State of the art of nanoforest structures and their applications

Boris I. Kharisov, Oxana V. Kharissova*, Beatriz Ortega García, Yolanda Peña Méndez and Idalia Gómez de la Fuente
Universidad Autónoma de Nuevo León, Monterrey, Mexico. E-mail: bkhariss@hotmail.com

Received 29th October 2015 , Accepted 24th November 2015

First published on 26th November 2015


Abstract

Forest-like nanostructures, their syntheses, properties, and applications are reviewed. Nanoforests are mainly represented by carbon nanotubes, zinc and titanium oxides, and gold, and much less frequently by other metals, metal oxides, arsenides and phosphides. These nanostructures generally consist of more simple 1D objects, such as nanowires, nanopillars, nanorods, nanotrees, nanofibers or nanotubes. Synthesis methods for nanoforests vary from catalytic pyrolysis or thermal decomposition of hydrocarbons to electrophoretic deposition, hydrothermal routes, electron beam lithography, focused-ion-beam techniques, vapor phase transport, facet-selective etching and pulsed deep reactive etching technologies, among others. A number of applications for the forest-like nanostructures are generalized, for instance as sensors/detectors, photoanodes in solar and fuel cells, supercapacitors and energy storage devices, in SERS applications, optical and MEMs switching devices, water splitting processes, CO2 fixation, and as supports or targets for biomolecules. In general, it is expected that more varieties of compounds and materials with exciting properties can be obtained in this form in the near future, thus expanding numerous applications of forest-like nanostructures.


Introduction

“Nanoforests”, or “forest-like nanostructures”, as a conglomerate of nanotrees and nanobushes and other “nanovegetation”-like nanostructures, as well as simple nanorods, nanopillars, nanowires, nanobars, nanoneedles, nanobelts and other related 1D nanostructures, have been reported for mainly carbon nanotubes,1 although they are known for a few metal oxides (for example ZnO (ref. 2) and TiO2 (ref. 3)), and organic nanoforests of peptide nanotubes are also known.4 The 1D nanostructures above are known emerging materials nowadays, possessing extraordinary properties and a variety of applications, particularly in nanophotonic and optoelectronic devices. Therefore, their agglomerations could be much more effective in comparison with the 1D precursors, in particular due to an increased surface area.5 That’s why a series of novel applications have been found for nanoforests, for instance for the preparation of solderless and durable electrical contacts,6 in the area of photovoltaic devices7 and in thermoelectric uses.8 It is also well-known that 1–3D nanomaterials are widely applied as catalysts in a variety of processes,9–13 in particular for methanol oxidation.14–17 In this respect, current and future achievements in novel catalytic processes on a nanoforest basis could be a big step in further development of this field.

Comprehensive reviews on nanotechnology, containing sections on nanoforests with particular aspects covered, have been recently published.18,19 In this review, we present a concise review of the state of the art of this type of nanostructure, their preparation, peculiarities, and current applications, taking into account that nanoforests (except those of CNTs) can be still considered as a relatively rare morphology, although very promising.

Most common nanoforests

Carbon nanotube forests (CNTFs)

The nanoforests made from carbon nanotubes are indubitably the most studied forest-like structures among the compounds, whose number is certainly limited, and have useful applications. The CNTFs’ peculiarity, in comparison with other carbon and non-carbon species, is that the forests of carbon nanotubes are currently known as the darkest artificially produced materials,20 being able to adsorb the entire visible range of electromagnetic waves much more efficiently than any other black material. They exhibit near-perfect optical absorption (reflectance ∼ 0.045%) due to low reflectance and nanoscale surface roughness. At the same time, carbon nanotubes are able to reflect light like a mirror when the CNTs in the forests are mechanically bent and flattened with proper control.21 Under a controlled mechanical processing of the CNTs, the mirror-like reflection from the processed area with 10–15% reflectivity was observed, having possible applications for the fabrication of monolithically integrated reflector-absorber arrays.

Carbon nanotube forests may include “regular”,22 bamboo-like,23 helical24 and branched structures, as well as a CNT forest-on-forest, where the number of layers in such a system can vary from one25 to two,26 four,27 eight28 or even forty,29 and second- and third-order CNT forest structures.30,31 Integrated simulations of active carbon nanotube forest growth, the diverse CNT forest morphologies, and mechanical compression is discussed in a recent report.32 In particular, CNT forest morphologies may be generally aligned to the growth axis or highly tortuous, with persistently wavy CNTs intermixed with aligned and straight carbon nanotubes. This depends on the height of the CNT forest, the CNT diameter, surface density, and growth conditions. The length of some CNTs may exceed the height of the carbon nanotube forest; different CNTs in the same nanotube forest can be different in tortuosity and length. Comprehensive information on CNT forests is present in a highly recommended excellent comprehensive recent review,33 citing 679 references. This shows an increasing interest in this type of nanostructure having a variety of applications, from catalysis to biosensors.

Synthesis. In general, CNT forests can be synthesized by catalytic pyrolysis on supported catalysts as described below; using various versions of the CVD method;34–36 by a template method with Al2O3 membranes; by graphite sputtering (this technique is used much more rarely); by electrophoresis or dielectrophoresis of CNTs from dispersions37,38 and by chemical grafting of CNTs onto substrates.39 Particular aspects of these methods for the synthesis of CNTFs are mentioned in several reviews.40,41 Among them, we note a wide use of mono- and polymetallic catalysts and supports for their preparation, mainly at high temperatures and using acetylene as a carbon source {although ethylene mixtures with H2 (ref. 42) (the equipment is shown in Fig. 1) or C2H4/H2/H2O/Ar (ref. 43) were also used}. Thus, the growth of dense CNT forests on metallic layers (Mo, Ta, W, and Ir) by thermal decomposition of C2H2 diluted in NH3 was studied.44 The growth process and resulting structure of the CNT forests were shown to depend on the metal substrates, process duration, temperature, and thickness of the stabilizer (Al). In a related report,45 dense CNT forests (height of ∼300 nm and a mass density of 1.2 g cm−3) were prepared on a copper support at 450 °C, using a Co/Al/Mo multilayer catalyst. The nanotubes exhibited very narrow inner spacing. The main characteristics of the formed material are high thermal effusivity and thermal conductivity, suitable for possible uses in heat dissipation devices. Another classic example consists of a vertically aligned SWCNT forest, grown using FePt alloy particles on a MgO substrate.46 Sometimes, a metal-based catalyst should be additionally activated prior to use. Thus, an oxidative pretreatment of an Fe, Co, or Ni growth catalyst on a SiO2 support can be used to switch the growth mode of CNTs from tip growth to root growth.47 Dense vertically aligned nanotube forests can be grown this way. The oxidative treatment effect was explained by the appearance of a strong “catalyst–support (SiO2)” interaction, limiting the surface diffusion, causing sinterization of the catalyst NPs, and then their binding to the surface of SiO2. In addition, vertically aligned small diameter (single- and few-walled) carbon nanotube forests were also grown by thermal CVD over the temperature range 560–800 °C and 10−5 to 14 mbar partial pressure range, using acetylene as the feedstock and Al2O3-supported Fe nanoparticles as the catalyst.48 The mechanism of their formation (Fig. 2) is described by reactions ((1)–(5)); an alternative mechanism is shown in Fig. 3.49 Curiously (Fig. 3G), nanoforest zones consisting of sinuous (Fig. 3G, left section, top) and almost straight (Fig. 3G, right section, bottom) carbon nanotubes can be sometimes formed in the same bulk growth conditions.
 
C2H2gas ↔ C2H2ads (1)
 
C2H2ads ↔ 2CHads (2)
 
CHads ↔ Cads + Hads (3)
 
2Hads → H2gas (4)
 
Cads → CCNT (5)

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Fig. 1 Obtaining CNT forests by CVD method. Reproduced with permission of the American Chemical Society.

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Fig. 2 Schematic of CNT growth process. Copyright. Reproduced with permission of the American Chemical Society.

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Fig. 3 Collective growth mechanism of a CNT forest: (A) growth stages and SEM images of (B) tangled crust at the top of a forest, (C) aligned and dense morphology near the top of a self-terminated forest, (D) less aligned and less dense morphology in the lower region of a self-terminated forest, and (E and F) randomly oriented morphology at the bottom, induced by the loss of the self-supporting forest structure. Reproduced with permission of the American Chemical Society. (G) View of the coexistence of sinuous-like and almost straight carbon nanotubes in the same image (authors’ own data).

Additionally to the metals above used as supports, CVD-assisted formation of a dense CNT (length 13–14 mm and diameter of 10–100 nm) forest is known using spin-coated iron oxide (Fe2O3) thin-film on a Si substrate starting from a mixture of acetylene, hydrogen and nitrogen for 45 min at 700 °C.50 Also, vertically aligned ohmic-conductive carbon nanotube forests were grown on TiSiN refractory conductive films and reached area densities of (5.1 ± 0.1) × 1012 tubes per cm2 and mass densities of approximately 0.3 g cm−3.51 The above support had a function as a diffusion barrier, and the resulting nanoforest grew according to the root growth mechanism. An additional discontinuous AlOx layer, inhibiting catalyst nanoparticle sintering, allowed maximization of the CNT area density.

To optimize high-temperature growth of CNTFs, several studies have been carried out, since an important obstacle of CNT fabrication in industrial mass production is the growth efficiency. Thus, in the case of using C2H2 as a precursor in water-assisted CVD, it was established52 that for 10 min, the optimum growth conditions gave SWCNT forests with ∼350 μm min−1 initial growth rates, ∼2500 μm height, and extended catalyst lifetimes could be reached by increasing the dwell time to ∼5 s. In other research, in order to tune the CVD-growth of CNT forests (DWCNTs or SWCNTs) on wafers, introduction of CO2 is a simple and controlled way.53 When the CO2 concentration is increased, the CNTs forests are transformed as follows: CNTs forests → radial blocks → bowl-shaped forests. It is possible to control the diameter distribution and wall number of the CNTs this way. At 36.8 mol% of CO2, the content of SWCNTs in the forest was found to be 70%. Also, under increased CO2 concentration, a smaller diameter and decreased wall number of CNTs were found. It was suggested that CO2 could be acting as a weak oxidant and generating water. Among other synthesis methods54 for CNTFs, we note high voltage electrophoretic deposition (HVEPD) was used to obtain forests of aligned MWCNTs on long strips of conductive substrates.55

Special studies on CNTF internal structure and other properties. XPS analysis of a CVD-grown forest of MWCNTs using monochromatic Al Kα radiation showed essentially only carbon present (1s peak at 284.5 eV).56 In order to evaluate properties of the “internal content” of the CNTFs, a precise and continuous control method for determining the average diameter of SWCNTs in a forest with diameters ranging from 1.3 to 3.0 nm, with 1 Å resolution was offered.57 This control was reached through the tuning of the catalyst state (composition, size, and density) by applying arc plasma deposition of nanoparticles. These results showed a direct relationship between the achievable height and the diameter of SWCNTs. On this basis, the fundamental difficulty in the fabrication of tall and small diameter SWCNT forests was suggested. In addition, the internal nanostructure of the CNT forest, consisting of mostly empty space between the nanotubes, allows effective capture of photons, yet also allows electrons to escape easily (Fig. 4).58 This makes the possible uses of CNT forests range from photocathodes to solar cells.
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Fig. 4 (Left) A photo of a carbon nanotube forest over 2 mm tall and 5 mm on each side (note the silicon substrate with a thickness of 0.5 mm), effectively forming a macroscopic object. Inset is a scanning electron micrograph of a side-wall of the forest, showing the overall alignment of the nanotubes and the significant inter-nanotube distance. (Right) A schematic of the experiment, showing the electrons exiting from the side-wall of the forest, as well as those emerging from the top surface of the forest. Reproduced with permission of the American Chemical Society.

Vertically aligned CNTs possess a peculiarity of waviness, regardless of the control of their fabrication process. Study of this phenomenon showed59 that this inherent waviness is the main mechanism by which the effective modulus of the CNTs is reduced by several orders of magnitude. The high compliance of the forests of carbon nanotubes was found to be because of the inherent waviness of individual CNTs, and not necessarily due to the bending and buckling of CNTs. The mechanical compression effect on the thermal conductivity of the closely-aligned parallel SWCNTs in the forest was investigated by molecular dynamics simulations.60 Among other findings, the thermal conductivity was shown to be linearly enhanced by increasing compression before the buckling of SWCNT forests. However, the thermal conductivity decreases quickly with further increase of compression after the forest is buckled. The inter-tube van der Waals interaction is strengthened by the compression and the smoothness of the inter-tube interface is maintained during compression. In addition, buckling-driven delamination of CNT forests from their growth substrates when subjected to compression was revealed.61 It was postulated that the post-buckling tensile stresses being developed at the base of the CNT forests, serve as the driving force for delamination. Also, the fundamental dependence of electrical conductance and thermal diffusivity on the diameter and defect level for the aligned SWCNT forests (fabrication scheme see Fig. 5) was evaluated.62 It was definitively concluded that high thermal diffusivity and electrical conductance would be extremely difficult to reach simultaneously by a single SWCNT forest structure using the CVD technique.


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Fig. 5 Schematic of synthesizing SWCNT forests by tailoring Fe thin film thickness and formation temperature. Reproduced with permission of the American Chemical Society.
Liquid flow slippage. Liquid flow slippage over superhydrophobic surfaces made of CNT forests, incorporated into microchannels, was also studied.63 We note that CNTF composites with highly hydrophobic properties can be created by special methods. Thus, a stable superhydrophobic surface was created using the nano-scale roughness inherent in a vertically aligned CNT forest.64 This became possible due to the use of a thin conformal hydrophobic polytetrafluoroethylene (PTFE), which coated the surface of the CNTs. In this material, essentially spherical, micrometer-sized water droplets can be suspended on top of the CNTs forest. This effect could be explained on the basis that the appearing difluorocarbene radicals may attach covalently to the nanotube surface and subsequently polymerize from these sites. The product could have several applications, in particular as fillers for nanocomposites and single strand conductors in molecular electronics. Among other important studies, we mention the linearized Gouy–Chapman–Stern theory of an electric double layer, generalized for morphologically complex and disordered electrodes,65 in particular its significance was illustrated for a forest of nanopillars. The theory allows analysis of the effect of compact layer thickness, concentration, shapes and their fluctuations, developing a general understanding of capacitance in complex interfacial systems.
Applications of CNTFs. The applications of CNTFs and their composites belong to a variety of areas, from academic to technical and medicinal. CNT nanoforests, each with a thin conformal dielectric coating, can provide an economic fabrication of sensitive and uniform SERS templates.66 CNTFs are also considered as particularly promising templates for the formation of porous metal oxides (Al2O3, TiO2, V2O5 and ZnO).67 A bi-layer Au–carbon nanotube composite (a vertically aligned MWCNT forest, sputtered with an Au layer) was fabricated as a potential low-force electrical contact surface for possible applications in MEMs switching devices.68 The penetration of Au atoms into the forest directly influences the electrical characteristics of the composite, depending on loading conditions (the effective resistivities are in the range from 303 nΩ m down to 54 nΩ m).

Sensing uses are also common. As such, a unique combination process of a sharp Si microneedle array and MWCNT forest has been developed and applied as a reference electrode for non-enzymatic glucose sensing.69 The registered sensitivity was found to be 17.73 ± 3 μA mM−1 cm−2. This electrode can be used for painless diabetes testing applications. Also, carbon nanofiber forests (CNFFs) grown on glass microballoons are able to directly detect Plasmodium falciparum histidine rich protein-2 (PfHRP-2) antigens at as low as 0.025 ng mL−1 concentrations in phosphate buffered saline.70 This effect can be applied for the early diagnosis of malaria and other infectious diseases. Among the technical applications, CNT forests have been revealed to reduce the access of abrasive particles to the compressible sealing elements of joints.71 In addition, a spray-based coating technique was applied for the deposition of nanoscale coatings of polystyrene and poly-3-hexylthiophene onto carbon nanotube forests, to act as a bonding medium that produces thermal resistance by expanding the area available for heat transfer at the CNT contacts.72

Diamond-based nanoforests. Diamond-based nanoforests are known, in addition to the CNTFs and CNFFs above. Thus, an electrode with a 3D structure on the basis of a boron-doped diamond nanorod forest (BDDNF) was fabricated by a hot filament CVD method (HFCVD) method (Fig. 6).73 In its preparation, an electroless metal deposition (EMD) method and HFCVD technique were combined for growing the BDDNF on Si nanowires; as a result, a 2D B-doped diamond electrode was transformed into the 3D analogue. This electrode was found to exhibit a better selectivity and sensitivity for biomolecule detection (for example, for glucose oxidation in basic conditions) in comparison with the conventional planar B-doped diamond electrodes.
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Fig. 6 Schematic of the fabrication of BDDNF. Reproduced with permission of the American Chemical Society.

Zinc oxide nanoforests

Hydrothermal-assisted synthesis. Zinc oxide, an extraordinary compound in nanotechnology, which can exist in a huge variety of structural types, from classic to less-common, is widely presented in forest-like forms, similarly to the CNTFs above. The ZnO nanoforests can be prepared by a series of techniques, mainly by a hydrothermal route.74 The methods for the controlled growth of ZnO nanowires, leading, in particular, to nanoforest formation, are reviewed in ref. 75. In a typical synthesis, 3D ZnO willow-like nanoforests (Fig. 7) were prepared via a facile hydrothermal route using ZnO nanobranches on preformed ZnO nanowire arrays, thus representing a wonderful morphology-controlled synthesis as by tuning the ammonia and PEI (polyethylenimine) concentrations systematically the architecture of ZnO nanoforests can be influenced.76 This unique product has a prominent PEC water splitting performance: a high photoconversion efficiency of 0.299% at 0.89 V (vs. RHE) was observed. In a related report,77 a high density nanoforest containing long branched tree-like multigeneration hierarchical ZnO nanowire photoanodes was hydrothermally fabricated (Fig. 8 and 9). For this branched material, the light conversion was found to be 5 times more than the efficiency of the materials based on upstanding ZnO nanowires, due to highly enhanced surface area and reduced charge recombination. Additionally, flexible solar cells could be made using this obtained nanoforest. In addition, the hydrothermal methods can be combined with other techniques, as it was reported, for example,78 for electron beam lithography (EBL). The EBL in combination with subsequent hydrothermal synthesis (Fig. 10 and 11) was applied to fabricate patterned ZnO nanorod arrays with different spacing distances and densities on a silicon (Si) substrate. The geometric parameters of the ZnO nanorod arrays can be expediently controlled in this process. The purpose of the use of EBL was to fabricate the patterned ZnO seed layers with different spacing distances and areas with high precision, and the next step, a hydrothermal growth method, was used to control the density and morphologies of the ZnO nanorod arrays. Such a combination allows the integration of patterned arrays into real devices.
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Fig. 7 SEM images of the obtained ZnO nanoforests after growing ZnO nanobranches on the preformed ZnO nanowire arrays in precursor solutions with different concentrations of ammonia but without PEI. Reproduced with permission of the Royal Society of Chemistry.

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Fig. 8 Two routes for hierarchical ZnO NW hydrothermal growth. Length growth (LG) (a–c), branched growth (BG) (a, b and d), and hybrids (a–e). Notice polymer removal and seed NPs for branched growth. Reproduced with permission of the American Chemical Society.

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Fig. 9 SEM pictures of ZnO nanowire forest: (a) tilted view, (b) cross section view, (d) magnified view of backbone (red dotted line) and first generation branches (blue dotted line), and (e) magnified view of a branch. (c) TEM picture and selected area electron diffraction pattern of a ZnO nanowire. Reproduced with permission of the American Chemical Society.

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Fig. 10 Schematics of the experimental procedures of patterned ZnO nanorod arrays. (a) Spincoating PMMA on a Si substrate; (b) pattern fabrication by EBL method; (c) magnetron sputtering a ZnO seed layer; (d) strip PMMA in acetone solution; (e) hydrothermal growth of ZnO nanorods on the patterned areas. Reproduced with permission of Springer.

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Fig. 11 Schematics of the apparatus used for the synthesis of ZnO nanorod arrays. Reproduced with permission of Springer.

Other methods for ZnO nanoforest fabrication have also been used, although more rarely. Multiple-generation-step deposits of branched ZnO nanostructures were carried out by vapor phase transport.79 The second generation of ZnO nanowires was grown on the first one. In depositing the third generation, the diameter of the branches decreases and the number of branches increases, and the fourth generation leads to the nanoforest-like morphology. On comparing such branched ZnO forest-like nanostructures with ZnO upstanding nanowire analogues it was established80 that the light-conversion efficiency for the dye-sensitized solar cells (DSSCs) of the high-density ZnO nanoforest, made of branched ZnO nanowire photoanodes, was found to be about 5 times higher than that of DSSCs composed of upstanding ZnO nanowires. Applying structural approaches to reach a large increase of the surface area for ZnO nanostructure, ZnO 3D nanostructures ({1011}-stacked nanocones and {1010}-nanoforest) were fabricated by facet-selective etching and oriented nanocrystal growth, respectively.81 In comparison with the original ZnO hexagonal nanocone structures, the 3D structures above exhibited much better photocatalytic properties for the photodegradation of rhodamine B. Growth control can also be carried out by other methods. The control of ZnO nanowire growth with uniform height, diameter, and high crystalline quality was studied by a focused-ion-beam (FIB) assisted approach, using a Au–Ga alloy catalyst at 880–940 °C.82 Observing the differences in growth behavior and mechanisms for ZnO nanowires using Au and Au–Ga, it was revealed that, in particular, the FIB-assisted process led to improved nanowire uniformity. In addition, a straightforward method, based on a two-photon absorption of a gating photon and a probe photon, was developed to measure the diffusive dwell time of light inside ZnO nanowire forests.83 It was suggested that the light dwell time can be predicted well from SEM images.

Doped or mixed composite nanoforests of ZnO with other metal oxides are also known. 3D core/shell ZnO/MnO2 branched nanowire arrays (Fig. 12), fabricated as shown in Fig. 13, exhibited an areal capacitance five times larger, smaller inner resistance and better rate performance than their nanowire array counterparts.84 Electrodes on this 3D nanoforest basis possess considerable application potential in miniaturized energy storage devices. In addition, nanosecond pulsed laser deposition (Fig. 14 and 15),85 used for the preparation of a forest of Al-doped ZnO nanotrees at high O2 pressures, can be also applied to fabricate other metal oxides important for technological applications such as Nb2O5 (see below), TiO2, Al2O3, WO3 and Ag4O4.


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Fig. 12 Typical SEM images of the synthesized 3D ZnO@MnO2 core@shell (a & b) nanowire arrays and (c and d) forest of nanotrees. Reproduced with permission of the Royal Society of Chemistry.

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Fig. 13 Schematic illustration of the fabrication process for the designed 3D ZnO/MnO2 core/shell nanowire array electrode and the nanoforest counterpart. Reproduced with permission of the Royal Society of Chemistry.

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Fig. 14 Scheme of the pulsed laser deposition apparatus. Reproduced with permission from Journal of Visualized Experiments.

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Fig. 15 Pictorial view of the deposition process in vacuum and in the presence of inert and reactive gases. Reproduced with permission from Journal of Visualized Experiments.

Silicon nanoforests

Silicon nanowires (SiNW) are known as excellent materials for use in highly-efficient and cost effective thermoelectric devices.86 These applications were reviewed recently, for instance, for the application of electrical SiNW-based devices in the gas phase.87 The top-down techniques of their production including oxidation, lithography, and anisotropic etching (plasma, wet, and metal-assisted) are well-established.88 Si nanowire nanoforests should therefore have much demand and many applications in distinct device types.89,90 A 3D symmetric micro-supercapacitor was created with polypyrrole (PPy) coated silicon nanotree (SiNTr) hybrid electrodes fabricated by a two-step process including Si nanotree CVD-assisted growth on Si substrates and further electrochemical deposition of the conducting polymer.91 A resulting remarkable cycling stability after thousands of cycles showing a loss of approximately 30% was revealed. A low-reflective “black silicon” surface was produced by a pulsed deep reactive etching technology at r.t., varying the bias power duty cycle and etching window size.92 At a 0.25 duty cycle, the height of the Si forest was found to be to about 10 μm and had 0.9% reflectance. Also, nanopillar-forests (heights of several microns, density 20 μm−2, and tip diameters 5–10 nm) with numerous nanoscale gaps were fabricated on the basis of the advantage of convex areas on poly-Si surfaces to act as support structures in side-wall technology.93 Showing a high SERS-active capability, they may have applications in biological monitoring and chemical detection.

Water splitting continues nowadays to be a hot topic in nanotechnology and the creation of novel ecomaterials, in particular those made with silicon. As such, an artificial photosynthesis system containing an interfacial layer for charge transport, two semiconductor light absorbers with large surface area, and spatially separated co-catalysts to facilitate the water reduction and oxidation, based on a Si nanowire array (Fig. 16) was used to reach a 0.12% solar-to-fuel conversion efficiency. The efficiency of these results is comparable with natural photosynthesis processes.94


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Fig. 16 Schematics of the asymmetric nanoscale tree-like heterostructures used for solar-driven water splitting: (a) structural schematics of the nanotree heterostructure. The small diameter TiO2 nanowires (blue) were grown on the upper half of a Si nanowire (gray), and the two semiconductors absorb different regions of the solar spectrum. The two insets display the photoexcited electron–hole pairs that are separated at the semiconductor–electrolyte interface to carry out water splitting with the help of co-catalysts (yellow and gray dots on the surface); (b) energy band diagram of the nanotree heterostructure for solar-driven water splitting. The photogenerated electrons in Si and the holes in TiO2 move to the surface to perform water splitting, while the holes in Si and electrons in TiO2 recombine at the ohmic contact between the two semiconductors. Reproduced with permission of the American Chemical Society.

TiO2 and other transition metal oxides and their hydrated forms

Similarly to the ZnO nanoforests described above, titanium dioxide nanoforests are frequently prepared by hydrothermal synthesis95 and applied mainly for solar cell purposes as photoanodes with high conversion energy efficiency.96 Thus, this technique was used to obtain TiO2 hierarchical nanoforest structures without the use of any template or additive (reactions (6) and (7)).97 The transformation mechanism of “nanorod → nanotree” arrays was proposed and shown in Fig. 17. These types of structures combining the properties of 1D and 3D nanostructures could have more interesting properties than the simple arrays of nanorods because of their higher porosity and specific surface areas, where the nanobranches have good connections to the trunk. Maximum energy conversion efficiency was observed using DSSCs made of these films containing a thin “adhesive” layer of nanocrystalline TiO2, for higher dye loading and light harvesting.
 
H2SO4 + H2Ti2O5·H2O → Ti(SO4)2 + H2O (6)
 
Ti(SO4)2 + H2O → TiO2 + H2SO4 (7)

image file: c5ra22738k-f17.tif
Fig. 17 (a)–(d) Schematic procedure for the formation of the forest-like hierarchical photoanodes. (e) A cartoon of the presumed preferential electron pathway in the hierarchical photoanodes. Reproduced with permission of the Royal Society of Chemistry.

Flexible DSSCs (reviewed in ref. 98) are also currently an area of elevated interest. Highly efficient Ti substrate-based all-flexible DSSCs, containing a TiO2 nanoforest underlayer (prepared by 3-step process using acid, H2O2, and TiCl4 treatment), have been prepared.99 These DSSCs showed relatively high conversion energy efficiency of 8.46%. As an example of another battery anode application, the nanoforests of parallel self-organized sodium titanate/titania nanotrees were obtained by a several-step process including an anodic oxidation of Ti foil to form amorphous TiO2 nanotubes, sodium insertion, thermal dehydration and crystallization of Na2Ti6O13/rutile nanotrees.100 The height (about 8.0 μm) of the nanotubes was found to be similar to the nanoforest, but the morphology varied from aligned nanotubes to complex-textured nanotrees. This material can be applied as a high-performance anode for Na-ion microbatteries.

Other transition metal oxides are represented by a few examples, as, for instance Co3O4@NiCo2O4 nanoforests.101 Single-crystalline β-cobalt hydroxide {β-Co(OH)2} hexagonal-phase nanostructures with distinct morphologies, in particular having a forest-like shape, were hydrothermally prepared in a large scale in supercritical water (SCW) and triethylamine (an alkali and a complexing reagent) from Co powder (the metal source).102 Varying the ratio “SCW[thin space (1/6-em)]:[thin space (1/6-em)]triethylamine”, branched (nanoforest, Fig. 18, ratio 2[thin space (1/6-em)]:[thin space (1/6-em)]1) or non-branched nanowires (ratio 1[thin space (1/6-em)]:[thin space (1/6-em)]0), and nanobelts having branched nanoneedles (ratio 1[thin space (1/6-em)]:[thin space (1/6-em)]2) were obtained.


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Fig. 18 Images of a forest-like cobalt hydroxide structure. Reproduced with permission of Elsevier Science.

Nanoforests of vertically aligned Nb2O5 nanocrystals were prepared by pulsed laser deposition (see description above).103 It was found that a partial pressure of oxygen is needed to develop this growth, revealing the importance of gas composition and pressure. The formed material was tested in a DSSC as a photoanode material. Also, a forest structure on the basis of vertically aligned VO2(B) nanobelts (NBs) was solvothermally prepared using a vertically oriented graphene (VOG) network as the underlying support (Fig. 19).104 After its further expansion to a 3D folded forest using folded conductive Ni foam (Fig. 20), the final material was tested as an electrode for energy storage, showing an excellent performance confirmed by a stable discharge capacity and high cycling stability (>2000 cycles).


image file: c5ra22738k-f19.tif
Fig. 19 (a) Schematic showing the synthesis of a VO2(B) NB-based forest structure on a VOG-coated flat substrate. (b) Lattice structure illustrating the double-layered crystal structure of VO2(B). Crystal directions a, b and c represent [100], [010], [001] directions, respectively. (c) Cross-sectional SEM image of VOG. (d and e) SEM images of a representative VO2(B) NB forest with the width of an individual NB of 1–2 mm. The inset of (e) depicts a photo of artificial turf for comparison. (f) High-resolution TEM image indicating the single-crystal structure of an individual NB. The lattice spacing of 0.35 nm corresponds to the (110) crystal plane of VO2(B). Reproduced with permission of the Royal Society of Chemistry.

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Fig. 20 (a) Schematic of 3D VO2(B) NB forest synthesis showing that the EOG structure is first grown around the Ni foam skeleton, and then, using the EOG/Ni foam as a support, a 3D VO2(B) NB forest structure is synthesized inside the foam. (b and c) SEM images of the EOG/Ni foam. The inset of (c) is the TEM image of the EOG flake. (d and e) SEM images of the 3D VO2(B) NB forest. The inset of (d) is the cross-sectional view SEM image of the VO2(B) NB forest in the EOG/Ni foam that indicates most space in the foam is occupied by the VO2(B) active material. The top inset of (e) is the TEM image of an individual NB, and the bottom inset of (e) is the cross-sectional view SEM image of the NB forest. Reproduced with permission of the Royal Society of Chemistry.

Mono- and polymetallic nanoforests

Metal-based nanoforests are limited to Au, Sn and some Pt/Ag nanostructures. Highly unconventional anisotropic growth of a forest made of gold nanowires from nanocrystals was recently discussed.105 In particular, (a) nanowires cannot grow from colloidal seeds, but they can grow from substrate-bound seeds, (b) nanowires can only grow from one side of the seeds, and their diameter is independent to the seed size, (c) ultrathin, vertically aligned nanowires were observed on substrates with use of aqueous solution at r.t. It was suggested that, in this system, the strong binding of ligands leads to the selective deposition of gold at the ligand-deficient interface between the Au seeds and oxide substrates (Fig. 21). Gold nanoforests have certain applications. Densely packed Au nanowires were vertically grown on 3-aminopropyltriethoxy silane functionalized glass slides106 and used as a sensor for the multiple detection of malachite green, 1-naphthalenethiol and rhodamine 6G in aqueous solution (limit of detection of 10−6 M). A complex 3D nanoforest on the basis of Sn nanorods having a core–shell structure was grown on genetically engineered viral scaffolds.107 The resulting material exhibited supreme capacity utilization and cycling stability toward Na-ion storage and release.
image file: c5ra22738k-f21.tif
Fig. 21 (a and b) Schematics illustrating (a) the specific conditions used for the syntheses of AuNWs on Si/SiO2 substrates; (b) because of the strong ligands, the Au deposition selectively occurs at the Au substrate interface. Reproduced with permission of the American Chemical Society.

Pt–Ag tubular dendritic nanoforests (tDNFs) were grown by a two-step galvanic replacement reaction (Fig. 22) at r.t. and applied in a cost-effective methanol oxidation reaction under solar irradiation.108 The first step was the growth of Ag nanoforests (10 μm in thickness) on a silicon wafer in AgNO3 solution, and then H2PtCl6 was used as a precursor for Pt deposition, converting the silver dendritic nanoforests into Pt–Ag tDNFs. The observed solar response (6.4% enhancement of the oxidation current) was attributed to the strong LSPR due to the Ag DNFs, so this material can be used in photo-electro-chemical fuel cells. In addition, the nanodendrite forest-like trimetallic structures composed of Pt, Au, and Ag showed excellent catalytic activity for a methanol oxidation reaction in comparison with bare Pt electrodes due to a larger active surface area.109


image file: c5ra22738k-f22.tif
Fig. 22 The schematic drawing of the two-step GRR process. Reproduced with permission of Springer.

Ga/P and Ga/As nanoforests

These nanoforests can be applied for distinct uses, in particular, as supports for biomolecules. Thus, the formation of fluid supported fluorescence labeled phospholipid bilayers, used as models for biomembranes, was observed as self-assembly from vesicles in solution onto GaP vertical nanowire forests.110 Another application was reported111 in the area of functional optical devices, which was fabrication based on a GaAs/AlGaAs nanowire forest by sectioning quantum-dot-in-nanowire systems by a “nanoskiving” process (Fig. 23). The quantum dots inside the nanowires are functional, exhibiting a photoluminescent emission (wavelengths 650–710 nm).
image file: c5ra22738k-f23.tif
Fig. 23 Nanoskiving process. (a) The GaAs/AlGaAs nanowire forest is grown by MBE on a silicon wafer. (b) The wires are hexagonal with a GaAs core and an Al0.75Ga0.25As shell. (c) The nanowire forest is embedded in epoxy and UV-cured. (d) The block of epoxy with the embedded nanowires is then cleaved and sliced on an ultramicrotome with a diamond blade. (e) The slices are transferred onto a new substrate. (f) Optionally, the epoxy can be etched by oxygen plasma, leaving the slices freestanding on the wafer. (g) The core can also be etched by citric acid, leaving hollow-core slices. Reproduced with permission of the American Chemical Society.

Organic nanoforests

Nanoforests based on organic compounds are very rare as mentioned in reviews112,113 and are made mainly of peptides, for example diphenylalanine.114 Thus, peptide nanoforest (Fig. 24) biosensors, tested for phenol detection, were found to be more sensitive than those modified with carbon nanotubes or combined coating and 17-fold more sensitive than the uncoated electrode.115 Several applications for organic nanoforests, for instance for tobacco mosaic virus nanoforest arrays, were achieved in preparation of 3D patterned LiFePO4 nanorods, used in lithium batteries.116
image file: c5ra22738k-f24.tif
Fig. 24 FF (diphenylalanine) peptide nanoforest deposited on an electrode, illustration and SEM image, respectively. Reproduced with permission of John Wiley & Sons.

Conclusions

Nano- and micrometric forest-like structures are generally made of more simple 1D nanostructures, such as nanowires, nanopillars, nanorods, nanotrees, nanofibers or nanotubes. They are generally known for simple inorganic compounds and well-studied mainly for carbon nanotubes and zinc oxide. In case of CNTs, their nanoforests are currently known as the darkest artificially produced materials. For different compounds, synthesis methods can vary. For CNT nanoforests, the main methods are CVD/spray pyrolysis/thermal decomposition of hydrocarbons such as C2H2, growing the nanoforest on metal catalysts, or electrophoretic deposition. For ZnO nanoforests the principal method is a hydrothermal route. Other methods include electron beam lithography (as well as a lithography-free approach117), focused-ion-beam technique, vapor phase transport, facet-selective etching and oriented nanocrystal growth. For the formation of silicon nanoforests, pulsed deep reactive etching technology and pulsed laser deposition are preferable.

Nowadays, nanoforests possess a variety of applications, for instance as sensors/detectors for glucose, phenol, malachite green, 1-naphthalenethiol, and rhodamine 6G, among others (CNT, Au, and organic nanoforests), as photoanodes in solar118 and fuel cells with high photoconversion efficiency, supercapacitors and energy storage devices (ZnO, Pt–Ag, Si, TiO2), in SERS applications (CNTs, Si), and as optical and MEMs switching devices (CNTs, GaAs/AlGaAs), as well as for water splitting processes (Si), CO2 fixation,119 and as supports for biomolecules (GaP). Vertical nanowires can be applied to study proteins (targeting a range of different drugs), which are active in the cell membranes, so nanoforests could be of great importance for both pharmaceutical research as basic cell research.120 One of causes for their advantages is that nanoforests have shown much larger surface areas in comparison with non-branched/upstanding nanowires and nanorods. As a concluding remark, forest-like nanostructures are currently actively studied121,122 and it is expected that more varieties of compounds can be obtained in this form and find useful applications.

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