Role of the Nitrogen Source in Determining Structure and Morphology

Feb 20, 2014 - The photon beam was focused on the sample with a spot size as .... the Ti oxidation states, and the underlying exchange-correlation fun...
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Role of the Nitrogen Source in Determining Structure and Morphology of N‑Doped Nanocrystalline TiO2 Leonardo Lo Presti,*,†,§,∥ Michele Ceotto,†,‡ Francesca Spadavecchia,† Giuseppe Cappelletti,†,‡ Daniela Meroni,†,‡ Robert G. Acres,⊥ and Silvia Ardizzone†,‡ †

Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy Consorzio Interuniversitario Nazionale per la Scienza e la Tecnologia dei Materiali (INSTM), Via Giusti 9, 50121 Firenze, Italy § Istituto di Scienze e Tecnologie Molecolari, Italian CNR, Via Golgi 19, 20133 Milano, Italy ∥ Centre for Materials Crystallography, Århus University, Langelandsgade 140, 8000 Århus, Denmark ⊥ Elettra - Sincrotrone Trieste S.C.p.A. S.S, 14 - km 163,5 in AREA Science Park, 34149 Basovizza, Trieste, Italy ‡

S Supporting Information *

ABSTRACT: The photocatalytic activity of N-doped nanostructured TiO2 (TiO2:N) in the visible region strongly depends on the close, yet not fully understood, interplay among crystal structure distortions, nature, and concentration of lattice defects and bulk electronic states. In this work, we study correlations among the chemical identity of the nitrogen source and the microscopic features of biphasic (anatase: brookite) TiO2:N nanoparticles through a broad starting doping range. Triethylamine, urea, and ammonia were considered as independent nitrogen supplies. Synchrotron X-ray photoelectron spectroscopy measurements confirmed the presence of nitrogen within the nanoparticles, while X-ray powder diffraction experiments performed at both synchrotron light sources and conventional laboratory-based instruments found that the dopant monotonically lengthens the cell edge module |c| along the unique C4-axis, until a plateau is reached for starting N/Ti ratios greater than 0.2. The chemical nature of the precursor determines (i) the morphology of the powder at the mesoscale, (ii) the actual magnitude of the maximum lengthening of the c-vector module, and (iii) the anatase phase enrichment. Overall, we found useful hints on possible routes to control and tailor one or more of the specific features of the material (polymorph enrichment, dopant levels, surface area).

1. INTRODUCTION Among the numerous varieties of titania doping for the development of visible-active second-generation photocatalysts, nitrogen (N) dopant is by far the most employed one, more than fluorine, sulfur, carbon, boron, metals, or rare-earth transition metals.1−3 After Asahi’s pioneering photocatalytic experiments on N-doped TiO2 (TiO2:N),4 we can safely state that anatase N-doped titania represents a class of photoversatile materials per se, as its optical properties can be easily tuned depending on the synthetic strategy. For example, doping titania using ammonia, triethylamine, or urea as the nitrogen source gives different visible light activity.5−7 Actually, a large number of publications dealing with the preparation of TiO2:N by both physical and chemical methods have been reported. These include sol−gel,8−14 sputtering ion implantation,15 mechanochemical,16 and plasma-enhanced chemical vapor deposition methods.17 Such a popularity is mainly due to the promising photocatalytic activity of this material and to the many affordable synthetic routes given for nitrogen inclusion. Manifold syntheses and multiple availability of N precursors are a clear advantage for the dissemination and usage of this kind of doping, especially in the perspective of massive scaling-up for © 2014 American Chemical Society

the industrial production and commercialization of this kind of catalysts. However, it is difficult to make direct and strict comparisons, and general considerations on the microscopic photoinduced electron transfer mechanisms associated with the performances of these materials when widely varying experimental setups, sample preparation, and even determination of photoreactivity and characterization methods are employed. Moreover, within the same doping procedure but increasing nitrogen concentration, the quantum yield decreases when irradiating with UV light, indicating that the doping sites may act as recombination centers or favor recombination by generating oxygen vacancies.18−23 In general, because of the widely different employed experimental conditions, it is hard to make direct comparisons among different N-doped anatase titania and draw unifying conclusions about the optimal value of dopant loading with respect to the photocatalytic performances. It is even harder to estimate the actual dopant concentration within the titania nanoparticles, since it is not Received: December 18, 2013 Revised: February 10, 2014 Published: February 20, 2014 4797

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synthesized by a sol−gel route. Titanium(IV) isopropoxide was used as the starting material, and three different nitrogen species (triethylamine, ammonia, and urea) were adopted as N sources. All reactants employed in this work were purchased from Aldrich and used without further purification; doubly distilled water passed through a Milli-Q apparatus was used to prepare solutions. First, titanium precursor (30.7 mL) and 2propanol (37.6 mL) were put into a 500 mL reactor with stirring for about 10 min to form a solution. A variable amount of N dopants was used, and then a KOH aqueous solution (180 mL) was added dropwise under vigorous stirring (300 rpm). A transparent fluid gel was formed at high N/Ti ratio. The molarity of the basic solution was adjusted to fix the pH around 9, while the water/alkoxide molar ratio was 100 and the water/ 2-propanol molar ratio was 20. The wet precursor was dried in oven as a xerogel (80 °C overnight) and subsequently calcined at 400 °C for 6 h under oxygen stream (9 NL h−1). Doped titania samples from triethylamine, urea, and NH3 precursors are named “TNT_x”, “TNU_x”, and “TNN_x”, respectively, with x standing for the nominal N/Ti molar ratio, whereas the undoped one is labeled as “T”. Overall, x ranges from N/Ti = 0.05 to N/Ti = 0.5. 2.2. X-ray Photoelectron Spectroscopy (XPS). XPS spectra on selected TNT samples were collected on TiO2:N nanopowders supported on thin conductive In foils at the Materials Science Beamline (MSB) of the Elettra synchrotron in Basovizza (IT). The beamline was equipped with a SX-700 plane grating monochromator with an energy range of 22− 1000 eV,30,31 and the endstation employs aSPECS PHOIBOS 150 hemispherical energy analyzer,32 overall granting a nominal maximum spectral resolution of 25 meV. The photon beam was focused on the sample with a spot size as large as roughly 100 μm in diameter. Repeated scans were carried out on different zones of the foil to avoid sample charging. The characteristic core-level XPS N 1s and Ti 2p peaks were carefully scanned by multiple acquisitions at 0.05 eV energy intervals for different nominal dopant concentrations. The base pressure in the analysis chamber was 2 × 10−10 mbar. 2.3. X-ray Powder Diffraction (XRPD). For the TNT and TNN sample series, high-resolution X-ray powder diffraction (HR-XRPD) experiments were performed at the bending magnet BM01B station of the Swiss-Norwegian beamline at ESRF, Grenoble (FR). Data were collected from 3° to 45° in 2ϑ at 2°/min in Debye−Scherrer capillary geometry using Si111 monochromatized X-rays with λ = 0.505 81(5) Å. A total of five acquisitions have been carried out for each doped sample, employing a six-counting chains multidetector system. The Xrays wavelength and the goniometer zero offset were calibrated using the NIST SRM 640b Si standard. The peak profile analysis of the same standard was carried out by the WinPLOTR program,33 and it allowed us to estimate an average intrinsic instrumental resolution as low as 2.4(1) × 10−4 rad (≈0.01°) in the 2ϑ range of interest. As for the TNU series, the diffractograms were recorded in our home laboratory on a Philips PW 3710 Bragg−Brentano goniometer equipped with a scintillation counter and 1° divergence slit, 0.2 mm receiving slit, and 0.04° Soller slit systems. We employed graphite-monochromated Cu Kα radiation at 40 kV × 40 mA nominal X-rays power. ϑ:2ϑ scans were performed between 20° and 90°. A step size 0.08° wide, for a total counting time of 4 h, was selected on the basis of our past experience on similar samples to maximize the trade-off between the desired accuracy and the available

possible on most synthetic approaches to control the amount of nitrogen that will be included into the lattice given an unavoidable partial loss of N species during the synthetic procedure. Actually, the real molar N/Ti ratio has been reported to be significantly smaller than the nominal one and even 1 order of magnitude lower.24 In general, the actual dopant concentration is very difficult to be accurately determined and neither clear nor unambiguous trends are even recognized in the literature among the starting dopant:Ti molar ratio and the final intensive properties of the material.25 Further complications arise when the electronic structure of the material is considered, as density functional theory (DFT) calculations suggest that shallow midgap states are created under nitrogen substitutional doping and deeper ones under interstitial one.26 In particular, it is believed that N 2p states allow higher photocatalytic activity to be achieved because these orbitals partially combine with titania valence band orbitals to generate midgap states. On the contrary, other authors27,28 conclude that the presence of the nitrogen alone cannot be cited as a direct means of inducing visible light activity and that a determinant role is played by the generation of extra oxygen vacancies with respect to the pristine material. In summary, crucial factors for determining the performances of these materials can be considered to be (i) the synthetic strategy (preparative method, choice of the nitrogen precursor), (ii) the amount of starting N content, and (iii) the chemical nature of the lattice point defects (substitutional/interstitial N, occurrence of extra oxygen vacancies). To the best of our knowledge, however, a comprehensive picture on how these factors correlate and influence each other is still lacking. In our group we are currently carrying out a broad series of experimental and theoretical investigations of this class of compounds,29 with the final purpose of identifying and rationalizing the key factors to be taken into account to optimize the photocatalytic performances in TiO2:N-based materials. In this context, the present contribution focuses on understanding the role played by the adopted nitrogen source in the resulting structure of the material. Specifically, we studied N-doped samples obtained from three independent nitrogen sources, i.e., triethylamine, urea, and ammonia. A rather wide nominal doping range, varied within 0 ≤ N/Ti ≤ 0.5, was taken into account. The specimens were analyzed through highresolution X-ray powder diffraction (XRPD) experiments performed at synchrotron light sources and partially complemented by conventional laboratory results. Moreover, synchrotron X-ray photoelectron spectroscopy (XPS) was employed to explore the surface state of the N-dopant. Specific surface area estimates were also carried out to provide information on the morphology and porosity of the powder at the mesoscale. At the same time, periodic DFT calculations were employed to obtain a solid-state quantum-mechanical interpretative basis for the lattice distortions detected by XRPD experiments. The next section presents the experimental and theoretical methods employed. Section 3 shows our results and discusses them by proposing a possible interpretative scheme to elucidate the mutual role of the doping amount and the chemical precursor in determining the observed material (micro)structure and morphology. Section 4 concludes the paper.

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Synthesis of Bare and Nitrogen-Doped TiO2 Nanoparticles. Pure and doped titania samples were 4798

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to 8 eV, as a result of the U dependence on the oxide, the Ti oxidation states, and the underlying exchange-correlation functional.50−52 Consequently, we chose to perform our calculations with U = 0 eV and U = 5 eV as a realistic interval of U values. To provide a least biased comparison against experimental results, we also estimated cell distortions for both doped and defective anatase structures resorting to a completely different DFT approach, i.e., by employing a basis of atom-centered linear combination of Gaussian-type functions (LCGTF) with the hybrid B3LYP53,54 and PBE055 Hamiltonians. Localized triple-ζ basis sets previously optimized for calculations of inorganic solids were chosen, namely 8-411Gd1 for oxygen,56 7-311G for nitrogen (slightly modified with respect to Pandey et al.57,58), and 86-411Gd41 for titanium.59 The same computational procedure as reported in full detail elsewhere29 has been applied by means of the CRYSTAL06 code.60

beamtime. A microcrystalline Si-powdered sample was used as a standard to correct for instrumental line broadening effects. The Rietveld method as implemented in the GSAS-EXPGUI program suite 34,35 was applied throughout to provide quantitative estimates of the phase composition and lattice parameters as a function of the doping extent. More in detail, experimental profiles were always modeled with pseudo-Voigt functions,36 but different refinement strategies were applied to synchrotron and home laboratory data sets. As for the latter, the background was described by power series in Q2n/n! and n!/Q2n. A surface roughness correction for microabsorption effects was also applied,37 while preferred orientation of crystallites was taken into account by a spherical harmonic model.38 Eventually, a parameter describing the shift of the sample surface from the diffractometer axis was refined. When synchrotron data were considered, a Chebyshev polynomial of the first kind was employed to fit the background, and neither microabsorption nor sample shift parameters were included in the model. For both data sources, anisotropic contributions for the Lorentzian broadening of experimental profiles were taken into account. During the last cycles of the refinement, scale coefficient(s), cell parameters, positional coordinates of anatase, and thermal factors were all allowed to vary as well as background and profile coefficients. Final agreement factors are reported within the Supporting Information (Table S1) and range among 0.031−0.043 (background-subtracted wRp), 1.7− 2.2 (reduced χ2), and 0.013−0.026 (R(F2)) throughout the TNT and TNN sample series, while the same parameters range between 0.034 and 0.047 (wRp), 1.3−1.5 (reduced χ2), and 0.009−0.019 (R(F2)) for the TNU series. The integral breadths of individual reflections up to sin ϑ/λ = 0.5 Å−1 were computed from the refined profile coefficients39 and then employed to estimate the average volume-weighted crystallite dimensions, ⟨Dv⟩, by means of the Williamson−Hall method. 2.4. BET Sample Characterizations. The BET surface area was determined by a multipoint BET method using the adsorption data in the relative pressure (p/p0) range of 0.05− 0.3 (Coulter SA3100 apparatus). Desorption isotherms were used to determine the total pore volume using the Barrett− Joyner−Halander (BJH) method with cylindrical pore size. 2.5. Computational Details. Spin-polarized DFT calculations40 were performed within the generalized gradient approximation (GGA)41 with the Perdew−Burke−Ernzerhof (PBE) exchange correlation functional42,43 and the PW91 one. Plane wave basis with projected augmented wave method (PAW)44 implemented in the Vienna Ab-initio Simulation Package code (VASP)45,46 was employed with an energy cutoff of 400 eV. The ground state optimizations were obtained by minimizing the free energy with respect to the atomic position, including the Harris−Foulkes correction to forces,47 using the conjugate-gradient scheme.48 Iterative relaxation of atomic positions was converged for a total energy change between successive steps less than 0.001 eV. The supercell and atomic relaxations were carried out until the residual forces were below 0.01 eV Å−1. The bulk doped systems were constructed from the relaxed 3 × 3 × 3 anatase TiO2 primitive supercell and a 2 × 2 × 2 one. Reciprocal space sampling was restricted to the Γpoint in the first case and sampled on a 7 × 7 × 7 grid generated using the Monkhorst−Pack method in the second one. We have applied the GGA+U method49 to account for the strong on-site Coulomb repulsion amid the localized Ti 3d electrons. There is no agreement on a precise value of U for all oxidation states of Ti, and the values of U span a range from 2

3. RESULTS AND DISCUSSION 3.1. Chemical State of the Dopant Nitrogen. XPS is nowadays a mature technique, able to accurately probe the chemical state of both surface and near-subsurface species.61 In conjunction with the exceptionally high photon flux provided by modern synchrotron light sources in a broad spectral range, it is also extremely sensitive even to very low amounts of contaminants, and it is therefore widely employed to investigate nanostructured materials. Here, we are mainly interested in exploring the electronic state of the N-dopant within the nanoparticles. Figure 1 reports the 1s core level X-ray photoelectron spectra for the guest nitrogen species in selected TNT samples at nominal concentration N/Ti = 0 (reference T compound) (Figure 1a), N/Ti = 0.1 (Figure 1b), and N/Ti = 0.5 (Figure 1c). NH terminal groups attributable to sample surface contamination62 are responsible for the peak detected at ≈401 eV, which is present even in the undoped specimen. It should be stressed, however, that some amount of surface impurities is to be expected, as our materials are designed for operating in air near/at the standard ambient pressure and temperature conditions. Because of high surface area (see below) and powdery nature of the material, this surface contamination cannot be completely removed even when the In-foil-supported powder is sputtered for 1/2 h with Ar+ ions (Figure S1 in the Supporting Information). Nevertheless, when the same spectral region is scanned on N-doped samples, a clearly recognizable satellite peak invariably appears at ≈399.8 eV (Figure 1b,c in the text and Table S2 in the Supporting Information). Even though there is no general consensus on the chemical state of nitrogen ions in TiO2:N compounds,63 signals at binding energies slightly lower than 400 eV are usually attributed to interstitial N.64,65 In any case, it is important to stress that the side peak at ≈399.8 eV unequivocally bears the signature of the chemically added dopant nitrogen, whatever its specific state within the nanoparticle is. Interestingly, no appreciable changes were detected in the chemical shifts of these N dopant species as a function of the N/Ti ratio (Figure 1b,c). This suggests that the dopant chemical nature is invariant with respect to the initial amount of precursor: in our opinion, this is a very interesting result. At the same time, a small (≈+0.5 eV) change in the binding energy of the N 1s main peak upon doping occurs (Figure 1 and Table S2). It is hard, however, to say whether this shift is due to some kind of dopant-induced surface distortion/ 4799

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reconstruction or it also implies a change in the chemical bonding environment of chemisorbed nitrogen species. As concerns Ti species, a secondary peak (≈459.7 eV, Table S2 and Figure S2) is always present near the main (J = 3/2) component of the Ti(IV) 2p doublet at ≈460.2 eV, which may be attributed to partially reduced titanium(III).67 As this feature is already present in the undoped specimen (Figure S2) and becomes higher and broader upon Ar+ sputtering (Figure S3), we believe that it should be mainly due to oxygen vacancies on the basis of the highly defective nature of these nanoparticles.29 Because of the problems highlighted above, it is virtually impossible to accurately quantify the amount of surface −NH species deriving from undesired contaminants and those due to the synthetic precursor. Actually, even taking into account just the side peak due to dopant nitrogen (see above), no obvious correlations are recognizable among the nominal dopant concentration and the A(Nb, 1s)/A(Ti, 2p) ratios, A(X,Y) being the integrated peak area for the signal due to the quantum level Y of the species X (Table S2). Interestingly, essentially identical results were obtained when the N 1s XPS signal was recorded using an Al Kα X-ray source in our home laboratory from specimens obtained by employing NH3 as the nitrogen source,68 with no significant changes of the total peak areas as a function of the nominal doping extent. In any case, on the basis of previous XPS investigations on similar compounds,24 it appears reasonable to conclude that the true amount of nitrogen within the nanoparticles should be at least 1 order of magnitude lower than the nominal one. Incidentally, we also note that great attention should be paid in reporting XPS-derived concentrations of dopant species in nanostructured materials, as it is known that the relative peak areas may be subject to very different sources of error, such as partial neglecting of secondary electrons, self-sputtering,69 and, of course, the presence of surface contaminants. In general, claims of accurate quantifications of near-surface dopants should be

Figure 1. XPS scans of the characteristic spectral region of the N 1s peak at source energy of 500 eV for three nominal N/Ti ratios: (a) 0.0, (b) 0.1, and (c) 0.5. Measured intensity (gray circles) is reported in arbitrary units as a function of the binding energy (B.E.). Purple curve: background; yellow and violet curves: individual peak contributions; red curve: total sum signal. The least-squares fitting was performed through the XPSPeak41 program.66

Figure 2. Diffractograms of the titania-based nanopowders at 0.0 and 0.4 nominal N/Ti ratios for different nitrogen sources (T: undoped; TNT: triethylamine; TNN: NH3; TNU: urea). The diffracted intensity is given in arbitrary units (au) with a maximum value of 24 600 (T and TNT), 13 200 (TNN), or 9600 (TNU) and divisions as large as 800 au (T and TNT) or 200 au (TNN and TNU). Empty circles: experimental observation; red line: fitting function; purple line: background; green line: point-by-point difference Iobs − Icalc between the measured and the computed intensities. 4800

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substantiated by as much accurate experimental procedures, such as the use of suitable internal standards and, when possible, avoiding of interfering contaminants. Otherwise, we believe that a quantitative XPS analysis is in general not possible. In conclusion, assuming that all the specimens are affected by surface contamination to a similar extent, the XPS analysis confirms that the N dopant is, at least in part, included within the nanoparticles. To see whether the guest nonmetal has an effect on the material microstructure, as well as whether there are differences upon varying the nominal N/Ti ratio and the chemical nature of the precursor, we performed a series of accurate X-ray diffraction experiments on the nanopowders (vide inf ra). 3.2. Crystallographic and Phase Composition Analysis. Among other techniques applied in nanotechnology research, XRPD is one of the most popular, as it is able to simultaneously retrieve information on the long-range bulk structure of crystalline nanoparticles and investigate their microstructure. In this context, the Rietveld method is widely employed for quantitative phase and microstructure studies.70,71 Moreover, as synchrotron radiation combines a high signal/noise ratio with a narrow instrumental contribution to the diffraction profiles, the HR-XRPD lattice parameters are usually both very precise and accurate.72,73 Figure 2 shows the least-squares fitting results versus the observed diffraction intensities for the undoped T specimen and for TNT, TNN, and TNU nanopowders at nominal N/Ti = 0.4 ratio. Figure S4 reports analogue plots for all the sample series here considered. Small intensity residuals are present in all the diffractograms. These are originated by the small inaccuracies of the final leastsquares structural model in the description of profile asymmetry caused by axial beam divergence at low angles and by the very defective nature of our nanostructured samples, which implies small anisotropies in the line profile broadening at higher angles. However, the residuals are well-centered around the peaks and they average to zero; i.e., cell parameters and main peak intensities are satisfactorily matched. The unique phases we identified in our samples were the anatase and brookite TiO2 polymorphs. Actually, the relatively weak and broad peak near 2ϑ ≈ 10° appearing in the TNN and TNT series, and corresponding to that at ≈30.8° in the TNU series, is entirely due to the (211) reflection of brookite, and it can be considered as a specific signature of the presence of this phase. It is worth noting that this peak fades away with respect to the undoped reference at high N/Ti ratios in the TNT and TNN series, but not in the TNU one (Figure 2). This proves a clear influence of the precursor on the phase composition of the material (vide inf ra). The structural models here employed take into account both the tetragonal anatase (I41/amd) and orthorhombic brookite (Pbca) phases. A clear trend is observed in the phase composition (Figure 3a), which is considerably affected by both the amount of N dopant and the nature of the nitrogen precursor. The undoped TiO2 sample contains ≈26% of brookite, which decreases almost monotonically at increasing dopant loadings in both the TNT and TNN series, until it falls below 10% when the nominal concentration is above N/Ti = 0.2. On the contrary, the TNU samples show a completely different behavior, with a slight (≈4%) increase in the amount of brookite across the low doping regime (N/Ti < 0.1), followed by a nominal concentration range where the composition remains essentially constant (≈70% of anatase

Figure 3. (a) Weight percent of the brookite phase vs the nominal N/ Ti molar ratio in nanostructured TiO2:N powders, as obtained from XRPD experiments. (b) Average volume-weighted crystallite size in anatase and brookite as a function of precursor and doping extent. Plotted curves serve as eye guidelines. When vertical error bars are not visible, symbol size roughly correspond to ±1 estimated standard deviations (esd’s).

and 30% of brookite, Figure 3a). At the same time, the volumeweighted average anatase crystallite sizes, ⟨Dv⟩, show a general tendency toward increasing values at higher N/Ti ratios. For example, in the TNN and TNT series this parameter ranges from ≈25−30 Å to ≈40−45 Å (Figure 3b), whereas TNU samples provide considerably higher estimates (up to ≈60 Å for N/Ti = 0.2). Even though the latter values could be slightly biased by the fact that they derive from different experimental settings (in-home vs synchrotron), it is reassuring that the general parameter trends are qualitatively similar in all the three sample series. It should be also noted that volume-weighted crystallite dimensions of anatase are in most cases greater than those of brookite (Figure 3b). As for the anatase crystallographic structure (Figure 4 and Table S3), analogies and differences are evident when the TNT,

Figure 4. Relative change of the |c| cell edge length of the anatase phase with respect to the undoped reference sample as a function of the nitrogen source and the nominal N/Ti dopant concentration, as retrieved from Rietveld refinement against X-ray diffraction data. Full (TNT), dashed (TNN), and dotted (TNU) lines are nonlinear leastsquares fittings against experimental estimates (see text). Error bars correspond to 1 estimated standard deviation (esd).

TNN, and TNU series are compared throughout the dopant concentration range. First of all, the |c| edge length always exhibits a well-defined monotonic trend as a function of N/Ti. Specifically, it undergoes an overall lengthening, which is generally significant in terms of the corresponding estimated 4801

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respect to the maximum distortion achievable by the anatase cell (|c|TNT, MAX > |c|TNN, MAX > |c|TNU, MAX). As the elongation of the |c|-axis is related to the starting N/Ti ratio, the following conclusions can be drawn: (i) the anatase cell distortion unequivocally marks the presence of dopant nitrogen within the core structure of the nanoparticles; (ii) the more N dopant is included in the material, the lower the quantity of brookite phase and (iii) the smaller are the corresponding crystallite dimensions (brookite). For example, the lowest ⟨Dv⟩ values for brookite occurred in the allegedly N-rich TNT series, while the highest ones were found in the allegedly N-poor TNU one (Figure 3b). In other words, the fact that an increasing nominal N/Ti ratio is not reflected in a significant distortion of the c edge implies that urea is poorly effective in including the dopant into the nanoparticles, providing a rationale for the relatively high and almost constant brookite content in the TNU series. Finally, (iv) the presence of a plateau in the maximum length of the c parameter as a function of the nominal dopant loading for all samples implies that there is an intrinsic structural limit to the ability of the lattice of including the guest nitrogen and preserving, at the same time, its translational invariance relations. This limit, at least according to the synthetic routes considered here, roughly corresponds to a nominal N/Ti ratio of 0.2. Some of these conclusions are not unexpected, as it is known that N hosting stabilizes the anatase phase.2 This in turn implies that, in order to provide a microscopic structural model accounting for the observed correlations among the c distortions and the N/Ti concentration, it is reasonable to focus just on the anatase structure. 3.2. Surface Area and Pore Volumes. Figure 5a reports the trend of the specific surface area of the oxide particles as a function of the N starting amount for the three tested sources. The surface area decreases for all samples with increasing initial N, more so in the TNT series. The nanoparticles promoted by urea (TNU samples) show the smallest surface area variation with respect to the undoped sample (about 15%), while those promoted by NH3 (TNN) and triethylamine (TNT) show much larger effects (25% and 45%, respectively). Figure S6 reports the adsorption isotherms of N2 obtained in subcritical conditions for the bare sample and TNT_0.1, with their relative hysteresis loops, as a representative comparison. The observed hysteresis is characteristic of a mesoporosity.75 By applying the BJH (Barrett, Joyner, Halenda) model76 based on capillary condensation in mesopores, the total pore volume can be obtained. The essential features of the hysteresis loops were explained by de Boer77 in terms of pore shape and the location and form of the liquid meniscus. Type E loops (like the ones observed in Figure S6) are often associated with capillary condensation in “ink bottle” pores, i.e., pores having narrow necks and wide bodies. All the curves from the present samples present the same shape but with different total pore volumes (Figure 5b). The comparison among the different pore volumes is striking: the porosity of the urea doped samples shows almost no variation, while in the case of higher amounts (N/Ti > 0.2) of triethylamine the total pore volume collapses completely. The trends, observed in Figure 5a,b, can be considered to mirror the structural effect produced by the addition of the three different N sources. In particular, by comparing Figures 3a and 5a, it can be generally inferred that the higher the anatase enrichment, the lower the surface areas. The dramatic loss of both surface area and pore volume observed in TNT samples can possibly be traced back to the

standard deviations. A plateau value is reached above 0.2 N/Ti nominal concentration, indicating that further inclusion of the dopant has no effect on the long-range crystallographic structure or that there is no actual inclusion for further nominal dopant concentrations. It is of paramount importance to find that the same structural trend is qualitatively recognizable in crystalline nanoparticles obtained from three independent synthesis procedures in conjunction with chemically different nitrogen precursors, even though the distortions detectable in the TNU series are poorly significant from a statistical viewpoint. Such a trend can be more precisely quantified upon noting that our experimental estimates for the |c| distortion can be modeled, for example, through an incremental law of the form y = p0 arctan(p1x), where p0 and p1 are adjustable parameters and x = 2π (N/Ti). Nonlinear least-squares fittings shown in Figure 4 were performed with the aid of the Fityk program74 and led to p0 =2.22(8) × 10−3, p1 = 2.8(5), R2 = 0.976 for the TNT series, p0 =1.8(6) × 10−3, to p1 = 1.0(7), R2 = 0.846 for TNN, and to p0 =0.41(2) × 10−3, p1 = 5(1), R2 = 0.986 for TNU. Second, the chemical nature of the nitrogen precursor has an important effect on the actual magnitude of the lengthening of the c vector module, with |c|TNT > |c|TNN > |c|TNU as a general trend (Figure 4). Interestingly, the maximum |c| elongation follows the same trend, with the TNT series providing the largest increment (up to ≈0.3%), the TNU one shows the smallest increase (≈0.05%), and the TNN samples exhibit an intermediate maximum expansion (≈0.2%). As concerns the two symmetry-equivalent a- and b-axes of anatase, they always undergo just minor (≤0.06%) fluctuations (see Figure S5 and Table S3). These variations are barely significant from a statistical viewpoint and without a clear trend. Because of its greater length, the c parameter plays a major role in governing the behavior of the anatase cell volume (see Figure S5), and its changes just mirror those experienced by the |c| module itself. For example, in the TNN (TNT) series, the cell undergoes a +0.3 (+0.4) Å3 large overall expansion in the heavily doped samples (N/Ti ≥ 0.2) with respect to the undoped specimen. However, this is no longer true for the TNU series, where the |c| lengthening is very small (Figure 4) and comparable in magnitude to the fluctuations affecting the other cell edges. Therefore, in this case the cell volume exhibits a nonmonotonic behavior, oscillating around an average value of 135.60(2) Å3. In other words, the doping has just a minimal effect, if any, on this series. On the other hand, the brookite cell edges and volume (Figure S5) remain constant or are subject to rather erratic fluctuations, no matter the nature of the nitrogen precursor. Even though a possible tendency toward higher |c| values for N/Ti > 0.2 may be inferred in the TNT and TNN series, it should be also noted that the precision of the refined brookite parameters is quite low at high dopant concentrations, as for N/Ti > 0.2 the phase composition is overbalanced toward anatase (see above). Therefore, in our opinion the changes we detected in this phase do not allow unambiguous determination of trends, as their magnitude is barely significant from a statistical viewpoint. Intriguingly, changes of the magnitude of the anatase |c| edge vector lengthening can be related to the material phase composition commented above. The weight percent of brookite, wb, increases through the sequence wbTNT < wbTNN < wbTNU at comparable nominal N/Ti concentrations (Figure 3a). This trend follows a qualitative inverse correlation with 4802

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magnitude of the actual dopant amount we expect in the bulk (see above). Figure 6 shows the relative changes affecting the c

Figure 5. Results from the BET analysis. (a) Specific surface area (m2 g−1) of the TNT, TNN, and TNU series of materials as a function of the nominal N/Ti ratio. (b) Specific pore volume (mL g−1) distributions of the TNT, TNN, and TNU series of materials as a function of the nominal N/Ti ratio, as obtained from N2 adsorption isotherms (see text).

Figure 6. (a) Relative changes affecting the computed c cell edge length of anatase with respect to the stoichiometric structure for different defect types (see text). Open triangles: PAW calculations at the PBE(U = 0) theory level. Open rhombi: PAW calculations at the PBE(U = 5) theory level. Full squares: LCGTF calculations, PBE0 Hamiltonian. Full circles: LCGTF, B3LYP Hamiltonian. The lines serve only as guides for the eye. (b) Same as (a), for the crystallographic cell volume. The horizontal bold bar highlights the undoped, fully relaxed anatase structure.

high local temperatures produced by the combustion of the organic chains of the dopant molecule and to the nature of the C-bonded N species. The opposite case is represented by TNU samples in which the dopant molecule is unstable at the temperature of calcination; moreover, the CO2, coming from the decomposition reaction, can act as a gas carrier in taking away the N species from the titania lattice. Lastly, in the case of TNN samples, intermediate sequences are appreciable due to concomitant chemisorption of ammonia at Broensted and Lewis acid sites of the oxide78 and its easy evaporation at the heating temperature used (400 °C). 3.3. DFT Simulations. Bulk theoretical simulations allow us the investigation of the effect of local distortion on the coordination polyhedron around Ti on the long-range lattice structure, if all the translation-related metal centers in the crystal were close to the same doping-induced point defect. In other words, within this picture each local distortion is amplified throughout the lattice by the exploitation of the crystal translational symmetry through the Born−Von Karmàn periodic conditions. As concerns the present work, significant 2ϑ shifts of the anatase reflections, resulting in the elongation of the c lattice vector, unequivocally appear in XRPD diffractograms (see above). In other words, nitrogen ions are able to significantly distort the average long-range anatase lattice structure. In part, this is due to higher dimensions of nitride ions substituting oxides, but the occurrence of a plateau unequivocally shows that some other structural effects, such as N-induced formation of vacancies,29 are at stake. To perform DFT quantum mechanical simulations, we took into account both interstitial and substitutional nitrogen, together with oxygen vacancies. The same defect/Ti ratio of 0.0625 was employed throughout, as it is somewhat of the same order of

cell edge length and volume with respect to the undoped TiO2 anatase structure, as estimated from the theoretical methods. Table S4, on the other hand, summarizes all the corresponding cell parameters as a function of the defect nature (oxygen vacancy, Ovac; nitrogen interstitial/substitutional, Ni/Ns) and the theoretical recipe employed for the periodic structure optimization. First, it should be noted that PAW and LCGTF show only a general qualitative agreement in describing the long-range structure distortions. On average, the LCGTF approach predicts systematically higher changes (in absolute value) upon doping than the PAW one. As a general consideration, we do not demand here that the outcomes from the PAW and LCGTF approaches are quantitatively comparable due to their inherent conceptual differences together with the different Hamiltonians (either hybrid or not) here spanned. Instead, we intend to see if some model-independent conclusions, at least at a qualitative level, hold true. Some discrepancies between the two methods are indeed observed for the behavior of the c-axis in the Ovac and Ns scenarios (Figure 6a). It is worth noting, however, that almost all the optimizations find cell volume changes of the same sign upon the various kinds of doping, no matter the changes affecting the individual lattice constants (Figure 6b). In our opinion, this is a remarkable conformity of views. Second, both the Ovac and the Ns models do not imply significant changes from the PAW perspective, with the cell volume showing at most a slight contraction in both the defective compounds. On the one hand, these scenarios are neatly differentiated by the LCGTF method (full squares and circles in Figure 6b), which finds either a contraction (from 4803

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−0.6 to −0.8%, depending on the Hamiltonian) or an expansion (from +0.3 to +0.4%) of the cell volume in Ovac and Ns optimizations, respectively. On the other hand, when interstitial nitrogen (Ni) is considered, both the PAW and LCGTF methods agree in predicting an expansion of the cell volume. It is also noteworthy that (i) PAW and LCGTF recipes both associate this volume expansion with a possible increment of the c edge length (Figure 6a) and that (ii) other doping scenarios imply quite different distortions. For example, from the PAW viewpoint the substitutional nitrogen dopant would imply a very small contraction of the c-axis (see the open dots in Figure 6a), whereas the LCGTF calculations find an increment of the c module, the latter being significantly smaller for the Ns model (+0.6, +0.7%) with respect to the Ni one (+1.1, +1.4%). In conclusion, theoretical simulations found that a simultaneous increment of the c edge module and of the cell volume can be associated with an interstitial (PAW) or to either an interstitial or substitutional (LCGTF) nitrogen guest. In general, it is worth remarking that it is very difficult to determine the real nature of point defects in these materials, as it cannot be excluded that interstitial N may coexist with substitutional N and oxygen vacancies in the lattice. Actually, photovoltage24 and EXAFS29 experiments strongly suggest that a significant amount of oxygen vacancies should be present in the TNT sample series at high dopant concentration. The average crystal structure observed by XRPD is likely due to the compromise among the various competitive cell distortions that the dopant inclusion triggers. Accordingly, differences in the maximum |c| lengthening discussed above should primarily depend on the amount of N dopant included into the anatase lattice together with the relative concentration of oxygen vacancies and the chemical nature of the dopant (Ni/Ns). More detailed investigations on these issues are in order and will be the subject of further studies.

XRPD experiments allowed us to investigate the crystallographic structure of these materials as a function of both the dopant concentration and the nature of the nitrogen precursor. The main structural effect we detected was a systematic and monotonic lengthening of the |c| cell edge vector module at increasing nominal N/Ti concentration up to a plateau that is reached for N/Ti ≈ 0.2. The maximum c lengthening is determined by the nature of the starting N source, with |c|triethylamine > |c|ammonia > |c|urea. At the same time, the diffraction analysis provided compelling evidence that the brookite content of the material is inversely correlated to the amount of effective dopant N included in the lattice. In this respect, the most effective precursors are triethylamine and ammonia, while urea produced essentially (or at least very slightly) undistorted unit cells in conjunction with high brookite weight fractions, invariant with respect to the nominal N/Ti ratio. Solid-state quantum-mechanical simulations confirmed that these kinds of lattice deformations are associated with the presence of bulk nitrogen, either substitutional or interstitial, as prevailing of oxygen vacancies would result in a neat contraction of the cell. From the theory level employed here, however, it was not possible to unequivocally discriminate the chemical nature of the defective N, as LCGTF and PAW simulations gave conflicting results in predicting unit cell distortions for the substitutional case. As for the material morphology at the mesoscale, BET investigations showed a dramatic loss of both surface area and pore volume observed in TNT samples at increasing nominal N/Ti concentrations. The latter can probably be traced back to the high local temperatures produced by the combustion of the organic chains of the dopant molecule and to the nature of the C-bonded N species. On the contrary, urea instability at the calcination temperature seems to hamper the effective inclusion of N dopant in the TNU series, whereas the intermediate behavior of the TNN samples may be rationalized in terms of concomitant chemisorption of ammonia at acid sites of the oxide and its easy evaporation. In conclusion, probing the various materials at different length scales as a function of (i) the nominal doping extent and (ii) the nature of the chemical precursor allowed us to rationalize the mutual role of these factors in determining the observed (micro)structure and morphology. We demonstrated that, upon selecting the proper precursor and the desired nominal N/Ti concentration, it is possible to tailor with good confidence crucial microscopic and mesoscopic parameters of the material, such as (i) phase composition, (ii) surface area and morphology, and (iii) crystallographic cell distortions. Although it was not possible to accurately quantify the dopant concentration within the nanoparticles, we showed that XRPD results do not suffer from undesired contaminant N, that is mainly localized at the surface and it is expected to be essentially invariant with respect to the nature and stoichiometry of the precursor. On the contrary, the effective bulk loadings of chemically provided N can at least qualitatively be inferred from the maximum distortion of the |c| module that is detected with respect to the undoped sample. Nevertheless, the specific and mutual roles of phase composition, microstructure and nature of N-induced point defects remain to be investigated from the viewpoint of band gap engineering, in the context of developing new materials for environmental and industrial applications.

4. CONCLUSIONS In this work, we performed a thorough and multidisciplinary (spectroscopic, diffractometric, and quantum-mechanical) investigation of the microstructure and morphology of three series of nanostructured TiO2:N samples in a broad (0.0 ≤ N/ Ti ≤ 0.5) dopant concentration range. More in detail, triethylamine, urea, and NH3 were taken into account as nitrogen precursors to provide hints on possible routes to control specific features of the material (polymorph enrichment, dopant levels, surface area). Through accurate synchrotron XPS measurements on the triethylamine-doped series, the chemical states of N species in nanotitania matrices were investigated. We found that in standard ambient conditions a non-negligible amount of surface N contaminant is always to be expected, while guest dopant nitrogen included in the near-surface layers of the nanoparticles was invariably detected in the doped materials. At the same time, the highly defective nature of these specimens was confirmed. No appreciable changes were detected in the chemical state of the N dopant ions as a function of the N/Ti ratio. On the contrary, the presence of surface contaminants and the impossibility of quantifying the relative contributions of (unwanted) ambient and (desired) chemical sources of guest N at the surface hampered any XPS-based accurate estimation of the actual dopant concentration in the nanoparticles. In any case, XPS allows the N/Ti atomic ratio to be fixed at values at least 1 order of magnitude lower than the starting amounts. 4804

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(8) Diwald, O.; Thompson, T. L.; Zubkov, T.; Goralski, E. G.; Walck, S. D.; Yates, J. T. Photochemical Activity of Nitrogen-Doped Rutile TiO2(111) in Visible Light. J. Phys. Chem. B 2004, 108, 6004−6008. (9) Burda, C.; Lou, Y. B.; Chen, X. B.; Samia, A. C. S.; Stout, J.; Gole, J. L. Enhanced Nitrogen Doping in TiO2 Nanoparticles. Nano Lett. 2003, 3, 1049−1051. (10) Gole, J. L.; Stout, J. D.; Burda, C.; Lou, Y. B.; Chen, X. B. Highly Efficient Formation of Visible Light Tunable TiO2‑XNx Photocatalysts and Their Transformation at the Nanoscale. J. Phys. Chem. B 2004, 108, 1230−1240. (11) Cong, Y.; Zhang, J. L.; Chen, F.; Anpo, M. Synthesis and Characterization of Nitrogen-Doped TiO2 Nanophotocatalyst with High Visible Light Activity. J. Phys. Chem. C 2007, 111, 6976−6982. (12) Sakthivel, S.; Janczarek, M.; Kisch, H. Visible Light Activity and Photoelectrochemical Properties of Nitrogen-Doped TiO2. J. Phys. Chem. B 2004, 108, 19384−19387. (13) Xing, M. Y.; Zhang, J. L.; Chen, F. New Approaches to Prepare Nitrogen-Doped TiO2 Photocatalysts and Study on Their Photocatalytic Activities in Visible Light. Appl. Catal., B 2009, 89, 563−569. (14) Nakamura, R.; Tanaka, T.; Nakato, Y. Mechanism for Visible Light Responses in Anodic Photocurrents at N-Doped TiO2 Film Electrodes. J. Phys. Chem. B 2004, 108, 10617−10620. (15) Kitano, M.; Funatsu, K.; Matsuoka, M.; Ueshima, M.; Anpo, M. Preparation of Nitrogen-Substituted TiO2 Thin Film Photocatalysts by the Radio Frequency Magnetron Sputtering Deposition Method and Their Photocatalytic Reactivity under Visible Light Irradiation. J. Phys. Chem. B 2006, 110, 25266−25272. (16) Yin, S.; Yamaki, H.; Komatsu, M.; Zhang, Q. W.; Wang, J. S.; Tang, Q.; Saito, F.; Sato, T. Preparation of Nitrogen-Doped Titania with High Visible Light Induced Photocatalytic Activity by Mechanochemical Reaction of Titania and Hexamethylenetetramine. J. Mater. Chem. 2003, 13, 2996−3001. (17) Maeda, M.; Watanabe, T. Visible Light Photocatalysis of Nitrogen-Doped Titanium Oxide Films Prepared by Plasma-Enhanced Chemical Vapor Deposition. J. Electrochem. Soc. 2006, 153, C186− C189. (18) Irie, H.; Watanabe, Y.; Hashimoto, K. Nitrogen-Concentration Dependence on Photocatalytic Activity of TiO2‑XNx Powders. J. Phys. Chem. B 2003, 107, 5483−5486. (19) Napoli, F.; Chiesa, M.; Livraghi, S.; Giamello, E.; Agnoli, S.; Granozzi, G.; Pacchioni, G.; Di Valentin, C. The Nitrogen Photoactive Centre in N-Doped Titanium Dioxide Formed Via Interaction of N Atoms with the Solid. Nature and Energy Level of the Species. Chem. Phys. Lett. 2009, 477, 135−138. (20) Katoh, R.; Murai, M.; Furube, A. Electron−Hole Recombination in the Bulk of a Rutile TiO2 Single Crystal Studied by SubNanosecond Transient Absorption Spectroscopy. Chem. Phys. Lett. 2008, 461, 238−241. (21) Umezawa, N.; Ye, J. Role of Complex Defects in Photocatalytic Activities of Nitrogen-Doped Anatase TiO2. Phys. Chem. Chem. Phys. 2012, 14, 5924. (22) D’Arienzo, M.; Siedl, N.; Sternig, A.; Scotti, R.; Morazzoni, F.; Bernardi, J.; Diwald, O. Solar Light and Dopant-Induced Recombination Effects: Photoactive Nitrogen in TiO2 as a Case Study. J. Phys. Chem. C 2010, 114, 18067−18072. (23) Spadavecchia, F.; Ardizzone, S.; Cappelletti, G.; Falciola, L.; Ceotto, M.; Lotti, D. Investigation and Optimization of Photocurrent Transient Measurements on Nano-TiO2. J. Appl. Electrochem. 2013, 43, 217−225. (24) Spadavecchia, F.; Cappelletti, G.; Ardizzone, S.; Ceotto, M.; Falciola, L. Electronic Structure of Pure and N-Doped TiO2 Nanocrystals by Electrochemical Experiments and First Principles Calculations. J. Phys. Chem. C 2011, 115, 6381−6391. (25) Bellardita, M.; Addamo, M.; Di Paola, A.; Palmisano, L.; Venezia, A. M. Preparation of N-Doped TiO2: Characterization and Photocatalytic Performance under UV and Visible Light. Phys. Chem. Chem. Phys. 2009, 11, 4084. (26) Di Valentin, C.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Giamello, E. Characterization of Paramagnetic Species in N-Doped TiO2

ASSOCIATED CONTENT

S Supporting Information *

Characteristic Ti and N XPS parameters and spectra before and after sputtering by Ar+ ion; XRPD parameters and diffractograms; XRPD cell parameters at room temperature as a function of (i) precursor, (ii) dopant loading, and (iii) crystallographic phase; DFT cell parameters at different theory levels; N2 adsorption/desorption isotherms in subcritical conditions; LGTCF basis sets. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +390250314252; Fax +390250314300 (L.L.P.). Present Address

F.S.: eni S.p.A - Refining & Marketing Division, San Donato Milanese Research Center, Via F. Maritano 26, I-20097 San Donato Milanese (MI), Italy. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by the University of Milan Research Funds (PUR) and a starting grant from the same University (“Cinque per mille”). The European Synchrotron Radiation Facility (ESRF) and the Elettra synchrotron are acknowledged for beamtime provision. We also warmly thank Dr. W. van Beek for his experimental support. Eventually, the LISA program “MATGREEN” sponsored by CINECA and Regione Lombardia is gratefully acknowledged for providing us computational time. Partial funding by the Danish National Research Foundation (DNRF) through the Center for Materials Crystallography (CMC) has been also very much appreciated.

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