Benzyl Alcohol-Mediated Versatile Method to Fabricate

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Benzyl Alcohol-Mediated Versatile Method to Fabricate Non-Stoichiometric Metal Oxide Nanostructures Mohammad Qamar, Alaaldin Adam, Abdul-Majeed Azad, and Yong-Wah Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09515 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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ACS Applied Materials & Interfaces

Benzyl Alcohol-Mediated Versatile Method to Fabricate NonStoichiometric Metal Oxide Nanostructures Mohammad Qamar,*† Alaaldin Adam,† Abdul-Majeed Azad,§ Yong-Wah Kim‡ †

Center of Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals Dhahran 31261, Kingdom of Saudi Arabia.

§ ‡

Acense LLC, Twinsburg, OH 44087, USA. Department of Chemistry and Biochemistry, University of Toledo, Toledo, OH 43606, USA.

ABSTRACT: Nanostructured metal oxides with cationic or anionic deficiency find applications in a wide range of technological areas including energy sector and the environment. However, a facile route to prepare such materials in bulk with acceptable reproducibility is still lacking; many synthesis techniques are still only bench-top and cannot be easily scaled-up. Here, we report that benzyl alcohol-mediated method is capable of producing a host of nanostructured metal oxides (MOx, where M = Ti, Zn, Ce, Sn, In, Ga or Fe) with inherent non-stoichiometry. It employs multifunctional benzyl alcohol (BA) – as a solvent, a reducing agent as well as a structure- directing agent. Depending on the oxidation states of metals, elemental or non-stoichiometric oxide forms are obtained. Augmented photoelectrochemical oxidation of water under visible light by some of these non-stoichiometric oxides highlights the versatility of the BA-mediated synthesis protocol. KEYWORDS: metal oxide, oxygen vacancy, benzyl alcohol, non-aqueous synthesis, photocatalysis, water oxidation 1. INTRODUCTION Metal oxides are the most abundant materials in Earth's crust and constitute an important class of inorganic solids. Their applications encompass a number of areas, such as, catalysis, energy storage and conversion and, functional ceramics including electronics, sensors and biomedical.1,2 Their electronic and chemical properties are invariably a function of surface or bulk defects, whose nature and concentration is predominantly determined by the preparative technique. The relevance of non-stoichiometric oxides in materials science has been amply reviewed.3,4 Methods that are predominantly employed to induce defects in oxides are based on solid state reactions such as calcination under high pressure of H22,5 or vacuum annealing at high temperature.6 On the contrary, material preparation through liquidphase routes is appealing as they enable controlling nucleation and growth at the molecular level (hence size and well-defined uniform crystal morphologies), reaction pathway, homogeneity, etc. However, soft-chemistry protocol requires strong reducing agents, such as NaBH4.7 The presence of reducing agents in the solution media could alter the precursor and solution chemistry, thereby endowing the end product with undesired structural features. Therefore, the development of a mild and benign liquid-phase protocol without the aid of a strong reducing agent is highly desirable; such a strategy, however, to a great extent still remains elusive. Here, we report that benzyl alcohol-mediated method could be pursued for preparing a host of non-stoichiometric metal oxides at nanoscale. Extensive work using benzyl alcohol (BA) for the preparation of a wide variety of organic– inorganic hybrid nanomaterials has been carried out by Niederberger and co-workers,8-23 and other researchers.24-27 Metallic nanoparticles of cobalt,28 nickel29 and copper30 were ob-

tained by BA-method, indicating reductive feature of benzyl alcohol. However, this method yet remained to be generalized for preparing metal oxides with inherent non-stoichiometry. Here, a distinctive characteristic of this route is demonstrated – benzyl alcohol introduces non-stoichiometric defects in metal oxides during crystal growth. Such defects are present both on the surface as well as in the bulk. The efficacy of BA to produce non-stoichiometric nanostructures was demonstrated in the case of a number of oxides (MOx, where M = Ti, Zn, Ce, Sn, In, Ga or Fe). The photoelectrochemical oxidation of water at λ > 420 nm was also investigated. 2. EXPERIMENTAL SECTION Experimental details of syntheses are described in the Supporting Information. In a typical synthesis, metal precursors of desired molarity were added to 20 mL of benzyl alcohol under constant stirring to allow complexation between the two, at the end of which the clear solution was transferred to a Teflon vessel and heated at 245 °C for 48 h. After completion of the solvothermal reaction, the solid residue was collected and washed with acetone and ethanol repeatedly and dried under vacuum at 240 °C. The photoelectrocatalytic (PEC) activity was investigated in a 3-electrode cell assembly connected to a potentiostat. A 0.1 M sodium sulphate solution was used as the electrolyte. Saturated calomel electrode and coiled platinum wires were used as the reference and counter electrode, respectively. The working electrode was prepared from a homogeneous suspension of Nafion® and the catalyst in ethanol by sonication and deposited on tin doped indium oxide (ITO) substrate. A 300 W Xenon lamp with a cut-off filter to obtain radiation of λ >420 nm was used. This illuminated ~1 cm2 of the photocatalyst surface.

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ACS Applied Materials & Interfaces 3. RESULTS AND DISCUSSION Detailed mechanism involving complexation, nucleation with concomitant growth of oxide materials through benzyl alcohol method has been extensively discussed earlier by Niederberger and co-workers.8-11 The XRD patterns shown in Fig. 1 confirm the formation of each of the intended formulations with high crystallinity. XRD patterns of Fe3O4, cobalt and copper are shown in Fig. S2. The formation of elemental Co and Cu nanoparticles (with traces of oxide impurities) validates the earlier and current notion that in some instances, BA is capable of acting as a potent reducing agent under solvothermal conditions.

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Figure 1. XRD patterns of BA-derived oxides. The structure-directing trait of BA was ascertained by detailed microscopic analyses conducted by HR-TEM, as shown in Fig. 2. The images highlight the size, morphology and homogeneity aspects of the TiOx, ZnOx, CeOx and Ga2Ox nanostructures. Formation of predominantly planar (square- or rectangle-shaped) nanoplates was observed in most of the cases. For instance, in the case of TiOx, nearly monoshaped square nanoplates with size in the range of 10-20 nm were formed, as can be easily ascertained from the HR-TEM image in Fig. 2b, which also corroborates the

high crystallinity seen in its diffraction pattern (Fig. 1). The calculated interplanar distance was 0.352 nm, which is same as that computed from the XRD reflection of plane of anatase TiO2. The morphology of ZnO nanoparticles is seen to be spherical with size in the range of 20 to 40 nm (Fig. 2c); the d-spacing computed from the HR-TEM image was 0.24 nm, which agrees well with that derived from the XRD data for the plane of ZnO. Predominantly homogeneous and uniform nanoplates of CeOx (< 10 nm) are formed (Fig. 2e), whose interplanar distance for the plane was calculated to be 0.299 nm (Fig. 2f). In the case of Ga2Ox (Fig. 2g), a mixture of quasi-spherical nanoparticles and nanoplates was formed; the d-spacing (Fig. 2h) of 0.277 nm agrees well with that for the plane of β-Ga2O3. The TEM/HR-TEM images and corresponding SAED patterns for SnOx, Fe3O4, In2Ox and Cu are shown in Figs. S3-S6. Predominant square nanoplates of elemental copper co-existed with some irregular morphology (Fig. S6). Metallic cobalt crystallizes in different morphologies (Fig. S7), reiterating the role of benzyl alcohols as a structure-directing agent, in addition to being a powerful reductant. Earlier investigation reported the formation of Cobased products with different compositions and oxidation states with spherical shape employing BA.28 Based on earlier and current findings, it seems reasonable to infer that the shape of cobalt and cobalt oxide could also be manipulated by

Figure 2. TEM and HR-TEM images of: TiOx (a, b), ZnOx (c, d), CeOx (e, f) and Ga2Ox (g, h).

tuning synthesis conditions such as metal precursor and process temperature. BET surface area of BA-derived and commercial samples were measured. Surface areas of TiOx, ZnOx, In2Ox, SnOx, CeOx, Ga2Ox, Fe3O4, Co/CoO and Cu/Cu2O were 85.7, 3.6, 29.1, 86.9, 175.8, 72.8, 10.3, 38.7 and 2.1 m2 g−1, respectively. The surface area of commercial TiO2 and ZnO was measured to be 93.4 and 11.2 m2 g−1, respectively. Surface composition and oxidation states were probed by Xray photoelectron (XPS) and the results for TiOx, Ga2Ox, SnOx and CeOx are shown in Fig. 3 and also summarized in Table S10. The XPS spectra for Ti and O in as-prepared TiOx are presented in Figs. 3a and b. Moreover, the XPS profiles of Ti in TiOx and commercial TiO2 are compared in Fig. 3c. The difference in the oxidation states of Ti indicates the existence of non- stoichiometry with respect to oxygen in titania made by BA-route. For example, the binding energy peaks at 458.8 and 464.2 eV in commercial titania (Ti4+O2) correspond to the Ti2p3/2 and Ti2p1/2 states, respectively.31,32 On the other hand, the Ti2p3/2 peak in the BA-derived TiOx is slightly shifted (458.2 eV), suggesting the existence of Ti3+.31,32 As alluded above, this shift, though modest, could be an artifact of the change in chemical environment around Ti ions. In the O1s spectrum (Fig. 3b), the peak at 529.6 eV belongs to O1s, structurally bonded to Ti. The peak at 531.3 eV is usually assigned to hydroxyl groups or O2− ions in the TiO2 lattice.31,32 Two distinct 2p3/2 peaks corresponding to Ga3+ (1121.8 eV) and Ga2+ (1119.7 eV) in gallium oxide are shown in Fig. 3d;33,34 the corresponding O1s peak at 531.6 eV is assigned to the OGa3+ bond (Fig. 3e).33,34 The spin orbit doublets of Sn3d3/2 appear at 487.6 and 496.1 eV for tetravalent tin (Sn4+), while the peaks at 486.9 and 495.3 eV are representative of Sn2+ (Fig. 3f).35 The value of spin orbit splitting energy (∆E) is approximately 8.5 and 8.6 eV which agrees well with those reported for Sn4+ and Sn2+, respectively.36 The peaks at 530.8 and 531.4 eV in the O1s spectra (Fig. 3g) corroborate the presence of O-Sn4+ and O-Sn2+ bonding in the compound.37 The deconvoluted Ce3d XPS profile of CeOx consists of several peaks (Fig. 3h). Those at 882.6 eV (Ce3d5/2) and 901.3 eV


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Figure 3. XPS signatures of: TiOx and anatase TiO2 (a, b, c), Ga2Ox (d, e), SnOx (f, g), and CeOx (h, i).

(Ce3d3/2) are assigned to Ce3+,38 while those appearing at 897.6 eV and 913.8 eV are ascribed to Ce4+.38 In addition, there are other peaks known as “shake up” peaks, usually observed in CeO2.38 Analysis of O1s spectra in CeOx (Fig. 3i) identifies two principal signals at ~525 and 527.3 eV, known respectively as Oα and Oβ; the Oα signal is usually associated with lattice oxygen while Oβ belongs to oxygen vacancies.39 A similar analysis of the XPS data collected on In2Ox and ZnOx revealed the existence of metals in different chemical states (Figs. S8 and S9). The electron paramagnetic resonance (EPR) spectroscopy was employed to further probe and substantiate the presence of intrinsic defects, together with reduced or paramagnetic metal centers (Fig. 4). The spectra for titania were collected on both the commercial and BA-derived sample without any heattreatment. However, as-prepared ZnOx, SnOx, In2Ox and CeOx were heated in static air at 500 °C for 2 h and EPR was recorded for comparison. A weak EPR signal in commercial TiO2 (trace a) signifies a lower level of intrinsic Ti3+. On the other hand, two stronger signals (trace b) were recorded in the case of BA-derived TiOx. The peak at g =1.942 is characteristic of paramagnetic Ti3+ centers in a distorted rhombic oxygen ligand field, whereas the peak at g = 2.002 is assigned to the oxygen vacancy created as a result of paramagnetic Ti3+ ion.40,41 Detection of Ti3+ at room temperature analysis indicated the presence Ti3+ in the bulk structure as well. Similarly, the EPR of ZnOx (trace b) consists of two prominent signals, which are usually ascribed to bulk and surface defects. In the core-shell model, the bulk defects are located in the core and the surface forms in the outer shell.42 The weaker signal at g = 2.003 indicates a lower concentration of surface defects,42 compared to defects originating from the bulk at g = 1.958.42 These defects are formed as a results of partial reduction of ZnO in the BAsolution during their forming stage. However, the very nature and precise origin of these defects is not clear. It is also not discernible if these defects are shallow donors, singly-ionized oxygen atoms, zinc vacancies, or oxygen and/or zinc interstitials.43,44 The EPR spectrum of heat-treated ZnOx (trace a) shows substantial increase in shell defect at g = 2.003, while the signal at g = 1.958 became weaker suggesting a decrease

Figure 4. EPR spectrums of TiOx, ZnOx, SnOx, In2Ox, and CeOx: (a) – recorded after heat treatment and (b) – as-synthesized, except for TiOx in which spectrum (a) corresponds to commercial TiO2.

in the concentration of bulk defects. A visual inspection showed that the heat-treated ZnOx did not return to its natural white color. The reason for retaining the light gray-brown color is unknown at this stage, and requires a more in-depth investigation. Likewise, the EPR spectra of SnOx also indicate the presence of oxygen vacancies and paramagnetic species, substantiating the inferences made above with the XPS data. The peak at g = 2.002 is indicative of the formation of oxygen vacancies,45,46 while the broader signal at g = 1.885 could be ascribed to the metal ion centers as a result of in-situ reduction. These peaks almost disappeared in the sample annealed in air at 500 °C. A similar explanation holds for In2Ox, where the resonance at g = 2.005 is attributed to the presence of singly charged oxygen vacancies, formed either intrinsically or created as a result of partial reduction of In3+.47 The In2Ox spectra shows a second paramagnetic resonance at g = 1.874 that is attributed to surface defects. This resonance disappeared in the heat-treated sample due to the oxidation of paramagnetic metallic center to In3+. In3+ is diamagnetic and does not have an EPR response. The EPR signature of ceria is rather complex, suggesting the presence of Ce3+ under two different chemical environments. The g = 2.005 peak is attributed to a Ce3+ cation chemically bonded to O2- and O- species. While that at g = 1.966 is assigned to Ce3+ ions stabilized by the lattice defects in the distorted fluorite structure of ceria.48 The signal at g = 2.005 disappeared in the heated samples, indicating that the above-stated defects were annealed. Heat treatment has a relatively minor effect on the signal intensity at g = 1.966, indicating the presence of inherent lattice defects both in the original and heat-treated samples by virtue of BAmediated synthesis. One of the most important applications of such nonstoichiometric oxides is in solar energy conversion.8,31,37,40,47 Hence, the photoelectrochemical (PEC) activity of selected oxides was investigated by studying water oxidation. The results of polarization studies under visible light for BA-derived TiOx, In2Ox, ZnOx and pure ITO coated glass are shown in Fig. 5A. Figures 5B-D show the variation of photocurrent (under illumination and in dark; λ ≥ 420 nm) in the presence of TiOx, In2Ox, and ZnOx. The photocurrent was generated instantaneously upon illumination and reached a steady state while very low current was recorded under dark, even at an applied voltage of 0.9 V. Furthermore, PEC activities of TiOx and ZnOx were compared with those of commercial TiO2 (anatase) and


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AUTHOR INFORMATION Corresponding Author

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Figure 5. (a) Comparative potentiodynamic (I-V) response of TiOx, In2Ox, ZnOx and bare ITO, and variation in photocurrent under intermitted light illumination: (b) In2Ox, (c) TiOx, and (d) ZnOx. The temperature-dependent variation in the photocurrent is shown in (e) for TiOx in (f) for ZnOx. The a, b and c traces correspond to the photocurrent density at 210, 235 and 245 °C, respectively. λ = > 420 nm. nanoscale ZnO (Figs. 5C and D). Apparently, the BA-made TiOx and ZnOx show significantly higher photocurrent than their commercial counterparts; in the case of TiOx, the increment was ~ 17 fold. In spite of the fact that the surface area of BA-derived TiOx (85.7 m2 g−1) and ZnOx (3.6 m2 g−1) was lower than that of the commercial TiO2 (93.4 m2 g−1) and ZnO (11.2 m2 g−1), yet the photoelectrocatalytic efficiency of the BA-derived samples was far better. This could be attributed to the synergy brought about by higher light absorption (Fig. S11); unique morphologies also played a significant role. Moreover, temperature-dependent PEC of TiOx and ZnOx increased with increase in the processing temperature (Figs. 5E and F respectively). This could likely be ascribed to the improved crystallinity, as indicated by the XRD patterns (Fig. S12). 4. CONCLUSION In summary, the preparation of non-stoichiometric metal oxides at nanoscale using benzyl alcohol in the absence of an additional reducing agent is described. The method is simple with the potential to scale-up. Its efficacy for the synthesis of a variety of metal oxides (TiOx, ZnOx, In2Ox, SnOx, CeOx, Fe3O4 and Ga2Ox) was demonstrated. Benzyl alcohol was shown to possess multifunctionality; it served as a solvent, a strong reductant and, a structuredirecting agent in solution. Metals with low (≤2) oxidation states, such as cobalt and copper, tend to transform into their elemental form, whereas those with higher (≥3) oxidation states, such as titanium, zinc, indium, gallium, cerium, iron, and tin, convert into non-stoichiometric oxide forms. Superior photoelectrochemical performance towards water oxidation under visible light indicates the potential of these non-stoichiometric materials in a host of industrially important applications. We believe that this strategy is likely to open new avenues for the manipulation of chemical attributes of technologically relevant nanostructured materials, as nucleation and growth of particles in homogeneous solution environment plays a critical role in imparting the desired functionality to the surface as well as the bulk.

ASSOCIATED CONTENT Supporting Information. Details of materials synthesis, structural and photoelectrochemical characterization, powder XRD of Fe3O4, copper and cobalt, TEM/HR-TEM images and SAED of SnOx, Fe3O4, In2Ox, copper and cobalt, XPS of In2Ox and ZnOx, diffuse reflectance spectra of TiOx and ZnOx, and temperaturedependent evolution in XRD patterns of TiOx and ZnOx. This

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. 13NAN1627-04 as part of the National Science, Technology and Innovation Plan.

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