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N-codoped SrTiO3
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Efficient visible-light-driven photocatalytic H2 production over Cr/N-codoped SrTiO3 He Yu a,c, Shicheng Yan a,b,*, Zhaosheng Li a,b, Tao Yu a,c, Zhigang Zou a,b,c,* a
National Laboratory of Solid State Microstructure, Nanjing University, 210093, PR China Eco-materials and Renewable Energy Research Center (ERERC), College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, PR China c Department of Physics, Nanjing University, 210093, PR China b
article info
abstract
Article history:
Visible-light-response Cr/N-codoped SrTiO3 was prepared by a solegel hydrothermal
Received 13 March 2012
method. The comparison studies indicate that Cr-doped and Cr/N-codoped SrTiO3 can be
Received in revised form
synthesized by this means, but not the N-doped SrTiO3. The theoretical calculations exhibit
14 May 2012
the defect formation energy of the Cr/N codoping into SrTiO3 is much smaller than that of
Accepted 18 May 2012
the N doping into SrTiO3, illuminating that the incorporation of Cr can promote the N
Available online 12 July 2012
doping into the O sites in the SrTiO3. Compared to the Cr-doped SrTiO3, the Cr/N-codoped
Keywords:
the quantum efficiency of 3.1% at 420 nm, due to the smaller band gap and much less
Photocatalysis
vacancy defects.
SrTiO3
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
SrTiO3 photocatalyst shows the high photocatalytic activities for hydrogen production with
Visible light response
reserved.
Water splitting
1.
Introduction
Hydrogen, as an environmentally friendly fuel, is attracting more and more attention. Since the production of hydrogen through photoinduced water splitting on titanium oxide was discovered [1], semiconductor-based photocatalysis has prompted many investigations, because this technique makes use of abundant, long lasting and clean solar energy. In order to construct an efficient photocatalytic water splitting reaction, some photocatalytic materials, such as oxides, oxynitrides and sulfides, have been explored [2]. To apply the photocatalysis technique, an ideal photocatalyst is expected to be having good photostability, having a narrow band gap to achieve the efficient use of solar energy, and having highly
efficiency in separating, collecting and transporting charges for the chemical processes. SrTiO3 is a promising perovskite-type photocatalyst for water decomposition. However, SrTiO3 has a wide band gap of 3.2 eV, only responds to ultraviolet light, which accounts for only 4% of the incoming solar energy. Element doping usually is an efficient route to broaden the light absorption of the wide-band-gap semiconductors into the visible light region, which possesses 43% of the solar energy. To obtain the visible light response, doping with foreign elements into SrTiO3 has been studied by many research groups [3e7]. Some threevalence metal ions, such as Rh3þ, Ir3þ and Ru3þ, were incorporated into the Ti4þ sites of SrTiO3. The strong hybridization of orbital of the doping elements with the O 2p states leads to
* Corresponding authors. National Laboratory of Solid State Microstructure, Nanjing University, 210093, PR China. Tel.: þ86 25 83686630; fax: þ86 25 83686632. E-mail addresses: [email protected] (S. Yan), [email protected] (Z. Zou). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.05.097
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the band gap narrowing of SrTiO3. Recently, the Cr3þ doping into the Sr2þ sites of SrTiO3 induces the visible light response due to the formation of new valence band by the occupied Cr3þ level, which is about 1.0 eV higher than the valence band top formed by O 2p orbital [8]. However, doping by only one element with different valence from the hosts usually induces the low photocatalytic activities, probably due to the formation of defects in the crystal, for example the oxygen vacancy, which usually acts as recombination center of the photogenerated electrons and holes [9]. Codoping may be a promising method to suppress the formation of defects, and thus improving the activities of photocatalysts. Kato and Kudo found that the codoping of Sb or Ta and Cr into SrTiO3 showed a quite high activity in H2 production. This can be attributed to that the charge balance was kept by codoping of Sb5þ or Ta5þ and Cr3þ in Ti4þ sites of SrTiO3 [6,7]. Another efficient method to decrease the formation of defects is codoping by anion and cation metals [10,11]. The high activity of La/N-codoped SrTiO3 was attributed to the decrease in the oxygen vacancies because codoping of La3þ ions in Sr2þ sites and N3 in O2 sites maintained the charge balance. For La/N-codoped SrTiO3, the visible light response results from the mixing of the N 2p states with the O 2p states. La element did not contribute to the formation of the band gap of SrTiO3. Here, we report the Cr/N-codoped SrTiO3, both the Cr and N doping not only maintained the charge balance in SrTiO3, but also contributed to the band gap narrowing of SrTiO3. The high photocatalytic activity in water splitting was achieved due to the visible light absorption and less crystal defects in SrTiO3.
2.
Experimental
The Cr/N-codoped SrTiO3 powder samples were synthesized by a solegel hydrothermal method. Analytical grade Sr(NO3)2, Cr(NO3)3$9H2O, Ti(C4H9O)4, CO(NH2)2, NaOH and ethylene glycol (EG) were used as raw materials without further purification. In the synthesis process of the Cr/N-codoped SrTiO3, firstly, Sr(NO3)2(0.0095 mol), Cr(NO3)3$9H2O (0.005 mol) and CO(NH2)2 (0.01 mol) were dissolved in EG (20 mL), and heated to 80 C under continuously stirring. Subsequently, Ti(C4H9O)4 (0.01 mol) was dropped into the above-mentioned solution to form a sol. With the evaporation of EG, a gel was obtained. Then 30 mL of NaOH solution (5 M) was added into the gel. After stirred a while, the mixed solution was put into a 40 mL Teflon-lined stainless autoclave. The autoclave was heated up from room temperature to 200 C and maintained for 24 h. The obtained precipitate was washed with distilled water until the pH value was 7e8, and then dried at 80 C overnight. As a comparison, the undoped SrTiO3, Cr-doped SrTiO3 and N-doped SrTiO3 by using urea as the nitrogen source (denoted as SrTiO3eU) were synthesized by this means in presence of Sr(NO3)2 (0.01 mol) and Ti(C4H9O)4 (0.01 mol), in presence of Cr(NO3)3$9H2O (0.005 mol), Sr(NO3)2 (0.0095 mol) and Ti(C4H9O)4 (0.01 mol), and in presence of Sr(NO3)2 (0.01 mol), Ti(C4H9O)4 (0.01 mol) and CO(NH2)2 (0.01 mol), respectively. In addition, N-doped SrTiO3 was synthesized by nitriding pristine SrTiO3 at 900 C for 10 h in a tube furnace with NH3 flow rate at 200 mL min1.
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The crystalline phases of the as-prepared samples were identified by a powder X-ray diffraction (XRD, Ultima III, ˚ , 40 kV, Rigaku Corp., Japan) using Cu Ka radiation (l ¼ 1.54178 A 40 mA) with a scan rate of 10 min1. The diffuse reflectance spectra were obtained using a UVevisible spectrophotometer (Shimadzu Corp., UV-2500PC, Japan) and X-ray photoelectron spectroscopy (XPS) was carried out on Thermo ESCALAB 250 spectrometer using monochromated Al Ka (1486.6 eV) source operated at 110 W, and the photoelectrons were detected by a hemispherical analyzer operating at a pass energy of 20 eV. The binding energy was calibrated on the reference C 1s peak at 284.8 eV. The morphologies of the as-prepared samples were observed by scanning electron microscopy (SEM). Photocatalytic reactions for H2 evolution were conducted over the Pt-loaded photocatalyst using CH3OH as the sacrificial reagent. The Pt cocatalyst was loaded on the catalyst surface by an in situ photodeposition method [12]. The Pt-loaded photocatalyst powders (0.25 g) were dispersed in the aqueous CH3OH solution (50 mL of CH3OH þ 220 mL of H2O) in an outer irradiation Pyrex glass cell. A 300 W Xe arc lamp was focused on the side window of the cell with a long-pass cutoff filter (l 420 nm, L42, HOYA) to achieve the visible light. The reaction cell was connected to a closed gas circulation system. Amount of the evolved gas was determined using gas chromatograph (Shimadzu GC-8A, argon carrier). Apparent quantum yields were obtained by the equation QY (%) ¼ Ne/Np ¼ 2NH2/Np, where Ne is the number of reacted electrons, Np is the number of incident photons, and NH2 is the number of evolved H2 molecules. Np can be calculated from the light intensity which was measured with a spectroradiometer to be 2.15 mV cm2. NH2 is determined by using a 420 nm band-pass filter. In this study, the first-principle density functional theory calculations were carried out with plane wave pseudopotential method as implemented in CASTEP [13]. The electronic wave functions were expanded in terms of a discrete plane wave basis set. The exchange and correlation interactions were modeled by the generalized gradient approximation (GGA) with PerdeweBurkeeErnzerhof (PBE) scheme. Interaction between electrons and ion core was replaced by ultra-soft pseudopotential. The k-space integrations were performed with Monkhorst-Pack grid with 4 4 4 k-points in the Brillouin zone to obtain the accurate electronic density. The kinetic energy cutoff for wave function expansion was 340 eV. Geometry optimization was performed before electronic and total energy calculations and the self-consistent convergence accuracy was set at 5 105 eV atom1. The convergence ˚ 1, the criterion of the largest force on atoms was 0.1 eV A 4 maximum displacement was 5 10 nm and the stress was not more than 0.2 GPa. All calculations were based upon a 2 2 2 SrTiO3 supercell with a size of 40-atom. We substituted a nitrogen atom for an oxygen atom to simulate the nitrogen doping. In the case of Cr/N-codoping, we substituted a nitrogen atom and a chromium atom for an oxygen atom and a strontium atom, respectively.
3.
Results and discussion
Fig. 1 shows that the XRD patterns of all samples can be indexed as the cubic SrTiO3 (JCPDS No. 89-4934). This implied
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Fig. 1 e XRD patterns of (a) SrTiO3, (b) SrTiO3eU, (c) N-doped SrTiO3, (d) Cr-doped SrTiO3 and (e) Cr/N-codoped SrTiO3. Inset shows the (110) diffraction peak positions in the range of 2q [ 32e33 .
that doping with foreign elements in SrTiO3 did not introduce impurity phases. A careful comparison of the (110) diffraction peaks in the range of 2q ¼ 32e33 (the inset of Fig. 1) shows that the SrTiO3, SrTiO3eU and N-doped SrTiO3 have the same peak position, but the peak positions of the Cr-doped SrTiO3 and Cr/N-codoped SrTiO3 slightly shifted toward a higher 2q value. XPS analysis shows that the Cr cation is mainly present as Cr3þ in the Cr-doped or Cr/N-codoped SrTiO3 (see supporting information Fig. S1) [7]. If substituting Cr3þ cations for Ti4þ sites in SrTiO3 lattice, the (110) diffraction peaks for XRD pattern should shift toward a lower 2q value or did not shift because the ionic radius of the Cr3þ (0.0615 nm) is slightly larger than that of Ti4þ (0.0605 nm). Therefore the shift of the (110) diffraction peak (see the inset of Fig. 1) resulted from that
the Cr3þ (ionic radius: 0.0615 nm) was incorporated into the Sr2þ (ionic radius: 0.118 nm) sites in SrTiO3 lattice. However, the diffraction peaks of the N-doped SrTiO3 and SrTiO3eU samples are very similar to those of the SrTiO3 sample. This probably is because a low content of nitrogen was doped into the SrTiO3 lattice for the N-doped SrTiO3 prepared by hightemperature nitriding, and the nitrogen element does not be incorporated into the SrTiO3 lattice for the SrTiO3eU sample obtained by solegel hydrothermal method. Fig. 2 displays the UVeVis diffuse reflectance spectra of the as-prepared samples. The band gaps of the SrTiO3, SrTiO3eU, N-doped SrTiO3, Cr-doped SrTiO3 and Cr/N-codoped SrTiO3 were estimated from the Tauc plots to be 3.2, 3.2, 3.2, 2.45 and 2.39 eV, respectively. The absorption edge of the SrTiO3eU is the same as the SrTiO3, further demonstrating that the N does not be doped into the SrTiO3 lattice by the solegel hydrothermal method. Compared to the SrTiO3, the N-doped SrTiO3 shows a weak absorption tail, indicating that a low content of nitrogen can be incorporated into the SrTiO3 lattice by the high-temperature nitriding. The absorption edge of the Cr/Ncodoped SrTiO3 exhibits further red shift comparing with Crdoped SrTiO3, probably suggesting that N was incorporated into O sites of the SrTiO3. In order to determine the oxidation state and content of Cr or N dopant in these photocatalysts, we performed the inductive coupled plasma emission spectrometer (ICP) and X-ray photoelectron spectroscopy (XPS) analysis, as listed in Table 1. From Table 1, we can find that the difference in metal element content existed between the XPS and ICP results. By XPS and ICP measurements, the difference in Sr contents was small for these as-prepared samples, but large Cr content difference was observed for the Cr-doped and Cr/N co-doped samples. As we all know, XPS results mainly show sample surface information. The ICP analysis is able to detect the element contents in bulk materials. The Cr content on the surface is much higher than that in bulk. The variation of Cr element content is attributed to that the residual Cr after hydrothermal reaction adsorbed on the surface of samples. Here we are inclined to trust that the ICP results are more close to the actual element contents for the asprepared samples. Fig. 3 shows the N 1s XPS spectra for the SrTiO3eU, N-doped SrTiO3 and Cr/N-codoped SrTiO3. The N 1s XPS peaks for the SrTiO3eU (Fig. 3a) and Cr/N-codoped SrTiO3 surfaces (Fig. 3c) are located at 406.9 eV, which can be assigned to NO 3 species [14]. NO3 may result from the surface adsorption due
Table 1 e Element ratios for the as-prepared photocatalysts analyzed by ICP and XPS. Sr* SrTiO3 SrTiO3eU N-doped SrTiO3 Cr-doped SrTiO3 Cr/N-codoped SrTiO3
Fig. 2 e UVeVis diffuse reflectance spectra of (a) SrTiO3, (b) SrTiO3eU, (c) N-doped SrTiO3, (d) Cr-doped SrTiO3 and (e) Cr/N-codoped SrTiO3. Inset shows the Tauc plots of these as-prepared samples.
a
0.83 /0.97 1.1a/0.96b 0.97a 0.85a/0.9b 0.8a/0.89b
Cr* b
0.16a/0.04b 0.17a/0.05b
Ti** 1 1 1 1 1
O*
N* a
3.06 3.07a 0.09a/0.00c 2.87a 0.11a/0.07c 3.06a 2.91a 0.18a/0.012c
*a: measured by XPS, b: measured by ICP, c: measured by XPS depth profile at 6 nm depth. **The element contents were determined by using Ti content as the reference.
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to the use of nitrate precursors such as Sr(NO3)2 and Cr(NO3)3. XPS depth profile measurements indicated that the binding energy of N in the Cr/N-codoped SrTiO3 (see Fig. 3c) and the N-doped SrTiO3 (see Fig. 3b) at a depth of 6 nm is 398.7 eV, which can be assigned to the OeN bonds [15]. The N/O ratio for the Cr/N-codoped SrTiO3 at the depth of 6 nm is about 0.004:1. This provides a direct evidence to support the argument that the N was doped into the crystal lattice of SrTiO3 for the Cr/N-codoped SrTiO3. However, no any peaks in N1s spectrum for the SrTiO3eU at a depth of 6 nm can be detected, further demonstrating that the N element cannot be incorporated into the crystal lattice of SrTiO3 in absent of Cr by solegel hydrothermal route. For N-doped SrTiO3, the N/O ratio at the depth of 6 nm is 0.02:1, further indicating that for SrTiO3 the N doping was difficult. The above-mentioned results indicate that the Cr-doped and Cr/N-codoped SrTiO3, but not N-doped SrTiO3, were prepared by solegel hydrothermal method. An interesting is to understand why the formation of the N-doped SrTiO3 is difficult compared to the Cr-doped and Cr/N-codoped SrTiO3. Theoretical calculation was performed to study the defect formation energy of the doped SrTiO3 samples, according to the following equations: Ef ¼ Edoped þ mo Eundoped mN ðfor N doped SrTiO3 Þ
(1)
Ef ¼ Edoped þ mSr Eundoped mCr ðfor Cr doped SrTiO3 Þ
(2)
Ef ¼Edoped þ mo þ mSr Eundoped mN mCr ðfor Cr=N codoped SrTiO3 Þ
Fig. 3 e XPS spectra of N1s for (a) SrTiO3eU, (b) N-doped SrTiO3 and (c) Cr/N-codoped SrTiO3 (inset shows N1s peaks in the range of 402e396 eV).
ð3Þ
where Edoped and Eundoped are the total energies of SrTiO3 with and without dopant(s). mo, mN, mSr and mCr are the chemical potentials of O, N, Sr and Cr, respectively. mo and mN are the energies of O and N atom taken from the energies of molecular O2 and N2. mSr and mCr are calculated from the bulk Sr and Cr crystal. It is noted that the substitution doping is energetically more favorable as the Ef value becomes smaller. The calculated defect formation energy for N-doped SrTiO3, Cr-doped SrTiO3 and Cr/N-codoped SrTiO3 was 5.876, 0.58 and 0.906 eV, respectively. Obviously, the energy cost of N-doped SrTiO3 is about 10 and 6.5 times larger than that of Cr-doped SrTiO3 and Cr/Ncodoped SrTiO3, respectively, indicating that the Cr doping or Cr/N codoping occurs easily. Compared to Cr doping into SrTiO3 the energy cost for Cr/N codoping into SrTiO3 presents an increase of about 50%. Obviously, this increase results from the N introduction into the Cr-doped SrTiO3, indicating that the Cr/ N codoping is slightly difficult than the Cr doping. However, compared with the N doping, the Cr/N codoping occurs easily due to a very low energy cost. This means that the Cr doping into SrTiO3 can efficiently promote the N doping. Photocatalytic properties of the as-prepared samples were investigated by visible light illuminating the Pt-loaded photocatalysts dispersed in the aqueous CH3OH solution, and the results are shown in Table 2. Note that the unloaded asprepared samples exhibited the neglectable photocatalytic activity in H2 production from water splitting, probably due to the serious recombination of photogenerated electron and hole pairs. By loading Pt as cocatalyst, the N-doped, Cr-doped and Cr/N-codoped SrTiO3 samples exhibited the photocatalytic activity for hydrogen evolution under visible light
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Table 2 e Physical and photocatalytic properties of the as-prepared samples.
SrTiO3 SrTiO3eU N-doped SrTiO3 Cr-doped SrTiO3 Cr/N-codoped SrTiO3
Grain size* (nm)
SBET (m2 g1)
Eg (eV)
H2 evolution rate** (mmol h1)
100e150b 100e150b >100 35.5a/40b 31.2a/40b
17.73 11.25 14.12 33.27 33.97
3.2 3.2 3.2 2.45 2.39
1.2 82.6 106.7
*a: grain size of Cr-doped SrTiO3 and Cr,N-codoped SrTiO3 were estimated by using the Scherrer equation according to XRD data. b: grain sizes of all samples obtained through the SEM images. **Photocatalytic H2 evolution was performed in aqueous CH3OH solution (50 mL of CH3OH þ 220 mL of H2O) over 0.5 wt.% Pt-loaded photocatalysts under visible light irradiation (l 420 nm) of 300 W Xe lamp.
irradiation because the Pt loading can efficiently promote separation of the photogenerated electronehole pairs [16]. The Pt-loaded SrTiO3 and SrTiO3eU samples had no photocatalytic activity under visible light irradiation, due to their wide band gap of 3.2 eV. The H2 evolution rate over the N-doped SrTiO3 is very low to be 1.17 mmol h1 (see supporting information Fig. S6) due to the weak visible light absorption, resulting from the weak absorption tail in visible light region when a little amount of N element was infiltrated into SrTiO3. The H2 evolution rate over 0.5 wt.% Pt-loaded Cr/N-codoped SrTiO3 could reach 106.7 mmol h1, which is 28% higher than 82.6 mmol h1 over the 0.5 wt.% Pt-loaded Cr-doped SrTiO3 sample. The results demonstrate that codoping with Cr and N greatly improves the photocatalytic activity under visible light irradiation. The wavelength dependence of H2 evolution rate over 0.5 wt.% Pt-loaded Cr/N-codoped SrTiO3 was carried out and the results were shown in Fig. 4. The H2 evolution rate increased with a decrease of the incident light wavelength, confirming that the H2 evolution inherently results from photocatalytic reaction. It is of importance to understand the enhancement in photocatalytic activity of the Cr/N-codoped sample comparing with the Cr-doped sample. Many factors, such as crystalline size, specific surface area, and crystal defects, affect the photocatalytic activity of catalysts. SEM was used to study the microstructure of the as-prepared samples (as shown in Fig. 5). From the Fig. 5, we can see that the particle sizes for SrTiO3 and SrTiO3eU are about 100e150 nm. Comparing shows that the particle sizes of the Cr/N-codoped and Cr-doped SrTiO3 to be about 30 nm, which is in good agreement with the XRD results calculated by the Scherrer equation, are smaller than those of the SrTiO3 and SrTiO3eU samples. This is because many crystal defects may be formed when the dopant ions occupy regular lattice sites of SrTiO3. As a result, the formed defects inhibit the growth of the crystal, and therefore decreasing the crystal size [17]. The specific surface area calculated from the linear region of the BrunauereEmmetteTeller (BET) plots ranging from P/Po ¼ 0.05 to P/Po ¼ 0.15 was listed in Table 2. We can know that the Cr/Ncodoped and Cr-doped SrTiO3 samples have the same specific surface area to be about 33 m2 g1, which is about two times higher than the SrTiO3. The porosity was calculated using the formula q ¼ V1/(V1þV2) (q: porosity, V1: total pore volume, obtained by using the BarreteJoynereHalenda (BJH) model via nitrogen physisorption, V2: the bulk volume of the sample, obtained by the equation: V ¼ m/r, m and r are the mass and density of samples, respectively). The calculated results
indicate that the porosity is about 1.6 times higher for Crdoped SrTiO3 (porosity: 0.41) than Cr/N-codoped SrTiO3 (porosity: 0.25). From these results, we can conclude that microstructure and specific surface area are not the mainly reasons for higher photocatalytic activity for Cr/N-codoped SrTiO3 than Cr-doped SrTiO3. As is well-known, doping elements into the semiconductor materials always bring the crystal defects, which may influence the photocatalytic activity. For Cr-doped SrTiO3 and Cr/ N-codoped SrTiO3, the defect reactions are shown in Eqs. (4) and (5), respectively. SrTiO3
Cr2 O3 / 2CrSr þ 3Oo þ V00Sr SrTiO3
CrN / CrSr þ N0O
(4) (5)
where CrSr means Sr2þ was replaced by Cr3þ, with a positive charge. V00Sr is Sr vacancy with two negative charges, Oo means oxygen on oxygen site. N0O means O2 was replaced by N3, with a negative charge. These vacancy defects in Cr-doped SrTiO3 will act as the recombination centers of photogenerated electronehole pairs. For the Cr/N-codoped SrTiO3, the charge balance was maintained without forming vacancies, because O2 was substituted by N3, while Cr3þ replaced the Sr2þ. The less crystal defects in the Cr/N-codoped SrTiO3 are beneficial for suppressing the recombination of
Fig. 4 e Wavelength dependence of H2 evolution rate over Cr/N-codoped SrTiO3 photocatalyst. Light source: a 300 W Xe lamp (operated at 20 A). Reaction time: 2 h. Cocatalyst: Pt (0.5 wt.%).
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Fig. 5 e SEM images of (a) SrTiO3, (b) SrTiO3eU, (c) Cr/N-codoped SrTiO3, and (d) Cr-doped SrTiO3.
photogenerated electronehole pairs, thus improving the photocatalytic activities. Another possible reason for the enhancement in photocatalytic activity of the Cr/N-codoped sample can be attributed to the band gap narrowing. Valence band X-ray photoelectron spectroscopy (VB XPS) was used to investigate the electronic structure of the as-prepared samples. Fig. 6 shows the VB XPS spectra of the SrTiO3, Cr-doped and Cr/Ncodoped SrTiO3. The VB edge of Cr-doped SrTiO3 shifted toward negative energy than that of SrTiO3, meaning that the VB top of Cr-doped SrTiO3 is indeed lifted up by Cr3þ level,
Correspondingly, a new absorption edge around 506 nm (2.45 eV) is formed in Cr-doped SrTiO3 sample (Fig. 2), which could be primarily ascribed to the excitation from the occupied Cr 3d orbitals to the Ti 3d orbitals. When the N element was incorporated into the O sites in the Cr-doped SrTiO3, the electronic structure of Cr-doped SrTiO3 was further narrowed by N 2p hybridizing with the O 2p [18]. In addition, the excitation was not only from the occupied Cr 3d orbitals to the Ti 3d orbitals, but also from the N 2p to the Ti 3d orbitals. Thus the absorption of visible light is stronger. Indeed, as observed by UVeVis spectra (Fig. 2), the Cr/N codoped SrTiO3 sample clearly shows considerable absorption of visible light in the wavelength range up to about 550 nm. According to the VB XPS and UVeVis diffuse reflectance spectra analysis, the schematic electronic band structures of SrTiO3, Cr-doped SrTiO3 and Cr/N-codoped SrTiO3 were drawn and shown in
Fig. 6 e VB-XPS spectra of (a) SrTiO3, (b) Cr-doped SrTiO3 and (c) Cr/N-codoped SrTiO3.
Fig. 7 e The electronic band structures for (a) SrTiO3, (b) Crdoped SrTiO3 and (c) Cr/N-codoped SrTiO3.
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results indicated that the incorporation of Cr is in favor of N-doping in the preparation process of Cr/N-codoped SrTiO3. Under visible light irradiation, the Cr/N-codoped SrTiO3 photocatalyst exhibits higher photocatalytic activities for hydrogen production than Cr-doped SrTiO3, which can be attributed to that the Cr/N-codoped SrTiO3 has smaller band gap and much less vacancy defects than Cr-doped SrTiO3.
Acknowledgments
Fig. 8 e Recycle course of photocatalytic H2 evolution from the aqueous CH3OH solution (50 mL of CH3OH D 220 mL of H2O) over Cr/N-codoped SrTiO3 photocatalyst under visible light irradiation (l ‡ 420 nm). Before the next cycle of reaction, the H2 gas evolved in the last cycle was evacuated from the reaction cell. Light source: a 300 W Xe lamp; cocatalyst: 0.5 wt.% Pt.
Fig. 7. The valence band top of SrTiO3 is made up predominately of the O 2p states, and its conduction band bottom is determined by the Ti 3d states [19].When Cr3þ was doped into Sr2þ sites in SrTiO3, the occupied Cr3þ level is about 1.0 eV higher than the valence band top formed by Ti 3d [8]. Therefore, the band gap for the Cr-doped SrTiO3 became small. The electronic band structure of the Cr/N-codoped SrTiO3 is further narrowed compared to the Cr-doped SrTiO3, because the N elements also affected the electronic structure by the orbital hybridization of N 2p and O 2p. The expanding light absorption region can effectively improve the photocatalytic activity of Cr/N-codoped SrTiO3. The stability of catalyst is a crucial factor for the practical applications. Fig. 8 shows the recycle course of H2 evolution over Cr/N-codoped SrTiO3 photocatalyst under visible light irradiation (l 420 nm). About 20% decrease in H2 evolution rate was observed after the three reaction cycles. During the process of photocatalytic H2 evolution, no N2 evolution was detected and the XPS spectrum of Cr 2p was not changed (see supporting information Fig. S2), indicating that the asprepared photocatalyst has good stability. The molar amount of photocatalyst used in the reaction was 1.38 mmol. The overall molar amount of H2 evolved during the three cycles reaction was 3.3 mmol. In terms of the reacted electrons to the amount of Cr/N-codoped SrTiO3 photocatalyst, the turnover number reached over 2.4, further indicating that the H2 evolution inherently resulted from the photocatalytic reaction. The apparent quantum yield was determined as described in Experimental Section to be 3.1% at l ¼ 420 nm.
4.
Conclusions
In summary, we have obtained Cr-doped and Cr/N-codoped SrTiO3 photocatalyst by solegel hydrothermal method. Our
This work is supported by the National Natural Science Foundation of China (Nos. 51102132, 11174129 and 21073090), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Jiangsu Provincial Science and Technology Research Program (BK2011056).
Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. ijhydene.2012.05.097.
references
[1] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37e8. [2] Chen XB, Shen SH, Mao SS. Semiconductor-based photocatalytichydrogen generation. Chem Rev 2010;110: 6503e70. [3] Wang JS, Yin S, Komatsua M, Zhang Q, Saito F, Sato T. Preparation and characterization of nitrogen doped SrTiO3 photocatalyst. J Photochem Photobio A Chem 2004;165: 149e58. [4] Konta R, Ishii T, Kato H, Kudo A. Photocatalytic activities of noble metal ion doped SrTiO3 under visible light irradiation. J Phys Chem B 2004;108:8992e5. [5] Wei W, Dai Y, Guo M, Huang BB. Density functional characterization of the electronic structure and optical properties of N-doped, La-doped and N/La-codoped SrTiO3. J Phys Chem C 2009;113:15046e50. [6] Kato H, Kudo A. Visible-light-response and photocatalytic activities of TiO2 and SrTiO3 photocatalysts codoped with antimony and chromium. J Phys Chem B 2002;106:5029e34. [7] Ishii T, Kat H, Kudo A. H2 evolution from an aqueous methanol solution on SrTiO3 photocatalysts codoped with chromium and tantalum ions under visible light irradiation. J Photochem Photobi A Chem 2004;163:181e6. [8] Yu H, Ouyang SX, Yan SC, Li ZS, Zou ZG. Solegel hydrothermal synthesis of visible-light-driven Cr-doped SrTiO3 for efficient hydrogen production. J Mater Chem 2011; 21:11347e51. [9] Hwang DW, Kim HG, Lee JS, Oh SH. Photocatalytic hydrogen production from water over M-doped La2Ti2O7 (M ¼ Cr, Fe) under visible light irradiation (l > 420 nm). J Phys Chem B 2005;109:2093e102. [10] Miyauchi M, Takashio M, Tobimatsu H. Photocatalytic activity of SrTiO3 codoped with nitrogen and lanthanum under visible light illumination. Langmuir 2004;20:232e6.
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[11] Wang JS, Yin S, Komatsu M, Sato T. Lanthanum and nitrogen co-doped SrTiO3 powders as visible light sensitive photocatalyst. J Eur Ceram Soc 2005;25:3207e12. [12] Siemon U, Bahnemann D, Testa Juan J, Rodrıguez D, Litter Marta I, Bruno N. Heterogeneous photocatalytic reactions comparing TiO2 and Pt/TiO2. J Photochem Photobio A Chem 2002;148:247e55. [13] Segal MD, Lindan Philip JD, Probert MJ, Pickard CJ, Hasnip PJ, Clark SJ, et al. First-principles simulation: ideas, illustrations and the CASTEP code. J Phys Condens Matter 2002;14: 2717e44. [14] Su WY, Chen JX, Wu L, Fu XZ. Visible light photocatalysis on praseodymium(III)-nitrate-modified TiO2 prepared by an ultrasound method. Appil Catal B Environ 2008;77:264e71.
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[15] Liu CM, Zu XT, Zhou WL. Photoluminescence of nitrogen doped SrTiO3. J Phys D Appl Phys 2007;40:7318e22. [16] Nosaka Y, Norimatsu K, Miyama H. The function of metals in metal-compounded semiconductor photocatalysts. Chem Phys Lett 1984;106:128e31. [17] Qin Y, Wang GY, Wang YJ. Study on the photocatalytic property of La-doped CoO/SrTiO3 for water decomposition to hydrogen. Catal Commun 2007;8:926e30. [18] Yang K, Dai Y, Huang BB. Study of the nitrogen concentration influence on N-doped TiO2 anatase from first-principles calculations. J Phys Chem C 2007;111:12086e90. [19] Mo SD, Ching WY, Chisholm MF, Duscher G. Electronic structure of a grain-boundary model in SrTiO3. Phys Rev B 1999;60:2416e24.
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Efficient visible-light-driven photocatalytic H2 production over Cr/N-codoped SrTiO3 He Yu a,c, Shicheng Yan a,b,*, Zhaosheng Li a,b, Tao Yu a,c, Zhigang Zou a,b,c,* a
National Laboratory of Solid State Microstructure, Nanjing University, 210093, PR China Eco-materials and Renewable Energy Research Center (ERERC), College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, PR China c Department of Physics, Nanjing University, 210093, PR China b
article info
abstract
Article history:
Visible-light-response Cr/N-codoped SrTiO3 was prepared by a solegel hydrothermal
Received 13 March 2012
method. The comparison studies indicate that Cr-doped and Cr/N-codoped SrTiO3 can be
Received in revised form
synthesized by this means, but not the N-doped SrTiO3. The theoretical calculations exhibit
14 May 2012
the defect formation energy of the Cr/N codoping into SrTiO3 is much smaller than that of
Accepted 18 May 2012
the N doping into SrTiO3, illuminating that the incorporation of Cr can promote the N
Available online 12 July 2012
doping into the O sites in the SrTiO3. Compared to the Cr-doped SrTiO3, the Cr/N-codoped
Keywords:
the quantum efficiency of 3.1% at 420 nm, due to the smaller band gap and much less
Photocatalysis
vacancy defects.
SrTiO3
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
SrTiO3 photocatalyst shows the high photocatalytic activities for hydrogen production with
Visible light response
reserved.
Water splitting
1.
Introduction
Hydrogen, as an environmentally friendly fuel, is attracting more and more attention. Since the production of hydrogen through photoinduced water splitting on titanium oxide was discovered [1], semiconductor-based photocatalysis has prompted many investigations, because this technique makes use of abundant, long lasting and clean solar energy. In order to construct an efficient photocatalytic water splitting reaction, some photocatalytic materials, such as oxides, oxynitrides and sulfides, have been explored [2]. To apply the photocatalysis technique, an ideal photocatalyst is expected to be having good photostability, having a narrow band gap to achieve the efficient use of solar energy, and having highly
efficiency in separating, collecting and transporting charges for the chemical processes. SrTiO3 is a promising perovskite-type photocatalyst for water decomposition. However, SrTiO3 has a wide band gap of 3.2 eV, only responds to ultraviolet light, which accounts for only 4% of the incoming solar energy. Element doping usually is an efficient route to broaden the light absorption of the wide-band-gap semiconductors into the visible light region, which possesses 43% of the solar energy. To obtain the visible light response, doping with foreign elements into SrTiO3 has been studied by many research groups [3e7]. Some threevalence metal ions, such as Rh3þ, Ir3þ and Ru3þ, were incorporated into the Ti4þ sites of SrTiO3. The strong hybridization of orbital of the doping elements with the O 2p states leads to
* Corresponding authors. National Laboratory of Solid State Microstructure, Nanjing University, 210093, PR China. Tel.: þ86 25 83686630; fax: þ86 25 83686632. E-mail addresses: [email protected] (S. Yan), [email protected] (Z. Zou). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.05.097
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the band gap narrowing of SrTiO3. Recently, the Cr3þ doping into the Sr2þ sites of SrTiO3 induces the visible light response due to the formation of new valence band by the occupied Cr3þ level, which is about 1.0 eV higher than the valence band top formed by O 2p orbital [8]. However, doping by only one element with different valence from the hosts usually induces the low photocatalytic activities, probably due to the formation of defects in the crystal, for example the oxygen vacancy, which usually acts as recombination center of the photogenerated electrons and holes [9]. Codoping may be a promising method to suppress the formation of defects, and thus improving the activities of photocatalysts. Kato and Kudo found that the codoping of Sb or Ta and Cr into SrTiO3 showed a quite high activity in H2 production. This can be attributed to that the charge balance was kept by codoping of Sb5þ or Ta5þ and Cr3þ in Ti4þ sites of SrTiO3 [6,7]. Another efficient method to decrease the formation of defects is codoping by anion and cation metals [10,11]. The high activity of La/N-codoped SrTiO3 was attributed to the decrease in the oxygen vacancies because codoping of La3þ ions in Sr2þ sites and N3 in O2 sites maintained the charge balance. For La/N-codoped SrTiO3, the visible light response results from the mixing of the N 2p states with the O 2p states. La element did not contribute to the formation of the band gap of SrTiO3. Here, we report the Cr/N-codoped SrTiO3, both the Cr and N doping not only maintained the charge balance in SrTiO3, but also contributed to the band gap narrowing of SrTiO3. The high photocatalytic activity in water splitting was achieved due to the visible light absorption and less crystal defects in SrTiO3.
2.
Experimental
The Cr/N-codoped SrTiO3 powder samples were synthesized by a solegel hydrothermal method. Analytical grade Sr(NO3)2, Cr(NO3)3$9H2O, Ti(C4H9O)4, CO(NH2)2, NaOH and ethylene glycol (EG) were used as raw materials without further purification. In the synthesis process of the Cr/N-codoped SrTiO3, firstly, Sr(NO3)2(0.0095 mol), Cr(NO3)3$9H2O (0.005 mol) and CO(NH2)2 (0.01 mol) were dissolved in EG (20 mL), and heated to 80 C under continuously stirring. Subsequently, Ti(C4H9O)4 (0.01 mol) was dropped into the above-mentioned solution to form a sol. With the evaporation of EG, a gel was obtained. Then 30 mL of NaOH solution (5 M) was added into the gel. After stirred a while, the mixed solution was put into a 40 mL Teflon-lined stainless autoclave. The autoclave was heated up from room temperature to 200 C and maintained for 24 h. The obtained precipitate was washed with distilled water until the pH value was 7e8, and then dried at 80 C overnight. As a comparison, the undoped SrTiO3, Cr-doped SrTiO3 and N-doped SrTiO3 by using urea as the nitrogen source (denoted as SrTiO3eU) were synthesized by this means in presence of Sr(NO3)2 (0.01 mol) and Ti(C4H9O)4 (0.01 mol), in presence of Cr(NO3)3$9H2O (0.005 mol), Sr(NO3)2 (0.0095 mol) and Ti(C4H9O)4 (0.01 mol), and in presence of Sr(NO3)2 (0.01 mol), Ti(C4H9O)4 (0.01 mol) and CO(NH2)2 (0.01 mol), respectively. In addition, N-doped SrTiO3 was synthesized by nitriding pristine SrTiO3 at 900 C for 10 h in a tube furnace with NH3 flow rate at 200 mL min1.
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The crystalline phases of the as-prepared samples were identified by a powder X-ray diffraction (XRD, Ultima III, ˚ , 40 kV, Rigaku Corp., Japan) using Cu Ka radiation (l ¼ 1.54178 A 40 mA) with a scan rate of 10 min1. The diffuse reflectance spectra were obtained using a UVevisible spectrophotometer (Shimadzu Corp., UV-2500PC, Japan) and X-ray photoelectron spectroscopy (XPS) was carried out on Thermo ESCALAB 250 spectrometer using monochromated Al Ka (1486.6 eV) source operated at 110 W, and the photoelectrons were detected by a hemispherical analyzer operating at a pass energy of 20 eV. The binding energy was calibrated on the reference C 1s peak at 284.8 eV. The morphologies of the as-prepared samples were observed by scanning electron microscopy (SEM). Photocatalytic reactions for H2 evolution were conducted over the Pt-loaded photocatalyst using CH3OH as the sacrificial reagent. The Pt cocatalyst was loaded on the catalyst surface by an in situ photodeposition method [12]. The Pt-loaded photocatalyst powders (0.25 g) were dispersed in the aqueous CH3OH solution (50 mL of CH3OH þ 220 mL of H2O) in an outer irradiation Pyrex glass cell. A 300 W Xe arc lamp was focused on the side window of the cell with a long-pass cutoff filter (l 420 nm, L42, HOYA) to achieve the visible light. The reaction cell was connected to a closed gas circulation system. Amount of the evolved gas was determined using gas chromatograph (Shimadzu GC-8A, argon carrier). Apparent quantum yields were obtained by the equation QY (%) ¼ Ne/Np ¼ 2NH2/Np, where Ne is the number of reacted electrons, Np is the number of incident photons, and NH2 is the number of evolved H2 molecules. Np can be calculated from the light intensity which was measured with a spectroradiometer to be 2.15 mV cm2. NH2 is determined by using a 420 nm band-pass filter. In this study, the first-principle density functional theory calculations were carried out with plane wave pseudopotential method as implemented in CASTEP [13]. The electronic wave functions were expanded in terms of a discrete plane wave basis set. The exchange and correlation interactions were modeled by the generalized gradient approximation (GGA) with PerdeweBurkeeErnzerhof (PBE) scheme. Interaction between electrons and ion core was replaced by ultra-soft pseudopotential. The k-space integrations were performed with Monkhorst-Pack grid with 4 4 4 k-points in the Brillouin zone to obtain the accurate electronic density. The kinetic energy cutoff for wave function expansion was 340 eV. Geometry optimization was performed before electronic and total energy calculations and the self-consistent convergence accuracy was set at 5 105 eV atom1. The convergence ˚ 1, the criterion of the largest force on atoms was 0.1 eV A 4 maximum displacement was 5 10 nm and the stress was not more than 0.2 GPa. All calculations were based upon a 2 2 2 SrTiO3 supercell with a size of 40-atom. We substituted a nitrogen atom for an oxygen atom to simulate the nitrogen doping. In the case of Cr/N-codoping, we substituted a nitrogen atom and a chromium atom for an oxygen atom and a strontium atom, respectively.
3.
Results and discussion
Fig. 1 shows that the XRD patterns of all samples can be indexed as the cubic SrTiO3 (JCPDS No. 89-4934). This implied
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Fig. 1 e XRD patterns of (a) SrTiO3, (b) SrTiO3eU, (c) N-doped SrTiO3, (d) Cr-doped SrTiO3 and (e) Cr/N-codoped SrTiO3. Inset shows the (110) diffraction peak positions in the range of 2q [ 32e33 .
that doping with foreign elements in SrTiO3 did not introduce impurity phases. A careful comparison of the (110) diffraction peaks in the range of 2q ¼ 32e33 (the inset of Fig. 1) shows that the SrTiO3, SrTiO3eU and N-doped SrTiO3 have the same peak position, but the peak positions of the Cr-doped SrTiO3 and Cr/N-codoped SrTiO3 slightly shifted toward a higher 2q value. XPS analysis shows that the Cr cation is mainly present as Cr3þ in the Cr-doped or Cr/N-codoped SrTiO3 (see supporting information Fig. S1) [7]. If substituting Cr3þ cations for Ti4þ sites in SrTiO3 lattice, the (110) diffraction peaks for XRD pattern should shift toward a lower 2q value or did not shift because the ionic radius of the Cr3þ (0.0615 nm) is slightly larger than that of Ti4þ (0.0605 nm). Therefore the shift of the (110) diffraction peak (see the inset of Fig. 1) resulted from that
the Cr3þ (ionic radius: 0.0615 nm) was incorporated into the Sr2þ (ionic radius: 0.118 nm) sites in SrTiO3 lattice. However, the diffraction peaks of the N-doped SrTiO3 and SrTiO3eU samples are very similar to those of the SrTiO3 sample. This probably is because a low content of nitrogen was doped into the SrTiO3 lattice for the N-doped SrTiO3 prepared by hightemperature nitriding, and the nitrogen element does not be incorporated into the SrTiO3 lattice for the SrTiO3eU sample obtained by solegel hydrothermal method. Fig. 2 displays the UVeVis diffuse reflectance spectra of the as-prepared samples. The band gaps of the SrTiO3, SrTiO3eU, N-doped SrTiO3, Cr-doped SrTiO3 and Cr/N-codoped SrTiO3 were estimated from the Tauc plots to be 3.2, 3.2, 3.2, 2.45 and 2.39 eV, respectively. The absorption edge of the SrTiO3eU is the same as the SrTiO3, further demonstrating that the N does not be doped into the SrTiO3 lattice by the solegel hydrothermal method. Compared to the SrTiO3, the N-doped SrTiO3 shows a weak absorption tail, indicating that a low content of nitrogen can be incorporated into the SrTiO3 lattice by the high-temperature nitriding. The absorption edge of the Cr/Ncodoped SrTiO3 exhibits further red shift comparing with Crdoped SrTiO3, probably suggesting that N was incorporated into O sites of the SrTiO3. In order to determine the oxidation state and content of Cr or N dopant in these photocatalysts, we performed the inductive coupled plasma emission spectrometer (ICP) and X-ray photoelectron spectroscopy (XPS) analysis, as listed in Table 1. From Table 1, we can find that the difference in metal element content existed between the XPS and ICP results. By XPS and ICP measurements, the difference in Sr contents was small for these as-prepared samples, but large Cr content difference was observed for the Cr-doped and Cr/N co-doped samples. As we all know, XPS results mainly show sample surface information. The ICP analysis is able to detect the element contents in bulk materials. The Cr content on the surface is much higher than that in bulk. The variation of Cr element content is attributed to that the residual Cr after hydrothermal reaction adsorbed on the surface of samples. Here we are inclined to trust that the ICP results are more close to the actual element contents for the asprepared samples. Fig. 3 shows the N 1s XPS spectra for the SrTiO3eU, N-doped SrTiO3 and Cr/N-codoped SrTiO3. The N 1s XPS peaks for the SrTiO3eU (Fig. 3a) and Cr/N-codoped SrTiO3 surfaces (Fig. 3c) are located at 406.9 eV, which can be assigned to NO 3 species [14]. NO3 may result from the surface adsorption due
Table 1 e Element ratios for the as-prepared photocatalysts analyzed by ICP and XPS. Sr* SrTiO3 SrTiO3eU N-doped SrTiO3 Cr-doped SrTiO3 Cr/N-codoped SrTiO3
Fig. 2 e UVeVis diffuse reflectance spectra of (a) SrTiO3, (b) SrTiO3eU, (c) N-doped SrTiO3, (d) Cr-doped SrTiO3 and (e) Cr/N-codoped SrTiO3. Inset shows the Tauc plots of these as-prepared samples.
a
0.83 /0.97 1.1a/0.96b 0.97a 0.85a/0.9b 0.8a/0.89b
Cr* b
0.16a/0.04b 0.17a/0.05b
Ti** 1 1 1 1 1
O*
N* a
3.06 3.07a 0.09a/0.00c 2.87a 0.11a/0.07c 3.06a 2.91a 0.18a/0.012c
*a: measured by XPS, b: measured by ICP, c: measured by XPS depth profile at 6 nm depth. **The element contents were determined by using Ti content as the reference.
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to the use of nitrate precursors such as Sr(NO3)2 and Cr(NO3)3. XPS depth profile measurements indicated that the binding energy of N in the Cr/N-codoped SrTiO3 (see Fig. 3c) and the N-doped SrTiO3 (see Fig. 3b) at a depth of 6 nm is 398.7 eV, which can be assigned to the OeN bonds [15]. The N/O ratio for the Cr/N-codoped SrTiO3 at the depth of 6 nm is about 0.004:1. This provides a direct evidence to support the argument that the N was doped into the crystal lattice of SrTiO3 for the Cr/N-codoped SrTiO3. However, no any peaks in N1s spectrum for the SrTiO3eU at a depth of 6 nm can be detected, further demonstrating that the N element cannot be incorporated into the crystal lattice of SrTiO3 in absent of Cr by solegel hydrothermal route. For N-doped SrTiO3, the N/O ratio at the depth of 6 nm is 0.02:1, further indicating that for SrTiO3 the N doping was difficult. The above-mentioned results indicate that the Cr-doped and Cr/N-codoped SrTiO3, but not N-doped SrTiO3, were prepared by solegel hydrothermal method. An interesting is to understand why the formation of the N-doped SrTiO3 is difficult compared to the Cr-doped and Cr/N-codoped SrTiO3. Theoretical calculation was performed to study the defect formation energy of the doped SrTiO3 samples, according to the following equations: Ef ¼ Edoped þ mo Eundoped mN ðfor N doped SrTiO3 Þ
(1)
Ef ¼ Edoped þ mSr Eundoped mCr ðfor Cr doped SrTiO3 Þ
(2)
Ef ¼Edoped þ mo þ mSr Eundoped mN mCr ðfor Cr=N codoped SrTiO3 Þ
Fig. 3 e XPS spectra of N1s for (a) SrTiO3eU, (b) N-doped SrTiO3 and (c) Cr/N-codoped SrTiO3 (inset shows N1s peaks in the range of 402e396 eV).
ð3Þ
where Edoped and Eundoped are the total energies of SrTiO3 with and without dopant(s). mo, mN, mSr and mCr are the chemical potentials of O, N, Sr and Cr, respectively. mo and mN are the energies of O and N atom taken from the energies of molecular O2 and N2. mSr and mCr are calculated from the bulk Sr and Cr crystal. It is noted that the substitution doping is energetically more favorable as the Ef value becomes smaller. The calculated defect formation energy for N-doped SrTiO3, Cr-doped SrTiO3 and Cr/N-codoped SrTiO3 was 5.876, 0.58 and 0.906 eV, respectively. Obviously, the energy cost of N-doped SrTiO3 is about 10 and 6.5 times larger than that of Cr-doped SrTiO3 and Cr/Ncodoped SrTiO3, respectively, indicating that the Cr doping or Cr/N codoping occurs easily. Compared to Cr doping into SrTiO3 the energy cost for Cr/N codoping into SrTiO3 presents an increase of about 50%. Obviously, this increase results from the N introduction into the Cr-doped SrTiO3, indicating that the Cr/ N codoping is slightly difficult than the Cr doping. However, compared with the N doping, the Cr/N codoping occurs easily due to a very low energy cost. This means that the Cr doping into SrTiO3 can efficiently promote the N doping. Photocatalytic properties of the as-prepared samples were investigated by visible light illuminating the Pt-loaded photocatalysts dispersed in the aqueous CH3OH solution, and the results are shown in Table 2. Note that the unloaded asprepared samples exhibited the neglectable photocatalytic activity in H2 production from water splitting, probably due to the serious recombination of photogenerated electron and hole pairs. By loading Pt as cocatalyst, the N-doped, Cr-doped and Cr/N-codoped SrTiO3 samples exhibited the photocatalytic activity for hydrogen evolution under visible light
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Table 2 e Physical and photocatalytic properties of the as-prepared samples.
SrTiO3 SrTiO3eU N-doped SrTiO3 Cr-doped SrTiO3 Cr/N-codoped SrTiO3
Grain size* (nm)
SBET (m2 g1)
Eg (eV)
H2 evolution rate** (mmol h1)
100e150b 100e150b >100 35.5a/40b 31.2a/40b
17.73 11.25 14.12 33.27 33.97
3.2 3.2 3.2 2.45 2.39
1.2 82.6 106.7
*a: grain size of Cr-doped SrTiO3 and Cr,N-codoped SrTiO3 were estimated by using the Scherrer equation according to XRD data. b: grain sizes of all samples obtained through the SEM images. **Photocatalytic H2 evolution was performed in aqueous CH3OH solution (50 mL of CH3OH þ 220 mL of H2O) over 0.5 wt.% Pt-loaded photocatalysts under visible light irradiation (l 420 nm) of 300 W Xe lamp.
irradiation because the Pt loading can efficiently promote separation of the photogenerated electronehole pairs [16]. The Pt-loaded SrTiO3 and SrTiO3eU samples had no photocatalytic activity under visible light irradiation, due to their wide band gap of 3.2 eV. The H2 evolution rate over the N-doped SrTiO3 is very low to be 1.17 mmol h1 (see supporting information Fig. S6) due to the weak visible light absorption, resulting from the weak absorption tail in visible light region when a little amount of N element was infiltrated into SrTiO3. The H2 evolution rate over 0.5 wt.% Pt-loaded Cr/N-codoped SrTiO3 could reach 106.7 mmol h1, which is 28% higher than 82.6 mmol h1 over the 0.5 wt.% Pt-loaded Cr-doped SrTiO3 sample. The results demonstrate that codoping with Cr and N greatly improves the photocatalytic activity under visible light irradiation. The wavelength dependence of H2 evolution rate over 0.5 wt.% Pt-loaded Cr/N-codoped SrTiO3 was carried out and the results were shown in Fig. 4. The H2 evolution rate increased with a decrease of the incident light wavelength, confirming that the H2 evolution inherently results from photocatalytic reaction. It is of importance to understand the enhancement in photocatalytic activity of the Cr/N-codoped sample comparing with the Cr-doped sample. Many factors, such as crystalline size, specific surface area, and crystal defects, affect the photocatalytic activity of catalysts. SEM was used to study the microstructure of the as-prepared samples (as shown in Fig. 5). From the Fig. 5, we can see that the particle sizes for SrTiO3 and SrTiO3eU are about 100e150 nm. Comparing shows that the particle sizes of the Cr/N-codoped and Cr-doped SrTiO3 to be about 30 nm, which is in good agreement with the XRD results calculated by the Scherrer equation, are smaller than those of the SrTiO3 and SrTiO3eU samples. This is because many crystal defects may be formed when the dopant ions occupy regular lattice sites of SrTiO3. As a result, the formed defects inhibit the growth of the crystal, and therefore decreasing the crystal size [17]. The specific surface area calculated from the linear region of the BrunauereEmmetteTeller (BET) plots ranging from P/Po ¼ 0.05 to P/Po ¼ 0.15 was listed in Table 2. We can know that the Cr/Ncodoped and Cr-doped SrTiO3 samples have the same specific surface area to be about 33 m2 g1, which is about two times higher than the SrTiO3. The porosity was calculated using the formula q ¼ V1/(V1þV2) (q: porosity, V1: total pore volume, obtained by using the BarreteJoynereHalenda (BJH) model via nitrogen physisorption, V2: the bulk volume of the sample, obtained by the equation: V ¼ m/r, m and r are the mass and density of samples, respectively). The calculated results
indicate that the porosity is about 1.6 times higher for Crdoped SrTiO3 (porosity: 0.41) than Cr/N-codoped SrTiO3 (porosity: 0.25). From these results, we can conclude that microstructure and specific surface area are not the mainly reasons for higher photocatalytic activity for Cr/N-codoped SrTiO3 than Cr-doped SrTiO3. As is well-known, doping elements into the semiconductor materials always bring the crystal defects, which may influence the photocatalytic activity. For Cr-doped SrTiO3 and Cr/ N-codoped SrTiO3, the defect reactions are shown in Eqs. (4) and (5), respectively. SrTiO3
Cr2 O3 / 2CrSr þ 3Oo þ V00Sr SrTiO3
CrN / CrSr þ N0O
(4) (5)
where CrSr means Sr2þ was replaced by Cr3þ, with a positive charge. V00Sr is Sr vacancy with two negative charges, Oo means oxygen on oxygen site. N0O means O2 was replaced by N3, with a negative charge. These vacancy defects in Cr-doped SrTiO3 will act as the recombination centers of photogenerated electronehole pairs. For the Cr/N-codoped SrTiO3, the charge balance was maintained without forming vacancies, because O2 was substituted by N3, while Cr3þ replaced the Sr2þ. The less crystal defects in the Cr/N-codoped SrTiO3 are beneficial for suppressing the recombination of
Fig. 4 e Wavelength dependence of H2 evolution rate over Cr/N-codoped SrTiO3 photocatalyst. Light source: a 300 W Xe lamp (operated at 20 A). Reaction time: 2 h. Cocatalyst: Pt (0.5 wt.%).
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Fig. 5 e SEM images of (a) SrTiO3, (b) SrTiO3eU, (c) Cr/N-codoped SrTiO3, and (d) Cr-doped SrTiO3.
photogenerated electronehole pairs, thus improving the photocatalytic activities. Another possible reason for the enhancement in photocatalytic activity of the Cr/N-codoped sample can be attributed to the band gap narrowing. Valence band X-ray photoelectron spectroscopy (VB XPS) was used to investigate the electronic structure of the as-prepared samples. Fig. 6 shows the VB XPS spectra of the SrTiO3, Cr-doped and Cr/Ncodoped SrTiO3. The VB edge of Cr-doped SrTiO3 shifted toward negative energy than that of SrTiO3, meaning that the VB top of Cr-doped SrTiO3 is indeed lifted up by Cr3þ level,
Correspondingly, a new absorption edge around 506 nm (2.45 eV) is formed in Cr-doped SrTiO3 sample (Fig. 2), which could be primarily ascribed to the excitation from the occupied Cr 3d orbitals to the Ti 3d orbitals. When the N element was incorporated into the O sites in the Cr-doped SrTiO3, the electronic structure of Cr-doped SrTiO3 was further narrowed by N 2p hybridizing with the O 2p [18]. In addition, the excitation was not only from the occupied Cr 3d orbitals to the Ti 3d orbitals, but also from the N 2p to the Ti 3d orbitals. Thus the absorption of visible light is stronger. Indeed, as observed by UVeVis spectra (Fig. 2), the Cr/N codoped SrTiO3 sample clearly shows considerable absorption of visible light in the wavelength range up to about 550 nm. According to the VB XPS and UVeVis diffuse reflectance spectra analysis, the schematic electronic band structures of SrTiO3, Cr-doped SrTiO3 and Cr/N-codoped SrTiO3 were drawn and shown in
Fig. 6 e VB-XPS spectra of (a) SrTiO3, (b) Cr-doped SrTiO3 and (c) Cr/N-codoped SrTiO3.
Fig. 7 e The electronic band structures for (a) SrTiO3, (b) Crdoped SrTiO3 and (c) Cr/N-codoped SrTiO3.
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results indicated that the incorporation of Cr is in favor of N-doping in the preparation process of Cr/N-codoped SrTiO3. Under visible light irradiation, the Cr/N-codoped SrTiO3 photocatalyst exhibits higher photocatalytic activities for hydrogen production than Cr-doped SrTiO3, which can be attributed to that the Cr/N-codoped SrTiO3 has smaller band gap and much less vacancy defects than Cr-doped SrTiO3.
Acknowledgments
Fig. 8 e Recycle course of photocatalytic H2 evolution from the aqueous CH3OH solution (50 mL of CH3OH D 220 mL of H2O) over Cr/N-codoped SrTiO3 photocatalyst under visible light irradiation (l ‡ 420 nm). Before the next cycle of reaction, the H2 gas evolved in the last cycle was evacuated from the reaction cell. Light source: a 300 W Xe lamp; cocatalyst: 0.5 wt.% Pt.
Fig. 7. The valence band top of SrTiO3 is made up predominately of the O 2p states, and its conduction band bottom is determined by the Ti 3d states [19].When Cr3þ was doped into Sr2þ sites in SrTiO3, the occupied Cr3þ level is about 1.0 eV higher than the valence band top formed by Ti 3d [8]. Therefore, the band gap for the Cr-doped SrTiO3 became small. The electronic band structure of the Cr/N-codoped SrTiO3 is further narrowed compared to the Cr-doped SrTiO3, because the N elements also affected the electronic structure by the orbital hybridization of N 2p and O 2p. The expanding light absorption region can effectively improve the photocatalytic activity of Cr/N-codoped SrTiO3. The stability of catalyst is a crucial factor for the practical applications. Fig. 8 shows the recycle course of H2 evolution over Cr/N-codoped SrTiO3 photocatalyst under visible light irradiation (l 420 nm). About 20% decrease in H2 evolution rate was observed after the three reaction cycles. During the process of photocatalytic H2 evolution, no N2 evolution was detected and the XPS spectrum of Cr 2p was not changed (see supporting information Fig. S2), indicating that the asprepared photocatalyst has good stability. The molar amount of photocatalyst used in the reaction was 1.38 mmol. The overall molar amount of H2 evolved during the three cycles reaction was 3.3 mmol. In terms of the reacted electrons to the amount of Cr/N-codoped SrTiO3 photocatalyst, the turnover number reached over 2.4, further indicating that the H2 evolution inherently resulted from the photocatalytic reaction. The apparent quantum yield was determined as described in Experimental Section to be 3.1% at l ¼ 420 nm.
4.
Conclusions
In summary, we have obtained Cr-doped and Cr/N-codoped SrTiO3 photocatalyst by solegel hydrothermal method. Our
This work is supported by the National Natural Science Foundation of China (Nos. 51102132, 11174129 and 21073090), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Jiangsu Provincial Science and Technology Research Program (BK2011056).
Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. ijhydene.2012.05.097.
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