research [24], the porous-like struc-ture forms on the surface of Ti coatings during SAP, shown

X-r

ay i

nte

nsi

ty /

arb

. u

nit

2θ /deg

30 40 50

Ti R

SA0.5h

SA-O673

(a)

SA-O773

SA-O873

SA-O973

SA-O1073

A

R+

A

A

Raman shift /cm-1In

ten

sity

/ a

rb. u

nit

0

5

10

Rutile

Anatase

R

R+

A

200 400 600

(b)

A

Mas

s fr

icti

on

of

ruti

le p

has

e /

%

40

80

SA-O873 SA-O973 SA-O1073

(c)

Fig. 2 XRD patterns (a) and Raman spectroscopy (b) of the SA-Ox samples, c mass friction of rutile phase from XRD. R is rutile, and A is anatase

Fig. 3 Comparison of the surface morphologies of the SA-Ox samples. a SA0.5  h, b SA-O673, c SA-O773, d SA-O873, e SA-O973, f SA-O1073

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as red arrow in Fig. 4a. The reaction between Ti and H2SO4 solution has been confirmed. When the oxidation tempera-ture is under 873 K in air, the SA-Ox samples clearly show the porous-like structure, and the changing trend of its size is becoming larger (Figs. 3b, 4d). However, the porous-like structure disappears at 973 K, and it shows columnar-like structure at 1073 K. The surface morphology change of the SA-Ox samples would affect the accessible surface area, and the samples of SA-O773 and SA-O873 should be relatively larger compared with those of others.

3.3 Bonding environment

Furthermore, to gain insight into the surface chemical bond-ing of the SA-Ox samples, XPS spectra have also been exam-ined and are shown in Fig. 4. Form Fig. 4a, c, it can be found that the main spectra of C 1s and Ti 2p are with the typical

peaks after oxidation at 773 K and 1073 K, respectively. It indicates that the influence of the oxidation temperature on C 1s and Ti 2p is slight. Notably, from Fig. 4b, the slight shift of O 1s peak and the change of the formed hydroxyl groups are found. The slight peak shift would be related to the phase transformation from anatase with 3.2 eV and rutile with 3.0 eV and its XR [25–27], while the hydroxyl groups are due to the influence of the reaction between Ti and H2SO4 during SAP, which favors the enhancement of pho-tocatalytic performance at relatively low oxidation tempera-ture, similar to the previous report [9–11]. Meanwhile, with increasing the oxidation temperature, the S 2p peak would be found from the SA-O773 sample, which could be related to the formed compounds during SAP, while the peak disap-pears at 1073 K, because the formed compounds had been oxidized or the too thicker TiO2 had formed on the outer surface during the oxidation in air at a higher temperature.

Fig. 4 XPS spectra of the SA-Ox samples. a C 1s, b O 1s, c Ti 2p, d S 2p

Norm

aliz

ed

inte

nsi

ty

Binding energy (eV)

532 528

(b)O 1s

Norm

aliz

ed

inte

nsi

ty

Binding energy (eV)

288 284

(a)C 1s

Norm

aliz

ed

inte

nsi

ty

Binding energy (eV)

172 168

(b)S 2p

Norm

aliz

ed

inte

nsi

ty

Binding energy (eV)

464 460

(c)Ti 2p

468 456

- OH Ti-OSA-O773

SA-O1073

284.6

529.88

168.5

530.08

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3.4 Photocatalytic activity

The photocatalytic activity of the SA-Ox samples was evalu-ated by the degradation of MB solution at room temperature, as shown in Fig. 5. To avoid the adsorption of the TiO2 coatings, the adsorption of MB solution was carried out in the dark for 18 h. Almost no degradation of MB solution could be observed during the irradiation in the absence of the photocatalyst coatings (Fig. 5a). Compared with that of MB solution without photocatalyst coatings, all samples show photocatalytic activity, according to the concentration changes of the MB solution. With enough energy absorption from the irradiation, the electrons are generated and trans-ferred from valence band to the conduction band, with excit-ing the holes valence band, which could be named the gen-eration of electron–hole pair. The formed electron–hole pair could degrade the MB solution [28–30]. Therefore, the con-centration changes of the MB solution with the photocatalyst coatings significantly decrease. In general, with increasing the oxidation temperature, Fig. 5b clearly shows the degra-dation rate constants of the SA-Ox samples first increases, then decreases, and reaches the highest at 873 K. Compared with those without SAP [22, 24], the enhancement of the SA-Ox samples on photocatalytic activity is remarkable and the degradation constant rate increases around 2 to 3 times, which could be related to the mixed-phase of anatase and rutile [25, 27], and the formed hydroxyl groups [9–11], as well as the increased accessible surface area via the porous-like structure [31–33]. The formed mixed-phase of anatase and rutile and hydroxyl groups favor the enhancement of electron–hole separation, while the increased accessible sur-face area will promote the energy absorption from the irradi-ation. In other words, the enhanced photocatalytic activity of TiO2 coatings has been achieved at relatively low oxidation

temperature of 773 K. The effects of relatively low oxidation temperature on photocatalytic activity could be attributed to the two main points. One is the suitable mass fraction of anatase, and another is the surface morphology with porous-like structure. Anatase TiO2 is with higher catalytic activ-ity than that of rutile, and mixed-phase favors the charge transfer to suppress the recombination of electron–hole pairs [22–24]. The porous-like structure formed during SAP with oxidation at relatively low oxidation temperature (Fig. 3a–d). However, with higher oxidation temperature, the porous-like structure disappears (Fig. 3e–f), resulting in the decreased accessible surface area.

4 Conclusions

In summary, TiO2 photocatalyst coatings have been success-fully fabricated by sulfuric-acid-bath pretreatment (SAP) followed by simple oxidation in air at relatively low tem-perature for Ti coatings. The photocatalytic activity of TiO2 coatings has been significantly enhanced by the mixed-phase of anatase and rutile, formed hydroxyl groups, and increased accessible surface area via the porous-like structure. The mixed-phase of anatase and rutile has formed and the phase transformation occurred from anatase to rutile at 773 K, with increasing the oxidation temperature. Compared with those without SAP, the porous-like structure forms on the surface of Ti coatings during SAP. With increasing the oxidation temperature until 873 K, the changing trend of its size is becoming larger, which would affect the accessible surface area, while the porous-like structure totally disappears at 973 K, and it shows columnar-like structure at 1073 K. With relatively lower temperature, the slight peak shift of O 1s reveals the phase transformation from anatase and rutile,

Fig. 5 Comparison of the photocatalytic activity of the SA-Ox samples. a Degradation of MB solution; b degradation constant rate

UV irradiation time (min)

C /

C0

0 60 120 180

1.0

0.8

0

5

10

15

6.4

11.6

14.1

14.9

10.7

3.7

Deg

rad

atio

n c

onst

ant

, R/

nm

ol

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SA

0.5

h

SA

-O673

SA

-O773

SA

-O873

SA

-O973

SA

-O1073

(b)(a)

MB solutionSA0.5hSA-O673SA-O773SA-O873SA-O973SA-O1073

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while the formed hydroxyl groups during SAP also have been confirmed by XPS results. With raising the oxidation temperature, the photocatalytic activity of the TiO2 coatings first increases, then decreases, and reaches the highest at 773 K, due to the suitable mass fraction of anatase and the surface morphology with porous-like structure.

Acknowledgements The authors would like to express their apprecia-tion to Dr. Hyuma Masu and Dr. Sayaka Kado from Center for Ana-lytical Instrumentation, Chiba University. There is no funding source for this paper.

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