Black TiO2 synthesized via magnesiothermic reduction for enhanced photocatalytic activity

  • Xiangdong Wang
  • Rong Fu
  • Qianqian Yin
  • Han Wu
  • Xiaoling Guo
  • Ruohan Xu
  • Qianyun Zhong
Research Paper


Utilizing solar energy for hydrogen evolution is a great challenge for its insufficient visible-light power conversion. In this paper, we report a facile magnesiothermic reduction of commercial TiO2 nanoparticles under Ar atmosphere and at 550 °C followed by acid treatment to synthesize reduced black TiO2 powders, which possesses a unique crystalline core–amorphous shell structure composed of disordered surface and oxygen vacancies and shows significantly improved optical absorption in the visible region. The unique core–shell structure and high absorption enable the reduced black TiO2 powders to exhibit enhanced photocatalytic activity, including splitting of water in the presence of Pt as a cocatalyst and degradation of methyl blue (MB) under visible light irradiation. Photocatalytic evaluations indicate that the oxygen vacancies play key roles in the catalytic process. The maximum hydrogen production rates are 16.1 and 163 μmol h−1 g−1 under the full solar wavelength range of light and visible light, respectively. This facile and versatile method could be potentially used for large scale production of colored TiO2 with remarkable enhancement in the visible light absorption and solar-driven hydrogen production.


Black TiO2 Magnesiothermic reduction Photocatalysis Hydrogen production Degradation of MB Nanostructured catalyst Solar energy 


Titanium dioxide (TiO2) nanomaterials have attracted enormous interest as a promising solar-driven photocatalyst for hydrogen production and environmental pollution removal (Fujishima and Honda 1972; Chen et al. 2010; Sun et al. 2015; Farbod et al. 2013; Liu and Chen 2014; Lian et al. 2017; Diker et al. 2011; Nowotny et al. 2015; Qu and Duan 2013). However, the wide bandgaps of 3.0–3.2 eV for different crystal phases of TiO2 make them only absorb ultraviolet light, which only accounts for less than 5% of the entire solar spectrum. To improve the optical absorption properties of TiO2, many persistent attempts for band structure modification have been made by doping metal and nonmetal elements, introducing oxygen vacancies, the formation of dopant band, and so on (Asahi et al. 2001; Khan et al. 2002; Wang et al. 2009, 2017; Liu et al. 2013a, b, c, 2014; Zhang et al. 2014; Dahlman et al. 2015; Li et al. 2015). Nevertheless, the limited solubility of doping elements in bulk TiO2 and the introduction of dopants, usually acting as recombination centers for photogenerated electrons and holes, have reduced the efficiency of doping elements (Liu et al. 2012). Despite many efforts, up until today, it is a major challenge to fabricate the TiO2 photocatalyst with wide spectrum solar absorption and highly efficient power conversion.

Recently, Chen et al. reported that black TiO2 nanocrystals with a narrow band gap were prepared from hydrogenating anatase TiO2 in a 20-bar H2 atmosphere at 200 °C for 5 days (Chen et al. 2011). The black TiO2 nanocrystals exhibit substantial visible-light photocatalytic activities, including the photo-oxidation of organic molecules in water and the production of hydrogen. Unsurprisingly, this discovery has triggered extensive research interest in black TiO2 nanomaterials and has led to a new direction in photocatalytic material modification (Zheng et al. 2012; Wang et al. 2011, 2013a, b; Zuo et al. 2012; Myung et al. 2013; Chen et al. 2013; Liu et al. 2013a, b, c; Zhou et al. 2014). However, their method for H2 reduction needs high annealing temperature and high pressure, and H2 is flammable and explosive. Therefore, a number of following investigations attempt to explore more effective and environment-friendly preparation methods of black TiO2, to avoid the harsh reaction conditions to reduce TiO2 at high hydrogen pressure and high temperature (Naldoni et al. 2012; Zhu et al. 2013, 2014a, b; Teng et al. 2014; Tan et al. 2014; Cui et al. 2014; Ullatti and Periyat 2016; Liu et al. 2015; Zhu et al. 2016; Chen et al. 2015).

Among the above methods, chemical reduction, a facile and inexpensive method, received a lot of attention. Wang et al. reported that black TiO2 nanoparticles were prepared by using melted Al as a reductant in an evacuated two-zone vacuum furnace (Wang et al. 2013a, b). The prepared black TiO2 nanoparticles possess a unique crystalline core–amorphous shell structure (TiO2@TiO2-x) and shows intense absorption in the visible-light and near-infrared regions. A series of nonmetal-doped Al-reduced titania nanocrystals were fabricated with the two-step strategy (Lin et al. 2014). The incorporation of the nonmetal element X (X = H, N, S, I) in the oxygen-deficient amorphous layers of the nanoparticles caused color darken and displayed enhanced absorption in both visible light and near-infrared regions. Yang et al. prepared black core–shell rutile TiO2 nanoparticles with sulfided surface from both anatase and rutile nanoparticles. The black rutile TiO2 nanoparticles exhibited remarkably enhanced absorption in visible and near-infrared regions (37 Yang et al. 2013). Kim et al. reported that the black TiO2 NTA was fabricated by the electrochemical self-doping of amorphous TiO2 NTA. The fabricated black TiO2 NTA exhibited stable and highly capacitive and electrocatalytic properties resulting in its good applications as a supercapacitor and an oxidant generating anode (Kim et al. 2015). Zhao et al. prepared reduced gray rutile TiO2 nanoparticles with zinc reduction via a solvothermal route, and the reduced TiO2 nanoparticles showed a broad visible-light absorption band (Zhao et al. 2014). Xin et al. reported that black brookite TiO2 single-crystalline nanosheets with outstanding photocatalytic activity toward CO2 reduction was prepared by a facile oxidation-based hydrothermal reaction method combined with postannealing treatment (Xin et al. 2016). Sinhamahapatra et al. developed a magnesiothermic reduction to synthesize black TiO2 under a 5% H2/Ar atmosphere and at 650 °C. The prepared materials show evidently improved optical absorption in the visible region (Sinhamahapatra et al. 2015). However, the method needs H2/Ar atmosphere and high annealing temperature.

In the present work, we have developed a facile method, using Mg reduction of commercial TiO2 nanoparticles in the presence of Ar atmosphere and at 550 °C followed by acid treatment to prepare reduced black TiO2 powders, which exhibit significantly improved photocatalytic hydrogen production in the methanol–water system and photocatalytic degradation of MB under visible light irradiation. By using various analytical characterization techniques, the reduced black TiO2 obtained by reducing TiO2 with Mg powder in the magnesiothermic reaction was demonstrated to possess a unique crystalline core–amorphous shell structure composed of disordered surface and oxygen vacancies, which enhances the light absorption and reduces the recombination rate of photogenerated electrons and holes in the samples.


Preparation of reduced black TiO2

Commercially available chemicals were purchased from Shanghai Chemical Reagent Co. Ltd., Shanghai, China. The reduced black TiO2 was prepared as following: well-mixed sample of TiO2 and Mg powder was placed in a controlled atmosphere furnace and then heated at 550 °C for 4 h under Ar atmosphere. After the calcining treatment, the sample was dispersed in 2.0 M HCl solution and stirred for 8 h. The sample was washed with sufficient amount of water to remove the acid and dried at 80 °C for 12 h. Different reduced black TiO2 samples were prepared with varying the molar ratio of TiO2 and Mg and denoted as RT-n (where n (= 0.5, 1, 1.5, and 2) is the molar ratio of Mg with respect to TiO2). As a comparison, the white TiO2 was calcined under Ar atmosphere at 550 °C to study the effect of temperature and Ar atmosphere. The sample is denoted as PT. All the prepared samples were surface-deposited with Pt nanoparticles by photo-reduction method.

Catalyst characterization

Powder X-ray diffraction (XRD) data were recorded in the range of 10–80° (2θ) using a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation and a 2θ scan rate of 2° min−1. Raman spectra were collected on a thermal dispersive spectrometer using a laser with an excitation wavelength of 632 nm at laser power of 10 mW. High-resolution transmission electron microscopy (HR-TEM) images were collected using JEOL FE-2010, operated at 200 kV. Diffuse reflectance UV-visible absorption spectra of the powder samples were obtained using a Shimadzu-2501 spectrophotometer. BaSO4 was the reference sample, and the spectra were recorded at room temperature in air within the range of 200–1000 nm. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a PHI 5300 ESCA instrument with an Mg Kα X-ray source at a power of 250 W. The binding energy scale was calibrated with respect to the C1s peak of hydrocarbon contamination fixed at 284.6 eV. Photoluminescence (PL) emission spectra were measured on a Fluoromax-4 (Horiba Jobin Yvon) spectrofluorometer, excited with 330 nm light.

Photocatalytic water splitting

Twenty milligrams of photocatalyst loaded with ~ 1 wt% Pt was added to an aqueous methanol solution (100 ml, 20%) in a closed gas circulation system. The full solar wavelength light irradiation was obtained from a 300-W Xenon lamp embedded with IR filter. Methanol was used as a sacrificial reagent, and the anodic reaction generating O2 from H2O did not occur. The amount of H2 generated was determined by online gas chromatography system connected to the reactor. The reaction was carried out at room temperature (25 °C) under Ar atmosphere. The visible light irradiation was obtained from a 300-W Xenon lamp by using a 400-nm cutoff filter, and the photocatalytic hydrogen production was carried by keeping all other parameters constant.

Photocatalytic degradation of MB

The photocatalytic degradation activity of the black TiO2 samples was evaluated by monitoring the decomposition of MB in an aqueous solution under visible light irradiation. The catalyst (50 mg) was added into a quartz reactor (100 mL), which contained 50 mL dye solution. Prior to irradiation, the suspension was stirred for 30 min in the dark to reach an adsorption–desorption equilibrium. A 300-W xenon lamp through a UV-cutoff filter (≤ 400 nm) was used as the visible light source. The photocatalytic degradation was conducted in a BLGHX-V multifunctional photochemical reactor. The analytic suspension (4 mL) was taken out of the reactor at regular intervals, and centrifuged immediately, before being filtered to separate the black TiO2 from the solution. The residual concentration of the dyes in the remaining clear liquid was analyzed by a spectrophotometer (UV-7220, Beifenruili, China). The change of relative absorbance was used to record the change of concentration of MB in the solution, that is C/C0 = A/A0 (C, A referred to the concentration and absorbance of MB in the solution at time t and C0, A0 referred to the concentration and absorbance of MB in the solution at the initial time, respectively).

Results and discussions

The XRD patterns of the different samples are presented in Fig. 1. It is confirmed that all samples are composed only anatase phase and are not found other phases, because of the heating temperature at 550 °C. The XRD patterns indicate that the samples do not contain Mg or MgO as no related peak is observed. The XRD patterns also do not show any peak related to Ti2O3 or any other titanium sub-oxide. Although the XRD analysis does not indicate any structural change during magnesium reduction, it can be assumed that Mg changes the surface of the TiO2 particles, which results in color changes from white to gray. The crystallite sizes of samples calculated using the Scherrer equation from XRD data are shown in Table 1. As shown in the table, the crystallite sizes of the sample RT-0.5, RT-1, and PT are similar, but the crystallite sizes of the sample RT-1.5 and RT-2 are higher than that of PT. This suggests that the increment of crystallite size for RT is due to increasing the amount of Mg. When higher amount (mole ratio > 1) of Mg is used, the crystallite size increases evidently, which may be induced by doping of Mg (Sinhamahapatra et al. 2015).
Fig. 1

Powder X-ray diffraction patterns of different samples

Table 1

The crystallite sizes of samples using the Scherrer equation from XRD data







Crystallite size (nm)






Raman spectroscopy was used to investigate the reduced black TiO2 shown in Fig. 2. As shown in Fig. 2a, the Raman spectrum of PT shows six (3Eg + 2B1g + A1g) characteristic Raman bands of the typical anatase TiO2 phase with the strongest Eg band around at 144 cm−1. Compared with PT, the bands of the reduced black TiO2 samples exhibit a blue shift accompanied by peak broadening. It is clear that RT-2 has much more blue shift (153.2 cm−1) than that of RT-1(144.8 cm−1) as shown in Fig. 2b, indicating much more changes for microstructure, which is demonstrated by the observations of the TEM results discussed above. As reported in the previous studies (Chen et al. 2011, 2015; Zheng et al. 2012; Wang et al. 2011; Naldoni et al. 2012; Sinhamahapatra et al. 2015), this is ascribed to the crystal domain size and nonstoichiometry over the surface of the reduced TiO2 samples and can be directly correlated to oxygen deficiency and disordered layer at the surface of the crystal particle. It could be assumed that during magnesiothermic reduction, the lattice periodicity and the octahedral symmetry of TiO6 are destroyed on the surface (Sinhamahapatra et al. 2015).
Fig. 2

a Raman spectra of the sample PT, RT-1, and RT-2 and b the magnified view of the most intense Eg peak of the three samples

The morphology of reduced black TiO2 sample was observed by HR-TEM analysis, shown in Fig. 3. The PT nanocrystal is highly crystallized, as the well-resolved lattice features are shown in the HR-TEM image (Fig. 3a). The reduced black TiO2 samples display a core–shell structure consisting of a crystalline core of TiO2 with a disordered shell layer. The sample RT-1 (Fig. 3b) has a 1.5-nm-thick disordered surface layer coating on a crystalline core of TiO2. The thickness of the disordered layer increases with increasing the amount of Mg. As shown in the Fig. 3c, the sample RT-2 has a thicker disordered layer than that of RT-1, and the nanocrystals of the sample RT-2 reveal more imperfections and more amorphous shell structure, which may be the results caused by the excessive magnesiothermic reduction (Chen et al. 2015).
Fig. 3

HR-TEM images of the sample a PT, b RT-1, and c RT-2

XPS was performed to investigate the change of the surface bonding and electronic valence band position of the sample PT and the reduced black TiO2 sample (RT-1) as shown in Fig. 4. Ti 2p XPS spectra (Fig. 4a) show characteristic Ti 2p3/2 and Ti 2p1/2 peaks centered at 459.1 and 464.9 eV for the PT, which are typical for Ti4+ in TiO2 (Chen et al. 2011). As shown in Fig. 4b, similar to the PT, the high-resolution XPS spectra of Ti 2p3/2 and Ti 2p1/2 centered at binding energies of 459.3 and 465.1 eV for RT-1 are typical for the Ti4+–O bonds in TiO2. In addition, according to the previous reports, the binding energies of Ti3+ are located at 457.6 and 463.5 eV, respectively (Cui et al. 2014; Rahman et al. 1999; Jiang et al. 2012). In contrast with the PT, there are two small peaks centering separately at 457.6 and 463.4 eV in the RT-1 sample, implying the existing of Ti3+ in the surface of the sample. This result clearly indicates the formation of Ti3+ during the magnesiothermic reduction of TiO2. The emerging Ti3+ signals suggest that oxygen vacancies are introduced into the reduced sample during the magnesiothermic reduction process.
Fig. 4

Ti 2p XPS spectra of the sample a PT and b RT-1

The absorption spectra and the color of the samples are displayed in Fig. 5. Figure 5a shows the color of the samples. The color of the samples turned from white (PT) to gray (RT) and the light absorption increased obviously with increasing the amount of Mg, which shows the more amount of Mg, the more reduction of the sample and the darker the sample. As seen in the Fig. 5b, the absorption edge of PT at the wavelength of 400 nm is attributed to the intrinsic bandgap absorption of crystalline nano anatase TiO2, but the reduced black TiO2 samples can extend the photoresponse from UV light to visible and infrared light region. It is clearly observed that the absorption intensity of visible and infrared light increases as the amount of Mg increases. The extended absorbance in visible region can be correlated to the color change trend of the samples from white to gray and brown. Therefore, magnesiothermic reduction of TiO2 nanoparticles leads to surface modification, which is reflected in enhanced light absorption as well as the color change.
Fig. 5

a The color and b absorbance spectra of different samples

The photoluminescence (PL) emission spectra is useful to understand the behavior of light-generated electrons and holes in the samples since PL emission results from the recombination of free carriers. As is shown in Fig. 6, the shapes of the emission spectra of three samples are similar. Four main emission peaks are observed at 402 nm (3.12 eV), 426 nm (2.88 eV), 450 nm (2.75 eV), and 472 nm (2.65 eV), respectively. The first one is attributed to the emission of band gap transition of antase. The peaks at 450 and 472 nm are attributed to the free excitons at the band edge. In addition, several small PL peaks observed in the wavelength range are mainly due to the surface defects of the TiO2 samples. The PT sample shows much stronger light emission in the range from 380 to 500 nm than the reduced black TiO2 samples (RT-1 and RT-2). The PL intensity can be directly correlated to the charge recombination. The higher intensity corresponds to the faster recombination. The tests results clearly show much faster charge recombination for PT than for RT-1, indicating that the RT-1sample has a relatively low recombination rate of electrons and holes. Generally, a low recombination rate of electrons and holes are the prerequisites for high photocatalytic activity.
Fig. 6

Photoluminescent spectra of the samples

The photocatalytic activity of the samples was investigated by monitoring hydrogen evolution from water splitting in the presence of methanol (20%) using Pt nanoparticles as cocatalysts. Pt (1 wt%) was loaded on the photocatalyst by a UV-vis light induced reduction procedure. The continuous hydrogen production profile and the rate of H2 production under the full solar wavelength range of light for different samples are shown in Fig. 7a. It is found that the rate of hydrogen evolution significantly increases for the reduced samples compared the sample PT. The active sequence of hydrogen evolution is RT-1 > RT-0.5 > RT-1.5 > RT-2, and the sample RT-1 steadily produced hydrogen gas at 16.1 mmol h−1 g−1, which is about 14.5 times higher than that of PT (1.1 mmol h−1 g−1) and comparable to the reported excellent TiO2 photocatalysts. Interestingly, the rate increases with the amount of Mg up to 1, and then, it starts to decrease for 1.5 and 2 although it is observed that with the increase of the Mg amount, the sample turns darker and the absorption of light also increases. These results can be ascribed to the generation of new recombination sites due to over-reduction in the presence of a high amount of Mg (Yang et al. 2013; Sinhamahapatra et al. 2015; Zhang et al. 2011). Therefore, the hydrogen production decreases due to more hole–electron recombination although the absorption of light is high. The photoluminescence spectra (Fig. 6) of the samples also indicate faster charge recombination for sample RT-2 compared with RT-1. In addition, the crystallite size of the reduced TiO2 is increased with increasing Mg. The small crystallite size is beneficial to the photoactivity.
Fig. 7

Photocatalytic H2 generation by samples under the a full solar wavelength range of light and b visible light irradiation

The visible light photocatalytic water splitting of the samples is shown in Fig. 7b. The activity was decreased in comparison to that of the full solar wavelength range of light. The sample RT-1 steadily produced hydrogen gas at about 163 μmol h−1 g−1, superior to other reduced samples and no hydrogen gas for sample PT. The active sequence of hydrogen evolution under visible light is RT-1 > RT-1.5 > RT-0.5 > RT-2, slightly different from the active sequence of hydrogen evolution under full solar wavelength range of light. The sample RT-1.5 exhibits more activity than RT-0.5, which may be assigned to the more absorption of visible light for RT-1.5 compared to RT-0.5. The results clearly indicate that RT-1 overmatches the other reduced samples, demonstrating high photocatalytic hydrogen production activity under both full solar wavelength and visible light conditions. Table 2 lists some important photocatalytic hydrogen production data of the reported black TiO2 for the comparison. It is clearly found from the table that compared with the recent results of the reported black TiO2, our hydrogen production rates are superior to those of most of other reported black TiO2 materials under both full solar wavelength and visible light irradiation.
Table 2

Rate of hydrogen generation obtained by using different black TiO2 materials

Black TiO2 photocatalyst

Reactant solution


Light source

H2 generation under full solar wavelength of light (mmol h−1 g−1)

H2 generation under visible light (μmol h−1 g−1)

Hydrogenated black TiO2 (Chen et al. 2011)

50% CH3OH

0.6% Pt

AM-1.5 solar stimulator



Al-reduced black TiO2 (Wang et al. 2013a, b)

25% CH3OH

0.5% Pt

Hg lamp, 300 W



Black TiO2 reduced by NaBH4 (Tan et al. 2014)

25% CH3OH


Xe lamp, 300 W



Mg-reduced black TiO2 (Sinhamahapatra et al. 2015)

20% CH3OH


Xe lamp, 400 W



TiH2-reduced TiO2 (Zhu et al. 2016)

20% CH3OH

0.5% Pt

Xe lamp, 300 W


Current work

20% CH3OH


Xe lamp, 300 W



The visible photocatalytic degradation activity of reduced black TiO2 samples was evaluated by measuring the decomposition of MB solutions under visible light irradiation at room temperature. The photocatalytic behavior of pure TiO2 was also measured as a comparison. Figure 8a shows the concentration change of MB for the samples against the reaction time. Compared with the sample PT, the reduced samples (RT) exhibit excellent visible light catalytic activity. The active sequence of degradation reaction is RT-1 > RT-1.5 > RT-0.5 > RT-2 > PT, which is similar to that of hydrogen evolution under visible light. These results indicate that there is an optimal Mg amount that leads to maximal photocatalytic efficiency. Using an appropriate amount of Mg, the photocatalytic activity of the reduced black TiO2 is enhanced, but an excessive amount of Mg may produce more recombination sites, which retard the photocatalytic activity. The photocatalytic degradation reaction of MB solution with the reaction time is a first order pattern, which can be well confirmed by the linear transforms of ln(C0/C) ~ t shown in Fig. 8b, from which the rate constants can be obtained. Results show that under visible light irradiation for 8 h, the degradation rate contents of MB are 46.3, 32.5, 24.9, 15.6, and 2.0 (× 10−2 h−1) for RT-1, RT-1.5, RT-0.5, RT-2, and PT, respectively. The test results shows that the photocatalytic activities under visible light illumination of reduced black TiO2 samples were significantly improved compared to pure TiO2 sample.
Fig. 8

Photocatalytic degradation of MB solution under the full solar wavelength range of light


In conclusion, a facile process of magnesiothermic reduction in the presence of Ar atmosphere followed by acid treatment was used firstly to prepare reduced black TiO2, which has core–shell structured antase TiO2 nanoparticles with the disordered and oxygen-deficient shell and antase TiO2 core. The disordered and oxygen-deficient shells are well-controlled by reduction conditions and are responsible for enhanced wide-spectrum light absorption and photocatalysis. The reduced black TiO2 samples exhibit much greater photocatalytic hydrogen production ability and visible photocatalytic activity than pure antase TiO2. The maximum hydrogen production rates are 16.1 mmol h−1 g−1 in full solar wavelength and 163 μmol h−1 g−1 in visible region, respectively. In perspective, the simple treatment of pristine TiO2 by the magnesiothermic reduction represents large-scale photocatalyst fabrication method to produce efficient photocatalysts for technological implementation of solar photocatalytic reaction.



The authors acknowledge proofreading and revision of this manuscript by Ms. Yuxin Li.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.


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Copyright information

© Springer Science+Business Media B.V., part of Springer Nature 2018

Authors and Affiliations

  • Xiangdong Wang
    • 1
  • Rong Fu
    • 1
  • Qianqian Yin
    • 1
  • Han Wu
    • 1
  • Xiaoling Guo
    • 2
  • Ruohan Xu
    • 1
  • Qianyun Zhong
    • 1
  1. 1.School of ScienceXi’an Jiaotong UniversityXi’anPeople’s Republic of China
  2. 2.School of Textile and MaterialsXi‘an Polytechnic UniversityXi’anPeople’s Republic of China

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