Experimental and Theoretical Insights into Influence of Hydrogen and

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Experimental and Theoretical Insights into Influence of Hydrogen and Nitrogen Plasma on the Water Splitting Performance of ALD Grown TiO Thin Films 2

Alexander Sasinska, Danny Bialuschewski, Mazharul M Islam, Trilok Singh, Meenal Deo, and Sanjay Mathur J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03424 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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The Journal of Physical Chemistry

Experimental and Theoretical Insights into Influence of Hydrogen and Nitrogen Plasma on the Water Splitting Performance of ALD Grown TiO2 Thin Films Alexander Sasinska1, Danny Bialuschewski1, Mazharul M. Islam2, Trilok Singh1, Meenal Deo1, and Sanjay Mathur1, * 1) Institute of Inorganic Chemistry, University of Cologne, Greinstrasse 6, Cologne50939, Germany 2) Mulliken Center for Theoretical Chemistry, University of Bonn, Beringstrasse 4–6, Bonn-53115, Germany

Abstract Specific effects of hydrogen and nitrogen plasma treatment on anatase TiO2 photoanodes, grown via atomic layer deposition (ALD) and their respective impact on water splitting properties are reported. ALD grown TiO2 samples were exposed to reactive hydrogen and nitrogen plasmas and the photoelectrochemical efficiency of the modified samples were comparatively analyzed. Both H2 and N2 plasma treatment enhanced the photocurrent values compared to pristine TiO2. Electron density at oxygen vacancy sites was decreased upon nitrogen incorporation, reflected in band gap modulation and decreased recombination probability as confirmed by spectral data. Surface modification via H2 plasma induce more mid-gap states to augment the photoinduced charge carriers concentration that was supported by theoretical investigation performed on the electronic properties of both H and N-doped TiO2 (anatase). First principles calculations based on Hartree-Fock (HF)–Density Functional Theory (DFT) hybrid models showed good agreement with the experimental findings and confirmed

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electronic band gap reduction due to extra electron density introduced via H and N incorporation in TiO2. *Corresponding author: Prof. Dr. Sanjay Mathur ([email protected])

1. Introduction In order to fulfill world-wide growing energy demand and to reduce the CO2 emission in the atmosphere, exploration of renewable energy sources is a pressing need. A suitable energy carrier is hydrogen, which can be used in fuel cells, since it has a higher energy density than petrochemicals and moreover its combustion product is water.1 In this context, photoelectrochemical (PEC) water-splitting is a viable path to produce hydrogen, however, water electrolysis is an uphill reaction with a positive Gibbs Free Energy (228.71 kJ/mol), demanding new water oxidation catalysts. Since the early observation on the ability of titania (TiO2) to split water into hydrogen and oxygen under solar light illumination2, a vast variety of tailor made photocatalysts have been reported.3 Despite several intrinsic benefits of TiO2, such as low cost, non-toxic nature and large abundance, its applications in the conversion of solar energy into chemical energy is limited due to its unfavorable band gap (Eg > 3 eV) and limited photoconductivity.4 Therefore, research efforts have grown rapidly to develop new TiO2-based functional materials to be used in commercial photoelectrodes for solar hydrogen production.5,6 To enhance the photoelectrochemical performance of TiO2, several chemical and structural modifications have been reported.7 For instance, surface decoration with noble metal nanoparticles8,9, dye-sensitization10,11 and design of heterostructures12,13 as well as doping with either metal cations14-17 or non-metal anions18,19 have all shown band gap narrowing and thus a shift of the optical absorption into the visible range.


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Enhancement of the water splitting efficiency of titania nanoparticles via hydrogen plasma reduction by Chen et al20 as well as the findings of band gap narrowing due to nitrogen doping by Asahi et al21 indicated the potential of tuning the opto-electronic properties by appropriate modifications of TiO2 materials. Hydrogen treatment is known to reduce the TiO2 to form black TiOx, which becomes a good visible light absorber due to controlled creation of defect levels. However, the hydrogen treatment also creates electronic traps due to etching of oxygen atoms from the TiO2 lattice. Nitrogen doping in TiO2 is known to create energy levels above the valence band, possibly making it a p-type conductor and reducing its band gap.22 To the best of our knowledge, there is only one report on the simultaneous hydrogen treatment and nitridation on TiO2 nanostructures for water splitting application. Hoang et al. annealed the TiO2 nanowires in a mixture of H2 and Ar followed by annealing in ammonia atmosphere at 500 °C to shift the absorption maxima from ~550 nm to ~570 nm.23 Despite various efforts, the specific role of hydrogen and nitrogen as dopants in increasing the water splitting efficiency is not yet clearly understood. Nitrogen doping occurs predominantly by oxygen substitution that creates a charge imbalance in the lattice. Asahi et al. proposed overlapping of the N 2p states with O 2p states in the valence band, resulting in a narrowing of the optical band gap.21 Other reports claim that discrete energy levels slightly above the valence band are formed24-26, but no explanations on their water oxidation capability was given, since localized states tend to trap holes. In case of hydrogen modified TiO2 it is not clarified, whether the improved light harvesting properties arise from lattice incorporation of hydrogen due to formation of filled mid-gap states27 or due to band gap variation caused by the structural disorder.20 We report herein on the effect of hydrogen and nitrogen incorporation in ALD grown TiO2 via plasma treatment that showed specific influence on the water splitting efficiency of modified TiO2 films. The observed experimental results are compared with the theoretical investigations


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on electronic properties of the H-, N- and (H,N) co-doped anatase films by employing the range separated hybrid Heyd-Scuseria-Ernzehof (HSE06) approach.

2. Methods 2.1 Experimental Techniques: Atomic Layer Deposition of TiO2 films on FTO-coated glass substrates (10 /cm2) was performed in a BENEQ TFS 200. Titanium-tetraisopropoxide [Ti(OiPr)4] was heated to 90 °C and pulsed for 300 ms, whereas water was kept at 20 °C with 500 ms pulsing time. The purging time of the first precursor was 1 s, while the purging time of the second precursor was 2 s. All depositions have been carried out at 120 °C reactor temperature for 2000 cycles.28 All samples were annealed for 3 hours at 500 °C under ambient conditions. Plasma treatment was carried out in a plasma reactor with a RF power of 20 W and 200 sccm of hydrogen or nitrogen gas flow. In case of hydrogen/nitrogen mixture, the sccm values were varied from 150 sccm H2 and 50 sccm N2 (3:1 ratio) to 100 sccm (both, 1:1 ratio). The samples were exposed to the respective plasma gases for 10 - 60 minutes at 200 °C substrate temperature. The crystallinity and crystal phase have been characterized by X-ray diffraction (XRD; STOE) using Cu-Kα radiation (λ = 1.540 Å). The morphology was investigated by scanning electron microscopy (SEM; Nova Nano SEM 430, FEI Company). XPS analysis was performed with an ESCA M-Probe from SURFACE SCIENCE INSTRUMENTS. The samples were irradiated with Al-Kα rays (λ = 8.33 Å). After all measurements were done, all samples have been sputtercleaned with Argon plasma for 120 seconds and measured again. UV-Vis measurements were carried out on Lambda 950 spectrophotometer from PERKIN ELMER. Photoluminescence measurements were carried out with a PERKIN ELMER LS55 Fluorescence Spectrometer. All PL-spectra have been normalized with respect to the


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highest peak at 422 nm, originating from the quartz substrate. PEC current evaluation measurements were performed in a three electrode (saturated calomel, Pt wire and TiO2) photoelectrochemical cell and freshly produced 1M NaOH solution as electrolyte while the voltammograms were recorded by potentiostat (PAR, Model: Versa state IV, USA). The PEC photoanodes were illuminated with 100 mW cm-2 AM 1.5 G (1 SUN) simulated sun light from 450 W Xenon lamp (Oriel).

2.2

Computational Details: The electronic properties such as density of states (DOS)

of undoped, H- and N-doped anatase were investigated by employing the range separated hybrid

Heyd-Scuseria-Ernzehof

(HSE06)

exchange-correlation

functional29

as

implemented in VASP package.30-32 We have employed the generalized gradient approximation (GGA) within the Perdew-Burke-Ernzerhof (PBE) functional.33,34 The correlation part is defined by PBE, whereas a range-separation approach is used for the exchange part. At short range a mixing of 25 % of exact HF and 75 % of PBE exchange is used, while at long range the standard PBE exchange is maintained. The range separation parameter is fixed at 0.2 Å. The projector-augmented wave (PAW) method was used for the core electron representation.35,36 With plane-wave methods, the quality of the basis set is determined by a single parameter, the energy cutoff Ecut. We used a converged value of Ecut = 400 eV, as optimized for undoped anatase in the present study. The integration in reciprocal space was performed with a Monkhorst–Pack grid.37 The k-points grid was set to 4 × 4 × 2 for the bulk cell, as the energy convergence was achieved at 10–6 eV per cell with these values. For the simulation of N and H doping in anatase, we have employed 2 × 2 × 1 (Ti16O32) and 3 × 3× 1 (Ti36O72) supercell models. Similar to previous theoretical investigations38-41, we have considered both interstitial and substitutional doping. In case of interstitial doping


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the N and H are attached with oxygen as shown in Figure 1a and 1b, respectively. The models can be denoted as TinO2nN and TinO2nH respectively. The spin states for both cases are considered as open shell doublet. The substitutional doping is performed by single substitution of one oxygen with N or H giving the model system TinO2n-1N or TinO2n-1H (as shown in Figure 1c). For the case of TinO2n-1N, two different spin configurations are considered. The first one is paramagnetic with one unpaired electron where one lattice O2− is replaced by N2−, i.e., N3− ion with a localized hole on it. The second case is diamagnetic with two unpaired electrons due to the replacement of one lattice O2− by N3−. The substitutional N-doping in combination of O vacancy was modeled by replacing two neutral O atoms with two N atoms and removing one additional neutral O atom which leads to the stoichiometry TinO2n-3xN2x as shown in Figure 1d. Removal of one neutral lattice oxygen atom leaves two extra unpaired electrons (assuming a formal –2 oxidation state of O in TiO2). Due to the possible presence of unpaired electrons at various sites, several spin configurations were tested. First, a high spin open-shell configuration corresponding to four unpaired electrons (quintet state) in which two unpaired electrons are localized on the two N impurities and two other unpaired electrons on the two Ti3+ ions. Then, a low-spin closedshell configuration (singlet state) resulting from localization of an extra unpaired electron on the two initially paramagnetic substitutional N2- species is formed. The singlet configuration is more stable than the quintet state. In case of co-doping of H and N, we have substituted one oxygen by one N that is attached to interstitial H giving the model system TinO2n-xNxH. In this case, the spin state is considered as open shell triplet.


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Figure 1. (a) Interstitial N doping, (b) Interstitial H doping (c) Substitutional N doping and (d) Substitutional N doping in combination of oxygen vacancy. The blue, red, yellow, green and gray spheres represent Ti, O, H, N atoms and O vacancy respectively.

3. Results and Discussion 3.1 Experimental Results: The X-Ray diffraction data of annealed films (Figure 2a) revealed polycrystalline TiO2 deposit in the anatase modification (JCPDS-File 21-1272) that showed no phase change upon plasma treatment. We deduce lattice inter planar distance and lattice parameters using Bragg’s law nλ = 2d sinθ, where n is an integer, λ is wavelength, d is spacing between planes and θ is the angle between the incident wave and the scattered planes. The XRD indexing and inter planar distance of pristine and plasma treated TiO2 thin films is summarized in Table 1.


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Table 1. Peak indexing and d spacing of pristine and treated TiO 2 thin films.

Peak

Peak

No.

index

Pristine

H2

N2

H2+N2 (1:1)

H2+N2 (3:1)

(d spacing)/Å

(d/ Å)

(d/ Å)

(d/ Å)

(d/ Å)

1

(101)

3.526

3.505

3.501

3.507

3.514

2

(004)

2.370

2.365

2.359

2.365

2.363

3

(200)

1.888

1.886

1.881

1.886

1.884

4

(105)

1.748

1.748

1.748

1.752

1.751

5

(211)

1.664

1.664

1.662

1.664

1.661

6

(204)

1.493

1.492

1.495

1.494

1.495

It has been observed that the inter planar spacing distance of various planes is reduced in H2 and N2 plasma treated samples as compared to the pristine TiO2. Further for the gas mixture (H2+N2) a slight increase in d spacings is observed at higher diffraction angle only; however, at lower diffraction angles the d spacings showed a reduction. The unit cells parameters of pristine and plasma treated TiO2 samples showed a reduction of both a and c lattice parameters of tetragonal TiO2 (Table 2). The nitrogen plasma caused maximum reduction in lattice parameters whereas pure hydrogen plasma and mixed plasma (H2+N2) have comparatively moderate effect on the TiO2 films. According to the Scherrer equation, the crystallite size of untreated TiO2 is 27 nm. In case of H2 plasma, the average crystallite size is 18 nm and in case of pure N2 plasma treatment up to 15.8 nm, whereas for mixed (H2+N2) plasma the average crystallite size increased slightly (~ 20 nm) possibly due to etching and redeposition processes occurring in plasma treatment.


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Table 2. Lattice parameters, cell volume and average crystal size of pristine and treated TiO2 thin films calculated from XRD analysis.

S.No.

Lattice

Pristine

H2

N2

H2+N2

H2+N2

(1:1)

(3:1)

parameters TiO2

1

a (Å)

3.776

3.772

3.762

3.772

3.768

2

c (Å)

9.480

9.460

9.436

9.460

9.452

3

Cell Volumes

135.17

134.6

133.54

134.6

134.2

27

18

15.8

20.6

20

(Å3) 4

Avg. Crystal Size (nm)

Xia et al. established a crystal morphology modelling derived from XRD and compared with HRTEM data of hydrogen treated TiO2 nanocrystals to validate the perturbation in crystal structure resulting from modifications.42 Further, Chen et al. observed a crystalline –disordered core shell structure in HRTEM analysis of hydrogen treated black TiO2 nanocrystals, whereas well-defined lattice fringes of a crystalline anatase nanocrystal was observed for pristine samples.20 The XRD pattern of plasma treated films showed well resolved (101) peak in all treated samples but the relative intensity decreased with treatment time indicating the disordered nature of the treated TiO2 films. A shrinkage of cell volume due to the increased degree of crystal disorder was also observed in all thin films XRD data.


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Figure 2. (a) XRD patterns of untreated and plasma treated TiO2 films showing the deposition of pure anatase phase (JCPDS-File 21-1272). The asterisks show FTO-peaks. (b) Cross-Sectional SEM image of untreated ALD grown TiO2, (c) N2 plasma treated ALD-TiO2.

Cross-sectional SEM images (Figure 2b and 2c) of annealed ALD grown TiO2 confirmed smooth and homogeneous polycrystalline film with average layer thickness of ca. 180 nm, whereas after 1 hour of nitrogen plasma treatment resulted in the enhancement of film thickness to ca. 250 nm (Figure 2c). However, the treatment with pure hydrogen plasma under similar experimental conditions resulted in negligible increase in film thickness, suggesting that small hydrogen atoms take interstitial sites, whereas nitrogen atoms incorporated in the lattice change the lattice constants.43 The High resolution XPS spectrum of N 1s peak of nitrogen plasma treated sample (Figure 3) showed two different nitrogen species: the peak at 397.4 eV corresponded to the substitutional nitrogen44,45, whereas the minor fraction with a binding energy of 400.1 eV is possibly due to surface adsorbed or interstitial nitrogen.46-48


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397.4

400.1

Intensity (A.U.)

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385

390

395

400

405

410

415

Binding Energy (eV) Figure 3. High resolution N 1s spectrum after Ar+-etching for 120 seconds.

Depth profiling XPS experiments showed a dependence of the nitrogen uptake on the plasma gas composition: For pure N2 plasma treatment, the maximum value of nitrogen content in the near surface region was 1.3 at.-%, whereas H2-N2 mixed plasma treatment led to higher N contents subject to H2:N2 ratio with values of 2.4 at.-% (H2:N2 = 1:1) and 3.4 at.-% (H2:N2 = 3:1). Argon etching of the pure N2 plasma treated sample for 300 seconds did not show any traces of nitrogen in the film, whereas the (3:1)-mixed plasma showed 0.8 at.-% of nitrogen after 420 seconds of argon etching. Apparently, hydrogen enables a higher reactivity and penetration depth possibly due to formation of reactive N-H+ fragments. High resolution XPS peak of Ti 2p3/2 (Figure 4a) showed only one fitting peak maximum at 459.5 eV for the Ti4+ states in nano-crystalline TiO2. After sputter-cleaning, the amount of Ti3+ increased in all samples due to commonly observed reduction by Ar+-plasma.49 The sputtering step was essential to avoid misinterpretation of N 1s- and O 1s-spectra with surface adsorbed hydroxyl or nitrogen species. The shift of the binding energy from 457.6 eV (H2 plasma, Figure 4b) to 457.9 eV (pure N2 and H2-N2 mixed plasma) possibly resulted from the altered chemical


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environment due to the presence of additional nitrogen atoms on interstitial lattice sites, which tend to drain electron density from the Ti3+-centers.50 The use of nitrogen plasma (Figure 4c), showed a pronounced Ti3+ peak, which is supported by other reports on the formation of oxygen vacancies upon N incorporation.38

Figure 4. Ti 2p3/2 high resolution spectra of (a) untreated TiO2, (b) pure H2 plasma, (c) pure N2 plasma and (d) H2/N2 mixed plasma treated TiO2.


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The O 1s spectra of N2 plasma treated TiO2 (Figure 5c) exhibited two different peaks located at 530.9 eV for standard Ti-O bonds and 531.8 eV assigned to oxygen deficient suboxides51, respectively. Pure H2 plasma treated samples (Figure 5b) showed a maximum at 531.1 eV and another one at 532.4 eV for the regular Ti-O bond and for Ti-O-H formed after hydrogen treatment, respectively.52 Intriguingly, untreated TiO2 showed a maximum at 529.4 eV corresponding to the regular Ti-O bond.20,53

Figure 5. High resolution O 1s spectra of (a) untreated TiO2, (b) pure H2 plasma, (c) pure N2 plasma and (d) TiO2 treated with H2/N2 mixture (1:1).


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In the case of plasma treatment with H2 and N2 gas mixture (1:1 ratio, Figure 5d), an extreme peak broadening of the O 1s peak was observed. Deconvolution of this peak resulted in three different peaks centered at 530.8 eV, 531.6 eV and 532.8 eV indicating the existence of lattice oxygen in three different chemical environments by incorporating nitrogen atoms, as well as hydrogen atoms into the lattice forming O-Ti, O-N and O-H units. The substrate temperature during plasma treatment was found to have no influence on the dopant ratios. The UV absorbance spectra (Figure 6a) showed an improved absorption in visible light range for all plasma treated samples in comparison to pristine TiO2 films. A low-intensity absorption band at 410 nm corresponding to d-d transitions revealed a certain non-stochiometry even in untreated TiO2. Plasma induced reduction to Ti3+ (d1) and Ti2+ (d2) species led to additional electrons in the Ti 3d shell, which could account for observed higher charge carrier density. Increasing the proportion of H2 in the plasma gas resulted in higher absorption due to higher electron density in the lattice. However, the numerical density of such states is low in case of N2 plasma treatment. Tauc plots54,55 revealed narrowing of the optical band gap for all plasma treated samples (Figure 6b), however, the energy gap depended on the plasma gas composition: for instance, plasma treatment with H2 resulted in a band gap of 2.7 eV (from 3.6 eV), whereas an increase of N2 content in the plasma gas mixture did not significantly affect the optical band gap (3.3 - 3.5 eV). Plasma treatment with pure N2 gas caused a slight band gap narrowing from 3.60 eV to 3.55 eV. Compared to TiO2 sample treated in N2 plasma, the band gap in the visible light range decreased with increasing hydrogen content. The optical band gap was found to depend on the incorporation of dopant atoms56,57 rather than on the creation of oxygen vacancies.


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Figure 6. (a) Absorbance spectra of untreated and plasma treated TiO 2 and corresponding Tauc Plots (b).

The linear sweep voltammetry for pristine and plasma treated samples (Figure 7) showed an enhancement in photocurrent density upon plasma treatment as reported earlier.20,58 The small anodic shift in onset potential for H2 plasma treated TiO2 films was observed (Figure 7) compared to untreated TiO2 film, whereas no significant change in the onset potential was found for N2 or H2/N2 plasma treated films. The photocurrent density was found to increase with increasing hydrogen percentage in the reacting plasma. The photocurrent reached to ~0.7 mA/cm2 at 1.23 V vs. NHE for the sample treated in H2/N2 plasma (3:1), which can be attributed to increased visible light absorption for these samples as shown by Tauc plot. However, for pure hydrogen plasma treated film, the photocurrent decreased to ~0.5 mA/cm2 probably due to additional electronic states below the conduction band, which could promote recombination of photo-generated charges.


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Figure 7. I-V curves of ALD-grown and plasma treated TiO2 photoanodes in comparison to pristine TiO2 films.

Based on the XPS data, the impact on the light harvesting behavior depended on the plasma gas composition. To elucidate the impact of the plasma gas composition on the defect states, photoluminescence (PL) measurements (Figure 8) were carried out on pure and plasma treated TiO2. The increase of oxygen vacancies trigger non-radiative transitions of photons from the conduction band minimum to the sub-band states59,60 located in the energy region of 430 – 450 nm (~2.8 eV). Radiative recombination processes from these states gave photons with energies lower than the original band gap energy, therefore higher PL intensity for all plasma treated materials compared to pristine TiO2 films was observed. Remarkably, only in the case of H2 plasma treated TiO2 samples, the band-to-band transitions61 in the energy range of ~3.2 eV exhibited a higher PL intensity compared to the untreated


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material, whereas samples treated in N2-containing plasma only featured PL intensities as low as the untreated TiO2.

Figure 8. Photoluminescence spectra of pristine and plasma treated TiO2, excitation wavelength = 300 nm.

The anodic shift in the onset potential (Figure 7) and photoluminescence spectra (Figure 8) showed the recombination probability in H2 plasma treated TiO2 films to be higher when compared to pristine TiO2 films, indicating higher number of trapping states formed due to oxygen vacancies. This should also result in a higher recombination rate for the other (N2, H2/N2) plasma treated samples, but remarkably the PL intensity in the band-gap region (~3.2 eV) dropped, which suggested that nitrogen incorporation apparently improved the charge carrier separation by alleviating the electron flow from the oxygen vacancies. The improved electron flow facilitated the water splitting reaction, leading to lower on-setpotentials in PEC measurements.62


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Chen et al. related the enhanced light absorption properties of hydrogen treated TiO2 to lattice order induced mid gap-states.20 As opposed to their results, the contribution of the H 1s orbital leads to shallow donor states, which act as additional absorption sites. Comparable results were obtained by Wang et al. They have shown that hydrogen incorporation raised the energy level of Ti3+-states towards the conduction band, alleviating the localization of trapped Ti 3d1electrons.27 The formation of oxygen vacancies enables better charge carrier transport through the host matrix, allowing higher photocurrents. The dependence of the structural integrity with the host matrix has also been shown by Mathur et al. by applying a mild hydrogen plasma treatment on anatase nanofibers, leaving the crystal structure intact accompanied by additional midgap-states from Ti3+-sites.63 Further studies from Wang et al. on aluminum-reduced TiO2 confirmed Ti3+-states as light harvesting sites.64 In case of nitrogen doping, Hoang et al. attributed the increased light absorption to N 2p states, since no Ti3+ sites were observed in the samples.65 In combination with hydrogen mediated reduction they claimed a synergistic coupling between the charge transfer from Ti 3d1 electrons into the N 2p-orbitals, which leads to Coulomb repulsion of the paired electrons with the valence band, resulting in an upward shift of the N 2p-states.23,26 Nitrogen incorporation facilitates charge carrier separation by circumvention of recombination processes due to the charge transfer from Ti3+-states to the N 2p states. Its impact on neighboring oxygen vacancies decreases the recombination probability by a lower charge dwell time at the Ti3+-sites. In fact, a synergistic relation between hydrogen and nitrogen is observable in the nitrogen uptake depending on the plasma gas composition. A H2/N2 mixed plasma apparently enabled deeper penetration of the host matrix, allowing higher nitrogen incorporation.


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3.2 Computational Results: Prior to studying the doped systems, the structural and electronic properties of the undoped anatase were calculated and compared with the available experimental data (see Table 1). It is observed that the calculated lattice parameters are in reasonable agreement with the experiment with maximum deviation of a parameter of 0.01 Å (-0.29 %). The calculated Ti–O bond distances are in agreement with experiment66 with a maximum deviation of 0.003 Å (Table S1, see Supporting Information). The calculated fundamental band gap is 3.3 eV, calculated as the difference between valence band top (VBT) and conduction band bottom (CBB), is in well agreement with the experimental value of 3.4 eV (Figure 9a).67 The calculated density of states (DOS) showed that both the valence band (VB) and conduction band (CB) are mainly composed of O 2p states with some hybridization with Ti 3d orbitals. The bottom of the conduction band is mainly formed by Ti 3d states, their contribution being several times higher than that of O 2p states. 3.2.1 N Doping: The interstitial nitrogen preferably bonds to lattice oxygen to form O–N bond perpendicular to the Ti–O–Ti plane (Figure 1a). The N–O and N–Ti bond distances are 1.38 and 2.12 Å, respectively. The calculated electronic structures showed that impurity bands are formed within the band gap with the introduction of nitrogen interstitial narrowing the band gap. It can be seen from the PDOS of N-doped TiO2 that there are two impurity bands: one is close to the valence band top and the other crosses the Fermi level, into the gap regions (Figure 9b). These states are localized and mainly contributed to the unsaturated nitrogen p-electrons and partially to oxygen p-electrons that may act as recombination centers (Figure 9b). These impurity bands may enhance the sunlight absorption and reduce the photo-energy to excite electrons from valence band to conduction band. Consequently, the N-doping was found to narrow the band gap of anatase TiO2 from 3.3 eV to 1.8 eV (Table 2).


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In case of the paramagnetic substitution of one O2- by one N2-, the N2- ion is coordinated to three Ti4+ ions with two Ti−N bonds at 1.99 Å and one Ti−N bond at 2.29 Å, which are longer than regular Ti−O bonds (1.93 to 1.98 Å) in anatase TiO2. The calculated DOS shows the presence of partially occupied states composed of N 2p orbitals above the Fermi level (Figure 9c). These unoccupied states are located in the beta ladder. The band gap has reduced to 3.1 eV (α ladder) and 2.0 eV (β ladder) compared to 3.3 eV of pure anatase. For the diamagnetic substitution of one O2- by one N3-, the unpaired electrons are localized on N atom as the Ti–N bonds have reduced namely: two Ti−N bonds at 1.86 Å and one Ti−N bond at 1.89 Å. It is also observed in the DOS as there are presence of partially occupied states composed of N 2p orbitals above the Fermi level (Figure 9d). The band gap has reduced to 3.0 eV compared to 3.3 eV of pure anatase. Another possibility of N doping is realized by a combination of substitutional N-doping and one extra oxygen vacancy where the substitutional N-centers are separated with a large N–N distance (2.8 Å). Similar to the interstitial N-doping, here two impurity bands are formed within the band (Figure 9e). These impurity states are localized on top of the valence band narrowing the band gap to 1.7 eV (Table 2) and originating from the contribution of p-orbitals localized on the two N atoms and p-orbitals localized on neighboring O atoms with a weak contribution of d-orbitals localized on the three Ti atoms surrounding each N atom (Figure 9e).


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Figure 9. (a) PDOS of pure anatase TiO2, (b) Interstitial N doped, (c) Paramagnetic substitutional N doped, (d) Diamagnetic substitutional N doped, (e) Substitutional N doping in combination of oxygen vacancy. 0 eV corresponds to the Fermi energy.

3.2.2 H Doping: In case of the interstitial hydrogen doping, hydrogen prefers to bond with the oxygen with a bond length of 1.1 Å. To investigate the effect of hydrogen on the electronic properties of TiO2, the density of states (DOS) was calculated as shown in Figure 10a. The total density of states (TDOSs) of TiO2 with hydrogen interstitial are very similar to those without hydrogen (Figure 9a). The incorporation of hydrogen only shifts the Fermi level up by introducing electrons into the systems. The extra electron introduced by H is completely delocalized. The calculated band gap for H doped anatase is 3.0 eV which is smaller than the undoped anatase TiO2 (3.3 eV), indicating that H doping can improve the sunlight absorption. The analysis of the partial density of states (PDOS) shows that the s electrons of hydrogen have


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little contribution to the band edge states of conduction band bottom. The interstitial hydrogen leads to n-type TiO2. In case of the substitutional hydrogen doping, the band gap was reduced to 2.9 eV (Figure 10b) compared to the undoped anatase. The substitutional hydrogen leads to n-type semiconducting in TiO2, similar to the interstitial hydrogen doping. The s electrons of hydrogen have a little contribution to the valence top states because of the sd-hybridization between hydrogen s- and Ti d-electrons (Figure 10b). As the incorporation of hydrogen (for both the interstitial and substitutional) narrows the bandgap, it may enhance the visible-light absorption of TiO2. 3.2.3 H–N Co-doping: In the case of co-doping of H, N in TiO2, the interstitial hydrogen prefers to bond with the nitrogen where H is perpendicularly attached on Ti–N–Ti plane. The optimized H–N bond length is 1.09 Å. The electronic effect for H,N co-doping in TiO2 is investigated by the calculation of DOS as shown in Figure 10c. Electronically, the (H,N-) codoped TiO2 are intrinsic semiconductors with the Fermi levels within the gap regions (Figure 10c) as the holes contributed by nitrogen doping are neutralized by electrons from hydrogen atoms. The density of states near both the valence band and conduction band edges is smooth, indicating that the doping states are completely delocalized.


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Figure 10. (a) PDOS of Interstitial H doped, (b) Substitutional H doped and (c) H-N co-doped Anatase TiO2, 0 eV corresponds to the Fermi energy.

The PDOS of (H,N-) co-doped TiO2 shows that the top of the valence band is dominated by the N-p electrons, and the bottom of the conduction band is mainly attributed to Ti-d electrons (Figure 10c). The band gap has narrowed to 2.88 eV compared to undoped TiO2, indicating the enhancement of visible-light absorption and the improvement of photocatalytic performance. Although the band gap of the (H,N-) co-doped anatase TiO2 is in the same range or larger than that of the N-doped counterpart (Table 3), there is no additional impurity bands in the gap regions (Figure 10c). This indicates that the carrier mobility in the co-doped TiO2 is higher and the trap centers are less than those in the N-doped counterpart. These aspects may result in enhancement of photocatalytic performance of TiO2.


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Table 3. Comparison of calculated and experimental band gap (eV) for doped systems with undoped anatase TiO2 (aparamagnetic substitution, bdiamagnetic substitution).

System

Calc.

Exp.

TinO2nN

1.8

3.55

TinO2n-1N

2.0a, 3.0b

3.55

TinO2n-3xN2x

1.7

3.55

TinO2nH

3.0

2.7

TinO2n-1H

2.9

2.7

TinO2n-xNxHi

2.88

Undoped

3.3

3.3 – 3.5 3.6

Our theoretical investigation showed that hydrogen incorporation in TiO2 shifted the Fermi level up by introducing more electrons into the d-states below the conduction band. The extra electron introduced by H is completely delocalized and imparts to n-type semiconducting behaviour in TiO2. Due to N incorporation, two impurity bands were observed mainly contributed by the unsaturated nitrogen p-electrons and oxygen p-electrons acting as potential recombination centers. The (H,N-) co-doped TiO2 showed intrinsic semiconductor behaviour as the holes contributed by nitrogen doping are neutralized by electrons from hydrogen atoms. This indicated that the carrier mobility in the (H,N-) co-doped TiO2 is higher and the trap centers are less than in the N-doped counterpart. The band gap was found to narrow down to 2.88 eV compared to undoped TiO2. Therefore, the (H,N-) co-doping is a viable option to enhance visible-light absorption and improve the photocatalytic performance.


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4. Conclusion ALD grown TiO2 films were successfully modified by incorporation of nitrogen and/or hydrogen in the lattice through reactive plasma treatment as shown by XPS studies. The effect of the dopant on the optical properties resulted in a higher absorption in the visible light range manifested in higher photocurrent densities. Besides the enhanced production of photoinduced charge carriers, the hydrogen plasma treatment also entails a higher recombination rate and therefore resulted in a more positive on-set potential, while lattice incorporated nitrogen supported the electron flow through the TiOx lattice, reducing the on-set potential. The minimized dwell time on the oxygen vacancies prevent the charge carriers from recombination. The samples treated in a mixture of H2 and N2 plasma showed photocurrent values of ~0.7 mA/cm2 at 1.23 V vs. NHE, which correspond to ~250 % enhancement in photocurrent compared to bare TiO2 film (~0.2 mA/cm2). Theoretical studies revealed that H insertion introduces extra electrons in the system leading to stronger n-type behaviour in TiO2, whereas the (H,N)-co-doped TiO2 showed intrinsic semiconducting behavior as the holes contributed by nitrogen doping are neutralized by electrons from hydrogen atoms. Despite the large body of data available on chemical composition and structural (electronic) modification of TiO2, there exist enormous potential in the engineering of the optical properties of titania through an integrated experimental and modeling approach.


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Supporting Information Supporting Information document contains an XPS survey spectrum, a photoluminescence spectrum of uncoated quartz substrate and I-V curves of nitrogen plasma treated TiO2 with prolonged plasma exposure times. A Table with a comparison of calculated and experimentally obtained structural parameters such as lattice parameters, bond distances and band gap is also included.

Acknowledgments The authors gratefully acknowledge the financial support by the DFG in the frame of SPP 1839 EnLight and also by the BMBF in the frame of KMU-innovativ NANOFLEX (03X0125C). Dr. Yajun Gao and Prof. Paul van Loosdrecht are thanked for providing photoluminescence measurements and helpful discussions. Financial and infrastructural support from University of Cologne is also thankfully acknowledged. Meenal Deo would like to acknowledge Science and Engineering Research Board (SERB), India for fellowship under SERB-Overseas postdoc fellowship scheme. The authors would also like to thank Dr. Thomas Fischer and Mr. Senol Öz for valuable discussions.

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Experimental and Theoretical Insights into Influence of Hydrogen and Nitrogen Plasma on the Water Splitting Performance of ALD Grown TiO2 Thin Films Alexander Sasinska1, Danny Bialuschewski1, Mazharul M. Islam2, Trilok Singh1, Meenal Deo1, and Sanjay Mathur1, * 1) Institute of Inorganic Chemistry, University of Cologne, Greinstrasse 6, Cologne-50939, Germany 2) Mulliken Center for Theoretical Chemistry, University of Bonn, Beringstrasse 4–6, Bonn53115, Germany

Table of Contents (TOC)



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Figure 1. (a) Interstitial N doping, (b) Interstitial H doping (c) Substitutional N doping and (d) Substitutional N doping in combination of oxygen vacancy. The blue, red, yellow, green and gray spheres represent Ti, O, H, N atoms and O vacancy respectively. 254x190mm (96 x 96 DPI)


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Figure 2. (a) XRD patterns of untreated and plasma treated TiO2 films showing the deposition of pure anatase phase (JCPDS-File 21-1272). The asterisks show FTO-peaks. (b) Cross-Sectional SEM image of untreated ALD grown TiO2, (c) N2 plasma treated ALD-TiO2. 254x190mm (96 x 96 DPI)



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Figure 3. High resolution N 1s spectrum after Ar+-etching for 120 seconds. 254x190mm (96 x 96 DPI)


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Figure 4. Ti 2p3/2 high resolution spectra of (a) untreated TiO2, (b) pure H2 plasma, (c) pure N2 plasma and (d) H2/N2 mixed plasma treated TiO2. 254x190mm (96 x 96 DPI)



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Figure 5. High resolution O 1s spectra of (a) untreated TiO2, (b) pure H2 plasma, (c) pure N2 plasma and (d) TiO2 treated with H2/N2 mixture (1:1). 254x190mm (96 x 96 DPI)


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Figure 6. (a) Absorbance spectra of untreated and plasma treated TiO2 and corresponding Tauc Plots (b). 254x190mm (96 x 96 DPI)



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Figure 7. I-V curves of ALD-grown and plasma treated TiO2 photoanodes in comparison to pristine TiO2 films. 254x190mm (96 x 96 DPI)


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Figure 8. Photoluminescence spectra of pristine and plasma treated TiO2, excitation wavelength λ = 300 nm. 254x190mm (96 x 96 DPI)



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Figure 9. (a) PDOS of pure anatase TiO2, (b) Interstitial N doped, (c) Paramagnetic substitutional N doped, (d) Diamagnetic substitutional N doped, (e) Substitutional N doping in combination of oxygen vacancy. 0 eV corresponds to the Fermi energy. 254x190mm (96 x 96 DPI)


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Figure 10. (a) PDOS of Interstitial H doped, (b) Substitutional H doped and (c) H-N co-doped Anatase TiO2, 0 eV corresponds to the Fermi energy. 254x190mm (96 x 96 DPI)


Experimental and Theoretical Insights into Influence of Hydrogen and

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